JP2009230899A - Nonaqueous electrolyte secondary battery, and manufacturing method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excellent in charge and discharge characteristics, wherein normal temperature molten salt is contained in a nonaqueous electrolyte. <P>SOLUTION: In this nonaqueous electrolyte secondary battery equipped with a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a nonaqueous electrolyte containing room temperature molten salt, after injecting the nonaqueous electrolyte, the potential of the negative electrode is swept to a potential by which the cations of the normal temperature molten salt is reduced and decomposed by potential scanning before first time charging. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium secondary battery and a method for producing the same.

  In recent years, the reduction in size and weight of portable electric devices has been remarkably progressing, and the power consumption has increased with the increase in functionality. For this reason, there is a strong demand for lighter weight and higher capacity even for nonaqueous electrolyte secondary batteries used as a power source. Moreover, with the improvement of the energy density of a battery, the request | requirement of the safety improvement with respect to a battery is also increasing.

  For electrolytes of nonaqueous electrolyte secondary batteries, an organic electrolyte solution in which a lithium salt is dissolved in an organic solvent is generally used. Since these batteries contain an organic solvent, the vapor pressure is high at high temperatures. Since many organic solvents are flammable, it is more important to consider safety.

  For this reason, attention has been focused on a method for improving the safety by using a non-volatile room temperature molten salt instead of a volatile organic solvent to make the non-aqueous electrolyte incombustible.

  The room temperature molten salt is a salt composed of a cation and an anion and in a liquid state at room temperature (for example, 20 ° C.). This room temperature molten salt has the characteristics of being non-volatile and flame retardant due to its strong ionic bonding.

  However, a non-aqueous electrolyte secondary battery using a room temperature molten salt has a problem that sufficient charge / discharge characteristics cannot be obtained. This is probably because a decomposition reaction of the room temperature molten salt occurs during charging.

  A method is known in which additives such as ethylene carbonate (EC) and vinylene carbonate (VC) are added to room temperature molten salt to reduce the decomposition reaction of room temperature molten salt (Non-patent Document 1). This is probably because a film made of EC, VC or the like is formed on the negative electrode during the first charge, and the cation decomposition of the room temperature molten salt can be suppressed. However, solvent-based additives such as EC and VC are flammable substances, and thus have a problem of impairing the flame retardancy of the non-aqueous electrolyte.

  In Non-Patent Document 2, although the details of the reason are not clear by using a room temperature molten salt containing a bis (fluorosulfonyl) imide anion, good charge and discharge is obtained when graphite is used as a negative electrode active material. It has been reported that cycle characteristics can be obtained.

However, even when a room temperature molten salt containing a bis (fluorosulfonyl) imide anion is used, sufficient charge / discharge cycle characteristics cannot be obtained depending on the type of graphite used, the battery configuration, and the like.
Takaya Sato, Tatsuya Maruo, Shoko Marukane and Kentaro Takagi, Journal of Power Sources, 138,253-261 (2004). M. Ishikawa, T. Sugimoto, M. Kikuta, M. Kono, Journal of Power Sources, 162, 658-662 (2006).

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics and a method for producing the same in a nonaqueous electrolyte secondary battery containing a room temperature molten salt in a nonaqueous electrolyte.

  The present invention provides a first charge after injecting a nonaqueous electrolyte in a nonaqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte containing a room temperature molten salt. It is characterized in that, previously, the potential of the negative electrode is swept below the potential at which the cation of the room temperature molten salt undergoes reductive decomposition by potential scanning.

  According to the present invention, before the first charge, the potential of the negative electrode is swept to a potential lower than that at which the cation of the room temperature molten salt undergoes reductive decomposition by potential scanning, whereby the cation of the room temperature molten salt is reduced and decomposed to form a stable coating on the negative electrode surface. Can be formed. Thereby, decomposition | disassembly of the cation of the normal temperature molten salt by subsequent charge is suppressed, and it can be set as the nonaqueous electrolyte secondary battery excellent in charging / discharging cycling characteristics. Although the details of this reason are not clear, a stable coating that can suppress the decomposition of the cation by forcibly sweeping the negative electrode potential in a direction lower than the decomposition potential of the cation of the room temperature molten salt has a negative electrode surface. It is thought that it is formed.

  As a potential scanning method for sweeping the potential of the negative electrode, a general method such as a cyclic voltammetry method using a potentiostat can be used. Since it is necessary to sweep to below the potential at which the cation of the room temperature molten salt undergoes reductive decomposition, for example, it is preferable to sweep to 0.5 V or less, more preferably 0.0 V, based on metallic lithium.

  The potential sweep rate is preferably in the range of 0.01 mV / sec to 10 mV / sec, and more preferably in the range of 0.1 mV / sec to 2 mV / sec.

The nonaqueous electrolyte in the present invention contains a room temperature molten salt. As the room temperature molten salt used in the present invention, the viscosity of the room temperature molten salt can be reduced by containing a bis (fluorosulfonyl) imide anion ([N (SO ₂ F) ₂ ] ⁻ ) (FSI anion). Therefore, it is preferable.

In addition to the FSI anion, the anion component contained in the room temperature molten salt may be, for example, BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , ClO ₄ , NO ₃ ⁻ , CF ₃ SO ₃ ⁻ , [N (C ₁ F _{2l + 1 SO 2) (C} m F 2m + 1 SO 2) ] ^- (l, m is an integer of 1 or more), _{[C (C p F 2p + 1} SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2 )] ^— (P, q, r are integers of 1 or more), CF ₃ CO ₂ ^— , CH ₃ CO ₂ ^{— and the} like may be included, and these anions may include two or more.

  As a cation contained in the room temperature molten salt, any of N, P, S, O, Cl, Si, or two or more kinds of elements is included in the structure, and a chain or a ring such as a 5-membered ring or a 6-membered ring is included. Examples include compounds having a structure in the skeleton.

  Examples of cyclic structures such as 5-membered rings and 6-membered rings include furan, thiophene, pyrrole, pyridine, oxazole, isoxazole, thiazole, isothiazole, furazane, imidazole, pyrazole, pyrazine, pyrimidine, pyridazine, pyrrolidine, piperidine, etc. And heterocyclic heterocyclic compounds such as benzofuran, isobenzofuran, indole, isoindole, indolizine, and carbazole.

  Among these cations, a chain or cyclic compound containing a nitrogen element is particularly preferable because it is chemically and electrochemically stable.

  Examples of the cation containing nitrogen include alkylammonium such as triethylammonium, imidazolium such as ethylmethylimidazolium and butylmethylimidazolium, pyrrolidinium such as 1-methyl-1-propylpyrrolidinium, 1-methyl-1 Preferred examples include piperidinium such as -propylpiperidinium. Of these, ethylmethylimidazolium has a low viscosity and is particularly preferable.

  The melting point of the room temperature molten salt is preferably 20 ° C. or less, and more preferably 0 ° C. or less.

  The non-aqueous electrolyte in the present invention usually contains a lithium salt used for a non-aqueous electrolyte secondary battery.

These lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (FSO ₂ ) ₂ , LiN (C ₁ F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (l , m is an integer of 1 or _{more), LiC (C p F 2p} + 1 SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r is an integer of 1 or more), Li [B (C ₂ O ₄ ) ₂ ] (Bis (oxalate) lithium borate (LiBOB)), Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ] and the like, and these lithium salts may be used alone or in combination of two or more. Among these, bis (fluorosulfonyl) imidolithium (LiN (FSO ₂ ) ₂ ) (LiFSI) is preferable because it can reduce the viscosity of the nonaqueous electrolyte, and Li [B (C ₂ O ₄ ) ₂ ] (bis (Oxalate) lithium borate (LiBOB)), Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ] Oxalate compounds such as can form a stable coating on the surface of the negative electrode, and are preferable from the viewpoint of inhibiting cation decomposition of a room temperature molten salt.

  These lithium salts are used after being dissolved at a concentration of 0.1 to 2.0 mol / liter, preferably 0.2 to 1.0 mol / liter, with respect to the nonaqueous electrolyte.

  In addition, a surfactant such as trioctyl phosphate may be added to the nonaqueous electrolyte in the present invention in order to improve the wettability of the nonaqueous electrolyte with respect to the separator.

  The negative electrode active material used in the present invention is not particularly limited as long as it is a material capable of inserting and extracting lithium, and examples thereof include lithium metal, lithium alloy, carbonaceous material, and metal compound. These negative electrode active materials may be used alone or in combination of two or more.

  Examples of the lithium alloy include a lithium aluminum alloy, a lithium silicon alloy, a lithium tin alloy, and a lithium magnesium alloy.

  Examples of the carbonaceous material that occludes / releases lithium include natural graphite, artificial graphite, coke, vapor grown carbon fiber, mesophase pitch carbon fiber, spherical carbon, and resin-fired carbon.

  Alternatively, a carbon material having a multilayer structure in which a part or all of the surface of the first carbon material having high crystallinity as a core material is coated with a second carbon material having lower crystallinity than the first carbon material may be used. Is possible. By using such a multi-layer structure carbon material, the decomposition of the cation of the room temperature molten salt can be suppressed, which is more preferably used.

  Here, when covering a part or all of the surface of the first carbon material having high crystallinity as the core material with the second carbon material having lower crystallinity than the first carbon material, as described above, Examples thereof include a method in which the first carbon material to be mixed with a carbonizable organic compound is baked, a CVD method and the like. As a method of firing by mixing with an organic compound, the first carbon material as the core material is made of methanol, such as pitch form or tar, phenol formaldehyde resin, furfuryl alcohol resin, carbon black, vinylidene chloride, cellulose, etc. After being immersed in a solution dissolved in an organic solvent such as ethanol, benzene, acetone or toluene, and then carbonized at 500 to 1800 ° C., preferably 700 to 1400 ° C. in an inert atmosphere. Can do.

  Moreover, as a CVD method, it can manufacture by introduce | transducing an organic compound vapor | steam to the 1st carbon material used as a core material on high temperature conditions for a fixed time. As the organic compound used in this CVD method, hydrocarbons such as methane, ethane, propane, butane, ethylene, propylene, butene, benzene, toluene, ethylbenzene, cyclohexane, cyclopentene, or derivatives thereof can be used. After heating and evaporating these organic compounds, they can be produced by sending them into a reaction vessel containing a carbon material as a core material using nitrogen or an inert gas as a carrier. In addition, the processing temperature of the carbon material used as the core material at this time is preferably 500 to 1800 ° C, and more preferably 700 to 1400 ° C.

A graphite-based material is preferably used as the carbon material, and the (002) plane spacing (d ₀₀₂ ) determined by X-ray diffraction is in the range of 0.335 to 0.338 nm, and the crystallite in the c-axis direction. When the size (Lc) is 30 nm or more, a battery having a high discharge capacity can be produced. Preferably, d ₀₀₂ is in the range of 0.335 to 0.336 nm and Lc is 100 nm or more. More preferred.

As for the carbon material, since the peak intensity in the vicinity of 1580 cm ^-1 as determined by laser Raman spectroscopy using an argon ion laser having a wavelength of 514.5nm and _{(I G)} 1360cm ^-1 vicinity of the peak intensity and _{(I D)} It is preferable that the calculated R value (I _D / I _G ) is in the range of 0.05 to 0.8.

  The negative electrode active material is kneaded with a binder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), styrene butadiene rubber or the like according to a conventional method, and used as a mixture for the negative electrode.

As the positive electrode active material used in the present invention, those conventionally used for non-aqueous electrolyte secondary batteries can be used. For example, lithium cobalt oxide (Li _X CoO ₂ ), lithium nickel oxide (Li _X NiO ₂ ), lithium nickel cobalt manganese composite oxide (Li _X Ni _a Co _b Mn _c O ₂ ), lithium manganese oxide (Li _X Mn ₂ O _4), lithium iron phosphate _(Li X FePO ₄₎ or the like can be used. This is kneaded with a conductive agent such as acetylene black or carbon black, and a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVdF), and used as a mixture.

  The non-aqueous electrolyte secondary battery of the present invention generally includes a separator, a battery case as an outer package, and the like in addition to the positive electrode, the negative electrode, and the non-aqueous electrolyte. These are not particularly limited, and for example, conventionally known ones can be used.

  The method for producing a non-aqueous electrolyte secondary battery of the present invention is a method capable of producing the non-aqueous electrolyte secondary battery of the present invention, a step of preparing a positive electrode, a negative electrode, and a non-aqueous electrolyte, A step of storing the negative electrode in the outer package and injecting a non-aqueous electrolyte into the outer package; and a step of sweeping the potential of the negative electrode below the potential at which the cation of the room temperature molten salt is reduced and decomposed by potential scanning before the first charge and discharge; It is characterized by having.

  According to the manufacturing method of the present invention, the nonaqueous electrolyte secondary battery of the present invention can be efficiently manufactured.

  ADVANTAGE OF THE INVENTION According to this invention, it can be set as the nonaqueous electrolyte secondary battery excellent in the charge / discharge characteristic in the nonaqueous electrolyte secondary battery which contains normal temperature molten salt in a nonaqueous electrolyte.

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to the following examples in any way, and can be appropriately modified and implemented without departing from the scope of the present invention. Is.

Example 1
(Production of working electrode)
Using graphite powder (d ₀₀₂ = 0.336 nm, Lc> 100 nm, R = 0.08) as a negative electrode active material, 97.5 parts by weight of this carbon material, 1 part by weight of styrene butadiene rubber (SBR), carboxymethylcellulose ( CMC) is mixed at a ratio of 1.5 parts by weight, and then water is added to the mixture to form a slurry. The slurry is applied on one side of a current collector made of copper foil and dried. Then, it was rolled and cut into a disk shape having a diameter of 20 mm to produce a working electrode serving as a negative electrode.

[Production of counter electrode]
A counter electrode was produced by punching a 20 mm diameter disc from a lithium rolled plate having a predetermined thickness.

(Preparation of electrolyte)
A room temperature molten salt (EMI-FSI) containing ethylmethylimidazolium (EMI) as a cation and bis (fluorosulfonyl) imide (FSI) as an anion is bis (trifluoromethylsulfonyl) at a rate of 0.2 mol / liter. ) Imidolithium (LiTFSI) was dissolved to prepare a non-aqueous electrolyte.

[Production of test batteries]
A flat test battery A1 (battery dimensions: diameter 24.0 mm, thickness 3.0 mm) was produced using the above working electrode, counter electrode and electrolyte. As the separator, a microporous membrane made of polyethylene was used, and this was impregnated with the nonaqueous electrolyte described above.

[Preliminary charge / discharge]
Regarding this test battery A1, pretreatment by potential scanning was performed as follows before the first charge after the nonaqueous electrolyte injection.

  The test battery produced as described above was connected to a potentiostat, swept from the open circuit voltage to 0.0 V on the reduction side at a scanning speed of 0.2 mV / sec, and then oxidized to 2.0 V Swept to the side. The current-voltage curve at this time is shown in FIG.

  FIG. 4 is a schematic diagram showing a potentiostat device that has been pre-processed by potential scanning. As shown in FIG. 4, a power source 2 is connected to a two-electrode cell 1 composed of a working electrode W and a reference electrode (here, a counter electrode) R, and a recorder 3 is connected between the power source 2 and the two-electrode cell 1. ing. A potential scan is performed by applying the same voltage as the open circuit voltage and changing the applied voltage at a constant speed therefrom. Thereby, as shown by an arrow in FIG. 1, the negative electrode potential decreases, and after the negative electrode potential is decreased to 0.0 V, the negative electrode potential is increased to 2.0 V. The peak observed at around 0.6 V seems to be based on the reductive decomposition of the cation of the room temperature molten salt.

(Comparative Example 1)
A test battery X1 of Comparative Example 1 was produced in the same manner as in Example 1 except that no pretreatment by potential scanning was performed.

  Since the evaluation battery is configured to evaluate the charge / discharge characteristics of the negative electrode and the electrolyte, when a current is passed in the direction in which the working electrode is electrochemically discharged, lithium ion is applied to the negative electrode as the working electrode. Is occluded and charged. Further, when a current is passed in the direction in which the working electrode is electrochemically charged, lithium ions are released from the negative electrode and discharged. This evaluation battery is configured with a large excess of metallic lithium in terms of electric capacity, and the characteristics of this evaluation battery can be evaluated as indicating the characteristics of the negative electrode and the electrolyte of the present invention.

(Charge / discharge test)
Next, for each of the test batteries of Example 1 and Comparative Example 1 manufactured as described above, from the counter electrode until the voltage of the working electrode with respect to the counter electrode becomes 0.0 V at a current density of 0.1 mA / cm ² , respectively. Lithium ions were inserted into the carbon material as the working electrode. Then, when lithium ions were inserted into the carbon material of the working electrode in each test battery in this way, the capacity Q1 (mAh / g) in the carbon material of each test battery was obtained, and the result is shown in Table 1 below. It was shown to.

Thereafter, in each of the test batteries, a carbon material in which lithium ions are inserted as described above until the voltage of the working electrode with respect to the counter electrode reaches 1.0 V at a constant current of a current density of 0.1 mA / cm ^2. Lithium ions were desorbed from. When lithium ions are desorbed from the carbon material in each test battery in this way, the capacity Q2 (mAh / g) in each test battery is determined, and the charge / discharge efficiency of lithium ions in the carbon material is The ratio of the capacity Q2 to the capacity Q1 (Q2 / Q1) × 100 was obtained, and the results are shown in the following Table 1, FIG. 2 and FIG.

  As is clear from the results shown in Table 1, the battery A1 of Example 1 that had been subjected to pretreatment by potential scanning showed excellent charge / discharge characteristics compared to the battery X1 of Comparative Example 1 that had not been pretreated. .

FIG. 2 is a graph showing a charge / discharge curve of the battery A1 of Example 1. Charging is performed with a current of 0.1 mA / cm ² and a cut-off voltage of 0V. The electric discharge is 0.1 mA / cm ² and the cut-off voltage is 1.0V. As shown in FIG. 2, the negative electrode potential could be set to 0 V during charging, and discharging was possible. Therefore, in the battery A1 of Example 1, it was possible to insert and desorb lithium ions from the graphite negative electrode.

  FIG. 3 is a diagram illustrating a charge / discharge curve of the battery X1 of Comparative Example 1. As shown in FIG. 3, the negative electrode potential could not be set to 0 V during charging. This is probably because cation decomposition of the room temperature molten salt occurred and lithium ions could not be inserted into the graphite negative electrode.

  As described above, in the battery A1 of Example 1, as shown in FIG. 1, a reduction current considered to be a decomposition of the cation of the room temperature molten salt is observed from around 0.8V. After that, by forcibly sweeping the potential to 0 V, a film capable of transmitting lithium ions is formed on the surface of the negative electrode, and by the presence of this film, decomposition of the cation of the room temperature molten salt is suppressed, whereby the graphite negative electrode It is considered that lithium ion insertion / extraction from / to can be performed.

In the above embodiment, evaluation is performed using a two-electrode cell for evaluating the negative electrode and the electrolyte. However, the present invention is generally used for non-aqueous electrolyte secondary batteries such as lithium cobalt oxide (LiCoO ₂ ). Similar effects can be obtained even when a positive electrode active material is used.

  FIG. 5 shows a nonaqueous electrolyte secondary battery using a general positive electrode active material such as a cobalt oxide of lithium, and the potential of the negative electrode is reduced to a potential at which the cation of the room temperature molten salt is reduced and decomposed by forced potential scanning. It is a schematic diagram which shows an example of the apparatus of the potentiostat for sweeping.

  For example, a metal lithium plate is inserted as a reference electrode into the manufactured lithium ion secondary battery so that the potential based on the metal lithium can be measured. Next, the working electrode W is connected to a graphite negative electrode (a negative electrode as an actual completed battery), the counter electrode C is connected as, for example, a lithium cobalt oxide positive electrode (a positive electrode as an actual completed battery), and lithium is used as a reference electrode W. Metals are connected to produce the three-electrode cell 4 shown in FIG.

  Next, by applying a voltage from the power source 2 between the working electrode W and the reference electrode R and passing a current from the power source 2 between the working electrode W and the counter electrode C, the applied voltage is changed, Potential scanning can be performed. As in the above example, by sweeping the potential of the negative electrode below the potential at which the cation of the room temperature molten salt undergoes reductive decomposition, a film capable of transmitting lithium ions is formed on the negative electrode surface, as in the above example. It can be set as the nonaqueous electrolyte secondary battery of this invention.

The figure which shows the cyclic voltammogram of battery A1 of the Example according to this invention. The figure which shows the charging / discharging curve of battery A1 of the Example according to this invention. The figure which shows the charging / discharging curve of the battery X1 of a comparative example. The schematic diagram which shows the potentiostat for two electrode cells used in the Example according to this invention. The schematic diagram which shows the potentiostat for sweeping a negative electrode electric potential by electric potential scanning using a three electrode cell according to this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Two-electrode cell 2 ... Power supply 3 ... Recorder 4 ... Three-electrode cell W ... Working electrode R ... Reference electrode C ... Counter electrode

Claims (7)

In a non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte containing a room temperature molten salt,
A nonaqueous electrolyte secondary battery in which the potential of the negative electrode is swept to a potential lower than that at which the cation of the room temperature molten salt undergoes reductive decomposition by potential scanning after the nonaqueous electrolyte is injected and before the first charge.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the potential of the negative electrode is swept to 0.5 V or less based on metallic lithium.   The nonaqueous electrolyte secondary battery according to claim 2, wherein the potential of the negative electrode is swept to 0.0 V with respect to metallic lithium.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the room temperature molten salt contains a bis (fluorosulfonyl) imide anion.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the room temperature molten salt contains a cation composed of a cyclic compound containing a nitrogen element.   The non-aqueous electrolyte secondary battery according to claim 5, wherein the cation is an ethylmethylimidazolim cation. A method for producing the nonaqueous electrolyte secondary battery according to any one of claims 1 to 6,
Preparing the positive electrode, the negative electrode, and the non-aqueous electrolyte;
Storing the positive electrode and the negative electrode in an outer package, and injecting the non-aqueous electrolyte into the outer package;
And a step of sweeping the potential of the negative electrode below the potential at which the cation of the room temperature molten salt is reductively decomposed by potential scanning before the first charge, the method for producing a non-aqueous electrolyte secondary battery.

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