JP4296274B2 - Lithium manganate positive electrode active material and all-solid lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium manganate positive electrode active material suitable for an all solid lithium secondary battery using an inorganic solid electrolyte as an electrolyte, and an all solid lithium secondary battery using a positive electrode containing the active material.

  As mobile devices such as mobile phones, PDAs, and notebook computers have been rapidly spread and enhanced in recent years, the demand for higher performance, lower cost, and improved safety of secondary batteries as power sources has become severe. Is increasing.

  As a secondary battery capable of meeting such demand, the production amount of a lithium secondary battery having the maximum energy density among the current secondary batteries is rapidly increasing. A lithium secondary battery is usually composed of a positive electrode using a lithium-transition metal composite oxide such as lithium cobaltate, an organic electrolyte, and a carbon negative electrode.

  Among the constituent members of the lithium secondary battery, the positive electrode is one of important constituent members that generally determine the battery voltage and capacity. Lithium cobalt oxide, which is currently widely used as a positive electrode active material, has high operating voltage, high capacity, and high reliability. On the other hand, cobalt resources are scarce and unevenly distributed, leading to higher raw material costs and lower costs. There is a need for the development of inexpensive cathode active materials for

A lithium manganate (LiMn ₂ O ₄ ) -based material has been proposed as a material that can replace the lithium cobaltate-based positive electrode active material. The reason is that the lithium manganate-based material has charge / discharge characteristics similar to those of the lithium cobaltate-based material, and can use as a raw material a manganese compound that is resource-rich and inexpensive compared to the cobalt compound. .

  On the other hand, from the aspect of improving safety, all-solid lithium secondary batteries using an inorganic solid electrolyte that is chemically stable and has no risk of leakage as compared with current organic electrolytes have been proposed. Research and development is underway.

  For example, an all-solid lithium secondary battery using lithium cobaltate or lithium nickelate as a positive electrode active material and using a sulfide-based solid electrolyte is excellent charge / discharge comparable to a lithium secondary battery using an organic electrolyte. It has been clarified to have characteristics (see Patent Document 1). An all-solid lithium secondary battery using a solid electrolyte made of a crystalline oxide having lithium ion conductivity using lithium manganate as an active material for positive and negative electrodes has also been proposed (see Patent Document 2).

  However, in these all-solid lithium secondary batteries, in order to obtain charge / discharge cycle characteristics equivalent to or better than those of lithium secondary batteries using organic electrolytes, the positive electrode active material-solid electrolyte contact interface closely related to the charge / discharge cycle characteristics In view of the lithium ion migration in the battery, it has not been clarified yet what material characteristics are necessary for the lithium manganate positive electrode active material for the secondary battery.

Therefore, it is difficult to develop a low-cost all-solid lithium secondary battery having good charge / discharge cycle characteristics without elucidating such material characteristics.
JP-A-8-162151 JP 2001-210374 A

  The main object of the present invention is to provide a lithium manganate-based positive electrode active material suitable for use in an all-solid lithium secondary battery that has a high capacity, suppresses capacity deterioration, and has excellent charge / discharge cycle characteristics.

  As a result of intensive studies to achieve the above object, the present inventor has achieved excellent battery characteristics by precisely controlling the chemical composition and particle size distribution of lithium manganate having a cubic spinel structure. The inventors have found that a positive electrode active material suitable for a solid lithium secondary battery can be obtained, and have completed the present invention.

  The present invention provides the following lithium manganate-based positive electrode active material, a positive electrode including the same, and an all-solid lithium secondary battery.

1. It has a cubic spinel structure and has the composition formula Li _{1 + x} (Mn _{2- y My} ) _2-x O ₄ (I)
(In the formula, M represents Al, Mg, Cr, Mn, Fe, Co, Ni, Cu, or Zn. When M is Mn, x is a number in the range of 0 <x <0.33. In the case where M is other than Mn, the number is in the range of 0 ≦ x <0.33. Y is the number in the range of 0 ≦ y ≦ 1, and the particle size is 1 μm or less. A lithium manganate-based positive electrode active material having an abundance ratio of 40% by weight or less.

  2. A positive electrode for an all-solid lithium secondary battery, comprising the positive electrode active material according to Item 1.

  3. An all-solid lithium secondary battery using the positive electrode described in the above item 2 and using an inorganic solid electrolyte.

  4). Item 4. The all-solid lithium secondary battery according to Item 3, wherein the inorganic solid electrolyte is a sulfide-based inorganic solid electrolyte.

  The lithium manganate positive electrode active material of the present invention has a high capacity and suppresses the deterioration of capacity, and can be suitably used for an all solid lithium secondary battery excellent in charge / discharge cycle characteristics.

  Therefore, the all-solid-state lithium secondary battery using the lithium manganate positive electrode active material of the present invention can contribute to cost reduction of the lithium secondary battery.

Lithium manganate-based positive electrode active material The lithium manganate-based positive electrode active material of the present invention has a specific chemical composition focused on the Li / Mn ratio, and can produce particles with a specific particle size distribution, that is, 1 μm or less (submicron). Characterized by having as much reduced powder properties.

Chemical Composition The lithium manganate positive electrode active material of the present invention has a cubic spinel structure, and has the composition formula Li _{1 + x} (Mn _{2− y My} ) _2−x O ₄ (I)
(In the formula, M represents Al, Mg, Cr, Mn, Fe, Co, Ni, Cu, or Zn. When M is Mn, x is a number in the range of 0 <x <0.33. , M is other than Mn, the number is in the range of 0 ≦ x <0.33, and y is the number in the range of 0 ≦ y ≦ 1.

In the composition formula (I), when M is Mn, that is, in the case of lithium manganate not containing a different metal ion, it is desirable that the amount of excess lithium is not x = 0 (LiMn ₂ O ₄ ). This, when you try to prepare a composition of LiMn ₂ O _4, previously reported (T. Nakamura and A. Kajiyama, Solid State Ionics, 133, 195-202, (2000) and R. Kanno, M. Yonemura, T As described in Kohigashi, Y. Kawamoto, M. Tabuchi and T. Kamiyama, J. Power Sources, 97-98, 423-426, (2001)), the oxygen-deficient phase LiMn ₂ O _4-d ( This is because 0 <d <0.03) is likely to occur, and the cubic structure becomes unstable, and as a result, it has been reported that significant cycle deterioration of charge / discharge capacity is induced as compared with a sample of x> 0.

Further, when x = 0.33, the composition becomes Li _1.33 Mn _1.67 O ₄ , and the formal charge of manganese becomes 3.99, which is almost tetravalent. In the case of lithium manganate, trivalent manganese changes to tetravalent by charging, so that lithium can be removed and charge capacity can be obtained. Currently used lithium secondary batteries do not have lithium ions in the structure. Considering that carbon materials are used and cannot be used as a battery unless they are charged, Li _1.33 Mn _1.67 O ₄ composition with a tetravalent manganese valence cannot be used for charging, so it should be used as a positive electrode active material for lithium secondary batteries. I can't. Therefore, it is desirable that the x value is located at 0 <x <0.33 when M = Mn.

On the other hand, in the composition formula (I), when M is a metal other than Mn, that is, when a different metal other than Mn is included, for example, in LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiCoMnO ₄ , Mn ions are Although all are tetravalent, since Ni ions and Co ions work as chargeable / dischargeable elements, there is no problem even if x = 0. Such a compound charges and discharges in a 5 V region, which is higher than 4 V, which is the oxidation-reduction potential of Mn ions. Therefore, all-solid lithium secondary batteries using an inorganic solid electrolyte that is superior in chemical stability to organic electrolytes are used. It can be suitably used as a positive electrode active material for a secondary battery.

  The y value can vary between 0 and 1 depending on the valence of the metal species used and the operating voltage during charging and discharging. This is because it is defined by the manganese valence (changes only between trivalent and tetravalent) in the composition formula. When Ni ions are dissolved as described above, since Ni ions are divalent, y can be dissolved only up to a composition in which manganese is tetravalent in the composition formula (I). Is the solid solubility limit. On the other hand, since Co and Cr ions are trivalent and form a solid solution, all the manganese becomes tetravalent until y = 1.

Particle Size Distribution In the lithium manganate positive electrode active material of the present invention, the abundance ratio of particles of 1 μm or less is defined as 40% by weight or less. The reason is as follows.

  In the all-solid-state lithium secondary battery, the positive electrode active material exchanges lithium ions and electrons between the solid electrolyte and the current collector on the surface of the powder during charge / discharge, so that the larger the specific surface area of the powder is, the larger ( It can be expected that the smaller the particle size), the easier the exchange of ions and electrons at the interface. However, particles with a small particle size are poor in packing properties and contain many voids in the electrode. As a result, the interface contact area with the solid electrolyte and current collector is reduced, and the charge / discharge cycle characteristics may not be improved. Conceivable. Therefore, it is necessary to reduce the amount of fine particles of 1 μm or less (submicron) as much as possible.

Therefore, the lithium manganate positive electrode active material in the present invention is required to have an abundance ratio of particles of 1 μm or less, that is, a content of 40% by weight or less. Further, the particle abundance ratio of 1 μm or less of the substance is preferably 35% by weight or less.

  The average particle diameter of the lithium manganate positive electrode active material of the present invention is usually 10 μm or less. A method for estimating the submicron particle content in the powder sample is not particularly limited, and a normal particle size distribution meter, a method by observation with a scanning electron microscope, or the like can be employed. For example, using a laser diffraction / scattering particle size distribution analyzer, the sample is dispersed in a 0.2% by weight sodium hexametaphosphate aqueous solution, and then the particle size distribution is measured in the range from 0.03 μm to 280 μm to 1 μm or less. The particle content can be calculated.

Manufacturing method of lithium manganate positive electrode active material The manufacturing method of the lithium manganate positive electrode active material having a specific chemical composition and a specific particle size distribution of the present invention is not particularly limited.

  For example, a lithium source, a manganese source, and a dissimilar metal source are dry-mixed or wet-mixed at a predetermined molar ratio and fired in a temperature range of about 500 to 1,200 ° C. in the air, in a reducing atmosphere, in an oxidizing atmosphere, or the like. If necessary, it can be prepared by classification.

  In the above manufacturing method, in a firing temperature range of 800 ° C. or higher, Li volatilization becomes significant, and therefore, an excess lithium source from the target composition may be added and fired. Further, the amount of submicron particles may be further reduced by classifying the obtained sample using a sieve or the like.

  As the lithium source, for example, anhydrides such as lithium hydroxide, lithium carbonate, lithium acetate, lithium nitrate, and lithium chloride, hydrates, and the like can be used. As the manganese source, for example, manganese hydroxide, manganese oxide hydroxide, manganese oxalate, manganese oxide, manganese carbonate, manganese acetate, manganese nitrate, manganese chloride and other anhydrides and hydrates can be used. . Examples of the different metal source include metal hydroxides such as Al, Mg, Cr, Fe, Co, Ni, Cu, and Zn, oxides, oxide hydroxides, carbonates, oxalates, nitrates, and acetates. Further, anhydrides such as chlorides, hydrates, and the like can be used.

The obtained lithium manganate-based positive electrode active material is desirably a single phase of lithium manganate phase as much as possible, but lithium carbonate, lithium hydroxide, other lithium manganese, and so on, as long as the charge / discharge characteristics are not seriously deteriorated. Oxides (Li ₂ MnO ₃ , LiMnO _2, etc.), manganese oxides, dissimilar metal oxides, and the like may be included.

  The obtained lithium manganate-based positive electrode active material of the present invention has a high function as a positive electrode active material, and if necessary, a conductive material, a binder resin, etc. are added, and a positive electrode is produced according to a conventional method. Can do.

All-solid lithium secondary battery An all-solid lithium secondary battery having excellent charge / discharge cycle characteristics and the like is prepared using a positive electrode, a solid electrolyte, a negative electrode, etc., produced using the above-described lithium manganate-based positive electrode active material of the present invention. be able to.

  The battery component is not particularly limited except that the positive electrode is used and an inorganic solid electrolyte is used as the electrolyte. As the inorganic solid electrolyte, for example, a sulfide solid electrolyte, an oxyacid salt solid electrolyte, or the like can be used. Among them, the sulfide-based solid electrolyte is particularly preferable because it exhibits high ionic conductivity at room temperature and the battery can be easily manufactured because of its low grain boundary resistance.

Examples of the sulfide-based solid electrolyte include Li ₂ S—P ₂ S ₅ , Li ₂ S—B ₂ S ₃ , and Li ₂ S.
A glass-based electrolyte such as SiS ₂ or a crystalline electrolyte such as Li _4-x Ge _1-x P _x S ₄ (0.2 ≦ 0x ≦ 0.8) can be used. Further, as the oxyacid salt solid electrolyte, for example, Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.4 Al _0.4 Ge _1.6 (PO ₄ ) ₃ can be used.

  The solid electrolyte may be used alone or as a mixture, and may be mixed not only between the positive electrode and the negative electrode but also on the positive electrode side in order to increase the interface contact area with the lithium manganate positive electrode active material. As the mixing method, not only normal dry mixing but also wet mixing using a solvent such as acetonitrile can be used as needed. The type of the electrolyte separating the solid electrolyte and the positive electrode-negative electrode placed on the positive electrode side does not need to be matched as long as the charge / discharge characteristics are not significantly affected.

  As the negative electrode, in addition to the carbon material used for the lithium secondary battery, metallic lithium, metallic indium, metallic aluminum, lithium titanate, or the like can be used.

  The all solid lithium secondary battery of the present invention can be produced, for example, as follows. That is, lithium manganate-based positive electrode active material powder and sulfide-based solid electrolyte powder were weighed in a glove box at a weight ratio of 7: 3, placed in a reagent bottle, added with acetonitrile solvent and zirconia balls, and shaken well. Thereafter, the solvent is removed by vacuum drying at 65 ° C. for about 1 hour without exposure to the atmosphere to obtain a positive electrode mixture powder. A sulfide solid electrolyte powder is put into a polyethylene terephthalate (PET) tube, sandwiched between metal push bars from above and below, and pressed at 185 MPa to form a solid electrolyte layer. The positive electrode composite powder produced on one side is put and pressed in the same manner at 370 MPa, and then a titanium mesh and a titanium foil as current collectors are put on the positive electrode mixture and pressed again at 555 MPa.

  FIG. 1 shows a cross-sectional view of an example of an all-solid lithium secondary battery. An all-solid lithium secondary battery can be produced by incorporating the PET tube including the positive electrode and the electrolyte layer into the lithium secondary battery of FIG. 1, inserting an indium metal foil as the negative electrode on the side opposite to the positive electrode, and pressing again from above. .

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not limited by an Example.

  The production phase obtained in each example was confirmed by powder X-ray diffraction measurement, and the shape of the lithium manganate powder was observed with a scanning electron microscope (SEM). The particle size distribution of the lithium manganate powder was measured using a laser diffraction / scattering particle size analyzer.

Example 1
Manganese oxalate dihydrate 0.1 mol (17.90 g) was weighed and pyrolyzed in an electric furnace at 400 ° C. in the air for 5 hours to produce 0.05 mol of Mn ₂ O ₃ fine powder. Lithium hydroxide monohydrate was added to the obtained powder in an amount (0.566 g) to give a Li / Mn ratio of 0.54 (0.0135 mol), and mixed well by a dry method. The mixture was tableted at 500 kg / cm ² , placed in an electric furnace, and baked in the atmosphere at 850 ° C. for 10 hours. The obtained powder is put into a heatable pressure vessel, oxygen gas is introduced to 5 atm, heat treated at 400 ° C. for 20 hours, cooled to room temperature, taken out from the pressure vessel, and the target lithium manganate powder (Li _1.05 Mn _1.95 O ₄ ) was obtained.

  From the X-ray diffraction pattern of the obtained sample, all the peaks are

The unit cell of lithium manganate expressed by The obtained lithium manganate has a lattice constant of 8.227 、, and the lattice constant of lithium manganate described in the above document (J. Power Sources, 97-98, 423-426, (2001)) 8.23634 (13) Close to the soot, it can be seen that lithium manganate powder was obtained.

  FIG. 2 shows an SEM photograph of the obtained powder. This SEM observation revealed that the powder was composed of particles having a particle size of about 0.5 to 2 μm.

  FIG. 3B shows the measurement result of the particle size distribution of the obtained powder. In the figure, vertical bars indicate the fractions for each particle size, and black circles indicate the cumulative distribution. In the measurement, a powder sample was dispersed in a 0.2 wt% sodium hexametaphosphate aqueous solution, and then a particle size distribution measurement was performed in a range from 0.03 μm to 280 μm. From FIG. 3 (b), the abundance ratio of particles having an average particle diameter of 2.4 μm (the point where the cumulative particle size distribution in the figure intersects the cumulative frequency of 50%) is 1 μm or less is estimated to be 17% by weight. It can be seen that a lithium manganate powder having a small amount of micron particles is obtained.

210 mg of the obtained powder was weighed in a glove box, and placed in a sample bottle together with 90 mg of pulverized sulfide glass powder (0.01Li ₃ PO ₄ -0.63Li ₂ S-0.36SiS ₂ composition). To this sample bottle, zirconia beads and 5 ml of acetonitrile were added, capped and shaken for 10 minutes. After shaking, the sample bottle was placed in a vacuum dryer without exposing to the atmosphere, and vacuum dried at 65 ° C. for about 1 hour to evaporate acetonitrile. The obtained positive electrode mixture was cooled to room temperature and then used for battery testing.

70 mg of the ground sulfide glass powder (0.01Li ₃ PO ₄ -0.63Li ₂ S-0.36SiS ₂ composition) and 20 mg of the positive electrode mixture were weighed in a glove box.

  The PET tube was filled with sulfide glass powder using a PET tube having an inner diameter of 10 mm and a metal push rod in FIG. 1, and dry-pressed at 185 MPa to form a solid electrolyte layer. After pressing, one side of the metal push rod was opened, filled with the positive electrode mixture, and dry pressed at 370 MPa to form a positive electrode mixture layer on the solid electrolyte layer. After pressing, the positive electrode mixture filling side of the metal push bar was opened, a 10-mm diameter metal titanium mesh was put in, and dry pressing was performed at 555 MPa, and the positive electrode mixture was filled with the mesh. After pressing, remove the metal push rod on the opposite side of the positive electrode, confirm that there are no problems such as cracking and peeling of the solid electrolyte layer, and then insert a metal indium foil with a thickness of 0.3 mm and a diameter of 10 mm as shown in FIG. The sample was introduced into the electrochemical cell and further tightened from the top of the push rod to prepare a test cell.

FIG. 4 shows that the test cell has a potential range of 2.3 to 3.9 V and a current amount of 0.064 mA / cm ^2.
The charging / discharging curve obtained by performing a charging / discharging test to 10 cycles from charge is shown. The initial charge capacity was 55 mAh / g, the initial discharge capacity was 43 mAh / g, the charge capacity after 10 cycles was 41 mAh / g, and the discharge capacity was 39 mAh / g. From these charge / discharge characteristic data, it is clear that the obtained lithium manganate-based positive electrode active material exhibits good charge / discharge characteristics in an all-solid lithium secondary battery.

Example 2
0.05 mol (7.89 g) of Mn ₂ O ₃ obtained by heat-treating manganese carbonate in the atmosphere at 550 ° C. for 5 hours was weighed, and 0.015 mol of lithium hydroxide corresponding to a Li / Mn ratio of 0.60 Add 0.629 g of monohydrate, mix well by dry method, and form tablets in the same manner as in Example 1. After baking in an oxygen stream at 900 ° C. for 20 hours, cool to room temperature. As a result, a target lithium manganate powder (Li _1.13 Mn _1.87 O ₄ ) was obtained.

  From the X-ray diffraction pattern of the obtained sample, all the peaks are

The unit cell of lithium manganate expressed by The obtained lithium manganate has a lattice constant of 8.238 、, and the lattice constant of lithium manganate described in the above literature (J. Power Sources, 97-98, 423-426, (2001)) 8.23634 (13) Close to the soot, it can be seen that lithium manganate powder was obtained.

  FIG. 3C shows the result of measuring the particle size distribution of the obtained powder in the same manner as in Example 1. In the figure, vertical bars indicate the fractions for each particle size, and black circles indicate the cumulative distribution. FIG. 3C shows that the lithium manganate-based positive electrode active material has an average particle size of 6.6 μm and a submicron particle abundance ratio of 6% by weight.

  FIG. 4 shows the results of producing a positive electrode mixture and an all-solid lithium secondary battery and conducting a charge / discharge test in the same manner as in Example 1. As apparent from FIG. 4, the initial charge capacity was 52 mAh / g, the initial discharge capacity was 42 mAh / g, the charge capacity after 10 cycles was 39 mAh / g, and the discharge capacity was 38 mAh / g. From these charge / discharge characteristic data, it is clear that the obtained lithium manganate-based positive electrode active material exhibits good charge / discharge characteristics in an all-solid lithium secondary battery.

Comparative Example In the same manner as in Example 1, 0.1 mol (17.90 g) of manganese oxalate dihydrate was weighed and thermally decomposed in an electric furnace at 400 ° C. in the atmosphere for 5 hours to obtain 0.05 mol of Mn ₂ O. ₃ fine powders were prepared. Lithium hydroxide monohydrate was added to the obtained powder in an amount (0.525 g) to give a Li / Mn ratio of 0.50 (0.0125 mol), and mixed well by a dry method. The mixture was tableted at 500 kg / cm ² and then placed in an electric furnace and baked in the atmosphere at 850 ° C. for 10 hours to obtain the target lithium manganate powder (Li _1.00 Mn _2.00 O ₄ ).

  From the X-ray diffraction pattern of the obtained sample, all the peaks are

The unit cell of lithium manganate expressed by The obtained lithium manganate has a lattice constant of 8.243 mm, and the lattice constant of lithium manganate described in the above-mentioned document (J. Power Sources, 97-98, 423-426, (2001)) is 8.23634. (13) Close to the soot, it can be seen that lithium manganate powder was obtained.

  FIG. 2 shows an SEM photograph of the powder obtained in this comparative example. From this SEM observation, it was confirmed that although this particle had coarse particles having a particle diameter of 3 μm or more as compared with Example 1, submicron particles of 1 μm or less were considerably present.

  FIG. 3A shows the result of measuring the particle size distribution of the obtained powder in the same manner as in Example 1. In the figure, vertical bars indicate the fractions for each particle size, and black circles indicate the cumulative distribution. From FIG. 3 (a), it is estimated that the average particle size is 1.1 μm and the submicron particle abundance ratio is 42% by weight, and it can be confirmed that a large amount of submicron particles are present. Since it is insufficient, the interface contact area with the electrolyte is expected to be small.

  FIG. 4 shows the results of producing a positive electrode mixture and an all-solid lithium secondary battery and conducting a charge / discharge test in the same manner as in Example 1. As is apparent from FIG. 4, the initial charge capacity was 37 mAh / g, the initial discharge capacity was 19 mAh / g, the charge capacity after 10 cycles was 14 mAh / g, and the discharge capacity was 13 mAh / g. Compared with Examples 1 and 2 above, only a low capacity was obtained despite the large amount of Li that could be inserted and removed per weight of the positive electrode, and the obtained lithium manganate positive electrode active material Is clearly not showing good charge / discharge characteristics in an all-solid lithium secondary battery.

It is sectional drawing of an example of an all-solid-state lithium secondary battery. It is a scanning electron micrograph of the lithium manganate powder obtained in Example 1 and a comparative example. It is a measurement result of the particle size distribution of the lithium manganate powder obtained in Examples 1 and 2 and Comparative Example. (A) shows the result of the comparative example, (b) shows the result of Example 1, and (c) shows the result of Example 2. It is the result of having performed the charging / discharging test of the all-solid-state lithium secondary battery produced using the lithium manganate type | system | group powder obtained by Example 1, 2 and the comparative example.

Claims (1)

It has a cubic spinel structure and has the composition formula Li _{1 + x} (Mn _2−y M _y ) _2−x O ₄ (I)
(In the formula, M represents Al, Mg, Cr, Mn, Fe, Co, Ni, Cu, or Zn. When M is Mn, x is a number in the range of 0 <x <0.33. In the case where M is other than Mn, the number is in the range of 0 ≦ x <0.33. Y is the number in the range of 0 ≦ y ≦ 1, and the particle size is 1 μm or less. An all-solid lithium secondary battery using a sulfide-based inorganic solid electrolyte containing a lithium manganate-based positive electrode active material having an abundance ratio of 40% by weight or less.