Secondary battery

Abstract

The present application relates to a secondary battery. The secondary battery has a positive electrode, an electrolyte layer, a negative electrode current collector, and metal lithium as a negative electrode active material that is deposited between the electrolyte layer and the negative electrode current collector by charging, and a nitride of an element M is present between the electrolyte layer and the negative electrode current collector, the element M being an element that can be alloyed with Li, and the nitride being covalent.

Description

Technical Field

[0001] This application discloses a secondary battery. Background Technology

[0002] Japanese Patent Application Publication No. 2016-012495 discloses a lithium solid-state secondary battery comprising a positive electrode, a solid electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the solid electrolyte layer and the negative electrode current collector during charging. US Patent Publication No. 2017 / 0346099 discloses a lithium battery comprising: a negative electrode containing lithium metal or a lithium alloy, an ion-conductive amorphous metal nitride layer disposed on the surface of the negative electrode, an electrolyte, and a positive electrode. Summary of the Invention

[0003] According to the inventor's understanding, in the secondary battery with a lithium metal anode disclosed in Japanese Patent Application Publication No. 2016-012495, when lithium metal is repeatedly deposited and dissolved between the electrolyte layer and the negative electrode current collector, there is a problem of low coulombic efficiency of the lithium metal deposition and dissolution reaction.

[0004] As one of the means to solve the above-mentioned problems, this application discloses a secondary battery having a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the electrolyte layer and the negative electrode current collector during charging. A nitride of element M is present between the electrolyte layer and the negative electrode current collector. Element M is an element that can be alloyed with Li, and the nitride is covalently bonded.

[0005] In the secondary battery of this disclosure, the nitride may be coated on at least a portion of the surface of the negative electrode current collector.

[0006] In the secondary battery disclosed herein, the positive electrode may contain a lithium oxide as the positive electrode active material.

[0007] In the secondary battery of this disclosure, the electrolyte layer may comprise a sulfide solid electrolyte.

[0008] The secondary battery disclosed herein exhibits high coulombic efficiency in the lithium metal precipitation and dissolution reactions. Attached Figure Description

[0009] The features, advantages, and technical and industrial importance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and wherein:

[0010] Figure 1 The structure of the secondary battery 100 after charging and discharging is shown in a general way.

[0011] Figure 2A This is a simplified example of a process for manufacturing a secondary battery.

[0012] Figure 2B This is a simplified example of a process for manufacturing a secondary battery.

[0013] Figure 2C This is a simplified example of a process for manufacturing a secondary battery.

[0014] Figure 3 For comparison, the cross-sectional SEM and EDX images of the negative electrode after charging in Example 3 are shown.

[0015] Figure 4 For comparison, the cross-sectional SEM and EDX images of the negative electrode after charging in Example 5 are shown.

[0016] Figure 5 The image shows the cross-sectional SEM and EDX images of the negative electrode after charging in Example 1.

[0017] Figure 6 The changes in the XPS spectrum of Si3N4 after charging are shown. Detailed Implementation

[0018] 1. Secondary battery

[0019] Figure 1 An embodiment of the secondary battery of this disclosure is illustrated below. For example... Figure 1 As shown, one embodiment of the secondary battery 100 includes a positive electrode 10, an electrolyte layer 20, a negative electrode current collector 31, and metallic lithium 32, which is deposited as a negative electrode active material between the electrolyte layer 20 and the negative electrode current collector 31 during charging. Here, a nitride 33 of element M exists between the electrolyte layer 20 and the negative electrode current collector 31. Element M is an element that can alloy with Li. The nitride 33 is covalently bonded.

[0020] 1.1 Positive electrode

[0021] The positive electrode 10 contains at least a positive electrode active material. During charging of the secondary battery 100, lithium ions released from this positive electrode active material travel through the electrolyte layer 20 to the space between the electrolyte layer 20 and the negative electrode current collector 31, where they accept electrons and are deposited as metallic lithium. Furthermore, during battery discharge, the metallic lithium 32 between the electrolyte layer 20 and the negative electrode current collector 31 dissolves (ionizes) and returns to the positive electrode 10. The positive electrode 10 can be any of the forms known as positive electrodes for secondary batteries. For example, such as... Figure 1 As shown, the positive electrode 10 may have a positive current collector 11 and a positive active material layer 12.

[0022] 1.1.1 Positive Current Collector

[0023] The positive current collector 11 can be any conventional positive current collector used in secondary batteries. The positive current collector 11 can be a metal foil or a metal mesh. In particular, metal foils offer excellent processability. The positive current collector 11 can be composed of multiple metal foils. Examples of metals constituting the positive current collector 11 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. Especially from the viewpoint of ensuring oxidation resistance, the positive current collector 11 may contain Al. For purposes such as adjusting resistance, the positive current collector 11 may have some coating on its surface. Furthermore, when the positive current collector 11 is composed of multiple metal foils, some layers may be formed between these multiple metal foils. There is no particular limitation on the thickness of the positive current collector 11. For example, it can be 0.1 μm or more, or 1 μm or more, or 1 mm or less, or 100 μm or less.

[0024] 1.1.2 Positive Electrode Active Material Layer

[0025] The positive electrode active material layer 12 contains positive electrode active material and may optionally further contain electrolyte, conductive additives, binders, etc. Furthermore, the positive electrode active material layer 12 may also contain various other additives. The respective contents of the positive electrode active material, electrolyte, conductive additives, and binders in the positive electrode active material layer 12 can be appropriately determined according to the target battery performance. For example, if the total solid component of the positive electrode active material layer 12 is set to 100% by mass, the content of the positive electrode active material can be 40% or more by mass, 50% or more by mass, or 60% or more by mass, or less than 100% by mass or less than 90% by mass. The shape of the positive electrode active material layer 12 is not particularly limited; for example, it can be a sheet with a generally planar surface. The thickness of the positive electrode active material layer 12 is not particularly limited; for example, it can be 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, or less than 2 mm, less than 1 mm, less than 500 μm, or less than 100 μm.

[0026] The positive electrode active material can be any material known as a positive electrode active material for secondary batteries that can supply lithium to the negative electrode side during charging. For example, lithium cobalt oxide, lithium nickel oxide, and LiNi oxide can be used as positive electrode active materials. 1 / 3 Co 1 / 3Mn 1 / 3Various lithium-containing oxides, such as O2, lithium manganese oxide, and spinel-based lithium compounds, can be used. A single positive electrode active material can be used alone, or two or more can be used in combination. The positive electrode active material can be in particulate form, and its size is not particularly limited. The particles of the positive electrode active material can be solid, hollow, or porous. The particles of the positive electrode active material can be primary particles or secondary particles formed by the aggregation of multiple primary particles. The average particle size (D50) of the positive electrode active material particles can be, for example, 1 nm or more, 5 nm or more, or 10 nm or more; alternatively, it can be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. It should be noted that the average particle size D50 mentioned in this application refers to the particle size (median particle size) at the 50% cumulative value of the particle size distribution on a volume basis determined by laser diffraction and scattering.

[0027] The surface of the positive electrode active material can be coated with a protective layer containing an ion-conducting oxide. That is, the positive electrode active material layer 12 may also contain a composite comprising the aforementioned positive electrode active material and the protective layer disposed on its surface. This facilitates the suppression of reactions between the positive electrode active material and sulfides (e.g., sulfide solid electrolytes described later). Examples of ion-conducting oxides used to coat and protect the surface of the positive electrode active material include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, and Li4Ti5O. 12 The materials used are Li₂Ti₂O₅, Li₂ZrO₃, LiNbO₃, Li₂MoO₄, and Li₂WO₄. The coverage ratio (area ratio) of the protective layer relative to the surface of the positive electrode active material can be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer can be, for example, 0.1 nm or more, 1 nm or more, less than 100 nm or less than 20 nm.

[0028] The electrolyte that may be included in the positive electrode active material layer 12 can be a solid electrolyte, a liquid electrolyte (electrolyte), or a combination thereof. In particular, when the positive electrode active material layer 12 includes a solid electrolyte (especially a sulfide solid electrolyte), the technology disclosed herein is expected to bring greater benefits.

[0029] For solid electrolytes, any known solid electrolyte used in secondary batteries can be used. Solid electrolytes can be inorganic or organic polymer electrolytes. In particular, inorganic solid electrolytes exhibit excellent ion conductivity and heat resistance. Examples of inorganic solid electrolytes include: lithium lanthanum zirconate, LiPON, and Li... 1+X Al X Ge 2-X(PO4)3, Li-SiO glass, Li-Al-SO glass and other oxide solid electrolytes, Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5-GeS2 and other sulfide solid electrolytes. In particular, sulfide solid electrolytes containing at least Li, S and P as constituent elements exhibit high performance. Solid electrolytes can be amorphous or crystalline. Solid electrolytes can be, for example, in particle form. A single solid electrolyte can be used alone, or two or more can be used in combination.

[0030] The electrolyte may contain lithium ions as charge carrier ions. The electrolyte may be a non-aqueous electrolyte. For example, an electrolyte in which lithium salts are dissolved at a specified concentration in a carbonate-based solvent can be used. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of lithium salts include hexafluorophosphate.

[0031] Examples of conductive additives that can be included in the positive electrode active material layer 12 include carbon materials such as fumed carbon fiber (VGCF), acetylene black (AB), Ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The conductive additives can be in particle or fibrous form, and their size is not particularly limited. A single conductive additive can be used alone, or two or more can be used in combination.

[0032] Examples of adhesives that can be included in the positive electrode active material layer 12 include butadiene rubber (BR) based adhesives, butene rubber (IIR) based adhesives, acrylate butadiene rubber (ABR) based adhesives, styrene butadiene rubber (SBR) based adhesives, polyvinylidene fluoride (PVdF) based adhesives, polytetrafluoroethylene (PTFE) based adhesives, polyimide (PI) based adhesives, and polyacrylic acid based adhesives. One type of adhesive may be used alone, or two or more types may be used in combination.

[0033] 1.2 Electrolyte Layer

[0034] The electrolyte layer 20 contains at least an electrolyte. The electrolyte layer 20 may contain a solid electrolyte, and optionally further contain binders, various additives, etc. There is no particular limitation on the content of the electrolyte and binders, etc., in the electrolyte layer 20. The electrolyte layer 20 may also contain liquid components such as an electrolyte solution. The electrolyte layer 20 may have a separator or the like for preventing contact between the positive and negative electrodes, and the electrolyte solution may be retained in the separator. The thickness of the electrolyte layer 20 is not particularly limited; for example, it may be 0.1 μm or more, or 1 μm or more, or 2 mm or less, or 1 mm or less.

[0035] The electrolyte contained in the electrolyte layer 20 can be appropriately selected from electrolytes exemplified as electrolytes that can be contained in the aforementioned positive electrode active material layer. In particular, when the electrolyte layer 20 contains a solid electrolyte (especially a sulfide solid electrolyte), the technology disclosed herein is expected to bring greater benefits. Furthermore, regarding the adhesive that may be contained in the electrolyte layer 20, it can also be appropriately selected from adhesives exemplified as adhesives that can be contained in the aforementioned positive electrode active material layer. The electrolyte and adhesive can each be used individually or in combination of two or more. In the case of a secondary battery being an electrolyte battery, the separator used to hold the electrolyte can be any separator commonly used in secondary batteries, such as separators made of resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator can be a single-layer structure or a multi-layer structure. Examples of multi-layer separators include, for example, a two-layer PE / PP separator, or a three-layer PP / PE / PP or PE / PP / PE separator. The separator can also be made of nonwoven fabrics such as cellulose nonwoven fabric, resin nonwoven fabric, and glass fiber nonwoven fabric.

[0036] 1.3 Negative current collector

[0037] The negative electrode current collector 31 can be any conventional negative electrode current collector used as a secondary battery. The negative electrode current collector 31 can be a metal foil or a metal mesh, or it can be a carbon sheet. In particular, metal foils offer excellent processability. The negative electrode current collector 31 can be composed of multiple metal foils or sheets. Examples of metals constituting the negative electrode current collector 31 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. Specifically, from the viewpoint of ensuring resistance to reduction and preventing alloying with lithium, the negative electrode current collector 31 can contain at least one metal selected from Cu, Ni, and stainless steel. The negative electrode current collector 31 can have some coating on its surface. For example, as described later, nitride 33 can be applied to at least a portion of the surface of the negative electrode current collector 31. Furthermore, when the negative electrode current collector 31 is composed of multiple metal foils, layers can be formed between these multiple metal foils. There is no particular limitation on the thickness of the negative current collector 31. For example, it can be 0.1 μm or more, or 1 μm or less, or 1 mm or less.

[0038] 1.4 Lithium Metal as an Anode Active Material

[0039] The secondary battery 100 has a lithium-deposited negative electrode. Specifically, such as... Figure 1 As shown, during charging, metallic lithium 32, which serves as the negative electrode active material, is deposited between the electrolyte layer 20 and the negative electrode current collector 31. Furthermore, the metallic lithium 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31 dissolves (ionizes) during discharge and returns to the positive electrode 10.

[0040] There is no particular limitation on the amount of lithium metal 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31. It can be adjusted appropriately according to the target battery performance. However, if the amount of deposited lithium metal 32 is too large, there are concerns about pressure concentration, etc. Regarding this, a reference value for the amount of lithium metal 32 deposited can be, for example, 1 mAh / cm³ to achieve the charging capacity of the secondary battery 100. 2 Above and 5mAh / cm 2 The following quantities.

[0041] According to the inventors' understanding, conventional secondary batteries with lithium deposition-type negative electrodes suffer from the following problem: the coulombic efficiency of lithium deposition and dissolution reactions is low during repeated deposition and dissolution between the electrolyte layer and the negative electrode current collector. According to the inventors' new understanding, one reason for this problem is the oxidation of the deposited lithium metal. Specifically, during repeated deposition and dissolution of lithium metal, voids and irregularities are created in the lithium metal due to uneven deposition, easily increasing the specific surface area of ​​the lithium metal. Therefore, during repeated charge-discharge cycles, trace amounts of oxygen present in the battery react with the lithium metal, gradually turning it into lithium oxide. Lithium oxide is electrochemically inactive and cannot dissolve. Therefore, in conventional secondary batteries, due to repeated charge-discharge cycles, the lithium metal gradually oxidizes, the amount of active lithium gradually decreases, the coulombic efficiency decreases, and the battery capacity decreases. Furthermore, the low electronic and ionic conductivity of lithium oxide may hinder electrochemical reactions within the battery, which may also be a major cause of reduced coulombic efficiency.

[0042] 1.5 Nitride

[0043] To address the aforementioned issues, in the secondary battery 100, a predetermined nitride 33 exists between the electrolyte layer 20 and the negative electrode current collector 31, thereby suppressing the oxidation of lithium metal 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31. Specifically, when lithium metal 32 is deposited, a conversion reaction A occurs between a portion of the lithium metal 32 and the nitride 33, alloying a portion of the lithium metal 32 and simultaneously nitriding a portion of the lithium metal 32. Unlike lithium oxide, lithium nitride possesses both electronic conductivity and lithium-ion conductivity, thus not hindering electrochemical reactions. In the following reaction formula, for convenience, it is assumed that Li3N is generated as lithium nitride, but in reality, the Li to N composition ratio is not fixed. It is assumed that N is dispersed over a large area of ​​the lithium metal 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31, making the lithium metal 32 generally difficult to oxidize.

[0044] (Reaction A)M x N y +zLi + +ze - →Li (z-3y) M x +yLi3N

[0045] By alloying a portion of the lithium metal 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31, and partially nitriding it, the reactivity of lithium metal 32 with oxygen is reduced, suppressing the formation of lithium oxide. Furthermore, alloyed and nitrided lithium exhibit superior electronic and ionic conductivity compared to lithium oxide, and even if they remain between the electrolyte layer 20 and the negative electrode current collector 31 during the charging and discharging of the secondary battery 100, they are unlikely to hinder electrochemical reactions within the secondary battery 100. Therefore, the secondary battery 100 with a defined nitride 33 between the electrolyte layer 20 and the negative electrode current collector 31 exhibits improved coulombic efficiency compared to the case where a defined nitride 33 is absent between the electrolyte layer 20 and the negative electrode current collector 31 (conventional technology).

[0046] Here, in the secondary battery 100, the nitride 33 needs to meet the following requirements (1) and (2).

[0047] (1) The element M that constitutes nitride 33 is an element that can be alloyed with Li. (2) Nitride 33 is covalent.

[0048] Regarding the above requirement (1), assuming that element M does not alloy with lithium, the above-mentioned conversion reaction A will not occur. Whether element M is an element that alloys with lithium can be determined by using known databases (phase diagrams), etc.

[0049] Regarding requirement (2) above, assuming that nitride 33 is non-covalent (e.g., ionic), it is difficult for element M to dissociate from nitrogen N in nitride 33, making conversion reaction A difficult to proceed. Furthermore, compared to covalent nitride 33, non-covalent nitrides tend to have lower electronic conductivity; from this perspective, conversion reaction A is also considered difficult to occur. Whether nitride 33 is covalent can be determined based on the difference in Pauling's electronegativity between element M and nitrogen N. That is, the smaller the difference in Pauling's electronegativity between element M and nitrogen N constituting the nitride, the more likely the nitride is to become covalent. For example, the difference in Pauling's electronegativity between element M and nitrogen N can be less than 1.2.

[0050] By employing nitride 33 of element M that satisfies both of the above requirements (1) and (2), the above-mentioned transformation reaction A can be carried out effectively. For example, at least one element selected from Si, Ga, Sn, In, etc. can be cited as such element M. Among them, high efficiency is easily obtained when at least one of Si and Ga, especially Si, is used as element M.

[0051] In the secondary battery 100, the morphology of the nitride 33 is not particularly limited, and various morphologies capable of carrying out the aforementioned conversion reaction A can be adopted. For example, the nitride 33 can be layered. Furthermore, the nitride 33 can be coated on at least a portion of the surface of the negative electrode current collector 31. Specifically, a layer (film) of nitride 33 can be laminated on at least a portion of the surface of the negative electrode current collector 31. In this case, the thickness of the nitride 33 layer (film) is not particularly limited. The amount of product generated by conversion reaction A can be controlled according to the thickness of this layer (film). The thickness of this layer (film) can be determined based on factors such as the amount of lithium metal 32 deposited in the secondary battery 100. For example, the thickness of this layer (film) can be 10 nm or more and 10 μm or less. Furthermore, if the layer (film) is too thick, the lithium metal will be excessively alloyed and nitrided, potentially resulting in an excessive reduction in the amount of electrochemically active lithium.

[0052] 1.6 Other Components

[0053] The secondary battery 100 only needs to have at least the above-described structures, but may also have other components. The components described below are examples of other components that the secondary battery 100 may have.

[0054] 1.6.1 Outer Packaging

[0055] The secondary battery 100 can also be a structure in which the aforementioned components are housed inside an outer packaging body. More specifically, all parts except for the tabs or terminals used to extract power from the secondary battery 100 to the outside can be housed inside the outer packaging body. The outer packaging body can be any known outer packaging body for batteries. For example, a laminated film can be used as the outer packaging body. Furthermore, multiple secondary batteries 100 can be electrically connected and can be arbitrarily stacked to form a battery pack. In this case, the battery pack can be housed inside a known battery casing.

[0056] 1.6.2 Sealing Resin

[0057] In the secondary battery 100, each of the above-described components can be sealed with resin. For example, Figure 1 At least one side (the side along the lamination direction) of each layer shown can be sealed with resin. This easily prevents moisture from seeping into the interior of each layer. Known solid resins or thermoplastic resins can be used as the sealing resin.

[0058] 1.6.3 Constraint Members

[0059] The secondary battery 100 may or may not have a constraint member for constraining the aforementioned components in the thickness direction. By employing a constraint member to apply constraint pressure, the internal resistance of the battery can be easily reduced. There are no particular limitations on the constraint pressure generated by the constraint member.

[0060] 2. Negative current collector for lithium deposition type negative electrode

[0061] The technology disclosed herein also has the aspect of serving as a negative electrode current collector for lithium deposition-type negative electrodes. Specifically, at least a portion of the surface of the negative electrode current collector for lithium deposition-type negative electrodes of this disclosure is coated with a nitride of element M, where element M is an element capable of alloying with Li, and the nitride is covalently bonded. As described above, by coating the surface of the negative electrode current collector 31 with nitride 33, a conversion reaction A occurs between lithium metal 32 and nitride 33 when lithium metal 32 is deposited, resulting in a partial alloying and partial nitriding of lithium metal 32, making it difficult for lithium metal 32 to be oxidized. There are no particular limitations on the method of coating the surface of the negative electrode current collector 31 with nitride 33. For example, nitride 33 can be deposited and layered on the surface of the negative electrode current collector 31 by sputtering with nitride 33 as a target. In this case, by adjusting the sputtering time, a layer of nitride 33 of desired thickness can be formed on the surface of the negative electrode current collector 31.

[0062] 3. Manufacturing method of secondary batteries

[0063] The aforementioned secondary battery 100 can be manufactured, for example, as described below. That is, as... Figure 2A , Figure 2B , Figure 2C As shown, one embodiment of the manufacturing method of the secondary battery 100 includes:

[0064] The surface of at least one of the surfaces of the negative electrode current collector 31 and the electrolyte layer 20 is coated with nitride 33. Figure 2A ),

[0065] Using the negative electrode current collector 31 or electrolyte layer 20 coated with the nitride 33, a laminate 50 having a positive electrode 10, the electrolyte layer 20, the nitride 33, and the negative electrode current collector 31 in sequence is obtained. Figure 2B ),as well as

[0066] The laminate 50 is charged, causing lithium metal 32 to deposit between the electrolyte layer 20 and the negative electrode current collector 31, while simultaneously causing the lithium metal 32 to react with the nitride 33. Figure 2C ).

[0067] 3.1 Coating with nitrides

[0068] like Figure 2A As shown, in the manufacturing method of this embodiment, at least one surface of the negative electrode current collector 31 and the electrolyte layer 20 is coated with nitride 33. From the viewpoint of superior processability, etc., Figure 2AAs shown, it is preferable to coat the surface of the negative electrode current collector 31 with the nitride 33. There are no particular limitations on the method of coating the surface of the negative electrode current collector 31 or the surface of the electrolyte layer 20 with the nitride 33. For example, sputtering can be used, as described above.

[0069] 3.2 Fabrication of Layered Bodies

[0070] like Figure 2B As shown, in the manufacturing method of this embodiment, as described above, a laminate 50 having a positive electrode 10, an electrolyte layer 20, a nitride 33, and a negative electrode current collector 31 are obtained by using a negative electrode current collector 31 coated with nitride 33 or an electrolyte layer 20. The laminate 50 can be easily obtained, for example, by coating or transferring the aforementioned materials in a manner that sequentially stacks the positive electrode current collector 11, the positive electrode active material layer 12, the electrolyte layer 20, the nitride 33, and the negative electrode current collector 31, thereby forming and stacking them. The laminate 50 only needs to include at least one positive electrode current collector 11, one positive electrode active material layer 12, one electrolyte layer 20, one nitride 33, and one negative electrode current collector 31. That is, the laminate 50 only needs to have at least one of the above-described stacked units of positive current collector 11, positive active material layer 12, electrolyte layer 20, nitride 33 and negative current collector 31, or it may have multiple such stacked units. In this case, the multiple stacked units may be electrically connected in series, in parallel, or not electrically connected.

[0071] After obtaining the aforementioned laminate 50, pressure can be applied to the laminate 50 along its thickness direction (lamination direction). For example, the layers constituting the laminate 50 can be pressed together to achieve integration, or gaps between the layers constituting the laminate 50 can be eliminated to reduce interfacial resistance. The laminate 50 can be pressurized using known methods. For example, various pressing methods such as CIP, HIP, rolling, uniaxial pressing, and molding can be used to pressurize the laminate 50 in the lamination direction. The magnitude of the pressure applied to the laminate 50 in the lamination direction can be appropriately determined according to the performance of the target battery. For example, when the laminate 50 contains a sulfide solid electrolyte, from the viewpoint of easily achieving the aforementioned integration and gap elimination by plastically deforming the sulfide solid electrolyte, the pressure can be 100 MPa or more, 150 MPa or more, 200 MPa or more, 250 MPa or more, 300 MPa or more, or 350 MPa or more. There are no particular limitations on the pressurization time and temperature of the laminate 50.

[0072] 3.3 Charging

[0073] like Figure 2CAs shown, in the manufacturing method of this embodiment, the laminate 50 obtained as described above is charged, causing lithium metal 32 to deposit between the electrolyte layer 20 and the negative electrode current collector 31. Specifically, by charging the laminate 50, lithium ions are conducted from the positive electrode active material contained in the positive electrode active material layer 12 through the electrolyte layer 20 to the negative electrode current collector 31. Between the electrolyte layer 20 and the negative electrode current collector 31, the lithium ions accept electrons and are deposited as lithium metal 32. At this time, a portion of the lithium metal 32 reacts with the nitride 33, a portion is alloyed, and a portion is nitrided. Thus, the lithium metal 32 becomes a state in which it is difficult to oxidize as a whole. The charging can be the first charging after the laminate 50 is prepared, or it can be a second or subsequent charging. The laminate 50 can be charged using the same method as conventional battery charging. That is, charging can be performed simply by connecting an external power source to the positive electrode current collector 11 and the negative electrode current collector 31 of the laminate 50.

[0074] 3.4 Other processes

[0075] In addition to the steps described above, the manufacturing method described in this embodiment may also include conventional steps for manufacturing secondary batteries. For example, steps such as housing the laminate 50 inside an outer packaging body such as a laminated film, and connecting the current collector tabs to the laminate 50. Specifically, for example, the laminate 50 may be housed inside the laminated film that serves as the outer packaging body, with the current collector tabs connected to the current collectors 11 and 31 of the laminate 50 (a portion of the current collectors 11 and 31 may protrude and be used as tabs). The laminated film may be sealed while the tabs are extended to the outside of the laminated film, and then the laminate 50 may be charged via the tabs outside the laminated film.

[0076] 4. Supplement

[0077] As described above, in a secondary battery with a lithium deposition-type negative electrode, by placing a predetermined nitride between the electrolyte layer and the negative electrode current collector, when lithium metal is deposited between the electrolyte layer and the negative electrode current collector during battery charging, a portion of the lithium metal is alloyed and a portion is nitrided, resulting in a state where the lithium metal is not easily oxidized. Therefore, the reaction between oxygen present in the battery and lithium metal is suppressed, making it difficult to form lithium oxide, and improving the coulombic efficiency involved in the deposition and dissolution of lithium metal. Here, examples of oxygen present in the battery include oxygen from the battery material and oxygen that enters the battery from outside. For example, when the positive electrode active material is a lithium oxide, oxygen can be released from this lithium oxide. In the secondary battery of this disclosure, when the positive electrode contains a lithium oxide as the positive electrode active material, oxygen is released in small amounts from this lithium oxide, and even if it reaches the lithium metal on the negative electrode side, the reaction between this oxygen and the lithium metal can be suppressed.

[0078] As described above, for lithium metal deposited between the electrolyte layer and the negative electrode current collector, its reactivity with oxygen increases as its specific surface area increases due to gaps and unevenness. Gaps and unevenness in lithium metal are easily generated due to uneven deposition of lithium metal between the electrolyte layer and the negative electrode current collector. In particular, uneven deposition of lithium metal is more likely to occur when the electrolyte layer contains a solid electrolyte (especially a sulfide solid electrolyte). This is because point contacts and localized pressure concentrations between the battery materials and the negative electrode current collector easily lead to uneven reaction. In the secondary battery of this disclosure, even if uneven deposition of lithium metal occurs between the electrolyte layer and the negative electrode current collector when the electrolyte layer contains a solid electrolyte (especially a sulfide solid electrolyte), the oxidation of lithium metal can be suppressed.

[0079] The foregoing has described one embodiment of the technology disclosed herein, but various modifications beyond the above-described embodiment are possible without departing from the spirit of the subject. The following examples illustrate the technology of this disclosure in further detail, but the technology of this disclosure is not limited to the following examples. It should be noted that in the following examples, the processing of solid electrolytes, active materials, and conductive additives is carried out in a glove box with an Ar gas atmosphere and a dew point below -70°C.

[0080] 1. Evaluation of the fabrication of battery cells.

[0081] 100 mg of a sulfide glass solid electrolyte containing Li, P, and S was weighed and placed into a cylindrical barrel with a diameter of 11.28 mm. The mixture was then pressed under 6 tons of pressure to produce an electrolyte pellet. A lithium metal foil (150 μm thick) was placed on one side of the electrolyte pellet, and various current collector foils (described later) were placed on the other side. The pellet was then pressed under 1 ton of pressure to obtain a laminate. The laminate obtained under 1 MPa of pressure yielded an evaluation battery cell.

[0082] 1.1 Comparative Example 1

[0083] SUS304 foil (10μm thick, the same below) was used as the current collector foil.

[0084] 1.2 Comparative Example 2

[0085] As the current collector foil, SUS304 foil coated with boron nitride (BN) was used. BN coating was performed by sputtering, forming a BN layer with a thickness of 1000 nm on the surface of the SUS304 foil.

[0086] 1.3 Comparative Example 3

[0087] As the current collector foil, SUS304 foil coated with copper nitride (Cu3N) was used. Cu3N coating was performed by sputtering, forming a Cu3N layer with a thickness of 1000 nm on the surface of the SUS304 foil.

[0088] 1.4 Comparative Example 4

[0089] As the current collector foil, SUS304 foil coated with magnesium nitride (Mg3N2) was used. Mg3N2 coating was performed by sputtering, forming a 1000 nm thick Mg3N2 layer on the surface of the SUS304 foil.

[0090] 1.5 Comparative Example 5

[0091] As the current collector foil, SUS304 foil coated with aluminum nitride (AlN) was used. AlN coating was performed by sputtering, forming an AlN layer with a thickness of 1000 nm on the surface of the SUS304 foil.

[0092] 1.6 Example 1

[0093] As the current collector foil, SUS304 foil coated with silicon nitride (Si3N4) was used. The Si3N4 coating was performed by sputtering, forming a 1000 nm thick Si3N4 layer on the surface of the SUS304 foil.

[0094] 2. Charge-discharge cycle test

[0095] The manufactured evaluation battery cell was connected to a charge-discharge test chamber and subjected to a charge-discharge test at 60°C with a voltage of +1V to -1V and an A / cm charge of 0.435mA. 2 Conduct a cyclic test. Set the number of cycles to 50. In each cycle, calculate the coulombic efficiency as a ratio of discharge capacity to charge capacity. Calculate the average coulombic efficiency after 10 cycles of charge-discharge reaction stabilization.

[0096] 3. Results

[0097] The properties of the nitrides on the surface of the current collector foil and the results of charge-discharge cycle tests are shown in Table 1 below.

[0098] [Table 1]

[0099]

[0100] As shown in Table 1, the results indicate that, based on the nitride M present between the electrolyte tablet and the current collector (SUS304), x N yThe type of element in the battery cell significantly affected the coulombic efficiency of the evaluation cells. As shown in Table 1, when the element M constituting the nitride could not alloy with Li (Comparative Examples 2 and 3) or when the nitride was ionic (Comparative Examples 4 and 5), the coulombic efficiency was worse than that of the absence of nitride (Comparative Example 1). Conversely, when the element M constituting the nitride was an element capable of alloying with Li and the nitride was covalent (Example 1), the coulombic efficiency was significantly improved compared to that of the absence of nitride (Comparative Example 1).

[0101] If metallic lithium is deposited on the surface of the nitride of element M, the following thermodynamic transformation reaction A can occur.

[0102] (Reaction A)M x N y +zLi + +ze - →Li (z-3y) M x +yLi3N

[0103] As shown in Table 1 above, it is assumed that, among various nitrides, the above reaction A proceeds when (1) the element M constituting the nitride is an element capable of alloying with Li and (2) the nitride is covalently bonded. A more detailed examination follows.

[0104] Figure 3 The image shows cross-sectional SEM and EDX images of the negative electrode after charging in Comparative Example 3 (where Cu3N is present). Figure 3 As shown, neither Cu nor N diffused from the surface of the current collector foil, and no conversion reaction A with metallic lithium occurred. It is believed that although Cu3N is covalently bonded, the conversion reaction A did not occur because Cu and Li were not alloyed. In Comparative Example 3, it is believed that oxygen in the system reacted with metallic lithium to form lithium oxide, resulting in a decrease in coulombic efficiency.

[0105] Figure 4 The cross-sectional SEM and EDX images of the negative electrode after charging are shown for Comparative Example 5 (with AlN present). Figure 4 As shown, neither Al nor N diffused from the surface of the current collector foil, and no conversion reaction A with metallic lithium occurred. It is believed that although Al alloyed with Li, the Al-N bond was ionic, making it difficult to cause dissociation of the Al-N bond, thus the conversion reaction did not proceed. In Comparative Example 5, similarly to Comparative Example 3, it is believed that oxygen in the system reacted with metallic lithium to form lithium oxide, resulting in a decrease in coulombic efficiency.

[0106] Figure 5 The image shows a cross-sectional SEM and EDX image of the negative electrode after charging in Example 1 (with Si3N4 present). Figure 5 It can be seen that N diffuses in Li, and the conversion reaction A is expected to proceed. Figure 6 The image shows the changes in the XPS spectrum of Si3N4 after charging. Both Si and N shift to the lower energy side and are reduced, approaching the literature values ​​for Li-Si alloys and Li3N, respectively. That is, the aforementioned transformation reaction A occurred, and Si and N were reduced. This is believed to be because Si3N4 is covalently bonded, easily causing the reducing dissociation of the Si-N bond. Furthermore, Si can alloy with Li, thus stabilizing the products of reaction A. Through reaction A, metallic lithium is partially nitrided. Figure 5 It is known that lithium nitriding extends across a large range of lithium metals. Therefore, it is believed that by nitriding a portion of the lithium metal, it becomes difficult to oxidize. That is, in Example 1, it is believed that oxygen in the system is unlikely to react with lithium metal, and lithium oxide is unlikely to form, thereby ensuring high coulombic efficiency.

[0107] It should be noted that in Example 1 above, silicon nitride was exemplified as the nitride of element M, but the technology disclosed herein is not limited thereto. As described above, it is believed that when (1) the element M constituting the nitride is an element capable of alloying with Li and (2) the nitride is covalently bonded, the coulombic efficiency is improved due to the same mechanism as in Example 1.

[0108] Furthermore, in Example 1 described above, a cell with a structure of lithium metal / electrolyte sheet / (nitride) / current collector foil was fabricated as the evaluation battery cell for the purpose of simple evaluation. However, this structure is only for simple evaluation and is not entirely consistent with the structure of an actual secondary battery. In the case of actually constructing a secondary battery, an appropriate structure for a secondary battery can be used as the positive electrode, electrolyte layer, nitride, and negative electrode current collector.

Claims

1. A secondary battery comprising a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium, which is deposited as a negative electrode active material between the electrolyte layer and the negative electrode current collector during charging. A nitride of element M exists between the electrolyte layer and the negative electrode current collector. The nitride coats at least a portion of the surface of the negative electrode current collector. The element M is selected from at least one element chosen from Si, Ga, Sn, and In. The nitride is covalently bonded.

2. The secondary battery according to claim 1, wherein The positive electrode contains a lithium oxide as the positive electrode active material.

3. The secondary battery according to claim 1 or 2, wherein The electrolyte layer contains a sulfide solid electrolyte.

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