JP5677181B2 - Solid electrolyte and secondary battery using the same - Google Patents

Description

  The present invention relates to a solid electrolyte and a secondary battery using the same.

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

  What is commonly required for these batteries is an increase in capacity that is an indicator of long-term use. Methods for increasing the capacity of the secondary battery include a method using an electrode material having a large capacity, application of a positive electrode material exhibiting a high discharge voltage, solidification of an electrolyte, and the like.

Among them, a secondary battery using a solid electrolyte as an electrolyte has many advantages in terms of safety and energy density. For example, in Patent Document 1, lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ) is glass. Further, in Patent Document 2, the Li ion conductivity is increased by adding Al ₂ O ₃ to LiNbO ₃ . Patent Document 3 proposes using a LiNbO ₃ —LiNb ₃ O ₈ complex as a solid electrolyte.

JP 59-71264 A JP 2009-218124 A JP 2010-251257 A

  However, the solid electrolytes described in Patent Documents 1 and 2 have a problem that the Li ion conductivity is lowered when heat treatment is performed at a temperature exceeding 400 ° C. in which components such as organic compounds are hardly left in the molded body.

  In addition, in such a secondary battery using a solid electrolyte, the solid electrolyte and the electrode are joined at a low temperature in order to maintain the Li ion conductivity of the solid electrolyte itself, and the electrode is used because of insufficient joining. Li ion conductivity at the interface between the active material and the solid electrolyte to be formed is lowered. Therefore, in Patent Document 1, a Li ion conduction path between the positive electrode and the solid electrolyte is formed by adding a solid electrolyte material to the positive electrode active material. However, in such a method, since the ratio of the active material in the positive electrode is low, there is a problem that the energy density as a battery is reduced.

  The present invention has been made in view of the above problems, and an object thereof is to provide a lithium niobate-based solid electrolyte capable of maintaining high Li ion conductivity and a high-capacity secondary battery using the same.

The solid electrolyte of the present invention is a solid electrolyte composed of an oxide of Li and Nb, and the oxide contains Zr, and the ratio of Zr to Li contained in the oxide is It is characterized by being 3 mol% or more .

A secondary battery according to the present invention includes a power generation element having the above-described solid electrolyte and a positive electrode and a negative electrode arranged so as to face each other with the solid electrolyte interposed therebetween. It is a ligation.

According to the present invention, it is possible to realize a solid electrolyte high have Li ion conductivity can be maintained stably. In the secondary battery using such a solid electrolyte, the solid electrolyte and the electrode made of the oxide sintered body can be firmly joined to increase the Li ion conductivity at the interface. realizable.

It is sectional drawing which shows an example of the secondary battery of this invention. It is sectional drawing which shows the other example of the secondary battery of this invention. It is a graph which shows the measurement result of the charge transfer resistance by the alternating current impedance method of the solid electrolyte containing 3 mol% of Zr. It is a graph which shows the measurement result of the charge transfer resistance by the alternating current impedance method of the solid electrolyte containing 10 mol% of Zr. It is a graph which shows the measurement result of the charge transfer resistance by the alternating current impedance method of the solid electrolyte containing 30 mol% of Zr. It is a graph which shows the measurement result of the charge transfer resistance by the alternating current impedance method of the solid electrolyte which does not contain Zr, (a) is a case where it heat-processes at 400 degreeC, (b) is a case where it heat-processes at 500 degreeC and 600 degreeC.

The solid electrolyte of the present embodiment is made of an oxide of Li and Nb containing Zr, and the Zr content is desirably 3 mol% or more and 50 mol% or less with respect to Li. By setting the Zr content to 3 mol% or more, even when the solid electrolyte is heat-treated at a temperature exceeding 400 ° C., a decrease in Li ion conductivity of the solid electrolyte due to crystallization can be suppressed. Further, the Li ion conductivity of the oxide of Li and Nb is maintained even when 50 mol% of Zr is contained with respect to Li, but from the viewpoint of realizing higher ion conductivity, the Zr content is It is desirable that it is 30 mol% or less. Such a solid electrolyte has high Li ion conductivity of the material itself, and crystallization at high temperature is suppressed. Therefore, even when heat treatment is performed at a temperature exceeding 400 ° C. It can be set as the solid electrolyte which has. The Zr content in the solid electrolyte can be confirmed by elemental analysis such as X-ray fluorescence and ICP.

The oxide of Li and Nb refers to LiNbO ₃ , LiNb ₃ O ₈ or the like and oxides in which these are mixed, and may be crystalline or amorphous, I do not care. However, it is preferable that the crystallinity of the oxide of Li and Nb is low from the viewpoint of ion conductivity. The crystallinity of the oxide of Li and Nb can be judged from X-ray diffraction (XRD) measurement. As crystallization progresses, the diffraction peak intensity of the crystal plane increases and the half-value width of the diffraction peak decreases. The crystallite diameter was calculated from the half-width of the diffraction peak, and the smaller the crystallite diameter, the lower the crystallinity.

In addition, Zr in the solid electrolyte in the present embodiment is considered to be dissolved in an oxide of Li and Nb because a diffraction peak due to a Zr compound such as ZrO ₂ is not confirmed in XRD measurement.

  Moreover, the solid electrolyte which consists of the oxide of Li and Nb containing Zr of this embodiment does not add positively except a material required for formation of the oxide of Li and Nb containing Zr as a raw material. However, the amount of impurities due to raw materials, binders, and processes is 1% by mass or less, further 0.5% by mass or less, and preferably 0.2% by mass or less. Examples of the constituent elements of impurities include Ca, Na, Zn, and C.

  The thickness of the solid electrolyte needs to be a thickness that does not cause an electrical short between the positive electrode and the negative electrode, and is preferably at least 30 nm, further 50 nm, more preferably 80 nm. Moreover, since resistance of Li ion conduction will become large when too thick, it is preferable to set it as 1 micrometer or less, Furthermore, it is 500 nm or less.

  The solid electrolyte of the present embodiment can be produced by a technique such as a sol-gel method or a precipitation method. In the sol-gel method, a solid electrolyte precursor sol is prepared from an alkoxide of Nb, Li, and Zr, and a thin film formed using the sol is processed at a temperature exceeding 400 ° C.

  For example, Nb alkoxides include pentamethoxyniobium, pentaethoxyniobium, penta-i-propoxyniobium, penta-n-propoxyniobium, penta-i-butoxyniobium, penta-n-butoxyniobium, penta-sec-butoxyniobium, Niobium aluminum-i-propoxide or the like can be used.

  As the alkoxide of Li, methoxy lithium, ethoxy lithium, i-propoxy lithium, n-propoxy lithium, i-butoxy lithium, n-butoxy lithium, sec-butoxy lithium, t-butoxy lithium, dipivaloylmethanato lithium Etc. can be used.

  These Nb and Li alkoxides are dissolved in alcohols such as methanol, ethanol, and IPA, and then added with water using an acid such as nitric acid or hydrochloric acid, or an alkali such as ammonia as a catalyst, and a solid electrolyte precursor sol containing no Zr. Is made.

  Zr alkoxides include tetramethoxy zirconium, tetraethoxy zirconium, tetra-i-propoxy zirconium, tetra-n-propoxy zirconium, tetra-i-butoxy zirconium, tetra-n-butoxy zirconium, tetra-sec-butoxy zirconium, tetra -T-Butoxyzirconium, (isopropoxy) tris (dipivaloylmethanato) zirconium, tetrakis (dipivaloylmethanato) zirconium, tetrakis (ethylmethylamino) zirconium and the like can be used.

  A Zr-containing solid is obtained by adding a Zr alkoxide dissolved in alcohol to a solid electrolyte precursor sol containing no Zr so that Zr is 3 mol% or more and 50 mol% or less with respect to Li. An electrolyte precursor sol (hereinafter also simply referred to as a precursor sol) is obtained. At this time, in order to stabilize the precursor sol contained in the precursor sol, reflux may be performed at 40 to 70 ° C.

  Using the obtained precursor sol, a solid electrolyte precursor thin film is formed on the electrode by, for example, a dip method or a spin coat method, and heat-treated at a temperature exceeding 400 ° C., so that Li and Nb containing Zr A solid electrolyte composed of the oxide can be obtained. The solid electrolyte thus obtained is a solid electrolyte having a high Li ion conductivity by removing impurities such as organic compounds from the inside.

  As described above, the method for producing the solid electrolyte of the present embodiment by the sol-gel method has been described. However, the method for producing the solid electrolyte of the present embodiment is not limited to the above-described method. A known thin film forming method capable of forming an oxide solid electrolyte with a thickness of submicron can be applied.

The solid electrolyte thus obtained is used as the solid electrolyte layer 2 of the power generation element 8 in which the positive electrode 1, the solid electrolyte layer 2, and the negative electrode 3 are sequentially laminated as shown in FIG. Thus, a secondary battery having a high energy density and a high capacity can be obtained. In addition, as shown in FIG. 2, a plurality of power generation elements 8 may be connected via the current collector 4.

The positive electrode 1 is composed of at least a positive electrode active material. As the positive electrode active material, a layered compound such as LiCoO ₂ , LiNiO ₂ , LiCrO ₂ , LiVO ₂ , LiM _x Mn _{2 -x} O ₄ (M is C
o, at least one of Ni, Fe, Cr and Cu), and olivine-based materials such as LiFePO ₄ and LiCoPO ₄ can be used.

  The negative electrode 3 is made of at least a negative electrode active material, and a transition metal oxide, a lithium-containing transition metal oxide, or the like can be used as the negative electrode active material.

  The positive electrode 1 and the negative electrode 3 may contain materials other than the active material. However, it is desirable that the positive electrode 1 and the negative electrode 3 are substantially formed of the active material from the viewpoint of improving the energy density and increasing the capacity.

  The positive electrode 1 and the negative electrode 3 are preferably dense sintered bodies made of an active material. By using the positive electrode 1 and the negative electrode 3 as dense sintered bodies, the bonding area with the solid electrolyte layer 2 facing each other can be increased. That is, in an electrode having many defects such as voids, a bonding interface with the solid electrolyte layer 2 is not formed in the defective portion, so that the ion conduction path is reduced, the internal resistance is increased, and the battery performance is lowered. Ideally, the porosity of the sintered body is desirably 0%, but the acceptable porosity is preferably 15% or less, and more preferably 10% or less.

  Next, an example of the manufacturing method of the secondary battery of this embodiment will be described. The secondary battery of the present embodiment includes, for example, a solid electrolyte precursor sol containing Nb, Li, and Zr produced by, for example, a sol-gel method on the surfaces of the positive electrode 1 and the negative electrode 3, and the first solid electrolyte precursor film and Prepared by forming a thin film as a second solid electrolyte precursor film by dipping or spin coating, and bonding the solid electrolyte precursor films together by heat treatment at a temperature exceeding 400 ° C., preferably at 500 ° C. or higher. it can.

  The method for producing the positive electrode 1 or the negative electrode 3 is not particularly limited. For example, each active material is press-molded, or a green sheet of the active material is shaped and degreased and fired to produce a sintered body. And it can be set as each electrode. Moreover, after forming the 1st solid electrolyte precursor film | membrane and the 2nd solid electrolyte precursor film | membrane in the molded object and green sheet of the electrode active material used as the positive electrode 1 or the negative electrode 3, respectively, the solid electrolyte precursor film | membranes are bonded together. The electrode may be densified and the electrode and the solid electrolyte may be joined simultaneously by firing.

First, LiCoO ₂ powder is used as the positive electrode active material, and Li ₄ Ti ₅ O ₁₂ powder is used as the negative electrode active material, and these active materials and a binder such as butyral are used as a dispersant or a plasticizer as necessary. Using water or an organic solvent such as toluene as a solvent, they are mixed by a known method to prepare slurry for positive electrode and negative electrode. B, Li, Si oxide or the like may be added to the powder as the active material as a sintering aid. In this case, the addition amount of the sintering aid is preferably 5% by weight or less with respect to the active material from the viewpoint of increasing the active material filling rate of the electrode.

  The produced slurry is coated and dried on a polyethylene terephthalate (PET) film by a known method such as a doctor blade or a coater to produce a green sheet having a thickness of about 200 μm, for example. The obtained green sheet is punched into a desired shape, degreased as necessary, and then fired to obtain a sintered body that becomes the positive electrode 1 and the negative electrode 3. The firing is usually performed at about 500 ° C. to 1100 ° C., but the firing temperature is not limited to this range, and may be appropriately selected according to the sinterability of the active material as the raw material powder.

  The solid electrolyte precursor sol obtained by the above-described sol-gel method is applied to the surfaces of the positive electrode 1 and the negative electrode 3 thus produced by a dip method, a spin coating method, or the like, and the first solid electrolyte precursor film and the second A thin film that is a solid electrolyte precursor film is formed. The thickness of the thin film needs to be a thickness that does not cause an electrical short between the positive electrode 1 and the negative electrode 3, and the thickness of the solid electrolyte layer 2 including the first solid electrolyte precursor film and the second solid electrolyte precursor film. The thickness is 30 nm or more, preferably 50 nm or more, and more preferably 80 nm or more. Moreover, since resistance of Li ion conduction will become large when too thick, it is preferable to set it as 1 micrometer or less, Furthermore, it is 500 nm or less.

  Next, the power generation element 8 can be obtained by bonding the first solid electrolyte precursor film and the second solid electrolyte precursor film formed on the positive electrode 1 and the negative electrode 3 and bonding them by heat treatment. The heat treatment temperature is higher than 400 ° C., and preferably 500 ° C. or higher in view of element diffusion. Moreover, in order to improve the adhesiveness of a joining interface, you may pressurize at the time of heat processing by methods, such as a hot press.

  The power generation element 8 formed in this way is used by being accommodated in a storage container. As the storage container, any of the outer casings 5 and 7 and the current collector 4 used in a laminated lithium ion battery or a conventional coin battery as shown in FIG. 1 can be applied. For example, as shown in FIG. 2, an exterior body 7 that is a lid material obtained by processing a metal plate having electronic conductivity made of aluminum, zinc, iron, nickel, stainless steel, or the like by a press molding method, and a plurality of power generation units therein. It is also possible to seal the exterior body 5, which is a battery case body in which the elements 8 are stacked and accommodated, with an insulating material 6 (insulating packing) interposed therebetween. In FIG. 2, three power generation elements 8 in which a solid electrolyte is disposed and stacked so as to be sandwiched between a positive electrode and a negative electrode are connected in series via a current collector. The number and connection method differ depending on the application and are not particularly limited.

(Preparation of Li ion conductivity measurement sample)
First, a solid electrolyte precursor sol containing Nb, Li and Zr was prepared. Pentaethoxyniobium was used as the alkoxide of Nb, lithium ethoxide was used as the alkoxide of Li, and these were mixed using ethanol as a solvent. The mixing ratio of Nb, Li, and ethanol was 1: 1: 100 as a molar ratio. Nitric acid having a molar ratio of 0.1 with respect to Li was added to and mixed with this mixed solution to prepare a solid electrolyte precursor sol containing no Zr. Next, using zirconium tetra-n-butoxide as the alkoxide of Zr and adding 90% by weight of this in 1-butanol solution to a solid electrolyte precursor sol containing no Zr, and mixing well. A solid electrolyte precursor sol containing Zr (hereinafter also simply referred to as precursor sol) was prepared. Table 1 shows the amount of Zr added to Li in the precursor sol.

Using the obtained precursor sol, a solid electrolyte precursor film was formed by spin coating on an electrode formed by vapor-depositing Pt on glass, and heat-treated at each temperature of 400 ° C. to 600 ° C., A solid electrolyte membrane having an area of 1.77 cm ² and a thickness of 0.3 μm was prepared, and Pt was deposited on the surface as a measurement electrode. In addition, it confirmed that Zr content of the produced solid electrolyte membrane was equivalent to the addition amount by ICP analysis.

(Measurement of Li ion conductivity)
Li ion conductivity of the obtained solid electrolyte membrane was measured by an AC impedance method (common name: Cole-Cole plot). The measurement apparatus used was VSP manufactured by France Bio-Logic. In an ion blocking state, a resistance value from a frequency of 1 MHz to 100 mHz was measured at room temperature, and Li ion conductivity was calculated by the following equation.

  The resistance measurement value R of the solid electrolyte membrane corresponds to the value of the real axis intercept (arc radius) of the graph plot. The thickness t of the solid electrolyte membrane is an average value obtained by observing the cross section of the solid electrolyte membrane with a scanning electron microscope (SEM) and measuring the thickness at three locations.

  3 to 6 show Sample No. It is a graph which shows the alternating current impedance measurement result of 1-4. Sample No. containing no Zr. 4, the electrical resistance value (about 900Ω) when the heat treatment temperature is 400 ° C. shown in FIG. 6A is 100 kΩ or more as shown in FIG. 6B when the heat treatment temperatures are 500 ° C. and 600 ° C. The electric resistance value was different from the order, and the electric resistance was greatly increased.

  On the other hand, sample no. In any of 1 to 3, as shown in FIGS. 3 to 5, the charge transfer resistance increases as the temperature of the heat treatment increases, but the order does not change, and the rate of increase of the charge transfer resistance due to the heat treatment temperature is small. .

  When the amount of Zr added is small, the charge transfer resistance tends to increase when heat-treated at a high temperature, and when the amount of Zr added is large, the charge transfer resistance tends to increase even when heat-treated at a low temperature such as 400 ° C. It was.

  As described above, it can be seen that by adding Zr to an oxide of Li and Nb, an increase in charge transfer resistance due to heat treatment at a high temperature is significantly suppressed, and a solid electrolyte exhibiting high Li ion conduction can be obtained. .

Based on the above results, Sample No. Table 1 shows the Li ion conductivity calculated from the above formula and the crystallite diameter of LiNbO ₃ obtained from the XRD measurement for the solid electrolyte membranes 1 to 4. The crystallite diameter was calculated from the following formula using the half width of the diffraction peak of the (012) plane of LiNbO ₃ .

Thus, the sample No. containing Zr. In the solid electrolytes 1 to 3, a practical solid exhibiting a high Li ion conductivity of 1 × 10 ⁻⁷ S / cm or more because the crystallite size is small and crystallization is suppressed even when heat treatment is performed at 600 ° C. It was an electrolyte.

DESCRIPTION OF SYMBOLS 1 ... Positive electrode 2 ... Solid electrolyte layer 3 ... Negative electrode 4 ... Current collector 5, 7 ... Exterior body 6 ... Insulating material

Claims (3)

A solid electrolyte comprising an oxide of Li and Nb , wherein the oxide contains Zr, and the ratio of Zr to Li contained in the oxide is 3 mol% or more. A solid electrolyte. A power generation element comprising the solid electrolyte according to claim 1 and a positive electrode and a negative electrode arranged so as to face each other with the solid electrolyte interposed therebetween, wherein the positive electrode and the negative electrode are oxide sintered bodies. A secondary battery characterized by that. The secondary battery according to claim 2 , wherein a plurality of the power generation elements are connected via a current collector.

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