JP6973775B2 - Manufacturing method of negative electrode for lithium ion secondary battery, negative electrode, and negative electrode active material - Google Patents

Description

The present invention relates to a method for manufacturing a negative electrode for a lithium ion secondary battery, a negative electrode, and a negative electrode active material.

Lithium-ion secondary batteries are lighter and have a higher capacity than conventional nickel-cadmium batteries and nickel-metal hydride batteries, and are therefore widely used as power sources for electronic devices such as mobile phones. Lithium-ion secondary batteries are also promising candidates for power sources installed in hybrid vehicles and electric vehicles.

The negative electrode of the lithium ion secondary battery is formed of a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, a binder, and a conductive auxiliary agent. Conventionally, graphite has been used as the negative electrode active material, but since the theoretical capacity of graphite is 372 mAh / g, there is a limit to increasing the capacity of the lithium ion secondary battery. Therefore, it is expected that the capacity of the lithium ion secondary battery will be increased by applying Si (silicon), which has a theoretical capacity of about 4200 mAh / g, which is 10 times or more that of graphite, to the negative electrode active material.

When a negative electrode active material made of Si is used, the amount of lithium ions that react with the active material is large, so that the capacity can be increased, but the volume change during charging and discharging is large. Si undergoes a volume change of about 400% in the alloying / dealloying reaction with lithium in the charge / discharge process. The volume change may cause destruction of the negative electrode active material, fragmentation of the conductive path in the negative electrode active material layer, separation of the negative electrode current collector and the negative electrode active material layer, etc., and as a result, the cycle characteristics of the lithium ion secondary battery. Causes a decrease in. Technologies such as nano-size, porous structuring, alloying, and compounding with carbon materials have been proposed in order to suppress volume changes, but there are major technical issues for material production with high throughput. There is.

Non-Patent Document 1 proposes a negative electrode active material in which conductive Sn (tin) nanoparticles are supported on the surface of nano-sized Si nanoparticles. The particle size of the Si nanoparticles is less than 100 nm, and the particle size of the Sn nanoparticles is 10 to 20 nm.

L.Zhong et al., SCIENTIFIC REPORTS. 6, 30952 (2016)

It has been reported that the cycle characteristics are stabilized by using the negative electrode active material described in Non-Patent Document 1. However, the cycle characteristics are still insufficient, and the irreversible capacitance at the beginning of the cycle is also large.

An object of the present invention is to provide a method for manufacturing a negative electrode for a lithium ion secondary battery, a negative electrode, and a negative electrode active material, in which the irreversible capacity at the initial stage of the cycle is reduced and the cycle characteristics are excellent.

In the method for manufacturing a negative electrode for a lithium ion secondary battery according to the present invention, a slurry containing a negative electrode active material particle in which a Sn metal body is provided on the surface of Si nanoparticles is applied to a negative electrode current collector to obtain a negative electrode precursor. It is characterized by having a negative electrode precursor forming step of forming and a heating step of heating the negative electrode precursor at a temperature of 50 ° C. or higher and 350 ° C. or lower in an atmosphere containing oxygen to oxidize the surface of the Sn metal body. And.

The negative electrode for a lithium ion secondary battery according to the present invention includes a negative electrode current collector and a negative electrode active material layer containing a plurality of negative electrode active materials provided on the negative electrode current collector, and the negative electrode active material is , Si nanoparticles are characterized in that oxidized Sn nanoparticles, which are Sn nanoparticles whose surface is oxidized, are provided on the surface of the Si nanoparticles.

The negative electrode active material for a lithium ion secondary battery according to the present invention is characterized in that the oxidized Sn nanoparticles, which are Sn nanoparticles whose surface is oxidized, are provided on the surface of the Si nanoparticles.

According to the present invention, since a negative electrode containing a negative electrode active material in which the surface of the Si nanoparticles is provided with the oxidized Sn nanoparticles, which are Sn nanoparticles whose surface is oxidized, the irreversible capacity at the initial stage of the cycle is reduced. , A lithium ion secondary battery having excellent cycle characteristics can be obtained.

It is the schematic of the appearance of the lithium ion secondary battery which carried out this embodiment. It is an enlarged sectional drawing of the main part of a lithium ion secondary battery. It is a flowchart explaining the manufacturing method of a negative electrode. It is a schematic diagram which shows the cross section of the negative electrode precursor. It is a schematic diagram which shows the cross section of a negative electrode. It is a graph which shows the result of the charge / discharge test. It is a figure which shows the XPS analysis result of the negative electrode of Example 1. FIG. It is a figure which shows the XPS analysis result of the negative electrode of the comparative example 3. FIG. It is a figure which shows the IR analysis result of the negative electrode of Examples 1, 2 and Comparative Example 3. It is an SEM image of the negative electrode of Example 1. FIG. It is a Sn mapping image of the negative electrode of Example 1. FIG.

1. 1. Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic view of a lithium ion secondary battery 10 in which the present invention is carried out. The shape of the lithium ion secondary battery 10 is not limited to the square shape in this example, but may be a cylindrical shape, a sheet shape, or the like. The lithium ion secondary battery 10 includes a battery container 11, an electrode structure 12, a positive electrode terminal 13, a positive electrode lead 14, a negative electrode terminal 15, and a negative electrode lead 16.

The battery container 11 has a flat box shape and is made of, for example, a metal such as aluminum. The battery container 11 is hermetically sealed with an electrode structure 12, a positive electrode lead 14, a negative electrode lead 16, and an electrolytic solution (not shown) in which an electrolyte is dissolved. The electrolytic solution is not particularly limited, and one or more of cyclic carbonate, chain carbonate and the like can be used. Examples of the cyclic carbonate include ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC). Examples of the chain carbonate include diethyl carbonate (DEC) and dimethyl carbonate (DMC). The electrolyte is not particularly limited, and lithium salts such as _{LiPF 6} and LiBF _{4 can be used. In this embodiment, a solution obtained by dissolving 1.0 mol / liter of LiPF 6} in a solution in which EC, DEC, and FEC are mixed at a volume ratio of 1: 1: 0.2 is used as an electrolytic solution. In FIG. 1, the electrode structure 12, the positive electrode lead 14, and the negative electrode lead 16 are shown by dotted lines.

The positive electrode terminal 13 is provided on the outside of the battery container 11 and is connected by welding to one end of the positive electrode lead 14. The other end of the positive electrode lead 14 is connected to the positive electrode 17 described later. The positive electrode terminal 13 and the positive electrode 17 are electrically connected via the positive electrode lead 14.

The negative electrode terminal 15 is provided outside the battery container 11 and is connected by welding to one end of the negative electrode lead 16. The other end of the negative electrode lead 16 is connected to the negative electrode 18 described later. The negative electrode terminal 15 and the negative electrode 18 are electrically connected via the negative electrode lead 16.

The electrode structure 12 includes a positive electrode 17, a negative electrode 18, and separators 19 and 20. The positive electrode 17, the negative electrode 18, and the separators 19 and 20 are formed in a sheet shape. In the electrode structure 12, the negative electrode 18 is laminated on one surface of the separator 20, and the separator 19 and the positive electrode 17 are laminated in this order on the negative electrode 18. It is formed by winding in the state of being wound. The electrode structure 12 may be a wound type or a laminated type as described above.

FIG. 2 is an enlarged cross-sectional schematic view of a part of the electrode structure 12 shown in FIG. As shown in FIG. 2, the positive electrode 17 has a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode current collector 22 is formed of, for example, a metal foil such as an aluminum foil, a Ni (nickel) foil, or a stainless steel foil. In this embodiment, the positive electrode current collector 22 is made of aluminum foil. The positive electrode current collector 22 is connected to the positive electrode lead 14.

The positive electrode active material layer 24 is provided on one surface of the positive electrode current collector 22. The positive electrode active material layer 24 contains a positive electrode active material that occludes and desorbs lithium. As the positive electrode active material, a lithium transition metal composite oxide is used. Lithium transition metal composite oxides include, for example, at least one of the group consisting of Co (cobalt), Ni, and Mn (manganese). In addition to the positive electrode active material, the positive electrode active material layer 24 further contains, for example, a conductive auxiliary agent and a binder for the positive electrode. The conductive auxiliary agent is, for example, a carbon material such as graphite or carbon black. The binder for the positive electrode is polyvinylidene fluoride (PVDF) or the like.

The negative electrode 18 has a negative electrode current collector 26 and a negative electrode active material layer 28. The negative electrode current collector 26 is formed of Cu (copper) foil and is connected to the negative electrode lead 16. The negative electrode active material layer 28 is provided on the surface of the negative electrode current collector 26 facing the positive electrode active material layer 24. The details of the negative electrode 18 will be described later with reference to another drawing.

The separators 19 and 20 are formed of a porous membrane and hold an electrolytic solution in the pores. The separator 19 prevents a short circuit between the positive electrode 17 and the negative electrode 18, and lithium ions pass through the separator 19 during charging and discharging. As the material of the separators 19 and 20, for example, a microporous polypropylene film can be used. Polyolefin-based, polyester-based, polyacrylonitrile-based, polyphenylene sulfide-based, polyimide-based or fluororesin-based micropore membranes and non-woven fabrics may be used.

As shown in FIG. 3, the negative electrode 18 uses a mixed powder of Si powder 30 and Sn powder 31 as a raw material powder to form negative electrode active material particles 32 in which Sn metal bodies are provided on the surface of Si nanoparticles. The active material particle forming step 40, the negative electrode precursor forming step 41 for applying the slurry containing the negative electrode active material particles 32 to the negative electrode current collector 26 to form the negative electrode precursor 33, and the heating step for heating the negative electrode precursor 33. Manufactured by 42.

In the negative electrode active material particle forming step 40, the negative electrode active material particles 32 are formed by a plasma spray PVD (PS-PVD; Plasma Spray − Physical Vapor Deposition) method using the mixed powder. In the PS-PVD method, high-temperature steam is generated by supplying raw material powder into plasma and completely evaporating it, and nanoparticles are formed by rapid condensation generated in the cooling process of the high-temperature steam. In this embodiment, a PS-PVD apparatus (not shown) that performs the PS-PVD method is used. As the PS-PVD device, in the present embodiment, a hybrid plasma spray device manufactured by JEOL Ltd. is used. The average particle size of the Si powder 30 in this embodiment is 19 μm. The purity of Si powder 30 is 99.5%. As the Sn powder 31, in the present embodiment, a commercially available powder, for example, a powder manufactured by High Purity Chemical Co., Ltd. is used. The particle size of the Sn powder 31 is 38 μm or less. The purity of Sn powder 31 is 99.9%.

The PS-PVD apparatus includes, for example, a raw material supply unit, a plasma generation unit, a plasma gas supply unit, and a chamber. The raw material supply unit is installed above the plasma generation unit, and is for supplying the raw material of the negative electrode active material particles 32 to the plasma generation unit. The plasma generation unit has a plasma torch and generates plasma in the plasma torch. In the present embodiment, the plasma generating unit uses a hybrid plasma torch having a DC plasma torch and a high frequency plasma torch. The DC plasma torch has a cathode electrode and an anode electrode, and discharges a DC voltage between both electrodes from a DC power source. A high-frequency induction coil is wound around the high-frequency plasma torch, and a high frequency is applied from a high-frequency power source to the high-frequency induction coil. The plasma gas supply unit supplies the DC plasma generated gas to the DC plasma torch and supplies the high frequency plasma generated gas to the high frequency plasma torch. Ar (argon) gas is used as the DC plasma generating gas. _{Ar gas and H 2} (hydrogen) gas are used as the high frequency plasma generating gas. The chamber is installed directly under the plasma torch.

In such PS-PVD apparatus thus configured, the Ar gas was supplied to the direct current plasma torch as direct current plasma generation gas, a plasma jet is formed by igniting the high-frequency Ar gas and H ₂ gas as a high frequency plasma generating gas Plasma is generated by supplying it to a plasma torch and applying a high frequency. This produces a high speed and high temperature gas flow. The temperature of the gas flow in the plasma torch is, for example, 10000 ° C. at the center of the plasma torch and 5000 ° C. on average at the outlet of the plasma torch. The central part of the plasma torch is the maximum temperature region. The average velocity of the gas flow in the plasma torch is several tens of m / sec. The gas stream is supplied into the chamber. The chamber has a cooling collector. The cooling collector cools the gas stream and collects nanoparticles formed by rapid condensation as negative electrode active material particles 32. The cooling collector is controlled by, for example, a control unit (not shown), and has a function of adjusting the temperature in the chamber, the temperature of the gas flow, the cooling rate, and the like. The plasma generating unit is not limited to the hybrid plasma torch, and may be a DC plasma torch alone or a high frequency plasma torch alone.

The atomic number concentration of Sn with respect to the total amount of the mixed powder, that is, the atomic number concentration of Sn powder 31 is larger than 0 at% and in the range of 10 at% or less, and preferably in the range of 1 at% or more and 5 at% or less. In this embodiment, the atomic number concentration of Sn powder 31 is set to, for example, 1 at%. In the PS-PVD method using such a mixed powder, Si nanoparticles are first formed by homogeneous nucleation in the high-temperature steam in the cooling process of the high-temperature steam, and then inhomogeneous on the surface of the Si nanoparticles. Sn nanoparticles are formed by nucleation. That is, in the negative electrode active material particle forming step 40, the negative electrode active material particles 32 are formed by supporting the Sn nanoparticles 62 as Sn metal bodies on the surface of the Si nanoparticles 60 (see FIG. 4).

The atomic number concentration of the Sn nanoparticles 62 with respect to the negative electrode active material particles 32 is equivalent to the atomic number concentration of the Sn powder 31 with respect to the total amount of the mixed powder, which is larger than 0 at% and within the range of 10 at% or less, and is 1 at% or more and 5 at. It is preferably in the range of% or less. When the atomic number concentration of the Sn nanoparticles 62 is larger than 10 at%, the amount of the active material made of Si in the negative electrode active material layer 28 decreases, and the battery capacity decreases. If the atomic number concentration of the Sn nanoparticles 62 is low, the effect of the ligament 72 that connects the plurality of Si nanoparticles 60 described later is not exhibited.

The particle size of the Si nanoparticles 60 is in the range of 20 nm or more and 40 nm or less. The particle size of the Sn nanoparticles 62 is in the range of 5 nm or more and 20 nm or less. In the present embodiment, the average particle size of the Si nanoparticles 60 is 30 nm, and the average particle size of the Sn nanoparticles 62 is 10 nm. The particle sizes of the Si nanoparticles 60 and the Sn nanoparticles 62 are determined by, for example, Rietveld analysis using an XRD (X-ray diffraction) apparatus.

The negative electrode active material particle forming step 40 will be described more specifically. The negative electrode active material particle forming step 40 includes a mixed steam generation step 50, a first cooling step 51, and a second cooling step 52.

In the mixed steam generation step 50, the mixed powder is heated by being supplied into the plasma. The mixed powder is completely pyrolyzed and evaporated by passing through the highest temperature region of the plasma torch together with the gas flow. In this way, a high-temperature mixed steam containing Si and Sn is generated. The mixed steam is supplied into the chamber together with the gas flow.

In the first cooling step 51, Si nanoparticles 60 are homogeneously nucleated in the mixed steam by cooling the mixed steam supplied into the chamber. The homogeneous nucleation temperature of Si under typical PS-PVD conditions is determined by the atomic number concentration of Sn powder 31 with respect to the total amount of mixed powder. For example, the homogeneous nucleation temperature of Si is 2175K when the atomic number concentration of Sn powder 31 is 1 at% and 2195K when the atomic number concentration is 5 at%. Therefore, in the first cooling step 51, it is necessary to cool from the temperature of the mixed steam of 2200 K or more that Si nanoparticles 60 can form by homogeneous nucleation. As a result, in the first cooling step 51, Si nanoparticles 60 are formed by homogeneous nucleation in the mixed steam in the cooling process of the mixed steam.

In the second cooling step, the mixed steam that has undergone the first cooling step is further cooled to generate inhomogeneous nuclei of Sn nanoparticles 62 on the surface of the Si nanoparticles 60. That is, in the second cooling step, the mixed steam that has undergone the first cooling step is brought to an inhomogeneous nucleation temperature at which Sn nanoparticles 62 are formed by inhomogeneous nucleation on the surface of the Si nanoparticles 60. The Sn heterogeneous nucleation temperature is determined according to the atomic number concentration of the Sn powder 31 and the Si nanoparticle diameter with respect to the total amount of the mixed powder. For example, the inhomogeneous nucleation temperature of Sn is 1375 K at 1 at% and 1480 K at 5 at% when the typical Si nanoparticles are 30 nm. On the other hand, the homogeneous nucleation temperature of Sn is 1345K when the atomic number concentration of Sn powder 31 is 1 at% and 1460K when the atomic number concentration is 5 at%. From this, in the second cooling step, a structure in which Sn nanoparticles are directly supported on Si nanoparticles is expected. Therefore, the second cooling step, in order to suppress the coarsening of Si and Sn nanoparticles, the cooling rate of the vapor mixture, it is preferable to 10 4 K ^/ sec or more.

Next, the negative electrode precursor forming step 41 will be described. The negative electrode precursor forming step 41 includes a slurry making step 53, a coating step 54, and a drying and pressing step 55, and the negative electrode precursor obtained through all these steps is the negative electrode precursor shown in FIG. 33. In the slurry making step 53, a slurry containing the negative electrode active material particles 32 is made. In the coating step 54, the coating film 63 is formed by coating the slurry on the negative electrode current collector 26. In the drying and pressing step 55, the coating film 63 formed on the surface of the negative electrode current collector 26 is dried and pressed. As shown in FIG. 4, in the negative electrode precursor 33, the negative electrode active material particles 32 are uniformly dispersed in the coating film 63.

Each step in the negative electrode precursor forming step 41 will be described more specifically.

In the slurry making step 53, the negative electrode active material particles 32, the binder material 46 for the negative electrode, and the conductive auxiliary agent 48 are mixed, and an appropriate amount of a solvent is added to the mixture to prepare a slurry. As the binder material 46 for the negative electrode, for example, PVDF, a polyimide precursor, or the like is used. Examples of the polyimide precursor include polyamic acid. As the conductive auxiliary agent 48, for example, a carbon-based material such as graphite, carbon black, or carbon fiber is used. As the solvent, N-methyl-2-pyrrolidone (NMP) or the like is used. In this embodiment, polyamic acid is used as the binder material 46 for the negative electrode, carbon black is used as the conductive auxiliary agent 48, and NMP is used as the solvent. In the above mixture, the binder material 46 for the negative electrode is in the range of 1 to 20 wt%, the conductive auxiliary agent 48 is in the range of 1 to 20 wt%, and in the present embodiment, the content is 5 wt%.

In the coating step 54, the coating film 63 is formed on the surface of the negative electrode current collector 26 by applying the slurry prepared in the slurry making step 53 onto the negative electrode current collector 26. Examples of the method of applying the slurry on the negative electrode current collector 26 include a doctor blade method, a dipping method, and a spray method. In the present embodiment, the slurry is coated on the negative electrode current collector 26 in the form of a film by the doctor blade method. In the coating step 54, the thickness of the coating film 63 is set to, for example, 75 μm, and the thickness of the negative electrode current collector 26 is set to, for example, 12 μm.

The drying and pressing step 55 includes a first drying step, a pressing step, and a second drying step.

In the first drying step, the coating film 63 is dried in the atmosphere. In the first drying step, Sn of the Sn nanoparticles 62 and Cu of the negative electrode current collector 26 are dried under the condition that they are not oxidized. Specifically, the product is dried at a temperature within the range of room temperature (for example, 25 ° C.) or higher and 110 ° C. or lower for a time within the range of 3 minutes or longer and 15 minutes or lower. The temperature of the first drying step is more preferably 80 ° C. or higher and 110 ° C. or lower. In this embodiment, the coating film 63 is dried at 110 ° C. for 15 minutes. As a result, the oxidation of Sn and Cu is suppressed, and the volatilization of the solvent is promoted.

In the pressing step, for example, a roll press machine is used to press the coating film 63 that has undergone the first drying step. In the pressing process, for example, the coating film 63 is pressed at 10 kN. By the pressing step, the flat surface of the coating film 63 is flattened, and the coating film 63 is more closely adhered to the negative electrode current collector 26. The porosity of the negative electrode active material layer 28 can be adjusted by the pressing process. After the pressing step, the thickness of the coating film 63 is set to, for example, 10 μm, and the thickness of the negative electrode current collector 26 is set to, for example, 10 μm.

In the second drying step, the negative electrode precursor 33 is formed by drying the coating film 63 that has undergone the pressing step in the atmosphere. In the second drying step, Sn of the Sn nanoparticles 62 and Cu of the negative electrode current collector 26 are dried under the condition that oxidation does not proceed. Specifically, it is dried at a temperature in the range of room temperature or more and 110 ° C. or less for a time in the range of 3 minutes or more and 60 minutes or less. The temperature of the second drying step is more preferably 80 ° C. or higher and 110 ° C. or lower. In this embodiment, the coating film 63 is dried at 110 ° C. for 45 minutes. As a result, the oxidation of Sn and Cu is suppressed and the solvent is removed. If the solvent is removed in the first drying step, the second drying step may not be performed.

Next, the heating step 42 will be described. In the heating step 42, the negative electrode precursor 33 is heated at a temperature at which Sn melts in an atmosphere containing oxygen, and the surface of the Sn nanoparticles 62, particularly the surface, is oxidized. Specifically, the melting point of Sn is about 230 ° C. when the particle size of the Sn nanoparticles 62 is 100 nm or more, but is lower than 50 ° C. when the particle size of the Sn nanoparticles 62 is about 10 nm. Therefore, the heating temperature in the heating step 42 is preferably 50 ° C. or higher, and more preferably 200 ° C. or higher at which the large Sn nanoparticles 62 are also melted. The negative electrode precursor 33 is immediately melted in the heating process of the heating step 42, and the Sn nanoparticles 62 are oxidized from the surface in the subsequent holding process to form the ligament 72 formed by the oxidized Sn nanoparticles 71 described later. (See FIG. 5). Utilizing the diffusion reaction of oxygen molecules, in order to make the surface of the Sn nanoparticles 62 SnOx having a large amount of oxygen x (x is in the range of 0 <x ≦ 2), the heating temperature in the heating step 42. Is preferably at a higher temperature. However, the temperature of the heating step 42 is preferably 350 ° C. or lower so that the imidization reaction of the polyamic acid as the binder material 46 for the negative electrode does not proceed too much. That is, the heating temperature in the heating step 42 is preferably 50 ° C. or higher and 350 ° C. or lower, and more preferably 200 ° C. or higher and 230 ° C. or lower. Further, in order to suppress the oxidation of Cu in the negative electrode current collector 26, it is necessary to shorten the heating time as the heating temperature in the heating step 42 becomes higher. Therefore, the heating time of the heating step 42 is preferably 1 minute or more and 5 minutes or less. In this embodiment, the negative electrode precursor 33 is heated at 230 ° C. for 1 minute.

The heating step 42 is performed using an annealing device (tubular furnace) (not shown). The annealing device has, for example, an annealing chamber in which the negative electrode precursor 33 is installed, a gas supply unit that supplies Ar gas to the annealing chamber, and a control unit that controls each unit of the annealing device (none of which is shown). A heater and a thermometer (neither shown) are provided inside the annealing chamber. The control unit controls each unit of the annealing device based on the setting information set by the setting means (not shown). For example, the control unit controls the calorific value of the heater based on the measured temperature information acquired from the thermometer and the set temperature information set by the setting means. As a result, the temperature in the annealing chamber is maintained at the set temperature corresponding to the set temperature information. Further, the control unit controls the rate at which the temperature in the annealing chamber is raised to the target temperature (hereinafter referred to as the temperature raising rate), the Ar gas supply start timing, the Ar gas supply stop timing, the Ar gas flow rate, and the like. I do.

In the present embodiment, after the negative electrode precursor 33 is installed in the annealing chamber under the atmospheric atmosphere, the supply of Ar gas and the temperature rise in the annealing chamber are started at the same time by the control unit. The flow rate of Ar gas is 300 sccm (Standard Cubic Centimeter per Minutes). The rate of temperature rise is set to 30 ° C./min. The set temperature is 230 ° C. After the temperature of the annealing chamber reaches 230 ° C., it is held for 1 to 5 minutes to heat the negative electrode precursor 33. When the predetermined heating time is reached, the negative electrode precursor 33 is immediately moved to a temperature range of 50 ° C. or lower and cooled while still in the Ar gas flow. At the time of introducing the negative electrode precursor 33 into the annealing chamber, the atmosphere enters the annealing chamber. The atmosphere remains even when the negative electrode precursor 33 is heated, and the negative electrode 18 is formed by heating the negative electrode precursor 33 in an atmosphere containing oxygen in this way. ^{The oxygen partial pressure during heating is preferably 0.1 × 10 2} Pa or more and 2 × 10 ⁴ Pa or less in the temperature range where the heating temperature is 50 ° C. or higher and 350 ° C. or lower. When the oxygen partial pressure is 0.1 × 10 ² Pa or more, Sn can be more reliably oxidized as compared with the case where the oxygen partial pressure is ^{less than 0.1 × 10 2 Pa.} When the oxygen partial pressure is 2 × 10 ⁴ Pa or less, the oxidation of Cu is more reliably suppressed as compared with the case where the oxygen partial pressure is ^{larger than 2 × 10 4 Pa.}

As shown in FIG. 5, the negative electrode 18 includes a plurality of negative electrode active materials 70 in the negative electrode active material layer 28. The negative electrode active material 70 is obtained by oxidizing the surface of the Sn nanoparticles 62 in the heating step 42. That is, in the negative electrode active material 70, the oxidized Sn nanoparticles 71, which are Sn nanoparticles whose surface has been oxidized, are supported on the surface of the Si nanoparticles 60. The oxidized Sn nanoparticles 71 have conductivity because they contain SnOx, which is an oxide of Sn. The oxidized Sn nanoparticles 71 maintain the conductivity of the negative electrode active material 70 even if the oxidation progresses.

Since the oxidized Sn nanoparticles 71 are formed by oxidation from the surface of the Sn metal body (Sn nanoparticles 62) directly supported on the Si nanoparticles 60, a SnOx layer having an oxidation amount different from that of the central portion is provided on the surface. (Neither is shown). The deeper the SnOx layer is from the surface, the smaller the value of x. _{For example, in the SnOx layer, SnO 2} is contained in a portion from the outermost surface to a specific depth, and SnO is contained in a deeper portion. By enclosing the low oxygen content SnO, the amount of Li 2 _O with respect to Sn in comparison with the case constituted only SnO ₂ is decreased and the irreversible capacity of the initial cycle is reliably reduced. By contain SnO ₂ on the outermost surface of the oxide Sn nanoparticles 71, aggregation because although SnO ₂ surface to form a Sn / Li ₂ O, the oxide Sn nanoparticles 71 interspersed between Si nanoparticles Is suppressed, so the cycle characteristics are superior.

In the negative electrode active material layer 28, the oxidized Sn nanoparticles 71 are contained in a uniformly distributed state. This is confirmed by mapping Sn by EDS (Energy Dispersion Spectroscopy) using an image of a scanning electron microscope (SEM).

The plurality of Si nanoparticles 60 are connected by a ligament 72 formed by the directly supported Sn oxide nanoparticles 71. The ligament 72 is formed by melting the Sn nanoparticles 62 in the heating step 42, placing them in the voids between the Si nanoparticles by the capillary effect, and immediately oxidizing the Sn nanoparticles 62 from the surface. Since the oxidized Sn has conductivity, the conductive network N is formed by the ligament 72, the plurality of Si nanoparticles 60, and the carbon material of the conductive auxiliary agent. The conductive network N is formed in the negative electrode active material layer 28 infinitely vertically and horizontally, whereby a three-dimensional negative electrode structure can be obtained. The conductive network N improves the electrical conductivity in the direction from the surface of the negative electrode active material layer 28 toward the negative electrode current collector 26 (depth direction) and in the direction parallel to the surface of the negative electrode active material layer 28. By connecting the plurality of Si nanoparticles 60 with the ligament 72, it is possible to prevent the conduction path from being divided due to the volume change of the negative electrode active material 70 during charging and discharging. Therefore, a lithium ion secondary battery 10 having particularly excellent initial efficiency can be obtained.

In order to form the conductive network N, the porosity of the negative electrode active material layer 28 is preferably in the range of 25% or more and 65% or less. If the porosity is larger than 65%, the plurality of Si nanoparticles 60 may not be sufficiently linked by the oxidized Sn nanoparticles 71 formed in the heating step 42. If the void ratio is smaller than 25%, the three-dimensional negative electrode structure cannot absorb the volume change during charging and discharging of the negative electrode active material 70, and the negative electrode active material 70 is destroyed or the negative electrode current collector 26 and the negative electrode active material layer 28 are formed. Peeling occurs. The porosity of the negative electrode active material layer 28 was determined from the volume and weight of the components of the negative electrode 18 cut out for the test.

The negative electrode active material layer 28 has an alloy portion 73 formed of an alloy of Cu and Sn at the interface between the negative electrode active material layer 28 and the negative electrode current collector 26. In the alloying portion 73, a part of the Sn nanoparticles 62 and the negative electrode current collector 26 is alloyed in the heating step 42. The negative electrode active material 70 is fixed to the negative electrode current collector 26 via the alloy portion 73. This improves the adhesion between the negative electrode current collector 26 and the negative electrode active material layer 28. When Sn is formed on the current collector in the form of a film, a layered Cu—Sn alloy layer is formed between the current collector and the active material layer, and the active material layer is peeled off due to repeated stress caused by the volume change. Is known to occur. However, in the present embodiment, the plurality of negative electrode active materials 70 are uniformly dispersed in the negative electrode active material layer 28, and the plurality of alloy portions 73 are provided in a state of being spaced apart from each other. Therefore, peeling of the negative electrode current collector 26 and the negative electrode active material layer 28 is more reliably suppressed, and a lithium ion secondary battery 10 having excellent cycle characteristics can be obtained.

The negative electrode active material layer 28 further includes a negative electrode binder 74 in addition to the conductive auxiliary agent 48 and the negative electrode active material 70. The negative electrode binder 74 corresponds to the "binder" in the claims. The binder 74 for the negative electrode contains polyimide. The polyimide is imidized by heating the polyamic acid, which is a polyimide precursor as the binder material 46 for the negative electrode, in the heating step 42. The imidization reaction of the polyamic acid proceeds by heating at 200 ° C. or higher. As described above, since it is not necessary to separately provide a step for converting the polyimide precursor into polyimide, the negative electrode 18 can be efficiently manufactured.

2. 2. Example The negative electrode manufactured by heating the negative electrode precursor 33 at 230 ° C. for 5 minutes in the heating step 42 was designated as Example 1. Example 2 was a negative electrode manufactured under the same conditions as in Example 1 except that the heating time in the heating step 42 was set to 1 minute.

Three types of negative electrodes were manufactured by changing the presence or absence of the heating step 42 and the raw material powder of the negative electrode active material particles, and used as Comparative Examples 1 to 3. Comparative Example 1 is a negative electrode manufactured by using only Si powder 30 as a raw material powder and without performing a heating step 42. Comparative Example 2 is a negative electrode manufactured under the same conditions as in Comparative Example 1 except that the heating step 42 was performed at 230 ° C. for 5 minutes. Comparative Example 3 is a negative electrode manufactured under the same conditions as in Example 1 except that the heating step 42 is not performed.

A charge / discharge test was performed on each of the negative electrodes of Examples 1 and 2 and Comparative Examples 1 to 3 using an electrochemical cell. The electrochemical cell was produced by using a lithium metal foil as the counter electrode of the negative electrode and using an electrolytic solution prepared by the same method as in the above embodiment. Charging and discharging were performed under a constant current condition of 0.1 mA (0.02C) for 1 to 3 cycles and a constant current condition of 0.5 mA (0.1 C) for 4 to 100 cycles. The cutoff voltage was 0 to 1.5V. The results of the charge / discharge test are shown in FIG. In this figure, the vertical axis shows the charge / discharge capacity and the horizontal axis shows the number of cycles. The white marks indicate the charge capacity, and the filled marks indicate the discharge capacity. In this figure, a legend about the discharge capacity is displayed. In Examples 1 and 2, the charge capacity and the discharge capacity at the initial stage of the cycle and the charge capacity and the discharge capacity after 100 cycles are higher than those of Comparative Examples 1 to 2. Comparing Example 1 and Example 2, the longer the heating time, the more the decrease in capacity due to the charge / discharge cycle is suppressed, and the cycle characteristics are improved. Comparing Example 1 and Comparative Example 2, in Comparative Example 2 in which only Si powder 30 was used without adding Sn powder 31 to the raw material powder even after the heating step 42 for 5 minutes, the charge capacity at the initial stage of the cycle was used. The discharge capacity is also low, and the rate of decrease in capacity due to the charge / discharge cycle is large. Comparing Examples 1 and 2 with Comparative Example 3, even if Sn powder 31 is added to the raw material powder, in Comparative Example 3 in which the heating step 42 is not performed, the capacity is significantly reduced due to the charge / discharge cycle, and 100 cycles. The subsequent volume is reduced to the same level as in Comparative Examples 1 and 2 in which Sn powder 31 is not added to the raw material powder. In Examples 1 and 2, the plurality of Si nanoparticles 60 are connected by the ligament 72 formed by the oxidized Sn nanoparticles 71, so that the conductive path is divided due to the volume change of the negative electrode active material 70 during charging and discharging. It is considered that this was suppressed and excellent cycle characteristics were obtained.

XPS (X-ray Photoelectron Spectroscopy) analysis was performed on the negative electrodes of Example 1 and Comparative Example 3 to examine the oxidation state of the oxidized Sn nanoparticles 71 and the Sn nanoparticles 62. For XPS analysis, an ESCA850 device manufactured by Shimadzu Corporation was used. After setting the negative electrode in the XPS device, vacuum exhaust was performed, and the surface of the negative electrode was etched with argon ions in the depth direction. The ion current and the etching time were set so that the amount (depth) of one etching was several nm. After that, XPS spectrum measurement was performed. The XPS spectrum measurement was performed with the number of etchings being 2, 8, 14, and 20, respectively. FIG. 7 shows the XPS spectrum of Example 1, and FIG. 8 shows the XPS spectrum of Comparative Example 3. In FIG. 7, L1 shows a case where the number of etchings is 2 times, L2 shows a case where the number of times of etching is 8 times, L3 shows a case where the number of times of etching is 14 times, and L4 shows a case where the number of times of etching is 20 times. In FIG. 8, L5 shows a case where the number of etchings is 2, L6 shows a case where the number of etchings is 8, L7 shows a case where the number of etchings is 14, and L8 shows a case where the number of etchings is 20 times. Metallic peaks corresponding to Sn appear at binding energies 485.0 eV and 493.0 eV, _{oxidation state peaks corresponding to SnO 2} at binding energies 486.6 eV and 494.7 eV correspond to SnO at 485.9 eV. A peak appears. 7 and 8, the L1 of Example 1 is conspicuous peak oxidation state corresponding to SnO _2, L2, L3, L4 and toward the deep position of the negative electrode, the direction in which the peak of the metallic state appears It can be seen that the peak is spreading. From this, it was confirmed that in Example 1, SnOx was contained in the surface of the negative electrode active material 70, and the deeper the depth from the surface, the smaller the value of x. In particular, in Example 1, it was confirmed that _{SnO 2} was formed on the outermost surface of the negative electrode active material 70. On the other hand, from FIG. 8, it was confirmed that _{in Comparative Example 3, SnO 2 was scarce on the surface of the negative electrode active material.}

IR (infrared spectroscopy) analysis was performed on the negative electrodes of Examples 1 and 2 and Comparative Example 3 to investigate the progress of the imidization reaction of the polyamic acid as the binder material 46 for the negative electrode. FIG. 9 is an IR analysis result. For IR analysis, an FT-IR apparatus manufactured by JASCO Corporation was used. In FIG. 9, L10 shows the spectra of Example 1, L11 shows the spectra of Example 2, and L12 shows the spectra of Comparative Example 3. From this figure, in Example 1, absorption peaks derived from the imide group were confirmed at ^{1725 cm -1} and 1336 cm ^-1. From this, it was confirmed that the imidization reaction of the polyamic acid, which is a polyimide precursor, proceeded by the heating step 42, and the polyimide was obtained.

A cross section of the negative electrode active material layer 28 of Example 1 was photographed by SEM, and EDS analysis was performed using the obtained SEM image. In the EDS analysis, a Sn mapping image was generated based on the SEM image, and the distribution of the oxidized Sn nanoparticles 71 in the negative electrode active material layer 28 was observed using this Sn mapping image. FIG. 10 shows an SEM image, and FIG. 11 shows a Sn mapping image. From FIG. 11, it was confirmed that in Example 1, Sn was uniformly dispersed without agglomeration and coarsening. From this, it can be seen that the oxidized Sn nanoparticles 71 are uniformly dispersed in the negative electrode active material layer 28.

As described above, the negative electrode precursor formed by applying the slurry containing the negative electrode active material particles 32 having the Sn metal body provided on the surface of the Si nanoparticles 60 to the negative electrode current collector 26 is oxygenated in the heating step 42. By heating at a temperature close to the melting point of Sn in the containing atmosphere, a conductive network N in which a plurality of Si nanoparticles 60 are connected by a ligament 72 formed from the oxidized Sn nanoparticles 71 is formed, so that the electric conductivity is formed. In addition, it is possible to prevent the conduction path from being divided due to the volume change of the negative electrode active material 70 during charging and discharging. Further, since the Cu and Sn alloy portion 73 is formed at the interface between the negative electrode current collector 26 and the negative electrode active material layer 28 by the heating step 42, the adhesion between the negative electrode current collector 26 and the negative electrode active material layer 28 is formed. Is improved, and peeling between the negative electrode current collector 26 and the negative electrode active material layer 28 is more reliably suppressed. Therefore, the irreversible capacity at the initial stage of the cycle is reduced, and the lithium ion secondary battery 10 having excellent cycle characteristics can be obtained.

3. 3. Modification Example The negative electrode active material particles 32 in which Sn nanoparticles are supported on the surface of Si nanoparticles may be formed by a method other than the PS-PVD method. For example, it can be formed by a method of dispersing Si nanoparticles in a solution containing Sn and drying them, or a mechanical granulation method using a ball mill or the like. In the PS-PVD method, nucleation is performed in a relatively high-density mixed steam even when low-grade metallurgical powder is used as a raw material, so that the negative electrode has higher throughput and is easier than the above method. It is preferable because it can form active material particles.

As the Sn metal body, a Sn coating layer may be provided on the surface of the Si nanoparticles. For example, a Sn thin film can be deposited on the surface of Si nanoparticles by the CVD method to form a Sn coating layer. As a result, the negative electrode active material particles having the Sn coating layer provided on the surface of the Si nanoparticles are formed. A negative electrode may be manufactured by applying a slurry containing the negative electrode active material particles to a negative electrode current collector to form a negative electrode precursor and heating the negative electrode precursor in the heating step 42. The negative electrode thus obtained also has the same effect as the negative electrode 18 of the above embodiment.

In the above embodiment, in the heating step 42, the supply of Ar gas and the temperature rise in the annealing chamber are started at the same time, but these may be performed at different timings. For example, the temperature may be started after supplying Ar gas to the annealing chamber for 10 minutes.

As the negative electrode current collector, Ni foil, stainless foil, or the like may be used, but from the viewpoint of alloying with Sn nanoparticles 62, it is preferable to use a metal foil containing Cu, and it is more preferable to use Cu foil. preferable.

10 Lithium ion secondary battery 17 Positive electrode 18 Negative electrode 19, 20 Separator 26 Negative electrode current collector 28 Negative electrode active material layer 32 Negative electrode active material particles 40 Negative electrode active material particle formation process 41 Negative electrode precursor formation process 42 Heating process 60 Si nanoparticles 62 Sn Nanoparticles 70 Negative Electrode Active Material 71 Oxidized Sn Nanoparticles 72 Ligament 73 Alloy Part N Conductive Network

Claims (10)

A negative electrode active material particle forming step of supporting Sn nanoparticles as a Sn metal body on the surface of Si nanoparticles to form negative electrode active material particles, and
A negative electrode precursor forming step of applying the slurry containing the negative electrode active material particles to the negative electrode current collector to form a negative electrode precursor.
A heating step of heating the negative electrode precursor at a temperature of 200 ° C. or higher and 350 ° C. or lower in an atmosphere containing oxygen having an oxygen partial pressure of 2 × 10 ⁴ Pa or less to oxidize the surface of the Sn metal body.
A method for manufacturing a negative electrode for a lithium ion secondary battery. A negative electrode active material particle forming step of supporting Sn nanoparticles as a Sn metal body on the surface of Si nanoparticles to form negative electrode active material particles, and
A negative electrode precursor forming step of applying the slurry containing the negative electrode active material particles to the negative electrode current collector to form a negative electrode precursor.
A heating step of heating the negative electrode precursor at a temperature of 50 ° C. or higher and 350 ° C. or lower in an atmosphere containing oxygen to oxidize the surface of the Sn metal body.
Have,
The negative electrode active material particle forming step is
A mixed steam generation step of evaporating a mixed powder of Si and Sn to generate a mixed steam containing the Si and the Sn.
A first cooling step of cooling the mixed steam and forming the Si nanoparticles by homogeneous nucleation in the mixed steam in the cooling process of the mixed steam.
A second cooling step of further cooling the mixed steam that has undergone the first cooling step and forming the Sn nanoparticles by inhomogeneous nucleation on the surface of the Si nanoparticles.
It is having, features and be Brighter lithium ion anode manufacturing method for a secondary battery to form the anode active material particles by plasma spraying PVD method. A negative electrode active material particle forming step of supporting Sn nanoparticles as a Sn metal body on the surface of Si nanoparticles to form negative electrode active material particles, and
A negative electrode precursor forming step of applying the slurry containing the negative electrode active material particles to the negative electrode current collector to form a negative electrode precursor.
A heating step of heating the negative electrode precursor at a temperature of 50 ° C. or higher and 350 ° C. or lower in an atmosphere containing oxygen to oxidize the surface of the Sn metal body.
Have,
In the negative electrode active material particle forming step, the negative electrode active material particles are formed by a plasma spray PVD method using a mixed powder of Si and Sn.
The atomic concentration of the Sn to the total amount of the mixed powder, wherein the to Brighter lithium ion method of manufacturing a negative electrode for a secondary battery to be within the scope of the following or 1 at.% 5at%. In the first cooling step, the temperature of the mixed steam is set to 2200 K or higher.
The second cooling step, method for producing a negative electrode for a lithium ion secondary battery according to claim 2, characterized in that the cooling rate of the mixed vapor and 10 4 ^{K /} sec. The method for producing a negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4 , wherein the slurry contains a polyimide precursor. Negative electrode current collector and
A negative electrode active material layer containing a plurality of negative electrode active materials provided on the negative electrode current collector is provided.
The negative electrode active material is provided with oxidized Sn nanoparticles, which are Sn nanoparticles whose surface is oxidized, on the surface of Si nanoparticles .
The oxidized Sn nanoparticles have a central portion formed by Sn and a SnOx layer (0 <x≤2) provided on the surface of the central portion, and the SnOx layer has a deep depth from the surface. A negative electrode for a lithium ion secondary battery, characterized in that _{the value of x is as small as possible, SnO 2} is contained in a portion from the surface to a predetermined depth, and SnO is contained in a deeper portion. The negative electrode for a lithium ion secondary battery according to claim 6 , wherein the plurality of the Si nanoparticles are connected by a ligament formed of the oxidized Sn nanoparticles. The negative electrode current collector contains Cu and contains Cu.
The negative electrode active material layer has an alloy portion formed of an alloy of Cu and Sn, and has an alloy portion.
The negative electrode for a lithium ion secondary battery according to claim 6 or 7 , wherein the negative electrode active material is fixed to the negative electrode current collector via the alloy portion. The negative electrode for a lithium ion secondary battery according to any one of claims 6 to 8 , wherein the negative electrode active material layer has a binder containing polyimide. Oxidized Sn nanoparticles, which are Sn nanoparticles whose surface has been oxidized, are provided on the surface of the Si nanoparticles.
The oxidized Sn nanoparticles have a central portion formed by Sn and a SnOx layer (0 <x≤2) provided on the surface of the central portion, and the SnOx layer has a deep depth from the surface. A negative electrode active material for a lithium ion secondary battery, characterized in that _{the value of x is as small as possible, SnO 2} is contained in a portion from the surface to a predetermined depth, and SnO is contained in a deeper portion.

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