JP5502518B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, lithium ion secondary batteries have been used as power sources for portable devices such as mobile phones and laptop computers, and are expected to be used as power storage, large batteries for hybrids and electric vehicles. In particular, regarding automobile power supplies, safety against ignition and battery life are more important requirements than ever.

  Conventionally, many attempts have been made to make the electrolyte solution flame retardant in order to improve the safety of the lithium ion secondary battery. As one of the attempts, the addition of a flame retardant additive to the electrolyte solution has been studied. I came.

  For example, Patent Document 1 proposes a technique of adding a phosphate ester having a fluorine-substituted alkyl group to an electrolytic solution, and Patent Document 2 proposes a technique of adding trimethyl phosphite or triethyl phosphite. ing. Patent Document 3 discloses a technique of adding a fluorine-substituted alkyl group-substituted phosphite to an electrolytic solution, and Patent Document 4 adds an at least partially fluorinated phosphite. Technology is disclosed.

On the other hand, the electrode greatly affects the safety of the lithium ion secondary battery, and many attempts have been made to improve the electrode. For example, in Patent Document 5, an oxide positive electrode such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ is generally used as a positive electrode active material of a lithium ion secondary battery. It is disclosed that there is a risk of causing thermal runaway due to release of oxygen when heated to a high temperature, and on the other hand, if a positive electrode material having an olivine structure is used, oxygen release does not occur even at high temperatures and it is disclosed that it is excellent in safety. . Further, Patent Document 6 describes that a material obtained by substituting a part of iron atoms of olivine-type lithium iron phosphate with a different metal is used as a positive electrode material, thereby improving electrode performance.

JP 2007-258067 A JP-A-8-321313 JP 2002-25609 A US Pat. No. 6,939,647 JP 2009-134970 A JP 2001-85010 A

  Patent Document 3 describes the effect of improving the life (cycle characteristics) of the lithium ion secondary battery, and Patent Document 4 describes that the flame retardancy of the electrolytic solution is improved. However, when the present inventors examined prior arts including Patent Documents 3 and 4 and Patent Documents 1 and 2, those inventions are safety represented by the flame retardancy of the electrolyte. It has become clear that the improvement of the cycle characteristics is desired, in particular, that the balance between the life and the life is still insufficient. Moreover, since thermal runaway in a lithium ion secondary battery does not necessarily occur only by reaction at the positive electrode, improvement of only the positive electrode as described in Patent Documents 5 and 6 is still insufficient to establish safety. It is necessary to improve the safety of the electrolyte at the same time. As is clear from the above explanation, the conventional techniques including the above-mentioned patent documents are still insufficient from the viewpoint of compatibility between the safety represented by the flame retardancy of the electrolytic solution and the cycle characteristics, and further safety. Improvements in performance and cycle characteristics are desired.

  The present invention has been made in view of the above circumstances, and aims to provide a non-aqueous electrolyte for a lithium ion secondary battery and a lithium ion secondary battery in which both safety and cycle characteristics are improved. To do.

  As a result of studying the present inventors to achieve the above-mentioned object, as a result of using olivine type lithium iron phosphate as a positive electrode and using an electrolytic solution to which a specific phosphite is added as an electrolytic solution, The present inventors have found that it is possible to design a battery excellent in flame retardancy that contributes to the property and cycle characteristics, and have completed the present invention.

That is, the present invention is as follows.
[1] A positive electrode containing olivine type lithium iron phosphate, a negative electrode made of a material capable of occluding and releasing lithium ions, a non-aqueous solvent, a lithium salt dissolved in the non-aqueous solvent, phosphorous acid A lithium ion secondary battery comprising: an electrolytic solution containing 5 to 50% by mass of tri (trifluoroethyl) acid .
[ 2] The lithium ion secondary battery according to [1], containing 5 to 30% by mass of the tri (trifluoroethyl) phosphite .

  According to the present invention, it is possible to provide a non-aqueous electrolyte for a lithium ion secondary battery and a lithium ion secondary battery in which both safety and cycle characteristics are improved.

  Hereinafter, a mode for carrying out the present invention (hereinafter abbreviated as “this embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist. First, the lithium ion secondary battery of this embodiment will be described in detail.

  The lithium ion secondary battery of this embodiment includes a non-aqueous electrolyte for a lithium ion secondary battery (hereinafter also simply referred to as “electrolyte”), a positive electrode, a negative electrode, and preferably a separator. More specifically, the lithium ion secondary battery of the present embodiment includes a positive electrode containing olivine type lithium iron phosphate, a negative electrode capable of inserting and extracting lithium ions, and a general formula (I) described later. It is a lithium ion secondary battery provided with the electrolyte solution containing the phosphite ester represented.

The positive electrode used for the lithium ion secondary battery of this embodiment contains at least olivine type lithium iron phosphate as a positive electrode active material. The olivine type lithium iron phosphate is represented by the following general formula (3).
Li _z Fe _1-y X _y PO ₄ (3)
Here, in formula (3), X represents a metal element, preferably a metal element selected from the group consisting of Mn, Co, Cr, Ti, Cu, Ni, Zn, and Al. Further, y represents a number of 0 or more and less than 1, preferably 0 or more and less than 0.5, and more preferably 0 or more and less than 0.3. z is a number greater than 0 and less than or equal to 1. A preferred olivine-type lithium iron phosphate is Li _z FePO ₄ . Further, olivine type lithium iron phosphate supporting carbon by various known methods is also preferably used.

  The number average particle size of the positive electrode active material is preferably 0.01 μm to 100 μm, more preferably 0.1 μm to 10 μm. In the present specification, the “number average particle diameter” refers to the number average particle diameter of the aggregate when the particles are aggregated. The number average particle size of the positive electrode active material can be measured by a wet particle size measuring device (for example, a laser diffraction / scattering particle size distribution meter, a dynamic light scattering particle size distribution meter). Alternatively, 100 particles observed with a transmission electron microscope are randomly extracted and analyzed with image analysis software (for example, image analysis software manufactured by Asahi Kasei Engineering Co., Ltd., trade name “A Image-kun”). It can also be obtained by calculating an average. In this case, when the number average particle diameter differs between measurement methods for the same sample, a calibration curve created for the standard sample may be used.

  The positive electrode is obtained, for example, as follows. That is, first, a positive electrode mixture-containing paste is prepared by dispersing, in a solvent, a positive electrode mixture obtained by adding a conductive additive, a binder, or the like to the positive electrode active material as necessary. Next, this positive electrode mixture-containing paste is applied to a positive electrode current collector and dried to form a positive electrode mixture layer, which is pressurized as necessary to adjust the thickness, thereby producing a positive electrode.

  Here, the solid content concentration in the positive electrode mixture-containing paste is preferably 30 to 80% by mass, and more preferably 40 to 70% by mass.

  The positive electrode current collector is made of, for example, a metal foil such as an aluminum foil, a nickel foil, or a stainless steel foil.

  Examples of the conductive aid used as necessary in the production of the positive electrode include graphite, carbon black such as acetylene black and ketjen black, and carbon fiber. The number average particle diameter of the conductive assistant is preferably 0.1 μm to 100 μm, more preferably 1 μm to 10 μm, and is measured in the same manner as the number average particle diameter of the positive electrode active material. Examples of the binder include PVDF, PTFE, polyacrylic acid, styrene butadiene rubber, and fluorine rubber.

  The non-aqueous electrolyte for a lithium ion secondary battery contains a non-aqueous solvent, a lithium salt dissolved in the non-aqueous solvent, and 5 to 50% by mass of a phosphite represented by the following general formula (I) To do.

ここで、式（Ｉ）中、Ｒ¹、Ｒ²及びＲ³（「Ｒ¹〜Ｒ³」と表記する。以下同様。）は、互いに同一でも異なっていてもよく、フッ素原子で置換されていてもよい炭素数１〜４の直鎖状又は分岐状のアルキル基を示す。そのようなフッ素原子で置換されていてもよいアルキル基としては、例えば、メチル基、エチル基、ｎ−プロピル基、イソプロピル基、ｎ−ブチル基、イソブチル基、ｔｅｒｔ−ブチル基、トリフルオロメチル基及びトリフルオロエチル基が挙げられる。

Here, in formula (I), R ¹ , R ² and R ³ (represented as “R ^{1 to} R ³ ”, the same shall apply hereinafter) may be the same as or different from each other and are substituted with a fluorine atom. The C1-C4 linear or branched alkyl group which may be shown. Examples of the alkyl group optionally substituted with a fluorine atom include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a tert-butyl group, and a trifluoromethyl group. And a trifluoroethyl group.

Specific examples of the phosphite include trimethyl phosphite, triethyl phosphite, tripropyl phosphite, triisopropyl phosphite, and tributyl phosphite. Moreover, even if at least one of R ^{1 to} R ^{3 in} the general formula (I) is an alkyl group substituted with a fluorine atom, it is preferably used for a non-aqueous electrolyte for a lithium ion secondary battery. Can do. Specific examples of such phosphite esters include tri (trifluoromethyl) phosphite, tri (trifluoroethyl) phosphite, and tri (tetrafluoropropyl) phosphite. From the viewpoint of improving the flame retardancy of the electrolyte and improving the cycle characteristics of the battery, any of R ^{1 to} R ^{3 in} the general formula (I) is preferably an alkyl group substituted with a fluorine atom. In particular, the phosphite is preferably tri (2,2,2-trifluoroethyl) phosphite.

  The above phosphites are used alone or in combination of two or more.

  The proportion of the phosphite ester in the electrolyte can be appropriately selected depending on the balance between the required flame retardancy and cycle characteristics, but is 5 to 50% by mass from the viewpoint of combining high flame retardancy and cycle characteristics. 10 to 40% by mass is preferable. In addition, the proportion of the phosphite ester in the electrolyte solution is particularly preferably 15 to 50% by mass in terms of flame retardancy, more preferably 5 to 30% by mass, and still more preferably 5 to 20% by mass. Particularly preferably, the content of 5 to 15% by mass provides good cycle characteristics.

  Examples of the non-aqueous solvent contained in the electrolytic solution of the present embodiment include an aprotic solvent, and an aprotic polar solvent is preferable. Specific examples thereof include, for example, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, and trans-2. , 3-pentylene carbonate, cis-2,3-pentylene carbonate, trifluoromethylethylene carbonate, fluoroethylene carbonate, cyclic carbonates represented by 4,5-difluoroethylene carbonate (however, specific polymerizable functional groups described later) Lactones represented by γ-butyrolactone and γ-valerolactone; cyclic sulfones represented by sulfolane; cyclic ethers represented by tetrahydrofuran and dioxane; -Borate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, ethyl propyl carbonate, chain carbonate represented by methyl trifluoroethyl carbonate; nitrile represented by acetonitrile An ether typified by dimethyl ether; a chain carboxylic acid ester typified by methyl propionate; a chain ether carbonate compound typified by dimethoxyethane. These are used singly or in combination of two or more.

  In particular, in order to increase the ionization degree of the lithium salt that contributes to charge / discharge of the lithium ion secondary battery, the non-aqueous solvent preferably contains one or more cyclic aprotic polar solvents, and in particular, one cyclic carbonate. More preferably included. The cyclic compound has a high dielectric constant, and is effective for assisting the ionization of the lithium salt.

  In addition, various mechanisms can be considered for the phenomenon that the capacity of the lithium ion secondary battery decreases while it is repeatedly charged and discharged, and this is also due to the phenomenon that the solvent, additive, and electrolyte are decomposed on the electrode surface. it is conceivable that. On the other hand, suppressing the electrode surface activity by forming a film on the electrode surface is considered to contribute to the improvement of the cycle characteristics. From such a viewpoint, it is also preferable to use one or more compounds having a specific polymerizable functional group in combination. Specific polymerizable functional groups include, for example, carbon-carbon unsaturated double bond groups such as vinyl group, allyl group, acrylic group, and methacryl group; carbon-carbon unsaturated triple bond groups such as acetylenyl group and propargyl group; epoxy group Monofunctional groups such as nitro group, nitroso group, and silyl group. As such a compound, those having an intercarbon unsaturated double bond group in the molecule are suitable, and those having one intercarbon unsaturated double bond group in the molecule are more preferred.

  More specifically, as such compounds, vinylene carbonate compounds represented by the following general formula (IIa), carbonate compounds represented by the following general formula (IIb), methyl cyanate, bisphenol A type epoxy resin, bisphenol F Type epoxy resin. Among these, vinylene carbonate and vinyl ethylene carbonate are preferable in terms of flame retardancy and cycle characteristics, and vinylene carbonate is more preferable.

ここで、式（ＩＩａ）中、Ｒ⁴及びＲ⁵は互いに同一であっても異なっていてもよく、水素原子又は炭素数１〜３のアルキル基を示す。アルキル基としては、メチル基、エチル基、ｎ−プロピル基、イソプロピル基が挙げられる。また、式（ＩＩｂ）中、Ｒ⁶は炭素数１〜３のアルケニル基を示し、Ｒ⁷は水素原子又は炭素数１〜３のアルキル基を示す。アルケニル基としては、ビニル基、１−プロペニル基、２−プロペニル基、イソプロペニル基が挙げられ、アルキル基としては、上記式（ＩＩａ）中のＲ⁴及びＲ⁵と同様のものが挙げられる。

Here, in formula (IIa), R ⁴ and R ⁵ may be the same as or different from each other, and represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. In formula (IIb), R ⁶ represents an alkenyl group having 1 to 3 carbon atoms, and R ⁷ represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. Examples of the alkenyl group include a vinyl group, a 1-propenyl group, a 2-propenyl group, and an isopropenyl group. Examples of the alkyl group include the same groups as R ⁴ and R ⁵ in the above formula (IIa).

  As the compound having the specific polymerizable functional group, a compound having two or more specific polymerizable functional groups that may be the same as or different from each other (hereinafter referred to as “compound (C)”). Furthermore, it is also preferable to use together. As the compound (C), from the viewpoint of further improving the cycle characteristics of the obtained lithium ion secondary battery, it is preferable to use a compound having an atom having an unshared electron pair, and among them, a compound having a nitrogen atom is used. It is preferable. In particular, it is preferable to use a compound having a triazine ring structure and / or an isomer structure thereof. Examples of the compound (C) include triallyl cyanurate, triallyl isocyanurate, divinylbenzene, diallyl phthalate, phenol novolac type epoxy resin having three epoxy groups in the molecule, and cresol having three epoxy groups in the molecule. A novolak-type epoxy resin is mentioned.

The lithium salt contained in the electrolytic solution of the present embodiment is not particularly limited as long as it is used as a normal nonaqueous electrolyte, and any lithium salt may be used. Specific examples of such lithium salts include, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{2k + 1} [k is an integer of 1 to 8], LiN ( SO ₂ C _k F _{2k + 1} ) ₂ [k is an integer of 1 to 8], LiPF _n (C _k F _{2k + 1} ) _6-n [n is an integer of 1 to 5, k is an integer of 1 to 8] LiBF _n (C _k F _{2k + 1} ) _4-n [n is an integer of 1 to 3, k is an integer of 1 to 8], lithium bisoxalylborate represented by LiB (C ₂ O ₂ ) ₂ , LiBF ₂ lithium difluoro oxalyl borate represented by (C ₂ O _2), include lithium trifluoromethane oxalyl phosphate represented by _{LiPF 3 (C 2 O 2)} .

Moreover, lithium salt represented by the following general formula (1a), (1b), (1c) can also be used.
LiC (SO ₂ R ¹¹ ) (SO ₂ R ¹² ) (SO ₂ R ¹³ ) (1a)
LiN (SO ₂ OR ¹⁴ ) (SO ₂ OR ¹⁵ ) (1b)
LiN (SO ₂ R ¹⁶ ) (SO ₂ OR ¹⁷ ) (1c)
Here, in the formula, R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ , R ¹⁶ and R ¹⁷ may be the same or different from each other, and are a C 1-8 perfluoroalkyl group. Indicates.

These lithium salts are used singly or in combination of two or more. Among these lithium salts, LiPF ₆ , LiBF ₄ , and LiN (SO ₂ C _k F _{2k + 1} ) ₂ [k is an integer of 1 to 8] are particularly preferable from the viewpoint of improving battery characteristics and stability.

  The lithium salt is preferably contained in the electrolytic solution at a concentration of 0.1 to 3 mol / liter, more preferably 0.5 to 2 mol / liter.

  In addition, from the viewpoint of further improving safety and suppressing the deterioration of battery performance, a polymer-based gel electrolyte composed of a polymer matrix and the above-described electrolytic solution held in the structure is used as a lithium ion secondary. It is also suitable to be used as a battery electrolyte. The polymer matrix is not limited as long as it can form a gel while holding an electrolytic solution, and examples thereof include polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, and polyurethane, and these are particularly preferably used. Furthermore, a gel electrolyte composed of a gelling agent that can function as a gelling agent that can be added to an electrolytic solution to gel the electrolytic solution even if it is a low molecular compound and the above electrolytic solution is used for a lithium ion secondary battery. It is also suitable to use as an electrolytic solution. Examples of the gelling agent include compounds represented by the following general formulas (2a), (2b), (2c) and (2d), and these are preferable.

_{C m F 2m + 1 C n} H 2n SO 2 -Ph-OC x H 2x + 1 ... (2a)
C _{m ′} F _{2m ′ + 1} — (CH ₂ ) —CH (R ¹⁸ ) — (CH ₂ ) _p —O—Ar—R (2b)
C _{m ′} F _{2m ′ + 1} — (CH ₂ ) _q —S—Ar—O—R (2c)
_{C m 'F 2m' + 1} - (CH 2) q -Z-Ar-O-Ar-Z- (CH 2) q -C m 'F 2m' + 1 ... (2d)
Here, m, n and x are all positive integers, m ′ is 6 to 12, p is 1 to 2, and q is a natural number of 0 or 1 to 4. Ph represents a benzene ring (phenylene group), Ar represents a substituted or unsubstituted divalent aromatic hydrocarbon group having 5 to 30 nuclear atoms, R ¹⁸ represents a hydrogen atom or a hydroxyl group, and R represents saturated or unsaturated. The C1-C20 monovalent hydrocarbon group, Z represents a sulfur atom or an oxygen atom, respectively.

  Among these, the compound represented by the general formula (2a) is particularly preferable.

  The negative electrode is not particularly limited as long as it can occlude and release lithium ions, and may be a known one. The negative electrode preferably contains at least one material selected from the group consisting of a material capable of inserting and extracting lithium ions as a negative electrode active material and metallic lithium. Examples of such materials include lithium metal, amorphous carbon (hard carbon), artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, glassy carbon, organic polymer compound fired body, mesocarbon microbeads. , Carbon fibers, activated carbon, carbon colloid, and carbon materials represented by carbon black. Among these, examples of the coke include pitch coke, needle coke, and petroleum coke. Further, the fired body of the organic polymer compound is obtained by firing and polymerizing a polymer material such as a phenol resin or a furan resin at an appropriate temperature. In the present invention, a battery employing metallic lithium as the negative electrode active material is also included in the lithium ion secondary battery.

  Further, examples of a material capable of inserting and extracting lithium ions include a material containing at least one of a metal element and a metalloid element capable of forming an alloy with lithium. This material may be a single metal or semi-metal, an alloy or a compound, and may have at least a part of one or more of these phases. .

  In the present specification, the “alloy” includes an alloy having one or more metal elements and one or more metalloid elements in addition to an alloy composed of two or more metal elements. In addition, the alloy may have a nonmetallic element as long as the alloy has metallic properties as a whole. In the alloy structure, a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or two or more of these coexist.

  Examples of such metal elements and metalloid elements include titanium (Ti), tin (Sn), lead (Pb), aluminum, indium (In), silicon (Si), zinc (Zn), and antimony (Sb). Bismuth (Bi), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), hafnium (Hf), zirconium (Zr) and yttrium (Y).

  Among these, metal elements and metalloid elements of Group 4 or 14 in the long-period periodic table are preferable, and titanium, silicon, and tin are particularly preferable.

  As an alloy of tin, for example, as a second constituent element other than tin, silicon, magnesium (Mg), nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium (Ti), germanium, bismuth, Examples thereof include those having one or more elements selected from the group consisting of antimony and chromium (Cr).

  Examples of the silicon alloy include, as the second constituent element other than silicon, tin, magnesium, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. Those having one or more elements selected from the above are listed.

  Examples of the titanium compound, the tin compound, and the silicon compound include those having oxygen (O) or carbon (C), and in addition to titanium, tin, or silicon, the second constituent element described above is included. It may be.

  A negative electrode active material is used individually by 1 type or in combination of 2 or more types.

  The number average particle diameter of the negative electrode active material is preferably 0.1 μm to 100 μm, more preferably 1 μm to 10 μm. The number average particle size of the negative electrode active material is measured in the same manner as the number average particle size of the positive electrode active material.

  A negative electrode is obtained as follows, for example. That is, first, a negative electrode mixture-containing paste is prepared by dispersing, in a solvent, a negative electrode mixture prepared by adding a conductive additive or a binder to the negative electrode active material, if necessary. Next, the negative electrode mixture-containing paste is applied to a negative electrode current collector and dried to form a negative electrode mixture layer, which is pressurized as necessary to adjust the thickness, thereby producing a negative electrode.

  Here, the solid content concentration in the negative electrode mixture-containing paste is preferably 30 to 80% by mass, and more preferably 40 to 70% by mass.

  The negative electrode current collector is made of, for example, a metal foil such as a copper foil, a nickel foil, or a stainless steel foil.

  In producing the negative electrode, examples of the conductive aid used as necessary include graphite, carbon black such as acetylene black and ketjen black, and carbon fiber. The number average particle diameter of the conductive assistant is preferably 0.1 μm to 100 μm, more preferably 1 μm to 10 μm, and is measured in the same manner as the number average particle diameter of the positive electrode active material. Examples of the binder include PVDF, PTFE, polyacrylic acid, styrene butadiene rubber, and fluorine rubber.

  The lithium ion secondary battery of the present embodiment preferably includes a separator between the positive electrode and the negative electrode from the viewpoint of providing safety such as prevention of short circuit between the positive and negative electrodes and shutdown. The separator may be the same as that provided for a known lithium ion secondary battery, and is preferably an insulating thin film having high ion permeability and excellent mechanical strength. Examples of the separator include woven fabrics, nonwoven fabrics, and synthetic resin microporous membranes. Among these, synthetic resin porous membranes are preferable. As the synthetic resin microporous membrane, for example, a microporous membrane containing polyethylene or polypropylene as a main component or a polyolefin microporous membrane such as a microporous membrane containing both of these polyolefins is preferably used. As the nonwoven fabric, a porous film made of a heat-resistant resin such as ceramic, polyolefin, polyester, polyamide, liquid crystal polyester, or aramid is used.

  The separator may be a single microporous membrane or a laminate of a plurality of microporous membranes, or may be a laminate of two or more microporous membranes.

  In addition, when the above-mentioned gel electrolyte is used as an electrolytic solution, the above-mentioned separator is not necessarily used when the gel electrolyte itself has sufficient mechanical strength. However, in that case, it is also preferable to use a separator in combination. When using a gel electrolyte, a gel electrolyte may be provided between the positive electrode and the negative electrode, or a separator and a gel electrolyte may be used in combination.

  The lithium ion secondary battery of this embodiment is produced by a known method using the above-described electrolyte, positive electrode, negative electrode, and, if necessary, a separator. For example, the positive electrode and the negative electrode are wound with a separator and / or a gel electrolyte interposed therebetween to form a laminated structure of a wound structure, or formed into a laminated body by bending or laminating a plurality of layers. Next, the laminated body is accommodated in the battery case, the electrolytic solution according to the present embodiment is poured into the case, and the laminated body is immersed and sealed in the electrolytic solution, whereby the lithium secondary battery of the present embodiment is sealed. A secondary battery can be produced. The shape of the lithium ion secondary battery of the present embodiment is not particularly limited, and for example, a cylindrical shape, an elliptical shape, a rectangular tube shape, a button shape, a coin shape, a flat shape, a laminate shape, and the like are preferably used.

  As mentioned above, although the form for implementing this invention was demonstrated, this invention is not limited to the said this embodiment. The present invention can be variously modified without departing from the gist thereof.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(I) Measurement of capacity retention rate (charge / discharge cycle test)
The initial charge / discharge and charge / discharge cycle tests were performed using a charge / discharge device ACD-01 manufactured by Asuka Electronics Co., Ltd. and a thermostatic chamber KCL-1000 manufactured by Tokyo Rika Kikai Co., Ltd. In Example 1 and Comparative Example 1 described later, in the initial charge, the battery was charged with a constant current of 1 mA, reached 3.8 V, and then charged with a constant voltage of 3.8 V for a total of 8 hours. Thereafter, the battery was discharged at a constant current of 1 mA until the voltage reached 2.5V. In addition, the battery ambient temperature at the time of initial charge / discharge was 25 degreeC. In the charge / discharge cycle test, the battery was first charged with a constant current of 1 mA, reached 3.8 V, and then charged with a constant voltage of 3.8 V for a total of 8 hours. Thereafter, the battery was discharged at a constant current of 1 mA, and when it reached 2.7 V, charging was repeated again. One cycle means that charging and discharging are performed once. The ambient temperature of the battery was 60 ° C. The capacity retention rate was the ratio of the discharge capacity after 50 cycles when the initial discharge capacity of the cycle test was 100%.

  For Comparative Example 2, in the initial charge, the battery was charged with a constant current of 2 mA, and after reaching 4.2 V, the battery was charged with a constant voltage of 4.2 V for a total of 8 hours. Thereafter, the battery was discharged at a constant current of 2 mA until the voltage reached 3.0V. In addition, the battery ambient temperature at the time of initial charge / discharge was 25 degreeC. In the charge / discharge cycle test, first, the battery was charged with a constant current of 6 mA, and after reaching 4.0 V, the battery was charged with a constant voltage of 4.0 V for a total of 3 hours. Thereafter, the battery was discharged at a constant current of 12 mA, and when it reached 3.0 V, charging was repeated again. The ambient temperature of the battery was 60 ° C.

(Ii) Flame Retardancy Test of Electrolyte Solution A commercially available glass fiber filter paper cut into a strip shape having a width of 13 mm and a length of 110 mm was impregnated with 1 mL of the prepared electrolyte solution. Thus, the both ends of the length direction were fixed so that the glass fiber filter paper impregnated with electrolyte solution might become horizontal. One end of the test flame was exposed to a test flame for a maximum of 30 seconds until ignition, and the test flame was moved away immediately after ignition to visually observe the state of the flame. The electrolyte solution that did not ignite even if the indirect flame continued for 30 seconds was judged as “excellent” and marked as “◎”. The electrolyte that ignites but disappears when the flame is removed is judged as “good” and marked with “◎”. Combustion continued even after igniting and taking off, but the flame did not propagate to the other end of the filter paper and disappeared in the middle was judged as “almost good” and marked as “◯”. An item that ignited and continued to burn even after the flame was released and the flame propagated to the other end of the filter paper was judged as “bad” and indicated as “x”. Thus, the flame retardancy of the electrolyte was evaluated.

[Production Example 1]
(Preparation of positive electrode)
Lithium iron phosphate (LiFePO ₄ ) as the positive electrode active material, acetylene black powder with a number average particle diameter of 1 μm as the conductive auxiliary, and polyvinylidene fluoride (PVdF) as the binder in a mass ratio of 80:10:10 -Mixed in 2-pyrrolidone and mixed. Next, N-methyl-2-pyrrolidone was added to the obtained mixture and further mixed to prepare a slurry-like solution having a solid content of 35% by mass. This slurry-like solution was applied to one side of an aluminum foil having a thickness of 20 μm so as to have an initial charge / discharge capacity of 1.8 mAh / cm ² , and the solvent was dried and removed, followed by rolling with a roll press. Thereafter, it was punched into a disk shape having a diameter of 16 mm to obtain a positive electrode.

[Production Example 2]
(Preparation of positive electrode)
Lithium cobalt acid (LiCoO ₂ ) having an average particle diameter of 5 μm as a positive electrode active material, carbon powder having a number average particle diameter of 3 μm as a conductive assistant, and polyvinylidene fluoride (PVdF) as a binder were mixed in a mass ratio of 85: 10: 5. . N-methyl-2-pyrrolidone was added to the obtained mixture and further mixed to prepare a slurry-like solution so that the solid content was 60% by mass. This slurry solution was applied to one side of an aluminum foil having a thickness of 20 μm so as to have an initial charge capacity of 3 mAh / cm ² , and the solvent was removed by drying, followed by rolling with a roll press. Thereafter, it was punched into a disk shape having a diameter of 16 mm to obtain a positive electrode.

[Production Example 3]
(Preparation of negative electrode)
Mesocarbon microbeads having a number average particle diameter of 5 μm as the negative electrode active material, binder made of diene rubber as the binder (glass transition temperature: −5 ° C., particle size at drying: 120 nm, dispersion medium: water, solid content concentration: 40% ) Was mixed so that the solid content concentration of the negative electrode active material was 60% by mass to prepare a slurry-like solution. This slurry solution was applied to one side of a 10 μm thick copper foil so as to have an initial charge capacity of 1.8 mAh / cm ² , and the solvent was dried and removed, followed by rolling with a roll press. Thereafter, it was punched into a disk shape having a diameter of 16 mm to obtain a negative electrode.
[Production Example 4]
(Preparation of negative electrode)
Mesocarbon microbeads having a number average particle diameter of 5 μm as the negative electrode active material, binder made of diene rubber as the binder (glass transition temperature: −5 ° C., particle size at drying: 120 nm, dispersion medium: water, solid content concentration: 40% ) Was mixed so that the solid content concentration of the negative electrode active material was 60% by mass to prepare a slurry-like solution. This slurry solution was applied to one side of a 10 μm thick copper foil so as to have an initial charge capacity of 3 mAh / cm ² , and after removing the solvent by drying, it was rolled with a roll press. Thereafter, it was punched into a disk shape having a diameter of 16 mm to obtain a negative electrode.

[Example 1]
LiPF ₆ was added to a solvent in which ethylene carbonate and methyl ethyl carbonate were mixed at a volume ratio of 2: 1 so as to be 1 mol / L to obtain a basic electrolyte solution. Tri (trifluoroethyl) phosphite (hereinafter abbreviated as “TFEP (III)”) is further added to the basic electrolyte solution, and the ratio of TFEP (III) to the total amount is 10 mass. % Electrolyte solution was prepared. As a result of conducting a flame retardant test of this electrolytic solution, the evaluation was good. The results are shown in Table 1.

  Subsequently, after superposing the positive electrode and the negative electrode prepared in Production Examples 1 and 3 via a separator made of polyethylene (film thickness: 25 μm, porosity: 50%, pore diameter: 0.1 μm to 1 μm), the product made of SUS The battery was inserted into a disk-type battery, injected with 1 mL of the above electrolyte, and sealed.

  After the first charge / discharge evaluation was performed on the obtained battery, a charge / discharge cycle test was performed, and the capacity retention rate was measured. The results are shown in Table 1.

[Example 2]
An electrolyte solution was prepared in the same manner as in Example 1 except that TFEP (III) was added so that the ratio thereof was 5% by mass with respect to the total amount. A battery was fabricated in the same manner as in Example 1 except that the electrolytic solution was used. Table 1 shows the results of the flame retardant test of the electrolyte and the results of the charge / discharge cycle test.

[Example 3]
An electrolyte solution was prepared in the same manner as in Example 1 except that TFEP (III) was added so that the ratio thereof was 20% by mass with respect to the total amount. A battery was fabricated in the same manner as in Example 1 except that the electrolytic solution was used. Table 1 shows the results of the flame retardant test of the electrolyte and the results of the charge / discharge cycle test.

[Example 4]
An electrolyte solution was prepared in the same manner as in Example 1 except that TFEP (III) was added so that the ratio thereof was 30% by mass with respect to the total amount. A battery was fabricated in the same manner as in Example 1 except that the electrolytic solution was used. Table 1 shows the results of the flame retardant test of the electrolyte and the results of the charge / discharge cycle test.

[Comparative Example 1]
A battery was fabricated in the same manner as in Example 1 except that the basic electrolyte solution prepared in Example 1 was used as it was as the electrolytic solution. Table 1 shows the results of the flame retardant test of the electrolyte and the results of the charge / discharge cycle test.

[Comparative Example 2]
A battery as in Example 1 except that the positive electrode produced in Production Example 2 was used instead of the positive electrode produced in Production Example 1, and the negative electrode produced in Production Example 4 was used instead of the negative electrode produced in Production Example 3. Was made. The result of the flame retardancy test of the electrolytic solution was “good”, but the charge / discharge cycle test had a capacity retention rate of 50% in less than 30 cycles.

[ Reference Example 1 ]
An electrolytic solution was prepared in the same manner as in Example 1 except that trimethyl phosphite (hereinafter abbreviated as “TMP (III)”) was used instead of TEFP (III). A battery was fabricated in the same manner as in Example 1 except that the electrolytic solution was used. Table 1 shows the results of the flame retardant test of the electrolyte and the results of the charge / discharge cycle test.

  The lithium ion secondary battery of the present invention is expected to be used as a rechargeable battery for automobiles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles, in addition to portable devices such as mobile phones, portable audio devices, and personal computers.

Claims (2)

A positive electrode containing olivine-type lithium iron phosphate;
A negative electrode made of a material capable of inserting and extracting lithium ions;
An electrolytic solution containing a non-aqueous solvent, a lithium salt dissolved in the non-aqueous solvent, and 5 to 50% by mass of tri (trifluoroethyl) phosphite ,
A lithium ion secondary battery comprising: The lithium ion secondary battery of Claim 1 containing 5-30 mass% of said tri (trifluoroethyl) phosphites .