JP2017152122A - Negative electrode active material for lithium ion secondary batteries, negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a negative electrode active material for a lithium secondary battery and a negative electrode for a lithium ion secondary battery, which are superior in high-temperature cycle characteristics, and which are arranged to be able to suppress the reaction between a negative electrode active material and an electrolytic solution and gas generation during charge and discharge cycles; and a lithium ion secondary battery using the negative electrode.SOLUTION: A negative electrode active material for a lithium ion secondary battery comprises: a polyacrylic acid including a carboxy group and an acid anhydride group; and a negative electrode active substance of which the surface is coated with the polyacrylic acid. In the negative electrode active material for a lithium ion secondary battery, the polyacrylic acid is included preferably at 0.1-5 wt%. The ratio of a thickness of the polyacrylic acid coating to a particle diameter of particles of the negative electrode active substance is 0.0005-0.02.SELECTED DRAWING: Figure 1

Description

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

  Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. Moreover, it is also a promising candidate as a vehicle-mounted power source mounted for hybrid vehicles and electric vehicles. In recent years, along with the downsizing and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries serving as power sources.

The negative electrode active material generally has a low potential during charging, and side reactions that reduce and decompose the electrolyte generally occur. At that time, a film made of a decomposition product of a gas such as carbon dioxide or hydrogen or an electrolytic solution is formed on the surface of the negative electrode active material. The generated gas increases the internal resistance of the lithium ion secondary battery, and the coating does not participate in charging / discharging, so that the discharge capacity is reduced and the cycle characteristics are thus deteriorated. On the other hand, by forming this film, the reaction between the negative electrode active material and the electrolytic solution during charging is alleviated. However, excessive growth of the coating film due to the decomposition product of the electrolytic solution is not preferable because the internal resistance is increased.

The surface of the negative electrode active material of a lithium ion secondary battery using a carbon material is coated with a polymer film containing an alkali metal salt for the purpose of suppressing side reactions between the negative electrode active material and the electrolyte and improving cycle characteristics. For example, it has been proposed to cover a silicon material with a polymer film or the like (for example, Patent Document 2).

JP-A-8-306353 JP-A-2005-197258

However, the conventional method has not been sufficient to improve the cycle characteristics. In addition, the reactivity of the electrolyte solution becomes very high at high temperature cycles of 60 ° C. or higher, so the side reaction that occurs during the charge / discharge cycle becomes very active and tends to have a greater adverse effect on the lithium ion secondary battery. There is.
When used as an in-vehicle power source, the lithium ion secondary battery may be exposed to high temperatures, so it is essential to improve the high-temperature cycle characteristics.
As described above, even when the technique of coating the negative electrode active material with the polymer film is used, when the lithium ion secondary battery becomes high temperature due to external heating or the like, the negative electrode active material that occludes lithium and the electrolytic solution are used. It is necessary to obtain sufficient high-temperature cycle characteristics in an environment where side reactions are very active.

In view of the above problems, an object of the present invention is to provide a negative electrode active material for a lithium ion secondary battery excellent in high-temperature cycle characteristics, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the same. .

  In order to solve the above problems, the negative electrode active material of the present invention is characterized in that the surface of the negative electrode active material particles is coated with polyacrylic acid containing a carboxylic acid group and an acid anhydride group.

According to the negative electrode active material for a lithium ion secondary battery according to the present invention, the surface of the negative electrode active material is coated with polyacrylic acid in which a carboxy group and an acid anhydride are present. It is possible to suppress the reaction between the electrolyte solution and the gas generation, and to suppress deterioration due to the charge / discharge cycle.
This is because the acid anhydride obtained by dehydration condensation of the carboxy group is chemically stable, so that the reaction between the electrolytic solution and the negative electrode active material is suppressed even under a high temperature cycle where the electrolytic solution is likely to be active. It is considered that decomposition and gas generation can be suppressed. As a result, it has excellent high temperature cycle characteristics.
In addition, the carboxy group maintains good adhesion to the surface of the negative electrode active material, can follow the expansion and contraction of the negative electrode active material, and suppresses peeling of the polymer compound coated on the surface of the negative electrode active material. It is considered possible.

The polyacrylic acid according to the present invention is preferably contained in an amount of 0.1 to 5% by weight with respect to the negative electrode active material.

If it is in this range, the surface of the negative electrode active material is coated, and there is an effect of suppressing the reaction between the active material and the electrolyte more sufficiently, and the insertion / extraction of Li ions during charge / discharge is not alienated, It is suitable for suppressing the reaction between the negative electrode active material and the electrolytic solution.

In the negative electrode active material for a lithium ion secondary battery according to the present invention, the ratio of the coating thickness of the polyacrylic acid to the particle size of the negative electrode active material particles is preferably 0.0005 to 0.02.

According to this, the effect of buffering the stress in the negative electrode active material layer generated due to the expansion of the negative electrode active material during charging is increased, and the high-temperature cycle characteristics can be further improved.

ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material for lithium ion secondary batteries which is excellent in a high temperature cycling characteristic, the negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery can be provided.

It is a schematic cross section of a lithium ion secondary battery.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(Lithium ion secondary battery)
FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to this embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30. ing.

  The laminated body 30 is configured such that a pair of a positive electrode 10 and a negative electrode 20 are disposed to face each other with a separator 18 interposed therebetween. The positive electrode 10 is obtained by providing a positive electrode active material layer 14 on a plate-like (film-like) positive electrode current collector 12. The negative electrode 20 is obtained by providing a negative electrode active material layer 24 on a plate-like (film-like) negative electrode current collector 22. The main surface of the positive electrode active material layer 14 and the main surface of the negative electrode active material layer 24 are in contact with the main surface of the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 10 and the negative electrode 20 are collectively referred to as electrodes 10 and 20, and the positive electrode current collector 12 and the negative electrode current collector 22 are collectively referred to as current collectors 12 and 22, and the positive electrode active material layer 14 and the negative electrode The active material layer 24 may be collectively referred to as the active material layers 14 and 24. First, the electrodes 10 and 20 will be specifically described.

(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a conductive auxiliary agent in an amount as necessary.

(Positive electrode active material)
Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions (for example, PF ₆ ⁻ ) of the lithium ions. The electrode is not particularly limited as long as it can be reversibly advanced, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the general formula: LiNi _x Co _y Mn _z MaO ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Oxide, lithium vanadium compound (LiV ₂ O ₅ , LiVOPO ₄ ), olivine-type LiMPO ₄ (where M is one or more selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) elements), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1), Li 2 MnO 3 -LiM _{2 (wherein} M is Mn, Co, selected one or more elements from Ni) complex metal oxide such as Li excess solid solution represented by the like.

(Positive electrode binder)
The binder binds the positive electrode active materials and the current collector 12 together with the positive electrode active materials. The binder is not particularly limited as long as the above-described bonding is possible, and examples thereof include fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition to the above, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, or the like may be used as the binder. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant. As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ , or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

Although content of the binder in the positive electrode active material layer 14 is not specifically limited, It is preferable that it is 1-10 mass% on the basis of the sum of the mass of a positive electrode active material, a conductive support agent, and a binder. By making content of a positive electrode active material and a binder into the said range, in the obtained positive electrode active material layer 14, the amount of binders is too small and the tendency which cannot form a strong positive electrode active material layer can be suppressed. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

(Positive electrode conductive aid)
The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

The content of the conductive additive in the positive electrode active material layer 14 is not particularly limited, but when added, it is preferably 0.5 to 5% by mass with respect to the mass of the positive electrode active material.

(Negative electrode current collector)
The negative electrode current collector 22 may be any conductive plate material, and for example, a metal thin plate (metal foil) of copper, nickel, stainless steel, or an alloy thereof can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a negative electrode binder, and an amount of a conductive aid as required.
(Negative electrode active material)
Examples of the negative electrode active material include graphite, non-graphitizable carbon, graphitizable carbon, and low-temperature calcined carbon that can occlude / release (intercalate / deintercalate, or dope / dedope) lithium ions. Carbon materials, metals that can be combined with lithium such as Al, Si, SiO and Sn, crystalline and amorphous compounds mainly composed of oxides such as TiO ₂ , SnO ₂ and Fe ₂ O ₃ , titanic acid and lithium _{(Li 4} Ti _{5 O 12)} particles and the like are.

The negative electrode active material of this embodiment is characterized in that the surface of the negative electrode active material particles is coated with polyacrylic acid containing a carboxy group and an acid anhydride group.
The negative electrode active material of this embodiment is coated with polyacrylic acid in which a part of the carboxy group is dehydrated and condensed to form an acid anhydride group. Since the acid anhydride group is chemically stable, the reaction between the electrolyte and the active material is suppressed even during high-temperature cycles where the electrolyte is likely to become active, and the reductive decomposition of the electrolyte and the generation of gas are suppressed. be able to. Thereby, by avoiding direct contact between the negative electrode active material surface and the electrolytic solution, side reactions can be suppressed and high-temperature cycle characteristics can be improved.

In addition, the carboxy group of polyacrylic acid is characterized by strong adhesion to the active material surface. Thereby, the expansion and contraction of the negative electrode active material particles at the time of charging / discharging is followed, and it is considered good.
On the other hand, a part of the carboxy group in polyacrylic acid is polymerized to form an acid anhydride, thereby strengthening the three-dimensional bond network and collapsing the negative electrode active material particles due to expansion of the negative electrode active material. It is considered that the effect of suppressing the above can be obtained.

By covering the surface of the negative electrode active material particles with polyacrylic acid in which a carboxy group and an acid anhydride are present at the same time, the side reaction caused by the contact between the negative electrode active material and the electrolyte is suppressed, and the negative electrode active material particles are prevented from collapsing. Due to this effect, the high-temperature cycle characteristics can be improved.

The method for coating the surface of the negative electrode active material with polyacrylic acid is not particularly limited. For example, a method in which a slurry in which the negative electrode active material and polyacrylic acid are uniformly dispersed is spray-dried with a spray dryer apparatus or the like. And a method in which a slurry in which a negative electrode active material and polyacrylic acid are uniformly dispersed is dropped into a solution in which a cross-linking agent is dissolved. The polyacrylic acid resin layer can be coated on.

The negative electrode active material and polyacrylic acid are weighed so as to have an arbitrary coating amount, and kneaded while adding pure water to prepare a slurry in which the negative electrode active material and polyacrylic acid are uniformly dispersed. Then, after drying the slurry and sufficiently volatilizing the portion, the dried product is crushed and pulverized, passed through a sieve having a predetermined opening, and a negative electrode active material having a polyacrylic acid resin layer coated on the surface is obtained.

Moreover, heat-processing is mentioned as a method of introduce | transducing an acid anhydride group into a part of carboxy group in the coated polyacrylic acid. Thereby, it is considered that the carboxy group in the side chain of polyacrylic acid undergoes dehydration condensation to produce an acid anhydride group. As a heat treatment method, a batch-type drying furnace or an infrared drying furnace can be used. In particular, an infrared drying furnace is useful because it can heat-treat the polymer efficiently and in a short time. The amount of the acid anhydride group can be controlled by the heat treatment temperature. The heat treatment temperature is 180 ° C to 300 ° C, more preferably 200 ° C to 250 ° C. The heat treatment time is further preferably 1 to 24 hours, more preferably 2 to 12 hours. The timing of the heat treatment may be either when the slurry in which the negative electrode active material and polyacrylic acid are uniformly dispersed is dried or after the slurry is dried.

In addition, what is necessary is just to use the amount of a carboxy group and an acid anhydride group for a FT-IR measuring machine. In the measured IR absorption spectrum may be compared with the peak intensity in the vicinity of 1235cm ^-1 indicating the carboxyl group, the ratio of the peak intensity in the vicinity of 1800 cm ^-1 which represents an acid anhydride. At that time, the intensity ratio of the IR absorption spectrum, subtracting the background measured IR absorption spectrum, the peak intensity in the vicinity of 1235cm ^-1 indicating the carboxyl group, the peak intensity in the vicinity of 1800 cm ^-1 indicating an acid anhydride in a ratio It is obtained by expressing.

The covering amount of polyacrylic acid is preferably 0.05 to 6.5% by weight with respect to the negative electrode active material. If even a part of the surface of the negative electrode active material is coated with a polymer compound, the contact area with the electrolytic solution can be reduced, so that side reactions can be suppressed. It is preferable to uniformly and uniformly coat the surface. If the coating amount is within this range, the entire surface of the negative electrode active material tends to be covered easily, which is more suitable for suppressing the reaction between the negative electrode active material and the electrolytic solution. Furthermore, the coating amount is preferably 0.1 to 5% by weight because the above-described effects are improved.

Furthermore, if the coating amount is within this range, the high-temperature cycle characteristics can be further improved.

The ratio of the coating thickness of the polyacrylic acid to the particle size of the negative electrode active material particles (coating thickness / particle size) is preferably 0.0005 to 0.02. Within this range, the surface of the negative electrode active material is coated, and there is an effect of suppressing the reaction between the active material and the electrolyte, and there is no alienation of insertion / extraction of Li ions during charge / discharge. It is suitable for suppressing the reaction between the substance and the electrolytic solution. The diameter of the negative electrode active material particles is R (μm), the thickness of the polyacrylic acid film is T (μm), and the ratio of the coating thickness of the polyacrylic acid to the particle size of the negative electrode active material particles = T / R. It is defined as

Moreover, it is preferable that the coating state of the polyacrylic acid on the negative electrode active material particles in the present embodiment is a state in which approximately half or more is covered.

(Negative electrode binder)
The negative electrode binder is added for the purpose of maintaining the electrode structure by bringing the negative electrode active material layer 24 into close contact with each other or the negative electrode active material layer 24 and the negative electrode current collector 22. As the binder contained in the negative electrode 20 for a lithium ion secondary battery, known materials can be used. For example, polyimide, polyamideimide, polyacrylonitrile, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC) and the like are preferable. Can be used. SBR / CMC mixed resin is more preferable.

(Negative conductive auxiliary)
The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the negative electrode active material layer 24, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

In addition, the content of the conductive auxiliary agent is appropriately adjusted when the volume change of the negative electrode active material and the adhesion to the foil must be taken into account, and the same content as the content in the positive electrode 10 described above should be adopted. That's fine. It is preferable that the addition amount of a conductive support agent is 0.5-5 mass% with respect to the mass of a negative electrode active material.

With the components described above, the electrodes 10 and 20 can be produced by a commonly used method. For example, a paint containing an active material (a positive electrode active material or a negative electrode active material), a binder (a positive electrode binder or a negative electrode binder), a solvent, and a conductive additive (a positive electrode conductive additive or a negative electrode conductive aid) is placed on the current collector. It can manufacture by apply | coating and removing the solvent in the coating material apply | coated on the electrical power collector.

  As the solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylformamide, water and the like can be used.

There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

The method for removing the solvent in the paint applied onto the current collectors 12 and 22 is not particularly limited, and the current collectors 12 and 22 applied with the paint may be dried at a temperature at which the solvent volatilizes. Further, the heat treatment method for introducing the acid anhydride group into the polyacrylic acid is not particularly limited, and the current collectors 12 and 22 to which the paint is applied may be 180 ° C. to 300 ° C. using, for example, an infrared dryer. What is necessary is just to heat-process in the range.

  Then, the electrodes on which the active material layers 14 and 24 are formed in this way may then be pressed by a roll press device or the like as necessary. The linear pressure of the roll press can be, for example, 10 to 50 kgf / cm.

Next, other components of the lithium ion secondary battery 100 will be described.

(Separator)
The separator is not particularly limited as long as it is stable with respect to the electrolytic solution and has excellent liquid retention, but generally includes a porous sheet of polyolefin such as polyethylene and polypropylene, or a nonwoven fabric.

(Electrolytes)
The electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited, and for example, in the present embodiment, an electrolytic solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, so that the withstand voltage during charging is limited to a low level. As the electrolytic solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. It does not specifically limit as lithium salt, The lithium salt used as an electrolyte of a lithium ion secondary battery can be used. For example, as the lithium salt, an inorganic acid anion salt such as LiPF ₆ , LiBF ₄ , LiBETI, LiFSI, LiBOB, or an organic acid anion salt such as LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, or the like is used. Can do.

Moreover, as the organic solvent, for example, aprotic high dielectric constant solvents such as ethylene carbonate and propylene carbonate, aprotic low viscosity such as acetic acid esters and propionic acid esters such as dimethyl carbonate and ethyl methyl carbonate, etc. A solvent is mentioned. It is desirable to use these aprotic high dielectric constant solvents and aprotic low viscosity solvents in combination at an appropriate mixing ratio. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. The counter anion is not particularly limited, and examples thereof include BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ^{− and the} like. The ionic liquid can be used by mixing with the organic solvent described above.
The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 2.0 M from the viewpoint of electrical conductivity. The conductivity of the electrolyte at 25 ° C. is preferably 0.01 S / m or more, and is adjusted by the type of electrolyte salt or its concentration.

When the electrolyte is a solid electrolyte or a gel electrolyte, polyvinylidene fluoride or the like can be contained as a polymer material.
Furthermore, you may add various additives in the electrolyte solution of this embodiment as needed. Examples of additives include vinylene carbonate and methyl vinylene carbonate for the purpose of improving cycle life, biphenyl and alkyl biphenyl for the purpose of preventing overcharge, various carbonate compounds for the purpose of deoxidation and dehydration, Carboxylic anhydride, various nitrogen-containing and sulfur-containing compounds can be mentioned.

(Case)
The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

(Lead)
The leads 60 and 62 are made of a conductive material such as aluminum.
Then, the leads 60 and 62 are welded to the positive electrode current collector 12 and the negative electrode current collector 22 by a known method, respectively, and a separator is provided between the positive electrode active material layer 14 of the positive electrode 10 and the negative electrode active material layer 24 of the negative electrode 20. 18 may be inserted into the case 50 together with the electrolytic solution with the 18 interposed therebetween, and the entrance of the case 50 may be sealed.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment. For example, the lithium ion secondary battery is not limited to the shape shown in FIG. It may be a type or the like.

Example 1
<Preparation of negative electrode for lithium ion secondary battery>
First, as the negative electrode active material, a negative electrode active material containing SiO and graphite that was disproportionated by heat treatment at 1000 ° C. under reduced pressure at a weight ratio of 1: 1 was prepared. An aqueous solution in which this negative electrode active material was dispersed in an aqueous solution containing 0.055% by weight of polyacrylic acid based on the weight of the negative electrode active material was prepared. Next, a negative electrode active material coated with polyacrylic acid was produced from this dispersion by a spray drying method using a spray dryer. Next, the obtained negative active material coated with polyacrylic acid was heat-treated at 225 ° C. for 2 hours in an infrared drying furnace. When the negative electrode active material after the heat treatment was measured by FT-IR, it was confirmed that the abundance ratio of the carboxy group to the acid anhydride group was 72:28. As a result of observing the cross section of the negative electrode active material particles obtained above with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. In one negative electrode active material particle, the thickness of the resin layer formed on the surface was arbitrarily measured, and the ratio of the thickness of the resin layer to the particle diameter was calculated from the following formula, and was 0.0002.
Ratio of coating thickness of polyacrylic acid to particle diameter of negative electrode active material particles = T / R
However, the diameter of the negative electrode active material particles is R (μm), and the film thickness of the polyacrylic acid is T (μm).

<Preparation of negative electrode for lithium ion secondary battery>
Paste obtained by mixing and dispersing 90% by weight of the negative electrode active material for lithium ion secondary battery, 5% by weight of acetylene black as a conductive auxiliary agent, 5% by weight of SBR / CMC mixed resin as a binder, and water. A negative electrode slurry was prepared. Then, using a comma roll coater, the negative electrode active material layer was uniformly applied on both surfaces of the copper foil having a thickness of 10 μm so as to have a predetermined thickness. Next, the negative electrode active material layer was dried in an air atmosphere at 90 ° C. in a drying furnace. In addition, the thickness of the coating film of the negative electrode active material layer apply | coated on both surfaces of copper foil was adjusted to the substantially same film thickness. The negative electrode on which the negative electrode active material was formed was pressure-bonded to both surfaces of the negative electrode current collector by a roll press to obtain a negative electrode sheet having a predetermined density.

  The negative electrode sheet was punched into an electrode size of 21 × 31 mm using an electrode mold to produce a negative electrode for a lithium ion secondary battery according to Example 1.

<Preparation of positive electrode for lithium ion secondary battery>
96% by weight of lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, ₂ % by weight of ketjen black as a conductive additive, 2% by weight of PVDF as a binder, and N-methyl-2-pyrrolidone as a solvent By mixing and dispersing, a paste-like positive electrode slurry was produced. Then, using a comma roll coater, the positive electrode active material layer was uniformly applied so that the positive electrode slurry had a predetermined thickness on both surfaces of an aluminum foil having a thickness of 20 μm. Next, the N-methyl-2-pyrrolidone solvent was dried in an air atmosphere at 110 ° C. in a drying furnace. In addition, the thickness of the coating film of the positive electrode active material layer applied to both surfaces of the aluminum foil was adjusted to substantially the same film thickness. The positive electrode on which the positive electrode active material was formed was pressure-bonded to both surfaces of the positive electrode current collector by a roll press to obtain a positive electrode sheet having a predetermined density.

  The positive electrode sheet was punched into an electrode size of 20 × 30 mm using an electrode mold to produce a positive electrode for a lithium ion secondary battery.

<Production of lithium ion secondary battery>
The prepared negative electrode and positive electrode were laminated via a polypropylene separator having a thickness of 16 μm and a size of 22 × 33 mm to prepare an electrode body. Three negative electrodes and two positive electrodes were laminated via four separators so that the negative and positive electrodes were alternately laminated. Further, in the negative electrode of the electrode body, a nickel negative electrode lead is attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer, while the positive electrode of the electrode body is provided with aluminum without the positive electrode active material layer. An aluminum positive electrode lead was attached to the protruding end of the foil by an ultrasonic welding machine. Then, this electrode body is inserted into an aluminum laminate film outer package and heat-sealed except for one peripheral portion to form a closed portion, and FEC / DEC is blended at a ratio of 3: 7 in the outer package. Example 1 After injecting an electrolyte containing 1M (mol / L) LiPF ₆ as a lithium salt into the solvent, the remaining one part was sealed with a heat seal while reducing the pressure with a vacuum sealer. The lithium ion secondary battery which concerns on was produced.

(Example 2)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 2 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 0.11% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. The thickness of the resin layer formed on the surface of one negative electrode active material particle was arbitrarily measured, and the ratio of the thickness of the resin layer to the particle diameter was calculated from the formula of Example 1 and was 0.0005.

(Example 3)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 3 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 0.55% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.002.

Example 4
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 4 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 1.1% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.004. .

(Example 5)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 5 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 3.3% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode active material coated with polyacrylic acid was prepared by a spray drying method using a spray dryer, and a negative electrode for a lithium ion secondary battery was obtained in the same manner as in Example 1 except that the dispersion was obtained. And the lithium ion secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.013.

(Example 6)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 6 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 4.4% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.018. .

(Example 7)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 7 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 5.5% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.02.

(Example 8)
The negative electrode for a lithium ion secondary battery and the lithium ion secondary battery according to Example 8 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 6.6% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.027.

Example 9
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Example 9 are an aqueous solution in which this negative electrode active material is dispersed in an aqueous solution containing 7.15% by weight of polyacrylic acid based on the weight of the negative electrode active material. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer. A secondary battery was produced. In addition, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.029.

(Comparative Example 1)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Comparative Example 1 are prepared by dispersing an aqueous solution in which the negative electrode active material is dispersed in an aqueous solution containing 3% by weight of polyacrylic acid based on the weight of the negative electrode active material. Then, this dispersion was prepared by a spray drying method using a spray dryer, and a negative electrode active material coated with polyacrylic acid was prepared, and the resulting negative electrode active material coated with polyacrylic acid was not subjected to heat treatment. In the same manner as in Example 1, a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were produced. In addition, when the negative electrode active material after heat processing was measured by FT-IR, presence of the carboxy group was confirmed, but the abundance ratio of the acid anhydride group was not confirmed. Moreover, as a result of observing the cross section of the obtained negative electrode active material particle | grains with the scanning electron microscope (SEM), on the surface of each particle | grain of a negative electrode active material. As a result of arbitrarily measuring the thickness of the resin layer formed on the surface of one negative electrode active material particle and calculating the ratio of the thickness of the resin layer to the particle diameter from the formula of Example 1, it was 0.013.

(Comparative Example 2)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Comparative Example 2 are prepared by dispersing an aqueous solution in which the negative electrode active material is dispersed in an aqueous solution containing 3% by weight of polyacrylic acid based on the weight of the negative electrode active material. Then, a negative electrode active material coated with polyacrylic acid was prepared by spray drying using a spray dryer from this dispersion, and the obtained negative electrode active material coated with polyacrylic acid was heated at 350 ° C. for 2 hours in an infrared drying furnace. A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the heat treatment was performed in In addition, when the negative electrode active material after heat processing was measured by FT-IR, presence of an acid anhydride group was confirmed, but presence of a carboxy group was not confirmed. Moreover, as a result of observing the cross section of the obtained negative electrode active material particles with a scanning electron microscope (SEM), the particle interface on the surface could not be clearly distinguished, and the entire particle was uniformly coated. In one negative electrode active material particle, the thickness of the resin layer formed on the surface was arbitrarily measured, and the ratio of the thickness of the resin layer to the particle diameter was calculated from the following formula and found to be 0.013.

(Comparative Example 3)
A negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to Comparative Example 3 were prepared in the same manner as in Example 1 by using a negative electrode active material not coated with polyacrylic acid. A lithium ion secondary battery was produced.

  Moreover, about the negative electrode for lithium ion secondary batteries produced in Examples 1-6 and Comparative Examples 1-4, and a lithium ion secondary battery, the cycle when a capacity | capacitance maintenance factor will be 70% in the charging / discharging cycle in 60 degreeC The number and the presence or absence of gas generation were evaluated.

(High-temperature charge / discharge cycle test)
The lithium ion secondary batteries produced in Examples and Comparative Examples were repeatedly charged and discharged under the following charge / discharge test conditions, and evaluated for high-temperature charge / discharge cycle characteristics. In addition, charging / discharging was implemented at 60 degreeC. The charging / discharging test conditions were as follows: constant current charging until a constant current of 1.0 C was 4.2 V, then discharging until a battery voltage of 2.5 V was reached with a constant current of 1.0 C, and the above was performed for one cycle. The number of charge / discharge cycles when the discharge capacity retention rate after the charge / discharge cycle was less than 70% was defined as the high-temperature cycle life and evaluated. Note that 1C is a current value at which charging / discharging is completed in just one hour after constant current charging or constant current discharging of a battery cell having a nominal capacity value.

(Gas generation amount)
The amount of gas generated was determined by measuring the volume according to Archimedes' principle after performing 100 cycles of 60 ° C. charge / discharge cycles in the lithium ion secondary batteries produced in the examples and comparative examples. The amount of gas generated.

About the negative electrode for lithium ion secondary batteries in this invention, and a lithium ion secondary battery, it shows in Table 1 about the result of the high temperature cycle test of Examples 1-9 and Comparative Examples 1-3 and the presence or absence of gas generation.

In addition, about the coating amount in a table | surface, the negative electrode active material was extracted from the negative electrode sheet of the battery, and the coating amount was measured by TG-DTA.

  As is apparent from Table 1 above, the coating is coated with polyacrylic acid containing a carboxy group and an acid anhydride group, and the ratio of the polyacrylic acid to the active material particle size is 0.1 to 5% by weight. When it was in the range of 0.0005 to 0.02, it became clear that the high-temperature cycle characteristics were further improved.

  From Examples 1 to 9 and Comparative Example 3, when the negative electrode active material particles are coated with polyacrylic acid containing a carboxy group and an acid anhydride group, cycle characteristics are improved and the amount of gas generated is also suppressed. It became clear. This is considered to suppress the side reaction between the active material particles and the electrolytic solution.

Further, from Example 5 and Comparative Examples 1 and 2, the polyacrylic acid that is coated has both carboxy groups and acid anhydride groups, so that cycle characteristics are improved and the amount of gas generated can be suppressed. It became clear.
In Comparative Example 1, no heat treatment was applied to the negative electrode active material coated with polyacrylic acid, and no acid anhydride was introduced into the polyacrylic acid. This is because a part of the carboxy group in polyacrylic acid is not polymerized to form an acid anhydride, so that the three-dimensional bond network is not strengthened, and the negative electrode active material particles collapse due to the expansion of the negative electrode active material. It is considered that gas generation and cycle deterioration have occurred due to side reactions between the new surface of the negative electrode active material particles and the electrolyte solution.
In Comparative Example 2, since all of the carboxy groups in the polyacrylic acid are polymerized into acid anhydride groups by heat treatment, the adhesion between the polyacrylic acid film and the negative electrode active material surface is reduced. The negative electrode active material is peeled off, and it is considered that gas generation and cycle deterioration are caused by the contact between the electrolytic solution and the negative electrode active material.
In Example 5, both the carboxy group and the acid anhydride group exist in the polyacrylic acid that is coated, and the cycle characteristics are improved by the strong adhesion by the carboxy group and the effect of forming a strong film by the acid anhydride group. However, it is thought that gas generation was also suppressed.

Moreover, from Example 1 to Example 9, it is coated with polyacrylic acid containing a carboxy group and an acid anhydride group, and the ratio of the polyacrylic acid to the active material particle size is 0.1 to 5% by weight. Is in the range of 0.0005 to 0.02, it was revealed that the high-temperature cycle characteristics are further improved. If the amount of the polyacrylic acid coating is within this range, the surface of the negative electrode active material is well covered, and the reaction between the active material and the electrolyte is suppressed. This is considered to be because the reaction between the negative electrode active material and the electrolytic solution is further suppressed because the detachment is not excluded.

Claims (5)

A negative electrode active material for a lithium ion secondary battery, wherein the surface of the negative electrode active material particles is coated with polyacrylic acid containing a carboxy group and an acid anhydride group. The negative active material for a lithium ion secondary battery according to claim 1, wherein the polyacrylic acid is contained in an amount of 0.1 to 5 wt% with respect to the negative electrode active material for the lithium ion secondary battery. material.   3. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein a ratio of the coating thickness of the polyacrylic acid to the particle size of the negative electrode active material particles is 0.0005 to 0.02. material. A negative electrode for a lithium ion secondary battery, wherein a mixture of the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3 and a binder is formed on a negative electrode current collector. The lithium ion secondary battery provided with the negative electrode for lithium ion secondary batteries of Claim 4, a positive electrode, and electrolyte.

Images