JP5780834B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a high capacity, high output lithium secondary battery.

  In recent years, secondary batteries have been used not only for mobile phones and notebook PCs but also as batteries for electric vehicles.

As a negative electrode active material of these batteries, a battery using ramsdellite-type lithium titanate (Li ₂ Ti ₃ O ₇ ) has been proposed. For example, Patent Document 1 discloses an electron conduction assistant in Li ₂ Ti ₃ O ₇ powder. Using an electrode hardened by pressure molding by adding a binder and a dispersant, good rapid charge / discharge performance and repeated charge / discharge performance (cycle characteristics) are achieved.

JP 2008-91079 A

However, in the battery described in Patent Document 1, the electrode is obtained by adding an electron conduction assistant, a binder, and a dispersing agent to Li ₂ Ti ₃ O ₇ powder, and simply pressing and solidifying the battery characteristics. Since the binder and the electron conduction assistant that do not directly contribute to the content are contained, the active material filling rate in the electrode is low. Therefore, there are problems that the energy density of the battery is low and the battery capacity with respect to the volume of the electrode is small.

  The present invention has been proposed in view of such circumstances, and an object thereof is to provide a lithium secondary battery that has excellent cycle characteristics and a high battery capacity with respect to energy density and electrode volume.

Rams lithium secondary battery of the present invention has an electrolyte between the positive electrode and the negative electrode, the negative electrode, which Nb, Ta, at least one element selected from the element group consisting of Al and Fe in solid solution A lithium titanate sintered body composed of lithium titanate having a delite-type crystal structure , wherein at least one element selected from the element group is substituted with Ti element constituting the lithium titanate The amount of substitution is in the range of 1 to 20 mol% .

  The lithium secondary battery according to the present invention can increase the battery capacity with respect to the energy density and the volume of the electrode.

It is sectional drawing of the lithium secondary battery which is one Embodiment of this invention.

  FIG. 1 is a cross-sectional view showing a lithium secondary battery according to an embodiment of the present invention, in which an electrolyte 4 is sandwiched between a pair of electrodes composed of a positive electrode 3 and a negative electrode 7. The positive electrode can 1 is bonded to the positive electrode 3 via the positive electrode current collector 2, and is bonded to the negative electrode can 5 bonded to the negative electrode 7 via the negative electrode current collector 6 via the insulating packing 8. .

  The positive electrode current collector 2 or the negative electrode current collector 6 is disposed for collecting current of the positive electrode 3 or the negative electrode 7, and examples of the material for forming the current collectors 6 and 7 include carbon black, graphite, Conductive filler made of at least one of gold, silver, nickel, zinc oxide, tin oxide, indium oxide, titanium oxide, and titanium potassium oxide, acrylic resin, epoxy resin, silicon resin, polyamide resin, phenol The mixture which consists of at least 1 type of polymer adhesive material among resin, a polyester resin, and a polyimide-type resin can be mention | raise | lifted.

Negative 7, Nb, Ta, Ru lithium titanate sintered body Der comprising lithium titanate having a crystal structure of at least ramsdellite which one element forms a solid solution selected from the element group consisting of Al and Fe.

Lithium titanate having a ramsdellite-type crystal structure (Li ₂ Ti ₃ O ₇ , hereinafter sometimes simply referred to as lithium titanate) has excellent cycle characteristics, and spinel type compounds such as Li ₄ Ti ₅ O ₁₂ Compared with the lithium ion diffusibility is better. However, lithium titanate having a ramsdelite-type crystal structure is a high-temperature stable phase, and phase transition accompanied by volume expansion to a low-temperature stable phase occurs during cooling, but in this embodiment, it has a ramsdelite-type crystal structure. Since at least one element selected from the group consisting of Nb, Ta, Al and Fe is dissolved in lithium titanate, the thermal stability of the ramsdellite-type crystal structure is improved and phase transition occurs even at low temperatures. In addition, lithium titanate having a ramsdellite type crystal structure can be maintained. Therefore, for example, when a solid oxide is used for the electrolyte 4 and it is integrated with the electrolyte 4 by heat treatment, it can be suitably used even when heat treatment is required in battery production. In particular, lithium titanate in which Fe is in a solid solution does not undergo phase transition even during heat treatment at 700 ° C. Therefore, when heat treatment is performed at a high temperature, it is preferable to use a solution in which Fe is dissolved in lithium titanate.

  Further, since at least one element selected from the group consisting of Nb, Ta, Al and Fe is dissolved in lithium titanate having a ramsdellite type crystal structure, the sinterability is improved and the sintered body is improved. It is possible to reduce the internal pores and make a dense sintered body. In addition, a temperature of at least 1100 ° C. or higher is required to sinter the conventional lithium titanate in which the additive is not dissolved, but the ramsdellite-type lithium titanate sintered body in this embodiment is about 1000 ° C. A dense sintered body can be obtained at this firing temperature. In particular, in order to make the sintered body more dense, it is preferable to use a solution obtained by dissolving Nb or Ta in lithium titanate.

  Furthermore, the lithium titanate sintered body of the present embodiment is made of lithium titanate having a ramsdellite type crystal structure in which at least one element selected from the group consisting of Nb, Ta, Al and Fe is dissolved. The other components are not substantially contained, and even if other components are contained, the content is 1% by mass or less, further 0.5% by mass or less, preferably 0.2% by mass or less. Therefore, the lithium titanate sintered body does not substantially contain a binder or an electron conduction aid that does not directly contribute to the battery characteristics, so that the active material filling rate in the negative electrode 7 can be increased. As a result, the energy density of the secondary battery can be increased and the battery capacity per volume can be increased.

  Furthermore, the relative density of the lithium titanate sintered body is preferably high, more preferably 85% or more, and even more preferably 90% or more. By increasing the relative density of the sintered body, the negative electrode 7 having a high active material filling rate and a high energy density can be realized.

In the present embodiment, at least one element selected from the group consisting of Nb, Ta, Al and Fe constitutes the lithium titanate in the ratio of the element dissolved in lithium titanate having a ramsdellite type crystal structure. In the case of substitution with Ti element to be substituted, the substitution amount is preferably in the range of 1 to 20 mol%, particularly 1 to 10 mol% in Nb, Ta and Al, and 1 to 15 in Fe. It is preferable to make it the range of mol%. By setting it as such a range, the thermal stability of a ramsdellite structure improves more. Two or more elements may be dissolved. In this case, the amount of substitution with the Ti element is preferably in the range of 1 to 20 mol% of the total amount of substitution with two or more elements.

  The amount of substitution of the Ti element with the additive element in the sintered body is nothing but the solid solution of the unit cell volume of the ramsdellite-type crystal of the lithium titanate sintered body obtained by X-ray diffraction (XRD) measurement. It is obtained from the rate of change with respect to the case where it is not. In the present specification, a unit cell of lithium titanate having a ramsdelite type crystal structure containing a solid solution element without any crystal phase other than lithium titanate having a ramsdelite type crystal structure being confirmed by XRD measurement. When the volume is different from the unit cell volume of lithium titanate having an additive-free ramsdelite-type crystal structure, all of the additive elements detected by ICP are solidified in the crystal lattice of ramsdelite-type lithium titanate. It was considered that it was dissolved and replaced with Ti element.

  The thickness of the negative electrode 7 is preferably 20 μm to 200 μm. As a result, the absolute amount of the active material necessary for improving the energy density and battery capacity of the secondary battery can be secured, the charge / discharge characteristics are good, the handling property is good, and the negative electrode 7 is easy to handle.

  Next, examples of the active material used for the positive electrode 3 include lithium cobalt composite oxide, lithium manganese composite oxide, manganese dioxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium vanadium composite oxide, and vanadium oxide. Etc.

  Like the negative electrode 7, the positive electrode 3 is preferably used as a sintered body having a high relative density, and the relative density is preferably 85% or more, and more preferably 90% or more.

  As the electrolyte 4, any of an organic electrolyte, a polymer solid electrolyte, and an inorganic solid electrolyte can be used.

  Next, an example is shown about the method of producing the lithium secondary battery of this embodiment.

Any of the following (1) to (3) may be used for manufacturing the negative electrode 7 made of the lithium titanate sintered body of the present embodiment.
(1) A slurry is prepared by mixing an active material with a molding aid, water or a solvent to which a dispersant and a plasticizer are added if necessary, and this slurry is applied to a substrate film, dried, and then dried. Peel from the film and sinter.
(2) A material obtained by directly or granulating an active material is put into a mold, pressed with a press machine, and then sintered.
(3) The granulated active material is pressure-formed with a roll press machine, processed into a sheet shape , and sintered. The granulations (2) and (3) may be wet granulation or dry granulation from the slurry described in the method (1). In (1) to (3), the active material to be sintered may be the active material itself after sintering, or a material that forms the active material after sintering by a reaction in the sintering process. It does not matter.

As a raw material of the negative electrode 7, a material necessary for forming lithium titanate having a ramsdellite type crystal structure in which at least one element selected from the group consisting of Nb, Ta, Al and Fe is dissolved as an active material Other than these, for example, an electron conduction aid or a binder is not positively added.
However, there is a possibility that Ca, Na, Si, Zr, Zn, C, etc. are contained as impurities due to raw materials, binders, and processes, but even if such impurities are contained, the amount is 1 mass. % Or less, further 0.5% by mass or less, preferably 0.2% by mass or less.

As the raw material powder of lithium titanate as an active material, fine powder of lithium carbonate and titanium oxide having a specific surface area of 20 m ² / g or more and a primary particle size of 0.1 μm or less is used. By using such fine powder, a calcined powder having a small average particle diameter can be obtained in a process for producing a calcined powder to be described later, and a sintered body having few pores after sintering and having a high relative density can be obtained. . Further, when slurrying is performed in the process, it is preferable to use one having a specific surface area of 20 to 50 m ² / g and a primary particle size of 0.05 to 0.1 μm.

  Then, as an additive, at least one compound selected from the group consisting of Nb, Ta, Al and Fe, preferably an oxide powder, is added and mixed using a mixing means such as a ball mill, and then 450 to 850 ° C. Is calcined at a temperature of 5 to obtain a calcined powder. In addition, at the stage of the calcined powder before firing, it is not always necessary to have a ramsdellite structure or the additive is in solid solution. From the viewpoint of sinterability, Nb, Ta, Al and Any material may be used as long as it forms lithium titanate having a ramsdellite type crystal structure in which at least one element selected from the group consisting of Fe is dissolved.

  Next, a method for producing a lithium titanate sintered body by the production method (1) will be specifically described.

  First, a slurry is prepared by mixing the obtained calcined powder with a binder, a molding aid, and water. The binder is preferably a butyral binder. Since the butyral binder has high strength, the amount added can be reduced, and a high-density sintered body can be obtained. The amount of the binder is preferably 10% by volume or less with respect to the calcined powder that is the active material.

  Examples of the molding aid include one or a mixture of two or more of polyacrylic acid, carboxymethyl cellulose, polyvinylidene fluoride, polyvinyl alcohol, diacetyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, polyvinyl pyrrolidone, butyral, and the like.

  Then, the obtained slurry is applied to a base film and dried. As a base film, resin films, such as a polyethylene terephthalate, a polypropylene, polyethylene, a tetrafluoroethylene, can be used, for example.

  Next, the molded sheet obtained by drying the slurry is peeled off from the base film and sintered.

  What is necessary is just to select the maximum baking temperature of lithium titanate suitably in the range of 1000-1300 degreeC according to a composition. By setting the maximum firing temperature in such a range, lithium titanate having a ramsdellite-type crystal structure can be generated, and a sintered body having no heterogeneous phase can be obtained, and deformation due to melting can be prevented. Further, in this embodiment, it is possible to produce at a cooling rate that can be realized by ordinary furnace cooling, for example, a cooling rate of 1500 ° C./hour or less from the highest temperature to 300 ° C., and equipment for rapid cooling and high cooling ability There is no need to use media.

The lithium titanate sintered body thus obtained was used as the negative electrode 7, and a separately prepared sintered body such as a lithium cobalt composite oxide was used as the positive electrode 3, and an epoxy resin adhesive using carbon as a filler was used. After adhering to the negative electrode can 5 and the positive electrode can 1 respectively, the negative electrode can 5 and the positive electrode can 1 are arranged so that the negative electrode 7 and the positive electrode 3 face each other with a separator containing an organic electrolyte solution, A lithium secondary battery can be obtained by caulking and sealing through the insulating packing 8.

  In addition, the shape of the lithium secondary battery of the present invention is not limited to a square shape, a cylindrical shape, a button shape, a coin shape, a flat shape, and the like, and instead of the positive electrode can 1 and the negative electrode can 5, a positive electrode terminal An insulating container including a negative electrode terminal may be used.

  In this embodiment, a separator containing an organic electrolyte is used as the electrolyte. However, a solid electrolyte may be used. In this case, after applying a solid electrolyte slurry to the surfaces of the negative electrode 7 and the positive electrode 3, By firing at a temperature at which the solid electrolyte can be sintered in a state where the slurries of the electrodes 3 and 7 are in contact with each other, an all solid oxide lithium secondary battery can be obtained.

  Using lithium carbonate and titanium oxide as the lithium titanate raw material and any one of niobium oxide, iron oxide, tantalum oxide and aluminum oxide as the additive, the amount of additive added is the amount of substitution with the Ti element As shown in Table 1, they were weighed and mixed in a ball mill, and calcined at 550 to 750 ° C. to prepare a lithium titanate raw material having an average particle size of 0.2 μm. To this lithium titanate raw material, a molding aid, a plasticizer, a dispersant, and a solvent were added and mixed to prepare a slurry. This slurry was applied on a polyethylene terephthalate (PET) film by a doctor blade method and then dried to produce a green sheet having a thickness of 70 to 80 μm. This green sheet was punched out so that the size after firing was a circle with a diameter of 15 mm, fired at a maximum temperature of 1100 ° C. in the air, cooled under the cooling conditions shown in Table 1, and titanic acid having a thickness of 60 μm. A lithium sintered body was obtained. In Table 1, rapid cooling means that the sample is held at the maximum temperature, then taken out of the furnace and left at room temperature.

  Then, the relative density and crystal structure of the obtained lithium titanate sintered body, the content of lithium titanate, the unit cell volume, the solid solution element and the Ti element substitution amount were confirmed and calculated. In Table 1, the crystal structure is indicated by ◯ for the rams delite type and by x for the others.

The relative density of the sintered body was calculated from the sintered body density measured by the Archimedes method and the theoretical density of Ramus delite type lithium titanate 3.63 g / cm ³ . The crystal structure of the lithium titanate sintered body was confirmed by X-ray diffraction (XRD) measurement. Further, the unit cell volume of lithium titanate in the lithium titanate sintered body was calculated from a plurality of diffraction peaks of ramsdellite type lithium titanate in the XRD pattern by using the least square method. The amount of Ti element substitution in lithium titanate was measured by ICP analysis for the content of Nb, Ta, Fe and Al elements, and the crystal phase due to the additive component could not be confirmed in any sample by X-ray diffraction measurement. In addition, since the unit cell volume of ramsdelite type lithium titanate is changed as compared with the case of no addition, all of Nb, Ta, Fe and Al elements measured by ICP are in the lithium titanate crystal lattice. The solid solution was substituted for the Ti element.

Furthermore, using these sintered bodies as working electrodes, a counter electrode obtained by pressure-bonding a Li metal foil to a current-collecting metal plate was opposed to each other through a separator to assemble a battery cell. For the separator, a polyethylene non-woven fabric impregnated with an organic electrolyte solution is used, and hexafluorophosphorus is added to a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 3: 7 as the organic electrolyte solution. lithium acid LiPF ₆ was used dissolved in 1 mol / L.

Sample No. Ten working electrodes were produced as follows. Sample No. as a raw material. 9 calcined lithium titanate raw material was fired at 1100 ° C. and then rapidly cooled, pulverized powder, 10% by mass of polyvinylidene fluoride as a binder, and 5% by mass of carbon black as a conductive material. And a dispersant was added to prepare a slurry. Next, the obtained slurry was applied onto an aluminum foil and pressed, and then punched to a diameter of 15 mm to form an electrode. The active material filling rate of this electrode was 55%.

  The battery cell produced as described above was subjected to a charge / discharge test at a current value corresponding to a 10 hour rate by a constant current charge / discharge method.

Charge / discharge rate: 0.1C
End-of- charge voltage: 2.5V
End-of- discharge voltage: 1.0V
Measurement temperature: 30 ° C
Cycle: One discharge-charge is defined as one cycle, and three cycles are repeated. The charge / discharge rate of 0.1 C is a current value at which the entire battery capacity is discharged or charged in 10 hours.

  The results of the charge / discharge test are shown in Table 1. The discharge capacity and the 3-cycle capacity maintenance rate were calculated as follows.

Discharge capacity 1: Discharge time x current value / negative electrode weight Discharge capacity 2: Discharge time x current value / negative electrode volume Capacity retention ratio: Discharge capacity after cycle test / initial discharge capacity

  As shown in Table 1, sample no. 1 to 7, it was confirmed by XRD measurement that the structure of the ramsdellite-type lithium titanate was maintained. Further, in the ICP measurement, the same element as that used at the time of preparation was detected, and the detection of impurities was not confirmed. In particular, sample no. In Examples 1 and 3 to 7, dense electrodes having a relative density of 90% or more were obtained. The discharge capacity was measured according to Sample No. 1-5 showed a high value. Furthermore, sample no. All of 1 to 7 showed excellent cycle characteristics, and Examples 1, 2, 5, and 6 showed almost no deterioration in 3 cycles.

On the other hand, sample no. In No. 8, the sample was destroyed by furnace cooling at a cooling rate of 500 ° C./hour. This is due to the volume expansion accompanying the phase transition during cooling, and the generation of a heterogeneous phase was also confirmed in the XRD measurement. In addition, sample No. 1 was obtained by quenching lithium titanate that was not solid-solved at all after firing. In No. 9, a sintered body having a ramsdellite structure and a relative density of 89% was obtained, but the cycle characteristics were inferior. In addition, sample No. 1 was simply formed by pressure-molding lithium titanate powder. In No. 10, since the negative electrode contains a large amount of binder and conductive agent, the discharge capacity per unit volume of the negative electrode was low.

  1: positive electrode can, 2: positive electrode current collector, 3: positive electrode, 4: electrolyte, 5: negative electrode can, 6: negative electrode current collector, 7: negative electrode, 8: insulating packing

Claims (3)

A lithium secondary battery having an electrolyte between a positive electrode and a negative electrode, wherein the negative electrode is a ramsdellite type in which at least one element selected from the element group consisting of Nb, Ta, Al and Fe is dissolved. It is a lithium titanate sintered body made of lithium titanate having a crystal structure,
Lithium secondary, wherein the substitution amount is in the range of 1 to 20 mol% when at least one element selected from the element group is substituted with Ti element constituting the lithium titanate battery.   The lithium secondary battery according to claim 1, wherein a relative density of the lithium titanate sintered body is 90% or more. At least one element is a lithium secondary battery according to claim 1 or 2, characterized in that a solid solution in the lithium titanate of the N b and Ta.

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