JP5187502B2 - Lithium battery - Google Patents

Description

  The present invention relates to a lithium battery including a solid electrolyte layer.

  Lithium ion secondary batteries (hereinafter simply referred to as lithium 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.

  In recent years, an all solid-state lithium battery that does not use an organic electrolyte for lithium conduction between the positive and negative electrodes has been proposed as this lithium battery. All solid-state lithium batteries use a solid electrolyte layer as the electrolyte layer, and problems associated with the use of organic solvent-based electrolytes, such as safety problems due to electrolyte leakage, The heat resistance problem caused by volatilization exceeding the boiling point can be solved. For this solid electrolyte layer, a sulfide-based substance having high lithium ion conductivity and excellent insulating properties is widely used.

  While having the advantages described above, the all-solid-state lithium battery using the solid electrolyte layer has a problem that the capacity is low (that is, the output characteristics are poor) as compared with the lithium battery using the organic electrolyte. It was. The cause of such a problem is that lithium ions are more likely to be attracted to oxide ions in the positive electrode layer than sulfide ions in the solid electrolyte layer, so the lithium ion deficiency is present in the positive electrode layer side region of the sulfide solid electrolyte. This is because a layer (depletion layer) is formed (see Non-Patent Document 1 and Non-Patent Document 2). This depletion layer is deficient in lithium ions and thus has a high electrical resistance value, which reduces the capacity of the battery.

As a technique for solving such problems, in Non-Patent Documents 1 and 2, the surface of the positive electrode active material (LiCoO ₂ ) is coated with a lithium ion conductive oxide. This coating restricts the movement of lithium ions and suppresses the formation of a depletion layer in the sulfide solid electrolyte layer. As a result, the output characteristics of the lithium battery are improved.

Advanced Materials 2006.18,2226-2229 Proceedings of the 47th Battery Symposium p542-543 Published on November 20, 2006

  However, the lithium batteries described in Non-Patent Documents 1 and 2 have a low productivity, which is disadvantageous for increasing demand for lithium batteries accompanying the recent development of portable devices. Specifically, in these documents, a coating is formed on the surface of an active material by an electrostatic spraying method. However, coating by this electrostatic spraying method is technically difficult and complicated. In other words, the lithium battery described in this document has a high production cost and poor production efficiency, so it is difficult to meet the demand for increasing demand for lithium batteries.

  The present invention has been made in view of the above circumstances, and one of its purposes is to provide a lithium battery having high capacity and excellent productivity while using a solid electrolyte.

  Conventionally, the positive electrode active material has been composed of various compounds, but in recent years, the positive electrode active material is composed of lithium cobalt oxide that can increase the operating voltage and increase the capacity when a lithium battery is used. It is common. Therefore, even in Non-Patent Documents 1 and 2, lithium cobaltate is used as a positive electrode active material in a lithium battery including a sulfide solid electrolyte layer, and other compounds have not been studied. This inventor examined again the combination of the compound which comprises each layer in a lithium battery provided with a sulfide solid electrolyte layer. As a result, it became clear that when the positive electrode active material is lithium nickelate, the performance of the lithium battery is improved with respect to the sulfide solid electrolyte. Based on this finding, the present invention is defined.

  The lithium battery of the present invention comprises a positive electrode layer, a negative electrode layer, and a sulfide solid electrolyte layer that mediates lithium ion conduction between the two layers. Crystalline nickel is used as a positive electrode active material contained in the positive electrode layer. It is characterized by using lithium acid.

  Since the formation of a depletion layer in the sulfide solid electrolyte layer can be reduced by employing the configuration of the present invention, a lithium battery having a capacity comparable to a conventional lithium battery using an organic electrolyte can be obtained. . In addition, by using lithium nickelate for the positive electrode layer, there is no need to apply a coating for suppressing the formation of a depletion layer on the surface of the active material particles as in the conventional battery using lithium cobaltate, and the productivity is high. Lithium batteries can be manufactured. This is because lithium nickelate is a cause of formation of a depletion layer and lithium ions are less likely to be attracted to the oxide of the positive electrode layer than sulfide of solid electrolyte than lithium cobaltate. Inferred.

As one mode of the present invention, a sulfide solid electrolyte layer preferably contains Li ₂ SP ₂ S _5. The solid electrolyte in the present invention is not particularly limited as long as it is sulfide-based, and for example, it may be LPSO-based other than Li ₂ SP ₂ S ₅ , and in particular, Li ₂ SP ₂ S ₅ If so, the interface resistance value between the solid electrolyte layer and the negative electrode layer can be reduced, and as a result, the performance of the battery can be improved.

  Moreover, as one form of this invention, it is preferable that a positive electrode layer contains at least 1 type or more of 0.5 mass%-5 mass% aluminum and 5 mass%-20 mass% cobalt. According to this configuration, the electrical resistance of the positive electrode layer can be reduced and the capacity of the battery can be improved, so that the performance of the entire battery can be improved. Aluminum and cobalt contained in the positive electrode active material constitute a compound with lithium nickelate.

  Furthermore, as one form of this invention, it is preferable that the average particle diameter of a positive electrode active material is 3-10 micrometers. Since the positive electrode active material generally has a low electronic conductivity, the internal resistance of the entire positive electrode layer tends to be high. When the average particle diameter of the positive electrode active material is within the above range, the positive electrode layer can be formed with a small gap between the active material particles, and the contact area between the active material particles is increased, so that the electron conductivity between the particles is increased. Thus, the internal resistance of the positive electrode layer can be lowered.

  Although the lithium battery of the present invention includes a sulfide solid electrolyte layer, the lithium battery can suppress the formation of a depletion layer in the solid electrolyte layer, and has a capacity comparable to that of a conventional battery using an organic electrolyte. it can. In addition, the lithium battery of the present invention can be manufactured without complicated operations such as coating the surface of the positive electrode active material in order to suppress the formation of a depletion layer.

  Hereinafter, preferred embodiments of the present invention will be described in detail.

≪Overall structure≫
FIG. 1 is a longitudinal sectional view of a lithium battery according to the present embodiment. This lithium battery 1 has a configuration in which a positive electrode layer 13, a solid electrolyte layer (SE layer) 15, a negative electrode layer 14, and a negative electrode current collector layer 12 are laminated on a positive electrode current collector layer 11 in this order. .

≪Each component≫
(Positive electrode current collector layer)
The positive electrode current collector layer 11 is a thin metal plate having a predetermined thickness, and also serves as a substrate for supporting each layer described later. As the positive electrode current collector layer 11, one selected from aluminum (Al), nickel (Ni), alloys thereof, and stainless steel can be suitably used.

(Positive electrode layer)
The positive electrode layer 13 is a layer containing an active material that occludes and releases lithium ions. In the lithium battery of the present invention, lithium nickelate (LiNiO ₂ ) is used as the positive electrode active material. Normally, there is no significant difference in battery capacity regardless of whether the positive electrode active material is lithium nickelate or lithium cobaltate (LiCoO ₂ ). However, when the SE layer is composed of a sulfide system as described later, lithium nickelate is used. The capacity is extremely large when used.

  The lithium nickelate that is the positive electrode active material of the positive electrode layer 13 preferably contains at least one of aluminum and cobalt. The content of aluminum is preferably 0.5% by mass to 5% by mass. The cobalt content is preferably 5% by mass to 20% by mass.

  The positive electrode layer 13 may further contain a conductive additive. As a conductive support agent, what consists of metal fibers, such as carbon black, such as acetylene black, natural graphite, thermal expansion graphite, carbon fiber, ruthenium oxide, titanium oxide, aluminum, and nickel, can be utilized, for example. In particular, carbon black is preferable because it can secure high conductivity in a small amount.

  The positive electrode layer 13 may contain solid electrolyte particles in order to improve the lithium ion conductivity in the positive electrode layer. As the solid electrolyte particles, a compound constituting the SE layer 15 described later can be used.

  As a method for forming the positive electrode layer 13 described above, a dry method such as a PVD method or a CVD method, or a wet method such as a coating method or a screen printing method can be used. If a wet method is used, a binder is included in the positive electrode layer so that the active materials are bound to each other, or the active material and a material other than the active material (electrolyte particles and conductive aid). Also good. As such a binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or the like can be used.

(Solid electrolyte layer)
The solid electrolyte layer (SE layer) 15 is a lithium ion conductor made of sulfide. The SE layer 15 preferably has a lithium ion conductivity (20 ° C.) of 10 ⁻⁵ S / cm or more and a lithium ion transport number of 0.999 or more. In particular, the lithium ion conductivity is preferably 10 ⁻⁴ S / cm or more and the lithium ion transport number is preferably 0.9999 or more. The SE layer 15 preferably has an electronic conductivity of 10 ⁻⁸ S / cm or less. Examples of the material of the SE layer 15 include sulfides such as Li-PSO composed of Li, P, S, and O, and Li-PS amorphous film or polycrystalline film composed of Li ₂ S and P ₂ S _5. It is preferable to configure. In particular, when the SE layer is composed of Li-PS composed of Li ₂ S and P ₂ S ₅ , the interface resistance value between the SE layer and the negative active material layer can be reduced. Performance can be improved.

  As a method for forming the SE layer 15, 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 press forming the raw material powder. On the other hand, examples of the vapor deposition method include a PVD method and a CVD method. Specifically, examples of the PVD method include a vacuum deposition 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 SE layer is formed by the vapor deposition method, the thickness of the SE layer can be made thinner than when the SE layer is formed by the solid phase method.

(Negative electrode current collector layer)
The negative electrode current collector layer 12 is a metal film formed on the negative electrode layer 14. As the negative electrode current collector layer 12, one selected from copper (Cu), nickel (Ni), iron (Fe), chromium (Cr), and alloys thereof can be suitably used. The negative electrode current collector layer 12 can also be formed by a PVD method or a CVD method as in the case of the positive electrode current collector layer 11.

(Negative electrode layer)
The negative electrode layer 14 is composed of a layer containing an active material that occludes and releases lithium ions. For example, as the negative electrode layer 14, one selected from the group consisting of Li metal and an element capable of forming an alloy with Li metal, or a mixture or alloy thereof can be suitably used. The metal capable of forming an alloy with Li is at least one selected from the group consisting of aluminum (Al), silicon (Si), tin (Sn), bismuth (Bi), and indium (In) (hereinafter, referred to as “metal”). Alloyed material). 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.

  A vapor phase deposition method can be used as the method for forming the negative electrode layer 14 described above. In addition, a negative electrode layer may be formed by stacking a metal foil on the SE layer and closely contacting the SE layer with a press or an electrochemical technique.

≪Method of manufacturing lithium battery≫
To manufacture a lithium battery, the positive electrode layer 13, the SE layer 15, the negative electrode layer 14, and the negative electrode current collector layer 12 are stacked in this order on the positive electrode current collector layer 11 that also serves as a substrate that supports each layer. To do. In addition, a laminated body in which the positive electrode current collector layer 11, the positive electrode layer 13, and the SE layer 15 are laminated, and a laminated body including the negative electrode current collector layer 12 and the negative electrode layer 14 is produced separately from the laminated body. Then, the lithium battery 1 may be manufactured by superimposing these two laminates.

Hereinafter, by actually producing coin cell type lithium batteries (samples 1-5, 11-15, 101) having the configuration described in the embodiment, and measuring the internal resistance (Ωcm ² ) and capacity (μAh) of the battery The battery performance was evaluated.

<Sample 1>
≪Material preparation≫
First, sulfide particles were prepared by mechanical milling using lithium sulfide (Li ₂ S) and phosphorus pentasulfide (P ₂ S ₅ ) as raw materials. The lithium ion conductivity of the sulfide particles was 5 × 10 ⁻⁴ S / cm.

  A thin plate made of SUS316L having a thickness of 0.5 mm was prepared as a positive electrode current collector. This thin plate also serves as a base material for supporting each layer of the battery.

  As an active material for the positive electrode layer, crystalline lithium nickelate powder having an average particle diameter of 8 μm was prepared. The lithium nickelate powder and sulfide particles were mixed at a mass ratio of 70:30 using a mortar to produce a positive electrode material. Aluminum and cobalt were added to lithium nickelate, and the aluminum content was 1.5% by mass and the cobalt content was 9% by mass.

  An indium foil was prepared as the negative electrode layer. The thickness of the indium foil was 100 μm.

≪Battery fabrication≫
A positive electrode current collector having an outer diameter corresponding to the inner diameter of the mold is placed at the bottom of the mold having an inner diameter of 10 mm, and 20 mg of the positive electrode material, 50 mg of sulfide particles, and indium foil are laminated in this order and pressed. Thus, an all solid-state lithium battery was produced. The pressure during pressing was approximately 500 MPa.

  Finally, the outer periphery of the laminate was covered with an exterior material to produce a lithium battery. The lithium battery is designed to have a terminal from the current collector.

<Samples 2-5>
In Samples 2 to 5, lithium batteries having a positive electrode layer composition different from that of Sample 1 were prepared. In sample 2, only aluminum was added to the positive electrode layer, and the content was 0.5% by mass. In sample 3, only aluminum was added to the positive electrode layer, and the content was 5% by mass. Sample 4 added only cobalt to the positive electrode layer, and its content was 5 mass%. Sample 5 added only cobalt to the positive electrode layer, and its content was 20 mass%. In Samples 2 to 5, the composition and thickness of layers other than the positive electrode layer, the formation method of each layer, and the like are the same as Sample 1.

<Sample 11>
In Sample 11, a lithium battery was prepared in which aluminum and cobalt were not intentionally added to the positive electrode layer. The configuration other than the positive electrode layer is the same as that of Sample 1.

<Samples 12-15>
Sample 12 was a lithium battery with an aluminum content of 0.4% by mass in the positive electrode layer, and sample 13 was a lithium battery with an aluminum content of 10% by mass. In Sample 14, a lithium battery having a cobalt content of 4% by mass in the positive electrode layer was prepared. In Sample 15, a lithium battery having a cobalt content of 25% by mass was prepared.

<Sample 101>
For sample 101, a conventional lithium battery using lithium cobaltate powder instead of lithium nickelate powder was prepared during the preparation of the positive electrode material. Except this point, it is the same as Sample 11.

Using the lithium batteries of Samples 1 to 5, 11 to 15, and 101 described above, a charge / discharge test was performed, and the capacity (μAh) of each lithium battery was measured. The test conditions were a charge upper limit voltage of 3.7 V, a discharge lower limit voltage of 2.0 V, and a charge / discharge current of 0.05 mA. Further, the internal resistance (Ωcm ² ) of each battery was measured by an AC impedance method. The results of these measurements are shown in Table 1.

  As is clear from the results in Table 1, in the battery having a sulfide-based SE layer, the sample 11 in which the positive electrode active material is composed of lithium nickelate has a capacity higher than that of the sample 101 in which the positive electrode active material is composed of lithium cobalt oxide. The internal resistance was small. Samples 1 to 5 in which a predetermined amount of aluminum or cobalt was added to the positive electrode layer using lithium nickelate as an active material had a higher capacity and a lower internal resistance than sample 11. Further, Samples 12 to 15 in which the amount of aluminum or cobalt contained in the positive electrode layer is out of the predetermined range are inferior in overall battery performance compared to Samples 1 to 5, but the positive electrode active material is lithium cobaltate. The battery performance was superior to that of the constructed sample 101.

  The embodiments of the present invention are not limited to those described above, and can be appropriately changed without departing from the gist of the present invention.

  The lithium battery of the present invention can be suitably used as a power source for portable devices and the like.

It is a longitudinal cross-sectional view of this invention lithium battery of a perfect laminated structure.

Explanation of symbols

1 Lithium battery
11 Positive electrode current collector layer 12 Negative electrode current collector layer
13 Positive electrode layer 14 Negative electrode layer
15 Solid electrolyte layer (SE layer)

Claims (4)

A lithium battery comprising a positive electrode layer, a negative electrode layer, and a sulfide solid electrolyte layer that mediates conduction of lithium ions between the two layers,
The positive electrode layer includes positive electrode active material particles and solid electrolyte particles,
The positive active material particles with crystalline lithium nickelate containing at least one or more kinds of 0.5% to 5% by weight of aluminum and 5% to 20% by weight cobalt, an average particle diameter of 3 to A lithium battery having a thickness of 10 μm . The lithium battery according to claim 1, wherein the sulfide solid electrolyte layer includes Li ₂ S—P ₂ S ₅ . The lithium battery according to claim 1 or 2, wherein the positive electrode active material particles contain both aluminum and cobalt. The lithium battery according to claim 1, wherein the sulfide solid electrolyte layer is formed by a vapor deposition method.

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