JP2018129159A - Negative electrode for all-solid-state secondary battery and all-solid-state secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a novel and improved negative electrode for all-solid-state secondary batteries, capable of improving cycle characteristics of an all-solid-state secondary battery using metal lithium as a negative electrode; and an all-solid-state secondary battery.SOLUTION: According to an aspect of the present invention, there is provided a negative electrode for all-solid-state secondary batteries, which includes a negative electrode collector, and a coating layer which coats the negative electrode collector and on which metal lithium can be deposited via a lithium alloy layer that is fast in diffusion of lithium during charging. According to the aspect, metal lithium is substantially uniformly generated and lost on a surface of the coating layer along with charge and discharge, so that dead lithium is hardly occurs. Accordingly, capacity can be maintained even after repeated charging and discharging. More specifically, cycle characteristics can be improved.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode for an all solid state secondary battery and an all solid state secondary battery.

  Metallic lithium has the maximum energy density as a negative electrode active material. For this reason, the practical application of a lithium secondary battery using metallic lithium as a negative electrode has long been desired.

  On the other hand, secondary batteries using carbon as a negative electrode, a lithium-containing layered oxide as a positive electrode, and a non-aqueous liquid as an electrolyte have been widely put into practical use as lithium ion secondary batteries. When metallic lithium is used for the negative electrode of a non-aqueous electrolyte secondary battery, deposition and dissolution of metallic lithium are repeatedly performed by charging and discharging. By such repeated precipitation and dissolution process, dendritic crystals (dendrites) are generated, and this dendrites may cause a short circuit from the negative electrode to the positive electrode. For this reason, the safety and cycle life of the nonaqueous electrolyte secondary battery are insufficient.

  Furthermore, lithium deposited during charging of the non-aqueous electrolyte secondary battery reacts with the non-aqueous electrolyte, that is, the organic electrolyte, and a reductively decomposed film is formed on the negative electrode metal lithium. Thereby, there exists a problem that charging / discharging efficiency worsens. That is, since lithium metal is consumed by charging / discharging, it is necessary to mount a large amount of lithium metal on the negative electrode during the manufacture (that is, the initial stage) of the nonaqueous electrolyte secondary battery. Therefore, the energy density of the battery is reduced. For the above reasons, non-aqueous electrolyte secondary batteries using metallic lithium as a negative electrode have not been put into practical use.

  On the other hand, as a lithium ion secondary battery, for example, an all solid state secondary battery disclosed in Patent Document 1 is known. In the all solid state secondary battery, an inorganic sulfide solid electrolyte is used instead of the non-aqueous electrolyte. In the inorganic sulfide solid electrolyte, the formation of a film accompanying reductive decomposition cannot occur. Therefore, even if charging and discharging are repeated, consumption of lithium ions due to such a reaction does not occur. For this reason, charging / discharging efficiency becomes high, and the amount of metallic lithium to be initially mounted on the negative electrode can be extremely reduced. In other words, a battery structure using only lithium in the positive electrode active material can be realized. Therefore, it is possible to dramatically improve the energy density in the all solid state secondary battery.

International Publication No. 2013/141241

  As described above, when metallic lithium is used for the negative electrode of the all-solid-state secondary battery, the problem caused in the non-aqueous electrolyte secondary battery does not occur. However, in an all solid state secondary battery having a battery structure using only lithium in the positive electrode active material, metallic lithium is deposited at the contact portion between the negative electrode current collector and the solid electrolyte during charging. Moreover, the overvoltage at the time of depositing metallic lithium on the negative electrode current collector is large, and the overvoltage is lower when it is deposited and grown on the deposited metallic lithium, resulting in more coarsening locally. The negative electrode current collector is made of a metal that does not form an alloy with metallic lithium, such as Ni. Further, the deposited metallic lithium hardly grows in the surface direction of the negative electrode current collector, and grows in the thickness direction of the all solid state secondary battery. This metallic lithium dissolves as lithium ions during discharge, but if the current density is high in this process, conduction between the metallic lithium and the solid electrolyte is interrupted, and the metallic lithium may be isolated. Such metallic lithium can no longer be used for charging and discharging and is therefore referred to as dead lithium. For this reason, there existed a problem that a capacity | capacitance fell rapidly by repetition of charging / discharging. That is, when the all-solid-state secondary battery has a battery structure using only lithium in the positive electrode active material that does not have metallic lithium in the negative electrode, there is a problem that the cycle characteristics of the all-solid-state secondary battery are significantly deteriorated. It was.

  Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide the cycle characteristics of an all solid state secondary battery having a battery structure using only lithium in the positive electrode active material. It is an object of the present invention to provide a new and improved all-solid-state secondary battery negative electrode and all-solid-state secondary battery that can be improved.

  In order to solve the above-described problems, according to an aspect of the present invention, lithium metal can be deposited through a lithium alloy layer that covers a negative electrode current collector and a negative electrode current collector, and lithium diffuses quickly during charging. And a negative electrode for an all-solid-type secondary battery, comprising: a coating layer.

  According to this aspect, the coating layer having the above-described characteristics is formed on the negative electrode current collector. For this reason, at the time of charge, metallic lithium deposits substantially uniformly from the surface of the coating layer through an alloy layer that diffuses faster than lithium self-diffusion. At the time of discharging, metallic lithium gradually becomes lithium ions and dissolves. In this process, the thickness of the metallic lithium layer is reduced substantially uniformly, so that the contact between the metallic lithium layer and the solid electrolyte can be maintained. For this reason, it becomes difficult to produce dead lithium. Therefore, the capacity can be maintained even after repeated charging and discharging. That is, cycle characteristics are improved.

  Here, the coating layer may contain a metal capable of forming an alloy with lithium.

  According to this viewpoint, the lithium metal is generated and disappears substantially uniformly on the surface of the coating layer with charge / discharge, so that dead lithium is hardly generated. Therefore, the capacity can be maintained even after repeated charging and discharging. That is, cycle characteristics are improved.

  Moreover, the coating layer may contain any one or more selected from the group consisting of zinc, germanium, tin, antimony, platinum, gold, bismuth, and an alloy containing two or more of these.

  According to this aspect, since the metal lithium layer is generated and disappears substantially uniformly on the surface of the coating layer with charge / discharge, dead lithium is hardly generated. Therefore, the capacity can be maintained even after repeated charging and discharging. That is, cycle characteristics are improved.

  Further, the thickness of the coating layer may be 1 nm or more and less than 100 nm.

  According to another aspect of the present invention, there is provided an all solid state secondary battery comprising the above-described negative electrode for an all solid state secondary battery.

  As described above, according to the present invention, the lithium metal layer is generated and disappears substantially uniformly on the surface of the coating layer with charge / discharge, so that dead lithium is hardly generated. Therefore, the capacity can be maintained even after repeated charging and discharging. That is, cycle characteristics are improved.

It is sectional drawing which shows typically the layer structure of the all-solid-state secondary battery which concerns on one Embodiment of this invention. It is sectional drawing which shows typically the behavior of the negative electrode at the time of charging the all-solid-type secondary battery which does not have a coating layer. It is sectional drawing which shows typically the behavior of a negative electrode at the time of discharging the all-solid-type secondary battery which does not have a coating layer. It is sectional drawing which shows typically the behavior of the negative electrode at the time of charging the all-solid-type secondary battery which concerns on this embodiment. It is sectional drawing which shows typically the behavior of the negative electrode at the time of discharging the all-solid-type secondary battery which concerns on this embodiment. It is a graph which shows the cycling characteristics of the all-solid-type secondary battery which concerns on an Example and a comparative example. It is a SEM (scanning electron microscope) photograph which shows the surface state of the negative electrode of the all-solid-state secondary battery which concerns on a comparative example. It is a SEM (scanning electron microscope) photograph which shows the surface state of the negative electrode of the all-solid-type secondary battery which concerns on an Example. It is a graph which shows the change of the electric potential profile by the presence or absence of a negative electrode active material layer. It is a graph which shows the change of the charging / discharging profile by the kind and presence / absence of a coating layer.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

<1. Configuration of all-solid-state secondary battery>
First, based on FIG. 1, the structure of the all-solid-state secondary battery 1 which concerns on this embodiment is demonstrated. The all solid state secondary battery 1 is a secondary battery using a solid electrolyte as an electrolyte. The all solid state secondary battery 1 is a so-called all solid state lithium ion secondary battery in which lithium ions move between the positive electrode 10 and the negative electrode layer 30.

  As shown in FIG. 1, the all solid state secondary battery 1 includes a positive electrode layer 10, a solid electrolyte layer 20, and a negative electrode layer 30.

(1-1. Positive electrode layer)
The positive electrode layer 10 includes a positive electrode active material and a solid electrolyte. Further, the positive electrode layer 10 may further include a conductive auxiliary agent in order to supplement electronic conductivity. The solid electrolyte contained in the positive electrode layer 10 may or may not be the same type as the solid electrolyte contained in the solid electrolyte layer 20. Details of the solid electrolyte will be described in detail in the section of the solid electrolyte layer 20.

  The positive electrode active material may be any positive electrode active material capable of reversibly occluding and releasing lithium ions.

  For example, the positive electrode active material includes lithium cobaltate (hereinafter referred to as LCO), lithium nickelate, nickel cobaltate lithium, nickel cobalt lithium aluminumate (hereinafter referred to as NCA), nickel cobalt lithium manganate (hereinafter referred to as NCM). ), Lithium salts such as lithium manganate and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, vanadium oxide, and the like. These positive electrode active materials may be used alone or in combination of two or more.

  Moreover, it is preferable that a positive electrode active material is formed including the lithium salt of the transition metal oxide which has a layered rock salt type structure among the lithium salts mentioned above. Here, “layered” represents a thin sheet-like shape. The “rock salt structure” refers to a sodium chloride structure, which is a kind of crystal structure. Specifically, the face-centered cubic lattice formed by each cation and anion is a unit lattice. It represents a structure that is displaced by a half of the edge.

The lithium salt of a transition metal oxide having such a layered rock-salt structure, for _{example, LiNi x Co y Al z O} 2 (NCA), or _{LiNi x Co y Mn z O 2} (NCM) ( where 0 < and lithium salts of ternary transition metal oxides such as x <1, 0 <y <1, 0 <z <1, and x + y + z = 1).

  When the positive electrode active material includes a lithium salt of a ternary transition metal oxide having the above layered rock salt structure, the energy density and thermal stability of the all solid state secondary battery 1 can be improved. .

The positive electrode active material may be covered with a coating layer. Here, the coating layer of the present embodiment may be any material as long as it is a known coating layer for the positive electrode active material of the all solid state secondary battery. Examples of the coating layer, for _example, Li ₂ O-ZrO 2 or the like.

  Further, when the positive electrode active material is formed of a lithium salt of a ternary transition metal oxide such as NCA or NCM and contains nickel (Ni) as the positive electrode active material, the capacity of the all solid state secondary battery 1 It is possible to increase the density and reduce metal elution from the positive electrode active material in the charged state. Thereby, the all-solid-state secondary battery 1 according to the present embodiment can improve long-term reliability and cycle characteristics in a charged state.

  Here, examples of the shape of the positive electrode active material include particle shapes such as a true sphere and an oval sphere. The particle size of the positive electrode active material is not particularly limited, and may be in a range applicable to the positive electrode active material of a conventional all solid state secondary battery. The content of the positive electrode active material in the positive electrode layer 10 is not particularly limited as long as it is applicable to the positive electrode layer of a conventional all solid state secondary battery.

  In addition to the positive electrode active material and the solid electrolyte described above, for example, a conductive agent and a binder may be appropriately mixed in the positive electrode layer 10.

  Examples of the conductive agent that can be blended in the positive electrode layer 10 include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of the binder that can be blended in the positive electrode layer 10 include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene.

(1-2. Solid electrolyte layer)
The solid electrolyte layer 20 is formed between the positive electrode layer 10 and the negative electrode layer 30 and includes a solid electrolyte.

The solid electrolyte is made of a sulfide-based solid electrolyte material, for example. Examples of the sulfide-based solid electrolyte material include Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —LiX (X is a halogen element, such as I or Cl), Li ₂ S—P ₂ S _{5, for example. -Li 2 O, Li 2 S-} P 2 S 5 -Li 2 O-LiI, Li 2 S-SiS 2, Li2 S -SiS 2 -LiI, Li 2 S-SiS 2 -LiBr, Li 2 S-SiS 2 _{-LiCl, Li 2 S-SiS 2} -B 2 S 3 -LiI, Li 2 S-SiS 2 -P 2 S 5 -LiI, Li 2 S-B 2 S 3, Li 2 S-P 2 S 5 -Z _m S _{n (m, n} is any positive number, Z is Ge, and Zn, or _{Ga), Li 2 S-GeS} 2, Li 2 S-SiS 2 -Li 3 PO 4, Li 2 S-SiS 2 - Li _p MO _{q (p, q} is a positive number, M is P, Si, Ge B, Al, either Ga or In), and the like. Here, the sulfide-based solid electrolyte material is produced by processing a starting material (for example, Li ₂ S, P ₂ S _5, etc.) by a melt quenching method, a mechanical milling method, or the like. Further, heat treatment may be further performed after these treatments.

In the solid electrolyte, it is preferable to use a material containing at least sulfur (S), phosphorus (P) and lithium (Li) as constituent elements among the above-mentioned sulfide solid electrolyte materials, and in particular Li ₂ S—P _2. it is more preferable to use those containing S _5.

Here, the case of using those containing _Li 2 _S-P 2 _{S 5} as a sulfide-based solid electrolyte material to form a solid electrolyte, the mixing molar ratio of Li ₂ S and _P 2 _{S 5,} for example, _Li 2 S : P ₂ S ₅ = 50: 50 to 90:10 may be selected.

  In addition, examples of the shape of the solid electrolyte include particle shapes such as a true sphere and an oval sphere. Further, the particle diameter of the solid electrolyte is not particularly limited as long as it is within a range applicable to the solid electrolyte of a conventional all solid state secondary battery.

(1-3. Negative electrode layer)
As shown in FIG. 1, the negative electrode layer 30 includes a negative electrode current collector 40 and a coating layer 50. The negative electrode current collector 40 is a plate-like or foil-like member made of a conductive material. Examples of the material constituting the negative electrode current collector 40 include stainless steel (SUS), titanium (Ti), nickel (Ni), and the like.

  The coating layer 50 covers the surface of the negative electrode current collector 40 (more specifically, the surface facing the solid electrolyte). The coating layer 50 forms an alloy layer in which lithium is rapidly diffused during charging, and metallic lithium is deposited thereon. In the present embodiment, since the negative electrode current collector 40 is covered with the coating layer 50, since metal lithium is generated in the coating layer 50 through an alloy layer in which lithium is rapidly diffused during charging, the surface of the coating layer 50 Lithium metal is deposited almost uniformly over a wide area of the film.

  Specifically, the coating layer 50 includes a metal capable of forming an alloy with lithium. Examples of such metals include zinc, germanium, tin, antimony, platinum, gold, bismuth, and alloys containing two or more of these. The coating layer 50 may be composed of one or more of these. As a result, metallic lithium is deposited in a wider area on the surface of the coating layer 50. That is, the lithium metal is less likely to grow in the thickness direction of the all-solid-state secondary battery 1, and instead deposits substantially uniformly from a wider region of the surface of the coating layer 50. As a result, since the metal lithium layer is continuously formed in the surface direction of the coating layer 50, it is difficult to generate dead lithium during discharge.

(1-4. Behavior of negative electrode layer during charge / discharge)
Here, in order to clarify the effect of the present embodiment, the behavior of the negative electrode layer 30 during charging and discharging will be described. First, for comparison, the behavior during charging / discharging of the negative electrode layer 300 not having the coating layer 50 will be described with reference to FIGS. 2 and 3. First, at the time of charging, as shown in FIG. 2, the metallic lithium 200 is deposited only at the contact portion between the negative electrode current collector 40 and the solid electrolyte. Moreover, even if it contacts, since the overvoltage of precipitation is large, metallic lithium precipitates only locally. The deposited metal lithium 200 hardly grows in the surface direction of the negative electrode current collector 40 and grows in the thickness direction of the all solid state secondary battery. Arrow A indicates the growth direction of metallic lithium 200. For this reason, the metallic lithium 200 that has grown greatly in the thickness direction of the all solid state secondary battery is locally generated on the negative electrode current collector 40.

  At the time of discharging, as shown in FIG. 3, the metallic lithium 200 gradually dissolves as lithium ions and decreases in the arrow B direction. In this process, conduction between some of the metallic lithium 200 and the solid electrolyte may be interrupted. Such a metallic lithium 200 can no longer be used for charging and discharging, and is therefore referred to as dead lithium. In the example of FIG. 3, conduction between the metallic lithium 200a and the solid electrolyte is interrupted. Therefore, the metal lithium 200a becomes dead lithium. And since the metal lithium 200 is produced | generated locally on the negative electrode collector 40, dead lithium is easy to generate | occur | produce. For this reason, there exists a problem that a capacity | capacitance falls rapidly by repetition of charging / discharging.

  Next, based on FIG. 4 and FIG. 5, the behavior during charging / discharging of the negative electrode layer 30 according to the present embodiment will be described. First, at the time of charging, as shown in FIG. 4, lithium ions enter the coating layer 50 from the contact portion between the coating layer 50 and the solid electrolyte and alloy with the metal constituting the coating layer 50. Since lithium diffusion in the alloy is fast, alloying proceeds in the surface direction of the coating layer 50. That is, the coating layer 50 becomes a lithium alloy layer. Therefore, lithium ions can be precipitated from the entire region that is the lithium alloy layer. As a result, the metallic lithium 200 is deposited substantially uniformly from the surface of the coating layer 50. That is, on the surface of the covering layer 50, a layer made of the metallic lithium 200, that is, a metallic lithium layer is continuously formed in the surface direction. Arrow A indicates the deposition direction of metallic lithium 200.

  At the time of discharging, as shown in FIG. 5, the metallic lithium 200 gradually dissolves as lithium ions and decreases in the arrow B direction. In this process, the metal lithium 200 is a metal lithium layer that is continuous in the plane direction, and its thickness is reduced substantially uniformly, so that the contact between the metal lithium and the solid electrolyte can be maintained. For this reason, it becomes difficult to produce dead lithium. Therefore, the capacity can be maintained even after repeated charging and discharging. That is, cycle characteristics are improved.

  Here, the thickness of the coating layer 50 is not particularly limited, but if it is too thin, metallic lithium may be difficult to precipitate. On the other hand, when the coating layer 50 is too thick, the coating layer 50 itself can be a negative electrode active material. In this case, the amount of metallic lithium deposited decreases, making it difficult to use the high energy density characteristic of metallic lithium. Moreover, since the volume expansion accompanying alloying is large, there is a problem that cracking of the electrode occurs, and conversely the efficiency is lowered. For this reason, it is preferable that the thickness of the coating layer 50 is 1 nm or more and less than 100 nm. The upper limit value of the thickness is more preferably 95 nm or less, more preferably 90 nm or less, and more preferably 50 nm or less.

  Thus, in this embodiment, the coating layer 50 is used only to expand the deposition region of the metal lithium 200 and is not used as a negative electrode active material layer.

  Heretofore, the configuration of the all solid state secondary battery 1 according to the present embodiment has been described in detail.

<2. Manufacturing method of lithium ion secondary battery>
Then, the manufacturing method of the all-solid-type secondary battery 1 which concerns on this embodiment is demonstrated. The all solid state secondary battery 1 according to the present embodiment can be manufactured by stacking the above layers after manufacturing the positive electrode layer 10, the negative electrode layer 30, and the solid electrolyte layer 20, respectively.

(Preparation of positive electrode layer)
The positive electrode active material can be produced by a known method.

  Subsequently, the produced positive electrode active material, a solid electrolyte produced by a method described later, and various additives are mixed and added to a nonpolar solvent to form a slurry or paste. Furthermore, the obtained slurry or paste is applied onto a positive electrode current collector, dried, and then rolled, whereby the positive electrode layer 10 can be obtained.

  Here, examples of the material constituting the positive electrode current collector include aluminum and stainless steel. The positive electrode layer 10 may be produced by compacting a mixture of a positive electrode active material and various additives into a pellet shape without using a positive electrode current collector.

(Preparation of negative electrode layer)
The negative electrode layer 30 is produced by covering the negative electrode current collector 40 with a coating layer 50. Here, the method for coating the coating layer 50 on the negative electrode current collector 40 is not particularly limited, and examples thereof include an electroless plating method, a sputtering method, and a vacuum deposition method. Thus, in this embodiment, since it is not necessary to prepare a negative electrode active material, the negative electrode layer 30 can be produced easily. Furthermore, since metal lithium is not used, the cost of manufacturing equipment for dew point management and safety measures can be greatly reduced.

(Preparation of solid electrolyte layer)
The solid electrolyte layer 20 can be made of a solid electrolyte formed of a sulfide-based solid electrolyte material.

  First, the starting material is processed by a melt quenching method or a mechanical milling method.

For example, when the melt quenching method is used, a predetermined amount of starting materials (for example, Li ₂ S, P ₂ S _5, etc.) are mixed, and the pellets are reacted at a predetermined reaction temperature in a vacuum, and then rapidly cooled. By doing so, a sulfide-based solid electrolyte material can be produced. The reaction temperature of the mixture of Li ₂ S and P ₂ S ₅ is preferably 400 ° C. to 1000 ° C., more preferably 800 ° C. to 900 ° C. Moreover, reaction time becomes like this. Preferably it is 0.1 to 12 hours, More preferably, it is 1 to 12 hours. Furthermore, the quenching temperature of the reaction product is usually 10 ° C. or less, preferably 0 ° C. or less, and the quenching rate is usually about 1 ° C./sec to 10000 ° C./sec, preferably 1 ° C./sec to 1000 ° C. It is about ° C / sec.

In addition, when using the mechanical milling method, a sulfide-based solid electrolyte material can be produced by stirring and reacting starting materials (for example, Li ₂ S, P ₂ S _5, etc.) using a ball mill or the like. . In addition, although the stirring speed and stirring time in the mechanical milling method are not particularly limited, the faster the stirring speed, the faster the generation rate of the sulfide-based solid electrolyte material, and the longer the stirring time, the more the sulfide-based solid electrolyte material. The conversion rate of the raw material can be increased.

  Thereafter, the mixed raw material obtained by the melt quenching method or the mechanical milling method is heat-treated at a predetermined temperature and then pulverized to produce a particulate solid electrolyte.

  Subsequently, the solid electrolyte obtained by the above method is formed by using a known film formation method such as an aerosol deposition method, a cold spray method, or a sputtering method. The solid electrolyte layer 20 can be produced. In addition, the solid electrolyte layer 20 may be produced by pressurizing solid electrolyte particles alone. Moreover, the solid electrolyte layer 20 may produce the solid electrolyte layer 20 by mixing a solid electrolyte, a solvent, and a binder, apply-drying, and pressurizing.

(Manufacture of lithium ion secondary batteries)
Furthermore, the positive electrode layer 10, the solid electrolyte layer 20, and the negative electrode layer 30 manufactured by the above method are stacked so that the solid electrolyte layer 20 is sandwiched between the positive electrode layer 10 and the negative electrode layer 30, and then pressed. The all solid state secondary battery 1 according to the embodiment can be manufactured.

<1. Cycle characteristics evaluation>
First, in order to evaluate the cycle characteristics of the all solid state secondary battery 1 according to the present embodiment, the following tests were performed.

(1-1. Example 1)
In Example 1, first, the all solid state secondary battery 1 was manufactured by the following steps.

(1-1-1. Production of negative electrode layer)
Ni foil was prepared as the negative electrode current collector 40, and tin was plated on the surface of the negative electrode current collector 40 with a thickness of 1 nm by an electroless plating method. As a result, a coating layer 50 made of tin and having a thickness of 1 nm was formed on the negative electrode current collector 40.

(1-1-2. Production of all solid state secondary battery)
The negative electrode layer 30 produced above was punched out with φ13 (mm) and set in a cell container. On top of that, 70 mg of Li ₂ S—P ₂ S ₅ (molar ratio 80:20) subjected to mechanical milling (MM treatment) was laminated as solid electrolyte particles, and the surface was lightly adjusted with a molding machine. Thereby, the solid electrolyte layer 20 was formed on the negative electrode layer 30. Next, a mixture of Li (Ni, Mn, Co) O ₂ as a positive electrode active material, solid electrolyte particles, and vapor grown carbon fiber (VGCF) as a conductive agent at a ratio of 60/35/5% by mass. The positive electrode layer 10 was laminated on the solid electrolyte layer 20. Next, the test cell according to Example 1 was manufactured by pressurizing the laminate with a pressure of 3 t / cm ² .

(1-1-3. Evaluation of cycle characteristics)
A test cell was installed in a thermostatic bath at 25 ° C., and charge / discharge evaluation device TOSCAT-3100 manufactured by Toyo System Co., Ltd., 0.1C constant current charge, 0.5C constant current discharge, voltage range 4.0V-3.0V The battery was charged and discharged under the conditions, and the cycle characteristics were evaluated. The results are shown in FIG. The graph L1 in FIG. 6 shows the cycle characteristics of Example 1.

(1-2. Example 2)
In Example 2, the same process as in Example 1 was performed except that the negative electrode layer 30 was produced by the following steps. The results are shown in FIG. The graph L2 in FIG. 6 shows the cycle characteristics of Example 2.

(1-2-1. Production of negative electrode layer)
Ni foil was prepared as the negative electrode current collector 40, and zinc was coated on the surface of the negative electrode current collector 40 to a thickness of 50 nm by a sputtering method. As a result, a coating layer 50 made of zinc and having a thickness of 50 nm was formed on the negative electrode current collector 40.

(1-3. Example 3)
In Example 3, the same process as in Example 1 was performed except that the negative electrode layer 30 was produced by the following steps. The results are shown in FIG. A graph L3 in FIG. 6 shows the cycle characteristics of Example 3.

(1-3-1. Production of negative electrode layer)
Ni foil was prepared as the negative electrode current collector 40, and the surface of the negative electrode current collector 40 was coated with a thickness of 50 nm by sputtering. As a result, a coating layer 50 made of bismuth and having a thickness of 50 nm was formed on the negative electrode current collector 40.

(1-4. Comparative Example 1)
In Comparative Example 1, the same treatment as in Example 1 was performed except that the Ni foil used in Example 1 was used as the negative electrode layer 30. The results are shown in FIG. A graph L4 in FIG. 6 shows the cycle characteristics of Comparative Example 1.

(1-5. Evaluation of cycle characteristics)
As is clear from FIG. 6, in Examples 1 to 3, the cycle characteristics were good, whereas in Comparative Example 1, the cycle characteristics rapidly decreased from the initial stage. Therefore, in Comparative Example 1, it is considered that a large amount of dead lithium was generated along with charge / discharge. On the other hand, in Examples 1 to 3, since the metal lithium layer was formed and disappeared substantially uniformly on the surface of the coating layer 50, it is considered that almost no dead lithium was generated.

<2. Confirmation of precipitation form>
In order to confirm the deposition state of the metallic lithium 200 due to charging / discharging, after the cycle characteristics evaluation test of Example 3 and Comparative Example 1 was completed, these test cells were disassembled. And the surface of the negative electrode layer 30 was observed by SEM. FIG. 7 shows an SEM photograph of Comparative Example 1, and FIG. 8 shows an SEM photograph of Example 3. In FIG. 7, it can be seen that the metallic lithium 200 is locally deposited on the surface of the negative electrode current collector 40, and the solid electrolyte 20a is attached only to a part of the region. That is, it can be seen that the metallic lithium 200 is locally deposited. On the other hand, in Example 3, it can be seen that the metallic lithium 200 is deposited on almost the entire surface of the negative electrode current collector 40, and the solid electrolyte 20 a is also dispersed and adhered on the entire surface of the negative electrode current collector 40.

<3. Evaluation of capacitance of coating layer>
As described above, the coating layer 50 is used to expand the deposition region of the metallic lithium 200, and is not used as a negative electrode active material layer. That is, in this embodiment, metallic lithium precipitates and dissolves in the negative electrode layer 30 with charge / discharge. Therefore, the electric capacity of the coating layer 50 should be very low with respect to the electric capacity of the entire solid state secondary battery 1. In order to confirm this fact, the following test was conducted.

  That is, in this test, a plurality of Ni foils were prepared as the negative electrode current collector 40, and the surface of each of the negative electrode current collectors 40 was coated with tin, zinc, bismuth, and gold with a thickness of 1 nm. Tin and gold were coated by an electroless plating method, and germanium, antimony, zinc, and bismuth were coated by a sputtering method. Thereby, the some negative electrode layer 30 from which the kind of coating layer 50 differs was produced. Next, a test cell was produced using these negative electrode layers 30 in the same manner as in Example 1. However, as the positive electrode layer 20, a Li foil (thickness 0.03 mm) punched with φ13 (mm) was used.

Next, each test cell was set in a constant temperature bath at 25 ° C., and lithium was inserted into the coating layer at a current density of 0.25 mA / cm ² for 10 minutes by a Solartron potentio / galvanostat apparatus 1470E to deposit metallic lithium. The potential was lowered to the potential. Thereby, the electric capacity of the whole test cell was measured. On the other hand, the electric capacity of the catalyst layer 50 was measured based on the theoretical capacity per unit mass of the metal species constituting the coating layer 50. And the ratio of the electric capacity of the catalyst layer 50 to the electric capacity of the whole test cell was calculated. The results are shown in Table 1.

  According to Table 1, since the electric capacity of the coating layer 50 is significantly lower than the electric capacity of the entire test cell, the coating layer 50 is not as a negative electrode active material layer, but extends the deposition region of the metallic lithium 200 It was confirmed that it was used for. In other words, an all solid state secondary battery having a very large electric capacity can be realized by such a very thin coating layer 50.

<3. Change in potential profile of negative electrode with and without negative electrode active material layer>
As described above, the coating layer 50 is used to expand the deposition region of the metallic lithium 200, and is not used as a negative electrode active material layer. That is, in this embodiment, metallic lithium precipitates and dissolves in the negative electrode layer 30 with charge / discharge. Therefore, for example, the potential of the negative electrode layer 30 during discharge should drop to 0 V (vs. Li ^/ Li ⁺ ) immediately after the start of discharge. In order to confirm this fact, the following test was conducted.

  A test cell was produced in the same manner as in “3. Evaluation of electric capacity of coating layer” except that Ni foil was used as negative electrode layer 30. This test cell is a test cell for confirming the behavior of this embodiment. Further, two Ni foils were prepared as the negative electrode current collector 40, and tin and gold were plated on each surface of the negative electrode current collector 40 with a thickness of 100 nm by an electroless plating method. Thereby, the negative electrode layer 30 was produced. Since the metal layer produced on the negative electrode current collector 40 by this method has a large thickness, it functions as a negative electrode active material layer. Then, using these negative electrode layers 30, test cells were produced in the same manner as in “3. Evaluation of electric capacity of coating layer”.

Subsequently, these test cells were set in a thermostatic chamber at 25 ° C., and lithium was inserted into the negative electrode for 60 minutes at a current density of 0.05 mA / cm ² using a potentio / galvanostat device 1470E manufactured by Solartron. Thereby, the potential profile of the negative electrode layer 30 was measured. The results are shown in FIG. A graph L5 in FIG. 9 shows a potential profile of a test cell using Ni foil as the negative electrode layer 30. Graphs L6 and L7 show potential profiles of test cells in which a negative electrode active material layer is formed of tin and gold, respectively.

According to FIG. 9, in the test cell using the Ni foil as the negative electrode layer 30, the potential dropped to almost 0 V (vs. Li ^/ Li ⁺ ) immediately after the start of discharge. Therefore, it can be seen that metallic lithium is deposited on the negative electrode layer 30. On the other hand, in the test cell in which the negative electrode active material layer was formed of tin and gold, the potential was maintained at a relatively high value. These values correspond to the potential of the negative electrode active material layer. That is, tin and gold are used as the negative electrode active material. Therefore, it has been confirmed from this point of view that the coating layer 50 is used to expand the deposition region of the metallic lithium 200.

<4. Charge / Discharge Profile with and without Negative Electrode Active Material Layer>
In order to confirm that the coating layer 50 is not used as the negative electrode active material layer, the following test was further performed. That is, Ni foil was prepared as the negative electrode current collector 40, and gold, zinc, or bismuth was coated on the surface of the negative electrode current collector 40 with a thickness of 50 nm by a sputtering method. Thereby, a 50 nm-thick coating layer 50 made of gold, zinc, or bismuth bismuth was formed on the negative electrode current collector 40. Using the negative electrode 30 formed in this manner, a test cell similar to that in Example 1 was produced. For comparison, a test cell similar to that of Example 1 was produced using the negative electrode 30 made of only Ni foil. Then, these test cells were set in a thermostatic chamber at 25 ° C., and a current density of 0.05 mA / cm ² and a voltage range of 4.0 V to 3.0 V were measured by a charge / discharge evaluation apparatus TOSCAT-3100 manufactured by Toyo System. Charging / discharging was performed once under the conditions. The charge / discharge profile at this time is shown in FIG. A graph L8 shows a charge / discharge profile of a test cell in which the negative electrode 30 is made of Ni foil. A graph L9 shows a charge / discharge profile of a test cell in which the coating layer 50 is made of gold. A graph L10 shows a charge / discharge profile of a test cell in which the coating layer 50 is made of zinc. A graph L11 shows a charge / discharge profile of a test cell in which the coating layer 50 is made of bismuth. If the coating layer 50 functions as a negative electrode active material layer, capacitive components having different potentials depending on the metal species should be observed in the charging profile surrounded by the region A, but such capacitive components are almost observed. There wasn't. Therefore, also from this point, it was found that the coating layer 50 was not used as the negative electrode active material layer. In addition, in the test cell using Ni foil as the negative electrode 30, the amount of increase in potential is small compared to other test cells even when charging progresses. In the test cell using the Ni foil as the negative electrode 30, it is assumed that such a phenomenon occurs because a short circuit occurs midway. That is, in the test cell using Ni foil as the negative electrode 30, it is presumed that the lithium deposition size is large and a micro short-circuit occurs due to current concentration.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 All-solid-state secondary battery 10 Positive electrode layer 20 Solid electrolyte layer 30 Negative electrode layer 40 Negative electrode collector 50 Covering layer

Claims (5)

A negative electrode current collector;
An anode for an all-solid-state secondary battery, comprising: a coating layer that covers the anode current collector and that can deposit metallic lithium through a lithium alloy layer during charging.   The negative electrode for an all solid state secondary battery according to claim 1, wherein the coating layer includes a metal capable of forming an alloy with lithium.   The coating layer includes at least one selected from the group consisting of zinc, germanium, tin, antimony, platinum, gold, bismuth, and an alloy including two or more thereof. 2. The negative electrode for an all solid state secondary battery as described in 2.   4. The all-solid-state secondary battery negative electrode according to claim 1, wherein the coating layer has a thickness of 1 nm or more and less than 100 nm.   An all solid state secondary battery comprising the negative electrode for an all solid state secondary battery according to any one of claims 1 to 4.