JP5218745B2 - Battery structure and lithium battery using the same - Google Patents

Description

  The present invention relates to a battery structure including a solid electrolyte layer and a lithium battery using the same.

  Lithium ion secondary batteries are used as power sources for relatively small electric devices such as portable devices. The lithium battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer that mediates conduction of lithium ions between these layers.

  As this lithium battery, an all-solid-state lithium battery that does not use an organic electrolyte for lithium conduction between a positive electrode and a negative electrode has been proposed. All-solid-state lithium batteries use a solid electrolyte layer as the electrolyte layer. Organic electrolytes such as safety problems due to electrolyte leakage and heat resistance problems due to volatilization of the electrolyte beyond its boiling point at high temperatures Problems caused by using the liquid can be solved.

  On the other hand, an all-solid-state lithium battery using a solid electrolyte layer has a problem that its capacity is low, that is, output characteristics are poor, as compared with a lithium battery using an organic electrolyte. Therefore, Patent Document 1 discloses a lithium sulfide-based solid electrolyte excellent in ion conductivity.

However, since the sulfide-based solid electrolyte disclosed in Patent Document 1 is easily reduced, there is a problem that when it comes into contact with metallic lithium, a decomposition reaction occurs and the resistance value increases. For this reason, when metallic lithium is used for the negative electrode, the cycle characteristics deteriorate, and when metallic lithium is not used for the negative electrode, the negative electrode potential is higher than that of lithium, so that there is a problem that the battery voltage becomes small.
Japanese Patent Publication No. 06-054687

  An object of the present invention is to provide a battery structure for obtaining a solid electrolyte battery having a high capacity and excellent charge / discharge characteristics (high current density and stable charge / discharge cycle characteristics even at low temperatures). .

In order to solve the above problems, the present invention provides a battery structure including a configuration in which a positive electrode layer, a buffer layer, a solid electrolyte, and a negative electrode layer are sequentially laminated, wherein X is phosphorus (P) and boron (B). At least one element, Y is oxygen (O), at least one element of nitrogen (N), the sum of a, b, c and d is 1, a is 0.20 to 0.52, and b is When the solid electrolyte is a numerical value in the range of 0.10 to 0.20, c is 0.30 to 0.55, and d is 0 to 0.30, the chemical composition excluding inevitable impurities has the formula aLi · bX · cS · dY, and a portion A in contact with the negative electrode layer and a portion B in contact with the buffer layer, wherein a of the portion A is larger than a of the portion B, and the negative electrode layer is made of metallic lithium ( Ri alloy der containing li) or lithium, the buffer layer and the solid electrolyte and the positive electrode layer In the surface, the bias of charge is buffered, and the buffer layer is a composite oxide containing Li-Nb, a composite oxide containing Li-Ti, or a composite oxide containing Li-Ta. to provide a battery structure, characterized in that there.

The a is preferably in the range of 0.4 to 0.52 in the portion A. The buffer layer is more preferably LiNbO ₃ having an amorphous structure.

  Furthermore, the present invention includes a lithium battery using any of the battery structures described above.

  According to the present invention, a battery structure for obtaining a solid electrolyte battery having high capacity and excellent charge / discharge characteristics (high current density and stable charge / discharge cycle characteristics even at low temperatures) and a lithium battery using the same are provided. can do.

  The battery structure according to the present invention includes a positive electrode current collector layer, a positive electrode layer, a buffer layer, a solid electrolyte layer, a negative electrode layer, and a negative electrode current collector layer. The lithium battery of the present invention uses the battery structure, and a positive electrode layer, a buffer layer, a solid electrolyte layer, a negative electrode layer, and a negative electrode current collector layer are sequentially laminated on the positive electrode current collector layer. It has a configuration.

  In the lithium battery of the present invention, the positive electrode current collector layer, the positive electrode layer, the buffer layer, the solid electrolyte layer, the negative electrode layer, and the negative electrode current collector layer may be arranged stepwise on the insulating substrate. . Further, the lithium battery of the present invention comprises a positive electrode current collector layer, a positive electrode layer, a buffer layer, a solid electrolyte layer, a negative electrode layer, and a negative electrode current collector layer on an insulating and heat resistant substrate, and the battery is thick. The positive electrode layer and the negative electrode layer may be disposed at a position where they do not overlap when viewed from the side, and the solid electrolyte layer may be disposed so as to cover the whole of both the bipolar layers.

  The solid electrolyte layer in the present invention is composed of a composition having a chemical composition excluding inevitable impurities represented by the formula aLi · bX · cS · dY. Here, Li is a lithium metal, X is at least one element of phosphorus (P) or boron (B), S is sulfur, and Y is at least one element of oxygen (O) or nitrogen (N). a, b, c and d are the content ratios of these elements, the sum of which is 1 (a + b + c + d = 1), and the content ratios of the individual component elements when the total number of elements in the composition is 1. . Note that a is a value in the range of 0.20 to 0.52, b is 0.10 to 0.20, c is 0.30 to 0.55, and d is a value in the range of 0 to 0.30. The solid electrolyte layer of the present invention composed of the above components is composed of at least two layers of a portion A in contact with the negative electrode layer and a portion B in contact with the buffer layer, and a of the portion A is larger than a of the portion B. It is desirable that a is in the range of 0.4 to 0.52 in the part A and 0.2 to 0.4 in the part B. Note that a may be a gradient composition that gradually increases from part B to the point where it contacts the negative electrode layer of part A.

The basic composition of the solid electrolyte is a Li—PS system, Li—B—S system, or a solution in which these two component systems are dissolved. By setting the content ratios a, b, c, and d of the component elements within the above ranges, a battery structure having a lithium ion conductivity of 1 × 10 ⁻⁴ S / cm or more and a lithium ion ring ratio of 0.9999 or more is obtained. Can be obtained. In particular, when a is in the range of 0.2 to 0.4, a battery structure having a lithium ion conductivity of 1 × 10 ⁻³ S / cm or more and a lithium ion ring rate of 0.9999 or more can be obtained.

  When the battery structure of the present invention is configured as described above, the lithium ion conductivity of the entire solid electrolyte layer is lowered. However, by making a of part A larger than a of part B, part A becomes electrochemically stable against the active lithium-based metal of the negative electrode layer in contact therewith, and part B is also buffered. Since the layer is electrochemically stable, generation of a high resistance layer due to a decomposition reaction between the lithium metal of the negative electrode layer and the solid electrolyte layer due to repeated charge / discharge cycles can be suppressed. As a result, a decrease in charge / discharge capacity can be suppressed.

  Since the portion A has an ionic conductivity smaller than that of the portion B, the thickness of the portion A is preferably thin as long as the portion B does not contact the positive electrode. The total thickness of the solid electrolyte layer including the part A and the part B is preferably thin so that the positive electrode layer and the negative electrode layer are not short-circuited. Specifically, the thickness of the entire solid electrolyte layer is preferably 0.1 to 10 μm, and more preferably 0.5 to 5 μm. Moreover, 0.01-2 micrometers is preferable and, as for the thickness of the part A, 0.1-1 micrometer is more preferable.

  When a is less than 0.2 and more than 0.52, the internal resistance of the battery is increased due to low ionic conductivity, and the output is significantly reduced. As described above, the range of 0.4 to 0.52 in the portion A and the range of 0.2 to 0.4 in the portion B are desirable.

  As a method for forming the solid electrolyte layer, a solid phase method or a vapor deposition method can be used. Examples of the solid phase method include forming a raw material powder using a mechanical milling method and sintering the raw material powder. Examples of the vapor deposition method include a PVD method and a CVD method. Specifically, examples of the PVD method include a vacuum evaporation method, a sputtering method, an ion plating method, and a laser ablation method, and examples of the CVD method include a thermal CVD method and a plasma CVD method. When the solid electrolyte layer is formed by the vapor deposition method, the thickness of the solid electrolyte layer can be made thinner than when the solid electrolyte layer is formed by the solid phase method.

  Hereinafter, each layer other than the battery structure of the present invention and the solid electrolyte layer of the lithium battery will be described.

(Positive electrode current collector layer)
The positive electrode current collector layer is a thin metal plate having a predetermined thickness, and also serves as a substrate that supports each layer. As the positive electrode current collector layer, one selected from aluminum (Al), nickel (Ni), alloys thereof, and stainless steel can be suitably used. The positive electrode current collector layer can be formed on the substrate as a metal film by a PVD method (physical vapor deposition method) or a CVD method (chemical vapor deposition method). In particular, when a metal film (current collector) having a predetermined pattern is formed, the current collector having a predetermined pattern can be easily formed by using an appropriate mask. In addition, the positive electrode current collector layer may be formed by pressure bonding a metal foil to an insulating substrate.

(Positive electrode layer)
The positive electrode layer is a layer containing an active material that occludes and releases lithium ions. In particular, oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), olivine-type lithium phosphate (LiFePO ₄ ), or LiNi _0.5 Mn _{0 0.5} O ₂ or a mixture thereof can be preferably used. The positive electrode layer containing these compounds can improve lithium ion conductivity by defining its crystal structure. For example, when a compound having a layered rock salt structure (for example, LiCoO ₂ , LiNiO ₂ , LiNi _0.5 Mn _0.5 O ₂ ) is employed as the active material of the positive electrode layer, the ratio of the surface index of the positive electrode layer is ( 003) / (101) <10.

  Moreover, it is preferable that the compound contained in the buffer layer mentioned later is diffusing in the positive electrode layer. For example, when the concentration of the compound at a predetermined thickness is measured from the interface with the buffer layer in the positive electrode layer, the degree of diffusion of the compound from the buffer layer to the positive electrode layer can be specified.

  The positive electrode layer may further contain a conductive additive. As the conductive assistant, for example, carbon black such as acetylene black, natural graphite, thermally expanded graphite, carbon fiber, ruthenium oxide, titanium oxide, aluminum, nickel, or other metal fibers can be used. In particular, carbon black is preferable because high conductivity can be secured in a small amount.

  As a method for forming the positive electrode layer, a vapor deposition method such as a PVD method or a CVD method can be used. For example, vapor deposition, ion plating, sputtering, laser ablation, etc. can be used.

  An intermediate layer may be provided between the positive electrode current collector layer and the positive electrode layer for the purpose of improving adhesion. The intermediate layer is preferably an oxide or sulfide of an element contained in the current collector metal. Alternatively, the positive electrode current collector layer and the positive electrode layer are formed on the positive electrode current collector layer by forming the positive electrode layer to a thickness of about 1 μm by sputtering and then forming the positive electrode layer to a predetermined thickness by vapor deposition. The adhesion of the layer can also be improved.

(Buffer layer)
The buffer layer prevents a large amount of lithium ions from moving from the solid electrolyte layer to the positive electrode layer, buffers the charge bias at the interface between the solid electrolyte layer and the positive electrode layer, and depletes the solid electrolyte layer near this interface. It is a layer which prevents that a layer arises. The buffer layer is preferably made of an oxide, and is preferably any one of a composite oxide containing Li—Nb, a composite oxide containing Li—Ti, or a composite oxide containing Li—Ta. . Specifically, Li _x La _{(2-x) / 3} TiO ₃ (x = 0.1 to 0.5), Li ₄ Ti ₅ O ₁₂ , Li _3.6 Si _0.6 P _0.4 O ₄ Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.8 Cr _0.8 Ti _1.2 (PO ₄ ) ₃ , LiNbO ₃ , LiTaO ₃ , or Li _1.4 In _{0 .4} Ti _1.6 (PO ₄ ) ₃ or the like is preferably used. Among these compounds, when LiNbO ₃ is in an amorphous state, the lithium ion conductivity is improved. The lithium ion conductivity of amorphous LiNbO ₃ has an excellent lithium ion conductivity of 10 ⁻⁵ S / cm or more. As an index indicating that LiNbO ₃ is in an amorphous state, in X-ray diffraction, there is no peak in the range of 2θ of 22 to 25 ° and a half width of 5 ° or less. If the temperature of the above compound takes a crystal structure when the buffer layer is formed, the compound constituting the buffer layer may be excessively diffused into the positive electrode layer, and the buffer layer may become brittle.

It is preferable that a part of the compound constituting the buffer layer in contact with the positive electrode layer is diffused in the positive electrode layer. By adjusting the degree of diffusion of the compound into the positive electrode layer, formation of a depletion layer can be suppressed, and adhesion between the positive electrode layer and the buffer layer can be improved. For example, when the buffer layer contains LiNbO ₃ , the concentration of Nb diffused from the buffer layer at a point of 25 nm in thickness from the interface with the buffer layer in the positive electrode layer is 1 × 10 ⁻³ atom% or more and 25 atom%. The following may be mentioned. The Nb concentration can be measured by, for example, SIMS (secondary ion mass spectrometry). When the positive electrode layer is LiCoO ₂ , the Nb concentration is the atomic weight ratio at the measurement point, that is, Nb / (Nb + O + Li + Co).

  The thickness of the buffer layer is preferably 1 μm or less. If the buffer layer is too thick, it will hinder the thinning of the lithium battery. In order to suppress the formation of the depletion layer, a thickness of 2 nm or more is sufficient. In order to suppress the formation of the depletion layer more reliably, the thickness of the buffer layer may be 5 nm or more.

Further, the electron conductivity of the buffer layer is preferably 10 ⁻⁵ S / cm or less. By setting the electron conductivity in this way, polarization in the buffer layer can be suppressed, and thus formation of a depletion layer can be suppressed. In addition, if said compound is employ | adopted, it can be set as the buffer layer which satisfy | fills said electronic conductivity substantially.

  The buffer layer can be formed by a vapor deposition method such as a PVD method or a CVD method.

(Negative electrode layer)
The negative electrode layer is composed of a layer containing an active material that occludes and releases lithium ions. The negative electrode layer is either lithium metal or an alloy containing lithium metal. Examples of the metal capable of forming an alloy with lithium metal include aluminum (Al), silicon (Si), tin (Sn), bismuth (Bi), and indium (In).

  A negative electrode layer containing such an element is preferable because the negative electrode layer itself can have a function as a current collector and has a high ability to occlude and release lithium ions. In particular, Si has a higher ability to occlude and release lithium than graphite, and can increase the energy density of the battery.

  Moreover, by using an alloy phase with lithium metal as the negative electrode layer, there is an effect of reducing the migration resistance of lithium ions at the interface between the metal alloyed with lithium metal and the lithium ion conductive solid electrolyte layer, The increase in resistance of the alloyed metal in the initial charge of the first cycle is alleviated.

  Furthermore, in the case where the alloyed metal simple substance is used as the negative electrode layer, there is a problem that the discharge capacity becomes significantly smaller than the charge capacity in the charge / discharge cycle of the first cycle. By using a negative electrode material alloyed with metal, this irreversible capacity is almost eliminated. As a result, it is not necessary to fill the positive electrode active material by an irreversible capacity, and the capacity density of the lithium battery can be improved.

  The above-described negative electrode layer forming method is preferably a vapor deposition method. In addition, the negative electrode layer may be formed by superimposing a metal foil on the solid electrolyte layer and adhering the metal foil onto the solid electrolyte layer by pressing or an electrochemical technique.

(Negative electrode current collector layer)
As the negative electrode current collector layer, one selected from copper (Cu), nickel (Ni), iron (Fe), chromium (Cr), and alloys thereof can be preferably used. Note that the negative electrode current collector layer can also be formed by a PVD method or a CVD method, similarly to the positive electrode current collector layer.

A thin plate made of SUS316 having a thickness of 50 μm was prepared as a positive electrode current collector. This thin plate also serves as a substrate for supporting each layer. LiCoO ₂ was vapor-deposited at a substrate temperature of 480 ° C. on the positive electrode current collector by an electron beam evaporation method to form a positive electrode layer. The thickness of the positive electrode layer was 1 μm. A buffer layer was formed by depositing LiNbO ₃ on the positive electrode layer by excimer laser ablation. The thickness of the buffer layer was 10 nm.

A solid electrolyte layer consisting essentially of part A and part B was formed on the buffer layer by the following method by excimer laser ablation. As a raw material, lithium sulfide (Li ₂ S) phosphorus pentasulfide (P ₂ S ₅ ) powder was used.

In the part A of the solid electrolyte layer, the molar ratio of Li ₂ S / P ₂ S ₅ was 4/1, and the mixture was molded into a pellet target material. In the part B, the target material was similarly obtained with the molar ratio being 2/1. The target material was evaporated by irradiating the target material with the excimer laser, and a solid charge layer was formed on the buffer layer. The total thickness of the solid electrolyte layer was 5 μm.

  When forming the solid electrolyte layer, a dummy substrate was placed at the same time, and the same solid electrolyte layer was also formed on the dummy substrate. When the film composition and thickness of the solid electrolyte layer formed on the dummy substrate were examined by ion-coupled plasma spectroscopy, in part A, a = 0.42, b = 0.11, c = 0.47, d = In part B, a = 0.31, b = 0.15, c = 0.54, and d = 0. The thickness of the portion A was 1 μm, and the thickness of the portion B was 4 μm.

  On the solid charge layer, a negative electrode layer was formed by vapor-depositing Li by a resistance heating vapor deposition method. The thickness of the negative electrode layer was 5 μm. A negative electrode current collector layer was formed by laminating a thin plate of SUS316 having a thickness of 50 μm on the negative electrode layer. Finally, the outer periphery of these laminates was covered with an exterior material to produce a lithium battery.

When this lithium battery was subjected to a cycle charge / discharge test at a constant current density of 0.1 mA / cm ² at a charge end voltage of 4.2 V and a discharge end voltage of 3.0 V, the discharge capacity retention rate after 500 cycles was 80. %Met.

(Comparative example)
A lithium battery was fabricated in the same manner as in Example 1 except that the solid electrolyte layer was formed using only the raw material target of the solid electrolyte layer having a molar ratio of 2/1. The thickness of the solid electrolyte layer was 5 μm, and a = 0.31, b = 0.15, c = 0.54, and d = 0. When the same cycle charge / discharge test as that of Example 1 was performed, the capacity retention rate after 500 cycles was 40%. It was found that the cycle characteristics were inferior if a in part A and a in part B were the same.

  As described above, according to the present invention, it is possible to provide a battery structure for obtaining a solid electrolyte battery having a high capacity and excellent charge / discharge characteristics, and a lithium battery using the battery structure.

Claims (4)

A battery structure including a configuration in which a positive electrode layer, a buffer layer, a solid electrolyte, and a negative electrode layer are sequentially stacked, wherein X is at least one element of phosphorus (P) and boron (B), S is sulfur, and Y is The total of at least one element of oxygen (O) and nitrogen (N), a, b, c and d is 1, a is 0.20 to 0.52, b is 0.10 to 0.20, When c is a numerical value in the range of 0.30 to 0.55 and d is in the range of 0 to 0.30, the solid electrolyte has a chemical composition excluding inevitable impurities expressed by the formula aLi · bX · cS · dY. A part A in contact with the negative electrode layer and a part B in contact with the buffer layer, wherein a of the part A is larger than a of the part B, and the negative electrode layer is metallic lithium (Li) or an alloy containing lithium And
The buffer layer buffers a charge bias at the interface between the solid electrolyte and the positive electrode layer, and the buffer layer includes a composite oxide containing Li-Nb, a composite oxide containing Li-Ti, Or the battery structure characterized by being either the complex oxide containing Li-Ta.   2. The battery structure according to claim 1, wherein a is in a range of 0.4 to 0.52 in the portion A. 3. The battery structure according to claim 1, wherein the buffer layer is LiNbO ₃ having an amorphous structure.   A lithium battery using the battery structure according to any one of claims 1 to 3.