JP7329553B2 - Negative electrode for lithium ion secondary battery, lithium ion secondary battery, and method for manufacturing negative electrode for lithium ion secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing a negative electrode for a lithium ion secondary battery.

A lithium ion secondary battery is a secondary battery that can be charged and discharged by moving lithium ions in an electrolyte between a positive electrode and a negative electrode that absorb and release lithium ions. Patent Literature 1 discloses a technique related to a lithium ion secondary battery using a negative electrode active material having tungsten trioxide arranged on its surface.

JP 2018-045904 A

In a lithium ion secondary battery, lithium ions are intercalated and deintercalated in each of the positive electrode and the negative electrode during charging and discharging. At this time, the active material expands/contracts in each electrode, but the active material may expand/contract non-uniformly at this time. In this case, since the surface of the active material becomes non-uniform, there is a problem that the characteristics (such as cycle characteristics) of the lithium ion secondary battery deteriorate. In particular, this problem is conspicuous in the negative electrode of a lithium ion secondary battery.

In view of the above problems, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing a negative electrode for a lithium ion secondary battery that can improve battery characteristics. .

A negative electrode for a lithium ion secondary battery according to one aspect of the present invention includes a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material and formed on the negative electrode current collector. The negative electrode active material includes a particulate core material and a coating layer surrounding the core material, and the coating layer is made of a porous metal material.

A method for producing a negative electrode for a lithium ion secondary battery according to one aspect of the present invention includes coating a metallic material on the surface of a particulate core material, and acid-treating the metallic material coating the core material. forming a negative electrode active material in which the core material is coated with a coating layer made of a porous metal material; forming a negative electrode paste containing at least the negative electrode active material and a solvent; and a step of coating and drying on the electric body.

ADVANTAGE OF THE INVENTION By this invention, the manufacturing method of the negative electrode for lithium ion secondary batteries which can improve a battery characteristic, a lithium ion secondary battery, and the negative electrode for lithium ion secondary batteries can be provided.

1 is a cross-sectional view for explaining a configuration example of a negative electrode for a lithium ion secondary battery according to Embodiment 1; FIG. 1 is a cross-sectional view for explaining a configuration example of a negative electrode active material according to Embodiment 1; FIG. 3 is a cross-sectional view for explaining the state of the negative electrode active material according to Embodiment 1 during charging and discharging; FIG. FIG. 2 is a cross-sectional view for explaining the state of a negative electrode active material according to the prior art during charging and discharging. 4 is a flowchart for explaining an example of a method for manufacturing a negative electrode active material according to Embodiment 1; 4 is a flow chart for explaining an example of a method for manufacturing a negative electrode for a lithium ion secondary battery according to Embodiment 1. FIG. FIG. 10 is an enlarged cross-sectional view for explaining a configuration example of a negative electrode active material according to a second embodiment; 6 is a flowchart for explaining an example of a method for manufacturing a negative electrode active material according to Embodiment 2; FIG. 4 is a Ni—W state diagram.

<Embodiment 1>
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view for explaining a configuration example of a negative electrode for a lithium ion secondary battery according to Embodiment 1. FIG. As shown in FIG. 1 , the lithium ion secondary battery negative electrode 1 according to the present embodiment includes a negative electrode current collector 10 and a negative electrode mixture layer 11 formed on the negative electrode current collector 10 . Although FIG. 1 shows an example in which the negative electrode mixture layer 11 is formed on one side of the negative electrode current collector 10 , the negative electrode mixture layer 11 may be formed on both sides of the negative electrode current collector 10 .

The negative electrode current collector 10 is made of metal foil or metal plate. A metal material such as copper or nickel can be used for the negative electrode current collector 10 . The thickness of the negative electrode current collector 10 can be, for example, about 5 μm to 50 μm. A negative electrode mixture layer 11 is formed on the negative electrode current collector 10 . The thickness of the negative electrode mixture layer 11 can be, for example, about 10 μm to 200 μm. The thicknesses of the negative electrode current collector 10 and the negative electrode mixture layer 11 are examples, and in the present embodiment, the thicknesses of the negative electrode current collector 10 and the negative electrode mixture layer 11 may be other thicknesses.

The negative electrode mixture layer 11 contains a negative electrode active material capable of intercalating and deintercalating lithium. As shown in FIG. 2 , the negative electrode active material 20 includes a particulate core material 21 and a coating layer 22 covering the periphery of the core material 21 . The coating layer 22 is constructed using a porous metal material having a plurality of voids 23 .

The core material 21 is configured using a carbon-based material. Specifically, the core material 21 is configured using graphite, hard carbon, or the like. Also, the core material 21 may be configured using silicon. The grain size of the core material 21 used at this time can be, for example, about 10 μm to 50 μm. In the present invention, the particle diameter of each material is the median diameter D50, which is a value measured using a laser diffraction/scattering method.

The coating layer 22 covers the periphery of the core material 21 . The thickness of the coating layer 22 is preferably 10 nm or more and 100 nm or less. This is because if the thickness of the coating layer 22 is less than 10 nm, the coating layer 22 may break due to the force of expansion and contraction of the core material 21 . Also, if the coating layer 22 is thicker than 100 nm, the coating layer 22 may interfere with the insertion and extraction of lithium ions into and from the core material 21 . Furthermore, the thickness of the coating layer 22 is preferably 20 nm or more and 80 nm or less, more preferably 30 nm or more and 70 nm or less, and even more preferably 40 nm or more and 60 nm or less.

The porosity of the coating layer 22 is preferably 10% or more and 50% or less. This is because if the coating layer 22 has a porosity of less than 10%, the coating layer 22 may hinder the insertion and extraction of lithium ions into and from the core material 21 . Also, if the porosity of the coating layer 22 is higher than 50%, the coating layer 22 may break due to the expansion/contraction force of the core material 21 . Furthermore, the porosity of the coating layer 22 is preferably 20% or more and 40% or less, more preferably 25% or more and 35% or less, and even more preferably 25% or more and 30% or less.

The coating layer 22 is configured using a porous metal material. For example, at least one selected from the group consisting of tungsten, molybdenum, and titanium can be used as the metal material forming the coating layer 22 . A method of forming the plurality of voids 23 in the coating layer 22 will be described later.

Note that the negative electrode mixture layer 11 shown in FIG. 1 may contain a binder, a thickener, and the like in addition to the negative electrode active material 20 . Examples of binders that can be used include fluorine-based resins such as polyvinylidene fluoride, rubber-based binders such as styrene-butadiene rubber (SBR), and thermoplastic resins such as polyethylene. Moreover, carboxymethyl cellulose (CMC) etc. can be used for a thickener.

Next, the effects of the invention according to this embodiment will be described. FIG. 3 is a cross-sectional view for explaining the charge/discharge state of the negative electrode active material according to the present embodiment. FIG. 4 is a cross-sectional view for explaining a charge/discharge state of a conventional negative electrode active material.

As shown in the prior art of FIG. 4, when the negative electrode active material 120 is not provided with a coating layer, lithium ions are inserted into the negative electrode active material 120 during charging and the volume of the negative electrode active material 120 expands. Material 120 may expand unevenly. If charging and discharging are repeated in such a state, there is a problem that the surface of the negative electrode active material 120 becomes uneven and the characteristics (such as cycle characteristics) of the lithium ion secondary battery deteriorate.

On the other hand, in the negative electrode active material 20 according to the present embodiment, the coating layer 22 is formed around the core material 21 as shown in FIG. Here, since the coating layer 22 is made of a metal material with high hardness, when the core material 21 expands in volume due to the insertion of lithium ions into the core material 21 during charging, the core material 21 is evenly distributed. Can be inflated. Moreover, it is possible to prevent the volume of the core material 21 from expanding too much. Furthermore, when lithium ions are desorbed from the core material 21 during discharge and the volume of the core material 21 shrinks, the core material 21 can be uniformly shrunk.

Further, in the present embodiment, the coating layer 22 is made porous by providing a plurality of voids 23 . By making the coating layer 22 porous in this way, lithium ions can easily pass through the coating layer 22 , and the insertion and extraction of lithium ions into and from the core material 21 are promoted. Therefore, it is possible to suppress an increase in the resistance of the negative electrode 1 .

Therefore, the invention according to this embodiment can provide a negative electrode for a lithium-ion secondary battery capable of improving battery characteristics.

Next, a method for manufacturing a negative electrode for a lithium ion secondary battery according to this embodiment will be described. First, a method for manufacturing a negative electrode active material according to this embodiment will be described using the flowchart shown in FIG.

When manufacturing the negative electrode active material 20, first, the surface of the particulate core material 21 is coated with a metal material (step S1). The materials described above can be used for the core material 21 . At least one selected from the group consisting of an alloy of tungsten and nickel, an alloy of tungsten and titanium, an alloy of molybdenum and niobium, and an alloy of titanium and aluminum is used for the metal material that coats the surface of the core material 21. be able to. The thickness of the coating is 10 nm or more and 100 nm or less, preferably 20 nm or more and 80 nm or less, more preferably 30 nm or more and 70 nm or less, and still more preferably 40 nm or more and 60 nm or less.

The method of coating the surface of the core material 21 with the metal material is not particularly limited, but in the present embodiment, for example, a barrel sputtering method can be used. In the barrel sputtering method, powder is placed in a hollow polygonal container (barrel) provided in a vacuum chamber, and the powder in the container is sputtered while rotating the polygonal container. This is a film forming method for forming a film on the surface. The above-mentioned alloy can be used for the target used in the barrel sputtering method. In addition, the thickness of the coating can be adjusted by the film formation time of the barrel sputtering method, the RF output, and the like.

Next, the metal material coating the core material 21 is acid-treated (step S2). The acid treatment dissolves the secondary metals of the coated alloy, and the coating layer 22 (coating) becomes porous. Hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ), or the like can be used for the acid treatment, but acids other than these may also be used. The acid treatment time can be, for example, 1 to 3 hours. For example, the timing when the color of the metal eluted during acid treatment changes (that is, the timing when all the secondary metals are dissolved. When hydrochloric acid (HCl) is used for Ni, green is the color of NiCl _2. ) , the acid treatment may be terminated. The secondary metal is a metal having a lower standard electrode potential than the primary metal.

In the present embodiment, the porosity of the coating layer 22 is 10% or more and 50% or less, preferably 20% or more and 40% or less, more preferably 25% or more and 35% or less, and still more preferably 25% or more and 30% or less. do. The porosity of the coating layer 22 can be adjusted by changing the composition of the metal material used for the coating, that is, the proportion of the main metal and sub-metal in the alloy. For example, by setting the ratio of the main metal in the alloy to 60% to 90% by volume, the porosity of the coating layer 22 can be set to 10% to 50%.

Table 1 below shows an example of the material of the coating layer 22, the alloy used when forming the coating layer 22, and the composition (% by weight, molar ratio) of the alloy. Note that the materials shown in Table 1 are examples, and in the present embodiment, materials other than the materials shown in Table 1 may be used when forming the coating layer 22 .

As shown in Table 1, in the tungsten-nickel alloy, tungsten is the main metal and nickel is the sub-metal, and the coating layer 22 (tungsten) becomes porous when the nickel is dissolved by the acid treatment. In addition, in the tungsten-titanium alloy, tungsten is the main metal and titanium is the sub-metal, and the coating layer 22 (tungsten) becomes porous when the titanium is dissolved by the acid treatment. In the alloy of molybdenum and niobium, molybdenum is the main metal and niobium is the sub-metal, and the coating layer 22 (molybdenum) becomes porous when niobium is dissolved by acid treatment. In the titanium-aluminum alloy, titanium is the main metal and aluminum is the sub-metal, and the coating layer 22 (titanium) becomes porous when the aluminum is dissolved by the acid treatment.

Next, the acid-treated sample is washed with water (step S3), and then dehydrated and dried (step S4), whereby the negative electrode active material 20 can be produced. The drying conditions may be, for example, 350° C. for 1 hour, but other drying conditions may be used.

Next, a method for manufacturing a negative electrode for a lithium ion secondary battery according to this embodiment will be described with reference to the flowchart shown in FIG.

When manufacturing a negative electrode for a lithium ion secondary battery, first, a negative electrode paste is produced (step S11). Specifically, the mixture containing the negative electrode active material and the binder prepared as described above is mixed with a solvent, and the mixture is kneaded to prepare a negative electrode paste. In addition, about a binder, the material mentioned above can be used. Examples of solvents that can be used include isopropyl alcohol, N-methylpyrrolidone, and water. A thickener may be added to the negative electrode paste. The materials described above can be used as the thickening agent.

Next, the negative electrode paste prepared in step S11 is applied to the negative electrode current collector to form the negative electrode mixture layer 11 on the negative electrode current collector 10 (step S12). After that, the negative electrode mixture layer 11 applied to the negative electrode current collector 10 is dried (step S13). The drying temperature at this time can be, for example, 60.degree. C. to 100.degree. Then, the dried negative electrode mixture layer 11 is pressed (step S14). Through such steps, the negative electrode 1 for a lithium ion secondary battery as shown in FIG. 1 can be manufactured.

Next, a lithium ion secondary battery according to this embodiment will be described.
As an example, a lithium ion secondary battery including a wound electrode body will be described below. The wound electrode body is obtained by laminating a long positive electrode sheet (positive electrode) and a long negative electrode sheet (negative electrode) with a long separator interposed therebetween, and winding the laminate. It is formed by squeezing the body from the lateral direction. For the negative electrode sheet, the negative electrode for lithium ion secondary batteries described above can be used. As with the negative electrode sheet, a positive electrode in which positive electrode mixture layers containing a positive electrode active material are formed on both sides of a foil-shaped positive electrode current collector can be used as the positive electrode sheet.

A container for a lithium-ion secondary battery includes a flat rectangular parallelepiped container body with an open upper end, and a lid that closes the opening. Metal materials such as aluminum and steel are preferable as materials for forming the container. Alternatively, a container molded from a resin material such as polyphenylene sulfide resin (PPS) or polyimide resin may be used. A positive electrode terminal electrically connected to the positive electrode of the wound electrode body and a negative electrode terminal electrically connected to the negative electrode of the wound electrode body are provided on the upper surface of the container (that is, the lid).

Then, a positive electrode lead terminal and a negative electrode lead terminal are respectively provided at the portions where the positive electrode sheet and the negative electrode sheet are exposed (portions without the positive electrode mixture layer and the negative electrode mixture layer) at both ends of the wound electrode body, and the positive electrode terminal described above is provided. and the negative terminal, respectively. The wound electrode body thus produced is housed in a container body, and the opening of the container body is sealed with a lid. Thereafter, a lithium ion secondary battery can be fabricated by injecting an electrolytic solution through an injection hole provided in the lid and closing the injection hole with a sealing cap.

<Embodiment 2>
Next, Embodiment 2 of the present invention will be described.
FIG. 7 is an enlarged cross-sectional view for explaining a configuration example of the negative electrode active material 30 according to the second embodiment. Note that FIG. 7 also illustrates the negative electrode active material 20 described in the first embodiment.

As shown in FIG. 7 , in the negative electrode active material 20 according to the first embodiment, when forming the coating layer 22 , the core material 21 is coated with a metal material (alloy), and then an acid treatment is performed to form the voids 23 . was forming. At this time, in the negative electrode active material 20 according to the first embodiment, the porosity of the coating layer 22 is adjusted by changing the composition of the metal material used for coating (that is, the ratio of the main metal and the sub-metal of the alloy). Ta. However, in the negative electrode active material 20 according to Embodiment 1, since the size of the voids 23 was not controlled, the distribution of the hole diameters of the voids 23 tended to be narrow (that is, the variation in hole diameters was small. tended to be). When the hole diameter distribution of the voids 23 is narrow in this way, the probability that the paths formed by connecting the voids 23 (indicated by arrows in the figure) penetrate the coating layer 22 may be low. . In other words, when the hole diameter distribution of the voids 23 is narrow, voids 23 that do not contribute to the formation of paths necessary for permeation of the electrolytic solution (paths penetrating through the coating layer 22) may be generated.

On the other hand, in the negative electrode active material 30 according to the second embodiment, the hole diameter distribution of the voids 33 in the coating layer 32 is widened (that is, the hole diameter varies widely). In this way, when the distribution of hole diameters of the voids 33 is widened, the probability that a path formed by connecting the voids 33 (indicated by arrows in the figure) penetrates the coating layer 32 increases. is higher than in form 1 of In other words, when the distribution of hole diameters of the voids 33 is wide, the number of voids 23 that do not contribute to the formation of paths required for permeation of the electrolytic solution (paths penetrating the coating layer 32) can be reduced. Therefore, in the negative electrode active material 30 according to the second embodiment, many paths necessary for permeation of the electrolytic solution can be formed in the coating layer 32, so that the cycle characteristics of the lithium ion secondary battery can be improved. . It should be noted that since the material is the same as the negative electrode active material 20 described in Embodiment 1 except that the hole diameter distribution of the voids 33 is wide, redundant description will be omitted.

Next, a method for manufacturing a negative electrode active material according to this embodiment will be described using the flowchart shown in FIG.

When manufacturing the negative electrode active material 30, first, the surface of the particulate core material 21 is coated with a metal material (step S21). The same materials as those described in the first embodiment can be used for the core material 21 and the metal material for coating. The method of coating the surface of the core material 21 with the metal material is not particularly limited, but in the present embodiment, for example, a barrel sputtering method can be used. In addition, the thickness of the coating can be adjusted by the film formation time of the barrel sputtering method, the RF output, and the like.

Next, the metal material coating the core material 21 is heat-treated (annealed) (step S22). The heat treatment temperature at this time is, for example, 500° C. or higher and 1000° C. or lower. Heat treatment can change the structure of the metal material (that is, the alloy containing the primary metal and the secondary metal). That is, by heat-treating the metal material (alloy) after film formation, the structure (structure) of the alloy can be changed according to the phase diagram of the alloy containing the main metal and the sub-metal. For example, an alloy of tungsten and nickel changes its structure according to the phase diagram shown in FIG.

For example, when the molar ratio of tungsten and nickel is W:Ni=65:35, when the metal material (alloy) after film formation is heat treated at a temperature of 500° C. or more and 1000° C. or less, NiW and NiW ₂ A mixed phase (phases with different compositions from each other) is produced (corresponding to the part of Example 2 in FIG. 9). Here, since the amount of Ni differs between NiW and _NiW2 , the arrangement of Ni changes compared to the state of the alloy after film formation.

Next, the metal material coating the core material 21 is acid-treated (step S23). The acid treatment dissolves the submetal of the metal material (alloy), and the coating layer 32 (coating) becomes porous. In the case of an alloy of tungsten and nickel, since tungsten is the main metal and nickel is the sub-metal, the acid treatment dissolves the nickel to make the coating layer 32 (tungsten) porous. Hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ), or the like can be used for the acid treatment, but acids other than these may also be used. The acid treatment time can be, for example, 1 to 3 hours. For example, the timing when the color of the metal eluted during acid treatment changes (that is, the timing when all the secondary metals are dissolved. When hydrochloric acid (HCl) is used for Ni, green is the color of NiCl _2. ) , the acid treatment may be terminated. The secondary metal is a metal having a lower standard electrode potential than the primary metal.

Also in the present embodiment, the porosity of the coating layer 32 is 10% or more and 50% or less, preferably 20% or more and 40% or less, more preferably 25% or more and 35% or less, and still more preferably 25% or more and 30% or less. and The porosity of the coating layer 32 can be adjusted by changing the composition of the metal material used for the coating, that is, the ratio of the main metal and sub-metal in the alloy. For example, by setting the ratio of the main metal in the alloy to 60% to 90% by volume, the porosity of the coating layer 32 can be set to 10% to 50%.

Next, the acid-treated sample is washed with water (step S24), and then dehydrated and dried (step S25), whereby the negative electrode active material 30 can be produced. The drying conditions may be, for example, 350° C. for 1 hour, but other drying conditions may be used.

Note that the method for manufacturing the negative electrode for lithium ion secondary batteries according to the present embodiment is the same as the method for manufacturing the negative electrode for lithium ion secondary batteries described in Embodiment 1, so redundant description will be omitted. .

As described above, in the method for manufacturing a negative electrode active material according to the present embodiment, after the surface of the core material 21 is coated with the metal material, the metal material is heat-treated before the acid treatment (step S22). ). When the metal material is heat-treated in this way, the structure (structure) of the metal material can be changed according to the state diagram of the metal material (alloy) containing the main metal and the sub-metal. After that, by acid-treating the metal material (step S23), the sub-metal contained in the metal material is dissolved, and the covering layer 32 (coating) becomes porous. In the present embodiment, since the structure of the metal material is changed by the heat treatment, it is possible to widen the distribution of hole diameters of the voids 33 formed in the coating layer 32 (that is, to increase the variation in hole diameters). can be done).

For example, when the molar ratio of tungsten and nickel shown in the above example is W:Ni=65:35, a mixed phase of NiW and _NiW2 is generated by heat treatment (corresponding to the part of Example 2 in FIG. 9). . Since the amount of Ni (sub-metal) differs between NiW and _NiW2 , the arrangement of Ni (sub-metal) changes compared to the state of the alloy after film formation. Therefore, by subsequently dissolving Ni (submetal) using an acid treatment, it is possible to form the voids 33 with a wide distribution of hole diameters. That is, the distribution of hole diameters of the voids 33 corresponds to the distribution of NiW and _NiW2 .

As described above, in the negative electrode active material 30 according to the present embodiment, the hole diameter distribution of the voids 33 in the coating layer 32 is widened (that is, the hole diameter varies widely). ing. In this way, when the hole diameter distribution of the voids 33 is wide, many paths required for permeation of the electrolytic solution can be formed in the coating layer 32 . Therefore, the cycle characteristics of the lithium ion secondary battery can be improved.

In the above example, an alloy of tungsten and nickel is used as the metal material, but in the present embodiment, any metal material (alloy) that generates a mixed phase by heat treatment can be used. may In other words, if the metal material generates a mixed phase by heat treatment, the arrangement of the sub-metals can be changed by heat treatment. Therefore, by performing an acid treatment after that, it is possible to form the voids 33 having a wide distribution of hole diameters.

Examples of the present invention will now be described.
As shown below, lithium ion secondary batteries according to Example 1, Comparative Example 1, and Comparative Example 2 were produced.

<Example 1>
A lithium ion secondary battery according to Example 1 was produced using the following method. In Example 1, first, a coating layer having a thickness of 50 nm and a porosity of 25% was formed on the surface of a particulate core material having a particle size of 10 μm to prepare a negative electrode active material. The method shown in FIG. 5 was used to prepare the negative electrode active material.

Specifically, first, 10 g of graphite having a particle size of 10 μm was prepared as a core material. Also, 1 g of an alloy of tungsten and nickel (W—Ni alloy) was prepared as a metal material for forming the coating layer. The weight ratio of the W—Ni alloy was W:Ni=90:10 (wt.%). The molar ratio of the W—Ni alloy at this time is W:Ni=75:25 (=3:1).

Then, using a barrel sputtering method, the surface of the particulate core material was coated with a W—Ni alloy having a thickness of 50 nm. An apparatus manufactured by Sambac Co., Ltd. was used for the barrel sputtering method. The coating conditions were an RF output of 100 W, an Ar gas atmosphere, a sputtering time of 100 minutes, and a barrel rotation speed of 5 rpm.

Next, the coated core material was taken out and subjected to an acid treatment for 3 hours. Hydrochloric acid (pH=0) with a concentration of 1 mol/L was used for the acid treatment. After that, the core material after the acid treatment was washed with water, dehydrated and dried at a temperature of 350° C. for 1 hour to prepare a negative electrode active material.

After producing the negative electrode active material, a negative electrode was produced using the method shown in FIG. Specifically, first, 98% by weight of the negative electrode active material prepared as described above, 1% by weight of carboxymethyl cellulose (CMC) as a thickener, and 1% by weight of styrene-butadiene rubber (SBR) as a binder. , were mixed, and the mixture was added to water as a solvent and kneaded to prepare a negative electrode paste. After that, the prepared negative electrode paste was applied to a copper foil as a negative electrode current collector to form a negative electrode mixture layer on the negative electrode current collector. Then, the negative electrode mixture layer was dried at a drying temperature of 60° C. for 60 minutes, and then pressed. At this time, the thickness of the negative electrode mixture layer (thickness after pressing) was set to 50 μm.

Also, a positive electrode was produced using the same method as the method for producing the negative electrode. At this time, 90% by weight of NMC (weight ratio 1:1:1) was used as the positive electrode active material, 8% by weight of AB was used as the conductive material, 2% by weight of PVDF was used as the binder, and NMP was used as the solvent. Using. Aluminum foil was used for the positive electrode current collector.

After that, a terminal was welded to each of the positive electrode and the negative electrode. Then, after the positive electrode and the negative electrode were arranged in the battery container and opposed to each other, the electrolytic solution was injected. As the electrolyte, an electrolyte containing 1M LiPF ₆ as a salt and EC, DMC, and EMC (in a volume ratio of 1:1:1, respectively) as solvents was used. A lithium ion secondary battery according to Example 1 was fabricated using the above method.

<Example 2>
A lithium ion secondary battery according to Example 2 was produced using the following method. In Example 2, first, a coating layer having a thickness of 50 nm and a porosity of 35% was formed on the surface of a particulate core material having a particle size of 10 μm to prepare a negative electrode active material. The method shown in FIG. 8 was used to prepare the negative electrode active material.

Specifically, first, 10 g of graphite having a particle size of 10 μm was prepared as a core material. Also, 1 g of an alloy of tungsten and nickel (W—Ni alloy) was prepared as a metal material for forming the coating layer. The molar ratio of the W—Ni alloy was W:Ni=65:35.

Then, using a barrel sputtering method, the surface of the particulate core material was coated with a W—Ni alloy having a thickness of 50 nm. An apparatus manufactured by Sambac Co., Ltd. was used for the barrel sputtering method. The coating conditions were an RF output of 100 W, an Ar gas atmosphere, a sputtering time of 100 minutes, and a barrel rotation speed of 5 rpm.

Next, the coated core materials were taken out and put into an electric furnace for heat treatment. The heat treatment conditions were 800° C. and 3 hours. This heat treatment produced a mixed phase of NiW and NiW ₂ as shown in the phase diagram of FIG. After that, the core material after the heat treatment was taken out and subjected to an acid treatment for 1 hour. Hydrochloric acid (pH=0) with a concentration of 1 mol/L was used for the acid treatment. After that, the core material after the acid treatment was washed with water, dehydrated and dried at a temperature of 350° C. for 1 hour to prepare a negative electrode active material.

After producing the negative electrode active material, the same method as in Example 1 was used to produce a negative electrode and a lithium ion secondary battery.

<Comparative Example 1>
As Comparative Example 1, a lithium ion secondary battery was produced using a negative electrode active material in which a core material was coated with a coating layer having no voids. Specifically, first, 10 g of graphite having a particle size of 10 μm was prepared as a core material. Also, 1 g of tungsten was prepared as a metal material for forming the coating layer. Then, using a barrel sputtering method, the surface of the particulate core material was coated with tungsten having a thickness of 50 nm, and a negative electrode active material according to Comparative Example 1 was produced. After that, the same method as in Example 1 was used to produce a lithium ion secondary battery.

<Comparative Example 2>
As Comparative Example 2, a lithium ion secondary battery was produced using a negative electrode active material without a coating layer. Specifically, a lithium ion secondary battery was produced using graphite with a particle size of 10 μm as a negative electrode active material. In addition, the method similar to Example 1 was used about the manufacturing method of a lithium ion secondary battery.

<Measurement of battery characteristics>
Battery characteristics of lithium ion secondary batteries according to Example 1, Comparative Example 1, and Comparative Example 2 were measured using the following method. First, the initial resistance (DC-IR) was determined using the amount of voltage drop when a current of 1C was passed through the lithium ion secondary battery. Specifically, the voltage value is recorded after a certain period of time (10 seconds) has passed since the current of 1 C was applied. Then, the difference between the battery voltage (OCV) measured when no current is flowing and the voltage at the time of measurement is ΔV, and the initial resistance (DC -IR) was calculated. Table 2 shows the initial resistance of each sample. Table 2 shows the ratio of each initial resistance when the initial resistance of the lithium ion secondary battery according to Comparative Example 2 is used as a reference (Ref).

Also, cycle characteristics were measured using the following method.
First, a charging/discharging cycle of charging to 4.2 V at a charging rate of 1 C and discharging to 3.0 V at a discharging rate of 1 C was repeated 300 cycles. After repeating charging and discharging for 300 cycles, the battery resistance of the lithium ion secondary battery was measured. Table 2 shows the cycle characteristics (battery resistance after 300 cycles) of each sample. Table 2 shows the ratio of each resistance value when the resistance value of the lithium ion secondary battery according to Comparative Example 2 is used as a reference (Ref).

In Example 1, the battery resistance after 300 cycles was lower than in Comparative Example 2. In other words, the tungsten used in the coating layer of the negative electrode active material is a strong metal material, and by covering the surface of the core material with tungsten, it is possible to suppress uneven expansion and contraction during the insertion and extraction of lithium ions. Therefore, in Example 1, the cycle characteristics were improved. Further, in Example 1, tungsten used for the coating layer of the negative electrode active material has low electron conductivity, but the coating layer is porous (that is, there are a plurality of voids), so the decrease in electron conductivity is suppressed. We were able to. Therefore, the resistance did not increase even when a large current was applied.

Moreover, in Example 2, the cycle characteristics were improved as compared with Example 1. In Example 2, when the negative electrode active material was formed, the metal material was heat-treated to form a mixed phase of NiW and _NiW2 . It is considered that many paths necessary for permeation of the electrolytic solution could be formed in the coating layer. It is considered that the cycle characteristics of the lithium ion secondary battery were improved for such a reason.

Although the present invention has been described with reference to the above embodiments, the present invention is not limited to the configurations of the above embodiments. It goes without saying that various modifications, modifications, and combinations that can be made are included.

1 negative electrode for lithium ion secondary battery 10 negative electrode current collector 11 negative electrode mixture layers 20, 30 negative electrode active material 21 core materials 22, 32 coating layers 23, 33 voids

Claims (12)

a negative electrode current collector;
a negative electrode mixture layer containing a negative electrode active material formed on the negative electrode current collector;
The negative electrode active material comprises a particulate core material and a coating layer surrounding the core material,
The coating layer is configured using a porous metal material,
The metal material constituting the coating layer is at least one selected from the group consisting of tungsten, molybdenum, and titanium.
Negative electrode for lithium-ion secondary batteries. 2. The negative electrode for a lithium ion secondary battery in accordance with claim 1, wherein said coating layer has a thickness of 10 nm or more and 100 nm or less. 3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein said coating layer has a porosity of 10% or more and 50% or less. 4. The negative electrode for a lithium ion secondary battery according to claim 1 , wherein said core material is made of a carbonaceous material. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4 . The surface of a particulate core material is coated with a metal material, and the metal material coated with the core material is acid-treated to provide a negative electrode active layer in which the core material is coated with a coating layer made of a porous metal material. forming a substance;
forming a negative electrode paste containing at least the negative electrode active material and a solvent;
and a step of applying the negative electrode paste onto a negative electrode current collector and drying it.
A method for producing a negative electrode for a lithium ion secondary battery. 7. The method for producing a negative electrode for a lithium ion secondary battery according to claim 6 , wherein the coating layer has a thickness of 10 nm or more and 100 nm or less. 8. The method for producing a negative electrode for a lithium ion secondary battery according to claim 6 , wherein said coating layer has a porosity of 10% or more and 50% or less. The negative electrode for a lithium ion secondary battery according to any one of claims 6 to 8 , wherein after coating the surface of the core material with the metal material and before performing the acid treatment, the metal material is heat-treated. manufacturing method. The method for producing a negative electrode for a lithium ion secondary battery according to claim 9 , wherein the temperature of said heat treatment is 500°C or higher and 1000°C or lower. The metal material coated on the surface of the core material is at least one selected from the group consisting of an alloy of tungsten and nickel, an alloy of tungsten and titanium, an alloy of molybdenum and niobium, and an alloy of titanium and aluminum. Item 11. A method for producing a negative electrode for a lithium ion secondary battery according to any one of Items 6 to 10 . The method for producing a negative electrode for a lithium ion secondary battery according to any one of claims 6 to 11 , wherein the core material is a carbon-based material.

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