JP4224583B2 - Positive electrode material for lithium secondary battery - Google Patents

Description

  The present invention relates to a method for producing a positive electrode material for a lithium secondary battery, a positive electrode material obtained by the method, and a lithium secondary battery using the positive electrode material.

  With the development of mobile devices such as notebook computers and mobile phones in recent years, development of lithium secondary batteries as power sources has been actively conducted. A lithium secondary battery uses a lithium-containing transition metal oxide (positive electrode active material) such as lithium cobaltate as a positive electrode material, a carbon material as a negative electrode material, and an organic electrolyte as an electrolyte.

The lithium secondary battery achieves a battery operating voltage close to 4V (corresponding to an operating voltage approximately three times that of the nickel metal hydride battery) due to such a material structure. Therefore, the above-mentioned mobile device is the secondary battery having the highest energy density. It is an indispensable power source.

  In the battery configuration described above, the positive electrode material usually undergoes lithium ion desorption / insertion during charging / discharging under a constant current density, and at that time, transition metal ions are oxidized / reduced. That is, the amount of lithium ion desorption / insertion determines the charge / discharge capacity of a lithium secondary battery, and the redox potential of transition metal ions substantially determines the battery operating voltage of the lithium secondary battery. is important.

  In particular, when obtaining a certain charge / discharge capacity, the larger the amount of current (current density) that can be energized per positive electrode material is, the larger current charge / discharge is possible. In order to realize such high current density charging / discharging, it is necessary to increase the electron conductivity in the positive electrode, but the current positive electrode material alone has a low electron conductivity, so usually a conductive carbon material such as acetylene black. (Hereinafter, referred to as “conductive material”) is combined with a binder to improve current collection (for example, see Non-Patent Document 1).

However, it has been reported that the positive electrode surface becomes insulative in charge / discharge under a high current density, even though the above-described measures for improving current collection are being performed (for example, (See Non-Patent Document 2.) In this document, it is clearly stated that the carbon material is responsible for the electron conductivity of the positive electrode, and it is considered that the carbon material is peeled off from the surface of the positive electrode material during charge and discharge. This phenomenon is considered to be caused by the active material expanding and contracting the crystal lattice during charging and discharging. Therefore, it is necessary to develop a new technique that can impart high electron conductivity to the positive electrode active material.

  As described above, a lithium secondary battery has a higher energy density than a nickel metal hydride battery. However, since an organic electrolyte is used as an electrolyte, there are problems regarding battery safety during charging such as liquid leakage and explosion. is there.

  In order to solve this problem, it is necessary to use an inorganic solid electrolyte with high lithium ion conductivity instead of the organic electrolyte, and as such an inorganic solid electrolyte, all solid lithium using a sulfide-based solid electrolyte is used. Secondary batteries have been proposed (see, for example, Patent Document 1).

  A feature of the positive electrode for an all solid lithium secondary battery using a sulfide-based inorganic solid electrolyte is that the sulfide solid electrolyte is contained in the positive electrode. This is because the interfacial bonding area between the solid electrolyte and the positive electrode active material cannot be sufficiently widened in a liquid system configuration (almost only an active material and a conductive material and includes a small amount of a binder).

In this all-solid-state battery, the sulfide-based solid electrolyte decomposes due to the catalytic action of the carbon material, causing significant capacity deterioration (50% of the capacity is lost by adding 0.1% by weight of carbon to lithium cobaltate). (For example, refer to Patent Document 2), it is necessary to examine a method for introducing a conductive material in the coexistence of a sulfide-based solid electrolyte into the positive electrode of a sulfide-based all solid lithium secondary battery.

Furthermore, in order to solve the problem of capacity deterioration due to the reaction between the solid electrolyte and the conductive material in the positive electrode, the surface of the positive electrode is coated with a lithium ion conductive polymer, and a conductive material such as a carbon material is dispersed in the polymer. (See, for example, Patent Document 3). However, in this method, there is a concern about a decrease in charge / discharge capacity due to a decrease in the amount of the positive electrode material in the positive electrode accompanying the introduction of the polymer into the positive electrode, and another method for applying a conductive material is necessary.
JP-A-8-162151 JP-A-8-96836 Japanese Patent Laid-Open No. 11-7942 JP-A-10-251070 JP 2001-348277 A Yoshio Masao and Ozawa Akiya "Lithium-ion Secondary Battery 2nd Edition, Materials and Applications", Nikkan Kogyo Shimbun, p191, 2000 R. Kostecki and F. McLarnon, Electrochemical and Solid State Letters, 5, A164-A166 (2002).

  The main object of the present invention is to provide a positive electrode material for a lithium secondary battery, in which deterioration of capacity is suppressed and which is excellent in charge / discharge cycle characteristics.

  As a result of intensive studies on the method of imparting conductivity to the positive electrode active material of the lithium secondary battery, the present inventor has excellent battery characteristics by supplying current to the positive electrode active material and the conductive material in contact with each other. The inventors have found that a positive electrode material for a lithium secondary battery can be obtained, and have completed the present invention.

  That is, the present invention is as follows.

1. A method for producing a positive electrode material for a lithium secondary battery, comprising:
(1) a step of contacting positive electrode active materials for lithium secondary batteries and carbon sheets by alternately laminating them under pressure;
(2) A method comprising a step of supplying a pulse current to the positive electrode active material and the carbon sheet that are alternately contacted to adhere the carbon sheet to the surface of the positive electrode active material.

2 . Item 2. The method according to Item 1, wherein the positive electrode active material is at least one selected from the group consisting of lithium cobaltate and lithium nickelate.

3 . 3. A positive electrode material for a lithium secondary battery obtained by the method according to item 1 or 2 .

4). 4. A lithium secondary battery comprising the positive electrode material for a lithium secondary battery according to item 3.

  In the present invention, as a positive electrode material for a lithium secondary battery, a positive electrode active material (hereinafter referred to as “the positive electrode material of the present invention”) having a conductive material attached to its surface is supplied by supplying a pulse current. And

Positive electrode active type of material for lithium secondary batteries used in the positive electrode active material present invention is not particularly limited, usually it is possible to use a positive electrode material for use in lithium secondary batteries. Preferably, at least one selected from the group consisting of lithium cobaltate and lithium nickelate can be exemplified. These can be used alone or in combination (a mixture of the two or a LiCo _1-x Ni _x O ₂ solid solution (0 <x <1)). Furthermore, a cation different from nickel and cobalt such as Al and Mn may be included in the sample in order to improve charge / discharge characteristics and safety.

  In the present invention, a material obtained by attaching a conductive material to such a positive electrode active material can be used as a positive electrode material for a secondary battery.

Conductive Material The conductive material used in the present invention is not limited as long as it can impart conductivity to the positive electrode material. Preferably, carbon can be exemplified. Although it is not limited as a raw material of carbon, For example, graphite etc. are mentioned.

Production of positive electrode material of the present invention (adhesion of conductive material to positive electrode active material surface)
The present invention is characterized in that a positive electrode active material and a conductive material are brought into contact with each other under pressure and a pulse current is supplied thereto to attach the conductive material to the positive electrode active material.

The method for supplying current to the positive electrode active material and the conductive material and the apparatus used therefor are not limited. For example, a discharge plasma sintering apparatus used in the discharge plasma sintering method can be used. The type of the discharge plasma sintering apparatus is not limited, but apparatuses such as those described in Patent Document 4, Patent Document 5, and the like can be used.

Hereinafter, an example of the positive electrode material manufacturing method of the present invention will be described with reference to FIG. 1 showing a schematic diagram of a discharge plasma sintering machine.

The spark plasma sintering machine 1 includes a die 3 on which a sample 2 is loaded and a pair of upper and lower punches 4 and 5. The punches 4 and 5 are supported by punch electrodes 6 and 7, respectively, and a pulse current is supplied through the punch electrodes 6 and 7 while applying pressure to the sample 2 loaded on the die 3 as necessary. Can do. The material of the die 3 is not limited, and examples thereof include a carbon material such as graphite.

Examples of the sample 2 loaded in the die 3 include a positive electrode active material and a conductive material. When the positive electrode active material and the conductive material are loaded into the die 3, it is preferable to bring the positive electrode active material and the conductive material into contact with each other. Although it does not limit as a method to contact, For example, you may just mix a positive electrode active material and a electrically conductive material. Moreover, you may make it contact by laminating | stacking a positive electrode active material and a electrically conductive material alternately.

In order to increase the adhesion efficiency of the conductive material to the positive electrode active material, the amount of the positive electrode active material sandwiched between the conductive materials is made as small as possible, and the repetition (number of layers) of “conductive material / positive electrode active material / conductive material” is increased (for example, , Conductive material / positive electrode active material / conductive material / positive electrode active material / conductive material /. The number of stacked layers is not limited and can be appropriately selected according to the reduction resistance of the target positive electrode active material.

The mixing ratio of the positive electrode active material and the conductive material is preferably mixed so that the conductive material is about 0.01 to 30% by weight with respect to the total amount of the positive electrode active material and the conductive material. This is because the conductivity of the positive electrode active material is sufficiently improved, and a decrease in energy density per volume / weight of the positive electrode material is suppressed.

Further, when the conductive material is adhered by lamination, the positive electrode active material and the conductive material layer may be alternately laminated so that the above-mentioned weight percent ratio is taken into consideration, for example, as the conductive material. When a carbon sheet is used, the ratio of the thickness of the positive electrode active material layer to the thickness of the conductive material layer is 0.1 to 10
It is preferable to laminate so as to be about the same.

A pulse current is preferable as the type of current applied to the positive electrode active material and the conductive material. By performing pulse energization, the positive electrode active material, the conductive material, and the vicinity thereof (the die 3 and the upper and lower punches 4 and 5) are heated. Because of the effect of both the heating and the pulse current, the conductive material can be firmly attached to the surface of the positive electrode active material, so that the bond is maintained even after charging and discharging the lithium secondary battery containing the positive electrode active material of the present invention. Kept. As a result, it is possible to provide a positive electrode for a lithium secondary battery having excellent charge / discharge characteristics as a result of the decrease in conductivity of the positive electrode material and almost no reaction between the conductive material and the solid electrolyte.

On the other hand, in the conventional method, conductivity was imparted simply by mixing the conductive material with the positive electrode active material, and thus the adhesion between the conductive material and the positive electrode active material was weak. As a result, the conductive material is peeled off from the positive electrode active material during charge and discharge, causing a decrease in electronic conductivity in the vicinity of the positive electrode active material and a reaction between the conductive material and the solid electrolyte, and charge / discharge characteristics (charge / discharge capacity, charge / discharge efficiency). (The schematic diagram of the difference in the positive electrode material between the present invention and the conventional method is shown in FIG. 2).
In the present invention, conditions for supplying current to the positive electrode active material and the conductive material are not limited as long as the conductive material can firmly adhere to the surface of the positive electrode active material. It is preferable to apply a pressure to the positive electrode active material and the conductive material (under pressure) to supply a pulse current. The pressure is, for example, about 5 to 60 MPa, preferably about 10 to 50 MPa. This is because the conductive material adheres firmly to the positive electrode active material, and the positive electrode is not easily decomposed.

Further, the temperature of the die 3 when supplying current to the positive electrode active material and the conductive material can be appropriately selected according to the type of the positive electrode active material and the particle size thereof, but is usually about 400 to 700 ° C., preferably Is preferably about 500 to 600 ° C. Conductive material adheres firmly to the positive electrode active material, and decomposition of the positive electrode active material due to the reduction effect of carbon (for example, when LiCoO ₂ is used as the positive electrode active material, cobalt oxide CoO or Co ₃ O ₄ that is not charged or discharged) Are not likely to coexist.).

The strength and time of the pulse current to be supplied are not limited as long as the conductive material is sufficiently attached to the surface of the positive electrode active material, and is appropriately selected according to the size of the die 3 and the upper and lower punches 4 and 5, for example. Is done. In addition, the strength of the current to be supplied and the time thereof can be controlled by the electrical conductivity of the mold material, the positive electrode active material, and the conductive material used.

For example, when using a graphite die having an inner diameter of 15 mm and laminating a carbon sheet and a positive electrode active material as a conductive material and conducting an energization treatment, for example, it is about 100 to 1000 A, preferably about 200 to 800 A.
The frequency of the pulse current is, for example, about 1 to 500 Hz, preferably about 3 to 200 Hz.

  The current supply time is about 1 to 30 minutes, preferably about 3 to 15 minutes after the temperature reaches the above range (about 400 to 700 ° C.). The speed at which the temperature of the die is raised is not limited, and can be appropriately selected according to the type of positive electrode active material, conductive material, and the like. For example, it is about 10 to 300 ° C./min, preferably about 50 to 200 ° C./min.

As described above, the positive electrode active material and the conductive material supplied with the pulse current are taken out from the die 3 after cooling, and the upper and lower conductive materials are removed to obtain the positive electrode material of the present invention in which the conductive material is firmly attached. it can.

  In the positive electrode material of the present invention thus obtained, the weight ratio of the conductive material to the positive electrode active material is, for example, about 1: 0.0001 to 0.3, preferably about 1: 0.0005 to 0.2. If the amount of the conductive material is less than this range, it becomes difficult to impart sufficient conductivity in the positive electrode. On the other hand, if the amount of the conductive material is larger than this range, the amount of the active material in the positive electrode is reduced and the charge / discharge capacity as a battery is reduced. Cause a decline.

Lithium Secondary Battery The positive electrode material of the present invention obtained as described above can be used for a lithium secondary battery by a usual method.

  For example, a positive electrode material is mixed with a conductive material such as acetylene black and, if necessary, a solid electrolyte to form a positive electrode mixture (positive electrode), and an organic electrolyte or a solid electrolyte is sandwiched between the obtained positive electrode and negative electrode Thus, a lithium secondary battery can be manufactured.

  In the present invention, a secondary battery can be made by using a positive electrode material with a conductive material attached and a negative electrode material for which a conductive material is not attached. A secondary battery can also be made using a negative electrode material to which a material is attached.

As the organic electrolytic solution, an electrolytic solution usually used for a lithium secondary battery can be used. As a preferable organic electrolyte in the present invention, a solution in which at least one selected from the group consisting of LiPF ₆ , LiClO ₄ and LiBF ₄ as a supporting salt is dissolved in an organic mixed solvent (a concentration of about 1 mol / kg) is preferable. .

  As the organic mixed solvent, for example, a solution obtained by mixing ethylene carbonate (EC) or propylene carbonate (PC) with dimethyl carbonate (DMC) or diethylene carbonate (DEC) can be used. The mixing ratio of these organic solvents is not limited, but is preferably about ethylene carbonate (EC) or propylene carbonate (PC): dimethyl carbonate (DMC) or diethylene carbonate (DEC) = 1: 1 (volume ratio). It is done.

As the solid electrolyte, a solid electrolyte usually used for a lithium secondary battery can be used. Preferred solid electrolytes in the present invention include, for example, Li ₂ S—SiS ₂ , Li ₂ SP ₂ S ₅ , Li ₂ SB ₂ S _{3
, Li 2 S-SiS 2 -Li} 3 PO 4, Li 2 S-SiS 2 -P 2 S 5 -LiI, Li 2 S-GeS 2, Li 4 GeS 4 -Li 3 PS sulfide-based solid electrolyte such as ₄ , Ethylene oxide, polyester, polymer solid electrolyte with polysulfide polymer combined with Li salt, lithium halide, lithium nitride, lithium oxyacid salt (eg lithium aluminum aluminum phosphate, lithium aluminum germanium phosphate, lithium containing titanium) Lanthanum acid) or derivatives thereof. Among these, sulfide-based solid electrolytes are preferable. Furthermore, among the sulfide-based solid electrolytes, Li ₂ S—SiS ₂ —Li ₃ PO ₄ -based glassy solid electrolyte, Li ₄ GeS ₄ —Li ₃ PS ₄ crystalline solid electrolyte, and the like are particularly preferable.

Further, the negative electrode material is not limited, and a negative electrode material used for a normal lithium secondary battery can be used. For example, at least one selected from the group consisting of metallic lithium, metallic indium, tin, aluminum, alloys thereof, carbon materials, tin oxide, lithium titanate, and lithium titanate in which iron is dissolved is used. These may be used alone or as a mixture of two or more.

According to the present invention, the conductive material can be easily and firmly attached to the surface of the positive electrode active material. As a result, it is possible to obtain a positive electrode material for a lithium secondary battery in which the deterioration of capacity is suppressed and the charge / discharge characteristics are excellent. The reason is not clear, but it is conceivable that the electroconductivity of the positive electrode is increased due to the conductive material adhering to the active material or the particle size of the conductive material is increased by heating, and the reaction area of the conductive material-solid electrolyte is decreased.

  Hereinafter, examples will be shown to further clarify the features of the present invention. The present invention is not limited to these examples.

The crystal phases of the positive electrode materials obtained in the following examples and comparative examples were evaluated by X-ray diffraction analysis, and the C / Ni and C / Co ratios in the positive electrode material were determined by energy dispersive X-ray (EDX) analysis. The amounts of Li, Co, and C were calculated by chemical analysis. Moreover, the surface state of the positive electrode active material or the positive electrode material was evaluated by a scanning electron microscope (SEM).

Reference example 1
20.00 g of nickel hydroxide (II) (Ni (OH) ₂ ) was weighed on a polytetrafluoroethylene petri dish. In a glass beaker, a lithium hydroxide aqueous solution in which 9.05 g of lithium hydroxide monohydrate corresponding to a Li / Ni atomic ratio of 1.00 was dissolved in 100 ml of distilled water was prepared. In addition, after stirring well, it was dried at 100 ° C. for 12 hours.

The dried product is pulverized in a mortar, filled in an alumina crucible, and heated in an electric furnace at 750 ° C in an oxygen stream.
Baked for hours. After cooling to room temperature over 24 hours after firing, the powder was taken out from an electric furnace, pulverized by a pulverizer, and passed through a 330 mesh sieve to obtain a sample for subsequent evaluation tests.

  All peaks are unit cells of hexagonal lithium nickelate (

). The obtained lattice constants are a = 2.88001 (6) Å, c = 14.2061 (3) 、, and have been reported (H. Arai, S. Okada, H. Ohtsuka, M. Ichimura, and J. Yamaki, Solid State Ionics). , 80, 261-269, (1995).) Near the lattice constants of lithium nickelate a = 2.883 Å and c = 14.0205 で き, confirming the formation of lithium nickelate as the target substance.

A 0.4 g powder was sandwiched between carbon sheets (thickness 0.2 mm) and loaded into a graphite mold (die) with an inner diameter of 15 mm. A pulse current of about 400 A was supplied while pressing the upper and lower punches to about 30 MPa. . The vicinity of the graphite die was heated at a heating rate of about 130 ° C./min, and reached 400 ° C. 3 minutes after the start of pulse current supply. After holding at this temperature for about 5 minutes, the application of current and pressurization were stopped and allowed to cool naturally. After cooling, the carbon sheet / lithium nickelate laminate was removed from the die, the carbon sheet was removed, and the product was recovered.

The X-ray diffraction pattern of the obtained powder is shown in FIG. Samples obtained by the structural refinement program RIETAN-2000 (F. Izumi and T. Ikeda, Mater. Sci. Forum, 321-324 (2000) 198-203.) Were 96.8% lithium nickelate (by weight) ( In the figure, the top vertical bar group just below the actual measurement pattern is the estimated peak position), 2.0% lithium carbonate (Li ₂ CO ₃ , the central vertical bar group just below the actual measurement pattern in the figure is the estimated peak position), 1.2% Of nickel oxide (NiO, the lowest vertical bar group just below the measured pattern in the figure is the estimated peak position).

That is, it shows that lithium nickelate is present in the sample in a form close to a single phase. The obtained lithium nickelate has a lattice constant of a = 2.88179 (8) Å, c = 14.2190 (4) Å, and has already been reported (H. Arai, S. Okada, H. Ohtsuka, M. Ichimura, and J. Yamaki , Solid State Ionics, 80, 261-269, (1995).), The lattice constants of lithium nickelate are close to a = 2.883 mm, c = 14.0205 mm, confirming that the target nickel lithium oxide was formed. It was.

At the same time, the nickel ion distribution of the obtained lithium nickelate with the layered rock salt type was estimated to be x = 0.043 (2) in the composition formula of (Li _1-x Ni _x ) _3a [Ni] _3b O ₂ . From the viewpoint of improving charge / discharge characteristics, it is desirable that x corresponding to the amount of nickel ions coexisting at the lithium position (3a position) be as close to 0 as possible.

That is, it is necessary to have a more ideal ion distribution (corresponding to all nickel ions existing only at the normal position 3b). The x value obtained in the present invention is the value (0.095) of (H. Arai, S. Okada, H. Ohtsuka, M. Ichimura, and J. Yamaki, Solid State Ionics, 80, 261-269, (1995).) It was found that lithium nickelate having an almost ideal nickel ion distribution was produced.

When the powder state of the obtained sample was observed by SEM (Fig. 4), it was confirmed that particles of 1 (m or less aggregated into secondary particles of several microns, Comparative Example 1)
There was no difference.

On the other hand, when the C: Ni ratio (weight ratio) was estimated by EDX analysis, it was 8:92, and the amount of carbon was larger than the value 3:97 of the sample before the energization coating treatment (Comparative Example 1). Can confirm that
Formation of a lithium nickelate-carbon composite was confirmed.

EDX surface analysis was performed to estimate the presence of carbon on the powder (Fig. 5). In FIG. 5, the upper figure is an SEM image, in which the portion enclosed by a square was analyzed. When each element to be analyzed is present, it is displayed in white, and when there is a small amount, it is displayed in black. As a result of carbon surface analysis, it can be confirmed that carbon is present at substantially the same position as nickel and oxygen elements derived from lithium nickelate, and that carbon as a conductive material is present on the surface of lithium nickelate. The results supported the conductive material distribution model estimated in.

  Further, when the green compact of the obtained sample was prepared and the direct current resistance measurement was performed, 42 ((cm was estimated, and the value of the uncoated product shown in Comparative Example 1 below was 60Ω (cm It was confirmed that the sample of Example 1 had high electron conductivity.

Comparative Example 1
In the same manner as in Example 1, lithium nickelate was synthesized, and no energization coating treatment was performed. The particle shape (FIG. 4) was not particularly different from Example 1, and the electrical resistance of the green compact was higher than that of Example 1 as described above.

Test example 1
Using the sample obtained in Example 1 or Comparative Example 1 as the positive electrode material, a lithium secondary battery was produced according to the following method. To 20 mg of the obtained sample, acetylene black 2.2 mg and polytetrafluoroethylene powder 0.5 mg were added, mixed in a mortar, and pressure-bonded to a metal aluminum current collector. The obtained positive electrode mixture is vacuum-dried at 120 ° C. and then introduced into a glove box, and an electrolytic solution comprising a supporting salt LiPF ₆ and a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 1: 1) in the glove box. And a metal lithium negative electrode were used to produce a coin-type lithium secondary battery.

Connect this battery to a charge / discharge device, and charge / discharge potential range 2.75-4.2V constant current constant voltage charge-constant current discharge, charge / discharge rate is 1-10 cycles, 21C is 0.2C (55mA / g) , 11-20
It was set to 1C (275mA / g) until the cycle. FIG. 6 shows a discharge curve at the time of charge / discharge measurement.

When the positive electrode material of Example 1 was used, the initial discharge capacity at 0.2 C was about 170 mAh / g, which is lower than the 200 mAh / g of the carbon-uncoated sample of Comparative Example 1, but 10 and 21 The discharge capacity after the cycle is almost the same.

At 1C, the discharge capacity after 11 cycles (corresponding to the initial discharge capacity at 1C) of the sample of Example 1 is maintained at 150 mAh / g, whereas the sample of Comparative Example 1 is 135 mAh / g. Declined. After 20 cycles, the sample of the example maintained a discharge capacity of 145 mAh / g, whereas the sample of Comparative Example 1 decreased to 120 mAh / g.

  From these facts, it is clear that the sample obtained according to the present invention is excellent in electron conductivity, and therefore can provide a positive electrode material excellent in charge / discharge characteristics at a high current density in a lithium secondary battery.

Example 1
Cobalt oxide (Co ₃ O ₄ ) 1.00 g and lithium carbonate (Li ₂ CO ₃ ) 0.47 g corresponding to a Li / Co molar ratio of 1.02 were weighed and dry mixed. This mixture was calcined in the atmosphere at 850 ° C. for 5 hours. FIG. 7 (a) shows the X-ray diffraction pattern of the obtained sample.

  All peaks are unit cells of hexagonal lithium cobaltate (

). The obtained lattice constants are a = 2.81557 (6) Å, c = 14.0489 (3) Å, and have been reported (RJGummow, MMThackeray, WIFDavid and S.Hull, Mat. Res. Bull., 27, 327-337, ( 1992).) Lattice constant value of lithium cobaltate a = 2.8179 (1) Å, c = 14.0597 (8)
Close to the soot, it was confirmed that lithium cobaltate, the target substance, was produced.

  Two layers of this powder 0.2 g sandwiched between carbon sheets (thickness 0.2 mm) are laminated (carbon sheet / lithium cobaltate (0.2 g) / carbon sheet / lithium cobaltate (0.2 g) / carbon sheet), This was loaded into a graphite mold (die) having an inner diameter of 15 mm, and a pulse current of about 500 A was supplied while being pressurized to about 30 MPa with upper and lower punches.

The vicinity of the graphite die is heated at a rate of about 130 ° C / min, and 500 minutes after the start of pulse current supply.
C reached. After holding at this temperature for about 5 minutes, the application of current and pressurization were stopped and allowed to cool naturally.

After cooling, the carbon sheet / lithium cobaltate laminate was removed from the die, the carbon sheet was removed, and the product was recovered. The X-ray diffraction pattern of the obtained sample is shown in FIG. 7 (b). Not only were traces of graphite peaks detected, but the other peaks were hexagonal lithium cobalt oxide unit cells (

). The obtained lattice constants are a = 2.81604 (6) c, c = 14.0517 (2) 、 and have been reported (RJGummow, MMThackeray, WIFDavid and S.Hull, Mat. Res. Bull., 27, 327-337, 1992).) Lattice constant value of lithium cobaltate a = 2.8179 (1) Å, c = 14.0597 (8)
It was close to the niece.

The weight ratio between lithium cobaltate and carbon was determined by X-ray Rietveld analysis.
It was found that the sample contained about 4% carbon. Moreover, the carbon content in the obtained sample was 0.27% by chemical analysis, and it was confirmed that the sample contained carbon. Furthermore, the amount of lithium in the sample is 7.22% by weight, the amount of cobalt is 57.7% by weight, Li / Co
Atomic ratio 1.06, close to the Li / Co ratio 1.00 estimated from LiCoO ₂ composition, the LiCoO ₂ was found to be close to the stoichiometric composition.

When the surface state of the powder was observed with a scanning electron microscope (SEM), it was confirmed that particles that could not be seen on the powder surface of the comparative example (bottom of FIG. 8) were attached (top of FIG. 8). . When the C / Co ratio was calculated by EDX analysis, it was estimated to be 8/92, which was found to be increased compared to the value 1/99 of the carbon-uncoated sample shown in Comparative Example 2 below. Consistent with X-ray diffraction measurement results.

The distribution state of carbon in the powder was estimated by EDX plane analysis in the same manner as in Example 1 together with cobalt and oxygen elements. As shown in FIG. 9, it was found that the carbon element was present at almost the same position as cobalt and oxygen, and it was confirmed that lithium cobalt oxide with carbon attached was formed.

Comparative Example 2
The lithium cobaltate obtained in the same manner as in Example 1 has a ratio of lithium cobaltate to a sulfide-based (Li ₂ S—SiS ₂ —Li ₃ PO ₄ ) solid electrolyte of 7: 3 (210 mg: 90 mg). After adding 0.25% by weight of acetylene black, which is approximately the same amount as the energization coating amount (the amount of the carbon material attached to the positive electrode active material in Example 2) to lithium cobaltate, the acetonitrile solvent was added. The mixture was mixed by a wet method to obtain a positive electrode mixture.

Test example 2
Using the sample obtained in Example 2 or Comparative Example 2 as the positive electrode material, an all-solid lithium secondary battery was produced by the following procedure (a schematic diagram of the battery cell is shown in FIG. 10).

A sample and a sulfide-based (Li ₂ S-SiS ₂ -Li ₃ PO ₄ ) solid electrolyte are weighed to a ratio of 7: 3 (210 mg: 90 mg), mixed by a wet method in acetonitrile solvent, and positive electrode A mixture was obtained.

In a 10 mm diameter insulator tube, 50 mg of ground Li ₂ S—SiS ₂ —Li ₃ PO ₄ glassy electrolyte was pressed at 1.3 t / cm ² . From one side (lower side in FIG. 10), 20 mg of the positive electrode mixture was added, and the two layers were simultaneously pressed at 3.8 t / cm ² .

Further, Ti is added as a current collector from the lower side in FIG. 10, and metal indium is put on the other side (the upper side in FIG. 10), and then pressed at 0.6 t / cm ² to press all solid lithium. A secondary battery was obtained.

The charge / discharge test conditions were constant current measurement at a current of 0.25 mA (0.32 mA cm ⁻² ) and a cut-off potential of 3.9 to 2.0 V. FIG. 11 shows the charge / discharge characteristics of the all-solid lithium secondary battery using the carbon-attached lithium cobalt oxide obtained in Example 2 as a positive electrode.

In Japanese Patent Application Laid-Open No. 8-96836, a 50% reduction in charge / discharge capacity was observed when only 0.1% carbon was added. In contrast, despite the fact that the carbon-adhered lithium cobaltate of the present invention contains more than 0.1% of carbon, the initial charge / discharge capacity of 100 mAh / g or more is shown, and the charge capacity (■) and discharge capacity (□) There is almost no variation from cycle to cycle. The discharge capacity after 10 cycles was also 70 mAh / g or more.

Furthermore, the formula:
“Charge / discharge efficiency in the nth cycle = discharge capacity in the nth cycle / charge capacity in the nth cycle”
The charge / discharge efficiency (○) defined by the equation continued to maintain 0.95 or more from 2 cycles to 10 cycles.

  From this, it was found that the conductive material was firmly attached to the surface of the positive electrode material obtained in the present invention, and this positive electrode material can be suitably used as the positive electrode material for an all solid lithium battery.

On the other hand, when the secondary battery obtained in Comparative Example 2 was used, as is clear from FIG. 12, the charge capacity (■) and the discharge capacity were compared with the secondary battery obtained in Example 2. (□) showed large variations from cycle to cycle, and the initial charge / discharge capacity was 90 mAh / g or less. Also, the discharge capacity after 10 cycles was 30 mAh / g or less. The charge / discharge efficiency was maintained at 0.9 or more after the second cycle, but the variation for each cycle was larger than that in Example 2.

From these facts, in the positive electrode material obtained by the ordinary method as in Comparative Example 2, the conductive material did not adhere firmly to the surface, and the charge / discharge efficiency and charge / discharge capacity cycle stability were excellent. This shows that an all-solid lithium secondary battery cannot be produced.

It is the schematic which shows a discharge plasma sintering machine. It is a figure which shows the difference in the electrically conductive material-positive electrode active material distribution state in this invention and the conventional method. FIG. 3 is a diagram showing a comparison between an actual measurement X-ray diffraction pattern (+) and a calculated diffraction pattern (solid line) of the positive electrode material obtained in Example 1. The residual is shown below the figure. The estimated peak positions of lithium nickelate, nickel oxide, and lithium carbonate are indicated by vertical bars. It is a figure which shows the scanning electron microscope (SEM) photograph of the positive electrode material obtained in Example 1 (right figure) and the comparative example 1 (left figure). FIG. 3 is a diagram showing an SEM image of the positive electrode material obtained in Example 1 and the corresponding distribution state of carbon, nickel, and oxygen by EDX analysis. FIG. 3 is a diagram showing charge / discharge characteristics at 0.2 C and 1 C of a lithium secondary battery using the samples obtained in Example 1 and Comparative Example 1 as positive electrodes. 4 is a diagram showing X-ray diffraction of a positive electrode active material (a) and a positive electrode material (b) obtained in Example 2. FIG. It is a figure which shows the scanning electron microscope (SEM) photograph of the positive electrode material obtained in Example 2 (upper figure) and the comparative example 2 (lower figure). FIG. 6 is a diagram showing an SEM image of the positive electrode material obtained in Example 2 and the corresponding distribution state of carbon, cobalt, and oxygen by EDX analysis. It is a schematic diagram which shows a coin type all-solid-state lithium secondary battery. 6 is a graph showing charge / discharge characteristics of an all solid lithium secondary battery produced using the positive electrode material obtained in Example 2. FIG. The upward curve corresponds to the charge curve, and the downward curve corresponds to the discharge curve (upper figure). The figure below shows the cycle number dependence of charge / discharge capacity and charge / discharge efficiency. The upper axis of the above figure shows the amount of Li removed / inserted. 6 is a diagram showing charge / discharge characteristics of an all solid lithium secondary battery produced using the positive electrode material obtained in Comparative Example 2. FIG. The upward curve corresponds to the charge curve, and the downward curve corresponds to the discharge curve (upper figure). The figure below shows the cycle number dependence of charge / discharge capacity and charge / discharge efficiency. The upper axis of the above figure shows the amount of Li removed / inserted.

Claims (4)

A method for producing a positive electrode material for a lithium secondary battery, comprising:
(1) a step of contacting positive electrode active materials for lithium secondary batteries and carbon sheets by alternately laminating them under pressure;
(2) A method comprising a step of supplying a pulse current to the positive electrode active material and the carbon sheet that are alternately contacted to adhere the carbon sheet to the surface of the positive electrode active material.   2. The method according to claim 1, wherein the positive electrode active material is at least one selected from the group consisting of lithium cobaltate and lithium nickelate.   3. A positive electrode material for a lithium secondary battery obtained by the method according to claim 1 or 2. 4. A lithium secondary battery comprising the positive electrode material for a lithium secondary battery according to claim 3.