JP2011159484A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery improved in safety by restraining an additive for suppressing an exothermic reaction, from being reduced by a side reaction. <P>SOLUTION: A positive electrode including a positive electrode mixture containing a positive electrode active material, a conductive assistant and binder resin, and a current collector, is used, and a polymer compound chemically bonded to phosphate ester or cyclic phosphazene is used as the binder resin. The chemical bond is performed through a functional group of an acid chloride that is a phosphate ester or a cyclic phosphazene. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery.

  Advances in electronic technology have improved the performance of electronic devices, making them smaller and more portable, and secondary batteries with high energy density are desired as the power source. In response to these requirements, in recent years, non-aqueous electrolyte secondary batteries that can significantly improve energy density, that is, organic electrolyte lithium ion secondary batteries (hereinafter simply referred to as “lithium secondary batteries”) have been developed. Developed and rapidly spreading. In addition, lithium secondary batteries are expected to be used for electric vehicles and hybrid vehicles as a high-capacity, high-voltage power source.

  In applying the above lithium secondary battery, in addition to the long life and high output of the lithium secondary battery, it is also important to ensure the safety of the battery during abuse.

  Because current lithium secondary batteries use flammable organic electrolytes as electrolytes, it has become difficult to ensure safety during abuse, such as overcharging and internal short circuits, as the energy density of batteries increases. ing.

  In the lithium secondary battery, when the heat generation of the battery proceeds as in the case of an internal short circuit, both the positive electrode and the negative electrode can react with the electrolyte. If the heat generation cannot be suppressed at this time, the electrolytic solution is vaporized, and the vaporized reaction product is released from the battery can and may ignite.

  In order to improve safety against the heat generation and ignition of the lithium secondary battery due to the reaction and vaporization of the electrolytic solution, it has been studied to add an additive to the electrolytic solution.

  Conventionally, as additives used as flame retardants, phosphoric acid esters, phosphazenes, which are compounds of phosphorus and nitrogen, and additives for halogen compounds have been studied. These additives have the effect of trapping oxygen radicals generated during combustion and suppressing the combustion reaction.

  Patent Document 1 discloses a flame retardant electrolyte solution for a lithium battery using a solvent containing a phosphate ester.

  Non-Patent Documents 1 and 2 generally describe that an ester bond is formed by a condensation reaction between a hydroxyl group and an acid chloride, specifically, a phosphate group (—P (═O) O—) or a carboxyl group. It is described that an acid ester group (—C (═O) O—) is formed.

JP-A-4-184870

McMurray Organic Chemistry (Middle), p828, Tokyo Chemical Doujin, 1995 TEBIKI for Organic Chemistry Experiment 3 -Synthetic Reaction [I]-, p95

  An object of the present invention is to provide a lithium secondary battery in which an additive for suppressing an exothermic reaction is suppressed from decreasing due to a side reaction and safety is improved.

  The positive electrode of the present invention includes a positive electrode mixture containing a positive electrode active material, a conductive additive and a binder resin, and a current collector, and the binder resin is a polymer compound chemically bonded to a phosphate ester or cyclic phosphazene. It is characterized by being. The chemical bond is made through a functional group of the phosphate ester or the acid chloride of the cyclic phosphazene.

  According to the present invention, a lithium secondary battery excellent in safety can be provided.

It is an expanded sectional view which shows the positive electrode of the lithium secondary battery of an Example. It is a schematic sectional drawing which shows the lithium secondary battery of an Example. It is an expanded sectional view of the A section of FIG.

  Hereinafter, a positive electrode and a lithium secondary battery using the same according to an embodiment of the present invention will be described.

  The positive electrode includes a positive electrode mixture containing a positive electrode active material, a conductive additive and a binder resin, and a current collector, and the binder resin is a polymer compound chemically bonded to a phosphate ester or cyclic phosphazene. The chemical bond is made through a functional group of a phosphate ester or cyclic phosphazene acid chloride.

  In the positive electrode, the positive electrode active material is a composite oxide containing lithium and a transition metal.

  The positive electrode is obtained by chemically bonding the hydroxyl group of the binder resin and the functional group of the acid chloride.

  In the positive electrode, the binder resin includes polyvinyl alcohol or a copolymer of polyvinyl alcohol and another monomer.

  In the positive electrode, the binder resin includes cellulose, agar, dextrin, agarose, carrageenan, chitansan gum, curdlan or glucomannan.

  The lithium secondary battery uses a positive electrode having the above characteristics.

  The lithium secondary battery includes a non-aqueous electrolyte containing a lithium salt, and the non-aqueous electrolyte includes a phosphate ester or a cyclic phosphazene having an acid chloride functional group.

  The lithium secondary battery includes a positive electrode that occludes and releases lithium, a negative electrode that occludes and releases lithium, and a non-aqueous electrolyte containing a lithium salt. The positive electrode includes a positive electrode mixture and a current collector. The positive electrode mixture includes a composite resin having lithium and a transition metal, a conductive auxiliary agent, and a binder resin containing a hydroxyl group (—OH).

  Further, a phosphate ester or cyclic phosphazene having a phosphate chloride (—P (═O) Cl) or a carboxylate chloride (—C (═O) Cl) is added to the electrolytic solution, and the battery is manufactured. By using it for injection, the hydroxyl group of the binder resin and the phosphate or carboxylate undergo a condensation reaction, resulting in a phosphate ester group (—P (═O) O—) or a carboxylate ester group (—C (= O) O-) is formed. Thereby, the heat generation suppressing additive is fixed to the positive electrode through the binder resin.

  By taking the said structure, it becomes possible to prevent the side reaction in the surface of a negative electrode, and the effect which improves the safety | security of a lithium secondary battery is acquired.

  Specifically, decomposition of the additive on the surface of the negative electrode due to side reactions or the like can reduce the reaction between the electrolyte and the positive electrode active material without reducing the additive, and suppress the exothermic reaction in the battery. it can. Further, by suppressing the exothermic reaction, the evaporation of the electrolyte can be suppressed, and the safety can be improved.

  Here, it is thought that oxygen radicals generated from the positive electrode active material oxidize the electrolytic solution. However, the phenomenon in which the oxygen radicals react with the electrolytic solution is an oxidation reaction and generates heat. As a result, the exothermic reaction is more likely to occur.

  Therefore, an additive that acts as a radical scavenger that traps the radicals and makes them safe is fixed by chemically bonding to the binder resin in the positive electrode. As a result, radicals generated from the positive electrode at the time of abuse can be captured by the above-described additive present near the positive electrode surface, and the reaction between the radical and the electrolyte can be suppressed, thereby improving the safety of the lithium secondary battery. be able to.

  Therefore, considering the action of capturing oxygen radicals generated at the positive electrode, it is more effective to fix the additive in the positive electrode than in the electrolyte.

  Specifically, the additive is a substance for suppressing an exothermic reaction between the nonaqueous electrolytic solution and the positive electrode active material, and is solid or liquid. The additive is preferably dissolved and mixed in the electrolytic solution. Further, when the lithium secondary battery is used under normal conditions, it is preferable to exhibit a heat generation suppressing effect when the temperature of the lithium secondary battery becomes high at 200 ° C. or higher.

  Hereinafter, specific examples in the present embodiment will be described with reference to the drawings. The present invention is not limited to the following examples.

  FIG. 1 is an enlarged cross-sectional view showing a positive electrode of a lithium secondary battery of an example.

  In this figure, a positive electrode 100 is formed by mixing a positive electrode active material 1, a conductive agent 2, and a binder resin 3 (also simply referred to as a binder) on the surface of a current collector 5 formed of aluminum (Al). Agent 6 is applied. The binder resin 3 is a polymer compound having a hydroxyl group (—OH), and is chemically bonded to the additive 4 for suppressing an exothermic reaction between the nonaqueous electrolytic solution and the positive electrode active material. That is, the hydroxyl group of the binder resin 3 and the additive 4 are bonded by a condensation reaction.

  The particle diameter of the positive electrode active material 1 is about 10 μm, and the particle diameter of the binder resin 3 in which the additive 4 is chemically bonded to the surface is about 1 to 2 μm.

  The binder resin 3 binds the positive electrode active material 1 and the conductive agent 2 and binds the current collector 5 and the positive electrode mixture 6.

  Specific examples of the polymer compound include cellulose, agar, dextrin, agarose, carrageenan, curdlan, glucomannan and the like, or polyvinyl alcohol having a hydroxyl group or a copolymer of polyvinyl alcohol and another monomer. There is. One or two or more selected from these polymer compounds may be used in combination as the binder resin 3.

  The addition amount of the binder resin 3 is suitably in the range of 1 to 10 parts by weight in total, more preferably 3 to 7 parts by weight when the positive electrode active material 1 is 100 parts by weight. Here, the addition amount of the binder resin 3 is a value before the additive 4 is bonded.

  As the additive 4, a phosphate ester or a cyclic phosphazene containing a functional group of a phosphate compound can be used.

  Among these, the phosphate ester containing the functional group of phosphate compound is represented by the following chemical formula (1).

In the chemical formula (1), R ¹ and R ² preferably have a hydrocarbon group or a substituent composed of carbon, hydrogen, and oxygen as a skeleton, an alkyl group, a cycloalkyl group, an alkenyl group, an alkoxy-substituted alkyl group. Alternatively, it is more desirable to use an aryl group as a skeleton. As used herein, “with a skeleton” means that the hydrogen element constituting the hydrocarbon group or a substituent composed of carbon, hydrogen and oxygen is substituted with another element, for example, a halogen element such as fluorine or chlorine. Or the addition of another element to the oxygen element. For example, it means that the above-mentioned hydrocarbon group or a substituent composed of carbon, hydrogen and oxygen may contain —CF—, —CCl—, —CHF— and the like.

  Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, and a hexyl group. Examples of the cycloalkyl group include a cyclopropyl group and a cyclohexyl group. Examples of the alkenyl group include allyl group and methallyl group. Examples of the alkoxy-substituted alkyl group include a methoxyethyl group and a methoxyethoxyethyl group. Examples of the aryl group include a phenyl group, a methylphenyl group, and a methoxyphenyl group.

The hydrogen element contained in the above R ¹ and R ² may be substituted with a halogen element. Among these, a methyl group, an ethyl group, a propyl group, a trifluoroethyl group, a phenyl group, and a 3-fluorophenyl group are preferable in terms of excellent flame retardancy.

  Moreover, cyclic phosphazene is represented by the following chemical formula (2), and is a cyclic compound containing phosphorus and nitrogen.

In the chemical formula (2), at least one of the substituents R ^{3 to} R ⁸ bonded to phosphorus (P) is a phosphate chloride (—P (═O) Cl) or a carboxylate chloride ( It has a functional group of —C (═O) Cl) at the terminal. In this specification, the functional groups of phosphate chloride (—P (═O) Cl) or carboxylic acid chloride (—C (═O) Cl) are collectively referred to as “functional group of acid chloride”. To do.

R ^{3 to} R ⁸ described above preferably have a hydrocarbon group or a substituent composed of carbon, hydrogen, and oxygen as a skeleton, and have an alkyl group, a cycloalkyl group, an alkenyl group, an alkoxy-substituted alkyl group, or an aryl group as a skeleton. Is more desirable.

  When all the hydroxyl groups of the binder resin 3 contained in the positive electrode react with the additive 4, the safety of the lithium secondary battery is the highest.

  Even if the additive 4 remaining without reacting with the binder resin 3 remains in the electrolytic solution, there is no particular influence on safety.

  Since the condensation reaction between the hydroxyl group and the acid chloride shown in this example proceeds at room temperature, a wound group formed by winding a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode, or a positive electrode, a negative electrode, and An electrode group formed by laminating a separator sandwiched between a positive electrode and a negative electrode is placed in a battery can (battery container), and the above-described condensation reaction is caused by injecting an electrolytic solution containing additive 4. it can. The condensation reaction between the hydroxyl group and the acid chloride can also be achieved by reacting the positive electrode and the phosphate chloride in the step before producing the wound group.

  Here, the following chemical reaction formula (1) shows an example of a condensation reaction between the binder resin 3 having a hydroxyl group and a phosphate ester containing a functional group of a phosphate compound.

In the formula, R is a generalized representation of R ¹ and R ² described above.

  As shown in the chemical reaction formula (1), hydrogen of the hydroxyl group of the binder resin and chlorine of the phosphate ester are separated and bonded.

  An example of a condensation reaction between the binder resin 3 having a hydroxyl group and cyclic phosphazene is shown in the following chemical reaction formula (2).

In the formula, R is a generalized representation of the above R ^{3 to} R ⁸ .

  As shown in the chemical reaction formula (2), hydrogen of the hydroxyl group of the binder resin and chlorine of the phosphate ester are separated and bonded.

In the present embodiment, the case where the binder resin 3 has a hydroxyl group is described. However, the present invention is not limited to this, and may be a hydroxyl group as long as it is a functional group that causes a condensation reaction with an acid chloride. . Specific examples thereof include an amino group (-NH ₂₎ is.

The positive electrode active material 1 reversibly occludes and releases lithium, and is a layered compound such as lithium cobaltate (LiCoO ₂ ) or lithium nickelate (LiNiO ₂ ), or one or more types contained in these layered compounds. Transition metal substituted lithium manganate (Li _{1 + x} Mn _2−x O ₄ (where x = 0 to 0.33), Li _{1 + x} Mn _2−xy M _y O ₄ (where M is Ni, Including at least one metal selected from the group of Co, Cr, Cu, Fe, Al, and Mg, where x = 0 to 0.33, y = 0 to 1.0, 2-xy> 0), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , LiMn _2−x M _x O ₂ (where M includes at least one metal selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, x = 0 _{01~0.1), Li 2 Mn 3 MO} 8 ( although, M includes Fe, Co, Ni, at least one metal selected from the group of Cu and Zn.)), A layered type lithium manganese oxide (Li _{1 + a} Ni _x Co _y Mn _z O ₂ (where −0.1 <a <0.2, 0 <x <0.9, 0 <y <0.9, 0 <z <0.9, 0 .9 <x + y + z <1.1)), copper-lithium oxide (Li ₂ CuO ₂ ), iron-lithium oxide (LiFe ₃ O ₄ ), VSe, vanadium oxide (LiV ₃ O ₈ , V ₂ O ₅ , V ₆ O ₁₂ , Cu ₂ V ₂ O _7, etc.), disulfide compounds, Fe ₂ (MoO ₄ ) _3, or the like.

  The positive electrode active material 1 is not limited to the above example. In general, the positive electrode active material 1 is desirably a composite oxide having lithium and a transition metal.

  In addition, as a negative electrode active material that reversibly occludes and releases lithium, graphitized materials obtained from natural graphite, petroleum coke, coal pitch coke, etc. are heat-treated at a high temperature of 2500 ° C. or higher, mesophase carbon, amorphous Carbon, carbon fiber, lithium metal, a metal alloyed with lithium, a material having a metal supported on the surface of carbon particles, and the like are used. These negative electrode active materials include elements selected from lithium, aluminum, tin, silicon, indium, gallium, and magnesium, or alloys containing these elements. Further, these metals or oxides of these metals can also be used as the negative electrode active material.

  As the electrolytic solution, a lithium salt as an electrolyte, which is dissolved in an organic solvent can be used.

As the lithium salt, one or more of LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF _6, etc. Can be selected and used.

  As the organic solvent, carbonates, esters, ethers and the like can be used. Specific examples of the organic solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactron, methyl acetate, 1,3-dioxolane, 1,3-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran and the like. In addition to these, organic solvents such as sulfur compounds such as sulfolane, nitrogen-containing compounds, silicon-containing compounds, fluorine-containing compounds, and phosphorus-containing compounds may be used.

  However, it is not desirable to chemically react with the binder resin 3 having a hydroxyl group used for the positive electrode 100 or the additive 4 having an acid chloride functional group because the effect of the additive 4 is significantly impaired.

  Moreover, the lithium secondary battery of a present Example can be applied to the following electric equipment.

  That is, electric cars, electric bicycles, personal computers, mobile phones, digital cameras, video recorders, mini-disc portable players, personal digital assistants, watches, radios, electronic notebooks, electric tools, vacuum cleaners, toys, elevators, disaster robots, These include walking aids for medical care, wheelchairs for medical care, mobile beds for medical care, emergency power supplies, road conditioners, and power storage systems.

  In addition to the improved safety, the battery can be used as a home rechargeable battery, and the battery can be increased in size. Furthermore, in addition to these consumer uses, it can also be used for military use and space.

  The shape of the non-aqueous electrolyte (lithium) secondary battery in the present embodiment is not particularly limited, and various shapes such as a cylindrical shape, a square shape, a coin shape, and a button shape can be used as necessary. It can be made into the shape. Further, the battery outer packaging material is not limited to the stainless steel can but may be applied to a laminated one.

  FIG. 2 is a schematic cross-sectional view showing a lithium secondary battery of an example.

  In this figure, positive electrodes 11 and negative electrodes 12 are alternately stacked and enclosed in a battery can 10.

  FIG. 3 is an enlarged cross-sectional view of a portion A in FIG.

  In this figure, the positive electrode 11 is obtained by applying a powdery lithium composite oxide 13 and a binder resin 15 containing an additive to both surfaces of a current collector 16, and the negative electrode 12 is formed on both surfaces of the current collector 18. Are coated with a powdery carbon material 14 and a binder resin 17.

  A separator 19 is sandwiched between the positive electrode 11 and the negative electrode 12, and the battery is filled with a non-aqueous electrolyte 20.

  2 and 3, a lithium secondary battery having a structure in which the positive electrode 11, the separator 19, the negative electrode 12, and the separator 19 are simply laminated repeatedly is shown. However, the internal structure and the external shape are limited to this. is not. As another example of the internal structure, there is a structure in which a stack of the positive electrode 11, the separator 19, the negative electrode 12, and the separator 19 is wound to have a cylindrical or prismatic structure. Further, the external shape may be a cylindrical shape, a circular plate shape or a rectangular parallelepiped shape.

  More specific description will be given below.

  In this example, a phosphate ester containing a functional group of phosphate is used as an additive.

[Production of positive electrode]
The positive electrode was produced by the following procedure.

Agar as a binder was dissolved in pure water to prepare an aqueous solution having a concentration of 10 wt%. 60 parts by weight of the powder of the positive electrode active material LiNi _0.33 Co _0.33 Mn _0.33 O ₂ is mixed with 34 parts by weight of this solution, and pure water is added and kneaded to prepare a positive electrode mixture slurry. Then, this slurry was applied to one surface of an aluminum foil having a thickness of 0.02 mm as a current collector, and dried by blowing warm air at 120 ° C. Similarly, the positive electrode mixture slurry was applied to the other surface (back surface) of the current collector and dried.

  Then, the positive electrode was obtained by press-molding so that it might become predetermined thickness, and drying under reduced pressure. This was used as a positive electrode plate.

(Production of negative electrode)
About the negative electrode, it produced in the following procedures.

  Polyvinylidene fluoride (PVDF) as a binder resin was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a solution with a concentration of 10 wt%. To 50 parts by weight of this solution, 30 parts by weight of an amorphous carbon powder having an average particle size of 10 μm, which is a carbon material, and 50 parts by weight of an active material powder are mixed, and NMP is added to adjust the viscosity. A negative electrode mixture slurry was prepared by kneading, and this slurry was applied (coated) to one side of a 0.01 mm thick copper foil as a current collector, and dried by blowing hot air at 120 ° C.

  Similarly, the negative electrode mixture slurry was applied to the other surface (back surface) of the current collector and dried. Then, the negative electrode was obtained by press-molding to a predetermined thickness and drying under reduced pressure. This was used as a negative electrode plate.

[Production of cylindrical lithium secondary battery]
The obtained negative electrode and positive electrode are wound through a separator to form a roll-shaped electrode. This was stored in a battery can and dried under reduced pressure.

Next, LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate (2: 4: 4 (volume ratio)) at a rate of 1 mol / L (mol / liter), and 5 wt. % And a phosphate ester represented by the above chemical formula (1) (R ¹ and R ² are —C ₆ H ₅ ) was added as an additive to prepare an electrolytic solution.

  After injecting this electrolytic solution into the battery can, the lid was caulked to produce a cylindrical lithium secondary battery (diameter 15 mm, length 65 mm).

  About this cylindrical lithium secondary battery, charging / discharging was performed 20 cycles using the charger / discharger under the conditions of an ambient temperature of 25 ° C., a current of 300 mA, and a charge termination voltage of 4.2 to 3.0 V.

[Differential scanning calorimetry]
In order to investigate the exothermic behavior in the reaction between the non-aqueous electrolyte and the positive electrode active material, differential scanning calorimetry was performed.

  After performing the above charge and discharge, the lithium secondary battery charged to 4.2 V was disassembled, and the positive electrode plate holding the electrolyte solution was punched out to 3.5 mmφ to obtain a sample piece.

  This sample piece was sealed in a stainless steel pressure-resistant airtight container and heated from room temperature to 400 ° C. under a temperature rising condition of 5 ° C./min with a differential scanning calorimeter (DSC). In this process, the initial exothermic peak temperature was measured and used as an index of positive electrode heat resistance.

  As a result of DSC measurement, the exothermic peak height was 2.0 mW, and the exothermic peak temperature was 335 ° C. Here, the exothermic peak height represents the maximum value of the calorific value in the reaction between the non-aqueous electrolyte and the positive electrode active material.

  In this example, a cyclic phosphazene derivative is used as an additive.

[Production of positive electrode]
The positive electrode was produced in the same procedure as in Example 1.

(Production of negative electrode)
The negative electrode was prepared in the same procedure as in Example 1.

[Production of cylindrical lithium secondary battery]
A roll-shaped electrode was formed by winding a separator between the negative electrode and the positive electrode prepared by the above procedure. This was stored in a battery can and dried under reduced pressure.

Next, LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate (2: 4: 4 (volume ratio)) at a rate of 1 mol / L in the battery can, and N is a cyclic phosphazene derivative. ₃ P ₃ (C ₆ H ₄ -pC (═O) Cl) ₂ (C ₆ H ₅ ) ₄ (p in this chemical formula is N ₃ P _{3 with} a benzene ring (—C ₆ H ₄ —) as the center). And -C (= O) Cl are bonded to the para position.) To prepare an electrolytic solution in which the amount was 10 wt%.

In the cyclic phosphazene derivative used in this example, R ³ and R ⁴ in the chemical formula (2) are —C ₆ H ₄ -pC (═O) Cl, and R ^{5 to} R ⁸ are —C ₆ H ₅ . is there.

  After injecting this electrolytic solution into the battery can, the lid was caulked to produce a cylindrical lithium secondary battery (diameter 15 mm, length 65 mm).

  About this cylindrical lithium secondary battery, charging / discharging was performed 20 cycles using the charger / discharger under the conditions of an ambient temperature of 25 ° C., a current of 300 mA, and a charge termination voltage of 4.2 to 3.0 V.

[Differential scanning calorimetry]
In order to investigate the exothermic behavior in the reaction between the non-aqueous electrolyte and the positive electrode active material, differential scanning calorimetry was performed.

  After performing the above charge and discharge, the lithium secondary battery charged to 4.2 V was disassembled, and the positive electrode plate holding the electrolyte solution was punched out to 3.5 mmφ to obtain a sample piece.

  This sample piece was sealed in a stainless steel pressure-resistant airtight container and heated from room temperature to 400 ° C. under a temperature rising condition of 5 ° C./min with a differential scanning calorimeter (DSC). In this process, the initial exothermic peak temperature was measured and used as an index of positive electrode heat resistance.

  As a result of DSC measurement, the exothermic peak height was 1.8 mW, and the exothermic peak temperature was 330 ° C.

(Comparative Example 1)
Comparative Example 1 using 20% by weight of liquid trimethyl phosphate (TMP) as an electrolytic solution is shown below.

[Production of positive electrode]
The positive electrode was produced by the following procedure.

Polyvinylidene fluoride (PVDF), which is a binder resin, was dissolved in NMP to prepare a solution having a concentration of 10 wt%. To 40 parts by weight of this solution, 60 parts by weight of the powder of the positive electrode active material LiNi _0.33 Co _0.33 Mn _0.33 O ₂ was mixed, and further NMP was added and kneaded to prepare a positive electrode mixture slurry. The slurry was applied to one side of a 0.02 mm thick aluminum foil as a current collector, and dried by blowing warm air at 120 ° C. Similarly, the positive electrode mixture slurry was applied to the back surface of the current collector and dried. Thereafter, the positive electrode was obtained by press molding to a predetermined thickness and drying under reduced pressure. This was used as a positive electrode plate.

(Production of negative electrode)
The negative electrode was prepared in the same procedure as in Example 1.

[Production of cylindrical lithium secondary battery]
The obtained negative electrode and positive electrode were wound through a separator to form a roll-shaped electrode. This was stored in a battery can and dried under reduced pressure.

Next, 20 parts by weight of trimethyl phosphate is added to 80 parts by weight of a mixed solvent of ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate (2: 4: 4 (volume ratio)), and LiPF ₆ is added at a rate of 1 mol / L. After injecting the dissolved electrolyte, the lid was caulked to produce a lithium secondary battery (diameter 15 mm, length 65 mm). Moreover, charging / discharging was performed 20 cycles with the ambient temperature of 25 degreeC, the electric current of 300 mA, and the charge end voltage of 4.2-3.0V using the charger / discharger.

[Differential scanning calorimetry]
In order to investigate the exothermic behavior in the reaction between the non-aqueous electrolyte and the positive electrode active material, differential scanning calorimetry was performed.

  After performing the above charge and discharge, the lithium secondary battery charged to 4.2 V was disassembled, and the positive electrode plate holding the electrolyte solution was punched out to 3.5 mmφ to obtain a sample piece.

  This sample piece was sealed in a stainless steel pressure-resistant airtight container and heated from room temperature to 400 ° C. under a temperature rising condition of 5 ° C./min with a differential scanning calorimeter (DSC). In this process, the initial exothermic peak temperature was measured and used as an index of positive electrode heat resistance.

  As a result of DSC measurement, the exothermic peak height was 2.4 mW, and the exothermic peak temperature was 307 ° C.

(Comparative Example 2)
The comparative example 2 which is an example which does not add the additive which concerns on this invention to electrolyte solution and a positive electrode is shown below.

[Production of positive electrode]
The positive electrode was produced by the following procedure.

Polyvinylidene fluoride (PVDF), which is a binder resin, was dissolved in NMP to prepare a solution having a concentration of 10 wt%. To 40 parts by weight of this solution, 60 parts by weight of the positive electrode active material LiNi _0.33 Co _0.33 Mn _0.33 O ₂ powder was mixed, and further NMP was added and kneaded to prepare a positive electrode mixture slurry. The slurry was applied to one side of a 0.02 mm thick aluminum foil as a current collector, and dried by blowing warm air at 120 ° C. Similarly, the positive electrode mixture slurry was applied to the back surface of the current collector and dried. Then, after press-molding so that it might become predetermined thickness, the positive electrode was obtained by drying under reduced pressure. This was used as a positive electrode plate.

(Production of negative electrode)
The negative electrode was prepared in the same procedure as in Example 1.

[Production of cylindrical lithium secondary battery]
The obtained negative electrode and positive electrode were wound through a separator to form a roll-shaped electrode. This was stored in a battery can and dried under reduced pressure.

Next, an electrolytic solution in which LiPF ₆ was dissolved at a ratio of 1 mol / L was injected into a mixed solvent of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate (2: 4: 4 (volume ratio)) into the battery can. Then, a lithium secondary battery (diameter 15 mm, length 65 mm) was produced by caulking the lid. Moreover, charging / discharging was performed 20 cycles with the ambient temperature of 25 degreeC, the electric current of 300 mA, and the charge end voltage of 4.2-3.0V using the charger / discharger.

[Differential scanning calorimetry]
In order to investigate the exothermic behavior in the reaction between the non-aqueous electrolyte and the positive electrode active material, differential scanning calorimetry was performed.

  After performing the above charge and discharge, the lithium secondary battery charged to 4.2 V was disassembled, and the positive electrode plate holding the electrolyte solution was punched out to 3.5 mmφ to obtain a sample piece.

  This sample piece was sealed in a stainless steel pressure-resistant airtight container and heated from room temperature to 400 ° C. under a temperature rising condition of 5 ° C./min with a differential scanning calorimeter (DSC). In this process, the initial exothermic peak temperature was measured and used as an index of positive electrode heat resistance.

  As a result of DSC measurement, the exothermic peak height was 6.6 mW, and the exothermic peak temperature was 320 ° C.

  Table 1 summarizes the results of Examples 1-2 and Comparative Examples 1-2.

  In this table, the exothermic peak of Comparative Example 1 in which TMP is added to the electrolytic solution is lower than that of Comparative Example 2 in which no additive is contained in the electrolytic solution.

  On the other hand, Examples 1 and 2 in which an additive for suppressing heat generation was fixed to the positive electrode had an effect of further reducing the heat generation peak as compared with Comparative Example 1. Moreover, since Example 1 and 2 also becomes high exothermic peak temperature compared with the comparative example 1, it can improve safety | security.

  From the above results, it is possible to maintain the charge-discharge cycle after fixing the additive for suppressing heat generation to the positive electrode binder. In addition, the exothermic peak can be reduced and the exothermic peak temperature can be increased.

  In the above examples, the case where the additive is fixed to the positive electrode using the non-aqueous electrolyte solution in which the additive for suppressing the heat generation is described is not limited to this. A lithium secondary battery may be manufactured by previously preparing a positive electrode to which an additive for suppressing the adhesion is attached. In this case, it is not necessary to dissolve the above additives in the nonaqueous electrolytic solution.

  Therefore, there is an effect of preventing the additive dissolved in the non-aqueous electrolyte from reacting by contact with the negative electrode and damaging the additive. In addition, since it is not necessary to dissolve the additive in the non-aqueous electrolyte and there is no loss of the additive in the negative electrode, the amount of additive used can be reduced.

  As a method for attaching the additive to the positive electrode, there are a method of immersing the positive electrode in a solution of the additive, a method of applying the additive solution by spraying on the positive electrode, and the like. The solvent of the solution used here may be any solvent that can dissolve the above-described additives, and may be a liquid different from the nonaqueous electrolytic solution.

  1: positive electrode active material, 2: conductive agent, 3: binder resin, 4: additive, 5: current collector, 6: positive electrode mixture, 10: battery can, 11: positive electrode, 12: negative electrode, 13: lithium composite Oxide, 14: carbon material, 15: binder resin containing additive, 16, 18: current collector, 17: binder resin, 19: separator, 20: non-aqueous electrolyte, 100: positive electrode.

Claims (12)

  A positive electrode mixture containing a positive electrode active material, a conductive additive and a binder resin, and a current collector, wherein the binder resin is a polymer compound chemically bonded to a phosphate ester or cyclic phosphazene, and the chemical bond is A positive electrode characterized by being formed through a functional group of an acid chloride of the phosphate ester or the cyclic phosphazene.   The positive electrode according to claim 1, wherein the positive electrode active material is a composite oxide containing lithium and a transition metal.   The positive electrode according to claim 1 or 2, wherein a hydroxyl group of the binder resin and a functional group of the acid chloride are chemically bonded.   The positive electrode according to claim 3, wherein the binder resin contains polyvinyl alcohol or a copolymer of polyvinyl alcohol and another monomer.   4. The positive electrode according to claim 3, wherein the binder resin contains cellulose, agar, dextrin, agarose, carrageenan, chitansan gum, curdlan, or glucomannan. The said phosphate ester is represented by following Chemical formula (1), The positive electrode as described in any one of Claims 1-5 characterized by the above-mentioned.

(In the formula, R ¹ and R ² have a skeleton of a hydrocarbon group or a substituent composed of carbon, hydrogen and oxygen.) 7. The positive electrode according to claim 6, wherein R ¹ and R ² have an alkyl group, a cycloalkyl group, an alkenyl group, an alkoxy-substituted alkyl group, or an aryl group as a skeleton. The positive electrode according to claim 7, wherein R ¹ and R ² are a methyl group, an ethyl group, a propyl group, a trifluoroethyl group, a phenyl group, or a 3-fluorophenyl group. The said cyclic phosphazene is represented by following Chemical formula (2), The positive electrode as described in any one of Claims 1-5 characterized by the above-mentioned.

(In the formula, R ^{3 to} R ⁸ are hydrocarbon groups or substituents composed of carbon, hydrogen and oxygen, and at least one of R ^{3 to} R ⁸ is —P (= (It has a functional group represented by O) Cl or —C (═O) Cl.) The positive electrode according to claim 9, wherein R ^{3 to} R ⁸ have an alkyl group, a cycloalkyl group, an alkenyl group, an alkoxy-substituted alkyl group, or an aryl group as a skeleton.   A lithium secondary battery using the positive electrode according to claim 1.   The lithium secondary battery according to claim 11, comprising a non-aqueous electrolyte containing a lithium salt, wherein the non-aqueous electrolyte contains a phosphate ester or cyclic phosphazene having an acid chloride functional group.

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