JP2017103138A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery with superior cycle characteristics.SOLUTION: A lithium ion secondary battery comprises: a negative electrode represented by a compound including Fe and O; and an electrolyte including LiPFand Cl.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

Lithium ion secondary batteries are lighter and have higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. Therefore, it is widely applied as a power source for portable electronic devices. 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, there is a graphite-based carbon material as a typical negative electrode material in a lithium ion secondary battery. The amount of lithium ions that can be occluded by this material is limited by the amount that can be inserted between graphite layers. Therefore, it is difficult to increase the capacity to 370 mAh / g or more.
In view of the above-mentioned problems of such graphite materials, further improvement in battery capacity characteristics is demanded, and a negative electrode having a larger capacity than graphite materials is desired. In the prior art document 1, as a candidate for such a material, an oxide such as iron oxide is disclosed as a lithium ion storage material for the negative electrode. An iron oxide material is a material that performs Li insertion and desorption by charge / discharge through a conversion reaction, and has a larger charge / discharge capacity than carbon materials such as graphite, and is expected to have a theoretical capacity exceeding 1000 mAh / g. .
However, when an iron oxide material is used as the negative electrode and LiPF ₆ is used as the electrolyte, the cycle characteristics may be deteriorated. This is because LiPF ₆ in the electrolytic solution undergoes a decomposition reaction with moisture contained in the electrolytic solution during a charge / discharge cycle, thereby forming an SEI film (Solid Electrolyte Interface film; hereinafter referred to as “SEI film”) on the negative electrode surface. It is considered that this SEI coating is excessively produced, leading to deterioration of cycle characteristics.

International Publication No. 2011/061825

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a lithium ion secondary battery having excellent cycle characteristics.

A lithium ion secondary battery according to the present invention is a lithium ion secondary battery characterized by having a negative electrode containing a compound composed of Fe and O and an electrolyte containing LiPF ₆ and Cl.

By using the electrolytic solution, Cl in the electrolytic solution reacts with iron oxide to form iron oxychloride, and the formed iron oxychloride causes a chemical reaction with moisture (FeOCl + H ₂ O → FeOOH + HCl). Demonstrates the effect of removing excess water contained in the liquid. Therefore, since the SEI film formed during the charging reaction is prevented from being generated excessively, a lithium ion secondary battery having good cycle characteristics can be obtained.

The lithium ion secondary battery according to the present invention is preferable when the molar ratio of Cl to P (Cl / P) in the electrolytic solution is 0.005 or more and 0.3 or less because good cycle characteristics can be obtained.

It is considered that the water in the electrolytic solution causes a chemical reaction with iron oxychloride more selectively than the LiPF ₆ electrolyte salt. Therefore, the chemical reaction between iron oxychloride and moisture occurs more rapidly than the chemical reaction between moisture in the electrolyte and LiPF ₆ electrolyte salt. Therefore, an effect of removing excess water contained in the electrolytic solution is exhibited, and an SEI coating is prevented from being generated excessively, so that a lithium ion secondary battery having even better cycle characteristics can be obtained.

  The lithium ion secondary battery according to the present invention preferably includes a negative electrode, a positive electrode having a positive electrode active material, and a separator and a nonaqueous electrolyte between the positive electrode and the negative electrode.

According to the present invention, a lithium ion secondary battery having excellent cycle characteristics can be provided.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment. 3 is an XRD spectrum of a negative electrode active material shown in Example 1. 10 is an XRD spectrum of a negative electrode active material shown in Example 9.

  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)
An electrode and a lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a power generation element 30, a case 50 that houses the power generation element 30 in a sealed state, and a pair of leads 60 and 62 connected to the power generation element 30.

The power generation element 30 is configured such that a pair of electrodes 10 and 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. The positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18 contain an electrolyte solution. 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.

(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.

(Negative electrode active material)
The negative electrode active material according to this embodiment is iron oxide represented by a compound composed of Fe and O.

Examples of the compound composed of Fe and O include Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , LiFeO ₂ , LiFe ₅ O _{8 and the} like.

The particles constituting the iron oxide powder are particularly preferably Fe ₂ O ₃ . Among the compounds composed of Fe and O, Fe ₂ O _{3 has the} largest theoretical capacity of 1007 mAh / g, so that a battery exhibiting a better discharge capacity can be realized.

The negative electrode active material is preferably a rod-like or plate-like particle.

By being rod-like or plate-like particles, the reaction between Cl and iron oxide in the electrolytic solution is promoted, and iron oxychloride is easily formed. The formed iron oxychloride causes a chemical reaction with moisture (FeOCl + H ₂ O). → FeOOH + HCl), which can more effectively remove excess water contained in the electrolyte. As a result, a lithium ion secondary battery having good cycle characteristics can be obtained.

The rod-like or plate-like particles preferably have a ratio of the longest diameter in the direction intersecting with the shortest diameter of 3 or more.
When it is 3 or more, it is considered that the reaction between Cl and iron oxide in the electrolytic solution is further promoted and the cycle characteristics are improved.

(Measurement of shortest diameter and longest diameter)
The shortest diameter and longest diameter of the rod-like or plate-like particles can be measured by observing them using a scanning electron microscope (SEM), measuring the shortest diameter A and the longest diameter B, and then determining the ratio of B / A. Good. At this time, the shortest diameter and the longest diameter can be calculated by performing image processing on the secondary electron image or the reflected electron image observed by the SEM. Specifically, for example, the reflected electron image is binarized to calculate the shortest diameter and the longest diameter.

From the viewpoint of handling, the longest diameter of the rod-like particles is preferably 5 μm or less, and the longest diameter of the plate-like particles is preferably 30 μm or less.

(Method for producing negative electrode active material)
The manufacturing method of the negative electrode active material according to the present embodiment includes the following hydrothermal synthesis step, filtration / washing step, drying step, and firing step. As another synthesis method, it is possible to synthesize by a solid phase method or the like, but it is difficult to synthesize a sample having a uniform particle size and a small particle size as compared with the hydrothermal synthesis method. Therefore, this time, the hydrothermal synthesis process will be specifically described.

<Hydrothermal synthesis process>
In the hydrothermal synthesis step, first, an iron source, water, and a neutralizing agent are introduced into a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside to prepare a mixture in which these are dispersed. .

  As the iron source, iron chloride, iron sulfate, iron nitrate, iron citrate, iron phosphate, and the like can be used, and iron chloride, iron sulfate, and iron nitrate are more preferable from the viewpoint of solubility in water. Furthermore, these 1 type (s) or 2 or more types can be mixed and used.

  As the neutralizing agent, monoethanolamine, diethanolamine, triethanolamine, urea or the like can be used, and in order to suppress the generation of acid in the aqueous solution after the reaction, a large excess may be added to the iron source anion. preferable.

  In the hydrothermal synthesis step, the hydrothermal reaction is allowed to proceed in the mixture by heating the mixture in the sealed reactor while applying pressure. Thereby, the compound precursor comprised from Fe and O which are the negative electrode active materials in this embodiment is hydrothermally synthesized. In addition, what is necessary is just to adjust suitably the time which heats a mixture, pressurizing according to the quantity of a mixture.

  Thus, in a hydrothermal synthesis process, a mixture is heated at 100 degreeC or more under pressure. Heating to 120 to 200 ° C. is preferable, and a hydrothermal synthesis reaction is preferable at a relatively low temperature.

  The pressure applied to the mixture in the hydrothermal synthesis step is preferably 0.4 to 0.9 MPa. If the pressure applied to the mixture is too low, the reaction vessel is required to have high pressure resistance, and the production cost of the negative electrode active material tends to increase. By setting the pressure applied to the mixture within the above range, these tendencies can be suppressed.

<Filtering and washing process>
As a filtration method, a commercially available filter paper having a pore size capable of collecting the produced compound precursor composed of Fe and O can be used, and a membrane filter having a pore size of 1 μm or less is preferable.

  As a washing method, a compound precursor composed of Fe and O is dispersed in pure water in a Meyer flask, filtered, and washed with water. By changing the number of times of washing with water, the amount of chlorine and conductivity can be adjusted.

<Drying process>
The drying method is preferably performed at 150 ° C. or lower in order to suppress dry aggregation. More preferably, drying under reduced pressure for 6 to 12 hours is performed at 60 to 90 ° C.

<Baking process>
The method for producing a negative electrode active material according to the present embodiment needs to include a firing step of firing a compound precursor composed of Fe and O after drying. A compound composed of Fe and O is obtained by the firing treatment.

  In the firing step, the compound precursor composed of Fe and O is fired at a firing temperature of 200 ° C. or higher. More preferably, firing is performed at a firing temperature of 250 to 800 ° C. for 1 to 6 hours. If the firing temperature is too low, a compound composed of Fe and O cannot be obtained, and if the firing temperature is within the above range, grain growth of the compound composed of Fe and O is suppressed, and charging and discharging are accompanied. The negative electrode active material does not easily collapse due to expansion and contraction, and the charge / discharge capacity is further improved.

<Crushing process>
The method for producing a negative electrode active material according to the present embodiment may include a pulverization step of pulverizing a compound composed of Fe and O after firing. By the pulverization treatment, the compound composed of Fe and O can be further refined and the discharge capacity can be improved.

(Negative conductive auxiliary)
As the conductive aid, carbon-based materials such as graphite, carbon black, graphene, and carbon nanotubes are preferably used. Alternatively, conductive metal powder such as nickel powder may be used.

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. This is preferable.

The content of the conductive additive in the negative electrode active material layer 24 is not particularly limited, but when added, it is preferably 0.5 to 10% by mass with respect to the mass of the negative electrode active material.

(Negative electrode binder)
In addition to the materials used for the positive electrode 10 described above, the binder and the conductive auxiliary agent include, for example, a fluororesin of polyvinylidene fluoride (PVDF), an acrylic resin such as polyacrylic acid (PAA), polyamideimide (PAI), and thermosetting. Resin can also be used. In addition, the content of the binder and the conductive auxiliary agent is also adjusted as appropriate 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. When adding, it is preferable that the addition amount of a binder is 2-20 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 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.

(Electrolyte)
The electrolytic solution is contained inside the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolytic solution is not particularly limited. 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 electrolytic solution is preferably an electrolytic solution (non-aqueous electrolyte solution) using an organic solvent, because the electrochemical decomposition voltage is low, and thus the withstand voltage during charging is limited to a low level. As the electrolytic solution, a lithium salt dissolved in an organic solvent (non-aqueous solvent) is preferably used. For example, LiPF ₆ is particularly preferable as the lithium salt, and other organic acid anion salts such as LiBF ₄ , LiClO ₄ , LiFSI and LiBOB, and organic acids such as LiCF ₃ SO ₃ and (CF ₃ SO ₂ ) ₂ NLi. An anion salt etc. can be used.

Examples of the organic solvent include aprotic high dielectric constant solvents such as ethylene carbonate and propylene carbonate, and aprotic low viscosity solvents such as dimethyl carbonate and ethyl methyl carbonate. 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.

  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. In addition, as halogenated phosphate compounds generally used as flame retardants, tris (chloroethyl) phosphate, tris (β-chloropropyl) phosphate, tris (dichloropropyl) phosphate, tetrakis (2chloroethyl) dichloroisopentyldi It is preferable to add 0.01 mass% to 1 mass% of phosphate, polyoxyalkylene bis (dichloroalkyl) phosphate, and the like.

The molar ratio of Cl to P (Cl / P) present in the electrolytic solution can be adjusted by changing the addition amount of the halogenated phosphate ester system. Specifically, the molar ratio of Cl and P is preferably 0.005 or more and 0.3 or less. Within the above range, the cycle characteristics are further improved.
(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 , LiVOPO ₄ ), olivine-type LiMPO ₄ (where M is one or more selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) shows the element or VO), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1), Li excess based It includes complex metal oxides.

(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.

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, carbon black, graphene, and carbon nanotubes, 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.

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

(Separator)
The separator 18 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.

(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.
The leads 60 and 62 are welded to the positive electrode current collector 12 and the negative electrode current collector 22, respectively, by a known method, 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.

  The produced lithium ion secondary battery was evaluated by the following method.

このように上記の条件によって充放電を繰り返し、例えば１００サイクル後の容量維持率によって充放電サイクル特性を評価すればよい。 (Measurement of cycle characteristics)
Using a secondary battery charge / discharge test apparatus, charging is performed at a current value of 0.5 C when the voltage range is 4.0 V to 1.0 V and 1 C = 800 mAh / g per weight of the negative electrode active material. Discharge at a current value of 0 C to evaluate cycle characteristics.
The cycle characteristics are expressed as the discharge capacity retention rate (%), where the discharge capacity at the first cycle is the initial discharge capacity, and is the ratio of the discharge capacity at each number of cycles to the initial discharge capacity. Is. Note that 1C is a current value at which charging / discharging is completed in exactly one hour after a constant current charge or constant current discharge is performed on a battery cell having a nominal capacity value.

Thus, charging / discharging is repeated under the above conditions, and the charge / discharge cycle characteristics may be evaluated by, for example, the capacity maintenance rate after 100 cycles.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, these do not restrict | limit this invention at all.

  Lithium ion secondary batteries of Examples 1 to 13 and Comparative Examples 1 to 3 were produced by the following procedure.

[Example 1]
(Preparation of negative electrode active material)
(Hydrothermal synthesis process)
To a 500 ml Meyer flask, 38.92 g (0.14 mol) of iron (II) sulfate heptahydrate and 250 mL of distilled water were added and stirred with a magnetic stirrer to obtain a mixed solution. Subsequently, 55.95 g (0.37 mol) of triethanolamine was gradually added dropwise over 10 minutes. The obtained mixed solution was transferred into a 0.5 L autoclave glass cylindrical container, and the container was sealed and kept at 160 ° C. for 4 hours to perform hydrothermal synthesis. The obtained slurry was filtered, washed with water, and dried to obtain a precursor of a compound composed of Fe and O.

(Baking process)
A compound precursor composed of Fe and O was put in an alumina crucible and baked at 200 ° C. for 6 hours. The negative electrode active material of Example 1 was obtained.

(Measurement of XRD spectrum)
The XRD spectrum of the negative electrode active material of Example 1 was measured using a sample horizontal multipurpose X-ray diffractometer Ultima IV manufactured by Rigaku Corporation. The results are shown in FIG.

As apparent from FIG. 2, it was confirmed that the negative electrode active material obtained from Example 1 was Fe ₂ O ₃ .

(Preparation of negative electrode)
The negative electrode active material (80% by mass) produced by the above method, carbon black (5% by mass) as a conductive auxiliary agent, and polyamideimide (PAI) (15% by mass) as a binder are mixed, and N-methyl- A slurry was obtained by dispersing in 2-pyrrolidone (NMP). The obtained slurry was applied to an electrolytic copper foil as a current collector by a doctor blade method and dried at 110 ° C. The dried electrode was rolled by a roll press at a linear pressure of 2000 kgf / cm, and heat-treated at 350 ° C. for 3 hours in an argon atmosphere to obtain a negative electrode.

(Preparation of positive electrode)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode active material, Ketjen black as a conductive additive, polyvinylidene fluoride (PVdF) as a binder, and a mass ratio of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ : Ketjen black: PVdF = 87: 8: 5. Then, N-methyl-2-pyrrolidone (NMP) as a solvent was added to prepare a slurry, and then kneading was performed for 1 hour. Thereafter, NMP was added to adjust the viscosity to 5000 mPa · s. It apply | coated on the aluminum foil which is a collector with the doctor blade method, and dried for 10 minutes at 100 degreeC. Thereafter, rolling was performed at a linear pressure of 2 tcm ⁻¹ by a roll press heated to 100 ° C. to produce a positive electrode. The active material carrying amount of the positive electrode was adjusted to 11 mg / cm ² .

(Production of evaluation lithium-ion secondary battery)
A positive electrode and a negative electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (battery body). This laminated body was put into an aluminum laminated pack (a laminated sheet bag body in which two main surfaces of an aluminum foil were coated with polypropylene (PP) and polyethylene terephthalate (PET), respectively).

(Electrolyte preparation)
As the electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved as a supporting salt to a concentration of 1.0 mol / L. In addition, 0.01 mass% of tris (chloroethyl) phosphate was added as an additive to the electrolyte solution produced by the above method. The mixture was stirred for 1 h with a magnetic stirrer to obtain a uniform mixed solution.
Then, after inject | pouring the said electrolyte solution into the aluminum laminate pack in which the laminated body was put, it vacuum-sealed and the lithium ion secondary battery for evaluation of Example 1 was produced.

(IC and ICP-AES measurement)
In Example 1, the quantification of Cl and P present in the electrolyte was analyzed by ion chromatography (IC) and ICP emission spectroscopy (ICP-AES) measurement. Cl ^- is quantitation using Shimadzu IC analyzer HIC-SP suppressor ion chromatograph Co., quantification of P was measured using an ICP emission spectrometer ICPS-8100CL. From the values obtained in each analysis, the Cl / P molar ratio contained in the electrolytic solution was calculated, and the results obtained are shown in Table 1.

The Cl / P molar ratio obtained by IC and ICP-AES measurement was 0.005.
[Example 2-8]
In the same manner as in Example 1, a lithium ion secondary battery for evaluation was produced. Table 1 shows the measurement results of the Cl / P molar ratio obtained by IC and ICP-AES measurement by variously changing the amount of tris (chloroethyl) phosphate added in the electrolytic solution preparation.
[Example 9]
In Example 9, a compound precursor composed of Fe and O was put in an alumina crucible, 0.5 mass% of carbon was added, and heat treatment was performed under a nitrogen gas atmosphere at 500 ° C. under the same conditions as in Example 1. The reaction was carried out. Using the obtained negative electrode active material of Example 9, a secondary battery for evaluating a lithium ion battery was produced in the same manner as in Example 1. In the electrolytic solution preparation, after adding 0.8% by mass of tris (chloroethyl) phosphate, the Cl / P molar ratio obtained by IC and ICP-AES measurement was 0.430.

(Measurement of XRD spectrum)
The XRD spectrum of the negative electrode active material of Example 9 was measured using a sample horizontal multipurpose X-ray diffractometer Ultima IV manufactured by Rigaku Corporation. The results are shown in FIG.

As is clear from FIG. 3, it was confirmed that the negative electrode active material obtained from Example 9 was Fe ₃ 0 ₄ .

[Examples 10-12]
In Example 10-12, the negative electrode active material LiFeO ₂ was obtained by weighing Fe ₂ O ₃ and Li ₂ CO _{3 so} that the Fe / Li molar ratio was 1.1 and firing at 600 ° C. for 10 hours in the air. It was. Using the obtained negative electrode active material of Example 10-12, a secondary battery for evaluating a lithium ion battery was produced in the same manner as in Example 1. Table 1 shows the Cl / P molar ratio obtained by IC and ICP-AES measurement by variously changing the amount of tris (chloroethyl) phosphate added in the electrolytic solution preparation.

(Measurement of XRD spectrum)
The XRD spectrum of the negative electrode active material of Examples 10-12 was measured using a sample horizontal multi-purpose X-ray diffractometer Ultima IV manufactured by Rigaku Corporation.

It was confirmed that the negative electrode active material was a mixed phase of LiFe ₅ O ₈ and LiFeO ₂ .

[Comparative Example 1]
In Comparative Example 1, a lithium ion secondary battery for evaluation was produced in the same manner as in Example 1. However, no tris (chloroethyl) phosphate was added in the preparation of the electrolytic solution. Table 1 shows the measurement results of the Cl / P molar ratio obtained by IC and ICP-AES measurements. Cl was below the detection limit, and the Cl / P molar ratio could not be calculated.

  Table 1 shows the crystal phases of the negative electrode active materials of Examples 1 to 12 and Comparative Example 1, the Cl / P molar ratio in the electrolytic solution, and the capacity retention rate after 100 cycles.

[Comparative Example 2]
In Comparative Example 2, a lithium ion secondary battery for evaluation was produced in the same manner as in Example 10-12. However, no tris (chloroethyl) phosphate was added in the preparation of the electrolytic solution. Table 1 shows the measurement results of the Cl / P molar ratio obtained by IC and ICP-AES measurements. Cl was below the detection limit, and the Cl / P molar ratio could not be calculated.

上記の条件によって充放電を繰り返し、１００サイクル後の容量維持率によってサイクル特性を算出し、表１に容量維持率として示した。 (Measurement of cycle characteristics)
Using a secondary battery charge / discharge test apparatus, charging is performed at a current value of 0.5 C when the voltage range is 4.0 V to 1.0 V and 1 C = 800 mAh / g per weight of the negative electrode active material. Discharging was performed at a current value of 0 C, and cycle characteristics were evaluated.
The cycle characteristics are expressed as the discharge capacity retention rate (%), and the discharge capacity at the first cycle is the initial discharge capacity, and the ratio of the discharge capacity at each cycle number to the initial discharge capacity is based on the following formula (1). Calculated.

Charging / discharging was repeated under the above conditions, the cycle characteristics were calculated from the capacity retention rate after 100 cycles, and are shown in Table 1 as the capacity retention rate.

As is apparent from Table 1, the capacity retention rate of the one using Fe ₂ O ₃ as the negative electrode active material is improved when the Cl / P molar ratio is in the range of 0.005 mass% to 0.3 mass%. In particular, it was revealed that the capacity retention ratio was 80% or more and the characteristics were further improved when the content was in the range of 0.01% by mass to 0.1% by mass. During charging and discharging, Cl in the electrolyte and iron oxide react to form iron oxychloride. The formed iron oxychloride causes a chemical reaction with moisture. Therefore, it is presumed that good cycle characteristics were obtained in order to prevent the SEI film formed during the charging reaction from being generated excessively.

  According to the present invention, a lithium ion secondary battery having excellent 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 (2)

A negative electrode containing a compound composed of Fe and O, and an electrolytic solution containing LiPF ₆ and Cl,
A lithium ion secondary battery comprising: 2. The lithium ion secondary battery according to claim 1, wherein a molar ratio (Cl / P) of Cl and P in the electrolytic solution is 0.005 or more and 0.3 or less.

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