WO2009101506A1

WO2009101506A1 - Totally-solid lithium secondary battery

Info

Publication number: WO2009101506A1
Application number: PCT/IB2009/000240
Authority: WO
Inventors: Yasushi Tsuchida; Kazunori Takada
Original assignee: Toyota Jidosha Kabushiki Kaisha; National Institute For Materials Science
Priority date: 2008-02-14
Filing date: 2009-02-12
Publication date: 2009-08-20
Also published as: JP2009193803A

Abstract

A totally-solid lithium secondary battery which includes an anode layer (1), a sulfide-based solid electrolyte layer (2) which is provided on the anode layer (1), does not contain silicon and germanium, and is electrochemically stable with respect to the anode layer (1), and a Ii ion conductor modifying layer (4) which is provided between the anode layer (1) and the sulfide-based solid electrolyte layer (2), does not conduct electrons, and is electrochemically stable with respect to the anode layer (1).

Description

TOTALLY-SOLID LITHIUM SECONDARY BATTERY

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a totally-solid lithium secondary battery in which output is improved by reducing resistance to lithium ion conduction by inhibiting the formation of a space-charge layer on an anode layer side interface of a sulfide-based solid electrolyte layer. 2. Description of the Related Art

[0002] A rapid increase in the popularity of information related devices and communications devices and the like such as personal computers, video cameras and mobile phones in recent years has led to great emphasis being placed on the development of secondary batteries such as lithium secondary batteries which are excellent power supplies for such devices. The development of secondary batteries is also advancing in the fields other than those of information related devices and communications related devices. For example, the development of high output, high capacity lithium secondary batteries for electric vehicles and hybrid vehicles as low-emission vehicles is advancing in the automotive industry, [0003] However, lithium secondary batteries currently on the market use an organic electrolyte solution in which a flammable organic solvent is used as the solvent. As a result, fail-safe devices must be installed to keep the temperature from increasing in the event of an electrical short, and improvements with structure and materials are necessary to prevent electrical shorts from occurring. [0004] In contrast, a totally-solid lithium secondary battery in which liquid electrolyte is replaced with solid electrolyte such that the battery is totally solid does not use an flammable organic solvent in the battery. As a result, fail-safe devices can be simplified, which lowers manufacturing costs and increases productivity.

[0005] However, when a Li₂S-GeS₂-P₂Ss type crystalline sulfide lithium ion conductor or a Li₂S-SiS₂-LiPC^ glass electrolyte containing silicon, germanium or the like is used as the solid electrolyte, the silicon or germanium which is contained in the electrolyte is electrochemically reduced at the surface of the carbon electrode that conducts electrons during charging due to a low current density. This impairs the oxidation reaction and the electrochemical reduction of lithium ions used in charging and discharging of the battery, which reduces charge -discharge efficiency. If this reaction continues to be performed, the silicon or germanium in the solid electrolyte is reduced at the surface, and as a result, becomes a deposit of an ion-conducting dendritic Lithium silicon alloy or lithium germanium alloy that grows until it ultimately causes the cathode and anode to short.

[0006] In order to solve this kind of problem, Japanese Patent Application Publication No. 2004-206942 (JP-A-2004-206942), for example, describes a totally-solid lithium secondary battery in which a second solid electrolyte layer which does not contain silicon and germanium is provided between a first sulfide-based solid electrolyte and a lithium metal anode. This second solid electrolyte layer that does not contain silicon and germanium prevents the first sulfide-based solid electrolyte from being reduced. However, in this case, it is necessary to more reliably prevent a reaction from taking place between the first sulfide-based solid electrolyte and the lithium metal anode. Therefore, the entire interface of the sulfide-based solid electrolyte and the lithium metal anode is covered with the second solid electrolyte layer that does not contain silicon and germanium, and this second solid electrolyte layer that does not contain silicon and germanium has to be made thicker, which leads to problems such as lower lithium ion conductivity.

[0007] Therefore, "Solid State Ionics 177 (2006) 2753-2757", for example, describes a totally-solid lithium secondary battery that uses Li₂S-P-S solid electrolyte material which is electrochemically stable with respect to the anode layer and is not susceptible to reduction even if metallic lithium is used for the anode. However, the chemical potential difference between the solid electrolyte material and the anode layer causes the Li ions to move such that a space-charge layer forms at the interface of the metallic lithium and the Li₂S -P -S solid electrolyte material. As a result, resistance to lithium ion conduction increases.

[0008] "Electrochemistry Communications 9 (2007) 1486-1490" states that high output is achieved by preventing a space-charge layer from forming between the cathode layer and the solid electrolyte layer by interposing LiNbC>₃ between them. However, an oxide Li ion conductor such as LiNbO₃ is susceptible to reduction and- cannot be used as a modifying layer for the anode layer.

SUMMARY OFTHE INVENTION

[0009] This invention thus provides a totally-solid lithium secondary battery in which output is improved by reducing resistance to lithium ion conduction by inhibiting the formation of a space-charge layer on an anode layer side interface of a sulfide-based solid electrolyte layer. [0010] One aspect of the invention relates to a totally-solid lithium secondary battery. The totally-solid lithium secondary battery includes an anode layer, a sulfide-based solid electrolyte layer which is provided on the anode layer, does not contain silicon and germanium, and is electrochemically stable with respect to the anode layer, and a Ii ion conductor modifying layer which is provided between the anode layer and the sulfide-based solid electrolyte layer, does not conduct electrons, and is electrochemically stable with respect to the anode layer.

[0011] According to this aspect of the invention, providing the Li ion conductor modifying layer described above suppresses a loss of Ii ions due to the movement of Li ions to the anode layer from within the sulfide-based solid electrolyte layer near the anode layer as a result of the chemical potential difference, and therefore inhibits the formation of a space-charge layer at the anode layer side interface of the sulfide-based solid electrolyte layer. As a result, this aspect of the invention reduces the resistance to lithium ion conduction, thus enabling output to be increased.

[0012] In the structure described above, the anode layer may have an oxidation-reduction potential of 0.5 V (vs Li / Li⁺) or less. When the anode layer has this kind of oxidation-reduction potential, it is even easier for the space-charge layer to form so the resistance to lithium ion conduction also increases. Therefore, in this case, the excellent effects of this structure are even more evident. Also, having this kind of oxidation-reduction potential makes it possible to increase the output even more.

[0013] In the structure described above, the anode layer may include at least one selected from the group consisting of metallic Ii, graphite, Si, and Sn. When the anode layer includes at least one selected from the group consisting of metallic Li, graphite, Si, and Sn, the space-charge layer forms more easily, in turn increasing the resistance to lithium ion conduction. Therefore, in this case, the excellent effects of this structure are even more evident. Also, having this kind of anode layer enables the output of the totally-solid lithium secondary battery to be more reliably increased.

[0014] In the structure described above, the thickness of the Li ion conductor modifying layer may be 100 nm or less, and more preferably within a range between 1 and 30 nm, inclusive, when the anode layer consists of metallic Li. Moreover, in the structure described above, the thickness of the Ii ion conductor modifying layer may be 30 nm or less, and more preferably within a range between 1 and 30 nm, inclusive, when the anode layer includes graphite.

[0015] In the structure described above, the sulfide-based solid electrolyte layer may include Ii and S, and at least one selected from the group consisting of P, B, and O. This enables the sulfide-based solid electrolyte layer to be even more reliably electrochemically stable with respect to the anode layer. It also enables a totally-solid lithium secondary battery with greater output to be obtained.

[0016] In the structure described above, the Li ion conductor modifying layer may be formed of at least one selected from the group consisting of L.3N, IiCl, and IiF. This is very effective for inhibiting the formation of a space-charge layer at the anode layer side interface of the sulfide-based solid electrolyte layer.

[0017] In the structure described above, the conductivity of the Ii ion conductor modifying layer may be no more than 10^"* S/cm. Also, the conductivity of the Li ion conductor modifying layer may be within a range between 10^" S/cm and 10^" S/cm, inclusive. This makes it even more difficult for Li ions to move due to the chemical potential difference, and therefore more reliably inhibits the formation of the space-charge layer. As a result, the resistance to lithium ion conduction decreases so output can be improved.

[0018] The totally-solid lithium secondary battery having the structure described above may also include an anode collector that is provided on the opposite side of the anode layer from the Li ion conductor modifying layer side, a cathode layer that is provided on the opposite side of the sulfide-based solid electrolyte layer from the Li ion conductor modifying layer side, and a cathode collector that is provided on the opposite side of the cathode layer from the sulfide-based solid electrolyte layer side. Also, the totally-solid lithium secondary battery having this structure may also include a Li ion conductor modifying layer which is provided between the cathode layer and the sulfide-based solid electrolyte layer, does not conduct electrons, and is electrochemically stable with respect to the cathode layer.

[0019] The aspects and structures of the invention make it possible to obtain a totally-solid lithium secondary battery in which output is improved by reducing resistance to lithium ion conduction by inhibiting the formation of a space-charge layer on an anode layer side interface of a sulfide-based solid electrolyte layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a sectional view schematically showing a frame format of an example of a totally-solid lithium secondary battery according to an example embodiment of the invention; and FIG 2 is a graph showing the relationship between IJ₃N thickness and impedance in Example 1, Example 2, Example 3, Example 4, Example 5, Comparative Example 1, and Comparative Example 2.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] A totally-solid lithium secondary battery according to an example embodiment of the invention will now be described in detail.

[0022] The totally-solid lithium secondary battery according to the example embodiment of the invention is a totally-solid lithium secondary battery, in which a sυlfide-based solid electrolyte layer that does not contain silicon and germanium and is electrochemically stable with respect to an anode layer, is formed on the anode layer. This totally-solid lithium secondary battery has a Li ion conductor modifying layer which does not conduct electrons and is electrochemically stable with respect to the anode layer, provided between the anode layer and the sulfide-based solid electrolyte layer.

[0023] According to this example embodiment of the invention, forming the Ii ion conductor modifying layer inhibits an increase in the Li ion concentration within the sulfide-based solid electrolyte layer near the anode layer due to a chemical potential difference, thereby inhibiting the formation of a space-charge layer at the anode layer side interface of the sulfide-based solid electrolyte layer. As a result, resistance to lithium ion conduction decreases, thus enabling output to be increased. The reason for this is presumed to be as follows.

[0024] That is, an anode layer and a sulfide-based solid electrolyte layer in which the potential difference between the two is large are normally used to increase output. When the potential difference between the anode layer and a sulfide-based solid electrolyte layer is laige, that potential difference or the chemical potential difference causes the lithium to move at the interface of the solid (i.e., the anode layer) and the solid (i.e., sulfide-based solid electrolyte layer), such that a space-charge layer is formed in the sulfide-based solid electrolyte layer near the anode layer. As a result, the lithium is no longer able to move easily and resistance to lithium ion conduction increases. Also, a Schottky junction forms at the interface of the electron-ion mixed conductor (such as cathode active material or anode active material) and an ion conductor (such as sulfide-based solid electrolyte) so the space-charge layer on the sulfide-based solid electrolyte layer grows large.

[0025] In this example embodiment of the invention, the IJ ion conductor modifying layer which does not conduct electrons and is electrochemically stable with respect to the anode layer is formed between this anode layer and the sulfide-based solid electrolyte layer. The Li ion conductor modifying layer inhibits the movement of Li ions due to the chemical potential difference, and therefore inhibits the formation of the space-charge layer. Also, because the Schottky junction described above does not form, the space-charge layer on the sulfide-based solid electrolyte layer does not grow large. Therefore, resistance to the lithium ion conduction decreases, thereby enabling output to be increased. [0026] Hereinafter, the totally-solid lithium secondary battery according to this example embodiment of the invention will be described with reference to the drawings. The totally-solid lithium secondary battery shown in FIG 1 includes an anode layer 1, a sulfide-based solid electrolyte layer 2 which is formed on the anode layer 1 and does not contain silicon and germanium and is electrochemically stable with respect to the anode layer 1, a cathode layer 3 arranged such that the sulfide-based solid electrolyte layer 2 is sandwiched between it and the anode layer 1, and a Li ion conductor modifying layer 4 which does not conduct electrons and is electrochemically stable with respect to the anode layer 1, provided between anode layer 1 and the sulfide-based solid electrolyte layer 2. Normally, an anode collector 5 is provided on the anode layer 1 and an cathode collector 6 is provided on the cathode layer 3 so as to sandwich the anode layer 1, the sulfide-based solid electrolyte layer 2, the cathode layer 3, and the Ii ion conductor modifying layer 4 in between. In addition, an insulation (i.e., a battery case) portion 7 is arranged so as to cover the side surfaces.

[0027] The totally-solid lithium secondary battery according to this example embodiment of the invention is not particularly limited as long as it at least has the Li ion conductor modifying layer, the anode layer, and the sulfide-based solid electrolyte layer. Normally, however, it also has a cathode layer, a cathode collector, an anode collector, a battery case, and the like as described above. [0028] Hereinafter, each part of the totally-solid lithium secondary battery according to the example embodiment of the invention will be described in detail. [0029] 1. Li ion conductor modifying layer

First, the Li ion conductor modifying layer used in this example embodiment of the invention will be described. The Li ion conductor modifying layer in this example embodiment of the invention is formed between the anode layer and the sulfide-based solid electrolyte layer. This Li ion conductor modifying layer does not conduct electrons and is electrochemically stable with respect to the anode layer.

[0030] In this example embodiment of the invention, the Li ion conductor modifying layer which does not conduct electrons and is electrochemical] y stable with respect to the anode layer is formed between the anode layer and the sulfide-based solid electrolyte layer. Accordingly, this Li ion conductor modifying layer inhibits the movement of Li ions due to the chemical potential difference, as described above, thereby inhibiting the formation of the space-charge layer. Also, a Schottky junction does not form at the interface of the anode layer and the sulfide-based solid electrolyte layer so the space-charge layer on the sulfide-based solid electrolyte layer will not grow large. As a result, resistance to lithium ion conduction decreases, which enables output to be improved.

[0031] In this example embodiment of the invention, the Li ion conductor modifying layer is electrochemically stable with respect to the anode layer. More specifically, this means that the Li ion conductor modifying layer will not reductively degrade (i.e., the Lj ion conductor modifying layer will not take on electrons due to the potential difference between the anode layer and the Li ion conductor modifying layer). Reductive degradation of the Li ion conductor modifying layer can be checked by checking whether there is a reduction current using cyclic voltammetry. [0032] Also, the Li ion conductor modifying layer does not conduct electrons- More specifically, the electric conductivity of the Li ion conductor modifying layer is preferably no more than 10^'δ S/cm, and more preferably, within the range of 10^~15 S/cm to 10~⁷ S/crn, inclusive. This makes it even more difficult for Li ions to move due to the chemical potential difference, and therefore more reliably inhibits the formation of the space-charge layer. As a result, the resistance to lithium ion conduction decreases so output can be improved.

[0033] The material used for the Li ion conductor modifying layer is not particularly limited as long as it can inhibit the formation of the space-charge layer, does not conduct electrons, and is electrochemically stable with iespect to the anode layer. More specifically, the Li ion conductor modifying layer is preferably formed of at least one selected from the group consisting of U₃N, LiCl, and LiF because these are able to effectively inhibit the formation of the space-charge layer at the anode layer side interface of the sulfide-based solid electrolyte layer. In this example embodiment of the invention, L1₃N in particular is preferable.

[0034] The thickness of the Li ion conductor modifying layer is not particularly limited as long as it makes it possible to obtain a desired totally-solid lithium secondary battery in which output is improved by reducing the resistance to lithium ion conduction by inhibiting the formation of the space-charge layer. [0035] The thickness is preferably determined by the chemical potential difference between the anode layer and the Li ion conductor modifying layer, and the Li ion conductivity of the Li ion conductor modifying layer, and the like, and is preferably no more than 100 nm, more preferably no more than 50 nm, and even more preferably no more than 30 nm, for example. Incidentally, the lower limit may be 1 nm or more. If the range is large, the Li ion conductivity resistance of the Li ion conductor modifying layer itself is large so output may not be able to be improved. On the other hand, if the range is small, tunneling current may flow, thus inhibiting the formation of the space-charge layer so output may not be able to be improved,

[0036] More specifically, the thickness of the Li ion conductor modifying layer may normally be determined by the chemical potential difference between the anode layer and the Li ion conductor modifying layer, and the Li ion conductivity of the Li ion conductor modifying layer. For example, when the anode layer is metallic Li, the sulfide-based solid electrolyte layer is Li₇PsSn, and the Li ion conductor modifying layer is L4₃N, the thickness of the Li ion conductor modifying layer is preferably no more than 100 nrri, and more preferably within a range between 1 nra and 30 nm, inclusive. Also, when an anode mixture formed of metallic Ii, graphite, and sulfide-based solid electrolyte is used for the anode layer, the Li ion conductor modifying layer is Li₇PsSu, and the Li ion conductor modifying layer is Li;jN, the thickness of the Li ion conductor modifying layer is preferably no more than 30 nm, and more preferably within a range between 1 nm and 30 nm, inclusive.

[0037] In this example embodiment of the invention, a value measured based on image analysis using an electron microscope may be used for the thickness of the Li ion conductor modifying layer. [0038] In this example embodiment of the invention, the Ii ion conductor modifying layer is formed on the interface between the anode layer and the sulfide-based solid electrolyte layer. However, as long as the Li ion conductor modifying layer makes it possible to obtain a desired totally-solid lithium secondary battery in which output is improved by reducing the resistance to lithium ion conduction by inhibiting the formation of the space-charge layer, the Li ion conductor modifying layer may also be formed on part of the interface or on the entire interface. Normally, it is preferable that the Ii ion conductor modifying layer be formed on much of the interface, and even more preferable that the Ii ion conductor modifying layer be formed on the entire interface, because this makes it possible to inhibit the formation of the space-charge layer along the entire interface, thereby further reducing the resistance to lithium ion conduction, which enables output to be further improved.

[0039] 2. Anode layer

Next, the anode layer of this example embodiment of the invention will be described. The anode layer in this example embodiment of the invention is formed on a sulfide-based solid electrolyte layer that is electrochemical Iy stable with respect to the anode layer, as will be described later.

[0040] The anode layer may be one in which there is a large potential difference between it and the sulfide-based solid electrolyte layer in order to provide the totally-solid lithium secondary battery with large amount of electromotive force. When the difference in the U ion electrochemical potential between the anode layer and the sulfide-based solid electrolyte layer is large in this way, a space-charge layer such as that described above tends to form easily at the interface of the anode layer and the sulfide-based solid electrolyte layer, so the resistance to lithium ion conduction increases. In this example embodiment of the invention, even if this kind of anode layer is used, the Li ion conductor modifying layer which does not conduct electrons and is electrochemically stable with respect to the anode layer is provided at the interface of the anode layer and the sulfide-based solid electrolyte layer, so the space-charge layer is inhibited from forming, which in turn reduces the resistance to lithium ion conduction. As a result, a desired totally-solid lithium secondary battery having improved output is able to be obtained.

[0041] The anode layer used in this example embodiment of the invention is not particularly limited as long as it functions as an anode layer. Normally, however, as described above, the anode layer that is used has an oxidation-reduction potential in which the potential difference from the sulfide-based solid electrolyte layer is large in order to provide the totally-solid lithium secondary battery with large amount of electromotive force. More specifically, the anode layer preferably has an oxidation-reduction potential of 0.5 V (vs Li / Li⁺) or less. When the anode layer has this kind of oxidation reduction potential, it is even easier for the space-charge layer to form so the resistance to lithium ion conduction also increases. Therefore, in this case, the excellent effects of this example embodiment of the invention are even more evident. Also, having a large potential difference between the anode layer and the sulfide-based solid electrolyte layer makes it possible to increase the electromotive force of the totally-solid lithium secondary battery. [0042] In this example embodiment of the invention, the anode layer preferably has, as anode material, at least one selected from the group containing metallic Li, graphite, Si, and Sn. Of these, the anode layer preferably has at least one selected from the group containing metallic Ii and graphite. In this case, the anode layer is more reliably able to have an oxidation-reduction potential of 0.5 V (vs Li / Ii⁺) or less so the space-charge layer forms more easily, in turn increasing the resistance to lithium ion conduction. In this case, the excellent effects of this example embodiment of the invention are even more evident. Also, the output of the totally-solid lithium secondary battery is able to be increased even more. [0043] Also, the anode layer may be formed of only anode material, or it may be formed of an anode mixture in which anode material is mixed with solid electrolyte material, or the like. Also, in order to improve conductivity, the anode layer of this example embodiment of the invention may also include a conductive agent such as acetylene black, ketjen black, or carbon fiber. Incidentally, the anode layer may also be formed using a mixruτe of two or more types of the anode material.

[0044] The thickness of the anode layer is not particularly limited as long as it makes it possible to obtain a desired totally-solid lithium secondary battery in which output is improved by reducing the resistance to lithium ion conduction by inhibiting the formation of the space-charge layer. [0045] In this example embodiment of the invention, a value measured based on image analysis using an electron microscope may be used for the thickness of the anode layer.

[0046] 3. Sulfide-based solid electrolyte layer

Next, the sulfide-based solid electrolyte layer of this example embodiment of the invention will be described. The sulfide-based solid electrolyte layer in this example embodiment of the invention which is formed on the anode layer, does not contain silicon and germanium and is elecrxochemically stable with respect to the anode layer.

[0047] The sulfide-based solid electrolyte layer which does not contain silicon and germanium and is electrochemically stable with respect to the anode layer, so even if the anode layer has a low oxidation-reduction potential as described above, the formation of an alloy or the like from a reaction between the sulfide-based solid electrolyte layer and the anode layer, as well as electrical shorts caused by that alloy and the like, can be inhibited. [0048] In this way, the sulfide-based solid electrolyte layer which does not contain silicon and germanium and is electrochemically stable with respect to the anode layer. However, a space-charge layer such as that described above easily forms at the interface of the sulfide-based solid electrolyte layer and the anode layer so resistance to lithium ion conduction increases. In this example embodiment of the invention, having the Li ion conductor modifying layer that does not conduct electrons and is electrochemically stable with respect to the anode layer between the sulfide-based solid electrolyte layer and the anode layer inhibits the formation of this space-charge layer, thereby reducing the resistance to lithium ion conduction. As a result, a desired totally-solid lithium secondary battery having improved output can be obtained. [0049] The sulfide-based solid electrolyte layer which does not contain silicon and germanium and is electrochemically stable with respect to the anode layer More specifically, this means that the sulfide-based solid electrolyte layer will not reductively degrade (i.e., the sulfide-based solid electrolyte layer will not take on electrons due to the potential difference between the anode layer and the sulfide-based solid electrolyte layer). Reductive degradation of the sulfide-based solid electrolyte layer can be checked by checking whether there is a reduction current using cyclic voltammetry.

[0050] The sulfide-based solid electrolyte layer is not particularly limited as long as it functions as a sulfide-based solid electrolyte layer, is formed on the anode layer, and is electTOchemically stable with respect to the anode layer. [0051] Having no silicon and no germanium enables the sulfide-based solid electrolyte layer to be more electrochemically stable with respect to the anode layer. The sulfide-based solid electrolyte material used for this kind of sulfide-based solid electrolyte layer includes at least Li (lithium) and S (sulfur), and may also include an element such as P (phosphorus), B (boron), or O (oxygen) when necessary. More specifically, examples of the sulfide-based solid electrolyte material include Li₇PsSn, Li₂S, Li₃PO₄-U₂S-B₂S₃ system, and 80Li₂S-ZOP₂S₅, and the like. Of these, the sulfide-based solid electrolyte layer preferably includes Li and S and at least one selected from the group consisting of P, B, and O. This enables the sulfide-based solid electrolyte layer to be even more reliably electrochemically stable with respect to the anode layer. It also results in greater ion conductivity, and therefore enables a totally-solid lithium secondary battery with greater output to be obtained. Examples of this kind of material include Li₇P₃S_n, 8OU₂S-2OP₂S₅, and U₃PO₄-U₂S-B₂S₃ system and the like. [0052] The thickness of the sulfide-based solid electrolyte layer is not particularly limited as long as it makes it possible to obtain a desired totally-solid lithium secondary battery in which output is improved by reducing the resistance to lithium ion conduction by inhibiting the formation of a space-charge layer.

[0053] In this example embodiment of the invention, a value measured based on image analysis using an electron microscope may be used for the thickness of the sulfide-based solid electrolyte layer, [0054] 4. Other structure

The structure of the totally-solid lithium secondary battery other than the anode layer, the sulfide-based solid electrolyte layer, and the U ion conductor modifying layer described above, such as the cathode layer, the cathode side Li ion conductor modifying layer, the cathode collector, the anode collector, and the battery case, and the like will now be described in detail.

[0055] (1) Cathode layer

The cathode layer used in this example embodiment of the invention is not particularly limited as long as it functions as a cathode layer. That is, the cathode layer may be formed of only cathode material, or it may be formed of a cathode mixture in which cathode material is mixed with solid electrolyte material, or the like. The cathode layer may be the same as that used in a typical totally-solid lithium secondary battery. Also, in order to improve conductivity, the cathode layer of this example embodiment of the invention may also include a conductive agent such as acetylene black, ketjen black, or carbon fiber.

[0056] The thickness of the cathode layer is not particularly limited and may be the same as the thickness of a cathode layer used in an ordinary totally-solid lithium secondary battery.

[0057] (2) Cathode side Li ion conductor modifying layer

Next, the cathode side Li ion conductor modifying layer used in this example embodiment of the invention will be described.

[0058] In this example embodiment of the invention, a space-charge layer forms at the interface of the cathode layer and the sulfide-based solid electrolyte layer. In order to prevent the resistance to lithium ion conduction from increasing, a cathode side Li ion conductor modifying layer which does not conduct electrons and is electrochemically stable with respect to the cathode layer may be provided at the interface of the cathode layer and the sulfide-based solid electrolyte layer. Providing this kind of cathode side Li ion conductor modifying layer inhibits the formation of the space-charge layer at the interface of the cathode layer and the sulfide-based solid electrolyte layer, and thus reduces the resistance to lithium ion conduction. As a result, a desired totally-solid lithium secondary battery having improved output can be obtained.

[0059] The material used for this cathode side Ii ion conductor modifying layer that is used in this example embodiment of the invention is not particularly limited as long as it is able to inhibit the formation of the space-change layer, does not conduct electrons, and is electrochemically stable with respect to the cathode layer. One example of material that can be used is LiNbCb.

[0060] The thickness of this cathode side Li ion conductor modifying layer is not particularly limited as long as it makes it possible to obtain a desired totally-solid lithium secondary battery in which output is improved by reducing the resistance to lithium ion conduction by inhibiting the formation of the space-charge layer.

[0061] In this example embodiment of the invention, the cathode side Li ion conductor modifying layer is formed at the interface between the cathode layer and the sulfide-based solid electrolyte layer. However, as long as the cathode side Li ion conductor modifying layer makes it possible to obtain a desired totally-solid lithium secondary battery in which output is improved by reducing the resistance to lithium ion conduction by inhibiting the formation of the space-charge layer, the cathode side Li ion conductor modifying layer may also be formed on part of the interface or on the entire interface. Normally, it is preferable that the cathode side Ii ion conductor modifying layer be formed on much of the interface, and even more preferable that the cathode side Li ion conductor modifying layer be formed on the entire interface, because this makes it possible to inhibit the formation of the space-charge layer along the entire interface, thereby further reducing the resistance to lithium ion conduction, which enables output to be further improved.

[0062] (3) Cathode collector

The cathode collector used in this example embodiment of the invention collects the power of the cathode layer. This cathode collector is not particularly limited as long as it functions as a cathode collector. The material of the cathode collector is not limited as long as it is conductive. Some examples are SUS (stainless steel), aluminum, nickel, iron, titanium, and carbon. Of these, SUS is preferable. Moreover, the cathode collector may be a compact collector or a porous collector,

[0063] (4) Anode collector The anode collector used in this example embodiment of the invention collects the power of the anode layer. This anode collector is not particularly limited as long as it functions as an anode collector. The material of the anode collector is not limited as long as it is conductive. Some examples are SUS, copper, nickel, and carbon. Of these, SUS is preferable. Moreover, the anode collector may be a compact collector or a porous collector.

[0064] (5) Other structure

Next, structure other than the members described above, e.g., the insulation portion such as the battery case and the resin used to seal a coin type battery case and the like, will be described. [00651 The insulation portion and the resin and the like are not particularly limited and may be the same as those used in a typical totally-solid lithium secondary battery.

[0066] More specifically, examples of the insulation portion are an insulation ring and a battery case and the like. The battery case is typically a metal case and may be stainless steel, for example. Also, the battery case may also function as a collector. More specifically, a SUS battery case may be prepared and a portion of it may be used as a collector, for example. Also, the resin is preferably a resin with a low water absorption rate. One example is epoxy resin. [006η 5. Other

(1) Manufacturing method of the totally-solid lithium secondary battery

The manufacturing method of the totally-solid lithium secondary battery in the example embodiment of the invention is not particularly limited as long as it is a method by which the totally-solid lithium secondary battery described above can be obtained. For example, a sulfide-based solid electrolyte layer forming step is performed for forming the sulfide-based solid electrolyte layer by press forming sulfide-based solid electrolyte material. Then an anode layer forming step is performed for forming an anode layer by pressure bonding anode material onto an anode collector. Further, a Li ion conductor modifying layer forming step is performed for forming a Li ion conductor modifying layer on the anode layer by supplying a predetermined gas (such as nitrogen gas, chlorine gas, or fluorine gas) for a predetermined period of time using the anode layer.

[0068] Next, a cathode layer forming step is performed for forming a cathode layer by press forming either a cathode mixture made of cathode material and solid electrolyte material, or cathode material only onto a cathode collector. Then a battery cell forming step is performed for forming a battery cell by arranging the anode layer on the sulfide-based solid electrolyte layer so that the Li ion conductor modifying layer is contacting the sulfide-based solid electrolyte layer, arranging the cathode layer so that it and the anode layer sandwich the sulfide-based solid electrolyte layer, and then placing the resultant structure in a coin type battery case, for example, and sealing it with resin packing. These steps together constitute one example of a method for obtaining the desired totally-solid lithium secondary battery described above.

[0069] Incidentally, the sulfide-based solid electrolyte layer forming step, the anode layer forming step, the Ii ion conductor modifying layer forming step, the cathode layer forming step, and the battery cell forming step may all be performed simultaneously or in a different order or the like as long as the desired totally-solid lithium secondary battery described above is able to be obtained. Also, the method may also include steps other than those described above as long as the desired totally-solid lithium secondary battery described above is able to be obtained. [0070] (2) Use

The purpose for which the totally-solid lithium secondary battery in the example embodiment of the invention is to be used is not particularly limited. For example, the totally-solid lithium secondary battery may be used as a totally-solid lithium secondary battery for a vehicle or the like. [0071] (3) Shape

With regard to shape, the totally-solid lithium secondary battery in the example embodiment of the invention may be a coin type battery, a laminated type battery, cylindrical type battery, or square type battery, or the like. Of these, the totally-solid lithium secondary battery is preferably a square type battery or laminated type battery, and more preferably, a laminated type battery.

[0072] Incidentally, example embodiments of the invention are not limited to those described above. The foregoing example embodiments are only examples. Any and all example embodiments that have a structure with substantially the same technical features described in the scope of the claims of the invention and which display similar operation and effects are intended to be included within the technical scope of the invention.

[0073] Hereinafter, various example embodiments will be described in more detail using examples. [0074] [Example 1]

(Forming an evaluation cell)

First, an anode layer was formed by pressure bonding metallic Li that had been dried for at least one week in a glow box with a dew point of -7O⁰C onto a SUS sheet that serves as a collector and which had also been dried under the same, conditions. The resultant anode layer was then treated in nitrogen by supplying (at room temperature) dry nitrogen for five minutes such that a Li ion conductor modifying layer (LiβN) was formed on the anode layer.

[0075] A U7P3S₁₁ pellet was formed by press forming using Li₇PsSu as the sulfide-based solid electrolyte material.

[0076] The obtained anode collector (SUS), the anode layer (metallic Li), and the Li ion conductor modifying layer (U₃N) together make up an anode electrode body.

Two of these anode electrode bodies are arranged so as to sandwich the IΛ7P3SU pellet such that the Li ion conductor modifying layer contacts with the sulfide-based solid electrolyte, thus forming a symmetrical cell that was used as an evaluation cell.

[0077] [Example 2]

Another symmetrical cell which was used as another evaluation cell was formed just as in Example 1 except for that the treatment time in the nitrogen was 10 minutes.

[0078] [Example 3] Another symmetrical cell which was used as another evaluation cell was formed just as in Example 1 except for that the treatment time in the nitrogen was 30 minutes.

[0079] [Example 4]

180 mg of graphite was mixed with 3.8 mg of metallic Ii and the resultant mixture was heat-treated for 16 hours at 200⁰C in Ar such that the metallic Li was intercalated in the graphite. Then, the mixture is heat-treated again for 8 hours at 200⁰C in N₂ to form a lithium nitride film. Then 3.4 rag of obtained powder was mixed with 3.4 mg of sulfide-based solid electrolyte layer material, the mixture was placed on a SUS plate as a collector that had been dried for at least one week in a glow box at a dew point of -70⁰C or less, and the two were press formed to form an anode electrode body. [0080J Another symmetrical cell which was used as another evaluation cell was formed just as in Example 1 except for the use of this kind of anode electrode body.

[00811 [Example s]

Another symmetrical cell which was used as another evaluation cell was formed just as in Example 4 except for that the amount of metallic U used was 12 mg.

[0082] [Comparative example 1]

Another symmetrical cell which was used as another evaluation cell was formed just as in Example 1 except for that the anode layei was not treated in nitrogen.

[0083] [Comparative example 2] Another symmetrical cell which was used as another evaluation cell was formed just as in Example 4 except for that metallic Ii was not added.

[0084] [Evaluation]

(Impedance measurement)

The impedance was measured using the evaluation cells obtained in Examples 1 to 5 and Comparative examples 1 and 2, Moreover, in Examples 1 to 3 and

Comparative example 1, the interface resistance (i.e., the resistance when lithium ions move between the anode layer and the sulfϊde-based solid electrolyte) was obtained from the diameter of an arc of which approximately 100 kHz of measured impedance was the peak, and the thickness of the lithium nitride layer was obtained using the value of the ion conductivity at the lithium nitride interface. Also, in Examples 4 and 5 and

Comparative example 2, the volume of lijN was calculated with the assumption that all of the lithium was nitrided from the weight of the metallic lithium used. Then the thickness of the lithium nitride was obtained by dividing that calculated volume by the surface area of graphite. FIG 2 shows the relationship between the obtained impedance (Ω) and the thickness (nm) of IJ3N.

[0085] As shown in FIG 2, the resistance values of Examples 1 and 2 in which pressure-bonded metallic lithium was used as the anode layer were good, being much lower than that of Comparative example 1. Also, the resistance value of Example 3 is comparable to that of Comparative example 1, [0086J The resistance value of Example 4 in which the anode layer containing graphite was used was good, being lower than that of Comparative example 2. Also, the resistance value of Example 5 was greater than that of Comparative example 2.

[0087] In any case, when the IJ₃N film was formed, the resistance value tended to increase as the thickness of the U₃N film increased.

[0088] From these results, in the examples, the Li ion conductor modifying layer (i.e., the IJ₃N film) made it possible to inhibit the movement of lithium ions due to the chemical potential difference between the anode layer and the sulfide-based solid electrolyte layer, and thereby inhibit the formation of a space-charge layer at the anode layer side interface of the sulfide-based solid electrolyte layer. As a result, the resistance to lithium ion conduction decreased such that output was able to be improved.

[0089] Also, in the examples, with Examples 1, 2, and 3 that used the pressure-bonded metallic lithium as the anode layer, it was evident that having the thickness of the U ion conductor modifying layer (i.e., the U₃N film) be 100 nm or less resulted in a better resistance value. Further, with Examples 4 and 5 which used an anode layer containing graphite, it was evident that having the thickness of the IJ ion conductor modifying layer (i.e., the Li₃N film) be 30 nm or less resulted in a better resistance value.

Claims

CLAIMS:

1. A totally-solid lithium secondary battery comprising: an anode layer; a sulfide-based solid electrolyte layer which is provided on the anode layer, does not contain silicon and germanium, and is electrochemically stable with respect to the anode layer; and a Li ion conductor modifying layer which is provided between the anode layer and the sulfide-based solid electrolyte layer, does not conduct electrons, and is electrochemically stable with respect to the anode layer.

2. The totally-solid lithium secondary battery according to claim 1, wherein the anode layer has an oxidation-reduction potential of 0.5 V (vs Li / Ii⁺) or less.

3. The totally-solid lithium secondary battery according to claim 1 or 2, wherein the anode layer includes at least one selected from the group consisting of metallic Li, graphite, Si, and Sn.

4. The totally-solid lithium secondary battery according to claim 1 or 2, wherein the anode layer includes metallic Li.

5. The totally-solid lithium secondary battery according to claim 1 or 2, wherein the anode layer consists of metallic Ii.

6. The totally-solid lithium secondary battery according to claim 5, wherein the thickness of the Li ion conductor modifying layer is 100 nm or less.

7. The totally-solid lithium secondary battery according to claim 6, wherein the thickness of the Ii ion conductor modifying layer is within a range between 1 and 30 nra, inclusive.

8. The totally-solid lithium secondary battery according to claim 1, 2 or 4, wherein the anode layer includes graphite.

9. The totally-solid lithium secondary battery according to claim 8, wherein the thickness of the Li ion conductor modifying layer is 30 nm or less.

10. The totally-solid lithium secondary battery according to claim 9, wherein the thickness of the Li ion conductor modifying layer is within a range between 1 and 30 nm, inclusive.

11. The totally-solid lithium secondary battery according to any one of claims 1 to

10, wherein the sulfide-based solid electrolyte layer includes Li and S, and at least one selected from the group consisting of P, B, and O.

12. The totally-solid lithium secondary battery according to any one of claims 1 to

11, wherein the Li ion conductor modifying layer is formed of at least one selected from the group consisting of Li₃N₁ LiCl, and LiF.

13. The totally-solid lithium secondary battery according to any one of claims 1 to

12, wherein the conductivity of the Li ion conductor modifying layer is no more than 10^"6 S/cm.

14. The totally-solid lithium secondary battery according to any one of claims 1 to

13, wherein the conductivity of the Li ion conductor modifying layer is within a range between 10^"15 S/cm and 10^"7 S/cm, inclusive.

15. The totally-solid lithium secondary battery according to any one of claims 1 to 14, further comprising: an anode collector that is provided on the opposite side of the anode layer from the Li ion conductor modifying layer side; a cathode layer that is provided on the opposite side of the sulfide-based solid electrolyte layer from the Li ion conductor modifying layer side; and a cathode collector that is provided on the opposite side of the cathode layer from the sulfide-based solid electrolyte layer side.

16. The totally-solid lithium secondary battery according to claim 15, further comprising: a Li ion conductor modifying layer which is provided between the cathode layer and the sulfide-based solid electrolyte layer, does not conduct electrons, and is electrochemically stable with respect to the cathode layer.