JP2015056326A - Solid electrolyte, and all-solid-state ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid electrolyte for an all-solid-state ion secondary battery, in which resistance to reduction is compatible with ion conductivity.SOLUTION: This all-solid-state ion secondary battery includes a positive electrode active material layer which contains a solid electrolyte having a ramsdellite type crystal structure expressed by LiSnO(0<x<1.33), particles containing the solid electrolyte, a positive electrode material, and an oxide softened and fluidized at lower temperatures in comparison with the solid electrolyte while having ion conductivity, and in which the particles and the positive electrode active material are bound by the oxide, a negative electrode active material layer which contains particles containing the solid electrolyte, a negative electrode active material, and an oxide softened and fluidized at lower temperatures in comparison with the solid electrolyte, and in which the particles and the negative electrode active material are bound by the oxide, and a solid electrolyte layer containing the solid electrolyte. The solid electrolyte layer is bonded between the positive electrode active material layer and the negative electrode layer.

Description

  The present invention relates to a solid electrolyte and an all solid-state ion secondary battery.

  All solid-state ion secondary batteries using non-flammable or flame-retardant inorganic solid electrolytes can be heat-resistant and safer, so the module cost can be reduced and the energy density can be increased. It is. In recent years, sulfide-based solid electrolytes with high ion conductivity have been developed, but toxic and corrosive gases are generated by reaction with water, and there are concerns about stability. On the other hand, although the oxide-based solid electrolyte is excellent in stability, no material has been developed that has both resistance to negative electrode potential and high ionic conductivity comparable to sulfide-based solid electrolytes.

Non-Patent Document 1 discloses a ramsdellite type Li _{2 + x} (Li _x Mg _1-x Sn ₃ ) O ₈ (0 ≦ x ≦ 0.5) ion-conductive substance.

J.GRINS and A.R.WEST: J. Solid State Chem., 65, 265 (1986)

  However, the ionic conductivity of the ionic conductive material of the above non-patent document is a maximum value at x = 0.3, and when x> 0.3, the ionic conductivity is lowered and there is room for further improvement. No solution is disclosed.

  An object of the present invention is to achieve both ring resistance and high ion conductivity in a solid electrolyte of an all solid-state ion secondary battery.

In order to achieve the above object, in the present invention, a compound having a ramsdellite type crystal structure represented by Li _4x Sn _4-x O ₈ (0 <x <1.33) is used as a solid electrolyte.

  According to the present invention, it is possible to realize a solid electrolyte that achieves both ring resistance and high ion conductivity, and an all solid-state ion secondary battery using the same.

1 is a structural diagram showing a crystal of a solid electrolyte. It is sectional drawing which shows the principal part of an all-solid-type ion secondary battery.

  Embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

The solid electrolyte of the present invention exhibits a ramsdellite structure represented by _Li 4x _Sn 4x _O 8 as shown in FIG. 1 (0 <x <1.33) , together SnO ₂ octahedra and edge-sharing They are combined to form a double chain, and adjacent double chains are connected at the apex to form a three-dimensional skeleton. This configuration has a one-dimensional tunnel structure that is sufficiently larger than the lithium ion size. A part of the Sn site in the SnO ₂ octahedron can be replaced with Li. In the range of 0 <x <1.33, x shows a ramsdellite structure and has high ionic conductivity.

By adding an oxide having ion conductivity that softens and flows at a lower temperature than the solid electrolyte to the solid electrolyte powder, a dense sintered body can be easily formed. Examples of oxides that soften and flow at low temperatures include lithium borate (Li ₃ BO ₃ ), lithium vanadate, NASICON-type Li _{1 + x} Al _x Ti _2-x (PO ₄ ) ₃ , Li _{1 + x} Ge _x Ti ₂ (PO _{4 3} ) Crystalline oxides typified by ₃ and amorphous oxides obtained by quenching the above compounds.

  FIG. 2 is a cross-sectional view showing a main part of the all solid state ion secondary battery.

  In this figure, the all solid state ion secondary battery includes a positive electrode active material layer 207, a solid electrolyte layer 208, and a negative electrode active material layer 209. The solid electrolyte layer 208 is sandwiched between the positive electrode active material layer 207 and the negative electrode active material layer 209. These are sandwiched between the positive electrode current collector 201 and the negative electrode current collector 206.

The solid electrolyte layer 208 includes solid electrolyte particles 204 (Li _4x Sn _4-x O ₈ (0 <x <1.33)). The positive electrode active material layer 207 includes positive electrode active material particles 202 and solid electrolyte particles 204. The negative electrode active material layer 209 includes negative electrode active material particles 205 and solid electrolyte particles 204. In the positive electrode active material layer 207, the solid electrolyte layer 208, and the negative electrode active material layer 209, the constituent particles are bound by lithium borate 203 (Li ₃ BO ₃ ) (fired at 700 ° C. × 1 h). That is, the active material particles and the solid electrolyte particles are dispersed in the lithium borate 203.

  In the solid electrolyte layer 208, the solid electrolyte particles 204 are bound by the lithium borate 203, but a sintered body of the solid electrolyte particles 204 may be used without using the lithium borate 203. Note that the positive electrode active material layer 207 and the negative electrode active material layer 209 are completely electrically insulated by the solid electrolyte layer 208.

  Moreover, you may add a conductive support agent in order to improve the electroconductivity in the active material layer of each electrode. Conductive aids include carbon materials such as graphite, acetylene black, ketjen black, metal powders such as gold, silver, copper, nickel, aluminum, titanium, indium / tin oxide (ITO), titanium oxide, tin oxide And conductive oxides such as zinc oxide and tungsten oxide are preferred.

The amount of Li ₃ BO ₃ added to the active material or solid electrolyte is preferably 5% by volume or more and 40% by volume or less (5 to 40% by volume) in terms of volume. When the volume is 5% by volume or more, the space between the active material particles and the solid electrolyte particles can be sufficiently filled. When the volume is 40% by volume or less, the charge / discharge capacity and the charge / discharge rate associated with the decrease in the amount of the active material and the solid electrolytic mass are reduced. Decline can be prevented.

  As the positive electrode active material, a known positive electrode active material capable of occluding and releasing lithium ions can be used. Examples thereof include spinel, olivine, layered oxide, solid solution system, silicate system, and vanadium crystallized glass.

As the negative electrode active material, a known negative electrode active material capable of occluding and releasing lithium ions can be used. For example, carbon materials typified by graphite, alloy materials such as TiSn alloy and TiSi alloy, nitrides such as LiCoN, oxides such as Li ₄ Ti ₅ O ₁₂ , LiTiO ₄ and vanadium crystallized glass may be used. it can. Further, lithium metal may be used.

  Hereinafter, the present invention will be specifically described with reference to examples.

<Preparation of lithium ion conductive solid electrolyte>
A lithium ion conductive solid electrolyte represented by Li _4x Sn _4-x O ₈ (0 <x <1.33) was produced.

Table 1 shows a sample in which the mixing ratio (molar ratio) of lithium carbonate (Li ₂ CO ₃ ) and tin oxide (SnO ₂ ) as raw materials was changed in order to produce solid electrolytes having different x values. is there.

  Sample No. shown in this table. 1-No. A mixed powder of 10 raw material compositions (molar ratio) was formed into a pellet by cold pressing, and the pellet formed into the mixed powder of the same composition was buried, and kept at 1300 ° C. for 5 minutes with a microwave heater Baking in the air under conditions. Although it can be synthesized by ordinary electric furnace heating, it takes a long time for firing, volatilization of lithium with a high vapor pressure, and composition deviation tends to occur. Synthesis is more preferred.

As a result of examining the X-ray diffraction pattern of the pulverized powder of the baked pellets, ramsdellite type single-phase crystals were obtained in the range of 0 <x <1.33 in Table 1. When x ≧ 1.33, a diffraction pattern belonging to Li ₂ SnO ₃ was obtained.

  Table 1 shows the lithium ion conductivity at room temperature measured by the AC impedance method of the baked pellets.

From the results shown in this table, it can be seen that the ionic conductivity tends to be maximized in the vicinity of the intermediate value of x value of 0 to 1.33. Specifically, the ionic conductivity is 2 × 10 ³ S / cm or more when the x value is in the range of 0.5 to 0.75. This is because when the x value decreases, that is, when the lithium ion concentration as a carrier decreases, the ionic conductivity decreases, conversely, when the x value increases, that is, when the lithium ion concentration increases, between lithium ions. It is considered that the influence of the electrostatic repulsive force on the surface increases and lithium ions do not easily diffuse, indicating that the ionic conductivity decreases.

  Furthermore, the reduction resistance was evaluated by measuring the potential at which the reduction current of the solid electrolyte was generated. The pulverized powder of the baked pellets, carbon black as a conductive auxiliary agent, and polyvinylidene fluoride as a binder were each prepared in a volume ratio of 70:10:20, and N-methyl-2-pyrodoline ( An appropriate amount of NMP) was added to prepare a paste. This paste is applied to an aluminum foil having a thickness of 20 μm, dried by heating in the atmosphere of 90 ° C. × 1 hr, pressed, punched into a disk shape having a diameter of 15 mm, and then subjected to heat treatment in vacuum of 120 ° C. × 1 hr. A solid electrolyte electrode was prepared.

The solid electrolyte electrode and the counter electrode Li plate are stacked via a 30 μm-thick separator impregnated with an electrolytic solution, and sandwiched between two SUS jigs, and the Li metal potential is applied to the solid electrolyte electrode. In contrast, a potential of 5 V to 1 V was applied. As a result, no reduction current was observed in this potential scanning range. Therefore, it was found that at least the reduction potential was less than 1 V, and the reduction resistance was excellent. The electrolyte used was 1 mol / l of lithium hexafluorophosphate (LiPF ₆ ) dissolved in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 2. A thing was used.

  The lithium ion conductive solid electrolyte produced in Example 1 was applied to produce an all-solid battery by the following steps, and its charge / discharge characteristics were evaluated.

<Preparation of Li ₃ BO ₃ >
Li ₂ CO ₃ and B ₂ O ₃ were mixed at a weight ratio of 3: 1, mixed for 2 hours by a planetary ball mill using zirconia balls, and then heated and held in the atmosphere at 600 ° C. for 12 hours to obtain crystals. Li ₃ BO ₃ was obtained. An endothermic peak due to melting was confirmed at 700 ° C. by differential thermal analysis (DTA).

<Positive electrode>
LiCoO ₂ powder with an average particle diameter of 5 μm as a positive electrode active material, Li ₃ BO ₃ powder, and Li _0.2 Sn with an average particle diameter of 3 μm as a solid electrolyte prepared in Example 1 (No. 6 in Table 1). _3.8 O ₈ powder (hereinafter referred to as LSO), and conductive titanium oxide (rutile-type titanium oxide) in the form of needles (short axis: 0.13 μm, long axis: 1.68 μm), which is a conductive aid. The base material is coated with a SnO ₂ conductive layer doped with Sb) and the volume ratio is 53: 30: 10: 7, and an appropriate amount of a resin binder and a solvent are added to the mixed powder. Thus, a positive electrode paste was prepared. In addition, ethyl cellulose or nitrocellulose was used as the resin binder, and butyl carbitol acetate was used as the solvent. The positive electrode paste was applied to a stainless steel foil having a thickness of 20 μm, and after heat treatment for removing the solvent and removing the binder, the positive electrode paste was baked at 700 ° C. × 1 hr in vacuum to obtain a positive electrode sheet having a positive electrode active material layer thickness of 10 μm. This was punched out into a disk shape having a diameter of 14 mm to obtain a positive electrode.

<Negative electrode>
Li ₄ Ti ₅ O ₁₂ powder having an average particle diameter of 5 μm as a negative electrode active material, Li ₃ BO ₃ powder, LSO having an average particle diameter of 3 μm as a solid electrolyte, and acicular (short axis: 0) as a conductive aid .13 μm, long axis: 1.68 μm) conductive titanium oxide (a rutile-type titanium oxide base material coated with SnO ₂ conductive layer doped with Sb) in a volume ratio of 53: 30: 10: 7 An appropriate amount of a resin binder and a solvent was added to the mixed powder to prepare a negative electrode paste. This negative electrode paste was applied to a stainless steel foil having a thickness of 20 μm, and after heat treatment for removing the solvent and removing the binder, the negative electrode paste was fired in vacuum at 700 ° C. × 1 hr to obtain a negative electrode sheet having a negative electrode active material layer thickness of 10 μm. This was punched out into a disk shape having a diameter of 14 mm to obtain a negative electrode.

  Here, a negative electrode active material layer in which negative electrode active material particles and solid electrolyte particles are bound with glass is used. However, the present invention is not limited to this, and a lithium metal plate can also be used. The same applies to the following embodiments.

<Solid electrolyte layer>
LSO with an average particle diameter of 3 μm, which is a solid electrolyte, and Li ₃ BO ₃ were prepared so that the volume ratio would be 70:30, and an appropriate amount of a resin binder and a solvent were added to the mixed powder to obtain a solid electrolyte paste. Was made. After applying this solid electrolyte paste to either the positive electrode layer or the negative electrode layer, heat treatment for removing the solvent and removing the binder is performed, followed by baking in a vacuum at 700 ° C. × 1 hr to obtain a solid electrolyte layer having a thickness of 15 μm. Formed. This was punched into a disk shape having a diameter of 15 mm.

  Here, a solid electrolyte layer in which solid electrolyte particles are bound with glass is used as a solid electrolyte layer. However, the present invention is not limited to this, and a sintered plate-shaped solid electrolyte bulk can also be used. The same applies to the following embodiments.

<Production of battery>
In order to improve the adhesion at the interface of the positive electrode active material layer / solid electrolyte layer / negative electrode active material layer by laminating the electrode layer on which the solid electrolyte layer is formed and the other electrode layer, this laminate is added. While pressing, baking was performed in a vacuum at 700 ° C. for 15 minutes to sufficiently adhere the interfaces of the layers. The side surface of the obtained laminate was masked with an insulator, and this was incorporated into a CR2025 type coin battery to produce an all-solid battery.

  Instead of the method of forming each layer by applying and baking the mixed powder paste described above, the mixed powder does not melt or gasify, but collides with the base material in a solid state in supersonic flow with an inert gas. Using a spray spray (CS) method to form a film, or an aerosol deposition (AD) that forms a film by spraying an aerosol obtained by mixing a powder mixture with a gas through a nozzle onto a substrate. ) Law can also be applied.

  A battery manufacturing method by the CS method will be described below.

A mixed powder of the same LiCoO ₂ powder, Li ₃ BO ₃ powder, LSO powder, and conductive titanium oxide is sprayed onto a stainless steel foil having a thickness of 20 μm to form a positive electrode active material layer having a thickness of 10 μm. I let you. Each powder may be put into a separate feeder and sprayed at the same time.

A mixed powder of the same LSO powder and Li ₃ BO ₃ powder was sprayed onto the positive electrode active material layer to form a solid electrolyte layer having a thickness of 15 μm.

Next, a mixed powder of the same Li ₄ Ti ₅ O ₁₂ powder, Li ₃ BO ₃ powder, LSO powder, and conductive titanium oxide is sprayed onto the solid electrolyte layer, and a negative electrode active material having a thickness of 10 μm is injected. A material layer was formed.

  Furthermore, a stainless steel powder was sprayed on the negative electrode electrolyte layer to form a negative electrode current collector layer having a thickness of 20 μm.

<Battery characteristics evaluation>
As a result of measuring the discharge capacity at a rate of 0.1 C and 1 C for the battery manufactured in Example 2, the initial discharge capacities at 0.1 C and 1.0 C were 140 mAh / g and 110 mAh per weight of the positive electrode active material, respectively. / G. In addition, it has been confirmed that charging and discharging are performed in the same manner even in an all-solid battery to which an ion conductive solid electrolyte other than lithium is applied.

  201: Positive electrode current collector, 202: Positive electrode active material particles, 203: Lithium borate, 204: Solid electrolyte particles, 205: Negative electrode active material particles, 206: Negative electrode current collector, 207: Positive electrode active material layer, 208: Solid Electrolyte layer, 209: negative electrode active material layer.

Claims (10)

A solid electrolyte having a ramsdellite type crystal structure represented by Li _4x Sn _4-x O ₈ (0 <x <1.33).   A particle comprising the solid electrolyte according to claim 1 and an oxide having ion conductivity that softens and flows at a lower temperature than the solid electrolyte, wherein the particle is bound with the oxide. A solid electrolyte layer.   A particle including the solid electrolyte according to claim 1, a positive electrode active material, and an oxide having ion conductivity that softens and flows at a lower temperature than the solid electrolyte, and the particle and the positive electrode active material are the oxide. A positive electrode active material layer characterized by being bound by:   A particle comprising the solid electrolyte according to claim 1, a negative electrode active material, and an oxide having ion conductivity that softens and flows at a lower temperature than the solid electrolyte, wherein the particle and the negative electrode active material are the oxide. A negative electrode active material layer characterized by being bound by   A solid electrolyte layer containing the solid electrolyte according to claim 1, a positive electrode active material layer, and a negative electrode active material layer, wherein the solid electrolyte layer is between the positive electrode active material layer and the negative electrode active material layer. An all-solid-type ion secondary battery having a bonded configuration.   A configuration comprising the solid electrolyte layer according to claim 2, a positive electrode active material layer, and a negative electrode active material layer, wherein the solid electrolyte layer is bonded between the positive electrode active material layer and the negative electrode active material layer. An all-solid-state ion secondary battery comprising:   The all-solid-state ion secondary battery according to claim 5, wherein the positive electrode active material layer is the positive electrode active material layer according to claim 3.   The all-solid-state ion secondary battery according to claim 5, wherein the negative electrode active material layer is the negative electrode active material layer according to claim 4.   A method for producing a solid electrolyte according to claim 1, wherein raw materials of lithium and tin are mixed, molded, and fired by microwave heating.   The method for producing a solid electrolyte according to claim 9, wherein the raw materials are lithium carbonate and tin oxide.

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