JP2016081707A - Negative electrode and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode which enables the enhancement in cycle characteristics; and a lithium ion secondary battery using such a negative electrode.SOLUTION: A negative electrode 20 comprises: a negative electrode active material layer 24 including a Si-containing negative electrode active material and a binder; and a layer on the surface of the negative electrode active material layer, which includes Si, V, C, O, P and F. In the negative electrode 20, the film thickness of the layer including Si, V, C, O, P and F is 10-100 nm. A lithium ion secondary battery 100 comprises the negative electrode 20, a positive electrode 10 and an electrolyte which are contained in a container 50.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode and a lithium ion secondary battery using the negative electrode.

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. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.

Currently, many researches have been conducted on negative electrode active materials for electrochemical devices such as lithium ion secondary batteries, such as silicon and silicon oxide, which have a larger charge / discharge capacity than carbon materials such as graphite. However, when such a material is used as the negative electrode active material, lithium oxide is formed on the surface of the negative electrode by repeated charge and discharge. In this case, since the lithium oxide is deposited as an irreversible film, the absolute number of lithium (lithium ions) contributing to the charge / discharge reaction is reduced, leading to a decrease in discharge capacity and long-term charge. The problem is that the discharge cannot be repeated (low cycle characteristics).

In order to solve the above-described problems, the negative electrode active material layer is coated with CoO, FeO, NiO or the like, and a reversible film is formed, thereby improving cycle characteristics by suppressing a decrease in the absolute number of lithium. The technique which tries is proposed (refer patent document 1).

JP 2008-153078 A

However, in Patent Document 1, the cycle characteristics are not sufficiently improved, and further improvement is desired.

The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide a negative electrode capable of improving cycle characteristics and a lithium ion secondary battery using the negative electrode.

In order to solve the above-described problems, a negative electrode according to the present invention includes a layer composed of Si—V—C—O—P—F on the surface of a negative electrode active material layer containing a negative electrode active material containing Si and a binder. It is characterized by having.

According to the present invention, it is possible to rotate a long-term cycle while maintaining a high capacity by forming a film containing V capable of changing the valence. Although this mechanism has not been clearly understood, the average valence of V in the film decreases to 3 to 4 during charging and increases to 3.5 to 5 during discharging. Using this function, it is presumed that the reaction with the electrolytic solution is suppressed and the generation of lithium oxide is prevented by V receiving the charge generated when Li is inserted into the negative electrode active material.
Further, since Si is contained in the coating composed of Si—V—C—O—P—F on the negative electrode active material layer, the coating becomes strong. This coating is considered to improve cycle characteristics in order to follow the expansion and contraction and prevent detachment of the negative electrode active material.

In the negative electrode of the present invention, the thickness of the layer composed of Si—V—C—O—P—F is preferably 10 to 100 nm.

By using the negative electrode according to the present invention, the layer composed of Si—V—C—O—P—F has the strength to follow the expansion and contraction of the active material and not break the coating, and more excellent It can be.

Moreover, it is preferable that the lithium ion secondary battery concerning this invention is the structure by which the positive electrode, the said negative electrode, and the electrolyte were accommodated in the container.

By adopting such a configuration, it can be used as a lithium ion secondary battery having excellent cycle characteristics.

ADVANTAGE OF THE INVENTION According to this invention, the negative electrode which has the outstanding cycling characteristics, and a lithium ion secondary battery using the same can be provided.

It is a schematic cross section of the lithium ion secondary battery of this embodiment. 2 is a transmission electron microscope (TEM) image of a layer composed of Si—V—C—O—P—F on the negative electrode obtained in Example 1. FIG.

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.

(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 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 ), olivine type LiMPO ₄ (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr, or VO) shown), and composite metal oxides of lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) , etc.

(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 binder, and a conductive auxiliary agent in an amount as required.

  The negative electrode of the present embodiment has a layer composed of Si—V—C—O—P—F on the surface of a negative electrode active material layer containing a negative electrode active material containing Si and a binder. It is.

By forming a film containing V capable of changing the valence on the negative electrode active material layer, a long-term cycle can be performed while maintaining a high capacity. Although this mechanism has not been clearly understood, the average valence of V in the film decreases to 3 to 4 during charging and increases to 3.5 to 5 during discharging. It is presumed that, by using this function, V receives a charge generated when Li is inserted into the negative electrode active material, thereby suppressing the reaction with the electrolytic solution and preventing the formation of lithium oxide.
Further, since Si is contained in the coating composed of Si—V—C—O—P—F on the negative electrode active material layer, the coating becomes strong. This coating is considered to improve cycle characteristics in order to follow the expansion and contraction and prevent detachment of the negative electrode active material.

In the layer composed of Si—V—C—O—P—F, Si is 3 to 25 atm% (meaning element%), V is 0.1 to 1 atm%, C is 1 to 7 atm%, O Is preferably 10 to 50 atm%, P is 1 to 5 atm%, and F is 40 to 70 atm%.
The composition of the layer composed of Si—V—C—O—P—F can be measured by an existing method, and an energy dispersive X-ray analyzer (STEM-EDS) is applied to the negative electrode cross section. ) Can be used for measurement.
The range is not necessarily limited to the above range.

  The layer composed of Si—V—C—O—P—F contains Si (silicon), V (vanadium), C (carbon), O (oxygen), P (phosphorus), and F (fluorine). In addition, the layer may be crystalline, but it is preferably an amorphous structure.

  The thickness of the layer composed of Si—V—C—O—P—F is preferably 10 to 100 nm. With such a configuration, the layer composed of Si—V—C—O—P—F follows the expansion and contraction of the active material, and has a strength that prevents the coating from being broken.

  The film thickness of the layer composed of Si—V—C—O—P—F can be measured by an existing method. A transmission electron microscope (TEM) or X-ray is used for the negative electrode cross section. It can be measured by photoelectron spectroscopy (ESCA).

Further, the layer composed of Si—V—C—O—P—F does not necessarily need to be completely 100% coated on the surface of the negative electrode active material particles if the effect of the present invention occurs. % Or more is preferable.

(Negative electrode active material)
The negative electrode active material particles of the present embodiment contain Si, and examples thereof include Si simple substance and silicon oxide such as SiO and SiO2. Of these, silicon oxide is preferable.

(Negative conductive auxiliary)
The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. Moreover, you may add negative electrode active material materials, such as non-graphitizable carbon, easily graphitizable carbon, and graphite, changing a shape.

As carbon black, acetylene black, ketjen black and the like are particularly preferable, and ketjen black is particularly preferable. By including an electron-conductive porous body, pores can be formed at the interface between the particles of the negative electrode active material and the binder, and the penetration of the non-aqueous electrolyte into the negative electrode active material layer 24 is facilitated by the pores. Therefore, it is preferable.

(Negative electrode binder)
As the binder, in addition to the above-described materials exemplified as the positive electrode binder, an acrylic resin such as PAA (polyacrylic acid) can also be used. Further, the content of the binder is also appropriately adjusted when the volume change of the negative electrode active material and the adhesion with the foil must be taken into account, and if the same content as the content in the positive electrode 10 described above is adopted. Good. The added amount of the binder is preferably 2 to 20% by mass with respect to the mass of the 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 on the current collectors 12 and 22 is not particularly limited, and the current collectors 12 and 22 applied with the paint are dried, for example, in an atmosphere of 80 ° C. to 150 ° C. Just do it.

  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, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiFSI, and LiBOB, organic acid anion salts such as LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, and the like are used. be able to.

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 gel electrolyte, poly (vinylidene 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. 1, but a coin type in which an electrode punched into a coin shape and a separator are stacked, or a cylinder in which an electrode sheet and a separator are wound in a spiral shape. It may be a type or the like.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Example 1]
(Preparation of positive electrode)
84% by weight of LiVOPO ₄ as a positive electrode active material, 6% by weight of carbon black as a conductive assistant, 2% by weight of graphite, 8% by weight of PVDF as a binder, and a solvent of N-methyl-2-pyrrolidone Were mixed and dispersed to prepare a paste-like positive electrode slurry. Then, using a comma roll coater, the positive electrode active material layer was uniformly applied to both surfaces of an aluminum foil having a thickness of 20 μm. Next, the N-methyl-2-pyrrolidone solvent in the positive electrode active material was dried in an air atmosphere at 110 ° C. in a drying furnace. The coating amount of the positive electrode active material was 12.0 mg / cm ² . In addition, the thickness of the coating film of the positive electrode active material layer apply | coated to both surfaces of the said aluminum foil was adjusted to the substantially 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 produce a positive electrode so that the density of the positive electrode active material layer was 2.3 g / cm ³ . .

(Preparation of negative electrode)
A slurry for forming a negative electrode active material layer by mixing and dispersing 90 wt% of SiO _x , 2.5 wt% of acetylene black, 7.5 wt% of polyamideimide resin, and a solvent of N-methyl-2-pyrrolidone. Was prepared. This slurry was applied to both sides of a 10 μm thick copper foil so that the amount of the negative electrode active material applied was 1.5 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. The negative electrode on which the negative electrode active material is formed is pressure-bonded to both sides of the negative electrode current collector by a roll press, and the negative electrode is produced so that the density of the negative electrode active material layer is 1.5 g / cm ^3. did.

(Formation of a layer composed of Si-V-C-O-P-F)
Vanadium pentoxide and silicon dioxide were each dissolved in hydrofluoric acid, a mixed solution of phosphoric acid and PVA was added, and these were mixed to obtain a Si—V—C—O—P—F solution.
In this mixed solution, the negative electrode was dipped once and dried at 100 ° C. in an argon atmosphere to form a layer composed of Si—V—C—O—P—F on the negative electrode active material layer. .

Using a transmission electron microscope (TEM), the thickness of the layer composed of Si—V—C—O—P—F was evaluated (FIG. 2). The average value of 20 or more positions of the obtained film thickness was defined as the film thickness of the layer composed of Si—V—C—O—P—F.
Moreover, the structure of the film was evaluated using an energy dispersive X-ray analyzer (STEM-EDS). The layer composed of Si—V—C—O—P—F is composed of Si: 3 atm%, V: 0.2 atm%, C: 2.8 atm%, O: 42.7 atm%, P: 3.3 atm%, F: 47.9 atm%.

(Production of evaluation lithium-ion secondary battery)
Using the positive electrode and the negative electrode prepared above, a separator made of a polyethylene microporous film is sandwiched between them and placed in an aluminum laminate pack. In this aluminum laminate pack, a 1M LiPF ₆ solution (solvent: EC) is used as an electrolytic solution. / DEC = 3/7 (volume ratio)) was injected, and then vacuum-sealed to produce a lithium ion secondary battery for evaluation.
When the average valence of vanadium in the layer composed of Si—V—C—O—P—F in Example 1 was evaluated, it was 4.0.
The average valence of vanadium was evaluated using X-ray absorption fine structure analysis (XANES).

[Example 2]
A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1, except that the Si—V—C—O—P—F solution adhered to the tip of the glass rod was coated on the negative electrode. Then, the cycle characteristics were evaluated.

[Example 3]
A lithium ion secondary battery for evaluation was prepared in the same manner as in Example 1 except that it was dipped twice in the Si—V—C—O—P—F solution, and the cycle characteristics were evaluated.

[Example 4]
A lithium ion secondary battery for evaluation was prepared in the same manner as in Example 1 except that it was dipped three times in the Si—V—C—O—P—F solution, and the cycle characteristics were evaluated.

[Example 5]
The same method as in Example 1 except that the Si—V—C—O—P—F solution adhered to the tip of the glass rod was coated on the negative electrode and dried at 170 ° C. in an argon atmosphere. A lithium ion secondary battery for evaluation was prepared and the cycle characteristics were evaluated.
At this time, the layer composed of Si—V—C—O—P—F is Si: 2.8 atm%, V: 1.2 atm%, C: 3.7 atm%, O: 15.4 atm%, P: 2 .4 atm%, F: 74.5 atm%.

[Example 6]
A lithium ion secondary battery for evaluation was prepared in the same manner as in Example 1 except that it was dipped into the Si—V—C—O—P—F solution four times, and the cycle characteristics were evaluated.

[Example 7] * In Example 7, a lithium ion secondary battery for evaluation was prepared in the same manner as in Example 1 except that it was dipped into the Si—V—C—O—P—F solution 5 times. The cycle characteristics were evaluated.

[Example 8]
A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the negative electrode was dipped once in the Si-VCCOPF solution and dried at 130 ° C. in an argon atmosphere. Then, the cycle characteristics were evaluated.
At this time, the layer composed of Si—V—C—O—P—F is Si: 3.9 atm%, V: 0.5 atm%, C: 5.5 atm%, O: 21.7 atm%, P: 2 .8 atm%, F: 65.6 atm%.
When the average valence of vanadium in the layer composed of Si—V—C—O—P—F was evaluated, it was 4.5.

[Example 9]
A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the negative electrode was dipped once in the Si-VCCOPF solution and dried at 150 ° C. in an argon atmosphere. Then, the cycle characteristics were evaluated.
At this time, the layer composed of Si—V—C—O—P—F is Si: 3.1 atm%, V: 1.0 atm%, C: 3.5 atm%, O: 19.9 atm%, P: 2 .6 atm%, F: 69.9 atm%.
The average valence of vanadium in the layer composed of Si—V—C—O—P—F was evaluated to be 4.3.

[Example 10]
A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the negative electrode was dipped once in the Si-VCCOPF solution and dried at 80 ° C. in an argon atmosphere. Then, the cycle characteristics were evaluated.
At this time, the layer composed of Si—V—C—O—P—F is Si: 4.3 atm%, V: 0.1 atm%, C: 3.1 atm%, O: 45.2 atm%, P: 3 0.0 atm%, F: 44.3 atm%.
Further, when the average valence of vanadium in the layer composed of Si—V—C—O—P—F was evaluated, it was 3.9.

[Comparative Example 1]
Evaluation was performed in the same manner as in Example 1 except that the Si—V—C—O—P—F solution was not dipped and the layer composed of Si—V—C—O—P—F was not coated. Lithium ion secondary batteries were prepared and the cycle characteristics were evaluated.

[Comparative Example 2]
CoO was deposited on both sides of the negative electrode using electron beam evaporation, and it was composed of Si-V-C-O-P-F without dip in the Si-V-C-O-P-F solution. A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the layer to be coated was not coated, and the cycle characteristics were evaluated.

<Method for evaluating cycle characteristics>
About the lithium ion secondary battery for evaluation produced by the Example and the comparative example, the cycle characteristic was measured in the environment of 25 degreeC using the secondary battery charging / discharging test apparatus (made by Hokuto Denko Co., Ltd.). The charge / discharge cycle of constant current and constant voltage charge to 4.2V at 0.5C and constant current discharge to 2.8V at 1C was repeated 500 cycles, the capacity retention rate after 500 cycles was measured, and the cycle characteristics were evaluated.

Table 1 shows the film thickness and cycle retention rate of the layers composed of Si—V—C—O—P—F in Examples 1 to 10 and Comparative Examples 1 and 2.

The batteries of Examples 1 to 10 showed a high cycle retention rate. The battery of Comparative Example 1 did not have a layer composed of Si—V—C—O—P—F, and a decrease in cycle retention was observed. Further, in the battery of Comparative Example 2, when CoO was deposited on the negative electrode by the electron beam evaporation method, the cycle retention rate was improved, but Examples 1 to 10 showed higher cycle retention rates.

  By using the lithium ion secondary battery of the present invention, a lithium ion secondary battery having improved cycle characteristics can be provided.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 52 ... Metal foil, 54 ... Polymer film, 60, 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (3)

  A negative electrode comprising a layer composed of Si—V—C—O—P—F on the surface of a negative electrode active material layer containing a negative electrode active material containing Si and a binder.   2. The negative electrode according to claim 1, wherein a film thickness of the layer composed of Si—V—C—O—P—F is 10 to 100 nm. A lithium ion secondary battery in which a positive electrode, a negative electrode, and an electrolyte are contained in a container, wherein the negative electrode is the negative electrode according to any one of claims 1 to 2. Ion secondary battery.

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