JP4955951B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery using a nonaqueous electrolytic solution obtained by dissolving an electrolyte in a nonaqueous solvent such as an organic solvent.

  Lithium ion secondary batteries using non-aqueous electrolytes not only provide high voltage and high energy density, but can also be reduced in size and weight, so they are already in practical use in information communication equipment such as personal computers and mobile phones. It has become. In recent years, electric vehicles and hybrid electric vehicles have attracted attention due to environmental problems and resource issues, and the lithium ion secondary battery is also used as a power source mounted on electric vehicles and hybrid electric vehicles.

  A lithium ion secondary battery mainly includes a positive electrode, a negative electrode, and a non-aqueous electrolyte that moves lithium ions between the positive electrode and the negative electrode. Specifically, for example, a non-aqueous electrolysis in which a positive electrode containing a lithium transition metal composite oxide as a positive electrode active material, a negative electrode containing a carbon material as a negative electrode active material, and a lithium salt as an electrolyte dissolved in an organic solvent Lithium ion secondary batteries having a liquid are used.

LiPF ₆ is often used as the electrolyte lithium salt because of its high electrical conductivity. Further, as the positive electrode, a sheet-like electrode obtained by applying a positive electrode active material such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ to an aluminum current collector is generally used, and as the negative electrode, graphite or For example, a sheet-like electrode obtained by applying a negative electrode active material such as coke to a copper current collector is used. The lithium ion secondary battery can be manufactured by combining the above-described positive electrode and negative electrode, housed in a battery case made of aluminum, aluminate film, or the like, and injecting the non-aqueous electrolyte. Such a lithium ion secondary battery can exhibit a high energy density of 3 to 4 V class in a single cell.

In constructing a high-power lithium ion secondary battery as described above, it is becoming an essential condition that the lithium salt LiPF ₆ having high electrical conductivity is the main component of the electrolyte.
However, LiPF ₆ is known to react with a small amount of moisture that enters the battery during the production or use of the secondary battery to generate HF (hydrogen fluoride) (see Non-Patent Document 1). . And the produced | generated HF had a bad influence on the constituent material in a battery, and there existed a possibility of reducing battery characteristics, such as an output and a lifetime.

  Specifically, HF may corrode battery members made of aluminum such as a current collector and a battery case, particularly in a high temperature environment. This is because aluminum is an amphoteric metal and is easily attacked by acids and alkalis. As a result, the battery output may be reduced, or the battery capacity may be easily deteriorated by repeated charging and discharging. In addition, gas may be generated in the battery, which may damage the battery case.

In addition, in a lithium ion secondary battery using the above non-aqueous electrolyte, there is a risk that the organic solvent will ignite and burn if the battery is damaged or the pressure inside the battery rises due to some cause and the electrolyte leaks. There is sex. In particular, in applications such as automobile batteries, use in a high-temperature environment is inevitable, so it is an important issue to avoid the risk of ignition.
Therefore, research for imparting flame retardancy to the electrolytic solution is underway. Specifically, lithium ion secondary batteries using a non-aqueous electrolyte containing a room temperature molten salt have been developed (see Patent Documents 1 to 3).

  However, a lithium ion secondary battery using an electrolytic solution mixed with a room temperature molten salt has a problem that it cannot exhibit practically sufficient durability. In other words, since normal temperature molten salt has low reduction durability, a lithium ion secondary battery containing a normal temperature molten salt in the electrolyte solution may increase the internal resistance of the battery and decrease the output when it is repeatedly charged and discharged. was there.

JP 2003-288939 A JP 2003-203694 A JP 2003-331918 A “Journal of Power Sources”, USA, ELSEVIER, 1997, Vol. 68, p. 91-98

  The present invention has been made in view of such a conventional problem, and has an excellent flame retardancy and intends to provide a lithium ion secondary battery capable of exhibiting high output even when repeated charge and discharge are performed. Is.

The present invention includes a positive electrode containing an oxide containing lithium and a transition metal element or a polyanionic compound as a positive electrode active material, a negative electrode containing a carbon material capable of occluding and desorbing lithium as a negative electrode active material, In a lithium ion secondary battery having a non-aqueous electrolyte obtained by dissolving an electrolyte in a mixed solvent of an organic solvent and a room temperature molten salt,
The room temperature molten salt is composed of one or more selected from an imidazolium salt, a pyrrolidinium salt, or an aliphatic quaternary ammonium salt,
The mixed solvent consists of the organic solvent 20-80 vol% and the room temperature molten salt 80-20 vol%,
The electrolyte is a lithium ion secondary battery using at least LiPF ₆ and a salt of an anion compound represented by the following formulas (2) to (5) (Claim 1) .

In the lithium ion secondary battery of the present invention, the nonaqueous electrolytic solution contains the room temperature molten salt.
Therefore, the non-aqueous electrolyte exhibits flame retardancy and can improve the safety of the lithium ion secondary battery.

In the lithium ion secondary battery, the non-aqueous electrolyte contains the anionic compound represented by the above formulas (2) to (5) . The anionic compound can capture HF (hydrogen fluoride) generated by the reaction of LiPF ₆ as the electrolyte with moisture or the like.
Therefore, it can suppress that HF exerts a bad influence on the structural member of the said lithium ion secondary battery. That is, it is possible to prevent HF from increasing the internal resistance of the lithium ion secondary battery and reducing the battery life. Moreover, generation of gas in the lithium ion secondary battery can be suppressed.
Therefore, the lithium ion secondary battery can exhibit high output even when charging and discharging are repeated.
In addition, the increase in internal resistance and gas generation due to HF are particularly likely to occur in a high-temperature environment. However, in the lithium ion secondary battery, the increase in internal resistance and gas generation due to HF are suppressed even in a high-temperature environment. can do.

  In addition, the anionic compound can suppress an increase in internal resistance as described above even in the non-aqueous electrolyte containing the room temperature molten salt. Therefore, the lithium ion secondary battery is excellent in safety and durability in that it can exhibit excellent flame retardancy and can suppress an increase in internal resistance due to repeated charge and discharge. Moreover, the anionic compound hardly affects the charge / discharge capacity. Therefore, the lithium ion secondary battery can exhibit a high discharge capacity even after repeated charge and discharge.

Although the details of the mechanism for improving the battery characteristics by the anionic compound are not clear, it is presumed as follows.
That is, the anionic compound is considered to form a coating such as an electrochemically stable coating on the surface of the active material. And, by forming the covering, in the lithium ion secondary battery, even if charging / discharging is repeatedly performed in a high temperature environment, insertion / extraction of lithium ions is performed smoothly, and an increase in internal resistance is suppressed. Can do. Furthermore, the formation of the coating can prevent the room temperature molten salt from being reduced and adversely affecting the battery characteristics.

｛但し、Ｍは、遷移金属、周期律表のIII族、IV族、又はV族元素、ｂは１〜３、ｍは１〜４、ｎは０〜８、ｑは０又は１をそれぞれ表し、Ｒ ¹ は、Ｃ ₁ 〜Ｃ ₁₀ のアルキレン、Ｃ ₁ 〜Ｃ ₁₀ のハロゲン化アルキレン、Ｃ ₆ 〜Ｃ ₂₀ のアリーレン、又はＣ ₆ 〜Ｃ ₂₀ のハロゲン化アリーレン（これらのアルキレン及びアリーレンはその構造中に置換基、ヘテロ原子を持ってもよく、またｍ個存在するＲ ¹ はそれぞれが結合してもよい。）、Ｒ ² は、ハロゲン、Ｃ ₁ 〜Ｃ ₁₀ のアルキル、Ｃ ₁ 〜Ｃ ₁₀ のハロゲン化アルキル、Ｃ ₆ 〜Ｃ ₂₀ のアリール、Ｃ ₆ 〜Ｃ ₂₀ のハロゲン化アリール、又はＸ ³ Ｒ ³ （これらのアルキル及びアリールはその構造中に置換基、ヘテロ原子を持ってもよく、またｎ個存在するＲ ² はそれぞれが結合して環を形成してもよい。）、Ｘ ¹ 、Ｘ ² 、Ｘ ³ は、Ｏ、Ｓ、又はＮＲ ⁴ 、Ｒ ³ 、Ｒ ⁴ は、それぞれが独立で、水素、Ｃ ₁ 〜Ｃ ₁₀ のアルキル、Ｃ ₁ 〜Ｃ ₁₀ のハロゲン化アルキル、Ｃ ₆ 〜Ｃ ₂₀ のアリール、Ｃ ₆ 〜Ｃ ₂₀ のハロゲン化アリールをそれぞれ示す（これらのアルキル及びアリールはその構造中に置換基、ヘテロ原子を持ってもよく、また複数個存在するＲ ³ 、Ｒ ⁴ はそれぞれが結合して環を形成してもよい。）。｝ Next, a preferred embodiment of the present invention will be described.
The lithium ion secondary battery of the present invention has a positive electrode, a negative electrode, and a nonaqueous electrolytic solution obtained by dissolving an electrolyte in a mixed solvent of an organic solvent and a room temperature molten salt.
The non-aqueous electrolyte, a mixed solvent of an organic solvent and an ambient temperature molten salt, as an electrolyte, the use of a obtained by dissolving a salt of the anion compound represented by LiPF ₆ and the following general formula (1) Can do . In the non-aqueous electrolyte, at least a part of the electrolyte is ionized, and Li ⁺ ions, PF ₆ ^- ions, the anion compound, and a cation paired with the anion compound are present.

{However, M is a transition metal, a group III, IV or V element of the periodic table, b is 1 to 3, m is 1 to 4, n is 0 to 8, q is 0 or 1 respectively. , R ¹ is C ₁ -C ₁₀ alkylene, C ₁ -C ₁₀ halogenated alkylene, C ₆ -C ₂₀ arylene, or C ₆ -C ₂₀ halogenated arylene (the alkylene and arylene have the structure) And R ¹ may be bonded to each other.), R ² is halogen, C ₁ -C ₁₀ alkyl, C ₁ -C ₁₀ Alkyl halides, C ₆ -C ₂₀ aryls, C ₆ -C ₂₀ aryl halides, or X ³ R ³ (these alkyls and aryls may have substituents, heteroatoms in their structures, the n number R ² each may combine with each other to form a ring present.), X ^{1, 2,} X ³ is O, S, or NR ^4, R ^3, R ⁴ are, each independently, hydrogen, alkyl of C ₁ -C _10, alkyl halide C ₁ ~C _10, C ₆ ~C ₂₀ aryls and C ₆ -C ₂₀ aryl halides are shown respectively. (These alkyls and aryls may have a substituent or a hetero atom in the structure, and a plurality of R ³ and R ⁴ are respectively present. May combine to form a ring. }

  Examples of the salt of the anionic compound include lithium salt, sodium salt, potassium salt, magnesium salt, calcium salt, barium salt, cesium salt, rubidium salt, silver salt, zinc salt, copper salt, cobalt salt, iron salt, nickel salt. , Manganese salt, titanium salt, lead salt, chromium salt, vanadium salt, ruthenium salt, yttrium salt, lanthanoid salt, actinoid salt, tetraalkylammonium salt, triethylammonium salt, pyridinium salt, imidazolium salt, proton salt, tetraethylphosphonium salt Tetramethylphosphonium salt, tetraphenylphosphonium salt, triphenylsulfonium salt, triethylsulfonium salt and the like. Specific examples of the tetraalkylammonium salt include a tetrabutylammonium salt, a tetraethylammonium salt, a tetramethylammonium salt, and a triethylmethylammonium salt.

Moreover, in the said General formula (1), the valence b of an anion is 1-3 . When b is larger than 3, the crystal lattice energy of the salt of the anionic compound is increased, so that it is difficult to form the anionic compound by dissolving the salt of the anionic compound in the non-aqueous solvent. Therefore, b = 1 is most preferable.
For the same reason, the valence of the cation constituting the salt of the anionic compound is preferably 1 to 3, and most preferably the valence of the cation is 1.
In addition, the anion compound represented by the general formula (1) has an ionic metal complex structure, and M as the center is a transition metal, a group III, group IV, or group V element of the periodic table. Chosen from .

Preferably, M in the general formula (1) is Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As, Sc, Hf. Or a cation that is paired with the anion compound in the salt of the anion compound may be at least one of Li ^{+ and} Na ⁺ (Claim 2).
In this case, the nonaqueous electrolytic solution containing the anionic compound can be easily prepared by dissolving the salt of the anionic compound in the nonaqueous solvent. In this case, the anion compound or the electrolyte compound can be easily synthesized.

More preferably, M in the general formula (1) is Al, B, or P. In this case, the synthesis of the anionic compound or a salt thereof is further facilitated, and the production cost can be reduced and the toxicity can be lowered.

Next, the ligand part of the anionic compound (ionic metal complex) will be described.
Hereinafter, in the present specification, an organic or inorganic part bonded to M in the general formula (1) is referred to as a ligand.

R in general formula (1) ¹ Is C ₁ ~ C _Ten Alkylene, C ₁ ~ C _Ten An alkylene halide of C ₆ ~ C ₂₀ Arylene or C ₆ ~ C ₂₀ Of halogenated arylene. These alkylene and arylene may have a substituent or a hetero atom in the structure. Specifically, instead of hydrogen on alkylene and arylene, halogen, chain or cyclic alkyl group, aryl group, alkenyl group alkoxy group, aryloxy group, sulfonyl group, amino group, cyano group, carbonyl group, acyl group , An amide group, a hydroxyl group, and a structure in which nitrogen, sulfur, or oxygen is introduced instead of carbon on alkylene and arylene.
  Furthermore, R ¹ Are present (when q = 1 and m = 2 to 4), each may be bonded, and examples thereof include a ligand such as ethylenediaminetetraacetic acid.

R ² is selected from halogen, C ₁ -C ₁₀ alkyl, C ₁ -C alkyl halide _10, aryl of C ₆ -C _20, an aryl halide of C ₆ -C _20, or X ³ R ³ It consists of things. Similarly to R ¹ , alkyl and aryl may have a substituent or a heteroatom in the structure, and when there are a plurality of R ² (when n = 2 to 8), each R ² is They may combine to form a ring.
Preferably, R ² is an electron-withdrawing group, particularly fluorine. In this case, it is possible to improve the solubility and dissociation degree of the salt of the anionic compound, and to improve the ionic conductivity accordingly. Furthermore, in this case, the oxidation resistance is improved, thereby preventing the occurrence of side reactions.

X ¹ , X ² , and X ³ are each independently O, S, or NR ⁴ , and the ligand is bonded to M through these heteroatoms. Here, it is not impossible to combine other than O, S, and N, but it becomes very complicated in synthesis. As a feature of the compound represented by the general formula (1), there is a bond between X ¹ and M by X ² in the same ligand, and these ligands form a chelate structure with M. . The constant q in this ligand is 0 or 1 . When q = 0, the chelate ring becomes a five-membered ring, and the complex structure of the anionic compound is stabilized. Therefore, in this case, the anionic compound can be prevented from causing side reactions other than the formation of the coating.

R ^Three , R ^Four Are each independently hydrogen, C ₁ ~ C _Ten Alkyl, C ₁ ~ C _Ten Alkyl halides of C ₆ ~ C ₂₀ Aryl, C ₆ ~ C ₂₀ These alkyls and aryls may have a substituent, a hetero atom in the structure, and R ^Three , R ^Four When a plurality of are present, each may be bonded to form a ring.

The constants m and n related to the number of ligands described above are determined by the type of M at the center, and m is 1 to 4 and n is 0 to 8.
It in ^{R 1, R 2, R 3} , R 4 described above, C ₁ -C ₁₀ indicates 1 to 10 carbon atoms, C ₆ -C ₂₀ is 6 to 20 carbon atoms Indicates.

Next, as the anion compound represented by the general formula (1), it is preferable to use one or more kinds represented by the following formulas (2) to (5) .
In this case, the solubility and dissociation degree of the salt of the anionic compound can be improved, and the ionic conductivity of the non-aqueous electrolyte can be improved. Furthermore, in this case, oxidation resistance can be improved.

Preferably, as the anionic compound, a compound represented by the above formula (4) may be used (claim 3 ).
In this case, the ion conductivity of the non-aqueous electrolyte is further improved, and the output of the lithium ion secondary battery can be further improved.

In addition, as a method for synthesizing the anion compound, for example, in the case of a compound represented by the following formula (2), after reacting LiBF ₄ and 2 moles of lithium alkoxide in a non-aqueous solvent, oxalic acid And alkoxide bonded to boron is replaced with oxalic acid. In this case, a lithium salt of an anionic compound represented by the following formula (2) can be obtained.

  Further, the anion compound contained in the non-aqueous electrolytic solution is obtained by charging the lithium ion secondary battery at least once, whereby all or part of the anion compound is decomposed, and the positive electrode and / or A coating such as a film can be formed by coating the surface of the negative electrode or the surface of the positive electrode active material or / and the negative electrode active material. The coating can be detected by, for example, X-ray photoelectron spectroscopy (XPS) or IR analysis.

Further, as the electrolyte, further LiClO _4, LiBF _4, LiAsF _6, or it is preferable to use at least one member selected from LiSbF ₆ (claim 4).
In this case, relatively stable battery characteristics can be obtained.

The addition amount of the salt of the anionic compound with respect to the mixed solvent is preferably 1 mol% to 50 mol% in the total electrolytic mass (Claim 5 ).
When the added amount of the salt of the anionic compound is less than 1 mol%, the effect of suppressing the increase in internal resistance may not be sufficiently exhibited. The reason for this is considered to be that an abdomen such as a film formed on the surface of the active material by the anionic compound is not sufficiently formed. On the other hand, when the added amount of the salt of the anionic compound exceeds 50 mol%, the initial discharge capacity of the lithium ion secondary battery may be reduced. This is probably because the thickness of the coating formed on the surface of the active material becomes larger than necessary.

  Next, in the non-aqueous electrolyte, an aprotic organic solvent or the like can be used as the organic solvent. As such an organic solvent, for example, a mixed solvent composed of one or more selected from cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether, and the like can be used.

  Here, examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of the cyclic ester include gamma butyrolactone and gamma valerolactone. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane and ethylene glycol dimethyl ether. As the organic solvent, any one of these can be used alone, or two or more kinds can be mixed and used.

In the non-aqueous electrolyte, the room temperature molten salt is an ionic liquid that is melted at room temperature (25 ° C.).
Preferably, the room temperature molten salt is at least one selected from an imidazolium salt, a pyrrolidinium salt, or an aliphatic quaternary ammonium salt .
When an imidazolium salt or a pyrrolidinium salt is used as the room temperature molten salt, the room temperature molten salt easily reaches the decomposition potential on the negative electrode side containing the negative electrode active material made of a carbon material, and is decomposed to have battery characteristics. However, since the anionic compound represented by the general formula (1) can form a coating as described above, decomposition of the room temperature molten salt can be suppressed. That is, in this case, the anionic compound exhibits the effect of suppressing decomposition of the room temperature molten salt, and the battery can be operated more stably.
Further, when an aliphatic quaternary ammonium salt is used as the room temperature molten salt, the room temperature molten salt is hardly decomposed. Therefore, the charge / discharge capacity and output stability of the lithium ion secondary battery can be further improved.

Moreover, it is preferable that the said mixed solvent of the said organic solvent and the said normal temperature molten salt consists of 0-80 vol% of organic solvents, and 20-100 vol% of normal temperature molten salts.
When the organic solvent exceeds 80 vol% or the room temperature molten salt is less than 20 vol%, the flame retardancy may not be sufficiently exhibited.
Further, when the organic solvent is less than 20 vol% or the room temperature molten salt exceeds 80 vol%, the ionic conductivity of the non-aqueous electrolyte becomes insufficient, and the output may be lowered. Therefore, more preferably, the mixed solvent is composed of an organic solvent of 20 vol% to 80 vol% and an ambient temperature molten salt of 20 vol% to 80 vol%. In addition, when an imidazolium salt is used as the room temperature molten salt, the non-aqueous electrolyte exhibits sufficient ionic conductivity even in the absence of the organic solvent by taking advantage of its high ionic conductivity. The lithium ion secondary battery can exhibit a high output.

  Next, the lithium ion secondary battery includes the positive electrode and the negative electrode, a separator that is sandwiched between the positive electrode and the negative electrode, the non-aqueous electrolyte that moves lithium between the positive electrode and the negative electrode, and the like. Can be configured as main components.

  For example, a positive electrode active material is mixed with a conductive material and a binder, and an appropriate solvent is added to form a paste-like positive electrode mixture, which is applied to the surface of an aluminum current collector and dried. Accordingly, it can be formed by compression so as to increase the electrode density. Specifically, an aluminum foil or the like can be used as the current collector.

As the positive electrode active material, an oxide containing lithium and a transition metal element or a polyanionic compound can be used. Specifically, for example, there are lithium cobalt composite oxide, lithium nickel composite oxide, lithium manganese composite oxide, lithium iron composite phosphorus oxide, and the like.
When these positive electrode active materials are used, it is possible to increase the capacity of the lithium ion secondary battery and improve the thermal stability. That is, in this case, it is possible to further extend the life and improve the safety of the lithium ion secondary battery.

  Further, the conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or more carbon powder materials such as carbon black, acetylene black, natural graphite, artificial graphite, and cokes are used. Can be used.

The binder serves to bind the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. Etc. can be used. In addition, an aqueous dispersion of a cellulose-based or styrene-butadiene rubber that is an aqueous binder can also be used.
As a solvent for dispersing these active material, conductive material, and binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  Next, the negative electrode is prepared by mixing a binder with a negative electrode active material, applying a suitable solvent to paste a negative electrode mixture on the surface of a metal foil current collector such as copper, and then drying. It can be formed by pressing. Similarly to the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as a binder to be mixed with the negative electrode active material, and an organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent.

As the negative electrode active material, a carbon material capable of inserting and extracting lithium can be used.
In this case, the coating with the anionic compound is likely to be formed on the negative electrode side of the lithium ion secondary battery. Therefore, in this case, as described above, the effect of improving battery characteristics by the anion compound can be more remarkably exhibited.

  Examples of the carbon material include natural or artificial graphite, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber and a mixture thereof, vapor-grown carbonized fiber, organic compound fired body such as phenol resin, coke, Examples thereof include carbon black, pyrolytic carbons, and carbon fibers. These carbon materials can be used alone or in combination of two or more.

  The separator that is narrowly attached to the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte. For example, a thin microporous film such as polyethylene or polypropylene can be used.

Examples of the shape of the lithium ion secondary battery include a cylindrical shape, a stacked shape, a coin shape, and a square shape. As the battery case that accommodates the positive electrode, the negative electrode, the nonaqueous electrolytic solution, and the like, those corresponding to these shapes can be used.
In the lithium ion secondary battery, for example, an electrode body formed by sandwiching the separator between the positive electrode and the negative electrode is housed in a battery case having a predetermined shape, and the positive electrode current collector and the negative electrode current collector are accommodated. The body can be electrically connected to the positive electrode external terminal and the negative electrode external terminal via lead wires, and the electrode body is impregnated with the nonaqueous electrolyte solution, and the battery case is sealed.

Moreover, it is preferable to use a case made of an aluminum laminate film as the battery case. Further, it is preferable to use a current collector made of aluminum as the positive electrode current collector.
In this case, the lithium ion secondary battery can be reduced in size and weight. Furthermore, in this case, the above-described effect of suppressing the deterioration of battery characteristics by the anion compound can be obtained more remarkably.

That is, a battery case made of an aluminum laminate film and a current collector made of aluminum are easily damaged by HF generated by the electrolyte LiPF ₆ . As a result, gas may be generated in the battery and the battery case may swell, or the battery case may be damaged and the electrolyte may leak out.
The non-aqueous electrolyte contains the anionic compound, and HF can be captured by the anionic compound to suppress damage to the battery case and the current collector. Therefore, when the battery case is made of an aluminum laminate film or the current collector is made of aluminum, the effect of the present invention by the anion compound can be more remarkably exhibited.

Example 1
Next, a lithium ion secondary battery according to an embodiment of the present invention will be described with reference to FIG.
As shown in FIG. 1, the lithium ion secondary battery 1 of this example includes a positive electrode 2, a negative electrode 3, and a non-aqueous electrolyte. The positive electrode 2 contains LiNi _0.75 Co _0.15 Al _0.10 O ₂ having a layered structure as a positive electrode active material. The negative electrode 3 contains fibrous graphite as a negative electrode active material. The nonaqueous electrolytic solution is obtained by dissolving an electrolyte in a mixed solvent of an organic solvent and a room temperature molten salt. As the electrolyte, LiPF ₆ and an anion compound salt represented by the following formula (4) are used.

Hereinafter, the lithium ion secondary battery 1 of this example will be described in detail with reference to FIG.
As shown in FIG. 1, the lithium ion secondary battery 1 of this example includes a positive electrode 2, a negative electrode 3, a separator 4, a gasket 59, a battery case 6, and the like. The battery case 6 is a 18650-type cylindrical battery case, and includes a cap 63 and an outer can 65. In the battery case 6, a sheet-like positive electrode 2 and a negative electrode 3 are arranged in a wound state together with a separator 4 sandwiched between the positive electrode 2 and the negative electrode 3.
A gasket 59 is disposed inside the cap 63 of the battery case 6, and a non-aqueous electrolyte is injected into the battery case 6.

The positive electrode 2 contains LiNi _0.75 Co _0.15 Al _0.10 O ₂ as a positive electrode active material, and the negative electrode 3 contains fibrous graphite as a negative electrode active material.
The positive electrode 2 and the negative electrode 3 are respectively provided with a positive electrode current collecting lead 23 and a negative electrode current collecting lead 33 by welding. The positive electrode current collector lead 23 is connected to the positive electrode current collector tab 235 disposed on the cap 63 side by welding. Further, the negative electrode current collecting lead 33 is connected by welding to a negative electrode current collecting tab 335 disposed on the bottom of the outer can 65.

In addition, the non-aqueous electrolyte is LiPF in a mixed solvent of an organic solvent composed of ethylene carbonate (EC) and diethyl carbonate (DEC) and a room temperature molten salt composed of 1 ethyl 3-methylimidazolium bis (trifluoromethanesulfonyl) imide. ₆ and a lithium salt of an anionic compound represented by the above formula (4) (hereinafter referred to as LPFO as appropriate) are dissolved and injected into the battery case.

Next, the manufacturing method of the lithium ion secondary battery 1 of this example is demonstrated using FIG.
First, the nonaqueous electrolyte solution was prepared as follows.
That is, first, a mixed solvent prepared by mixing a room temperature molten salt (1 ethyl 3-methylimidazolium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 20:30:50 is prepared. did. That is, this mixed solvent is composed of 80% by volume of carbonate-based organic solvent (EC and DEC) and 20% by volume of room temperature molten salt. Further, as an electrolyte, and a LiPF ₆ and LPFO 95: was prepared mixed supporting salt were mixed in a molar ratio of 5.
This mixed support salt was dissolved in a non-aqueous solvent to a concentration of 1M to prepare a non-aqueous electrolyte. This is designated as Sample E1.

Next, the positive electrode 2 and the negative electrode 3 were produced as follows.
In preparing the positive electrode 2, first, LiNi _0.75 Co _0.15 Al _0.10 O ₂ is prepared as a positive electrode active material, and the positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride as a binder are mixed. Then, an appropriate amount of N-methyl-2-pyrrolidone as a dispersing agent was added and dispersed to prepare a slurry-like positive electrode mixture. The mixing ratio of the positive electrode active material, the conductive material, and the binder was, by weight ratio, positive electrode active material: conductive material: binder = 85: 10: 5.

Next, the positive electrode mixture obtained as described above was applied to both sides of an aluminum foil current collector having a thickness of 20 μm and dried. Then, it densified with the roll press, cut out in the shape of 52 mm width x 450 mm length, and produced the sheet-like positive electrode 2. The amount of positive electrode active material deposited was about 7 mg / cm ² per side.

  On the other hand, in producing the negative electrode 3, first, fibrous graphite as a negative electrode active material and polyvinylidene fluoride as a binder are mixed, and an appropriate amount of N-methyl-2-pyrrolidone is added and dispersed as a dispersing agent. Thus, a slurry-like negative electrode mixture was obtained. The mixing ratio of the negative electrode active material and the binder was, by weight, negative electrode active material: binder = 95: 5.

Next, the negative electrode mixture obtained as described above was applied to the surface of a copper foil current collector having a thickness of 10 μm and dried. Then, it densified with the roll press, cut out into the shape of 54 mm width x 500 mm length, and produced the sheet-like negative electrode 3. The adhesion amount of the negative electrode active material was about 5 mg / cm ² per side.

  Next, as shown in FIG. 1, a positive electrode current collecting lead 23 and a negative electrode current collecting lead 33 were welded to the sheet-like positive electrode 2 and negative electrode 3 obtained as described above, respectively. The positive electrode 2 and the negative electrode 3 were wound in a state where a polyethylene separator 4 having a width of 56 mm and a thickness of 25 μm was sandwiched between them, and a roll-shaped electrode body was produced.

  Subsequently, the roll-shaped electrode body was inserted into an 18650-type cylindrical battery case 6 including an outer can 65 and a cap 63. At this time, the positive electrode current collecting lead 23 is connected to the positive electrode current collecting tab 235 arranged on the cap 63 side of the battery case 6 by welding, and the negative electrode current collecting lead is arranged on the negative electrode current collecting tab 335 arranged on the bottom of the outer can 65. 33 was connected by welding.

  Next, the battery case 6 was impregnated with the nonaqueous electrolyte solution (sample E1) prepared as described above. A gasket 59 is disposed inside the cap 63, and the cap 63 is disposed in the opening of the outer can 65. Subsequently, the battery case 6 was sealed by caulking the cap 63, and the lithium ion secondary battery 1 was produced. This was designated as battery E1.

  In this example, eleven types of non-aqueous electrolytes (sample E2 to sample E9 and sample C1 to sample C3) that are different from the sample E1 in terms of the type of room temperature molten salt, the type of electrolyte, and the blending ratio are further prepared. Then, eleven types of lithium ion secondary batteries (battery E2 to battery E9 and battery C1 to battery C3) were produced using these nonaqueous electrolyte solutions.

Specifically, the battery E2 is a mixed solvent obtained by mixing a room temperature molten salt (1 ethyl 3-methylimidazolium PF ₆ ), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume ratio of 20:30:50. The above battery except that a non-aqueous electrolyte solution (sample E2) in which a mixed supporting salt obtained by mixing LiPF ₆ and LPFO at a molar ratio of 95: 5 was dissolved to a concentration of 1 M was used as the electrolyte. It was produced in the same manner as E1.
Further, the battery E3 has a room temperature molten salt (N-methyl-N′-propylpiperidinium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume of 20:30:50. a solvent mixture in a ratio, as an electrolyte, LiPF ₆ and the LPFO 95: in that the mixing supporting salt were mixed in a molar ratio of 5 with dissolved non-aqueous electrolyte at a concentration 1M (sample E3) The battery was manufactured in the same manner as the battery E1 except for the above.

Battery E4 is a mixed solvent in which a room temperature molten salt (1 ethyl 3-methylimidazolium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 20:30:50. The above battery except that a non-aqueous electrolyte solution (sample E4) in which a mixed supporting salt obtained by mixing LiPF ₆ and LPFO at a molar ratio of 90:10 was dissolved to a concentration of 1 M was used as the electrolyte. It was produced in the same manner as E1.
Further, the battery E5 is prepared by mixing a normal temperature molten salt (1 ethyl 3-methylimidazolium PF ₆ ), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume ratio of 20:30:50 as an electrolyte. The same as the battery E1 except that a non-aqueous electrolyte solution (sample E5) in which a mixed supporting salt obtained by mixing LiPF ₆ and LPFO at a molar ratio of 90:10 was dissolved to a concentration of 1M was used. It was made.

The battery E6 includes a room temperature molten salt (N-methyl-N′-propylpiperidinium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume ratio of 20:30:50. Except for using a non-aqueous electrolyte solution (sample E6) in which a mixed supporting salt prepared by mixing LiPF ₆ and LPFO at a molar ratio of 90:10 to a concentration of 1 M is used as an electrolyte in the mixed solvent mixture. In the same manner as the battery E1.

Battery E7 is a mixed solvent in which a room temperature molten salt (1 ethyl 3-methylimidazolium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 20:30:50. As an electrolyte, a mixed supporting salt prepared by mixing LiPF ₆ and a lithium salt of an anionic compound represented by the following formula (2) (hereinafter referred to as LBFO as appropriate) at a molar ratio of 90:10 is dissolved to a concentration of 1M. A battery was prepared in the same manner as the battery E1 except that the nonaqueous electrolyte solution (sample E7) was used.

The battery E8 was prepared by mixing LiPF as an electrolyte in a mixed solvent in which a room temperature molten salt (1 ethyl 3-methylimidazolium PF ₆ ), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed at a volume ratio of 20:30:50. ₆ except that a non-aqueous electrolyte solution (sample E8) in which a mixed supporting salt obtained by mixing ₆ and LBFO at a molar ratio of 90:10 was dissolved to a concentration of 1 M was used. Produced.
Further, the battery E9 has a room temperature molten salt (N-methyl-N′-propylpiperidinium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume of 20:30:50. a solvent mixture in a ratio, that as the electrolyte, LiPF ₆ was used and LBFO and a nonaqueous electrolytic solution in which the mixed supporting salt were mixed in a molar ratio of 90:10 was dissolved at a concentration 1M (sample E9) The battery was manufactured in the same manner as the battery E1 except for the above.

Battery C1 is a mixed solvent in which a room temperature molten salt (1 ethyl 3-methylimidazolium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed at a volume ratio of 20:30:50. The battery was prepared in the same manner as the battery E1 except that a non-aqueous electrolyte (sample C1) in which only LiPF ₆ was dissolved was used as the electrolyte.
In addition, the battery C2 is prepared as an electrolyte in a mixed solvent obtained by mixing a room temperature molten salt (1 ethyl 3-methylimidazolium PF ₆ ), ethylene carbonate (EC), and diethyl carbonate (DEC) at a volume ratio of 20:30:50. This was prepared in the same manner as the battery E1 except that a non-aqueous electrolyte solution (sample C2) in which only LiPF ₆ was dissolved was used.

In addition, the battery C3 includes a room temperature molten salt (N-methyl-N′-propylpiperidinium bis (trifluoromethanesulfonyl) imide), ethylene carbonate (EC), and diethyl carbonate (DEC) in a volume of 20:30:50. The battery was prepared in the same manner as the battery E1 except that a non-aqueous electrolyte solution (sample C3) in which only LiPF ₆ was dissolved was used as the electrolyte in the mixed solvent at a ratio.
About each non-aqueous electrolyte (sample E1-sample E9 and sample C1-sample C3) used for preparation of each battery (battery E1-battery E9 and battery C1-battery C3), kind of normal temperature molten salt and organic solvent, electrolyte Table 1 shows the types and blending ratios.

(Experimental example 1)
Next, the battery characteristics of 12 types of lithium ion secondary batteries (battery E1 to battery E9 and battery C1 to battery C3) produced as described above were evaluated. Specifically, as the battery characteristics, the capacity maintenance rate, initial IV resistance (initial internal resistance), and IV resistance increase rate (internal resistance increase rate) of each battery were examined. Moreover, the flame retardance of the electrolyte solution of each battery was evaluated.

"Initial IV resistance"
Each battery (battery E1-battery E9 and battery C1-battery C3) is adjusted to a battery capacity of 50% (SOC = 50%), and a current of 0.5A, 1A, 2A, 3A, 5A is applied. The battery voltage after 2 seconds was measured. The current and voltage passed through each battery were linearly approximated, and the initial resistance was determined from the slope. The initial resistance value of each battery was calculated as a relative value when the value of the battery C1 was 1. That is, the result of the initial resistance of each battery was expressed as a value normalized with respect to the value of the battery C1. The results are shown in Table 2.

"Charge / discharge cycle test"
The battery E1 to the battery E9 and the batteries C1 to C3 are charged at a constant current of a current density of 2.0 mA / cm ² under a temperature condition of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the battery. Charging / discharging which was charged to ² V and then discharged to a discharge lower limit voltage of 3 V at a constant current of 2.0 mA / cm ² was defined as one cycle, and this cycle was performed for a total of 100 cycles.

"Capacity maintenance rate"
Before and after the charge / discharge cycle test, when the discharge capacity before the charge / discharge cycle test was discharge capacity A and the discharge capacity after the charge / discharge cycle test was discharge capacity B, the capacity retention rate was calculated by the following equation (a). The results are shown in Table 2.
Capacity maintenance ratio (%) = discharge capacity B / discharge capacity A × 100 (a)

"IV resistance increase rate"
Each battery was adjusted to 50% of the battery capacity (SOC = 50%), a current of 0.5A, 1A, 2A, 3A, and 5A was applied to measure the battery voltage after 10 seconds. The applied current and voltage were linearly approximated, and IV resistance was obtained from the slope.
The IV resistance increase rate can be calculated by the following equation (b), where IV resistance after the charge / discharge cycle test is resistance Y and IV resistance before the charge / discharge cycle test is resistance X. The results are shown in Table 2.
IV resistance increase rate (%) = (resistance Y−resistance X) × 100 / resistance X (b)

"Flame retardance test"
Twelve types of nonaqueous electrolytes (sample E1 to sample E9 and sample C1 to sample C3) used for each battery (battery E1 to battery E9 and batteries C1 to C3) were prepared, and separators were provided for each nonaqueous electrolyte. Twelve types of separators were prepared by immersing (thickness 25 μm, width 6 mm, length 120 mm) and impregnating each non-aqueous electrolyte. Next, the upper end of each separator was sandwiched between tweezers and hung in the atmosphere, and a match fire was brought closer to the lower end. At this time, the presence or absence of ignition and the combustion state of the separator when ignited were visually observed. The case where ignition was not performed was evaluated as “◯”, and the case where ignition was performed and most of the separator burned out was evaluated as “X”. The results are shown in Table 2.

As is known from Table 2, all of the non-aqueous electrolytes (sample E1 to sample E9 and sample C1 to sample C3) containing the room temperature molten salt were not ignited in the flame retardancy test, and were excellent in difficulty. Showed flammability.
Further, when comparing the battery E1, the battery E4, the battery E7, and the battery C1 containing 1 ethyl 3 methylimidazolium bis (trifluoromethanesulfonyl) imide as a room temperature molten salt, the battery E1 and the battery E4 containing an anionic compound as an electrolyte are compared. It can be seen that the capacity retention rate and the initial IV resistance of the battery E7 are almost the same as the battery C1, but the increase rate of the IV resistance is greatly reduced.

Similarly, cell E2 containing 1-ethyl-3-methyl-imidazolium PF ₆ as ambient temperature molten salt, battery E5, a comparison of cell E8, and a battery C2, cell E2 containing an anionic compound as the electrolyte, batteries E5 and, It can be seen that the battery E8 has almost the same capacity maintenance rate and initial IV resistance as the battery C2, but the IV resistance increase rate is greatly reduced.

  Further, when comparing battery E3, battery E6, battery E9, and battery C3 containing N-methyl-N′-propylpiperidinium bis (trifluoromethanesulfonyl) imide as a room temperature molten salt, an anionic compound is contained as an electrolyte. It can be seen that the battery E3, the battery E6, and the battery E9 have almost the same capacity maintenance rate and initial IV resistance as the battery C3, but the IV resistance increase rate is greatly reduced.

  Thus, the lithium ion secondary battery (battery E1 to battery E9) containing a room temperature molten salt and an anion compound in the electrolyte solution can exhibit excellent flame retardancy, and can be repeatedly charged and discharged. An increase in internal resistance can be suppressed, and a high output can be exhibited over a long period of time.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the lithium ion secondary battery concerning Example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Positive electrode 3 Negative electrode 4 Separator 6 Battery case

Claims (5)

A positive electrode containing an oxide containing lithium and a transition metal element or a polyanionic compound as a positive electrode active material, a negative electrode containing a carbon material capable of occluding and desorbing lithium as a negative electrode active material, an organic solvent and room temperature In a lithium ion secondary battery having a nonaqueous electrolytic solution obtained by dissolving an electrolyte in a mixed solvent with a molten salt,
The room temperature molten salt is composed of one or more selected from an imidazolium salt, a pyrrolidinium salt, or an aliphatic quaternary ammonium salt,
The mixed solvent consists of the organic solvent 20-80 vol% and the room temperature molten salt 80-20 vol%,
As the electrolyte, at least LiPF ₆ and a salt of an anion compound represented by the following formulas (2) to (5) are used .

2. The lithium ion secondary battery according to claim 1, wherein the cation paired with the anion compound in the salt of the anion compound is at least one of Li ^{+ and} Na ⁺ . 3. The lithium ion secondary battery according to claim 1 , wherein a compound represented by the above formula (4) is used as the anion compound. In any one of claims 1 to 3, as the electrolyte, further _{LiClO 4, LiBF 4, LiAsF 6} , and lithium-ion secondary battery, which comprises using one or more selected from LiSbF _6. 5. The lithium ion secondary battery according to claim 1, wherein an addition amount of the salt of the anion compound to the mixed solvent is 1 mol% to 50 mol% in the total electrolytic mass.

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