JP5846031B2 - Lithium ion secondary battery and non-aqueous electrolyte - Google Patents

Description

  The present invention relates to a lithium ion secondary battery and a non-aqueous electrolyte.

  Lithium ion secondary batteries are lighter and have higher capacity than nickel cadmium batteries and nickel metal hydride batteries, and thus are widely applied as power sources for portable electronic devices. It is also a promising candidate as a power source for hybrid vehicles and electric vehicles. However, with the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries serving as these power sources. Lithium ion secondary batteries are mainly composed of a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, and various studies for further improvement of the battery characteristics have been made.

  For example, the non-aqueous solvent of the non-aqueous electrolyte has a relatively low melting point, a relatively high conductivity, a relatively wide potential window (electrochemical window), and high ions even at low temperatures when the electrolyte is dissolved. Those capable of obtaining conductivity are preferred. From this viewpoint, propylene carbonate is preferably used. However, when a negative electrode (anode) using a carbon material such as highly crystallized graphite as a constituent material is provided, the propylene carbonate at the cathode (referred to as an electrode functioning as a negative electrode during discharge) particularly during charging. There was a problem that decomposition progressed.

  As the decomposition of propylene carbonate proceeds, gas is generated, and the carbon material of the negative electrode is peeled off, decomposed, and the like, and the battery characteristics such as capacity and charge / discharge cycle characteristics gradually deteriorate during use. Further, as the decomposition of propylene carbonate proceeds, decomposition products are deposited on the negative electrode, and it is considered that the above-described deterioration in battery characteristics further proceeds due to the influence of this deposit.

  Therefore, by adding 1,3-propane sultone or 1,4-butane sultone to a non-aqueous electrolyte containing propylene carbonate as a non-aqueous solvent, it is possible to suppress the progress of the above-described decomposition reaction of propylene carbonate. Intended batteries have been proposed (see, for example, Patent Document 1 and Patent Document 2).

JP 2000-3724 A JP 2000-3725 A

  However, in the conventional lithium ion secondary batteries described in Patent Documents 1 and 2 described above, when a carbon material is used as the negative electrode, suppressing decomposition of propylene carbonate is sufficient as a lithium ion secondary battery. While an initial discharge capacity can be obtained, a sufficient capacity cannot be obtained at the time of discharge at a low temperature, which has recently been required particularly in temperature characteristics.

  The present invention has been made in view of the above problems, and provides a lithium ion secondary battery and a non-aqueous electrolyte that have an excellent initial discharge capacity and can obtain a sufficient discharge capacity even at a low temperature of −10 ° C. For the purpose.

［式（１）中、Ｒ_１、Ｒ_２、Ｒ_３、Ｒ_４はそれぞれ独立で、水素、ハロゲン、炭素数１〜６のアルキル基、または側鎖にハロゲンを有する炭素数１〜６のアルキル基である。］ A lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode including a carbon material, and a nonaqueous electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent. The nonaqueous solvent includes a cyclic carbonate and a chain carbonate. And a compound represented by the following formula (1), wherein the compound is contained in an amount of 0.1 to 7% by mass with respect to the non-aqueous electrolyte.

[In the formula (1), R ₁ , R ₂ , R ₃ and R ₄ are each independently hydrogen, halogen, an alkyl group having 1 to 6 carbon atoms, or an alkyl having 1 to 6 carbon atoms having halogen in the side chain. It is a group. ]

  The compound represented by the formula (1) contributes to film formation at the interface between the negative electrode of the lithium ion secondary battery and the non-aqueous electrolyte, and suppresses side effects such as decomposition of the non-aqueous solvent. And it is assumed that a sufficient discharge capacity at −10 ° C. can be realized.

In the lithium ion secondary battery of the present invention, R ₁ , R ₂ , R ₃ and R ₄ of the compound are preferably F.

In the lithium ion secondary battery of the present invention, it is preferable that R _{1 of} the compound is CF ₃ and R ₂ , R ₃ and R ₄ are F.

  ADVANTAGE OF THE INVENTION According to this invention, while having the outstanding initial discharge capacity, the lithium ion secondary battery and non-aqueous electrolyte which can obtain sufficient discharge capacity also at low temperature of -10 degreeC can be provided.

It is a schematic diagram of a lithium ion secondary battery.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. However, the non-aqueous electrolyte and the lithium ion secondary battery using the same according to the present invention are not limited to the following embodiments. In addition, the dimensional ratio of drawing is not restricted to the ratio of illustration.

(Lithium ion secondary battery)
FIG. 1 schematically shows a lithium ion secondary battery 10 of the present embodiment. The lithium ion secondary battery 10 in FIG. 1 is between the positive electrode 20 and the negative electrode 30 including materials (positive electrode active material, negative electrode active material) that occlude / release lithium ions, and between the positive electrode 20 and the negative electrode 30 and is related to the present invention. It is comprised from the separator 40 with which electrolyte solution was hold | maintained.

(Negative electrode)
The negative electrode 30 includes a negative electrode active material layer 31 on both sides of a negative electrode current collector 32. Further, the negative electrode active material layer 31 is formed by applying a paint containing a negative electrode active material, a conductive additive, and a binder to the negative electrode current collector.

The negative electrode active material includes at least one selected from carbon materials such as natural graphite and artificial graphite (non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, etc.).
A carbon material having an interlayer distance d002 of 0.335 to 0.338 nm and a carbon material crystallite size Lc002 of 30 to 120 nm is preferable because of its large capacity as a negative electrode and good cycle characteristics. . Examples of the carbon material satisfying such conditions include artificial graphite and MCF (mesocarbon fiber). The interlayer distance d002 and the crystallite size Lc002 can be obtained by an X-ray diffraction method.
In addition, for example, a metal capable of forming a compound with lithium such as Al, Si and Sn, an amorphous compound mainly composed of an oxide such as SiO ₂ and SnO ₂ , lithium titanate (Li ₄ Ti ₅ O _A known negative electrode active material such as ₁₂ ) may be mixed with a carbon material.

  The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. In addition, negative electrode active material such as non-graphitizable carbon, graphitizable carbon, and graphite may be used in different shapes.

  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 31 is facilitated by the pores. This is preferable.

  The binder is not particularly limited as long as it can bind the particles of the negative electrode active material and the particles of the conductive additive. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoro Examples thereof include fluorine resins such as ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF). Further, this binder contributes not only to the binding of the particles of the negative electrode active material and the particles of the conductive additive, but also to the binding to the negative electrode current collector 32.

  As the negative electrode current collector 32, various known metal foils used in current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use a copper foil.

(Positive electrode)
The positive electrode 20 includes a positive electrode active material layer 21 on both surfaces of a positive electrode current collector 22. Further, the positive electrode active material layer 21 is formed by applying a coating material including a positive electrode active material, a conductive additive, and a binder to the positive electrode current collector 22.

The positive electrode active material is composed of lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a lithium ion and a counter anion of the lithium ion (for example, ClO ⁴⁻ ). Is not particularly limited as long as it can be reversibly advanced, and a known positive electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) Composite metal oxides, lithium vanadium compounds (LiV ₂ O _5, LiVPO ₄ , LiV ₂ (PO ₄ ) ₃ , LiVOPO ₄ ), olivine-type LiMPO ₄ (where M represents Co, Ni, Mn, Fe or V) ) And composite metal oxides such as lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

  Furthermore, the same material as that used in the negative electrode 30 can be used for each constituent element (conductive aid and binder) other than the positive electrode active material. Therefore, the binder contained in the positive electrode 20 contributes not only to the binding of the particles of the positive electrode active material and the particles of the conductive additive, but also to the binding to the positive electrode current collector 22. Yes.

  As the positive electrode current collector 22, various known metal foils used for current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use an aluminum foil.

(Separator)
As long as the separator 40 is formed of an insulating porous material, the material, the manufacturing method, and the like are not particularly limited, and a separator used in a known lithium ion secondary battery can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

［式（１）中、Ｒ_１、Ｒ_２、Ｒ_３、Ｒ_４はそれぞれ独立で、水素、ハロゲン、炭素数１〜６のアルキル基、または側鎖にハロゲンを有する炭素数１〜６のアルキル基である。］ (Nonaqueous electrolyte)
In the nonaqueous electrolytic solution, an electrolyte is dissolved in a nonaqueous solvent. As a non-aqueous solvent, a cyclic carbonate, a chain carbonate, and a compound represented by the following formula (1) are included, and this compound is included in an amount of 0.1 to 7% by mass with respect to the non-aqueous electrolyte. .

[In the formula (1), R ₁ , R ₂ , R ₃ and R ₄ are each independently hydrogen, halogen, an alkyl group having 1 to 6 carbon atoms, or an alkyl having 1 to 6 carbon atoms having halogen in the side chain. It is a group. ]

  The compound represented by Formula (1) contributes to film formation on the surface of the negative electrode having a carbon material, and is considered to have an effect of suppressing unnecessary polymerization and decomposition of the compound constituting the nonaqueous solvent. It is also considered that there is an effect of lowering the melting point of the non-aqueous electrolyte and suppressing an increase in the viscosity of the non-aqueous electrolyte at low temperatures.

  When the content of the compound represented by the formula (1) is 0.1% by mass or more, a strong film can be formed, and the capacity can be maintained particularly at high temperature storage. Further, by suppressing the content of the compound represented by the formula (1) within 7% by mass, an increase in impedance can be suppressed, and a sufficient battery capacity can be obtained.

Examples of the compound represented by the formula (1) are as follows. Table 1 lists the combinations of R ₁ , R ₂ , R ₃ and R ₄ .

Specifically, preferred examples include compounds in which R ₁ , R ₂ , R ₃ , and R ₄ are all F. This compound has a low melting point of −30 ° C. or less, and its viscosity does not increase even at low temperatures. Therefore, the lithium ion secondary battery containing this compound can secure the ion conductivity of the non-aqueous electrolyte to some extent even at a low temperature, and can exhibit a sufficient discharge capacity. When the total amount of the non-aqueous electrolyte is 100% by mass, the content is preferably 0.5 to 5% by mass, and particularly preferably 1 to 3% by mass.

Another preferred example is a compound in which R ₁ is CF ₃ and R ₂ , R ₃ and R ₄ are F. Since this compound also has a low melting point and little increase in viscosity at low temperatures, it can exhibit properties even at low temperatures. When the total amount of the non-aqueous electrolyte is 100% by mass, the content is preferably 0.1 to 3% by mass, and particularly preferably 0.5 to 2% by mass.

  Examples of the cyclic carbonate in the non-aqueous solvent include ethylene carbonate, propylene carbonate, and butylene carbonate. Among them, it is preferable to contain ethylene carbonate, and it may be used by mixing with propylene carbonate or butylene carbonate.

  Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, and it is preferable to include one or more selected from these. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like may be mixed and used.

  The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 3: 7 to 1: 1 in terms of volume.

In particular, the electrolyte preferably contains LiPF ₆ from the viewpoint of conductivity. LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ A plurality of electrolytes such as LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) and LiN (CF ₃ CF ₂ CO) ₂ may be mixed and used.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the solute in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the conductivity of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. In addition, by suppressing the electrolyte concentration to within 2.0 mol / L, the viscosity of the non-aqueous electrolyte does not become excessively large and the mobility of lithium ions can be sufficiently secured. Capacity becomes easy to be obtained.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

  The non-aqueous electrolyte of the present invention is suitably used as a constituent member of a secondary battery, particularly as a constituent member of a lithium ion secondary battery. The constituent members other than the non-aqueous electrolyte constituting the lithium ion secondary battery are not particularly limited, and various conventionally used constituent members can be used.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

(Example 1)
Examples 1 to 14 and Comparative Examples 1 to 4 lithium ion secondary batteries were prepared according to the following procedure.
First, a negative electrode was produced. In the production of the negative electrode, artificial graphite (90% by mass) as a negative electrode active material, carbon black (2% by mass) as a conductive additive, and polyvinylidene fluoride (PVDF) (8% by mass) as a binder are mixed. A slurry was obtained by dispersing in N-methyl-2-pyrrolidone (NMP) as a solvent. The obtained slurry was applied to an electrolytic copper foil as a current collector by a doctor blade method and dried at 110 ° C. After drying, rolling was performed to obtain a negative electrode.

Next, a positive electrode was produced. Also in the production of the positive electrode, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (90% by mass) as the positive electrode active material, carbon black (6% by mass) as the conductive additive, and PVDF (4 as the binder) (Mass%) was mixed and dispersed in NMP to obtain a slurry. The obtained slurry was applied to an aluminum foil as a current collector, dried, rolled, and a positive electrode was obtained.

Next, a non-aqueous electrolyte was prepared. While mixing ethylene carbonate and diethyl carbonate as a non-aqueous solvent at a volume ratio of 3: 7, LiPF ₆ was added as an electrolyte at a rate of 1.0 mol / L. Furthermore, the compound (hereinafter abbreviated as T1) in which R ₁ , R ₂ , R ₃ , and R ₄ are F in Formula (1) is 0.1% by mass with respect to the non-aqueous electrolyte. This was added to obtain a non-aqueous electrolyte.

  A laminated body (element body) was obtained by laminating a separator made of polyethylene between the obtained negative electrode and positive electrode. The obtained laminate is placed in an aluminum laminate pack, a non-aqueous electrolyte is injected into the aluminum laminate pack, and then vacuum-sealed to produce a lithium ion secondary battery (length: 60 mm, width: 85 mm, thickness: 3 mm). did. In addition, on the film of the aluminum laminate pack, the innermost layer (layer made of modified polypropylene) made of synthetic resin that comes into contact with the non-aqueous electrolyte, the metal layer made of aluminum foil, and the layer made of polyamide are sequentially laminated in this order. The laminated body used was used. And two sheets of this composite packaging film were piled up, and the edge part was produced by thermocompression bonding.

(Examples 2-14 and Comparative Examples 1-4)
Examples 2 to 14 and Comparative Examples 1 to 1 were the same as Example 1 except that the combination and salt concentration of the nonaqueous solvent, the type and amount of the compound added to the nonaqueous electrolyte were changed as shown in Table 2. No. 4 lithium ion secondary battery was produced. In Table 2, “EC” represents ethylene carbonate, “DEC” represents diethyl carbonate, “EMC” represents ethyl methyl carbonate, “PC” represents propylene carbonate, T2 represents Formula (1) and R ₁ represents CF ₃ is a compound in which R ₂ , R ₃ and R ₄ are represented by F.

  Each battery characteristic was evaluated using the obtained lithium ion secondary battery of Examples 1-14 and Comparative Examples 1-3.

(Initial discharge capacity evaluation test)
After the production of the lithium ion secondary battery, the battery was charged in an environment set at 25 ° C. in a thermostatic chamber, and discharged immediately thereafter. The charging was performed at a constant current and constant voltage up to 4.2 V at 30 mA, and the discharging was performed at a constant current of 2.5 mA at 30 mA. The discharge capacity (mAh) of the entire lithium ion secondary battery at this time was defined as the initial discharge capacity, and the results are shown in Table 2. The lithium ion secondary batteries shown in the examples all show a large discharge capacity at 25 ° C. compared to the case where T1 or T2 is not added, and it is confirmed that the discharge capacity is practically sufficient. did it.

(Discharge temperature characteristic evaluation test)
After the initial discharge capacity evaluation test, the temperature of the thermostatic chamber containing the lithium ion secondary battery was set to −10 ° C., and after waiting for 2 hours, charging and discharging were performed at that temperature. The charging was performed at a constant current and constant voltage up to 4.2 V at 30 mA, and the discharging was performed at a constant current of 2.5 mA at 30 mA. Table 2 shows the capacity retention ratio obtained by dividing the discharge capacity at −10 ° C. by the discharge capacity at 25 ° C. An increase in discharge capacity at −10 ° C. was confirmed by using a nonaqueous electrolytic solution containing 0.1 to 7 mass% of T1 or T2.

10 lithium ion secondary battery, 20 positive electrode, 21 positive electrode active material layer, 22 positive electrode current collector, 30 negative electrode, 31 negative electrode active material layer, 32 negative electrode current collector, 40 separator

Claims (4)

A positive electrode;
A negative electrode containing a carbon material;
A non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, and
The non-aqueous solvent includes a cyclic carbonate, a chain carbonate, and a compound represented by the following formula (1),
In the compound, R ₁ , R ₂ , R ₃ and R ₄ are F, or R ₁ is CF ₃ , and R ₂ , R ₃ and R ₄ are F;
A lithium ion secondary battery comprising 0.1 to 7% by mass of the compound with respect to the non-aqueous electrolyte.

[In the formula (1), R ₁ , R ₂ , R ₃ and R ₄ are each independently hydrogen, halogen, an alkyl group having 1 to 6 carbon atoms, or an alkyl having 1 to 6 carbon atoms having halogen in the side chain. It is a group. ] The cyclic carbonate includes ethylene carbonate,
The chain carbonate includes at least one carbonate selected from diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate,
The lithium ion secondary battery according to claim 1, wherein the electrolyte includes LiPF ₆ . A non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent,
A cyclic carbonate, a chain carbonate, and a compound represented by the following formula (1),
In the compound, R ₁ , R ₂ , R ₃ and R ₄ are F, or R ₁ is CF ₃ , and R ₂ , R ₃ and R ₄ are F;
A nonaqueous electrolytic solution comprising 0.1 to 7% by mass of the compound.

[In the formula (1), R ₁ , R ₂ , R ₃ and R ₄ are each independently hydrogen, halogen, an alkyl group having 1 to 6 carbon atoms, or an alkyl having 1 to 6 carbon atoms having halogen in the side chain. It is a group. ] The cyclic carbonate includes ethylene carbonate,
The chain carbonate includes at least one carbonate selected from diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate,
The non-aqueous electrolyte according to claim 3, wherein the electrolyte contains LiPF ₆ .

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