JP4940617B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

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

  Along with the rapid miniaturization and diversification of consumer mobile phones, portable devices and personal digital assistants, etc., the battery that is the power source is small, lightweight, high energy density, and repeatedly charged and discharged for a long time. There is a strong demand for the development of secondary batteries that can be realized. Among them, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are the most promising and active as secondary batteries that satisfy these needs compared to lead batteries and nickel cadmium batteries that use aqueous electrolytes. Research has been conducted.

The positive electrode active material of the nonaqueous electrolyte secondary battery includes general formula Li _x such as titanium disulfide, vanadium pentoxide and molybdenum trioxide, lithium cobalt composite oxide, lithium nickel composite oxide and spinel type manganese oxide. Various compounds represented by MO ₂ (where M is one or more transition metals) have been studied. Among these, lithium cobalt composite oxide, lithium nickel composite oxide, spinel-type lithium manganese composite oxide, and the like can be charged and discharged at an extremely noble potential of 4 V (vs Li / Li ⁺ ) or more. Can be used to realize a battery having a high discharge voltage.

  Various negative electrode active materials for non-aqueous electrolyte secondary batteries, such as metallic lithium, lithium alloys, and carbon materials capable of occluding / releasing lithium, have been studied. There is an advantage that a battery having a long life can be obtained and safety is high.

For the electrolyte of a non-aqueous electrolyte secondary battery, a supporting salt such as LiPF ₆ or LiBF ₄ is generally mixed with a mixed solvent of a high dielectric constant solvent such as ethylene carbonate or propylene carbonate and a low viscosity solvent such as dimethyl carbonate or diethyl carbonate. A dissolved electrolyte is used.

  In order to improve the cycle life characteristics of non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries that require high energy density, positive electrode materials, negative electrode materials, and non-aqueous electrolytes are being developed. In recent years, a method for adding a small amount of an additive to an electrolyte has been actively studied. For example, Patent Document 1 exemplifies that a high discharge capacity can be obtained even when a cycle life test is performed by increasing the initial discharge capacity by reducing the initial irreversible capacity of the negative electrode using a phosphate additive. Patent Document 2 exemplifies that charge / discharge cycle life characteristics and high-temperature storage characteristics are improved by adding a silyl ester compound and tetrafluoroborate to a non-aqueous electrolyte.

  In recent years, the demand for non-aqueous electrolyte secondary batteries as power sources for mobile vehicles such as hybrid cars has been increasing, and in addition to the high energy density that has been regarded as important in the past, high input / output performance has been long-term. Has been demanded to last. Maintaining a high discharge capacity over a long period of time does not necessarily match maintaining a high input / output characteristic over a long period of time. This is mainly because the decrease in the discharge capacity of non-aqueous electrolyte secondary batteries used at low loads is due to the change in the balance between the charge levels of the positive and negative electrodes due to the decomposition reaction of the non-aqueous electrolyte on the electrodes. On the other hand, the decrease in input / output characteristics, that is, the increase in DC resistance, is mainly due to increases in current collection resistance, electrolyte resistance, and charge transfer resistance between the positive and negative electrodes and the electrolyte. is there. Therefore, in applications that require high input / output, a method for suppressing an increase in DC resistance is important. Patent Document 3 discloses a pulse discharge before conducting a long-life test by using nitrite, nitrate, carbonate, dicarbonate, phosphonate, phosphate, sulfate and fluorocarbon or a mixture thereof in the electrolyte. Although it is said that the increase in voltage delay, that is, the increase in polarization can be suppressed, there is only an example of dibenzyl carbonate as an example, and it is possible to suppress an increase in DC resistance after a charge / discharge cycle at high temperature or after standing for a long time. Not listed. Similarly, Patent Document 1 and Patent Document 2 described above do not describe that an increase in DC resistance after a charge / discharge cycle at a high temperature or after standing for a long time can be suppressed.

There is not yet a sufficient method for suppressing the increase in DC resistance of non-aqueous electrolyte secondary batteries in long-term use, and improvements in active materials, current collector structures, separators, non-aqueous electrolytes, and electrolyte additives There is a need for further improvement in life characteristics by adding and improving the mixing method.
JP 2000-331710 A JP 2004-342607 A JP 2000-164251 A

  The invention of claim 1 relates to a non-aqueous electrolyte secondary battery, which is a non-aqueous electrolyte secondary battery comprising a positive electrode for storing and releasing lithium, a negative electrode for storing and releasing lithium, and a non-aqueous electrolyte. The nonaqueous electrolyte contains 0.02 M to 0.2 M sulfate ester compound and 0.005 M to 0.04 M phosphoric acid silyl ester compound.

  The invention of claim 2 is the invention according to claim 1, wherein the sulfate ester compound contains a cyclic sulfate having a carbon number of 3 or more of 0.02 M to 0.2 M, and the phosphate silyl ester compound is 0.005 M to It contains 0.04M tristrimethylsilyl phosphate.

  According to the first aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery comprising a positive electrode that occludes and releases lithium, a negative electrode that occludes and releases lithium, and a non-aqueous electrolyte. A non-aqueous electrolyte secondary battery containing a 02M to 0.2M sulfate ester compound and a 0.005M to 0.04M silyl phosphate ester compound having a small increase in DC resistance in long-term use Can be obtained.

  According to invention of Claim 2, in Claim 1, the said sulfate ester compound contains 0.02M-0.2M C3 or more cyclic sulfate, and the said phosphoric acid silyl ester compound is 0. By containing 005M to 0.04M tristrimethylsilyl phosphate, a non-aqueous electrolyte secondary battery with a remarkably small increase in DC resistance can be obtained over a long period of use.

  The present invention relates to a nonaqueous electrolyte secondary battery comprising a positive electrode that occludes / releases lithium, a negative electrode that occludes / releases lithium, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte is 0.02 M to 0.2 M sulfuric acid. It contains an ester compound, and contains 0.005M to 0.04M phosphoric acid silyl ester compound. Here, specific examples of the sulfate ester compound include dimethyl sulfate, diethyl sulfate, ethyl methyl sulfate, ethyl propyl sulfate, methyl propyl sulfate, dipropyl sulfate, methyl phenyl sulfate, ethyl phenyl sulfate, phenyl propyl sulfate, benzyl methyl sulfate, and benzyl ethyl sulfate. Allyl methyl sulfate, allyl ethyl sulfate, allyl propyl sulfate, diallyl sulfate, allyl trimethyl silyl sulfate, methyl trimethyl silyl sulfate, ethyl trimethyl silyl sulfate, propyl trimethyl silyl sulfate, ditrimethyl silyl sulfate, ethylene glycol sulfate, vinyl ethylene glycol sulfate, 1,2-propane Diol sulfate, 1,2-butanediol sulfate, 1,2-pentanediol sulfate, 1,2-hexanediol sulfate 1,3-propanediol sulfate, 1,3-butanediol sulfate, 1,3-pentanediol sulfate, 1,3-hexanediol sulfate, 2,3-butanediol sulfate, 2,3 -Pentanediol sulfate, 2,3-hexanediol sulfate, 3,4-hexanediol sulfate, 2,4-pentanediol sulfate, 2,4-hexanediol sulfate, phenylethylene glycol sulfate, methylphenyl Examples include ethylene glycol sulfate, ethylphenylethylene glycol, allylethylene glycol sulfate, and the like. The hydrogen atom bonded to the alkyl group may be substituted with a halogen. Specific examples of the phosphoric acid silyl ester compound include, for example, tris (trimethylsilyl) phosphate, dimethyltrimethylsilyl phosphate, methylbis (trimethylsilyl) phosphate, diethyltrimethylsilyl phosphate, diethyltrimethylsilyl phosphate, ethylbis (trimethylsilyl) phosphate, and phosphoric acid. Dipropyltrimethylsilyl, propyl bis (trimethylsilyl) phosphate, dibutyl phosphate (trimethylsilyl), butyl bis (trimethylsilyl) phosphate, dioctyltrimethylsilyl phosphate, octylbis (trimethylsilyl) phosphate, diphenyltrimethylsilyl phosphate, phenylbis (trimethylsilyl) phosphate , Di (trifluoroethyl) phosphate (trimethylsilyl), trifluoroethylbisphosphate (trimethylsilyl) phosphate, A compound in which the trimethylsilyl group of a silyl ester is substituted with a triethylsilyl group, a triphenylsilyl group, a dimethylsilyl group, or the like, a structure of a so-called condensed phosphate ester in which phosphoric acid esters are condensed and phosphorus atoms are bonded via oxygen. And the like. These silyl phosphates may be used alone or in combination, and preferably trimethylsilyl phosphate is used alone as a silyl phosphate, or a mixture of trimethylsilyl phosphate and other silyl phosphates. The effect of suppressing an increase in DC resistance is great and more preferable.

  Further, it is preferable that the sulfuric acid ester compound is a cyclic sulfuric acid ester having 3 or more carbon atoms and the trialkylsilyl ester is tristrimethylsilyl phosphate, which can significantly suppress an increase in direct current resistance during long-term use. Here, the cyclic sulfate having 3 or more carbon atoms can be selected from the aforementioned sulfates, and more preferably 1,2-propanediol sulfate, 1,2-butanediol sulfate, 1,2 -Pentanediol sulfate, 1,2-hexanediol sulfate, 1,3-propanediol sulfate, 1,3-butanediol sulfate, 1,3-pentanediol sulfate, etc. The effect is particularly great.

  Even if each of the sulfate ester compound and the trialkylsilyl ester compound used in the present invention is added alone to the non-aqueous electrolyte, the effect of suppressing an increase in direct current resistance associated with long-term use cannot be obtained. Specifically, depending on the same slope or addition amount as in the case where these additives are not added, the direct current resistance increases with respect to the period of use with a further slope. In addition, if the amount of one additive is excessive, the effect of suppressing an increase in DC resistance cannot be obtained even if the amount of the other additive is changed. Resistance increases. However, by mixing these compounds in a specific concentration range, the increase in DC resistance can be largely suppressed for the first time, and the initial output characteristics can be maintained over a long period of time. The mechanism by which this surprising effect is manifested by mixing the ineffective ones properly is not clear at the present time, although the slope of the increase in DC resistance is not improved at all. In addition to acting on the decomposition mechanism of the nonaqueous electrolyte solution at the positive and negative electrodes or the thermal decomposition mechanism of the nonaqueous electrolyte, each compound has a deterioration promoting factor that causes no effect when used alone. It is thought that a specific synergistic effect is brought about by canceling. In addition, when tristrimethylsilyl phosphate and a cyclic sulfate compound having 3 or more carbon atoms were mixed at a specific ratio, a particularly remarkable effect was obtained because each compound formed a particularly stable mixed film on the electrode. This is probably because

  Here, since the additive is consumed on the electrode when the battery is produced and charged / discharged, the content here is the content when the liquid is injected into the battery, and the amount remaining in the battery Rather, the amount remaining in the battery is less than the amount normally added.

  As the non-aqueous electrolyte, either an electrolytic solution or a solid electrolyte can be used. In the case of using an electrolytic solution, as an electrolytic solution solvent, ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, γ-valerolactone, sulfolane, 1,2-dimethoxyethane, 1,2-di- Ethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, halogenated dioxolane, trifluoroethyl methyl ether, ethylene glycol diacetate, propylene glycol diacetate, ethylene glycol dipropionate, propylene glycol dipropio , Methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl fluoroacetate, full Ethyl oloacetate, propyl fluoroacetate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, methyl isopropyl carbonate, ethyl isopropyl carbonate, diisopropyl carbonate, dibutyl carbonate, acetonitrile, fluoroacetonitrile, ethoxy Alkoxy and halogen-substituted cyclic phosphazenes such as pentafluorocyclotriphosphazene, diethoxytetrafluorocyclotriphosphazene, phenoxypentafluorocyclotriphosphazene and phosphorus such as chain phosphazenes, triethyl phosphate, trimethyl phosphate, trioctyl phosphate Acid alkyl esters, triethyl borate, Boric acid esters such as c acid tributyl, N- methyl oxazolidinone, a non-aqueous solvent such as N- ethyl-oxazolidinones, or in itself can be used a mixture of these solvents.

The nonaqueous electrolyte is used by dissolving the supporting salt in these nonaqueous solvents. Examples of the supporting salt include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiCF ₃ CF ₂ CF ₂ SO ₃ , LiN (SO ₂ CF ₃ ). ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ , LiN (COCF ₂ CF ₃ ) ₂ LiBF ₂ C ₂ O ₄ , LiBC ₄ O ₈ , LiPF ₂ (C ₂ O ₄ ) ₂ and A salt such as LiPF ₃ (CF ₂ CF ₃ ) ₃ or a mixture thereof can be used.

  In order to improve battery characteristics, a small amount of additives may be mixed in the non-aqueous electrolyte. Vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, phenyl vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene Carbonates such as carbonate, dimethyl vinylene carbonate, diethyl vinylene carbonate, fluoroethylene carbonate, vinyl esters such as vinyl acetate and vinyl propionate, diallyl sulfide, allyl phenyl sulfide, allyl vinyl sulfide, allyl ethyl sulfide, propyl sulfide, diallyl disulfide , Sulfides such as allyl ethyl disulfide, allyl propyl disulfide, allyl phenyl disulfide, , Cyclic sulfonic acid esters such as 3-propane sultone, 1,4-butane sultone, 1,3-prop-2-ene sultone, methyl methanesulfonate, ethyl methanesulfonate, propyl methanesulfonate, methyl ethanesulfonate, ethane Ethyl sulfonate, propyl ethane sulfonate, methyl benzene sulfonate, ethyl benzene sulfonate, propyl benzene sulfonate, phenyl methane sulfonate, phenyl ethane sulfonate, phenyl propane sulfonate, methyl benzyl sulfonate, ethyl benzyl sulfonate, benzyl Chain sulfonates such as propyl sulfonate, benzyl methanesulfonate, benzyl ethanesulfonate, benzyl propanesulfonate, dimethyl sulfite, diethyl sulfite, ethyl methyl Rufite, methyl propyl sulfite, ethyl propyl sulfite, diphenyl sulfite, methyl phenyl sulfite, ethyl methyl sulfite, ethylene sulfite, vinyl ethylene sulfite, divinyl ethylene sulfite, propylene sulfite, vinyl propylene sulfite, butylene Sulfites such as sulfite, vinylbutylene sulfite, vinylene sulfite, phenylethylene sulfite, benzene, toluene, xylene, biphenyl, cyclohexylbenzene, 2-fluorobiphenyl, 4-fluorobiphenyl, diphenyl ether, tert-butylbenzene , Orthoterphenyl, metaterphenyl, naphthalene, fluoronaphthalene, cumene, fluorobenzene, 2,4- Aromatic compounds such as fluoro anisole, halogen-substituted alkanes such as perfluorooctane, trimethylsilyl borate, such as boric acid triethylsilyl, may be appropriately added depending on the purpose.

  When a solid electrolyte is used, a porous polymer solid electrolyte membrane may be used as the polymer solid electrolyte, and an electrolyte solution may be further contained in the polymer solid electrolyte. Moreover, when using a gel-like polymer solid electrolyte, the electrolyte solution which comprises gel and the electrolyte solution contained in the pore etc. may differ. When such a polymer solid electrolyte is used, the sulfuric acid ester and silyl phosphate ester of the present invention may be contained in the electrolytic solution. However, since a high output is required for HEV applications, it is more preferable to use a non-aqueous electrolyte alone as the electrolyte than to use a solid electrolyte or a polymer solid electrolyte.

There is no restriction | limiting in particular as a positive electrode active material of the nonaqueous electrolyte secondary battery to which this invention is applied, A various material can be used suitably. For example, transition metal compounds such as manganese dioxide and vanadium pentoxide, transition metal chalcogen compounds such as iron sulfide and titanium sulfide, and composite oxides of these transition metals and lithium LixMO _2-δ (where M is , Co, Ni, or Mn, and 0.4 ≦ x ≦ 1.2 and 0 ≦ δ ≦ 0.5), or these composite oxides may include Al, Mn, Fe, Ni, Co, A compound containing at least one element selected from Cr, Ti, and Zn, or a non-metallic element such as P and B can be used. Further, preferably a positive electrode active material represented by a composite oxide of lithium and nickel, that is, LixNipM1qM2rO _2-δ (where M1 and M2 are at least selected from Al, Mn, Fe, Ni, Co, Cr, Ti, Zn) It may be a kind of element or a nonmetallic element such as P or B. Furthermore, 0.4 ≦ x ≦ 1.2, 0.8 ≦ p + q + r ≦ 1.2, and 0 ≦ δ ≦ 0.5. Can be used. Examples of the organic compound include conductive polymers such as polyaniline. Furthermore, the above various active materials may be mixed and used regardless of whether they are inorganic compounds or organic compounds.

Further, as a negative electrode material compound, Al, Si, Pb, Sn, Zn, Cd, etc. and lithium alloy, LiFe ₂ O ₃ , WO ₂ , MoO ₂ , SiO, CuO etc. metal oxide, graphite, difficulty Amorphous carbonaceous materials such as graphitizable carbon and graphitizable carbon, crystalline carbon materials such as graphite, lithium nitride such as Li ₃ N, metallic lithium, or a mixture thereof may be used. The body may be used. It is particularly preferable to use an amorphous carbon material.

  Moreover, as a separator of the nonaqueous electrolyte battery according to the present invention, a woven fabric, a non-woven fabric, a synthetic resin microporous membrane, or the like can be used, and a synthetic resin microporous membrane can be particularly preferably used. Among them, polyolefin microporous membranes such as polyethylene and polypropylene microporous membranes, polyethylene and polypropylene microporous membranes combined with aramid and polyimide, or microporous membranes composited with these, thickness, membrane strength, membrane resistance, etc. It is suitably used in terms of

  Furthermore, a separator can also be used by using a solid electrolyte such as a polymer solid electrolyte. Further, a synthetic resin microporous membrane and a polymer solid electrolyte may be used in combination. In this case, a porous polymer solid electrolyte membrane may be used as the polymer solid electrolyte, and the polymer solid electrolyte may further contain an electrolytic solution. However, in this case, since the battery output is reduced, it is preferable that the amount of the solid polymer electrolyte is kept to a minimum.

  The shape of the battery is not particularly limited, and can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a rectangular shape, a long cylindrical shape, a coin shape, a button shape, a sheet shape, and a cylindrical battery.

  Hereinafter, specific embodiments to which the present invention is applied will be described. However, the present invention is not limited to the embodiments, and can be appropriately modified and implemented without departing from the scope of the present invention. .

[Example 1]
FIG. 1 is a schematic cross-sectional view of a prismatic nonaqueous electrolyte secondary battery of the present example. In this rectangular nonaqueous electrolyte secondary battery 1, a positive electrode 3 formed by applying a positive electrode mixture to an aluminum current collector and a negative electrode 4 formed by applying a negative electrode mixture to a copper current collector are interposed via a separator 5. The wound flat flat electrode group 2 and the non-aqueous electrolyte are housed in a battery case 6 and have a width of 34 mm × a height of 50 mm × a thickness of 5.0 mm.

  A battery lid 7 provided with a safety valve 8 is attached to the battery case 6 by laser welding, a negative electrode terminal 9 is connected to the negative electrode 4 via a negative electrode lead 11, and a positive electrode 3 is connected to the battery lid via a positive electrode lead 10. Has been.

The positive electrode plate is composed of 8% by weight of polyvinylidene fluoride as a binder, 6% by weight of acetylene black as a conductive agent, and a positive electrode active material LiCo _0.34 Ni _0.33 Mn which is a lithium-cobalt-nickel-manganese composite oxide. N-methylpyrrolidone is added to a positive electrode mixture obtained by mixing 86% by weight of _0.33 O ₂ to prepare a paste, which is then applied to both sides of a 20 μm thick aluminum foil current collector and dried. It was prepared by.

  A negative electrode plate was prepared by adding 95% by weight of non-graphitizable carbon and 5% by weight of polyvinylidene fluoride to N-methylpyrrolidone to prepare a paste, which was then applied to both sides of a 10 μm thick copper foil current collector and dried. Made by doing.

A polyethylene microporous membrane is used for the separator, and a mixed solvent of propylene carbonate (PC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 3: 2: 5 (volume ratio) is used for the electrolyte. Further, LiPF ₆ was further dissolved to a concentration of 1 mol / L after adjustment, and diethyl sulfate was added to a concentration of 0.02 M and tris (trimethylsilyl) phosphate to a concentration of 0.005 M, respectively.

  Three cells of the nonaqueous electrolyte secondary battery of Example 1 were produced by the above-described configuration and procedure.

[Examples 2-48 and Comparative Examples 1-99]
For the 146 types of batteries of Examples 2 to 48 and Comparative Examples 1 to 99, as shown in Tables 1 to 4, the amount or type of sulfate ester contained in the electrolyte, or Tris phosphate (a phosphate silyl ester) Except for changing the amount of trimethylsilyl), three non-aqueous electrolyte secondary batteries were produced in the same manner as in Example 1.

以上のようにして作製した実施例１〜４８および比較例１〜９９の角形非水電解質二次電池について、２５℃における初期放電容量を測定し、いずれも４７０ｍＡとほぼ同等の放電容量が得られていることを確認した。なお、放電容量は、２５℃において、充電電流９０ｍＡ、充電電圧４．２８Ｖの定電流−定電圧充電で６時間充電した後、すなわち、９０ｍＡの電流値で充電して電圧が４．２８Ｖに達したのち、４．２８Ｖで定電圧充電をおこない、合計で６時間の充電をおこなった後、放電電流９０ｍＡ、終止電圧２．５０Ｖの条件で定電流放電をおこなうことにより測定した。

With respect to the prismatic nonaqueous electrolyte secondary batteries of Examples 1 to 48 and Comparative Examples 1 to 99 manufactured as described above, the initial discharge capacity at 25 ° C. was measured, and a discharge capacity almost equal to 470 mA was obtained in all cases. Confirmed that. The discharge capacity was 25 ° C. After charging for 6 hours with a constant current-constant voltage charge with a charge current of 90 mA and a charge voltage of 4.28 V, that is, with a current value of 90 mA, the voltage reached 4.28 V. Thereafter, constant voltage charging was performed at 4.28 V, charging was performed for a total of 6 hours, and then constant current discharging was performed under conditions of a discharge current of 90 mA and a final voltage of 2.50 V.

  The DC resistance is measured by setting the SOC of the battery to 50% by charging with a current value of 1 CmA (470 mA) and a constant current-constant voltage charge of a charge voltage of 3.78 V for 3 hours, and 10% at 0.2 CmA. The voltage at the time of discharging for 1 second, the voltage at the time of discharging at 0.5 CmA for 10 seconds and the voltage at the time of discharging for 10 seconds at 1 CmA are measured, and the slope of the voltage drop (E) with respect to the discharge current (I) (R = E / I).

  The neglect test was conducted by the following method. After measuring the initial DC resistance Ra, charge the battery for 3 hours with a current value of 1 CmA (470 mA) and a constant current-constant voltage charge of 4.09 V, and set the SOC of the battery to 80%. And stored for 60 days. After cooling to 25 ° C., the battery was discharged in the same manner as in the condition where the initial discharge capacity was confirmed, and then the DC resistance Rb after the test was measured.

  The increase rate of the DC resistance was calculated from the DC resistance Rb before the 45 ° C. standing test and the DC resistance Ra after the 45 ° C. 60 day testing using the following formula.

DC resistance increase rate (%) = (Rb / Ra-1) * 100
Tables 5 to 8 show the DC resistance increase rates of Examples 1 to 48 and Comparative Examples 1 to 99.

表５より、添加剤をリン酸トリス（トリメチルシリル）のみとした比較例２〜比較例７では、未添加の比較例１と同等またはそれ以上の直流抵抗増加率を示した。これは、リン酸トリス（トリメチルシリル）のみの添加では、高温放置時における直流抵抗の増加率を抑制する効果はなく、添加量が増えた場合にむしろ直流抵抗の増加率が大きくなることを意味している。また、硫酸ジエチルのみを添加した比較例８、１５、１８、２１、２４では、やはり未添加の比較例１と同等またはそれ以上の直流抵抗増加率を示した。これは、硫酸ジエチルのみの添加では高温放置時における直流抵抗の増加率を抑制する効果はなく、添加量が増えた場合にむしろ直流抵抗の増加が大きくなることを意味している。しかし、実施例１〜４、５〜８、９〜１２のように、リン酸トリス（トリメチルシリル）と硫酸ジエチルの添加量を最適な範囲に規定することで直流抵抗の上昇率が特異的に小さくなることが確認できた。このように、単独では抵抗上昇率の低下に効果がないものの、それぞれをある濃度範囲で混合することで抵抗上昇率が低下した原因は現時点では不明であるが、それぞれ単独で添加した場合は直流抵抗上昇を抑制する効果と、促進する効果を併せ持っているが、両者を混合したときに直流抵抗上昇を促進する効果を打ち消しあうことでこのような結果が得られたことが考えられる。同様に、表６〜表８でも、ある濃度範囲で混合した場合に直流抵抗の上昇を大きく抑制できる結果が得られた。なお、実施例として記載はしていないが、リン酸シリルエステルとしてリン酸メチルビス（トリメチルシリル）やリン酸エチルビス（トリメチルシリル）等のリン酸シリルエステル化合物を用いても実施例と同様の効果が得られた。しかも、これらの効果は硫酸ジエチルや、エチレングリコール硫酸エステルなどのような炭素数が２の硫酸エステルよりも、１，２−プロパンジオール硫酸エステルや１，２−ペンタンジオール硫酸エステルのような炭素数が３以上の環状硫酸エステルを用いた場合に特に著しいものであった。この原因も現時点では明らかではないが、硫酸エステル中の炭素原子数が３以上となることで、特に安定な混合被膜を形成して抵抗上昇をより効果的に抑制したものと考えられる。

From Table 5, Comparative Example 2 to Comparative Example 7 in which only the additive was trisphosphate (trimethylsilyl) showed a DC resistance increase rate equal to or higher than that of Comparative Example 1 without addition. This means that the addition of tris (trimethylsilyl) phosphate alone has no effect of suppressing the increase rate of DC resistance when left at high temperature, and rather the increase rate of DC resistance increases when the amount added is increased. ing. Further, Comparative Examples 8, 15, 18, 21, and 24 to which only diethyl sulfate was added exhibited a DC resistance increase rate that was equal to or higher than that of Comparative Example 1 that was not added. This means that the addition of diethyl sulfate alone has no effect of suppressing the rate of increase in DC resistance when left at high temperatures, but rather increases the DC resistance when the amount added is increased. However, as in Examples 1 to 4, 5 to 8, and 9 to 12, the rate of increase in DC resistance is specifically small by regulating the addition amounts of tris (trimethylsilyl) phosphate and diethyl sulfate within an optimal range. It was confirmed that Thus, although it is not effective in reducing the resistance increase rate by itself, the cause of the decrease in the resistance increase rate by mixing each in a certain concentration range is unclear at this time. Although it has both the effect of suppressing the increase in resistance and the effect of promoting it, it is considered that such a result was obtained by canceling out the effect of promoting the increase in DC resistance when both were mixed. Similarly, in Tables 6 to 8, a result that the increase in DC resistance can be largely suppressed when mixed in a certain concentration range was obtained. Although not described as an example, the same effect as in the example can be obtained even when a phosphoric acid silyl ester compound such as methyl bis (trimethylsilyl) phosphate or ethyl bis (trimethylsilyl) phosphate is used as the phosphoric acid silyl ester. It was. Moreover, these effects are greater than the number of carbon atoms such as 1,2-propanediol sulfate and 1,2-pentanediol sulfate than sulfate esters having 2 carbon atoms such as diethyl sulfate and ethylene glycol sulfate. Was particularly remarkable when three or more cyclic sulfates were used. The cause of this is also not clear at the present time, but it is considered that the increase in resistance is more effectively suppressed by forming a particularly stable mixed film when the number of carbon atoms in the sulfate ester is 3 or more.

  From these results, by increasing the concentration of sulfate ester to 0.02M to 0.2M and the concentration of silyl phosphate ester to 0.005 to 0.04M, the direct current resistance increases with leaving at high temperature. It was found that can be suppressed. Moreover, it turned out that a raise of direct-current resistance can be suppressed remarkably by using C3 or more cyclic | annular sulfate ester. Such an effect cannot be obtained when silyl phosphate ester is added alone or when sulfate ester is added alone, but can be obtained specifically when mixed in a specific concentration range, and output The present invention is particularly suitable for applications such as HEV that require characteristics, and the industrial value of the present invention is very large.

  In the examples and comparative examples, a mixed system of the electrolyte solvent PC: DMC: EMC was described. However, when the ratio of the cyclic carbonate and the chain carbonate was changed, or the type and concentration of the electrolyte salt were changed. In this case, the same tendency was observed when diethyl carbonate or methylpropyl carbonate was used as the chain carbonate, or when a carboxylic acid ester such as ethyl acetate was used. A similar tendency was obtained when ethylene carbonate was used instead. However, when propylene carbonate was used as the cyclic carbonate, the increase in DC resistance was particularly suppressed.

The figure which shows the cross-section of the square battery of the Example and comparative example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Square type nonaqueous electrolyte secondary battery 2 Winding type electrode group 3 Positive electrode 4 Negative electrode 5 Separator 6 Battery case 7 Battery cover 8 Safety valve 9 Positive electrode terminal 10 Positive electrode lead 11 Negative electrode lead

Claims (2)

A nonaqueous electrolyte secondary battery comprising a positive electrode that occludes and releases lithium, a negative electrode that occludes and releases lithium, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte is a 0.02 M to 0.2 M sulfate compound And a non-aqueous electrolyte secondary battery comprising 0.005M to 0.04M phosphoric acid silyl ester compound. The sulfuric acid ester compound contains 0.02 M to 0.2 M cyclic sulfuric acid ester having 3 or more carbon atoms, and the phosphoric acid silyl ester compound contains 0.005 M to 0.04 M tristrimethylsilyl phosphate. The nonaqueous electrolyte secondary battery according to claim 1.

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