JP2006066298A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery which is superior in low temperature characteristics and cycle properties. <P>SOLUTION: The lithium secondary battery provided with a cathode, an anode, and nonaqueous electrolytic solution containing at least one kind of lactone of which the melting point is 0°C or less, and contains a carbon material and lithium titanate capable of storing and releasing lithium ions as anode active materials. It is preferable that a content of the carbon material to the total weight of the anode active material is 80 to 99% by mass, and a content of the lithium titanate is 1 to 20% by mass. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a negative electrode material for lithium secondary batteries, an electrolytic solution, and a lithium secondary battery including the same.

A lithium secondary battery includes a positive electrode and a negative electrode having a material (active material) capable of inserting and extracting lithium ions, and lithium ions travel back and forth between the electrolyte (typically a non-aqueous electrolyte) between the two electrodes. It is a secondary battery that is charged and discharged by the above, and has become increasingly important as a vehicle-mounted battery or a power source for personal computers and portable terminals.
One of the performances required for such a lithium secondary battery is excellent low temperature characteristics. Here, the low temperature characteristic refers to charge / discharge performance under low temperature conditions (for example, 0 ° C.). Therefore, excellent low temperature characteristics typically include the stable realization of a desired capacity (or output) discharge under low temperature conditions.
One means for obtaining such a lithium secondary battery having excellent low temperature characteristics is the use of a non-aqueous electrolyte mainly composed of a lactone such as γ-butyrolactone. γ-Butyrolactone has a low melting point (about −45 ° C.) and can maintain the performance required as a non-aqueous electrolyte at a high level even when used below freezing point (for example, −20 ° C.). For example, Patent Document 1 below describes a lithium secondary battery using a non-aqueous electrolyte containing γ-butyrolactone.

Japanese Patent Laid-Open No. 2003-257481 Japanese Patent Laid-Open No. 2002-8658

By the way, as a point to be improved in a lithium secondary battery using a low melting point lactone as a non-aqueous electrolyte component, there is an improvement in cycle characteristics when a carbon material such as graphite is used as the negative electrode active material. Here, the cycle characteristic is an index relating to the durability (life) of the secondary battery, and refers to the charge / discharge performance of the battery when charge / discharge is repeated. Therefore, excellent cycle characteristics typically include that a desired capacity (or output) discharge can be stably realized regardless of repeated charge and discharge. By improving the cycle characteristics, a lithium secondary battery realizing high durability (long life) can be provided.
The present invention has been developed to solve such conventional problems, and is a lithium secondary battery including a negative electrode using a carbon material as a main component of a negative electrode active material and a lactone-containing non-aqueous electrolyte, An object of the present invention is to provide a lithium secondary battery excellent in low temperature characteristics and cycle characteristics.

The lithium secondary battery provided by the present invention includes a positive electrode (typically a lithium transition metal composite oxide), a negative electrode, and a non-aqueous electrolyte solution (hereinafter referred to as “lactone-containing non-aqueous electrolyte) containing at least one lactone having a melting point of 0 ° C. Lithium secondary battery provided with an abbreviated “water electrolyte”), which includes a carbon material capable of inserting and extracting lithium ions and lithium titanate as a negative electrode active material.
And the suitable lithium secondary battery disclosed here has a content of the carbon material of 80 to 99% by mass and a content of lithium titanate of 1 to 20% by mass with respect to the total amount of the negative electrode active material. . It is particularly preferable that the content of the carbon material with respect to the total amount of the negative electrode active material is 90 to 95% by mass, and the content of lithium titanate is 5 to 10% by mass.

In the battery employing a lactone-containing non-aqueous electrolyte and a carbon material such as graphite as a negative electrode active material, the present inventor has found that a passivation film formed on the surface of the negative electrode active material, that is, SEI (Solid Electrolyte Interface) The adhesion of the interfacial coating produced by the thermal decomposition) to the surface of the negative electrode active material is poor, and therefore the coating can be destroyed and peeled off when the negative electrode potential suddenly rises at the end of discharge, and further reductive decomposition of the electrolyte solution occurs. It has been found that this can occur at the end of each discharge. Such excessive reductive decomposition of the electrolytic solution is not preferable because it leads to deterioration of cycle characteristics.
The present inventor can moderate the degree of increase in the negative electrode potential at the end of discharge by adding lithium titanate to a carbon material such as graphite at a predetermined ratio. As a result, it has been found that excessive reductive decomposition of the electrolytic solution can be prevented, and the present invention has been completed.
The lithium secondary battery of the present invention achieves both excellent low temperature characteristics and cycle characteristics by adopting a lactone-containing non-aqueous electrolyte and a negative electrode active material mainly composed of a carbon material containing lithium titanate at a predetermined ratio. can do.

One preferable lithium secondary battery disclosed herein is characterized in that the content of the lactone in the solvent constituting the non-aqueous electrolyte is 55 to 85% by volume.
By using the lactone-containing non-aqueous electrolyte having such a configuration, a higher output can be obtained while maintaining excellent low-temperature characteristics and cycle characteristics. Those containing γ-butyrolactone as the lactone are preferred.

  Moreover, this invention is another negative aspect which solves the said subject, It is a negative electrode active material (composition) which can be used for a lithium secondary battery, Comprising: The content rate of a carbon material is 80-99 mass% (preferably 90- 95% by mass) and the content of lithium titanate is 1 to 20% by mass (preferably 5 to 10% by mass). Moreover, the manufacturing method of the lithium secondary battery characterized by using the negative electrode active material of this composition and a lactone containing non-aqueous electrolyte is provided.

Furthermore, the present invention provides, as another aspect to solve the above problems, a lithium secondary comprising a positive electrode (typically a lithium transition metal composite oxide), a negative electrode mainly composed of a carbon material, and a lactone-containing non-aqueous electrolyte. A method for improving the cycle characteristics of a battery is provided.
The method for improving the cycle characteristics of a lithium secondary battery disclosed herein is characterized in that lithium titanate is added to a carbon material that is a negative electrode active material. Typically, the compound is added so that the content of lithium titanate with respect to the total amount of the negative electrode active material is 1 to 20% by mass (preferably 5 to 10% by mass). Typically, the content of the carbon material with respect to the total amount of the negative electrode active material may be 80 to 99% by mass (preferably 90 to 95% by mass).

  Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that the technical matters other than the matters specifically mentioned in the present specification (for example, the composition of the negative electrode active material and the composition of the non-aqueous electrolyte) and the matters necessary for the practice of the present invention (for example, lithium having various shapes) The general manufacturing technique of the secondary battery can be understood as a design matter of a person skilled in the art based on the prior art. The present invention can be implemented based on the technical contents disclosed in the present specification and the common general technical knowledge in the field.

The negative electrode active material provided in the negative electrode of the lithium secondary battery disclosed here is mainly composed of a carbon material. A carbonaceous material capable of occluding and releasing lithium ions can be used, but graphite materials such as highly crystalline natural graphite and artificial graphite are suitable because of their high capacity and excellent voltage flatness.
On the other hand, the composition ratio (atomic ratio) of lithium titanate added to the carbon material in order to prepare the negative electrode active material disclosed herein is not limited, and those having various composition ratios that can function as the active material are used. can do. For example, the general formula: Li _x Ti _y O ₄ (where x is a real number that typically satisfies 0.8 ≦ x ≦ 1.4, and y is typically 1.6 ≦ y ≦ 2.2). Lithium titanate having various composition ratios (atomic ratios) that can be represented by the following formula:

The amount of lithium titanate added is not particularly limited as long as the amount of increase in the negative electrode potential at the end of discharge of the target lithium secondary battery can be moderated. The content of lithium titanate with respect to the total amount of the negative electrode active material is generally 1 to 20% by mass, preferably about 3 to 15% by mass, and particularly preferably about 5 to 10% by mass. On the other hand, the content of the carbon material with respect to the total amount of the negative electrode active material is suitably about 80 to 99% by mass, preferably about 85 to 97% by mass, and particularly preferably about 90 to 95% by mass.
As long as the negative electrode potential is not increased excessively at the end of discharge, a substance other than the carbon material and lithium titanate may be added as the negative electrode active material.

The negative electrode of the lithium secondary battery may be constructed using the above-described negative electrode active material according to the present invention, and there are no particular limitations on the materials or construction methods other than the negative electrode active material used for that purpose.
Typically, a composition for forming a negative electrode active material layer in the form of a paste (slurry) is prepared by mixing and dispersing a negative electrode active material, a binder, and a conductive material used as necessary in an appropriate solvent. (Hereinafter referred to as “a negative electrode composite”). It is preferable that the negative electrode active material (including the carbon material and lithium titanate), the binder, and the conductive material to be used are in powder form.
As the solvent (dispersion medium), an aqueous solvent (typically water) or a non-aqueous solvent (organic solvent) can be used. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, toluene and the like. The use of NMP is preferred.
As the conductive material, carbon powder such as carbon black (acetylene black or the like), conductive metal powder such as nickel powder, or the like can be used. In addition, since the negative electrode active material disclosed here is mainly composed of a carbon material, the addition of a conductive material is not essential.

  As the binder, when a non-aqueous solvent is used, a polymer that is soluble in an organic solvent can be preferably used. Preferable examples include polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene oxide-propylene oxide copolymer (PEO-PPO), and the like. PVDF, PVDC and the like are preferable. Alternatively, when an aqueous solvent is used, a hydrophilic polymer that dissolves in water and / or a polymer that disperses in water can be preferably used. Preferable examples of such hydrophilic polymer include various cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose (MC), cellulose acetate phthalate (CAP), hydroxypropyl methylcellulose (HPMC), hydroxypropyl methylcellulose phthalate (HPMCP) and the like. It is done. The use of CMC is preferred. Suitable water-dispersed polymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene- Fluorine resins such as tetrafluoroethylene copolymer (ETFE), vinyl acetate copolymer, styrene butadiene block copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), rubbers such as gum arabic It is done. A fluorine resin such as PTFE is particularly preferable.

  Thus, a negative electrode having a negative electrode active material layer is obtained by applying a negative electrode mixture obtained by appropriately mixing the materials as described above to the surface of an appropriate current collector using an appropriate application device. Can be built. The shape of the current collector can be different depending on the shape of the battery, and is not particularly limited. For example, a sheet-like negative electrode can be constructed by applying a negative electrode mixture on the surface of a metal foil such as copper foil or nickel foil.

The positive electrode of the lithium secondary battery may be constructed by appropriately selecting and using a conventionally used positive electrode active material, and the material used or the construction method is not particularly limited.
As in the case of the negative electrode, a composition for forming a positive electrode active material layer in the form of a paste (slurry) is typically obtained by mixing and dispersing a positive electrode active material, a binder, and a conductive material in an appropriate solvent ( Hereinafter, it is referred to as “a positive electrode composite”). The positive electrode active material, the binder, and the conductive material to be used are preferably in the form of powder.
For example, as the positive electrode active material, an oxide-based positive electrode active material having a layered structure used for a general lithium secondary battery, an oxide-based positive electrode active material having a spinel structure, or the like can be preferably used. It is preferable to use various lithium transition metal composite oxides (also referred to as lithium-containing transition metal oxides). For example, a lithium cobalt complex oxide (typically LiCoO ₂ ), a lithium nickel complex oxide (typically LiNiO ₂ ), a lithium manganese complex oxide (LiMn ₂ O ₄ ), etc. A positive electrode active material can be used. A composite oxide containing two or more transition metal elements (for example, a composite oxide represented by the general formula: LiNi _x Co _1-x O ₂ , where x is a positive real number satisfying 0 <x <1). May be. The use of LiNiO ₂ is particularly preferred. As the conductive material, carbon powder such as carbon black (acetylene black or the like), conductive metal powder such as nickel powder, or the like can be used.
As the solvent (dispersion medium), the same organic solvent (non-aqueous solvent) or aqueous solvent (typically water) as the above-described negative electrode mixture can be used. In addition, an appropriate binder may be employed depending on the nature of the solvent (aqueous or non-aqueous). When a non-aqueous solvent is used, a polymer that is soluble in an organic solvent can be preferably used. A suitable example is the same as that of the negative electrode mixture. Alternatively, when an aqueous solvent is used, it is preferable to use a hydrophilic polymer that dissolves in water and / or a polymer that disperses in water. A suitable example is the same as that of the negative electrode mixture.

  Thus, a positive electrode having a positive electrode active material layer can be obtained by applying a positive electrode mixture obtained by appropriately mixing the materials described above to the surface of an appropriate current collector using an appropriate application device. Can be built. A conductive member made of a metal with good conductivity is preferred as the positive electrode current collector. For example, those made of aluminum are preferable. The shape of the current collector can be different depending on the shape of the battery, and is not particularly limited. For example, a sheet-like positive electrode can be constructed by applying a positive electrode mixture on the surface of a metal foil such as an aluminum foil.

Non-aqueous electrolytes used in the lithium secondary battery disclosed herein are γ-butyrolactone (C ₄ H ₆ O ₂ : hereinafter abbreviated as “γ-BL”), γ-barrel lactone (C ₅ H ₈ ). It is a nonaqueous electrolytic solution (organic electrolytic solution) characterized by containing at least one lactone having a melting point of 0 ° C. or lower, such as O ₂ ) and δ-barrel lactone (C ₅ H ₈ O ₂ ). Preferably γ-BL is included. Typically, from the viewpoint of ensuring suitable ionic conductivity for high output, an aprotic solvent preferably having a high dielectric constant and / or low viscosity other than a lactone is blended. One or more conventionally known ester-based, ether-based and other solvents that can be used as a solvent constituting the electrolyte solution of the lithium secondary battery can be used. For example, aprotic solvents such as cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, and sulfones can be used. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2 -Methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane and the like. Cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate are preferred.

  In preparing the lactone-containing non-aqueous electrolyte, the mixing ratio of the lactone and the other aprotic solvent is not particularly limited, considering the low temperature characteristics and high output characteristics of the battery depending on the type of solvent used. Set it appropriately. For example, when a lactone such as γ-BL and a cyclic carbonate such as EC are blended, the lactone content in the non-aqueous electrolyte is preferably 55 to 85% by volume, and preferably 60 to 80% by volume. Particularly preferred.

Various known lithium salts can be used as the supporting salt for the non-aqueous electrolyte. For _example, selected from _{LiPF 6, LiBF 4, LiClO 4} , LiAsF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (CF 3 SO 2) 2, LiC (CF 3 SO 2) 3, LiI , etc. One or more lithium compounds (lithium salts) can be used. In particular, the use of LiPF ₆ and / or LiBF ₄ is preferred. In addition, the density | concentration of the support salt in a nonaqueous electrolyte solution may be the same as that of the electrolyte solution used with the conventional lithium secondary battery, and there is no restriction | limiting in particular. An electrolytic solution (initial electrolytic solution) containing an appropriate lithium compound at a concentration of about 0.1 to 5 mol / L (preferably 0.2 to 3 mol / L, more preferably 0.5 to 2 mol / L) is used. be able to.

  The separator used in manufacturing the battery may be a conventionally known separator and is not particularly limited. A woven fabric, a nonwoven fabric, a porous film made of a synthetic resin, or the like can be used. In particular, as the synthetic resin porous membrane, a polyolefin-based porous membrane such as polyethylene and polypropylene is preferably used.

Using the materials as described above, lithium secondary batteries of various shapes and applications (capacities) can be manufactured by the same process as before.
That is, the lithium secondary battery disclosed herein has a composition (negative electrode active material) containing a carbon material capable of occluding and releasing lithium ions and lithium titanate at the negative electrode, and has a melting point such as γ-butyrolactone. A lithium secondary battery (lithium ion secondary battery) that can be manufactured and provided by using a non-aqueous electrolyte solution containing at least one lactone at 0 ° C. or lower (that is, a lactone-containing non-aqueous electrolyte solution), and other configurations There are no particular restrictions on the structure or shape (outer shape) of the element or the battery itself. The lithium secondary battery of the present invention uses conventional negative electrode active materials, electrode terminals, separators, laminate films and other casings, except that any of the negative electrode active materials and non-aqueous electrolytes mentioned here are used. And a desired shape (for example, a coin shape, a cylindrical shape, and a square shape) based on a manufacturing method similar to that of a conventional lithium secondary battery.

  Hereinafter, the present invention will be described in detail based on examples. In addition, this invention is not limited at all by the following Example, For example, lithium salt of electrolyte, positive electrode material, a separator, etc. can also be changed.

<Example 1: Production of lithium secondary battery (1)>
A cylindrical standard type 18650 type lithium secondary battery was manufactured as follows.
A negative electrode active material according to this example was prepared by mixing graphite powder and lithium titanate powder at a mass ratio of 95: 5. The obtained negative electrode active material and the binder SBR and CMC were mixed with ion-exchanged water to prepare a slurry-like negative electrode mixture according to this example. The approximate mass ratio of each material (other than water) contained in this negative electrode composite is 98% by mass for the negative electrode active material, 1% by mass for SBR, and 1% by mass for CMC.
This negative electrode mixture (slurry) was applied (attached) to both sides of a long copper foil having a thickness of about 15 μm as a negative electrode current collector and dried, and a negative electrode active material having a thickness of 120 μm on both sides of the copper foil current collector. A layer was formed. Subsequently, it pressed so that the whole thickness might be set to 85 micrometers. In this way, a negative electrode sheet was produced.

On the other hand, lithium nickelate (LiNiO ₂ ) powder as a positive electrode active material, acetylene black powder as a conductive material, and PTFE and CMC as binders are mixed with ion-exchanged water to form a slurry according to this example. A positive electrode mixture was prepared. The approximate mass ratio of each material (other than water) contained in the positive electrode composite is 90% by mass of lithium nickelate, 7% by mass of acetylene black, 2% by mass of PTFE, and 1% by mass of CMC.
This positive electrode mixture (slurry) is applied (attached) to both sides of a long aluminum foil having a thickness of about 15 μm as a positive electrode current collector and dried, and a positive electrode active material having a thickness of 120 μm on both sides of the aluminum foil current collector. A layer was formed. Subsequently, it pressed so that the whole thickness might be set to 85 micrometers. In this way, a positive electrode sheet was produced.

Ethylene carbonate (EC) and γ-butyrolactone (γ-BL) were mixed at a volume ratio of 30:70, and LiBF ₄ was dissolved in this mixed solvent to a concentration of 1.5 mol / L. A non-aqueous electrolyte solution according to an example was prepared.
And after attaching a current collection tab to the produced positive electrode sheet and negative electrode sheet, these sheets were laminated together with two separators (here, a porous polypropylene sheet was used), and this laminated sheet was wound. A wound electrode structure was produced. The electrode structure was accommodated in a predetermined battery container (casing), and the current collecting tab and the casing were welded. Next, the lactone-containing nonaqueous electrolytic solution was poured into the casing, and the inside of the casing was decompressed to impregnate the wound electrode structure disposed in the casing with the nonaqueous electrolytic solution. Thus, a cylindrical lithium secondary battery having a diameter of 18 mm and a height of 65 mm (that is, 18650 type) was manufactured.

<Example 2: Production of lithium secondary battery (2)>
A lithium secondary battery having the same shape was manufactured by the same material and process as in Example 1 except that a negative electrode active material having a mass ratio of graphite powder to lithium titanate powder of 90:10 was prepared and used. .

<Example 3: Production of lithium secondary battery (3)>
A lithium secondary battery having the same shape was manufactured by the same material and process as in Example 1 except that a negative electrode active material having a mass ratio of graphite powder to lithium titanate powder of 80:20 was prepared and used. .

<Example 4: Production of lithium secondary battery (4)>
A lithium secondary battery having the same shape was manufactured by the same material and process as in Example 1 except that a negative electrode active material having a mass ratio of graphite powder to lithium titanate powder of 99: 1 was prepared and used. .

<Example 5: Production of lithium secondary battery (5)>
Example 1 except that a non-aqueous electrolyte prepared using a mixed solvent having a volume ratio of EC to γ-BL of 40:60 (the type and concentration of the lithium salt is not changed) was used. A lithium secondary battery having the same shape was manufactured by the same material and process.

<Example 6: Production of lithium secondary battery (6)>
Example 1 except that a non-aqueous electrolyte prepared using a mixed solvent in which the volume ratio of EC to γ-BL is 20:80 (the type and concentration of the lithium salt are not changed) was used. A lithium secondary battery having the same shape was manufactured by the same material and process.

<Example 7: Production of lithium secondary battery (7)>
Example 1 except that a non-aqueous electrolyte prepared using a mixed solvent having a volume ratio of EC to γ-BL of 50:50 (the type and concentration of the lithium salt is not changed) is used. A lithium secondary battery having the same shape was manufactured by the same material and process.

<Example 8: Production of lithium secondary battery (8)>
Example 1 except that a non-aqueous electrolyte prepared using a mixed solvent having a volume ratio of EC and γ-BL of 10:90 (the type and concentration of the lithium salt is not changed) was used. A lithium secondary battery having the same shape was manufactured by the same material and process.

<Comparative Example 1: Production of lithium secondary battery (9)>
Only graphite powder (that is, lithium titanate is not used) is used as the negative electrode active material, and the volume ratio of EC and ethyl methyl carbonate (EMC) is 30:70 as the non-aqueous electrolyte, and as the supporting salt. The same as Example 1 except that LiPF ₆ was used instead of LiBF ₄ and the concentration thereof was 1.0 mol / L (that is, a non-aqueous electrolyte not containing lactones) was prepared and used. The lithium secondary battery of the same shape was manufactured by using the materials and processes.

<Comparative Example 2: Production of lithium secondary battery (10)>
A lithium secondary battery having the same shape is manufactured by the same material and process as in Example 1 except that only graphite powder (that is, lithium titanate is not used) is used as the negative electrode active material as in Comparative Example 1. did.

<Comparative Example 3: Production of Lithium Secondary Battery (11)>
Example except that a mixed carbon material (that is, lithium titanate is not used) having a mass ratio of graphite powder to hard carbon (non-graphitizable carbon) powder of 95: 5 was prepared and used as the negative electrode active material. A lithium secondary battery having the same shape was manufactured by the same material and process as in No. 1.
The composition (volume ratio) of the mixed solvent for electrolytic solution and the composition (mass ratio) of the negative electrode active material used in Examples 1 to 8 and Comparative Examples 1 to 3 are summarized in Table 1, respectively.

<Test Example 1: Measurement of initial output at 25 ° C.>
The initial output at 25 ° C. of the lithium secondary batteries according to the examples and comparative examples was measured by the following process.
That is, each of the lithium secondary batteries manufactured in each of the examples and comparative examples was first charged to reach 4.1 V at a constant current of 800 mA, and then discharged to 3 V at a constant current of 800 mA. Next, each battery was placed in a thermostat at 25 ° C. and left for 3 hours, and then charged to 3.7 V at a constant current of 800 mA. Thereafter, the current was appropriately changed with 3 V as the lower limit voltage, and pulse discharge / charge was performed for 10 seconds. Based on the current-voltage curve thus obtained, an initial output (W) at 25 ° C. was calculated. The results are shown in Table 2.

<Test Example 2: Measurement of 500 cycle capacity retention ratio>
The 500 cycle capacity retention rate of the lithium secondary batteries according to the examples and comparative examples was measured by the following process.
That is, each of the lithium secondary batteries manufactured in each of the examples and comparative examples was first charged to reach 4.1 V at a constant current of 800 mA, and then discharged to 3 V at a constant current of 800 mA. Next, each battery was placed in a thermostat at 60 ° C. and left for 3 hours, and then charged and discharged at a constant current of 1600 mA in a voltage range of 3V to 4.1V. This was made into 1 cycle, and charging / discharging was implemented by repeating a total of 500 cycles. The 500 cycle capacity retention rate (%) was calculated by dividing the discharge capacity at the 500th cycle by the discharge capacity at the first cycle and multiplying by 100. The results are shown in Table 2.

<Test Example 3: Measurement of negative electrode potential (discharge curve) with respect to discharge capacity>
For the lithium secondary batteries of Example 1, Comparative Example 2, and Comparative Example 3, the degree of change in the negative electrode potential with respect to the discharge capacity (that is, the discharge curve) was measured. In this test, a negative electrode of Example 1, Comparative Example 2 and Comparative Example 3 was used as a working electrode and an electrolytic solution, and a single electrode cell was produced using lithium metal as a counter electrode and a reference electrode. A constant current charge / discharge is performed at a charge rate of 0.2 C in the potential range of 0 V to 1.5 V on the basis of the reference electrode, and the transition of the negative electrode potential (V) with respect to the capacity (mAh / g) by negative electrode mass is measured. is there. The results are shown in FIG. The dotted line in the figure indicates the discharge curve for the negative electrode potential according to Example 1, the solid line indicates the negative electrode potential according to Comparative Example 2, and the alternate long and short dash line indicates the discharge curve for the negative electrode potential according to Comparative Example 3.
As is apparent from the illustrated discharge curve, the negative electrode potential of the battery according to Comparative Example 2 rapidly increased at the end of discharge. Further, the negative electrode potential of the battery according to Comparative Example 3 temporarily increased around 320 mAh / g, and then rapidly increased again at the end of discharge. In contrast to these comparative examples, the negative electrode potential of the battery according to Example 1 showed a gradual increase from around 320 mAh / g to the end of discharge.

As is clear from Table 2 showing the results of Test Examples 1 to 3 and FIG. 1, the lithium secondary battery provided by the present invention is excellent in cycle characteristics and can realize high output.
Moreover, although specific data are not shown, since the lactone containing non-aqueous electrolyte is used, it is excellent also in the low temperature characteristic.
In the lithium secondary battery provided by the present invention, the negative electrode potential rises slowly at the end of discharge, so that the SEI film formed on the negative electrode surface hardly breaks down, and an excessive amount of electrolyte remains even after repeated charge and discharge. Therefore, it is estimated that the cycle characteristics are improved.
Further, as apparent from Table 2, the cycle characteristics of the lithium secondary battery according to the present invention are clearly improved as compared with the battery according to Comparative Example 3 using the negative electrode active material added with hard carbon. Furthermore, the batteries according to Examples 1, 2, 5 and 6 exhibited excellent cycle characteristics and a high initial output of 25 ° C. of 15 W or more.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
Further, the technical elements described in the present specification exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in the present specification achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

It is a graph which shows the change (discharge curve) of the negative electrode potential with respect to the discharge capacity about each lithium secondary battery of Example 1, Comparative Example 2 and Comparative Example 3, and the horizontal axis is the discharge capacity per unit mass of the negative electrode (mAh / g). The vertical axis represents the negative electrode potential (V) relative to lithium metal (as a Li reference).

Claims (4)

A lithium secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte containing at least one lactone having a melting point of 0 ° C. or lower,
As a negative electrode active material, including a carbon material capable of occluding and releasing lithium ions and lithium titanate,
The lithium secondary battery whose content rate of the said carbon material with respect to the total amount of a negative electrode active material is 80-99 mass%, and whose content rate of lithium titanate is 1-20 mass%.   2. The lithium secondary battery according to claim 1, wherein a content ratio of the carbon material with respect to a total amount of the negative electrode active material is 90 to 95 mass% and a content ratio of lithium titanate is 5 to 10 mass%.   The lithium secondary battery according to claim 1 or 2, wherein a content of the lactone in a solvent constituting the non-aqueous electrolyte is 55 to 85% by volume.   The lithium secondary battery according to claim 1, wherein the lactone includes γ-butyrolactone.

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