JP2010160988A - Positive electrode sintered body and nonaqueous electrolyte battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode sintered body, which can be used as a positive electrode of a nonaqueous electrolyte battery to reduce the internal resistance of the battery and improve the discharge capacity, and to provide a nonaqueous electrolyte battery provided with a positive electrode using the positive electrode sintered body. <P>SOLUTION: The positive electrode sintered body to be used as a positive electrode of a nonaqueous electrolyte battery contains particles of a positive electrode active material, and has a porosity of 15 vol.% or less, the particle size of the active material particles being 10 μm or more but 100 μm or less. The nonaqueous electrolyte battery includes a positive electrode, a negative electrode, and an electrolyte layer interposed between the positive and negative electrodes, and the positive electrode includes the positive electrode sintered body. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a positive electrode sintered body used for a positive electrode of a nonaqueous electrolyte battery, and a nonaqueous electrolyte battery including a positive electrode using the positive electrode sintered body.

  Nonaqueous electrolyte batteries are used as power sources for relatively small electric devices such as portable devices. A nonaqueous electrolyte battery includes a positive electrode, a negative electrode, and an electrolyte layer interposed between the positive and negative electrodes. A typical example of this non-aqueous electrolyte battery is a lithium ion secondary battery using a lithium ion occlusion / release reaction in the positive and negative electrodes (see, for example, Patent Document 1).

  Patent Document 1 discloses a technique of using a lithium composite oxide sintered body (positive electrode sintered body) as a positive electrode of a nonaqueous electrolyte battery.

JP-A-8-180904

  However, as a result of intensive studies by the present inventors, the positive electrode using the conventional positive electrode sintered body has a large internal resistance even when used in a battery, and there is room for improvement in that a sufficient discharge capacity cannot be obtained. I found out.

Patent Document 1 teaches the making the volume density of the Seikyokusho sintered body and 2.0~4.3g / ml, Seikyokusho sintered body is LiCoO ₂ (LiCoO ₂ which is frequently used as a positive active material density: Assuming that it is composed only of about 5.0 g / cm ³ ), the porosity is about 16 to 60% by volume. Therefore, when such a positive electrode sintered body having a large porosity is used for the positive electrode, there are many voids, which may cause a decrease in discharge capacity.

  The present invention has been made in view of the above circumstances, and one of its purposes is to reduce the internal resistance of the battery and improve the discharge capacity by using it for the positive electrode of a non-aqueous electrolyte battery. The object is to provide a non-aqueous electrolyte battery including a positive electrode sintered body and a positive electrode using the positive electrode sintered body.

  (1) The positive electrode sintered body of the present invention is a positive electrode sintered body used for a positive electrode of a nonaqueous electrolyte battery. The positive electrode sintered body of the present invention is characterized by containing positive electrode active material particles, a porosity of 15% by volume or less, and an active material particle size of 10 μm or more and 100 μm or less.

  By setting the porosity to 15% by volume or less, voids that do not contribute to the battery reaction are reduced, and the discharge capacity can be improved. Moreover, if the porosity is within this range, it is easy to ensure the strength of the positive electrode sintered body. On the other hand, if the porosity exceeds 15% by volume, the strength of the positive electrode sintered body decreases and becomes brittle, and the diffusion resistance of lithium ions in the positive electrode sintered body generally increases to increase the internal resistance of the battery. The discharge capacity decreases. A more preferable porosity is 12% by volume or less. It is technically difficult to produce a positive electrode sintered body having a porosity of 0.1% by volume or less.

  Moreover, the following effects can be exhibited by setting the porosity to 15% by volume or less. For example, when a solid electrolyte is used for the electrolyte layer, the solid electrolyte layer may be formed on one surface of the positive electrode sintered body using a vapor phase method or the like. However, the positive electrode sintered body having a large porosity such as a porosity of more than 15% by volume tends to have a large surface irregularity, and pinholes are formed in the solid electrolyte layer formed on one surface due to the irregularities. Defects such as these are likely to occur, causing a frequent short circuit between the positive and negative electrodes. This is particularly noticeable in a thin film battery having a thin solid electrolyte layer (for example, the thickness of the solid electrolyte layer is 50 μm or less). On the other hand, if the porosity is 15% by volume or less, the surface unevenness due to the voids becomes small, so that defects are hardly generated in the solid electrolyte layer, and a short circuit between the positive and negative electrodes can be suppressed. As a result, a battery with high safety and reliability can be obtained.

  Furthermore, the present inventors have found that the particle size of the active material particles in the positive electrode sintered body is important in realizing a reduction in internal resistance, and the particle size of the active material particles is 10 μm or more and 100 μm or less. Propose that. In addition, in this invention, an active material particle is a crystal grain of the active material enclosed by the crystal grain boundary in a sintered compact. The particle size of the active material particles refers to a plurality (10 or more, preferably 20 or more) of active material particles selected from an arbitrary field of view by observing the surface structure of the sintered body with an electron microscope (SEM). It is the value which measured the longest major axis diameter about each, and averaged these.

  The mechanism for lowering resistance is considered as follows. When the particle size is less than 10 μm, the diffusion bonding between the crystal grains is not sufficiently developed, so that the lithium ion conductivity between the crystal grains is lowered and the internal resistance of the battery is increased. On the other hand, when the particle size exceeds 100 μm, the pore size between crystal grains increases, so that the diffusion distance of lithium ions and the conduction distance of electrons in the positive electrode sintered body increase, and the internal resistance of the battery increases. A more preferable particle size is 10 μm or more and 50 μm or less.

(2) The positive electrode active material is preferably at least one selected from LiαO ₂ and Liβ ₂ O ₄ . However, α and β include at least one element selected from Co, Mn, Al, and Ni, and the total ratio of the elements in α and β is 50% or more in atomic ratio.

Such an oxide is typical as a positive electrode active material of a nonaqueous electrolyte battery, and is preferable in securing the discharge capacity of the battery. Examples of LiαO ₂ and Liβ ₂ O ₄ include LiCoO ₂ , LiNiO ₂ , and LiNi _0.5 Mn _0.5 O ₂ . Further, the total ratio of Co, Mn, Al, and Ni in α and β only needs to satisfy 50% or more in atomic ratio. For example, a positive electrode active material such as LiCo _0.5 Fe _0.5 O ₂ may be used. In addition, a positive electrode active material having a spinel structure such as LiNi _0.5 Mn _1.5 O ₄ or LiMn ₂ O ₄ may be used as the positive electrode active material. The positive electrode active materials may be used alone or in combination.

  In addition to the positive electrode active material described above, the positive electrode sintered body may contain lithium ion conductive composite oxide. As the composite oxide, for example, an oxide containing Li and at least one element selected from Ti, Nb and Ta, or at least one element selected from La, Zr and Ti, and Li Examples thereof include oxides contained therein. The complex oxides may be used alone or in combination.

  (3) The nonaqueous electrolyte battery of the present invention includes a positive electrode, a negative electrode, and an electrolyte layer interposed between these positive and negative electrodes. In the battery of the present invention, the positive electrode includes the positive electrode sintered body of the present invention, and the electrolyte layer includes a solid electrolyte.

  As will be described later, the electrolyte layer can be constituted by, for example, a solid electrolyte, an organic electrolyte, or a combination thereof. When the electrolyte layer includes an organic electrolyte, the organic electrolyte enters the crystal grain boundaries and voids of the positive electrode sintered body and penetrates deep into the positive electrode sintered body, so that the active material particles constituting the positive electrode sintered body The small diffusion of lithium ions between crystal grains can be covered. On the other hand, when the electrolyte layer includes a solid electrolyte, since the solid electrolyte does not enter the crystal grain boundaries and voids of the positive electrode sintered body, lithium ions between the active material particles constituting the positive electrode sintered body The small diffusion greatly affects the lithium ion diffusion of the entire positive electrode sintered body.

  Therefore, even in a battery including a solid electrolyte as an electrolyte layer, the use of the positive electrode sintered body of the present invention as a positive electrode significantly improves the lithium ion conductivity and reduces the internal resistance of the battery. be able to.

  (4) The nonaqueous electrolyte battery of the present invention includes a positive electrode, a negative electrode, and an electrolyte layer interposed between the positive and negative electrodes. In the battery of the present invention, the positive electrode includes the positive electrode sintered body of the present invention and a metal thin film formed on the surface of the positive electrode sintered body.

  In the nonaqueous electrolyte battery of the present invention, the positive electrode sintered body functions as a positive electrode active material layer, and the metal thin film functions as a positive electrode current collector. The metal thin film is formed on the surface opposite to the surface on which the electrolyte layer of the positive electrode sintered body is disposed, and can be formed using, for example, a vapor phase method. As the metal thin film, Au, Al, Ni and alloys thereof, or stainless steel can be suitably used.

  Further, the positive electrode sintered body preferably has an average thickness of 0.05 to 0.3 mm, and the metal thin film preferably has an average thickness of 0.01 to 30 μm. When the average thickness of the positive electrode sintered body satisfies this range, it is easy to maintain the strength of the positive electrode sintered body and to secure the discharge capacity. When the average thickness of the metal thin film satisfies this range, the metal thin film can be made thinner than the positive electrode sintered body, and the proportion of the positive electrode sintered body (amount of the positive electrode active material) in the positive electrode is increased. The energy density of the entire battery can be increased. By the way, even if the positive electrode sintered body is made too thick, the diffusion capacity of lithium ions in the positive electrode sintered body becomes rate-determining, so the effect of improving the discharge capacity commensurate with the thickness may not be obtained. The upper limit of the average body thickness was set to 0.3 mm. Here, the average thickness is a value obtained by measuring the thickness at each of a plurality of locations (10 or more points) and averaging them.

  (5) In the nonaqueous electrolyte battery of the present invention, it is preferable to provide a buffer layer for reducing the interface resistance between the positive electrode and the electrolyte layer.

  For example, when an oxide is used as the positive electrode active material and a sulfide-based solid electrolyte is used as the electrolyte layer, the oxide ions attract lithium ions more strongly than sulfide ions, so the electrolyte at the junction interface between the positive electrode and the electrolyte layer Lithium ions may move from the layer to the positive electrode. As a result, a charge bias occurs near the interface of the electrolyte layer in contact with the positive electrode, and a charge depletion layer is formed, so that the interface resistance increases. Therefore, when the electrolyte layer is composed of a sulfide-based solid electrolyte, the interface resistance can be reduced by providing a buffer layer between the positive electrode and the electrolyte layer.

Examples of the buffer layer include a composite oxide containing Li and at least one element selected from Ti, Nb, Ta, and Si, specifically, Li ₄ Ti ₅ O ₁₂ , LiNbO ₃ , LiTaO. ₃ , Li _3.6 Si _0.6 P _0.4 O ₄ , Li _X La _{(2-X) / 3} TiO ₃ (X = 0.1 to 0.5), Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.8 Cr _0.8 Ti _1.2 ( PO ₄ ) ₃ , Li _1.4 In _0.4 Ti _1.6 (PO ₄ ) _{3 and the} like can be used alone or in combination.

  The buffer layer preferably has an average thickness of 5 to 100 nm. If the buffer layer is too thick, the internal resistance of the battery increases. Conversely, if the buffer layer is too thin, the effect of reducing the interface resistance may not be obtained.

(6) The positive electrode sintered body of the present invention has a lithium ion conductive composite oxide in the gap, and the composite oxide is at least one selected from [1] and [2] shown below. It is preferable.
[1] Oxides containing at least one element selected from Ti, Nb and Ta and Li
[2] An oxide containing La, at least one element selected from Zr and Ti, and Li

The presence of such a complex oxide in the void portion of the positive electrode sintered body helps the lithium oxide to conduct lithium ions in the void portion, thereby contributing to improvement in lithium ion conductivity. As a result, the internal resistance of the battery can be reduced and the discharge capacity can be improved. Examples of such complex oxides include LiNbO ₃ , Li ₂ Ti ₃ O ₇ , Li ₄ Ti ₅ O ₁₂ , Li _0.35 La _0.52 TiO _2.96 , Li _x La _1/3 Nb _1-x Ti _x O ₃ ( x <0.1), Li _5.5 La ₃ Nb _1.75 In _0.25 O ₁₂ , Li ₆ BaLa ₂ Ta ₂ O _{12 and the} like.

In addition, it is also effective to fill the voids with a polymer material (for example, polyethylene oxide) in which a lithium salt such as LiPF ₆ is dissolved. Also in this case, the composite oxides may be used alone or in combination.

  The positive electrode sintered body of the present invention has a porosity of 15% by volume or less and an active material particle size of 10 μm or more and 100 μm or less, and reduces the internal resistance of the battery when used for the positive electrode of a nonaqueous electrolyte battery. In addition, the discharge capacity can be improved. The nonaqueous electrolyte battery of the present invention includes a positive electrode using the positive electrode sintered body of the present invention, has a low internal resistance, and a high discharge capacity.

  The positive electrode sintered body and the nonaqueous electrolyte battery of the present invention will be described in more detail.

(Positive electrode sintered body)
The positive electrode sintered body can be manufactured by press-molding a powder containing a positive electrode active material as a raw material (hereinafter referred to as a raw material powder) and then firing it. The positive electrode sintered body may contain a conductive additive in addition to the active material. As the conductive aid, carbon black such as acetylene black, natural graphite, thermally expanded graphite, carbon fiber, ruthenium oxide, titanium oxide, metal fibers of Al or Ni, and the like can be used.

  In order to control the porosity to 15% by volume or less and the particle size of the active material particles to 10 μm or more and 100 μm or less, the particle size of the raw material powder, the press pressure at the time of pressure molding, the firing temperature and the firing time are appropriately set. It is important to set the conditions.

  The particle size of the raw material powder is preferably 0.1 to 8 μm, and more preferably 0.5 to 3 μm. When the positive electrode sintered body is formed, the active material particles in the sintered body grow and become coarser than before firing. Therefore, when the particle size of the raw material powder satisfies this range, the particle size of the active material particles in the final sintered body can be easily controlled to 10 μm or more and 100 μm or less.

The firing temperature varies depending on the type of active material used and the particle size of the raw material powder. For example, in the case of LiαO ₂ (α is at least one element selected from Co, Mn, Al and Ni), the firing temperature Is preferably 750 to 1100 ° C.

(Nonaqueous electrolyte battery)
The basic structure of the nonaqueous electrolyte battery of the present invention is a structure in which a positive electrode, an electrolyte layer, and a negative electrode are sequentially laminated. And a positive electrode is comprised from the above-mentioned positive electrode sintered compact and the metal thin film formed in the surface of this positive electrode sintered compact.

(Electrolyte layer)
In the nonaqueous electrolyte battery of the present invention, the electrolyte layer can be composed of, for example, a solid electrolyte or an organic electrolyte. When the electrolyte layer is composed of a solid electrolyte, it is preferable to use a sulfide-based solid electrolyte with high lithium ion conductivity. Examples of such sulfide-based solid electrolytes include Li-PS-based and Li-PSO-based ones. In addition, a Li-PO-based or Li-PON-based oxide-based solid electrolyte may be used. Specific examples of the Li-PS solid electrolyte include Li ₂ SP ₂ S ₅ and Li ₇ P ₃ S ₁₁ . When the electrolyte layer is composed of an organic electrolyte, a separator impregnated with an organic electrolyte in which a lithium salt is dissolved in an organic solvent may be used. As the organic solvent, ethylene carbonate, propylene carbonate, γ-butyrolactone, diethyl carbonate, tetrahydrofuran, acetonitrile, a mixed solvent thereof and the like can be used. As the lithium salt, LiPF ₆ , LiClO ₄ , LiBF ₄ , lithium halide and the like can be used. As the separator, a porous body (for example, non-woven fabric) such as polyethylene or polypropylene can be used. The electrolyte layer can be a combination of a solid electrolyte and an organic electrolyte.

(Negative electrode)
In the nonaqueous electrolyte battery of the present invention, as the negative electrode active material constituting the negative electrode, Li metal alone, an element that forms an alloy with Li metal, or a mixture or alloy thereof can be used. In particular, the use of a Li-containing material, particularly Li metal, is advantageous in terms of increasing the battery capacity and voltage. Examples of elements that form an alloy with Li include Al, Si, C, Sn, and In. In addition, composite oxides of Li and Ti, oxides of Si, Sn, and V can be used. Moreover, Cu, Ni, Fe, Cr, and these alloys can be utilized suitably as a collector.

Example 1
The positive electrode sintered body of the present invention was manufactured, a lithium ion secondary battery including a positive electrode using the positive electrode sintered body was produced, and the battery performance was evaluated.

(Preparation of raw material powder)
Lithium hydroxide (LiOH) and cobalt acetate (Co (CH ₃ COO) ₂ ) were mixed in equimolar amounts, poured into distilled water, mixed and stirred, and then the mixture was dried to obtain a precursor powder. Next, this precursor powder was formed into pellets of φ20 mm × thickness 1 mm by applying a pressure of 50 MPa with a cold isostatic press, and then calcined at 900 ° C. for 5 hours. The calcined body was pulverized with a mortar to obtain a LiCoO ₂ powder. When the particle size distribution of the powder was measured using a laser scattering method, the volume distribution center particle size D50 was 4 μm. Further, this powder was pulverized using a jet mill device (manufactured by Nissin Engineering Co., Ltd.), and finally LiCoO ₂ powder (raw material powder) having a volume distribution center particle size D50 of 1 μm was obtained. The volume distribution center particle diameter D50 is a particle diameter corresponding to 50% of a volume-based cumulative distribution curve.

(Prototype of positive electrode sintered body)
Next, D50 = 1μm raw material powder is placed in a mold, pressed at a pressure of 50MPa, and then press-molded. Got.

  About this positive electrode sintered compact, the porosity and the particle size of the active material particle were calculated | required. The porosity was obtained by polishing the surface of the positive electrode sintered body, taking an SEM photograph of the surface, and calculating the void area measured by binarizing the image. Further, the particle diameter of the active material particles was obtained by extracting any 20 active material particles in the SEM image, measuring the longest major axis diameter for each, and averaging these. As a result, the porosity of the positive electrode sintered body and the particle diameter of the active material particles were 5% by volume and 30 μm, respectively.

  This positive electrode sintered body was polished so that the final size would be φ15 mm × thickness 0.1 mm.

  Prototype Example 2 was produced in the same manner as Prototype Example 1 except that the firing temperature was changed to 1050 ° C. and the firing time was changed to 12 hours. The porosity of the positive electrode sintered body and the particle size of the active material particles were 5% by volume and 120 μm, respectively.

  Prototype Example 3 was produced in the same manner as Prototype Example 1, except that the firing temperature was 950 ° C. and the firing time was changed to 2 hours. The porosity of the positive electrode sintered body and the particle diameter of the active material particles were 11% by volume and 8 μm, respectively.

(Battery prototype)
In these positive electrode sintered bodies of Experimental Examples 1 to 3, an Al metal thin film (average thickness of 0.1 μm) was formed on one surface by resistance heating vapor deposition to produce a positive electrode. And using these positive electrodes, the thin film type lithium ion secondary battery (battery AD) which laminated | stacked the positive electrode, the buffer layer, the electrolyte layer, and the negative electrode in order was produced. Table 1 shows the configuration of each battery. However, in any battery, the electrolyte layer is made of a solid electrolyte, and the battery A is not provided with a buffer layer.

The buffer layer was formed by depositing LiNbO ₃ on the positive electrode, specifically on the other surface of the positive electrode sintered body on which the metal thin film was not formed, using the excimer laser ablation method. Here, the average thickness of the buffer layer was 10 nm. The buffer layer was formed in an oxygen atmosphere with an evaporation source output of 500 mJ and a pressure of 1 Pa.

The electrolyte layer was formed on the buffer layer as follows according to the constituent material. When the electrolyte layer was composed of a Li-PON-based solid electrolyte, the film was formed using a high-frequency magnetron sputtering method in a nitrogen atmosphere with a nitrogen partial pressure of 1 Pa using LiPO ₃ as a target. When the electrolyte layer is composed of a Li-PS solid electrolyte, batteries B, C, and D use Li ₂ S and P ₂ S ₅ as targets and excimer laser ablation to form a Li ₂ SP ₂ S ₅ solid. An electrolyte was deposited. Here, the average thickness of the electrolyte layer was 5 μm.

The negative electrode was formed by depositing Li metal on the electrolyte layer using a resistance heating vapor deposition method under a vacuum of 10 ⁻⁴ Pa or less. Here, the average thickness of the negative electrode was 4 μm. This Li metal film can have both a function as a negative electrode active material layer and a function as a negative electrode current collector.

(Evaluation of battery performance)
About each battery AD mentioned above, after charging to 4.2V with the constant current of 0.05 mA / cm < ² > current density, the charging / discharging test discharged to 3V with the same constant current was implemented, and the first time discharge capacity was measured. The internal resistance of the battery was calculated from the voltage drop at the start of discharge. The results are shown in Table 2.

  As is clear from Table 2, the batteries A and B equipped with the positive electrode using the prototype 1 had a large discharge capacity and both had a low internal resistance. On the other hand, the batteries C and D provided with the positive electrode using the prototype examples 2 and 3 had a small discharge capacity and both had a large internal resistance.

  From the above results, the positive electrode sintered body of the present invention has a porosity of 15% by volume or less and the particle size of the active material particles is 10 μm or more and 100 μm or less, and is used for a positive electrode of a nonaqueous electrolyte battery. It can be seen that the internal resistance of the battery can be reduced and the discharge capacity can be improved.

Note that the present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention. For example, the porosity and the particle size of the active material particles may be appropriately changed, or a material other than LiCoO ₂ may be used as the positive electrode active material.

  The positive electrode sintered body and the non-aqueous electrolyte battery of the present invention can be suitably used for a power source of an electric vehicle as well as a mobile phone, a notebook computer, and a digital camera.

Claims (4)

A positive electrode sintered body used for a positive electrode of a nonaqueous electrolyte battery,
Containing particles of positive electrode active material,
A positive electrode sintered body having a porosity of 15% by volume or less and a particle diameter of active material particles of 10 μm or more and 100 μm or less. The positive electrode sintered body according to claim 1, wherein the positive electrode active material is at least one selected from LiαO ₂ and Liβ ₂ O ₄ .
However, α and β include at least one element selected from Co, Mn, Al, and Ni, and the total ratio of the elements in α and β is 50% or more in atomic ratio. A non-aqueous electrolyte battery comprising a positive electrode and a negative electrode, and an electrolyte layer interposed between the positive and negative electrodes,
The positive electrode includes the positive electrode sintered body according to claim 1 or 2,
The non-aqueous electrolyte battery, wherein the electrolyte layer includes a solid electrolyte. A non-aqueous electrolyte battery comprising a positive electrode and a negative electrode, and an electrolyte layer interposed between the positive and negative electrodes,
The positive electrode is
The positive electrode sintered body according to claim 1 or 2,
A nonaqueous electrolyte battery comprising a metal thin film formed on the surface of the positive electrode sintered body.