JP2008123798A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery capable of preventing sudden decrease in discharge capacity with temperature drop and using graphite powder in a positive electrode material. <P>SOLUTION: The lithium secondary battery has a positive electrode containing graphite powder charging/discharging by storing/releasing electrolyte anions as the positive electrode material, a negative electrode containing lithium metal or a negative electrode material charging discharging by storing/releasing lithium ions, and an organic electrolyte comprising an organic solvent containing a lithium salt, and the lithium salt contains LiPF<SB>6</SB>and LiN(CF<SB>3</SB>SO<SB>2</SB>)<SB>2</SB>and the concentration of LiN(CF<SB>3</SB>SO<SB>2</SB>)<SB>2</SB>to the organic solvent is made 0.2-0.5 mol/L. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery.

  Non-aqueous electrolyte secondary batteries including a positive electrode made of a graphitized carbon material, an electrolyte containing a lithium salt, and a negative electrode made of lithium metal have been known for a long time (for example, Patent Documents 1 and 3). reference). In addition, attempts have been made to improve the charge / discharge cycle life by applying a carbon material capable of inserting and extracting lithium as the negative electrode of the battery (see, for example, Patent Documents 2 and 4). This is because lithium metal is repeatedly dissolved and precipitated by the charge / discharge cycle, and dendrite (dendritic precipitate) is generated and passivated, resulting in a short cycle life.

  The nonaqueous electrolyte secondary battery having such a configuration is assembled in a normal discharge state, and cannot be discharged unless it is charged. Hereinafter, the charge / discharge reaction will be described by taking as an example a case where a graphite material capable of reversibly occluding and releasing lithium is used as the negative electrode.

First, when charging in the first cycle, the anion in the electrolyte is occluded (intercalated) in the positive electrode (graphite material) and the cation (lithium ion) in the negative electrode. Then, a donor type carbon intercalation compound is formed. Thereafter, when discharging is performed, cations and anions stored in both electrodes are released (deintercalation), and the battery voltage decreases. The charge / discharge reaction can be expressed by the following equations 1 and 2.
Positive electrode: (discharge) Cx + A ⁻ = CxA + e ⁻ (charge) Equation 1
Negative electrode: (discharge) Cy + Li ⁺ + e ⁻ = LiCy (charge) Equation 2
Therefore, the positive electrode in this type of secondary battery undergoes a reaction in which the graphite layer compound of the electrolyte anion is reversibly formed by charging and discharging, and the carbon layer compound of the electrolyte cation is reversibly formed by charging and discharging. It can be expressed that each reaction is used.

As such a positive electrode material, the present inventors aim to improve cycle characteristics, and one or more materials selected from graphitizable carbon materials or starting materials or carbon precursors thereof have an average particle diameter of 50 μm or less. After pulverization, these were heat-treated at 1700 ° C. or higher in an inert gas atmosphere to develop graphitized graphite powder (see, for example, Patent Document 5). In addition, in order to improve capacity retention after floating charging at a high temperature of 60 ° C., the electron spin resonance method measured using the X band has a peak derived from carbon that appears in the range of 3200 to 3400 gauss, The relative ratio (ΔH _40K / ΔH _296K ) of the peak half-value width ΔH _40K measured at a temperature of 40 K to the peak half-value width ΔH ₂₉₆ K measured at _{296 K} is 2.6 or more. A graphite powder (see, for example, Patent Document 6) in which the electron density is significantly suppressed was developed.
JP 53-123841 JP 61-7567 JP 61-10882 JP 62-103991 A PCT / JP03 / 12906 PCT / JP2005 / 011720

  However, the discharge capacity of lithium secondary batteries using these graphite powders for the positive electrode has a very high temperature dependency, and there is a problem that the discharge capacity decreases linearly as the temperature decreases, particularly in a temperature range of 25 ° C. or lower.

  The present invention includes a positive electrode containing, as a positive electrode material, graphite powder that is charged / discharged by occluding / releasing electrolyte anions, and a negative electrode material that is charged / discharged by occluding / releasing lithium metal or lithium ions. An object of the present invention is to provide a lithium secondary battery having improved low-temperature characteristics, in a lithium secondary battery comprising a negative electrode to be prepared and an organic electrolytic solution comprising an organic solvent containing a lithium salt.

As lithium salts used in the lithium secondary battery, so _{far, LiPF 6, LiBF 4, LiClO} 4, LiGaCl 4, LiBCl 4, LiAsF 6, LiSbF 6, LiInCl 4, LiSCN, LiBrF 4, LiTaF 6, LiB ( CH ₃ ) ₄ , LiNbF ₆ , LiIO ₃ , LiAlCl ₄ , LiNO ₃ , LiI, LiBr, LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ and Various compounds such as LiB (C ₂ O ₄ ) ₂ have been investigated. Further, for example, a mixture of a plurality of compounds such as a mixture of LiPF ₆ (lithium hexafluorophosphate) and LiN (CF ₃ SO ₂ ) ₂ (lithium bis (trifluoromethanesulfone) imide, hereinafter abbreviated as LiTFSI). Has also been reported (Abstracts from the 41st Battery Discussion Meeting P322).

When the present inventors include LiPF ₆ and LiTFSI as lithium salts, the electrolyte anion reversibly enters between the graphite crystal layers by setting the concentration of LiTFSI to the organic solvent to 0.2 to 0.5 mol / L. In a lithium secondary battery using a positive electrode that is charged and discharged by occlusion, the present inventors have found that the low-temperature characteristics of the above problem can be improved and have completed the present invention.

That is, the lithium secondary battery according to the present invention includes a positive electrode containing, as a positive electrode material, graphite powder that is charged and discharged by performing occlusion / release of electrolyte anions, and a material that occludes / releases lithium metal or lithium ions. A negative electrode containing a negative electrode material that performs charge and discharge, and an organic electrolyte solution comprising an organic solvent containing a lithium salt, wherein the lithium salt contains LiPF ₆ and LiTFSI, and the concentration of LiTFSI relative to the organic solvent is 0 It is characterized by being 2 to 0.5 mol / L.

  According to the present invention, a positive electrode containing, as a positive electrode material, graphite powder that is charged / discharged by occluding / releasing electrolyte anions, and a negative electrode material that is charged / discharged by occluding / releasing lithium metal or lithium ions It is possible to provide a lithium secondary battery having improved low-temperature characteristics in a lithium secondary battery comprising a negative electrode to be prepared and an organic electrolytic solution comprising an organic solvent containing a lithium salt.

  Hereinafter, a preferred embodiment will be described in detail. However, the following are examples for carrying out the present invention, and are not intended to limit the present invention to the following embodiments.

== Configuration of lithium secondary battery of the present invention ==
The lithium secondary battery according to the present invention includes a positive electrode containing graphite powder as a positive electrode material that charges and discharges by occluding and releasing electrolyte anions, and charging and discharging by occluding and releasing lithium metal or lithium ions. A negative electrode containing a negative electrode material and an organic electrolyte solution comprising an organic solvent containing a lithium salt, wherein the lithium salt contains LiPF ₆ and LiN (CF ₃ SO ₂ ) ₂ , and the LiN with respect to the organic solvent The concentration of (CF ₃ SO ₂ ) ₂ is 0.2 to 0.5 mol / L.

  Examples of positive electrode materials for lithium secondary batteries include graphite materials such as various natural graphites, synthetic graphites, and expanded graphites that have been appropriately pulverized, carbonized mesocarbon microbeads, mesophase pitch carbon fibers, A synthetic graphite material obtained by graphitizing a carbon material such as phase-grown carbon fiber, pyrolytic carbon, petroleum coke, pitch coke, and needle coke, or a mixture thereof can be used.

  The positive electrode containing this positive electrode material is kneaded and molded together with a conductive agent and a binder, and is incorporated into the battery as a positive electrode mixture. In addition, although graphite material is originally highly conductive and a conductive agent or the like is considered unnecessary, a conductive agent or the like may be used as necessary in consideration of the use of the battery. As the conductive agent, various graphite materials and carbon black can be used. However, in the case of the organic electrolyte secondary battery according to the present invention, the graphite material functions as a positive electrode. It is preferable to use conductive carbon blacks rather than mixing another type of graphite material.

  As the carbon black used here, any of channel black, oil furnace black, lamp black, thermal black, acetylene black, ketjen black and the like can be used. However, carbon blacks other than acetylene black use a part of petroleum pitch or coal tar pitch as raw materials, so there are cases where many impurities such as sulfur compounds or nitrogen compounds may be mixed. It is preferable to use it afterwards. On the other hand, acetylene black uses only acetylene as a raw material and is produced by a continuous pyrolysis method, so it is difficult for impurities to be mixed in, has a chain structure of particles, has excellent liquid retention, and has low electrical resistance. Therefore, it is preferable as this type of conductive agent.

  The mixing ratio of the conductive agent and the graphite material according to the present invention may be appropriately set according to the use of the battery. As requirements for the finished battery, especially when improvement of quick charge characteristics and heavy load discharge characteristics is mentioned, together with the graphite material according to the present invention, it is possible to conduct electricity within a range where an effect of imparting conductivity can be sufficiently obtained. It is preferable to mix the agent to constitute the positive electrode mixture.

  Moreover, as a binder, since it does not melt | dissolve with respect to electrolyte solution, and is excellent in solvent resistance, fluorine-types, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), etc. Organic polymer compounds such as resin, alkali metal salt or ammonium salt of carboxymethyl cellulose, polyimide resin, polyamide resin, polyacrylic acid and sodium polyacrylate are preferred.

On the other hand, any material can be used for the negative electrode as long as it can electrochemically occlude and release lithium ions. For example, lithium metal, lithium aluminum alloy, graphite material, graphitizable carbon material, non-graphitizable carbon material, niobium pentoxide (Nb ₂ O ₅ ), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), silicon monoxide (SiO), tin monoxide (SnO), a composite oxide of tin and lithium (Li ₂ SnO ₃ ), a composite oxide of lithium, phosphorus and boron (for example, LiP _0.4 B _0.6 O _2.9 ), Etc. may be used. In particular, when carbon materials such as graphite materials, graphitizable carbon materials, and non-graphitizable carbon materials are used for the negative electrode, the potential for inserting and extracting lithium is low, reversibility is high, and the capacity is large. When the present invention is applied, a particularly great effect is exhibited. Examples of the carbon material include graphite materials such as various natural graphites, synthetic graphites, and expanded graphites that have been appropriately pulverized, carbonized mesocarbon microbeads, mesophase pitch-based carbon fibers, and vapor grown carbon. Carbon materials such as fibers, pyrolytic carbon, petroleum coke, pitch coke, and needle coke, and synthetic graphite materials obtained by subjecting these carbon materials to graphitization, or mixtures thereof can be used. The negative electrode is also mixed and molded with the materials exemplified above, a binder, and, if necessary, the conductive agent and the like to form a negative electrode mixture, which is incorporated into the battery. In this case, as the binder and the conductive agent, the materials as exemplified can be used as they are when producing the positive electrode mixture.

As the organic electrolyte, an organic solvent containing a lithium salt is used, and any organic solvent can be used as long as it is used in this type of battery. For example, propylene carbonate (PC), ethylene carbonate ( EC), 1,2-butylene carbonate (BC), γ-butyrolactone (GBL), vinylene carbonate (VC), acetonitrile (AN), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and These derivatives or a mixed solvent thereof can be used. On the other hand, the lithium salt contains LiPF ₆ and LiTFSI, and the concentration of LiTFSI with respect to the organic solvent is 0.2 to 0.5 mol / L.

  The lithium secondary battery to which the present invention is applied by constituting the electrode body in which the positive electrode part and the negative electrode part configured as described above constitute a stacked electrode body through a separator and is disposed in a container together with an organic electrolyte. Is completed.

== Experimental example 1 concerning low temperature characteristic problem peculiar to a conventional lithium secondary battery ==
The low temperature characteristic problem of the lithium secondary battery using graphite powder as the positive electrode will be described in detail based on an example in a 18650 type cell (diameter φ18 mm, height 65 mm).

  FIG. 1 is a cross-sectional view of the 18650 cell described above. In FIG. 1, 11 and 13 are a positive electrode part and a negative electrode part, respectively. In the positive electrode part 11, graphite powder (SFG-44 manufactured by Timcal) as a positive electrode material and carboxymethylcellulose (Daiichi Kogyo Seiyaku Co., Ltd., Cellogen 4H) as a binder are mixed at a weight ratio of 96: 4, After the replacement water was added to form a paste, it was applied to both sides of an aluminum foil having a thickness of 20 μm, dried and rolled, and cut into a width of 54 mm to form a strip-shaped sheet electrode. A part of the sheet electrode is scraped of the mixture perpendicular to the longitudinal direction, and an aluminum positive electrode lead plate 14 is attached to the current collector by ultrasonic welding.

  The negative electrode part 13 is a non-graphitizable carbon material (PIC manufactured by Kureha Chemical Co., Ltd.), which is a negative electrode material, and polyvinylidene fluoride resin (KF # 1100 manufactured by Kureha Chemical Co., Ltd.) in a weight ratio of 90: 10 and mixed with N-methyl-2-pyrrolidinone as a solvent, kneaded into a paste, coated on both sides of a 14 μm thick copper foil, dried and rolled, and cut into a width of 56 mm Sheet electrode. Part of this sheet is scraped of the mixture perpendicularly to the longitudinal direction of the sheet, and a nickel negative electrode lead plate 15 is attached to the current collector by ultrasonic welding.

  The positive electrode portion 11 and the negative electrode portion 13 are wound in a spiral shape via a separator 12. This wound electrode is inserted into a battery case 21 made of stainless steel. The separator 12 was a polyethylene microporous film. The negative electrode lead plate 15 was resistance welded to the center position of the circular bottom surface of the battery case 21. The battery case 21 serves as a negative electrode terminal and a negative electrode case. Reference numeral 23 denotes a polypropylene insulating bottom plate, which has a hole so as to have the same area as a space generated simultaneously with winding.

After the above steps, an electrolytic solution is injected. The electrolyte used was a solution in which LiPF ₆ (lithium hexafluorophosphate) was dissolved at a concentration of 2 mol / L in a solvent in which propylene carbonate (PC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 9. is there.

  Thereafter, an explosion-proof lid element provided with a current interruption mechanism is fitted together with the gasket 25 to seal the battery case 21. The lid element includes a positive electrode terminal plate 26 made of metal, an intermediate pressure-sensitive plate 27, conductive members (28, 29) including a protruding portion 28 and a base portion 29 protruding upward, and an insulating gasket 25. The positive electrode terminal plate 26 and the base portion 29 are formed with gas vent holes, and the conductive member (28, 29) is exposed to the upper surface portion of the base portion 29 and the upper surface portion of the protrusion 28 is exposed to the base portion. The lower surface of the base 29 is exposed on the lower surface side of the battery 29, the gasket 25 is fitted into the inner periphery of the opening of the battery case 21, the base 29 is fitted into the inner periphery of the gasket 25, The intermediate pressure-sensitive plate 27 and the positive terminal plate 26 are laminated, and the conductive members (28, 29) and the intermediate pressure-sensitive plate 27 are connected to each other by the protrusions 28 of the conductive members (28, 29). Both of them are conducted only at the contact portion including the connection portion 30. The tip of the positive electrode lead plate 14 is connected to the base 29 of the conductive member (28, 29), and the opening of the battery case 21 is caulked inward to compress the gasket 25, thereby Case 21 is sealed with the lid element. When the inside of the battery case 21 reaches a predetermined internal pressure, the intermediate pressure-sensitive plate 27 is activated, and the outside of the battery member 21 bulges outward, so that the periphery of the connection portion 30 of the protrusion 28 of the conductive member (28, 29). By being broken, the conductive path between the positive lead plate 14 and the positive terminal plate 26 is cut off.

  A charge / discharge test was performed on the 18650 type cell thus fabricated in a temperature range of 60 to 0 ° C. The charging / discharging method is as follows: a battery is placed in a thermostat kept at a predetermined temperature, and a constant current / constant voltage charging is performed with a current of 1 A, a voltage of 4.2 V, and a time of 30 minutes. After resting, the battery was discharged at a constant current of 0.5 A until the cell voltage reached 2.0 V, and the discharge capacity was measured. FIG. 2 shows the relationship between temperature and discharge capacity, and FIG. 3 shows the discharge curve obtained at each temperature.

  From FIG. 2, the discharge capacity obtained at a temperature lower than 25 ° C. as a boundary line decreases linearly and rapidly as the temperature decreases, and the discharge capacity obtained at 0 ° C. is obtained at 25 ° C. It decreased to 40% or less of the capacity. This result suggests that it cannot be used practically in a temperature atmosphere of 0 ° C., and means that the lower limit of the operating temperature range is greatly limited. Further, from FIG. 3, in the discharge curve obtained at a temperature of 20 ° C. or lower, a step portion is clearly recognized at the initial stage of discharge, and the voltage at which this step portion appears shifts to the low voltage side as the temperature decreases, The discharge capacity is decreasing rapidly. As described above, since there is a correlation between the behavior of the step appearing in the discharge curve and the decrease in the discharge capacity accompanying the decrease in temperature, the problem that the discharge capacity is decreased even at least at 20 ° C. has already occurred. Can be grasped.

== Experimental example 2 concerning low temperature characteristic problem peculiar to a conventional lithium secondary battery ==
Next, 18650 type cells A to E having the same specifications as described above were made on a trial basis and charged and discharged at 25 ° C. and 0 ° C. The charge / discharge method is a constant current / constant voltage charge in which a battery is installed in a thermostatic chamber maintained at a constant temperature, the current is 1 A, the voltage is 4.2 V, and the time is 30 minutes. After a pause, the battery was discharged at a constant current of 0.5 A or 0.1 A until the cell voltage reached 2.0 V, and the discharge capacity was measured. When the discharge and charge temperatures differed, the discharge temperature was set immediately in the thermostat after the charge was completed, and the battery temperature was adjusted to the discharge temperature during the 2-hour rest period.

The charge / discharge method (temperature, discharge current) of each cell and the obtained charge / discharge capacity are shown in Table 1 below.

  Moreover, the discharge curve of each cell is shown in FIG. Cells A and E which were discharged at 25 ° C. were almost the same regardless of the charging temperature, and were about 64 mAh. Similarly, the cells B, C and D which were discharged at 0 ° C. did not depend on the charging temperature and had almost the same discharge capacity, which was about 25 mAh. This capacity is about 39% of the capacity obtained when discharged at 25 ° C. It can be seen that the discharge capacity is remarkably reduced by discharging at 0 ° C.

  Cells C and D are both discharged at 0 ° C., but their current values are different. Cell C is 500 mA and cell D is 100 mA, but there is no change in the discharge capacity even when the current value is reduced. That is, it is understood that the cause of the decrease in the discharge capacity at 0 ° C. is not the increase in polarization caused by the large discharge current but the decrease in the capacity that can be discharged potentially. Cell E is charged at 0 ° C. and discharged at 25 ° C., but the discharge capacity is the same as cell A charged at 25 ° C. That is, at 0 ° C., it means that the battery can be charged by a capacity capable of obtaining about 64 mAh when discharged at 25 ° C. From the above phenomenon, it is understood that this type of lithium secondary battery can be charged at 0 ° C., but the capacity that can be discharged is decreased, so that the discharge capacity is reduced as compared with the case of 25 ° C. be able to.

== Experimental example 3 concerning low temperature characteristic problem peculiar to a conventional lithium secondary battery ==
In order to consider the reason why the discharge capacity decreases at low temperatures, a three-pole type 18650 type test cell was fabricated and the behavior of positive and negative electrodes was observed. FIG. 5 shows a cross-sectional view of a three-pole 18650 type test cell and a method for measuring positive and negative electrode potentials (schematic diagram). In this cell, a positive electrode portion 48 formed in a band shape and a negative electrode portion 49 also formed in a band shape are wound in a spiral shape via a separator 50, and the wound electrode body is stored in a stainless steel container 41. It has a structured. The container 41 is formed by cutting a cylindrical hole having a diameter of 18 mm and a depth of 65 mm in a cylindrical stainless steel block, and the electrode winding body is accommodated in the hole. A stainless steel lid 42 is fixed to the container 41 with screws 43 through two ethylene propylene rubber (EPDM) packings 51 at the opening of the container, and between the two rubber packings 51 is made of aluminum. Positive electrode lead plate 44 and nickel negative electrode lead plate 45 are taken out of the cell. The reference electrode uses a belt-like nickel substrate 52 with a belt-like lithium metal foil 53 attached to both sides thereof, is inserted into a cylindrical space formed by winding, and the nickel substrate part has two rubber packings 51. It is taken out of the cell from between.

  In the method for producing the cell, the positive electrode portion 48 and the negative electrode portion 49 are spirally wound via a polyethylene porous film separator 50 and inserted into the stainless steel container 41. After insertion, inject electrolyte. After injection of the electrolyte, the reference electrode is inserted into the cylindrical space generated by winding, and the nickel substrate portion 52 of the reference electrode together with the positive electrode lead plate 44 and the negative electrode lead plate 45 is taken out between the two rubber packings 51. The lid 42 is fixed with screws 43. In addition, the preparation methods of the positive electrode part and the negative electrode part, and the composition of the electrolytic solution are the same as those of the 18650 type cell described above.

  As shown in FIG. 5, the positive electrode potential is the voltage between the positive electrode lead plate 44 and the nickel lead portion 52 of the reference electrode, the negative electrode potential is the voltage between the negative electrode lead plate 45 and the nickel lead portion 52 of the reference electrode, The cell voltage is a voltage between the positive electrode lead plate 44 and the negative electrode lead plate 45.

  The completed 18650 type test cell was subjected to the same charge / discharge test as that of the 18650 type cell described above, and the positive / negative electrode potential was measured. The charging / discharging method is performed by performing constant current / constant voltage charging in which a battery is installed in a constant temperature bath maintained at a constant temperature, current is 1 A, voltage is 4.2 V, and time is 30 minutes. After resting for a minute, the battery was discharged at a constant current of 0.5 A until the cell voltage reached 2.0 V, and the discharge capacity was measured. The measurement temperatures were 25 ° C. and 0 ° C., and the obtained discharge curve is shown in FIG. At the end of discharge at 0 ° C., it is the positive electrode potential that rapidly changes in potential. That is, when discharging at 0 ° C., it can be grasped that the capacity decrease of the positive electrode is rate limiting, the cell voltage decreases, and the discharge capacity decreases. A phenomenon that the positive electrode potential when discharged at 25 ° C. rapidly decreases at the time when the discharge capacity is 30 mAh as in the case of 0 ° C. is not recognized.

== Experimental example for confirming the effect of the present invention ==
(1) Preparation of test cell A 2 wt% aqueous solution of graphite powder (SFG-44 manufactured by Timcal) and CMC (carboxymethylcellulose) (Daiichi Kogyo Seiyaku Co., Ltd., Cellogen 4H) is 97: 3 by weight. Then, distilled water was added to obtain a slurry. The CMC ratio in the weight ratio is a solid content ratio. The obtained slurry was applied to one side of an aluminum foil (thickness 20 μm) by a doctor blade method so that the amount of graphite material per unit area was about 12 mg / cm ² and dried at 60 ° C. for 20 minutes. A sheet electrode was prepared. Thereafter, the sheet was sandwiched between die sets, and the entire sheet was compressed and molded with a press so that the apparent density of the positive electrode mixture was about 1.0 g / cm ³ . The obtained sheet electrode was punched to 9 mm by a punching press, and used as a working electrode of a test cell.

  FIG. 7 shows a cross-sectional view of the test cell. This test cell is a three-electrode type in which a working electrode 64 and a counter electrode 66 are pressed by a spring 68 between a pair of upper and lower stainless steel fixing plates 80a and 80b, and the lower working electrode 64 has a diameter of 9 mm. The punched sheet electrode 64 was made of lithium metal for the upper counter electrode 66 and the reference electrode 70. The sheet electrode 64 is 120 ° C., the parafilm 72 is 45 ° C., and other resin parts and metal parts are dried at 60 ° C. under reduced pressure for 10 hours or more, and in a dry air atmosphere with a dew point of −40 ° C. or less. Assembled. A polypropylene insertion block 82 is interposed between the stainless steel fixing plates 80a and 80b, and is fastened and fixed by bolts and nuts 84 and 86. Stainless steel fixing plates 80a and 80b are attached to the upper and lower surfaces of the insertion block 82, respectively. A parafilm 72 is interposed between the two.

  As the separator 74 interposed between the working electrode 64 and the counter electrode 66, two 50 μm thick polyethylene microporous films 74 (porosity 67%) are used in an overlapping manner, and a lithium metal 70 serving as a reference electrode is interposed between the counter electrodes. 66 and the working electrode 64 are inserted so as not to contact each other. The reference electrode 70 is fixed to the insertion block 82 with fixing bolts 88. The sheet electrode 64 and the separator 74 were each placed in a container filled with an electrolytic solution and impregnated under reduced pressure, and then incorporated into a test cell.

  An aluminum plate 90, a parafilm 72, and a polypropylene plate 92 are interposed from the upper side between the lower working electrode 64 and the stainless steel fixing plate 80a. A stainless disk 94 is placed on the upper surface of the upper counter electrode 66, and a spring 68 is interposed between the stainless disk 94 and the upper fixing plate 80b.

(2) Measurement of charge / discharge cycle and low-temperature characteristics The electrolyte used in each test cell was lithium having a predetermined concentration shown in FIG. 8 in a solvent in which propylene carbonate and ethylmethyl carbonate were mixed at a volume ratio of 1: 9. Prepared by dissolving the salt. The abbreviations of the lithium salt shown in FIG. 8 are as shown in Table 2 below, and the unit of the lithium salt concentration is “mol / L” as “M”.

The charge / discharge cycle was measured after first injecting a predetermined electrolyte into the test cell and first in a constant temperature room at 25 ° C. in the atmosphere. The charge / discharge conditions of the first cycle are set to a current value of 30 mA / g in terms of graphite weight of the working electrode 64, charged until it reaches 20 mAh / g in terms of graphite weight, and rested for 1 minute. Thereafter, discharging was performed with the same current until the potential of the working electrode 64 was 3.0 (V vs Li ⁺ / Li) with respect to the reference electrode 10. Next, at the same current density of 30 mA / g, charging is performed until the working electrode 64 becomes 15 mAh / g in terms of graphite weight. After a pause of 1 minute, the potential of the working electrode 64 with respect to the reference electrode 10 with the same current is The charge / discharge cycle of discharging until 3.0 (V vs Li ⁺ / Li) was repeated 9 times. The number of charge / discharge cycles for the ninth time is the 10th cycle when recounted from the beginning, and the discharge capacity obtained in the 10th cycle is A.

Next, the temperature of the thermostatic bath was set to 0 ° C., and 5 hours after reaching 0 ° C., the 11th charge / discharge cycle was performed. The charging / discharging method is the same as the charging / discharging method performed in the 2nd to 10th cycles. The discharge capacity obtained in the eleventh cycle was defined as B, and the discharge capacity retention rate at 0 ° C. was calculated from the following formula 3.
[Discharge capacity maintenance rate at 0 ° C.] = B / A × 100 (%) Formula 3

(3) Test results of charge / discharge and low temperature characteristics of test cell FIG. 8 shows the lithium salt dissolved in the electrolyte, the discharge capacity at the 10th cycle (discharge capacity at 25 ° C.), and the end of charge at the 10th cycle. The potential of the working electrode that has reached the time point (potential of the working electrode at 25 ° C.), the discharge capacity of the eleventh cycle (discharge capacity at 0 ° C.), and the potential of the working electrode that has reached the end of charge in the eleventh cycle (The potential of the working electrode at 0 ° C.) and the discharge capacity maintenance rate at 0 ° C. are summarized.

  The electrolytic solution shown in Comparative Example 1 in FIG. 8 has the same composition as the electrolytic solution used in the aforementioned 18650 type cell. In the case of the 18650 type cell, the ratio of the discharge capacity obtained at 0 ° C. to the discharge capacity obtained at 25 ° C. can be calculated as 39% by comparing the cells A and C in Table 1. In this example, since the discharge capacity maintenance rate of Comparative Example 1 was 42%, it was considered that the same value as the discharge capacity maintenance rate of the battery was obtained, and the discharge capacity maintenance obtained by the test cell was obtained. It can be determined that the rate and the discharge capacity maintenance rate in the case of a battery are almost the same value.

9 and 10 show the measurement results of charge and discharge cycles at 25 ° C. and 0 ° C. in an electrolyte solution in which LiPF ₆ and LiTFSI are dissolved at various ratios so that the lithium ion concentration in the electrolyte solution is 2 mol / L. Show. At 25 ° C. (FIG. 9), it was found that any electrolyte solution showed a high discharge capacity. On the other hand, at 0 ° C. (FIG. 10), when LiTFSI was added to the electrolytic solution, it became clear that the discharge capacity significantly increased as the concentration of LiTFSI increased. Specifically, when LiPF ₆ was dissolved alone, the ratio of the discharge capacity obtained at 0 ° C. to the discharge capacity obtained at 25 ° C. was 42% (detailed numerical values are 8), when LiTFSI was dissolved at a concentration of 0.2 mol / L or more, the ratio of the discharge capacity obtained at 0 ° C. to the discharge capacity obtained at 25 ° C. was 70% or more ( For detailed numerical values, see FIG. 8 and Examples 1 to 3).

  In addition, when the density | concentration of LiTFSI was 0.5 mol / L, since positive electrode collectors, such as aluminum, melt | dissolved remarkably, it turned out that it is not preferable.

Therefore, the capacity retention rate is highest when an electrolytic solution in which 1.5 mol / L LiPF ₆ and 0.5 mol / L LiTFSI are dissolved is used, and the capacity retention rate is as high as 99.5%. (Fig. 8, Example 3).

Then, instead of LiTFSI, TFSI ^- had similar chemical structure as, BETI ^- anion _{(N (CF 3 SO 2)} 2) - anion _{(N (C 2 F 5 SO} 2) 2 -) and Methid ^- An electrolytic solution in which a lithium salt of an anion (C (CF ₃ SO ₂ ) ₃ ⁻ ) is dissolved at 0.5 mol / L, that is, 1.5 mol / L LiPF ₆ and various other lithium salts of 0.5 mol / L The charge / discharge cycle at 25 ° C. and 0 ° C. was measured. The results are shown in FIGS.

In both cases of LiBETI and LiMetheid, the ratio of the discharge capacity obtained at 0 ° C. to the discharge capacity obtained at 25 ° C. is as low as 38 to 41% (see FIG. 8 and Comparative Examples 4 and 5 for detailed numerical values). ), No improvement effect of low temperature characteristics such as LiTFSI was observed. Therefore, simply dissolving a salt other than LiPF ₆ does not improve the discharge capacity maintenance rate at 0 ° C., but dissolving LiTFSI at a predetermined concentration or more together with LiPF ₆ is indispensable for improving low-temperature characteristics. It became clear that there was.

Further, as a factor that increases the capacity retention rate when using an electrolytic solution in which 1.5 mol / L LiPF ₆ and 0.5 mol / L LiTFSI are dissolved, FIG. 8 Comparative Example 1 (2 mol / L LiPF ₆ ) It can also be considered that LiPF ₆ has a low concentration compared to the above. However, as shown in FIGS. 11 and 12, in the electrolytic solution in which only 1.5 mol / L LiPF ₆ was dissolved, the discharge capacity retention rate at 0 ° C. was almost the same as in the case of 2 mol / L, which was 45.2. (For detailed numerical values, see FIG. 8 and Comparative Example 2).

From this result, the reason why the capacity retention rate was high when using an electrolytic solution in which 1.5 mol / L LiPF ₆ and 0.5 mol / L LiTFSI were dissolved was not that the salt concentration of LiPF ₆ was lowered. Proved.

The electrolytic solution in which 1.5 mol / L LiPF ₆ and 0.5 mol / L LiTFSI which showed good results as described above were dissolved was compared with FIG. 8 Comparative Example 1 using only 2 mol / L LiPF ₆ . Thus, there was no significant change in the potential at the end of charging at 25 ° C. (V ₂₅ ), the potential at the end of charging at 0 ° C. (V ₀ ), and the difference (V ₀ −V ₂₅ ). Therefore, it has been clarified that by dissolving LiTFSI, only the low temperature characteristics are improved without affecting other characteristics.

Next, charging and discharging at 25 ° C. and 0 ° C. when using an electrolytic solution in which only 1 mol / L LiPF ₆ is dissolved and an electrolytic solution in which 1 mol / L LiPF ₆ and various concentrations of LiTFSI are dissolved are used. The curves are shown in FIGS.

From FIG. 13, a large difference was not recognized in the charge / discharge curve at 25 ° C. However, the curve at 0 ° C. varied greatly depending on the concentration of LiTFSI coexisting with 1 mol / L LiPF ₆ . This trend is similar to that observed in FIGS. When the electrolyte solute is only 1 mol / L LiPF ₆ , the ratio of the discharge capacity at 0 ° C. to the discharge capacity at 25 ° C. is 49%, and the potentially dischargeable capacity at 0 ° C. decreases. Therefore, it is not preferable. On the other hand, when an electrolyte solution in which LiTFSI is dissolved at a concentration of 0.2 mol / L or more is used, the ratio of the discharge capacity at 0 ° C. to the discharge capacity at 25 ° C. is as high as 79% or more, and LiTFSI is It has been found that there is a significant increase in potential discharge compared to using undissolved electrolyte.

As described above in detail, LiPF ₆ and LiTFSI are used as the electrolyte of a lithium secondary battery including a positive electrode made of graphite powder that is charged and discharged by occluding and releasing electrolyte anions, and the concentration of LiTFSI is 0. In the case of 2 to 0.5 mol / L, the low temperature characteristic peculiar to this type of battery, that is, the problem that the dischargeable capacity of the battery potentially decreases at a low temperature can be improved. It was revealed.

  A positive electrode containing, as a positive electrode material, graphite powder that is charged and discharged by occluding and releasing electrolyte anions, and a negative electrode containing a negative electrode material that is charged and discharged by occluding and releasing lithium metal or lithium ions; It is possible to provide a lithium secondary battery having improved low-temperature characteristics in a lithium secondary battery including an organic electrolyte solution made of an organic solvent containing a lithium salt.

The schematic diagram of the lithium secondary battery by a 18650 type cell is shown. The temperature dependence of the discharge capacity is shown. The discharge curve at each temperature is shown. The discharge curve in each temperature at the time of charge temperature change is shown. The schematic diagram of the lithium secondary battery by a 3 pole-type 18650 type | mold cell is shown. The discharge curve in each temperature of a 3 pole type 18650 type cell is shown. The schematic diagram of the test cell in one Example is shown. The solute and density | concentration of the electrolyte solution in one Example, and the charging / discharging test result in 25 degreeC and 0 degreeC are shown. The charging / discharging curve at 25 degreeC of the electrolyte solution containing LiTFSI in one Example is shown. The charging / discharging curve in 0 degreeC of the electrolyte solution containing LiTFSI in one Example is shown. The charging / discharging curve in 25 degreeC of the electrolyte solution containing the lithium salt in one Example is shown. The charging / discharging curve at 0 degreeC of the electrolyte solution containing the lithium salt in one Example is shown. The charging / discharging curve in 25 degreeC of the electrolyte solution which has various lithium salt density | concentration in one Example is shown. The charging / discharging curve in 0 degreeC of the electrolyte solution which has various lithium salt density | concentration in one Example is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Positive electrode part 12 Separator 13 Negative electrode part 14 Positive electrode lead board 15 Negative electrode lead board 21 Battery case 23 Insulation bottom board 25 Gasket 26 Positive electrode terminal board 27 Intermediate pressure-sensitive plate 28 Protrusion 29 Base 30 Connection part 41 Container 42 Cover 43 Screw 44 Positive electrode lead board 45 Negative electrode lead plate 48 Positive electrode portion 49 Negative electrode portion 50 Separator 51 Rubber packing 52 Nickel substrate 53 Banded lithium metal foil 64 Working electrode 66 Counter electrode 68 Spring 70 Reference electrode 72 Parafilm 74 Separator 80 (80a, 80b) Stainless steel fixing plate 82 Insertion Block 84 Bolt 86 Nut 88 Fixing bolt 90 Aluminum plate 92 Polypropylene plate 94 Stainless disc

Claims (1)

A positive electrode containing, as a positive electrode material, graphite powder that is charged and discharged by occluding and releasing electrolyte anions;
A negative electrode containing a negative electrode material that is charged and discharged by a material that absorbs and releases lithium metal or lithium ions;
A lithium secondary battery comprising an organic electrolyte solution made of an organic solvent containing a lithium salt,
The lithium salt includes LiPF ₆ and LiN (CF ₃ SO ₂ ) ₂ ,
The concentration of the LiN (CF ₃ SO ₂ ) ₂ with respect to the organic solvent is 0.2 to 0.5 mol / L,
Lithium secondary battery.