Secondary battery negative electrode and method for producing secondary battery negative electrode

Abstract

One embodiment of the present disclosure provides a secondary battery negative electrode comprising a negative electrode material layer that contains Si and a dopant, wherein the negative electrode material layer contains a surface region and an interior region having different dopant concentrations from each other and the dopant concentration in the surface region is higher than the dopant concentration in the interior region.

Description

Negative electrode for secondary battery and method for manufacturing the same

[0001] The present disclosure relates to a negative electrode for a secondary battery and a method for manufacturing a negative electrode for a secondary battery.

[0002] Secondary batteries, which can be repeatedly charged and discharged, have been used for various purposes. For example, secondary batteries are used as power sources for electronic devices such as smartphones and laptop computers.

[0003] A secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, and an electrolyte are housed in an exterior body. The electrodes include a current collector and an electrode material layer provided on at least one main surface of the current collector. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode material layer provided on at least one main surface of the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode material layer provided on at least one main surface of the negative electrode current collector.

[0004] JP 2016-21320 A JP 2015-128075 A JP 2010-192444 A

[0005] In recent years, in order to increase the capacity of batteries, a negative electrode material layer containing Si has been used in some secondary batteries. Furthermore, because Si has a relatively high electrical resistance, a negative electrode material layer containing a dopant in addition to Si may be used to reduce the resistance. In this regard, in order to effectively prevent a decrease in the charge / discharge characteristics of the battery, it is desirable to reduce the interfacial resistance between the electrolyte and the negative electrode material layer so that ions can easily diffuse uniformly from the electrolyte to the Si in the negative electrode material layer during the initial charge / discharge cycle.

[0006] The present disclosure has been devised in view of the above circumstances. Specifically, an object of the present disclosure is to provide a negative electrode for a secondary battery including a negative electrode material layer that can reduce the interfacial resistance with an electrolyte solution, and a method for manufacturing the same.

[0007] In order to achieve the above object, one embodiment of the present disclosure provides a negative electrode for a secondary battery, comprising a negative electrode material layer containing Si and a dopant, the negative electrode material layer including a surface region and an internal region having mutually different dopant concentrations, and the dopant concentration in the surface region is higher than the dopant concentration in the internal region.

[0008] In order to achieve the above object, one embodiment of the present disclosure provides a method for producing a negative electrode for a secondary battery, the method including: forming a Si-containing member containing Si and a dopant on a metal foil as a current collector; and applying a solution containing the dopant onto the Si-containing member, followed by heat treatment.

[0009] According to one embodiment of the present disclosure, it is possible to provide a negative electrode for a secondary battery including a negative electrode material layer that can reduce the interfacial resistance with the electrolyte, thereby making it possible to preferably suppress deterioration in the charge-discharge characteristics of the secondary battery.

[0010] 6A is a cross-sectional view schematically showing a negative electrode for a secondary battery according to one embodiment of the present disclosure; FIG. 6B is a cross-sectional view schematically showing a negative electrode for a secondary battery according to another embodiment of the present disclosure; FIG. 6C is a graph showing the solubility limit of impurities (dopants); FIG. 6D is a graph showing the diffusion distance of dopants at predetermined heat treatment temperatures and heat treatment times; FIG. 6E is a graph showing the relationship between the depth from the negative electrode surface and the dopant (Sb) concentration; FIG. 6F is a graph showing the difference in behavior of charge curves when charging at 0.01 C when a reference sample and samples 1 to 4 are used as negative electrodes; FIG. 6G is a graph showing an enlargement of the charge curve of the portion enclosed in dotted parentheses in FIG. 6A (at the initial stage of charge); FIG. 6G is a photograph showing the thickness of the negative electrode (negative electrode material layer) before and after charge; FIG. 6H is a photograph showing the presence or absence of cracks on the surface of the negative electrode at the initial stage of charge (left: comparative example / right: example); FIG. 6I is a graph showing the difference in behavior of charge curves when charging at 0.01 C when a reference sample and samples A to E are used as negative electrodes; FIG. 6I is a cross-sectional view schematically showing the basic configuration of an electrode constituent layer.

[0011] Hereinafter, a negative electrode for a secondary battery according to an embodiment of the present disclosure will be described in detail with reference to the drawings. The various elements in the drawings are merely shown schematically and exemplarily to facilitate understanding of the present disclosure, and the appearance, dimensional ratio, etc. may differ from the actual objects.

[0012] Before specifically describing the negative electrode for a secondary battery according to an embodiment of the present disclosure, the basic structure of a secondary battery will be described. As used herein, the term "secondary battery" refers to a battery capable of repeated charging and discharging. The term "secondary battery" is not limited to the name itself and may also encompass, for example, "energy storage devices." As used herein, "cross-sectional view" refers to a state when viewed from a direction substantially perpendicular to the thickness direction of the electrode material constituting the secondary battery, based on the stacking direction of the electrode material. The terms "vertical direction" and "horizontal direction" used directly or indirectly in this specification correspond to the vertical direction and horizontal direction in the drawings, respectively. Unless otherwise specified, the same reference numerals or symbols refer to the same components or parts or have the same meaning. In a preferred embodiment, the vertical downward direction (i.e., the direction in which gravity acts) can be considered to correspond to the "downward direction," and the opposite direction can be considered to correspond to the "upward direction."

[0013] The various numerical ranges mentioned in this specification are intended to include both the lower and upper limits. For example, a numerical range such as 1 to 10 can be interpreted as including the lower limit of 1 and the upper limit of 10.

[0014] [Basic Structure of Secondary Battery] A secondary battery has a structure in which an electrode assembly and an electrolyte are housed and sealed inside an exterior housing. The electrode assembly may include a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes. The electrode assembly may be a stacked electrode assembly or a wound (jelly roll) electrode assembly. A stacked electrode assembly is formed by stacking multiple electrode constituent layers, each including a positive electrode, a negative electrode, and a separator. A wound electrode assembly is formed by winding an electrode constituent layer, each including a positive electrode, a negative electrode, and a separator. Furthermore, for example, the electrode assembly may have a so-called stack-and-fold structure in which a positive electrode, a separator, and a negative electrode are stacked on a long film and then folded.

[0015] The positive electrode 10A is composed of at least a positive electrode current collector 11A and a positive electrode material layer 12A (see FIG. 10 ), with the positive electrode material layer 12A provided on at least one side of the positive electrode current collector 11A. A positive electrode side lead tab is located at a portion of the positive electrode current collector 11A where the positive electrode material layer 12A is not provided, i.e., at an end of the positive electrode current collector 11A. The positive electrode material layer 12A contains a positive electrode active material as an electrode active material. The negative electrode 10B is composed of at least a negative electrode current collector 11B and a negative electrode material layer 12B (see FIG. 10 ), with the negative electrode material layer 12B provided on at least one side of the negative electrode current collector 11B. A negative electrode side lead tab is located at a portion of the negative electrode current collector 11B where the negative electrode material layer 12B is not provided, i.e., at an end of the negative electrode current collector 11B. The negative electrode material layer 12B contains a negative electrode active material as an electrode active material.

[0016] The positive electrode active material contained in the positive electrode layer 12A and the negative electrode active material contained in the negative electrode layer 12B are materials directly involved in the transfer of electrons in the secondary battery and are the main materials of the positive and negative electrodes responsible for charge and discharge, i.e., the battery reaction. More specifically, the "positive electrode active material contained in the positive electrode layer 12A" and the "negative electrode active material contained in the negative electrode layer 12B" provide ions to the electrolyte, and these ions move between the positive electrode 10A and the negative electrode 10B, transferring electrons and causing charge and discharge. The positive electrode layer 12A and the negative electrode layer 12B are preferably layers capable of absorbing and releasing lithium ions. In other words, a secondary battery in which lithium ions move between the positive electrode 10A and the negative electrode 10B via the electrolyte to charge and discharge the battery is preferred. When lithium ions are involved in charge and discharge, the secondary battery corresponds to a so-called "lithium ion battery."

[0017] The positive electrode active material of the positive electrode layer 12A is, for example, made of granules, and preferably contains a binder to ensure sufficient contact between the particles and maintain their shape. Furthermore, the positive electrode layer 12A may contain a conductive additive to facilitate the transfer of electrons that drive the battery reaction. Similarly, the negative electrode active material of the negative electrode layer 12B is, for example, made of granules, and preferably contains a binder to ensure sufficient contact between the particles and maintain their shape. The negative electrode layer 12B may contain a conductive additive to facilitate the transfer of electrons that drive the battery reaction. Because they contain multiple components, the positive electrode layer 12A and the negative electrode layer 12B may also be referred to as a "positive electrode composite layer" and a "negative electrode composite layer," respectively.

[0018] The positive electrode active material is preferably a material that contributes to the absorption and desorption of lithium ions. From this perspective, the positive electrode active material is preferably, for example, a lithium-containing composite oxide. More specifically, the positive electrode active material is preferably a lithium transition metal composite oxide containing lithium and at least one transition metal selected from the group consisting of cobalt, nickel, manganese, and iron. In other words, such a lithium transition metal composite oxide is preferably contained as the positive electrode active material in the positive electrode layer 12A of the secondary battery. For example, the positive electrode active material may be lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, or a material in which part of the transition metal is replaced with another metal. Such positive electrode active materials may be contained alone or in combination of two or more types. In a more preferred embodiment, the positive electrode active material contained in the positive electrode layer 12A is lithium cobalt oxide.

[0019] The binder that can be contained in the positive electrode layer 12A is not particularly limited, but can include at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, etc. The conductive additive that can be contained in the positive electrode layer 12A is not particularly limited, but can include at least one selected from the group consisting of carbon black such as thermal black, furnace black, channel black, ketjen black, and acetylene black; carbon fibers such as graphite, carbon nanotubes, and vapor-grown carbon fibers; metal powders such as copper, nickel, aluminum, and silver; and polyphenylene derivatives. For example, the binder of the positive electrode layer 12A can be polyvinylidene fluoride. By way of example only, the conductive additive of the positive electrode layer 12A is carbon black. Furthermore, the binder and conductive additive of the positive electrode layer 12A may be a combination of polyvinylidene fluoride and carbon black.

[0020] The negative electrode active material is preferably a material that contributes to the absorption and release of lithium ions. In the present disclosure, the negative electrode active material may contain a Group 14 metal or metalloid as a main component. For example, at least one selected from the group consisting of Si, Ge, and Sn can be used. When a Group 14 metal or metalloid is used, stress generated by volume expansion during battery charge and discharge can be easily absorbed, thereby improving the capacity retention rate of the battery. In addition, graphite (natural graphite, artificial graphite), soft carbon, hard carbon, diamond-like carbon, etc. can also be used as the negative electrode active material.

[0021] The binder that can be contained in the negative electrode material layer 12B is not particularly limited, but can include at least one selected from the group consisting of styrene-butadiene rubber, polyacrylic acid, polyvinylidene fluoride, polyimide resin, and polyamide-imide resin. For example, the binder contained in the negative electrode material layer 12B can be styrene-butadiene rubber. The conductive additive that can be contained in the negative electrode material layer 12B is not particularly limited, but can include at least one selected from carbon black such as thermal black, furnace black, channel black, ketjen black, and acetylene black; carbon fiber such as graphite, carbon nanotubes, and vapor-grown carbon fiber; metal powder such as copper, nickel, aluminum, and silver; and polyphenylene derivatives. The negative electrode material layer 12B may also include a component derived from a thickener component (e.g., carboxymethyl cellulose) used during battery manufacture.

[0022] The positive electrode current collector 11A and the negative electrode current collector 11B used in the positive electrode 10A and the negative electrode 10B are components that contribute to collecting and supplying electrons generated in the active material due to the battery reaction. Such current collectors may be sheet-like metal members and may be porous or perforated. For example, the current collectors may be metal foil, punched metal, mesh, expanded metal, or the like. The positive electrode current collector 11A used in the positive electrode 10A is preferably made of a metal foil containing at least one selected from the group consisting of aluminum, stainless steel, nickel, and the like, and may be, for example, aluminum foil. On the other hand, the negative electrode current collector 11B used in the negative electrode 10B is preferably made of a metal foil containing at least one selected from the group consisting of copper, stainless steel, nickel, and the like, and may be, for example, copper foil.

[0023] The separator is a component provided from the viewpoint of preventing short circuits due to contact between the positive and negative electrodes and maintaining electrolyte retention. In other words, the separator is a component that allows ions to pass through while preventing electronic contact between the positive electrode 10A and the negative electrode 10B. Preferably, the separator is a porous or microporous insulating component, and has a membrane form due to its small thickness. Although merely exemplary, a microporous membrane made of polyolefin may be used as the separator. In this regard, the microporous membrane used as the separator may contain, for example, only polyethylene (PE) or only polypropylene (PP) as the polyolefin. Furthermore, the separator may be a laminate composed of a "PE microporous membrane" and a "PP microporous membrane." The surface of the separator may be covered with an inorganic particle coating layer and / or an adhesive layer, etc. The surface of the separator may have adhesive properties.

[0024] The separator should not be limited to a specific name, and may be a solid electrolyte, a gel electrolyte, insulating inorganic particles, or the like, which have similar functions. From the perspective of further improving the ease of handling of the electrodes, it is preferable that the separator and the electrode (positive electrode 10A / negative electrode 10B) are bonded together. Bonding between the separator and the electrode can be achieved by using an adhesive separator, applying an adhesive binder to the electrode material layer (positive electrode material layer 12A / negative electrode material layer 12B) and / or by thermocompression bonding. Examples of adhesive binder materials that provide adhesiveness to the separator or electrode material layer include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene polymer, and acrylic resin. The thickness of the adhesive layer formed by applying an adhesive binder or the like may be 0.5 μm or more and 5 μm or less.

[0025] When the positive electrode 10A and the negative electrode 10B have layers capable of absorbing and releasing lithium ions, the electrolyte is preferably a "non-aqueous" electrolyte such as an organic electrolyte and / or an organic solvent (i.e., the electrolyte is preferably a non-aqueous electrolyte). The electrolyte contains metal ions released from the electrodes (positive electrode 10A and negative electrode 10B), and therefore the electrolyte assists the migration of metal ions in the battery reaction.

[0026] The nonaqueous electrolyte is an electrolyte containing a solvent and a solute. A specific solvent for the nonaqueous electrolyte preferably contains at least a carbonate. Such a carbonate may be a cyclic carbonate and / or a chain carbonate. While not particularly limited, the cyclic carbonate may include at least one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC). The chain carbonate may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC). By way of example only, a combination of a cyclic carbonate and a chain carbonate may be used as the nonaqueous electrolyte, such as a mixture of ethylene carbonate and diethyl carbonate. In addition, a specific example of the solute of the non-aqueous electrolyte is preferably LiPF ６ , LiBF ４ As a specific example of the solute of the non-aqueous electrolyte, LiPF 6 is preferably used. ６ and / or LiBF ４ Li salts such as the above can be used.

[0027] Any current collecting leads used in the field of secondary batteries can be used as the positive electrode current collecting lead and the negative electrode current collecting lead. Such current collecting leads may be made of any material that allows electron transfer, such as conductive materials such as aluminum, nickel, iron, copper, and stainless steel. The positive electrode current collecting lead is preferably made of aluminum, and the negative electrode current collecting lead is preferably made of nickel. The shape of the positive electrode current collecting lead and the negative electrode current collecting lead is not particularly limited and may be, for example, wire-like or plate-like.

[0028] Any external terminal used in the field of secondary batteries can be used as the external terminal. Such external terminals may be made of any material that allows electron transfer, typically a conductive material such as aluminum, nickel, iron, copper, or stainless steel. The external terminals 5 may be electrically connected directly to the substrate, or electrically connected indirectly to the substrate via another device. However, this is not limited thereto. A positive electrode current collecting lead connected to each of the multiple positive electrodes may function as a positive electrode external terminal, and a negative electrode current collecting lead connected to each of the multiple negative electrodes may function as a negative electrode external terminal.

[0029] The exterior body may take the form of a conductive hard case or a flexible case (e.g., a pouch). When the exterior body is in the form of a flexible case (e.g., a pouch), each of the multiple positive electrodes is connected to a positive electrode external terminal via a positive electrode current collecting lead. The positive electrode external terminal is fixed to the exterior body by a seal portion, which prevents leakage of the electrolyte. Similarly, each of the multiple negative electrodes is connected to a negative electrode external terminal via a negative electrode current collecting lead. The negative electrode external terminal is fixed to the exterior body by a seal portion, which prevents leakage of the electrolyte. However, this is not limited thereto, and the positive electrode current collecting lead connected to each of the multiple positive electrodes may have the function of a positive electrode external terminal, and the negative electrode current collecting lead connected to each of the multiple negative electrodes may have the function of a negative electrode external terminal. When the exterior body is in the form of a conductive hard case, each of the multiple positive electrodes is connected to a positive electrode external terminal via a positive electrode current collecting lead. The positive electrode external terminal is fixed to the exterior body by a seal portion, and the seal portion prevents leakage of the electrolyte.

[0030] The conductive hard case consists of a main body and a lid. The main body consists of a bottom and side sections that form the bottom surface of the exterior body. The main body and lid are sealed together after housing the electrode assembly, electrolyte, current collecting leads, and external terminals. The sealing method is not particularly limited, and examples include laser irradiation. Any material capable of forming a hard case-type exterior body in the field of secondary batteries can be used as the material for the main body and lid. Such materials may be any material capable of achieving electron transfer, including conductive materials such as aluminum, nickel, iron, copper, and stainless steel. The dimensions of the main body and lid are determined primarily based on the dimensions of the electrode assembly. For example, it is preferable that the dimensions are large enough to prevent the electrode assembly from moving (shifting) within the exterior body when the electrode assembly is housed. Preventing electrode assembly movement prevents damage to the electrode assembly and improves the safety of the secondary battery.

[0031] The flexible case is composed of a soft sheet. The soft sheet is sufficient to allow the sealing portion to be folded, and is preferably a plastic sheet. The plastic sheet is a sheet that maintains its deformation due to an external force after the external force is applied and then removed, and a so-called laminated film can be used, for example. A flexible pouch made of a laminated film can be produced, for example, by overlapping two laminated films and heat-sealing their periphery. A typical laminated film is a film made by laminating a metal foil and a polymer film, and a specific example is a three-layer structure consisting of an outer layer polymer film / metal foil / inner layer polymer film. The outer layer polymer film is intended to prevent moisture permeation and damage to the metal foil due to contact, and polymers such as polyamide and polyester are suitable. The metal foil is intended to prevent moisture and gas permeation, and foils such as copper, aluminum, and stainless steel are suitable. The inner layer polymer film protects the metal foil from the electrolyte contained inside and serves to melt and seal the opening during heat sealing, and polyolefin or acid-modified polyolefin can be suitably used.

[0032] [Characteristics of the Present Disclosure] The present disclosure relates to characteristics of a negative electrode of a secondary battery. Specifically, the negative electrode has the following characteristics, assuming that the negative electrode material layer of the negative electrode contains Si element and a dopant as main components.

[0033] FIG. 1 is a cross-sectional view schematically illustrating a negative electrode for a secondary battery according to one embodiment of the present disclosure.

[0034] As shown in Fig. 1, a secondary battery negative electrode 10B according to the present disclosure includes a negative electrode material layer 12B provided on a negative electrode current collector 11B. In the embodiment shown in Fig. 1, the negative electrode material layer 12B is provided continuously on the negative electrode current collector 11B. The negative electrode material layer 12B also contains a dopant (impurity) in addition to Si as a constituent material.

[0035] If no dopant is contained, Si has a high electrical resistance value for use as a negative electrode active material, making it difficult to inject and desorb electrons, and therefore difficult to insert and desorb Li ions. Therefore, a dopant is contained in addition to Si in the negative electrode material layer 12B. The inclusion of a dopant reduces the electrical resistance value of the negative electrode active material, and therefore the negative electrode, making it easier to inject and desorb electrons and to insert and desorb Li ions.

[0036] The dopant element may include, for example, at least one of a Group 13 element and a Group 15 element. The Group 13 element may include at least one element selected from the group consisting of B, Al, Ga, and In. The Group 15 element may include at least one element selected from the group consisting of N, P, As, and Sb. In addition to or instead of these, transition elements such as Fe (Group 8 element), Cu, and Au (Group 11 element) may be used.

[0037] The inclusion of a dopant generates carriers, thereby reducing the electrical resistance. Specifically, the dopant element substitutes for the covalently bonded Si site and releases electrons or holes as carriers, thereby reducing the electrical resistance compared to when the dopant is not included. Generally, a higher dopant concentration can reduce the electrical resistance in proportion to the increase in dopant concentration.

[0038] During battery charge and discharge, lithium ions diffuse from the electrolyte to the negative electrode, i.e., the Si in the surface region of the negative electrode material layer, and then diffuse into the inner region of the negative electrode material layer. As described above, attempts have already been made to add dopants to improve battery characteristics (higher capacity and faster charge and discharge). These attempts aim to optimize the diffusion of lithium ions in the inner region of the negative electrode material layer.

[0039] In this regard, the present inventors considered it important to attempt to make the lithium ions diffuse uniformly not only in the internal region of the negative electrode layer but also in the Si in the surface region of the negative electrode layer. Specifically, the present inventors focused on the fact that such an attempt can be made by reducing the interfacial resistance between the electrolyte and the negative electrode surface at the initial stage of charge / discharge of the battery, and conducted extensive research into means for achieving this.

[0040] As a result, the present inventors have devised a negative electrode for a secondary battery 10B according to the present disclosure, which has the following characteristic: "a negative electrode material layer 12B is provided, which includes a surface region 13B and an internal region 14B having mutually different dopant concentrations, and the dopant concentration in the surface region 13B is made higher than the dopant concentration in the internal region 14B" (see FIG. 1).

[0041] In this specification, the "surface region of the negative electrode material layer" refers to a region that is in contact with the electrolyte and has a predetermined thickness or depth in the thickness direction of the negative electrode, and the "internal region of the negative electrode material layer" refers to a region that is located inside this surface region.

[0042] Considering that the negative electrode material layer 12B is disposed on the negative electrode current collector 11B, the negative electrode current collector 11B can be provided on the opposite side of the surface region 13B with respect to the position of the internal region 14B of the negative electrode material layer 12B. That is, the negative electrode current collector 11B, the internal region 14B of the negative electrode material layer, and the surface region 13B of the negative electrode material layer can be disposed in this order in the thickness direction of the negative electrode.

[0043] The "negative electrode material layer 12B on the negative electrode current collector 11B" includes not only the negative electrode material layer 12B that is in direct opposing contact with the negative electrode current collector 11B, but also the negative electrode material layer 12B that is arranged to (not directly) face the negative electrode current collector 11B via a separate intervening layer or intermediate layer.

[0044] Based on these characteristics, the negative electrode of the present disclosure differs in structure from conventional negative electrode layer structures in which a dopant is simply added to the negative electrode layer in that the negative electrode layer is configured to have an inner region 14B having a predetermined dopant concentration and a surface region 13B having a dopant concentration higher than the predetermined dopant concentration. That is, in the present disclosure, the surface region 13B of the negative electrode layer 12B can be referred to as a "heavily doped region" or a "high dopant concentration region" that is more doped with dopant than the inner region 14B.

[0045] As a result, the interfacial resistance between the electrolyte and the surface of the negative electrode 10B can be reduced during the initial charging and discharging of the battery, thereby enabling uniform diffusion of lithium ions into the Si in the surface region 13B of the negative electrode material layer 12B. This can prevent cracks from occurring in the negative electrode material layer 12B during the initial charging and discharging of the battery, thereby effectively preventing deterioration of the charge and discharge characteristics of the battery. In other words, the life of the charge and discharge characteristics of the battery can be extended.

[0046] Furthermore, the negative electrode 10B can be configured such that the surface region 13B and the internal region 14B of the negative electrode material layer 12B form a multi-layer structure. With such a multi-layer structure, the surface region 13B has a layered form, and therefore a high dopant concentration region can be suitably secured to contribute to reducing the interfacial resistance between the electrolyte solution and the surface of the negative electrode 10B.

[0047] Furthermore, from the viewpoint of suitably realizing a reduction in the interfacial resistance between the electrolyte and the surface of negative electrode 10B during the initial charging and discharging of the battery, the thickness of surface region 13B in the thickness direction of negative electrode 10B is preferably 15 nm or more. If the thickness is smaller than this, the interfacial resistance is difficult to reduce during the initial charging period, resulting in a drop in voltage.

[0048] The thickness of the surface region 13B in the thickness direction of the negative electrode 10B is more preferably 70 nm. In this case, the thickness of the negative electrode material layer 10B before charge / discharge and the thickness of the negative electrode material layer 10B after charge / discharge can be made substantially the same. Here, "substantially the same thickness" refers to a difference in thickness of about 5% or less in micrometers before and after charge / discharge. This makes it possible to suppress expansion and contraction of the negative electrode during charge / discharge. Furthermore, from the viewpoint of maintaining the charge / discharge characteristics of the battery, the thickness of the surface region 13B in the thickness direction of the negative electrode 10B is preferably less than 2 μm.

[0049] The dopant concentration in the surface region 13B is 5 to 15 times, preferably about 10 times, the dopant concentration in the inner region 14B. By being 5 times or more, the "heavily doped region" and "high dopant concentration region" can be suitably secured, and the interface resistance between the electrolyte and the surface of the negative electrode 10B during the initial charging and discharging of the surface region battery can be reduced. Furthermore, by being 15 times or less, a predetermined amount of active material can be secured in the surface region 13B of the negative electrode material layer, which can contribute to the absorption and release of lithium ions during charging and discharging of the battery.

[0050] In one example, in the secondary battery of the present disclosure, the negative electrode material layer 12B may contain only one type of dopant. In this case, the type of dopant in the surface region 13B of the negative electrode material layer 12B may be the same as the type of dopant in the inner region 14B.

[0051] Without being limited thereto, in another example, the negative electrode material layer 12B may contain two or more types of dopants, in which case the surface region 13B and the inner region 14B of the negative electrode material layer 12B may contain different types of dopants.

[0052] Specifically, the type of dopant in the surface region 13B of the negative electrode material layer 12B may be different from the type of dopant in the internal region 14B. Alternatively, the surface region 13B and the internal region of the negative electrode material layer 12B may contain the same type of dopant, and the surface region 13B and the internal region 14B of the negative electrode material layer 12B may contain different types of dopant.

[0053] 2 is a cross-sectional view schematically illustrating a negative electrode for a secondary battery according to another embodiment of the present disclosure. In another embodiment, as shown in FIG. 2, compared to the configuration shown in FIG. 1 (in which a negative electrode material layer 12B is provided continuously on a negative electrode current collector 11B), multiple negative electrode material layers 10BI are provided on a negative electrode current collector 11BI, and each negative electrode material layer 10BI may be spaced apart. In this case, each negative electrode material layer 10BI may have a pillar-like or tubular structure.

[0054] According to this other embodiment, a space is formed between adjacent negative electrode material layers 10BI through which the electrolyte can penetrate. The negative electrode material layer may expand and contract during charging and discharging of the battery, and the presence of this space ensures a region for expansion and contraction, making it possible to suitably suppress the occurrence of cracks in the negative electrode material layer.

[0055] Hereinafter, the method for producing a negative electrode for a secondary battery according to the present disclosure will be described in detail.

[0056] The method for producing a negative electrode for a secondary battery according to the present disclosure includes the steps of: providing a Si-containing member containing Si and a dopant on a metal foil as a current collector; and applying a solution containing a dopant onto the Si-containing member, followed by heat treatment.

[0057] At least one of the Group 13 elements and Group 15 elements can be used as the dopant. An example of the heat treatment is annealing in an annealing furnace. The heat treatment temperature can be 550°C or higher and 1500°C or lower, taking into account the graph of FIG. 3 showing the solid solubility limits of impurities (dopants). The step of providing the Si-containing member also includes applying a negative electrode layer slurry containing a negative electrode active material such as Si and impurities as dopants onto a metal foil to form a negative electrode precursor, followed by drying and pressing the negative electrode precursor. The negative electrode layer slurry can be applied to the metal foil continuously or intermittently at predetermined intervals. As a result of the above, a secondary battery negative electrode according to one embodiment of the present disclosure can finally be manufactured (see FIGS. 1 and 2).

[0058] Furthermore, a positive electrode can be obtained by applying a slurry for a positive electrode material layer containing the above-described positive electrode active material onto a metal foil to form a positive electrode precursor, and then drying and pressing the positive electrode precursor. After manufacturing the positive electrode and negative electrode for a secondary battery, an electrode assembly is formed. Specifically, after forming the positive electrode and negative electrode, the positive electrode and negative electrode are stacked in the stacking direction with a separator interposed therebetween to form an electrode component layer. By stacking at least two electrode component layers in the stacking direction, a stacked electrode assembly can be finally formed. Furthermore, by winding a single electrode component layer, a wound electrode assembly can be finally formed.

[0059] After forming a predetermined electrode assembly (wound type / stacked type), the electrode assembly is housed in an outer casing and a current collecting tab is welded. Next, an electrolyte solution is injected into the outer casing under reduced pressure. By going through the above steps, a secondary battery including a secondary battery negative electrode according to an embodiment of the present disclosure can finally be obtained.

[0060] Although one embodiment of the present disclosure has been described above, it is merely a typical example within the scope of application of the present disclosure. Therefore, it will be readily understood by those skilled in the art that the present disclosure is not limited thereto and that various modifications can be made.

[0061] Hereinafter, examples of the present disclosure will be described.

[0062] [Manufacturing Process of Secondary Battery Negative Electrode and Secondary Battery Including Secondary Battery Negative Electrode] Example 1 (corresponding to Sample 1 in FIGS. 6A and 6B) First, an Sb-doped Si-containing substrate (doping amount calculated from resistivity (0.001 Ω cm): 5×10 １８ / cm ３ ) to SbCl ３ A solution containing the dopant (OCD T-1 Sb-30230-SG manufactured by Tokyo Ohka Kogyo Co., Ltd.) was applied to the electrode, and the electrode was heat-treated in an annealing furnace at 900°C for 0.5 hours to obtain a negative electrode having a higher dopant concentration in the surface region than in the internal region. The positive electrode and negative electrode were then stacked in the stacking direction with a separator interposed between them to produce an electrode assembly, which was then housed in an outer casing and a current collecting tab was welded. An electrolyte solution was then injected into the outer casing. By going through the above steps, a secondary battery including the secondary battery negative electrode of Example 1 was finally manufactured.

[0063] In the following other embodiments, in order to avoid duplication of description, the description will focus on the differences from the first embodiment, and the description of the overlapping contents will be omitted or omitted.

[0064] Example 2 (corresponding to Sample 2 in FIGS. 6A and 6B) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ A solution containing the compound was applied to the negative electrode and heat-treated at 900° C. for 6.0 hours to obtain a negative electrode having a higher dopant concentration in the surface region than in the inner region. Thereafter, a secondary battery including the negative electrode for a secondary battery of Example 2 was finally manufactured through the same steps as those in Example 1.

[0065] In Example 2, the thickness (depth) and concentration distribution of the surface region of the negative electrode material layer in the thickness direction of the obtained negative electrode were measured according to secondary ion mass spectrometry (SIMS). The measurement device used in the secondary ion mass spectrometry was a CAMECA IMS-6f. The primary ion species was Cs+, the primary acceleration voltage was 15.0 kV, and the detection area was 30 μmφ. The measurement results are shown in FIG. 5.

[0066] Example 3 (corresponding to Sample 3 in FIGS. 6A and 6B) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ A solution containing the compound was applied onto the negative electrode and heat-treated at 1200° C. for 0.5 hours to obtain a negative electrode having a higher dopant concentration in the surface region than in the inner region. Thereafter, a secondary battery including the negative electrode for a secondary battery of Example 3 was finally manufactured through the same process as in Example 1.

[0067] Example 4 (corresponding to Sample 4 in FIGS. 6A and 6B) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ The resulting negative electrode had a higher dopant concentration in the surface region than in the inner region, and was then heat-treated at 1200° C. for 2.0 hours. Thereafter, the same steps as in Example 1 were carried out to finally manufacture a secondary battery including the negative electrode for a secondary battery of Example 4.

[0068] Example 5 (corresponding to sample D in FIG. 9) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl３ A solution containing the compound was applied to the negative electrode and heat-treated at 900° C. for 108 seconds to obtain a negative electrode having a higher dopant concentration in the surface region than in the inner region. After that, a secondary battery including the negative electrode for a secondary battery of Example 5 was finally manufactured through the same steps as those in Example 1.

[0069] Example 6 (corresponding to sample E in FIG. 9) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ A solution containing the compound was applied to the negative electrode and heat-treated at 900° C. for 360 seconds to obtain a negative electrode having a higher dopant concentration in the surface region than in the inner region. After that, a secondary battery including the negative electrode for a secondary battery of Example 6 was finally manufactured through the same steps as those in Example 1.

[0070] Comparative Example 1 (corresponding to Ctrl in FIGS. 6A, 6B, and 9) SbCl ３ A Sb-doped Si-containing substrate similar to that used in Example 1 was used as a negative electrode without applying a solution containing the same, and then the same steps as those in Example 1 were carried out to finally produce a secondary battery including the negative electrode for a secondary battery of Comparative Example 1.

[0071] Comparative Example 2 (corresponding to Sample A in FIG. 9) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ The negative electrode for a secondary battery of Comparative Example 2 was finally manufactured by applying a solution containing the compound (I) to the negative electrode and then heat-treating the negative electrode for a secondary battery of Comparative Example 2 at 900° C. for 4 seconds.

[0072] Comparative Example 3 (corresponding to Sample B in FIG. 9) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ The negative electrode for a secondary battery of Comparative Example 3 was finally manufactured by applying a solution containing the compound (I) to the negative electrode and heat-treating the negative electrode for a secondary battery of Comparative Example 3 at 900° C. for 11 seconds.

[0073] Comparative Example 4 (corresponding to Sample C in FIG. 9) The same Sb-doped Si-containing substrate as in Example 1 was doped with SbCl ３ The negative electrode for a secondary battery of Comparative Example 4 was finally manufactured by applying a solution containing the compound to the negative electrode and heat-treating the negative electrode for a secondary battery of Comparative Example 4 at 900° C. for 36 seconds.

[0074] [Secondary Battery Charging Step] Then, each of the manufactured secondary batteries was charged at 0.01C.

[0075] [Measurement Results] Measurement results based on secondary ion mass spectrometry in Example 2 are shown in Figure 5. Measurement results relating to the behavior of the charging curves in Examples 1 to 4 and Comparative Example 1 are shown in Figures 6A and 6B. Photographs relating to the thickness of the negative electrode (negative electrode material layer) before and after charging are shown in Figure 7. The difference in the presence or absence of cracks on the negative electrode surface at the beginning of charging between Example 1 and Comparative Example 1 is shown in Figure 8. Measurement results for Examples 5 and 6 and Comparative Examples 1 to 4 are shown in Figure 9. The behavior of the dopant diffusion distance (corresponding to the thickness of the surface region of the negative electrode material layer) at a given heat treatment temperature and heat treatment time, taking into account the diffusion coefficient of Sb, is shown in Figure 4.

[0076] From the measurement results in Fig. 5, it was found that Sb as a dopant was secured at a high concentration in the surface region from the surface of the negative electrode material layer to a depth of about 0.23 µm (230 nm) in the thickness (depth) direction. ３ Since the structure is basically the same as that of a commercially available Sb-doped Si-containing substrate before the application of the solution, it was found that the Sb concentration in the surface region of the negative electrode material layer was about 10 times higher than that in the inner region.

[0077] 4, the thicknesses of the surface regions with high Sb concentration in each Example and Comparative Examples 2 to 4 were determined as follows: (Thickness of the surface regions with high Sb concentration) Example 1: 70 nm Example 2: 230 nm Example 3: 500 nm Example 4: 1 μm Comparative Example 2: 3 nm Comparative Example 3: 5 nm Comparative Example 4: 10 nm Example 5: 16 nm Example 6: 30 nm

[0078] 6A and 6B, at the initial stage of charging the battery (capacity around 0.6 mAh / g), the voltage was higher than that of Comparative Example 1, which was not additionally doped, indicating that the interfacial resistance between the electrolyte and the surface of the negative electrode at the initial stage of charging and discharging was reduced. Furthermore, from the measurement results of FIG. 9, it was found from the results of Comparative Examples 2 to 4 and Examples 5 and 6 that if Sb is diffused in the thickness (depth) direction from the surface of the negative electrode material layer to a depth of about 16 nm, the voltage was higher than that of Comparative Example 1, which was not additionally doped, indicating that the interfacial resistance between the electrolyte and the surface of the negative electrode at the initial stage of charging and discharging was reduced.

[0079] From the measurement results in FIG. 9 and the results of Comparative Examples 2 to 4 and Examples 5 and 6, it was found that if Sb is diffused in the thickness (depth) direction from the surface of the negative electrode material layer to a depth of about 16 nm, the voltage is higher than that of Comparative Example 1 where no additional doping was performed, and therefore the interfacial resistance between the electrolyte and the surface of the negative electrode at the initial stage of charge and discharge is reduced.

[0080] From the measurement results in FIG. 8, it was found that if Sb is diffused from the surface of the negative electrode material layer to a predetermined depth in the thickness (depth) direction, the interfacial resistance between the electrolyte and the surface of the negative electrode at the initial stage of charge / discharge is reduced compared to Comparative Example 1 in which no additional doping is performed, thereby suppressing the occurrence of cracks.

[0081] 7, it was found that when the thickness of the surface region in the thickness direction of the negative electrode in Example 1 was 70 nm, the thickness of the negative electrode material layer before charge and discharge (53.6 μm) and the thickness of the negative electrode material layer after charge and discharge (56.3 μm) could be made substantially the same. On the other hand, in Comparative Example 1, in which no additional doping was performed, the thickness of the negative electrode material layer after charge and discharge increased from the thickness of the negative electrode material layer before charge and discharge (53.6 μm) to 65.9 μm, and expansion of the negative electrode was confirmed.

[0082] The present disclosure may take the following forms. <1> A negative electrode for a secondary battery, comprising a negative electrode material layer containing Si and a dopant, the negative electrode material layer including a surface region and an internal region having mutually different dopant concentrations, the dopant concentration in the surface region being higher than the dopant concentration in the internal region. <2> The negative electrode for a secondary battery according to <1>, wherein the surface region and the internal region of the negative electrode material layer form a multi-layer structure. <3> The negative electrode for a secondary battery according to <1> or <2>, wherein, in the negative electrode material layer, the dopant element includes at least one of a Group 13 element and a Group 15 element. <4> The negative electrode for a secondary battery according to any one of <1> to <3>, wherein the thickness of the surface region in the thickness direction is 15 nm or more. <5> The negative electrode for a secondary battery according to <4>, wherein the thickness of the surface region in the thickness direction is 70 nm. <6> The negative electrode for a secondary battery according to <4> or <5>, wherein the thickness of the surface region in the thickness direction is less than 2 μm. <7> The negative electrode for a secondary battery according to any one of <1> to <6>, wherein the negative electrode material layer is obtained by applying a solution containing the dopant to a Si-containing member containing the Si and the dopant, followed by heat treatment. <8> The negative electrode for a secondary battery according to <3>, wherein, in the negative electrode material layer, the dopant element includes the Group 13 element, and the Group 13 element includes at least one element selected from the group consisting of B, Al, Ga, and In. <9> The negative electrode for a secondary battery according to <3>, wherein, in the negative electrode material layer, the dopant element includes the Group 15 element, and the Group 15 element includes at least one element selected from the group consisting of N, P, As, and Sb. <10> The electrode for a secondary battery according to any one of <1> to <9>, wherein the type of dopant is one type, and the type of dopant in the surface region and the type of dopant in the inner region are the same. <11> The secondary battery electrode according to any one of <1> to <9>, wherein the negative electrode material layer contains two or more types of the dopant, and the surface region and the internal region contain different types of the dopant.<12> The negative electrode for a secondary battery according to any one of <1> to <11>, further comprising a current collector, the current collector being provided on the opposite side of the surface region with respect to the position of the internal region of the negative electrode material layer, and the negative electrode material layer being provided continuously on the current collector. <13> The negative electrode for a secondary battery according to any one of <1> to <11>, further comprising a current collector, the current collector being provided on the opposite side of the surface region with respect to the position of the internal region of the negative electrode material layer, and a plurality of the negative electrode material layers being provided on the current collector, the respective negative electrode material layers being spaced apart. <14> A secondary battery including the negative electrode for a secondary battery according to any one of <1> to <13>. <15> A method for producing a negative electrode for a secondary battery, the method comprising the steps of: forming a Si-containing member containing Si and a dopant on a metal foil as a current collector; and applying a solution containing the dopant onto the Si-containing member and performing a heat treatment.

[0083] A secondary battery including a secondary battery negative electrode according to an embodiment of the present disclosure can be used in various fields where power storage is required. By way of example only, a secondary battery according to an embodiment of the present disclosure, particularly a nonaqueous electrolyte secondary battery, can be used in fields such as electrical, information, and communications where mobile devices are used (e.g., mobile devices such as mobile phones, smartphones, laptops, digital cameras, activity monitors, arm computers, and electronic paper), household and small-scale industrial applications (e.g., power tools, golf carts, and household, nursing care, and industrial robots), large-scale industrial applications (e.g., forklifts, elevators, and port cranes), transportation systems (e.g., hybrid vehicles, electric vehicles, buses, trains, electrically assisted bicycles, and electric motorcycles), power system applications (e.g., various power generation systems, road conditioners, smart grids, and general-purpose household power storage systems), medical applications (e.g., medical devices such as earphones and hearing aids), medical applications (e.g., medication management systems), IoT, and space and deep-sea applications (e.g., space probes, submersible research vessels, and the like).

[0084] 10A Positive electrode 10B, 10BI Negative electrode 11A Positive electrode current collector 11B, 11BI Negative electrode current collector 12A Positive electrode material layer 12B, 12BI Negative electrode material layer 13B, 13BI Surface area 14B, 14BI Internal area

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode material layer containing Si and a dopant, the negative electrode material layer including a surface region and an internal region having mutually different dopant concentrations, the dopant concentration of the surface region being higher than the dopant concentration of the internal region.

2. The negative electrode for a secondary battery according to claim 1, wherein the surface region and the internal region of the negative electrode material layer form a multi-layer structure.

3. The negative electrode for a secondary battery according to claim 1 or 2, wherein in said negative electrode layer, said dopant element includes at least one of a Group 13 element and a Group 15 element.

4. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the thickness of the surface region in the thickness direction is 15 nm or more.

5. The negative electrode for a secondary battery according to claim 4, wherein the thickness of said surface region in the thickness direction is 70 nm.

6. The negative electrode for a secondary battery according to claim 4 or 5, wherein the thickness of the surface region in the thickness direction is less than 2 μm.

7. The negative electrode for a secondary battery according to any one of claims 1 to 6, wherein the negative electrode layer is obtained by applying a solution containing the dopant onto a Si-containing member containing the Si and the dopant, followed by heat treatment.

8. The negative electrode for a secondary battery according to claim 3, wherein in said negative electrode layer, said dopant element comprises said Group 13 element, and said Group 13 element comprises at least one element selected from the group consisting of B, Al, Ga, and In.

9. The negative electrode for a secondary battery according to claim 3, wherein in said negative electrode layer, said dopant element comprises said Group 15 element, and said Group 15 element comprises at least one element selected from the group consisting of N, P, As, and Sb.

10. An electrode for a secondary battery according to any one of claims 1 to 9, wherein the dopant is of one type, and the type of the dopant in the surface region is the same as the type of the dopant in the internal region.

11. The electrode for a secondary battery according to any one of claims 1 to 9, wherein the negative electrode layer contains two or more types of the dopant, and the surface region and the internal region contain mutually different types of the dopant.

12. The negative electrode for a secondary battery according to any one of claims 1 to 11, further comprising a current collector, said current collector being provided on the opposite side of said surface region with respect to the position of said internal region of said negative electrode material layer, and said negative electrode material layer being provided continuously on said current collector.

13. The negative electrode for a secondary battery according to any one of claims 1 to 11, further comprising a current collector, the current collector being provided on the opposite side of the surface region with respect to the position of the internal region of the negative electrode material layer, and a plurality of the negative electrode material layers being provided on the current collector, each negative electrode material layer being spaced apart.

14. A secondary battery comprising the negative electrode for secondary batteries according to any one of claims 1 to 13.

15. A method for manufacturing a negative electrode for a secondary battery, comprising: forming a Si-containing member containing Si and a dopant on a metal foil as a current collector; and applying a solution containing the dopant onto the Si-containing member and subjecting it to a heat treatment.

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