Lithium secondary battery manufacturing method

Abstract

Provided is a method for manufacturing a lithium secondary battery that comprises: a positive electrode; a negative electrode onto which lithium metal deposits during charging; a solid electrolyte layer that is interposed between the positive electrode and the negative electrode, the solid electrolyte layer containing a solid electrolyte; and a functional layer that is interposed between the solid electrolyte layer and the negative electrode, the functional layer having electron insulation properties and lithium-ion conductive properties, and being more stable than the solid electrolyte layer in terms of reductive decomposition due to physical contact with the lithium metal, the method comprising a first charging step for depositing a lithium metal layer to a thickness of 90% or more of the thickness of the functional layer, by charging a lithium secondary battery precursor that has the same configuration as the lithium secondary battery and is in an uncharged state at a first charging rate, and a second charging step for charging the lithium secondary battery precursor that has undergone the first charging step at a second charging rate, and when the maximum value of the first charging rate is represented by C1 and the maximum value of the second charging rate is represented by C2, the relationship C1 < C2 being satisfied.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a lithium secondary battery. [Background technology]

[0002] In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is hoping that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and there has been active development of non-aqueous electrolyte secondary batteries, such as secondary batteries for driving motors, which hold the key to putting these vehicles into practical use.

[0003] Secondary batteries for driving motors are required to have extremely high output characteristics and high energy compared to consumer secondary batteries used in mobile phones, laptops, etc. Therefore, lithium secondary batteries, which have the highest theoretical energy of all practical batteries, have attracted attention and are currently being rapidly developed.

[0004] Currently widely used lithium secondary batteries use flammable organic electrolytes, and these liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

[0005] Therefore, in recent years, there has been active research and development into all-solid-state secondary batteries that use oxide- or sulfide-based solid electrolytes. Solid electrolytes are materials primarily composed of ionic conductors that allow ionic conduction in a solid state. For this reason, all-solid-state secondary batteries do not, in principle, encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. Furthermore, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the output density and energy density of batteries.

[0006] One type of such all-solid-state secondary battery is known as a lithium deposition type, in which lithium metal is deposited on the negative electrode current collector during charging. During the charging process of a lithium deposition type all-solid-state lithium secondary battery, lithium metal is deposited between the solid electrolyte layer and the negative electrode current collector. However, it is known that communication between the lithium metal and the positive electrode causes a short circuit, resulting in a decrease in the performance of the all-solid-state lithium secondary battery. To address this issue, for example, Japanese Patent Application Laid-Open No. 2020-9724 discloses a charging method that involves multi-stage charging of an all-solid-state lithium secondary battery and forming a roughness coating layer of a predetermined thickness made of lithium metal between the solid electrolyte layer and the negative electrode current collector, thereby preventing short circuits in the battery and shortening the charging time. Summary of the Invention [Problem to be solved by the invention]

[0007] However, according to the studies of the present inventors, it has been found that the techniques described in the above patent documents may not achieve a sufficient discharge capacity when discharging an all-solid-state lithium secondary battery after a charging process.

[0008] Therefore, an object of the present invention is to provide a means for improving the discharge capacity of a lithium deposition type lithium secondary battery. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by stepwise charging a lithium secondary battery precursor having a predetermined functional layer provided between a solid electrolyte layer and a negative electrode, thereby completing the present invention.

[0010] That is, one embodiment of the present invention is a lithium secondary battery manufacturing method having a positive electrode in which a positive electrode active material layer containing a positive electrode active material capable of occluding and releasing lithium ions is disposed on the surface of a positive electrode current collector, a negative electrode having a negative electrode current collector, and lithium metal being deposited on the negative electrode current collector during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a functional layer interposed between the solid electrolyte layer and the negative electrode, having electron insulation and lithium ion conductivity, and being more stable than the solid electrolyte with respect to reduction decomposition due to contact with the lithium metal. The manufacturing method includes a first charging step of depositing the lithium metal until a thickness of 90% or more of the thickness of the functional layer is achieved by charging a lithium secondary battery precursor having the same configuration as the lithium secondary battery in an uncharged state at a first charging rate, and a second charging step of charging the lithium secondary battery precursor that has undergone the first charging step at a second charging rate. And when the maximum value of the first charging rate is C1 and the minimum value of the second charging rate is C2, it is characterized in that C1 < C2.

Brief Description of the Drawings

[0011] [Figure 1] FIG. 1 is a perspective view showing an external appearance of a flat laminated type lithium secondary battery which is an embodiment of a lithium secondary battery manufactured by the manufacturing method according to the present invention. [Figure 2] FIG. 2 is a cross-sectional view taken along the line 2-2 shown in FIG. 1. [Figure 3] FIG. 3 is a perspective view of a lithium secondary battery provided with a pressing member. [Figure 4] FIG. 4 is a side view seen from the direction A shown in FIG. 3.

Embodiments for Carrying Out the Invention

[0012] One embodiment of the present invention is a lithium secondary battery manufacturing method having a positive electrode in which a positive electrode active material layer containing a positive electrode active material capable of occluding and releasing lithium ions is disposed on the surface of a positive electrode current collector, a negative electrode having a negative electrode current collector, and lithium metal being deposited on the negative electrode current collector during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a functional layer interposed between the solid electrolyte layer and the negative electrode, having electron insulation and lithium ion conductivity, and being more stable than the solid electrolyte with respect to reduction decomposition due to contact with the lithium metal. The method includes a first charging step of charging a lithium secondary battery precursor having the same configuration as the lithium secondary battery and in an uncharged state at a first charging rate to deposit the lithium metal until a thickness of 90% or more of the thickness of the functional layer is achieved, and a second charging step of charging the lithium secondary battery precursor that has undergone the first charging step at a second charging rate. At this time, when the maximum value of the first charging rate is C1 and the minimum value of the second charging rate is C2, C1 < C2 is satisfied.

[0013] According to the manufacturing method of the lithium secondary battery according to this embodiment, in a lithium precipitation type lithium secondary battery, the discharge capacity can be improved.

[0014] Hereinafter, embodiments of the above-described embodiment will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the description in the claims and is not limited to only the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0015] FIG. 1 is a perspective view showing the appearance of a flat-layered lithium secondary battery, which is one form of lithium secondary battery manufactured by the manufacturing method according to the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1. The layered structure allows the battery to be compact and have a high capacity. In this specification, the flat-layered non-bipolar lithium secondary battery shown in FIGS. 1 and 2 (hereinafter also simply referred to as a "layered battery") will be described in detail as an example. However, in terms of the internal electrical connection form (electrode structure) of the lithium secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

[0016] 1, the stacked battery 10a has a flat, rectangular shape, and a negative electrode current collector 25 and a positive electrode current collector 27 for extracting power are pulled out from both sides of the stacked battery 10a. The power generating element 21 is wrapped in a battery exterior material (laminate film 29) of the stacked battery 10a, and the periphery is heat-sealed, with the negative electrode current collector 25 and positive electrode current collector 27 pulled out to the outside.

[0017] As shown in Fig. 2, the stacked battery 10a has a structure in which a flat, approximately rectangular power generating element 21, where the charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, a functional layer 12, and a negative electrode are stacked in this order. Note that Fig. 2 shows a cross section of the stacked battery during charging, and therefore, a negative electrode active material layer 13 made of lithium metal is present between the negative electrode current collector 11' and the solid electrolyte layer 17.

[0018] The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are laminated in this order such that one positive electrode active material layer 15 faces the adjacent negative electrode active material layer 13 with a solid electrolyte layer 17 interposed therebetween. In addition, a functional layer 12 is disposed between the solid electrolyte layer 17 and the negative electrode. As a result, the positive electrode, solid electrolyte layer, functional layer, and negative electrode constitute one unit cell layer 19. Therefore, the stacked battery 10a shown in FIG. 2 can be said to have a configuration in which a plurality of unit cell layers 19 are laminated and electrically connected in parallel. In addition, a restraining pressure is applied to the stacked battery 10a in the stacking direction of the power generating element 21 by a restraining member (pressure member) (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0019] The negative electrode current collector 11′ and the positive electrode current collector 11″ are respectively attached with a negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (positive and negative electrodes), and are structured so as to be sandwiched between the ends of a laminate film 29 that is a battery outer casing material and led out of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11″ and the negative electrode current collector 11′ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode lead and a negative electrode lead (not shown) as necessary.

[0020] The main components of the stacked battery 10a described above will now be described.

[0021] [Positive electrode current collector] The positive electrode current collector is a conductive member that functions as a flow path for electrons that are released from the positive electrode toward the power source as the battery reaction (charge / discharge reaction) progresses, or that flow from an external load toward the positive electrode. There are no particular limitations on the material that constitutes the positive electrode current collector. For example, metals and conductive resins can be used as the material that constitutes the positive electrode current collector. There are no particular limitations on the thickness of the positive electrode current collector, but an example is 10 to 100 μm.

[0022] [Cathode active material layer] The positive electrode constituting the lithium secondary battery according to this embodiment has a positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions. The positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11″ as shown in FIG. 2.

[0023] The positive electrode active material is not particularly limited as long as it can release lithium ions during the charging process of the secondary battery and absorb lithium ions during the discharging process. An example of such a positive electrode active material is a material containing an M1 element and an O element, where the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. Examples of such a positive electrode active material include layered rock salt active materials such as LiCoO2, LiMnO2, LiNiO2, and Li(Ni-Mn-Co)O2; LiMn2O4; LiNi 0.5 Mn 1.5 Examples of oxide active materials include spinel-type active materials such as LiFePO4 and LiMnPO4, olivine-type active materials such as LiFeSiO4 and LiMnSiO4, and Si-containing active materials such as LiFeSiO4 and LiMnSiO4. 12 , LiVO2. In some cases, two or more types of positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used. In a preferred embodiment, the positive electrode active material layer 15 constituting the lithium secondary battery according to this embodiment contains, from the viewpoint of output characteristics, a layered rock salt type active material containing lithium and cobalt (for example, Li(Ni-Mn-Co)O2) as the positive electrode active material.

[0024] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 30 to 99 mass %, more preferably in the range of 40 to 90 mass %, and even more preferably in the range of 45 to 80 mass %.

[0025] In the lithium secondary battery according to this embodiment, the positive electrode active material layer preferably further contains a solid electrolyte. Examples of the solid electrolyte include a sulfide solid electrolyte, a resin solid electrolyte, and an oxide solid electrolyte. Note that, as the solid electrolyte, a material having a desired bulk modulus can be appropriately selected according to the degree of volume expansion of the electrode active material during charge and discharge.

[0026] In another preferred embodiment of the secondary battery according to this embodiment, from the viewpoint that the solid electrolyte exhibits excellent lithium ion conductivity and can better follow the volume change of the electrode active material during charge and discharge, it is preferably a sulfide solid electrolyte containing S element, more preferably a sulfide solid electrolyte containing Li element, M element and S element, and the M element is at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl and I, and even more preferably a sulfide solid electrolyte containing S element, Li element and P element. The sulfide solid electrolyte may have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS4, and Li3PS4. Examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include, for example, a Li-P-S-based solid electrolyte called LPS. Also, as the sulfide solid electrolyte, for example, LGPS represented by Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1) such as LGPS may be used. More specifically, for example, LPS (Li2S-P2S5), Li7P3S 11 、Li 3.2 P 0.96 S、Li 3.25 Ge 0.25 P 0.75 S4、Li 10 GeP2S 12, or Li6PS5X (wherein X is Cl, Br, or I). The term "Li2S-P2S5" refers to a sulfide solid electrolyte obtained using a raw material composition containing Li2S and P2S5, and the same applies to other terms. Among these, sulfide solid electrolytes are preferably LPS (Li2S-P2S5), Li6PS5X (wherein X is Cl, Br, or I), Li7P3S, etc., from the viewpoint that they have high ionic conductivity and a low bulk modulus and can therefore follow the volume change of the electrode active material during charge and discharge. 11 , Li 3.2 P 0.96 S and Li3PS4.

[0027] The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is preferably within the range of 1 to 70 mass %, more preferably within the range of 10 to 60 mass %, and even more preferably within the range of 15 to 55 mass %.

[0028] The positive electrode active material layer may further contain at least one of a conductive additive and a binder in addition to the positive electrode active material and the solid electrolyte. The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within a range of, for example, 0.1 to 1000 μm, and more preferably 40 to 150 μm.

[0029] [Solid electrolyte layer] The solid electrolyte layer is a layer interposed between the positive electrode and the negative electrode, and contains a solid electrolyte (usually as a main component). More specifically, the solid electrolyte layer is a layer interposed between the positive electrode active material layer and the functional layer. The specific form of the solid electrolyte contained in the solid electrolyte layer is the same as that described above, and therefore a detailed description thereof will be omitted here.

[0030] The content of the solid electrolyte in the solid electrolyte layer is, for example, preferably in the range of 10 to 100 mass%, more preferably in the range of 50 to 100 mass%, and even more preferably in the range of 90 to 100 mass%, relative to the total mass of the solid electrolyte layer. The solid electrolyte layer may further contain a binder in addition to the above-mentioned solid electrolyte. The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but is, for example, preferably in the range of 0.1 to 1000 μm, and more preferably 10 to 40 μm.

[0031] [Negative electrode current collector] The negative electrode current collector is a conductive member that functions as a flow path for electrons that are released from the negative electrode toward an external load as the battery reaction (charge / discharge reaction) progresses, or that flow from a power source toward the negative electrode. There are no particular limitations on the material that constitutes the negative electrode current collector. For example, metals and conductive resins can be used as materials for the negative electrode current collector. There are no particular limitations on the thickness of the negative electrode current collector, but an example is 10 to 100 μm.

[0032] [Negative electrode active material layer] The lithium secondary battery according to the present embodiment is a so-called lithium deposition type in which lithium metal is deposited on the negative electrode current collector during the charging process. The layer of lithium metal deposited on the negative electrode current collector during this charging process is the negative electrode active material layer of the lithium secondary battery according to the present embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. Although the negative electrode active material layer may not be present during full discharge, in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be present during full discharge.

[0033] [Feature Layer] In the lithium secondary battery according to the present embodiment, a functional layer is provided between the solid electrolyte layer and the negative electrode. The functional layer is a layer having electronic insulation and lithium ion conductivity. Furthermore, the functional layer must be more stable than the solid electrolyte with respect to reductive decomposition due to contact with lithium metal.

[0034] Here, "more stable than a solid electrolyte with respect to reductive decomposition upon contact with lithium metal" means that the tendency of the solid electrolyte constituting the solid electrolyte layer to undergo reductive decomposition upon contact with lithium metal is smaller than the tendency of the constituent material of the functional layer to undergo reductive decomposition upon contact with lithium metal. Whether the constituent material of the functional layer satisfies this condition can be determined by whether the current flowing through the functional layer is smaller than the current flowing through the solid electrolyte layer when a voltage is swept around 0 V [vs. Li / Li+] by cyclic voltammetry using each of the solid electrolyte layer and the functional layer as the working electrode and lithium metal as the counter electrode.

[0035] The presence of such a functional layer between the solid electrolyte layer and the negative electrode prevents contact between the lithium metal deposited on the surface of the negative electrode current collector and the solid electrolyte layer during charging, thereby suppressing degradation of the solid electrolyte layer due to reductive decomposition. Furthermore, the presence of the functional layer at this location also prevents dendrites from growing from the lithium metal side when cracks occur in the solid electrolyte layer. Whether or not a functional layer is present in the lithium secondary battery according to this embodiment can be determined, for example, by observing the cross section of the lithium secondary battery with SEM-EDX to determine whether a layer corresponding to the functional layer is present on the main surface of the solid electrolyte layer, and then analyzing its composition by elemental analysis or the like. Furthermore, if the above-described method is difficult to determine due to the thinness of the functional layer, it can also be determined by analyzing the layer corresponding to the functional layer while etching it using the XPS method.

[0036] The constituent materials of the functional layer described above are not particularly limited, and any material that satisfies the above conditions can be suitably used. For example, the functional layer preferably contains one or more materials selected from the group consisting of lithium oxide (LiO), lithium halides (lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), and lithium iodide (LiI)), lithium ion conductive polymers, composite metal oxides represented by Li-MO (where M is one or more metal elements selected from the group consisting of Mg, Au, Al, Sn, and Zn), and Li-Ba-TiO composite oxides. All of these materials are particularly stable against reductive decomposition upon contact with lithium metal, making them suitable as constituent materials for the functional layer. In particular, it is preferable for the functional layer to contain one or more materials selected from the group consisting of lithium oxide (LiO), lithium chloride (LiCl), lithium fluoride (LiF), lithium bromide (LiBr), and lithium iodide (LiI), because this can improve the rate characteristics of the battery. This is thought to be because the activation barrier when lithium ions diffuse through the solid electrolyte layer and functional layer during charging and discharging is lowered, improving the interfacial diffusion rate of lithium ions and ensuring sufficient contact area between the functional layer and the negative electrode active material layer (lithium metal layer).

[0037] The average thickness of the functional layer is not particularly limited, as long as it is disposed at a thickness that allows the above-mentioned functions to be exhibited. However, from the viewpoint of suppressing an increase in internal resistance, the average thickness of the functional layer is preferably smaller than the average thickness of the solid electrolyte layer. Furthermore, from the viewpoint of fully exhibiting the protective effect of the functional layer, the average thickness of the functional layer is preferably equal to or greater than a predetermined value. From these viewpoints, the average thickness of the functional layer is preferably 0.1 nm to 30 μm, more preferably 0.5 nm to 25 μm, even more preferably 0.5 nm to 20 μm, even more preferably 10 nm to 1000 nm, and most preferably 100 nm to 500 nm. The "average thickness" of the functional layer refers to the value calculated by cutting the functional layer constituting the lithium secondary battery along the stacking direction, observing the cross section of the functional layer with a scanning electron microscope (SEM), measuring the thickness at several to several tens of different locations, and calculating the arithmetic average value of the measured values.

[0038] [Positive and negative current collector plates] The material constituting the current collector plate is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate and the negative electrode current collector plate may be made of the same material or different materials.

[0039] [Positive and negative electrode leads] The current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive electrode and negative electrode leads. The portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting the product (e.g., automobile parts, particularly electronic devices).

[0040] [Battery exterior materials] As the battery exterior material, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film that can cover the power generating element can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it can easily adjust the collective pressure applied to the power generating element from the outside.

[0041] [Method of manufacturing lithium secondary batteries] (Preparation of lithium secondary battery precursor) The lithium secondary battery produced by the production method according to the present embodiment has undergone a charging step. Herein, the structure to which the charging step is applied is referred to as a "lithium secondary battery precursor." This "lithium secondary battery precursor" has the same configuration as the lithium secondary battery produced by the production method according to the present embodiment (specifically, it essentially has the above-described positive electrode current collector, positive electrode active material layer, solid electrolyte layer, functional layer, and negative electrode current collector). Furthermore, this "lithium secondary battery precursor" can also be said to be in a state in which the charging step described below has not yet been performed.

[0042] The method for producing the lithium secondary battery precursor is not particularly limited, and can be, for example, produced according to the following method. First, a powder composition (cathode mixture) containing a cathode active material, and optionally a solid electrolyte, a binder, and a conductive additive, is prepared. Next, this powder composition is rolled using a roll press to produce a cathode active material layer. The cathode active material layer and a cathode current collector are stacked and pressed to produce a cathode. Next, a solid electrolyte slurry is prepared by mixing the solid electrolyte with a solvent, and the solid electrolyte slurry is applied to the surface of the support and dried to produce a solid electrolyte layer as a free-standing film. Then, a functional layer is formed on one side of the obtained solid electrolyte layer by a method such as sputtering. A solid electrolyte layer with the functional layer formed similarly as above is stacked on the cathode active material layer side of the obtained positive electrode so that the exposed surface of the solid electrolyte layer faces the cathode active material layer, and then pressed. Next, a negative electrode current collector is stacked on the exposed surface of the functional layer to produce a lithium secondary battery precursor.

[0043] (charging process) The manufacturing method of the lithium secondary battery according to this embodiment is characterized by performing a first charging step and a second charging step on a lithium secondary battery precursor having the configuration described above. The first charging step is a step of depositing lithium metal until the thickness becomes 90% or more of the thickness of the functional layer by charging at the first charging rate. The second charging step is a step of charging the lithium secondary battery precursor that has undergone the first charging step at the second charging rate. The first and second charging rates are determined so as to satisfy C1 < C2, where the maximum value of the first charging rate is C1 and the minimum value of the second charging rate is C2. By having these two charging steps, the manufacturing method of the lithium secondary battery according to the present invention can improve the discharge capacity of the lithium secondary battery. Although the mechanism by which such an effect is achieved has not been fully clarified, the following is presumed. First, as charging progresses, lithium metal is deposited from the functional layer toward the negative electrode current collector. That is, the layer of lithium metal deposited in the first charging step (hereinafter, the first lithium metal layer) will be located closer to the functional layer side than the layer of lithium metal deposited in the second charging step (hereinafter, the second lithium metal layer).

[0044] Here, by charging using a first charge rate with a relatively low current value in the first charging step, a first lithium metal layer with a uniform thickness is deposited. Then, in this first charging step, lithium metal is deposited until the thickness of this uniform first lithium metal layer is 90% or more of the thickness of the functional layer. Thereafter, in the second charging step, charging is performed using a second charge rate with a relatively high current value. At this time, since the first lithium metal layer with a uniform thickness is present on the surface facing the functional layer, when the thickness of the lithium metal layer increases and compresses the functional layer, large local pressure is not applied to the functional layer. As a result, cracking of the functional layer can be prevented. Furthermore, as described above, the first lithium metal layer has a thickness of 90% or more of the thickness of the functional layer. Therefore, even if the thickness of the second lithium metal layer increases and pressure is applied to other layers in the second charging step, the first lithium metal layer protects the functional layer from that pressure. As a result, cracking of the functional layer can also be prevented. Preventing cracking of the functional layer as described above can prevent reductive decomposition of the solid electrolyte layer due to contact between the lithium metal and the solid electrolyte layer. Furthermore, by preventing cracks in the functional layer, it is also possible to prevent short circuits caused by dendrites that form on the lithium metal side.

[0045] (1st charging process) The first charging step is a step of charging the lithium secondary battery precursor having the configuration described above at a first charging rate, thereby depositing lithium metal to a thickness that is 90% or more of the thickness of the functional layer. In this specification, the thickness of the lithium metal is calculated by the following method. First, the negative electrode active material layer, which is the layer in which lithium is deposited, is cut along the stacking direction of the battery. Next, the cross section of the negative electrode active material layer is observed with a scanning electron microscope (SEM), and the thickness is measured at several to several tens of different locations. The average value of these thicknesses is then calculated and used as the thickness of the lithium metal.

[0046] The thickness of the lithium metal deposited in the first charging step is at least 90% of the thickness of the functional layer, preferably 92% to 120%, more preferably 94% to 110%, and preferably substantially the same as the thickness of the functional layer. Here, "substantially the same" means that the thickness of the lithium metal is at least 95% to 105% of the thickness of the functional layer. By having the lithium metal thickness substantially the same as the thickness of the functional layer, cracking of the functional layer can be prevented, thereby improving discharge capacity, and charging time can be shortened, thereby improving production efficiency. Furthermore, the thickness of the lithium metal deposited in the first charging step is more preferably at least 97% to 103%, even more preferably at least 99% to 101%, and most preferably 100% of the thickness of the functional layer.

[0047] The first charge rate in the first charging step is not particularly limited as long as its maximum value C1 is smaller than the minimum value C2 of the second charge rate in the second charging step. Furthermore, the first charge rate in the first charging step may be constant or variable. From the viewpoint of shortening the charging time in the manufacturing process, it is preferable that the first charge rate in the first charging step is constant (i.e., the first charge rate remains constant at C1). Furthermore, the maximum value C1 of the first charge rate is preferably 0.03 [C] or less, and more preferably 0.01 [C] or less. By setting the maximum value C1 of the first charge rate within the above range, lithium metal is deposited in a more uniform state. Furthermore, the lower limit of the maximum value C1 of the first charge rate is not particularly limited, but is preferably 0.0001 [C] or more, and more preferably 0.0005 [C] or more. By setting the lower limit of the maximum value C1 of the first charge rate within the above range, the charging time in the first charging step can be made suitable for manufacturing lithium secondary batteries. Note that 1 C is the current value at which the battery reaches a fully charged or fully discharged state when charged from a fully discharged state or discharged from a fully charged state for 1 hour at that current value. In other words, the maximum value C1 of the first charge rate is preferably 0.0001 C or more and 0.03 C or less, and more preferably 0.0005 C or more and 0.01 C or less.

[0048] (2nd charging process) The second charging step is a step of charging the lithium secondary battery precursor that has been through the first charging step at a second charging rate that is higher than the first charging rate.

[0049] The second charge rate in the second charging step is not particularly limited as long as its minimum value C2 is greater than the maximum value C1 of the first charge rate in the first charging step. Furthermore, the second charge rate in the second charging step may be constant or variable. From the viewpoint of shortening the charging time in the manufacturing process, it is preferable that the second charge rate in the second charging step is constant (i.e., the second charge rate remains constant at C2). Furthermore, the minimum value C2 of the second charge rate is preferably greater than 0.03 [C], and more preferably 0.04 [C] or greater. Furthermore, the upper limit of the minimum value C2 of the second charge rate is not particularly limited, but is preferably 0.5 [C] or less, and more preferably 0.1 [C] or less. By keeping the minimum value C2 of the second charge rate within the above range, lithium metal can be deposited in a more uniform state while maintaining a sufficient charging speed. That is, the minimum value C2 of the second charging rate is preferably greater than 0.03 [C] and equal to or less than 0.5 [C], and more preferably greater than or equal to 0.04 [C] and equal to or less than 0.1 [C].

[0050] (Other charging processes) The method for producing a lithium secondary battery according to one embodiment of the present invention may include other charging steps in addition to the first and second charging steps. For example, charging step A may be included between the first and second charging steps, or charging step B may be included after the second charging step.

[0051] In the charging step A, a charge rate smaller than the first charge rate may be used, or a charge rate larger than the first charge rate and smaller than the second charge rate may be used, or a charge rate larger than the second charge rate may be used. However, from the viewpoint of uniformity of lithium metal deposition and production efficiency of lithium secondary batteries, it is preferable to use a charge rate equal to or larger than the first charge rate and equal to or smaller than the second charge rate. In the charging step B, a charge rate smaller than the first charge rate may be used, or a charge rate equal to or larger than the first charge rate and equal to or larger than the second charge rate may be used. However, from the viewpoint of production efficiency of lithium secondary batteries, it is preferable to use a charge rate equal to or larger than the first charge rate and equal to or smaller than the second charge rate, or a charge rate larger than the second charge rate.

[0052] However, from the viewpoint of depositing lithium metal in a more uniform state and achieving a sufficient charging rate in the production of a lithium secondary battery, it is preferable that the charging process be composed of two steps: a first charging step and a second charging step. In other words, it is preferable that charging is started in the first charging step, and the first charging step is terminated when lithium metal has been deposited to a thickness that is 90% or more of the thickness of the functional layer, and then the second charging step is initiated and terminated when the battery is fully charged. A charging rest period (interval) may be provided between each charging step. More preferably, the initial charging process performed on a lithium secondary battery precursor that has the same configuration as a lithium secondary battery and is in an uncharged state consists only of a first charging step in which lithium metal is deposited at a first charging rate until it reaches the same thickness as the functional layer, and a second charging step in which the lithium secondary battery precursor that has undergone the first charging step (i.e., in a state in which lithium metal has been deposited to a thickness that is the same as the thickness of the functional layer) is charged at a second charging rate, and the first charging rate in the first charging step is constant, and the second charging rate in the second charging step is constant.

[0053] (discharge process) The method for producing a lithium secondary battery according to one embodiment may further include a discharging step of discharging the lithium secondary battery that has been subjected to the second charging step. In this discharging step, it is preferable to discharge the battery so that the thickness of the lithium metal after discharge is not thinner than the thickness of the lithium metal deposited in the first charging step. By discharging within this range, it is possible to maintain uniformity in the deposition of lithium metal when the battery is charged again, and it is possible to prevent cracking of the functional layer, dendrite growth, and reductive decomposition of the solid electrolyte layer.

[0054] The above-described charging and discharging steps may be performed while applying a restraining pressure to the cell in the stacking direction using a pressure member. Charging and discharging while applying pressure results in a more uniform thickness of the deposited lithium metal. FIGS. 3 and 4 show an example of a power generating element equipped with a pressure member. A power generating element 100 equipped with a pressure member includes the power generating element 21 sealed in the laminate film 29 shown in FIG. 1 , two metal plates 200 sandwiching the power generating element 21 sealed in the laminate film 29, and bolts 300 and nuts 400 as fastening members. These fastening members (bolts 300 and nuts 400) function to secure the power generating element 21 sealed in the laminate film 29 while sandwiching it between the metal plates 200. Thus, the metal plates 200 and the fastening members (bolts 300 and nuts 400) function as pressure members that pressurize (restrain) the power generating element 21 in the stacking direction. The pressure members are not particularly limited as long as they can pressurize the power generating element 21 in the stacking direction. The pressure member typically uses a combination of a plate made of a rigid material such as the metal plate 200 and the above-mentioned fastening member. The fastening member may also be a tension plate that fixes the end of the metal plate 200 so as to restrain the power generating element 21 in the stacking direction, in addition to the bolts 300 and nuts 400.

[0055] The lower limit of the load applied to the power generating element 21 (confining pressure in the stacking direction of the power generating element) is, for example, 0.05 MPa or more, preferably 0.1 MPa or more, more preferably 0.5 MPa or more, and even more preferably 1 MPa or more. The upper limit of the confining pressure in the stacking direction of the power generating element is, for example, 10 MPa or less, preferably 7 MPa or less, more preferably 5 MPa or less, and even more preferably 4 MPa or less. That is, the confining pressure in the stacking direction of the power generating element is, for example, 0.05 MPa to 10 MPa, preferably 0.1 MPa to 7 MPa, more preferably 0.5 MPa to 5 MPa, and even more preferably 1 MPa to 4 MPa. When the confining pressure in the stacking direction of the power generating element is within the above range, lithium deposition becomes more sufficiently uniform, and cracking of each layer (particularly cracking of the functional layer) due to the confining pressure can be prevented.

[0056] (Discharge method) Another aspect of the present invention is a method for discharging the lithium secondary battery according to the above-described aspect of the present invention. This discharging method involves discharging the battery so that the thickness of the lithium metal after discharge is not thinner than the thickness of the lithium metal deposited in the first charging step. Discharging using this method makes it possible to maintain uniformity in the deposition of lithium metal when the battery is recharged, preventing cracking of the functional layer, and preventing dendrite growth and reductive decomposition of the solid electrolyte layer.

[0057] While one embodiment of the present invention has been described above, the present invention is not limited to the configurations described in the above embodiments and can be modified as appropriate based on the claims. The following embodiments also fall within the scope of the present invention: a method for manufacturing a lithium secondary battery as defined in claim 1 having the characteristics of claim 2; a method for manufacturing a lithium secondary battery as defined in claim 1 or claim 2 having the characteristics of claim 3; a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 3 having the characteristics of claim 4; a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 4 having the characteristics of claim 5; a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 5 having the characteristics of claim 6; a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 6 having the characteristics of claim 7; a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 7 having the characteristics of claim 8; a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 8 having the characteristics of claim 9; and a method for manufacturing a lithium secondary battery as defined in any one of claims 1 to 9 having the characteristics of claim 10. [Example]

[0058] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0059] <Example of evaluation cell production> [Preparation of Evaluation Cell for Example 1] (Preparation of positive electrode) The positive electrode active material is NMC composite oxide (LiNi 0.8 Mn 0.1 Co 0.1O2), a lithium ion conductive sulfide solid electrolyte (LPS (Li2S-P2S5)), acetylene black as a conductive additive, and styrene-butadiene rubber (SBR) as a binder were prepared. In a glove box with an argon atmosphere at a dew point of -68°C or below, the NMC composite oxide, solid electrolyte, binder, and conductive additive were weighed out to a mass ratio of 78.8:15.3:2.9:3.0, mixed in an agate mortar, and then further mixed and stirred in a planetary ball mill to obtain a powder composition (cathode mixture).

[0060] Next, the powder composition (positive electrode mixture) obtained above was supplied to a powder inlet set in a roll press. Then, the powder composition was rolled using the roll press (conditions are shown below) to form the powder composition into a sheet. Next, the sheet-shaped powder composition was folded in half and compressed using the roll press. This rolling process was repeated until the sheet thickness reached 100 μm, thereby producing a positive electrode active material layer.

[0061] (Roll press machine conditions) Roll size: 250mmφ×400mm Roll rotation speed: 1m / min Pressure: 10kN (Linear pressure: 25kN / m).

[0062] Next, the positive electrode active material layer and aluminum foil (thickness: 12 μm) serving as a positive electrode current collector were stacked together, and a press treatment was carried out to prepare a positive electrode.

[0063] A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene-butadiene rubber (SBR) to 100 parts by mass of sulfide solid electrolyte (LPS (Li2S-P2S5)) and mesitylene as a solvent. The solid electrolyte slurry prepared above was then applied to the surface of a stainless steel foil support and dried to obtain a solid electrolyte layer (30 μm thick) as a free-standing film. A functional layer (250 nm thick) made of lithium chloride (LiCl) was then formed on the entire surface of one side of the obtained solid electrolyte layer by sputtering.

[0064] (Assembly of cell precursor for evaluation) The solid electrolyte layer with the functional layer formed similarly to the above was transferred to the positive electrode active material layer side of the positive electrode prepared above by cold isostatic pressing (CIP) so that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer. Finally, a stainless steel foil (thickness 10 μm) was laminated as a negative electrode current collector on the exposed surface of the functional layer to assemble a precursor for an evaluation cell (lithium deposition type lithium secondary battery).

[0065] (Charging of the evaluation cell precursor) A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector, respectively, of the evaluation cell precursor of Example 1 prepared above, and charging was performed by a first charging step and a second charging step. First, in the first charging step, the first charging rate was set to 0.01 C, and charging was performed until the thickness of lithium metal deposited on the negative electrode current collector reached 250 nm. The lithium secondary battery precursor that had undergone the first charging step was cut along the stacking direction, and the cross section of the negative electrode active material layer, which was the layer where lithium was deposited, was observed with a scanning electron microscope (SEM). The thickness was measured at 10 different locations, and the arithmetic average value of these measurements was calculated.

[0066] Next, the lithium secondary battery precursor that had undergone the first charging step was charged in a second charging step at a second charging rate of 0.05 C and an upper limit voltage of 4.3 V until it reached a fully charged (100% charged) state, to obtain an evaluation cell for Example 1. In the first charging step and the second charging step, charging was performed while applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member.

[0067] [Preparation of Evaluation Cell for Example 2] An evaluation cell of Example 2 was produced in the same manner as Example 1, except that no restraining pressure was applied in the first charging step and the second charging step.

[0068] [Preparation of Evaluation Cell for Example 3] An evaluation cell of Example 3 was produced in the same manner as Example 1, except that the restraining pressure applied in the first charging step and the second charging step was 0.1 MPa.

[0069] [Evaluation cell for Example 4] The evaluation cell for Example 4 was prepared in the same manner as Example 1, except that the thickness of the functional layer was set to 5000 nm and charging was carried out until the thickness of the lithium metal deposited on the negative electrode current collector in the first charging step reached 5000 nm.

[0070] [Evaluation cell for Example 5] An evaluation cell of Example 5 was produced in the same manner as in Example 1, except that the thickness of the lithium metal deposited on the negative electrode current collector in the first charging step was set to 500 nm.

[0071] [Evaluation cell for Example 6] An evaluation cell for Example 6 was produced in the same manner as in Example 1, except that lithium fluoride (LiF) was used as the functional layer.

[0072] [Evaluation cell for Example 7] An evaluation cell of Example 7 was produced in the same manner as in Example 1, except that the first charge rate in the first charge step was set to 0.03 C.

[0073] [Evaluation Cell of Example 8] The evaluation cell of Example 8 was produced in the same manner as in Example 1, except that lithium oxide (LiO) was used as the functional layer, the thickness of the functional layer was set to 0.3 nm, and the thickness of the lithium metal deposited on the negative electrode current collector in the first charging step was set to 0.3 nm.

[0074] [Evaluation Cell of Example 9] The evaluation cell of Example 9 was prepared in the same manner as Example 1, except that the thickness of the functional layer was 25,000 nm and the thickness of the lithium metal deposited on the negative electrode current collector in the first charging step was 25,000 nm.

[0075] [Evaluation cell for Comparative Example 1] An evaluation cell of Comparative Example 1 was produced in the same manner as in Example 1, except that the thickness of the lithium metal deposited on the negative electrode current collector in the first charging step was set to 125 nm.

[0076] [Evaluation cell for Comparative Example 2] An evaluation cell of Comparative Example 2 was produced in the same manner as in Example 1, except that the first charging step was not carried out and charging was carried out only in the second charging step.

[0077] [Evaluation cell for Comparative Example 3] An evaluation cell of Comparative Example 3 was produced in the same manner as in Example 1, except that no functional layer was provided.

[0078] <Evaluation of discharge capacity> The discharge capacity of the evaluation cells prepared in each of the above examples and comparative examples was evaluated by carrying out the following discharge process. Measurements were carried out using a charge / discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C.

[0079] In the discharging step, constant current (CC) discharge was performed at a current value corresponding to 0.05 C with a lower limit voltage of 2.5 V. The capacity (discharge capacity) was measured during the discharge process and normalized by the mass of the positive electrode active material used in each evaluation cell to calculate the discharge capacity per mass of the active material. The percentage (discharge capacity retention rate (%)) of the discharge capacity per mass of the active material calculated in this manner relative to the theoretical discharge capacity was calculated and used as an evaluation index for the discharge capacity. The results are shown in Table 1 below. In the discharging step, charging was performed while applying a restraining pressure in the stacking direction of the evaluation cell using a pressure member. The applied pressure was 3 MPa in Examples 1, 4 to 9, and Comparative Examples 1 to 3, and 0.1 MPa in Example 3. No pressure was applied in Example 2.

[0080] [Table 1]

[0081] The results shown in Table 1 indicate that the discharge capacity of the lithium secondary battery manufactured by the manufacturing method of the present invention was improved. Furthermore, in the Example in which lithium metal was deposited to a thickness equivalent to that of the functional layer in the first charging step, the manufacturing time of the lithium secondary battery was shorter than in Example 5 in which lithium metal was deposited to a thickness twice that of the functional layer. [Explanation of symbols]

[0082] 10a stacked battery, 11' negative electrode current collector, 11” positive electrode current collector, 12 functional layers, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 cell layer, 21 power generation elements, 25 negative current collector plate, 27 positive current collector plate, 29 Laminating film, 100 Power generating element equipped with a pressure member; 200 metal plate, 300 volts, 400 Nuts.

Claims

1. a positive electrode including a positive electrode active material layer disposed on a surface of a positive electrode current collector, the positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions; a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; a functional layer interposed between the solid electrolyte layer and the negative electrode, the functional layer having electronic insulation and lithium ion conductivity, and being more stable than the solid electrolyte with respect to reductive decomposition due to contact with lithium metal, a first charging step of charging a lithium secondary battery precursor in an uncharged state having the same configuration as the lithium secondary battery at a first charging rate to deposit the lithium metal to a thickness of 90% or more of the functional layer; a second charging step of charging the lithium secondary battery precursor that has been subjected to the first charging step at a second charging rate, At this time, the maximum value of the first charging rate is C １ and the minimum value of the second charging rate is C ２ When this is done, C １ <C ２ A method for manufacturing a lithium secondary battery that satisfies the above requirements.

2. The method for producing a lithium secondary battery according to claim 1 , wherein the first charging step and the second charging step are performed while the lithium secondary battery precursor is pressurized in the stacking direction at a pressure of 0.1 MPa or more.

3. 2. The method for producing a lithium secondary battery according to claim 1, wherein the first charging step and the second charging step are performed while pressurizing the lithium secondary battery precursor in the stacking direction at a pressure of 0.5 MPa or more and 5 MPa or less.

4. The maximum value C of the first charging rate １ The method for producing a lithium secondary battery according to claim 1 or 2, wherein the current is 0.03 C or less.

5. The maximum value C of the first charging rate １ The method for producing a lithium secondary battery according to claim 1 or 2, wherein

6. 3. The method for producing a lithium secondary battery according to claim 1, wherein the thickness of the lithium metal deposited in the first charging step is substantially the same as the thickness of the functional layer.

7. 3. The method for producing a lithium secondary battery according to claim 1, wherein the average thickness of the functional layer is 0.5 nm to 20.0 μm.

8. The functional layer is made of lithium oxide (Li ２ 3. The method for producing a lithium secondary battery according to claim 1, wherein the lithium secondary battery contains one or more selected from the group consisting of lithium iodide (LiO), lithium chloride (LiCl), lithium fluoride (LiF), lithium bromide (LiBr), and lithium iodide (LiI).

9. an initial charging step performed on a lithium secondary battery precursor having the same configuration as the lithium secondary battery and in an uncharged state, the initial charging step being a first charging step of charging at a first charge rate to deposit the lithium metal until the lithium metal has a thickness equal to a thickness of the functional layer; a second charging step of charging the lithium secondary battery precursor that has been subjected to the first charging step at a second charging rate, a first charging rate in the first charging step is constant; and 3. The method for producing a lithium secondary battery according to claim 1, wherein the second charging rate in the second charging step is constant.

10. 3. The method for producing a lithium secondary battery according to claim 1, further comprising a discharging step of discharging the lithium secondary battery precursor that has been subjected to the second charging step, wherein the discharging step is performed so that the thickness of the lithium metal after discharge is not thinner than the thickness of the lithium metal deposited in the first charging step.

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