JP2011181427A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve low-temperature output characteristics and a preservation characteristic in a lithium secondary battery using an oxy-anion compound containing lithium and iron as a positive electrode active material. <P>SOLUTION: In the lithium secondary battery including a positive electrode 1 having the oxy-anion compound containing lithium and iron as a positive electrode active material, a negative electrode 2 having a carbon material as a negative electrode active material, wherein graphite and amorphous carbon come in contact with each other, and a nonaqueous electrolyte wherein an electrolyte containing fluorine is dissolved in a nonaqueous solvent, the nonaqueous electrolyte contains vinylene carbonate, and at least one of the positive electrode, the negative electrode, and the nonaqueous electrolyte contains lithium chloride. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery including a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte obtained by dissolving a solute in a non-aqueous solvent. Non-aqueous electrolysis in which an oxyanion compound containing lithium and iron is used as a material, a carbon material in which graphite and amorphous carbon are in contact is used as a negative electrode active material, and an electrolyte containing fluorine is dissolved in a non-aqueous solvent The lithium secondary battery using the liquid is characterized in that its low-temperature output characteristics and storage characteristics are improved.

  In recent years, lithium secondary batteries using a non-aqueous electrolyte and moving lithium ions between a positive electrode and a negative electrode for charging and discharging are widely used as new secondary batteries with high output and high energy density. It became so.

In such a lithium secondary battery, generally, lithium cobalt oxide LiCoO ₂ is used as the positive electrode active material in the positive electrode, and lithium metal, a lithium alloy, or a carbon material capable of occluding and releasing lithium is used as the negative electrode active material in the negative electrode. As a non-aqueous electrolyte, a solution obtained by dissolving an electrolyte composed of a lithium salt such as LiBF ₄ or LiPF ₆ in a non-aqueous solvent such as ethylene carbonate or diethyl carbonate is used.

Here, Co used for the positive electrode active material LiCoO ₂ is a scarce resource, and there are problems such as high production costs and difficulty in stable supply. In addition, lithium secondary batteries using LiCoO ₂ as a positive electrode active material generally have a problem that the thermal stability at high temperature in the charged state is poor and the battery characteristics at high temperature are deteriorated.

Conventionally, the use of lithium spinel manganate LiMn ₂ O ₄ as a positive electrode active material in place of LiCoO ₂ has also been studied.

However, in a lithium secondary battery using LiMn ₂ O ₄ as a positive electrode active material, it is difficult to obtain a sufficient discharge capacity, and when the battery temperature increases, manganese in the LiMn ₂ O ₄ of the positive electrode active material There is a problem that Mn is dissolved and battery characteristics such as cycle characteristics are greatly deteriorated.

Conventionally, as a substitute for the positive electrode active material as described above, the general formula LiMPO ₄ (wherein M is at least one element selected from Co, Ni, Mn and Fe). The use of a lithium-containing phosphate having an olivine structure represented by

Here, such a lithium-containing phosphate has a different operating voltage depending on the kind of the core metal element M, and the battery voltage can be arbitrarily selected by selecting M, and the theoretical capacity is 140 mAh / g to 170 mAh / g. Since it is relatively high, the battery capacity per unit mass can be increased. In particular, by using LiFePO ₄ using inexpensive iron Fe as the metal element M, the manufacturing cost of the lithium secondary battery can be greatly reduced.

However, in the case of a lithium secondary battery using LiFePO ₄ as a positive electrode active material, when charging and discharging are repeated in a high temperature environment, Fe in the positive electrode active material is eluted, and a negative electrode active material made of a carbon material or the like is obtained. There were problems such as adversely affecting the capacity and cycle characteristics.

In recent years, as shown in Patent Document 1, in a lithium secondary battery using LiFePO ₄ as a positive electrode active material, a mixed solvent of ethylene carbonate and diethyl carbonate, or a mixture of ethylene carbonate and ethyl methyl carbonate is used. There has been proposed a nonaqueous electrolytic solution in which an electrolyte mainly composed of LiPF ₆ is dissolved in a nonaqueous solvent mainly composed of a solvent so that vinylene carbonate or vinyl ethylene carbonate is contained therein.

  And in this patent document 1, the stable film is formed in the surface of a positive electrode active material or a negative electrode active material with the vinylene carbonate or vinyl ethylene carbonate contained in the non-aqueous electrolyte as mentioned above, and positive electrode active material It is shown that Fe in the inside is suppressed, and that the eluted Fe is prevented from adversely affecting the negative electrode active material made of a carbon material or the like, thereby preventing a decrease in capacity and cycle characteristics. Has been.

However, in the lithium secondary battery using LiFePO ₄ as the positive electrode active material in this way, even when vinylene carbonate or vinyl ethylene carbonate is contained in the non-aqueous electrolyte, the low-temperature output characteristics of the lithium secondary battery It has been difficult to sufficiently improve the storage characteristics.

JP 2009-4357 A

An object of the present invention is to solve the above-mentioned problems in a lithium secondary battery using a lithium-containing phosphate having an olivine structure such as LiFePO _{4 as} a positive electrode active material.

In particular, in the present invention, an oxyanion compound containing lithium and iron such as LiFePO ₄ is used as the positive electrode active material, and a carbon material in which graphite and amorphous carbon are in contact is used as the negative electrode active material. An object of the present invention is to improve the low-temperature output characteristics and storage characteristics of a lithium secondary battery using a nonaqueous electrolytic solution in which an electrolyte containing fluorine is dissolved in a solvent.

  In the present invention, in order to solve the above-mentioned problems, a positive electrode containing an oxyanion compound containing lithium and iron as a positive electrode active material, and a carbon material in which graphite and amorphous carbon are in contact as a negative electrode active material A lithium secondary battery comprising a non-aqueous electrolyte in which an electrolyte containing fluorine is dissolved in a non-aqueous solvent, and the non-aqueous electrolyte contains vinylene carbonate, and the positive electrode and the negative electrode And at least one of the non-aqueous electrolyte solution contains lithium chloride.

Here, as the oxyanion compound containing lithium and iron used for the positive electrode active material, LiFePO ₄ which is a lithium-containing phosphate having a large theoretical capacity and an olivine structure as described above is used. Is preferred.

  In addition, as a carbon material in which graphite and amorphous carbon used for the negative electrode active material are in contact, a material in which at least a part of the surface of graphite is coated with amorphous carbon is used in order to improve output characteristics at low temperature. It is preferable to use it. Here, if the amount of amorphous carbon covering the surface of the graphite is small, it will be difficult to sufficiently improve the low temperature output characteristics of the lithium secondary battery, while the amount of amorphous carbon will increase. If it is too high, the storage characteristics of the lithium secondary battery will be reduced. For this reason, it is preferable to make the quantity of the amorphous carbon in said carbon material into the range of 0.1-10 mass%.

  In addition, as the non-aqueous solvent in the non-aqueous electrolyte, those generally used in lithium secondary batteries can be used, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and dimethyl carbonate. , Chain carbonates such as methyl ethyl carbonate and diethyl carbonate, ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. can be used. A mixed solvent or a mixed solvent of the above cyclic carbonate and ether can be used.

Examples of the electrolyte containing fluorine dissolved in the non-aqueous solvent include, for example, a general formula LiXF _p (wherein X is P, As, Sb, Al, B, Bi, Ga, or In, and X is In the case of P, As, Sb, p is 6, and in the case where X is Al, B, Bi, Ga, In, p is 4.), the general formula LiN (C _m F _{2m + 1} SO ₂ ) (C _n F _{2n + 1} SO ₂ ) (wherein m is 1, 2, 3 or 4, n is 1, 2, 3 or 4), general formula LiC (C _l F _{2l + 1} SO _{2) (C m F 2m +} 1 SO 2) (C n F 2n + 1 SO 2) ( wherein, l is 1, 2, 3 or 4, m is 1, 2, 3 or 4, n is 1, 2, 3 or 4) and a mixture containing fluorine and a mixture thereof can be used. In the case of a compound containing fluorine represented by the above general formula LiXF _p, its concentration is preferably higher within a range not precipitated without being dissolved.

  When the amount of vinylene carbonate is too small to contain vinylene carbonate in the nonaqueous electrolytic solution, a sufficient film is not formed on the surface of the positive electrode active material, and Fe in the positive electrode active material is eluted. On the other hand, when the amount of vinylene carbonate increases, the interfacial resistance of the positive electrode active material increases and the charge / discharge characteristics deteriorate. For this reason, it is preferable to make the quantity of vinylene carbonate contained in a non-aqueous electrolyte into the range of 0.1 mass%-5 mass%.

  In addition, when lithium chloride is contained in at least one of the positive electrode, the negative electrode, and the non-aqueous electrolyte as described above, the above-described fluorine-containing electrolyte in the non-aqueous electrolyte reacts with moisture to generate HF. It will be suppressed. Here, in order to more efficiently suppress the electrolyte containing fluorine in the non-aqueous electrolyte from reacting with moisture, it is preferable to contain lithium chloride in the non-aqueous electrolyte.

  In addition, when lithium chloride is contained in the non-aqueous electrolyte as described above, if the amount of lithium chloride is small, the electrolyte containing fluorine cannot sufficiently suppress the generation of HF by reacting with moisture. If the amount of lithium chloride is too large, the conductivity of lithium ions in the non-aqueous electrolyte is lowered, and the charge / discharge characteristics of the lithium secondary battery are lowered. For this reason, it is preferable to make the quantity of lithium chloride contained in a non-aqueous electrolyte into the range of 0.01-0.1 mol / l.

In the lithium secondary battery of the present invention, since the oxyanion compound containing lithium and iron is used as the positive electrode active material, the cost is reduced as compared with the case where LiCoO ₂ is used as the positive electrode active material. _A sufficient battery capacity can be obtained as compared with the case of using ₂ O ₄ .

  In the lithium secondary battery of the present invention, since a carbon material in which graphite and amorphous carbon are in contact with each other is used as the negative electrode active material, lithium receiving and releasing performance is improved. Output characteristics are improved.

  Further, in the lithium secondary battery of the present invention, vinylene carbonate is contained in the non-aqueous electrolyte, so that a stable film is formed on the surface of the positive electrode active material or the negative electrode active material, and iron in the positive electrode active material Elution into the non-aqueous electrolyte is suppressed, and the eluted iron is prevented from adversely affecting the negative electrode active material, thereby preventing the capacity and cycle characteristics of the lithium secondary battery from deteriorating.

  Furthermore, in the lithium secondary battery of the present invention, when a non-aqueous electrolyte in which an electrolyte containing fluorine is dissolved in a non-aqueous solvent is used, lithium chloride is added to at least one of the positive electrode, the negative electrode, and the non-aqueous electrolyte. As a result, the fluorine-containing electrolyte in the nonaqueous electrolytic solution is inhibited from reacting with moisture to generate HF, thereby further suppressing the elution of iron in the positive electrode active material. Become.

  As a result, in the lithium secondary battery of the present invention, sufficient storage characteristics can be obtained even in a high temperature environment, and sufficient output characteristics can be obtained even in a low temperature environment.

It is a schematic sectional drawing of the lithium secondary battery produced in the Example and comparative example of this invention.

  Hereinafter, the lithium secondary battery according to the present invention will be specifically described with reference to examples. In the lithium secondary battery according to this example, an oxyanion compound containing lithium and iron as a positive electrode active material is used. Even when used, it will be clarified by giving a comparative example that sufficient storage characteristics can be obtained under a high temperature environment and sufficient output characteristics can be obtained under a low temperature environment. The lithium secondary battery of the present invention is not limited to those shown in the following examples, and can be implemented with appropriate modifications within a range that does not change the gist thereof.

Example 1
In Example 1, a cylindrical lithium secondary battery as shown in FIG. 1 having a diameter of 18 mm and a height of 65 mm using a positive electrode, a negative electrode, and a nonaqueous electrolyte prepared as described below was used. Produced.

[Production of positive electrode]
In obtaining the positive electrode active material, Fe ₃ (PO ₄ ) ₂ · 7H ₂ O, H ₃ PO ₄ and LiOH were weighed so as to have a molar ratio of 1: 1: 3.1.

Then, the Fe ₃ (PO ₄ ) ₂ · 7H ₂ O and water are mixed at a mass ratio of 1: 2 and mixed to dissolve Fe ₃ (PO ₄ ) ₂ · 7H ₂ O. The above H ₃ PO ₄ was dissolved.

Next, while stirring the above aqueous solution with a stirrer, a LiOH aqueous solution in which the above LiOH and water were mixed at a mass ratio of 1:10 was gradually added. Then, hydrothermal treatment was performed for 5 hours at a temperature of 160 ° C. in an autoclave to synthesize LiFePO ₄ .

Then, LiFePO ₄ synthesized in this way, sucrose, and water were adjusted to a mass ratio of 20: 6: 8, and this was mixed by a ball mill at 100 rpm for 18 minutes, and then dried at 50 ° C., Then, heat treatment was performed in a vacuum at 850 ° C. for 5 hours to obtain a positive electrode active material in which carbon was adhered to the surface of LiFePO ₄ . The positive electrode active material had an average particle size of 0.7 μm and a BET specific surface area of 14 m ² / g.

  Next, the positive electrode active material, the conductive agent acetylene black, and the binder polyvinylidene fluoride were mixed in a weight ratio of 90: 5: 5, and then N-methyl-2-pyrrolidone was mixed. An appropriate amount of was added to prepare a positive electrode mixture slurry.

  Then, this positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector made of aluminum foil by a doctor blade method, dried, then cut into a size of 55 mm × 750 mm, and rolled with a rolling roller. Then, a positive electrode lead was attached to produce a positive electrode.

[Production of negative electrode]
In producing the negative electrode, the mass ratio of the amorphous carbon-coated natural graphite in which a part of the surface of natural graphite is coated with amorphous carbon and the polyvinylidene fluoride as a binder is set to a mass ratio of 98: 2. Then, an appropriate amount of N-methyl-2-pyrrolidone was added to prepare a negative electrode mixture slurry.

  Then, this negative electrode mixture slurry was applied to both surfaces of a negative electrode current collector made of copper foil by the doctor blade method, dried, and then cut into a size of 58 mm × 850 mm, which was rolled by a rolling roller. After that, a negative electrode lead was attached to produce a negative electrode.

[Preparation of non-aqueous electrolyte]
In the preparation of the non-aqueous electrolyte, the solute LiPF ₆ is adjusted to a concentration of 1.0 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate, which are non-aqueous solvents, are mixed at a volume ratio of 3: 7. A non-aqueous electrolyte is prepared by dissolution, and 1% by mass of vinylene carbonate (VC) is added to the non-aqueous electrolyte. Further, 0.05 mol / l of lithium chloride (LiCl) is added to the non-aqueous electrolyte. Added.

[Production of battery]
In producing the battery, as shown in FIG. 1, a separator 3 made of a lithium ion-permeable polyethylene microporous membrane is interposed between the positive electrode 1 and the negative electrode 2 produced as described above, These are spirally wound and accommodated in the battery can 4, and the positive electrode current collecting lead 1 a provided on the positive electrode 1 is connected to the positive electrode lid 5 provided with the positive electrode external terminal 5 a and the above described provided on the negative electrode 2. The negative electrode current collecting lead 2 a is connected to the battery can 4, the nonaqueous electrolyte is poured into the battery can 4 and sealed, and the battery can 4 and the positive electrode lid 5 are electrically connected by the insulating packing 6. Separated.

(Example 2)
In Example 2, the amount of lithium chloride (LiCl) added to the non-aqueous electrolyte in the preparation of the non-aqueous electrolyte in Example 1 was changed to 0.1 mol / l. And the lithium secondary battery was produced like the case of said Example 1 except using this non-aqueous electrolyte.

(Comparative Example 1)
In Comparative Example 1, in the preparation of the non-aqueous electrolyte in Example 1 above, only 1% by mass of vinylene carbonate (VC) was added to the non-aqueous electrolyte, and lithium chloride (LiCl) was not added. . And the lithium secondary battery was produced like the case of said Example 1 except using this non-aqueous electrolyte.

(Comparative Example 2)
In Comparative Example 2, in the preparation of the non-aqueous electrolyte in Example 1 above, 0.05 mol / l of lithium chloride (LiCl) alone is added without adding vinylene carbonate (VC) to the non-aqueous electrolyte. I did it. And the lithium secondary battery was produced like the case of said Example 1 except using this non-aqueous electrolyte.

(Comparative Example 3)
In Comparative Example 3, natural graphite not coated with amorphous carbon was used for the negative electrode active material in the production of the negative electrode in Example 1, and in the production of the non-aqueous electrolyte in Example 1, As in Comparative Example 1, only 1% by mass of vinylene carbonate (VC) was added to the nonaqueous electrolytic solution, and lithium chloride (LiCl) was not added. And the lithium secondary battery was produced like the case of said Example 1 except using said negative electrode and non-aqueous electrolyte.

(Comparative Example 4)
In Comparative Example 4, LiNi _0.33 Co _0.33 Mn _0.33 O ₂ was used as the positive electrode active material in the production of the positive electrode in Example 1, and in the production of the non-aqueous electrolyte in Example 1, the above Comparative Example 1 was used. In the same manner as described above, only 1% by mass of vinylene carbonate (VC) was added to the nonaqueous electrolytic solution, and lithium chloride (LiCl) was not added. And the lithium secondary battery was produced like the case of said Example 1 except using said positive electrode and non-aqueous electrolyte.

  Next, the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 4 manufactured as described above were charged to 200 mAh at a current of 1000 mA under the condition of 25 ° C. It was allowed to stand for 1 day under the conditions.

Then, for each of the above lithium secondary batteries, the battery voltage Vo before being left and the battery voltage Va after being left are measured, and the voltage change (V) after being left is obtained based on the following formula, and the result is shown in Table 1. It was shown in 1.
Voltage change (V) = Va-Vo

  Each lithium secondary battery is charged at a constant current of 1000 mA at a current of 25 mA until the voltage reaches 4.2 V, and further fixed at a voltage of 4.2 V until the current value is reduced to 50 mA. After being charged with voltage, the battery was discharged at a current of 1000 mA until the voltage reached 2.0 V.

Thereafter, each of the lithium secondary batteries described above was charged to 500 mAh at a current value of 1000 mA, and then discharged at a current of 500 mA, 5000 mA, 10000 mA, and 15000 mA for 10 seconds under the condition of −20 ° C., respectively. The voltage after 10 seconds is obtained, the voltage against each current value is plotted, the current value Ia (A) at which the voltage is 2.2 V under the condition of −20 ° C. is obtained, and −20 ° C. based on the following formula: The low temperature output (W) at was calculated.
Low temperature output (W) = Ia (A) x 2.2 (V)

  Then, assuming the low temperature output of the lithium secondary battery of Example 1 calculated as described above as 100, the relative low temperature output in each of the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 4 was obtained, and the result Are shown in Table 1.

As a result, LiFePO ₄ is used as the positive electrode active material, amorphous carbon-coated natural graphite is used as the negative electrode active material, and vinylene carbonate (VC) and lithium chloride (LiCl) are added to the nonaqueous electrolyte solution in which the fluorine-containing electrolyte is dissolved. In each of the lithium secondary batteries of Examples 1 and 2, the cathode active material is LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , the anode active material is amorphous carbon-coated natural graphite, and the electrolyte contains fluorine. Compared with the lithium secondary battery of Comparative Example 4 in which only the vinylene carbonate (VC) is contained in the non-aqueous electrolyte in which the solution is dissolved, the storage characteristics at a high temperature are slightly improved, while the output characteristics at a low temperature are slightly Although decreased, characteristics equivalent to those using LiNi _0.33 Co _0.33 Mn _0.33 O ₂ as the positive electrode active material were obtained.

Moreover, when comparing the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3 using LiFePO ₄ as the positive electrode active material, natural graphite not coated with amorphous carbon was used as the negative electrode active material. The lithium secondary battery of Comparative Example 3 had greatly reduced output characteristics at low temperatures.

  In addition, even in the case of using amorphous carbon-coated natural graphite as a negative electrode active material, a comparative example in which only vinylene carbonate (VC) is contained in a nonaqueous electrolytic solution in which an electrolyte containing fluorine is dissolved. The lithium secondary battery of No. 1 and the lithium secondary battery of Comparative Example 2 containing only lithium chloride (LiCl) are made of vinylene carbonate (VC) and chloride with respect to a non-aqueous electrolyte in which an electrolyte containing fluorine is dissolved. Compared to each of the lithium secondary batteries of Examples 1 and 2 containing both lithium (LiCl), the storage characteristics at high temperatures and the output characteristics at low temperatures were greatly deteriorated.

  In the above examples, lithium chloride (LiCl) was included in the non-aqueous electrolyte. However, when lithium chloride (LiCl) is included in the positive electrode and the negative electrode, this lithium chloride is also included. (LiCl) is dissolved in the nonaqueous electrolytic solution, and the same effect is obtained.

DESCRIPTION OF SYMBOLS 1 Positive electrode 1a Positive electrode current collection lead 2 Negative electrode 2a Negative electrode current collection lead 3 Separator 4 Battery can 5 Positive electrode cover 5a Positive electrode external terminal 6 Insulation packing

Claims (3)

  A positive electrode containing an oxyanion compound containing lithium and iron as a positive electrode active material, a negative electrode containing a carbon material in which graphite and amorphous carbon are in contact as a negative electrode active material, and an electrolyte containing fluorine in a non-aqueous solvent A non-aqueous electrolyte solution containing vinylene carbonate in the non-aqueous electrolyte solution, and at least one of the positive electrode, the negative electrode, and the non-aqueous electrolyte solution containing lithium chloride. Rechargeable lithium battery.   The lithium secondary battery according to claim 1, wherein the lithium chloride is contained in a non-aqueous electrolyte. 3. The lithium secondary battery according to claim 1, wherein the oxyanion compound containing lithium and iron is LiFePO _{4. 4} .

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