JP4994628B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to an improvement in a non-aqueous electrolyte secondary battery, in particular, a negative electrode and a non-aqueous electrolyte of a high-power lithium ion secondary battery.

  In recent years, non-aqueous electrolyte secondary batteries have been put into practical use as a power source for driving portable electronic devices such as mobile phones, notebook computers, and video camcorders as secondary batteries with high operating voltage and high energy density. Have achieved. In particular, the production volume of lithium ion secondary batteries continues to increase as a battery system that leads small secondary batteries.

As a positive electrode active material of a lithium ion secondary battery, a lithium-containing composite oxide having a high voltage of 4V is used. For example, LiCoO ₂ having a hexagonal crystal structure, LiNiO ₂ , and LiMn ₂ O ₄ having a spinel structure are typical lithium-containing composite oxides. Among these, LiCoO ₂ having a high operating voltage and a high energy density is mainly used as the positive electrode active material.
As the negative electrode active material, a carbon material capable of inserting and extracting lithium ions is used. In particular, a graphite material is mainly used from the viewpoint of realizing a flat discharge potential and a high capacity density.

  Recently, non-aqueous electrolyte secondary batteries are being developed not only for small consumer applications, but also for large-capacity large batteries for power storage and electric vehicles. In particular, in a hybrid electric vehicle (HEV), vehicles equipped with nickel metal hydride batteries have already been mass-produced and marketed as a measure to overcome environmental problems.

  In addition, as an alternative to nickel metal hydride batteries, development of lithium ion secondary batteries for HEV has been rapidly advanced, and some of them have begun to be put into practical use. In the future, the spread of fuel cell vehicles is expected, but lithium ion secondary batteries are promising as secondary batteries with high output and long life for assisting fuel cells.

  As described above, a highly crystalline graphite material having a high capacity density and a high current efficiency is suitably used for the negative electrode active material of a lithium ion secondary battery in which high capacity is particularly desired. Propylene carbonate (PC) is used as a solvent for non-aqueous electrolytes because it has a high dielectric constant, a low melting point, and is generally stable in a wide redox potential region. However, PC has a large interaction with the graphite surface and is easily decomposed electrochemically on the graphite surface. Therefore, when a nonaqueous electrolyte containing PC is used, the lithium ion secondary battery may not be able to be charged.

  It has also been proposed to use ethylene carbonate (EC) instead of PC. By using EC, the battery can be charged / discharged even when a negative electrode using graphite as an active material is used. EC forms a stable coating on the graphite surface. This is believed to facilitate the intercalation of lithium ions. However, EC has a high melting point and is solid at room temperature. For this reason, it is usually necessary to use EC in a mixture with a low-melting chain carbonate or ether.

  It is also possible to use a mixed solvent of EC and PC. In this case, in order to form a stabilization coating derived from EC, relax the interaction between PC and the graphite surface, and sufficiently advance the charging reaction, the ratio of EC in the total of EC and PC is increased. It is required to do.

  Such a lithium ion battery using a non-aqueous electrolyte containing EC as a main component of a non-aqueous solvent has a lower temperature characteristic than a lithium ion battery using a non-aqueous electrolyte containing PC as a main component of a non-aqueous solvent. Tend to be inferior. In addition, a non-aqueous electrolyte containing EC as a main component of a non-aqueous solvent exhibits significant gas generation due to decomposition when a lithium ion battery is stored at a high temperature. In particular, in the case of a battery having a square structure or a battery having an exterior made of a laminate sheet, the swelling of the battery is a serious problem.

In order to solve such a problem, it is conceivable to use low crystalline carbon or non-graphitizable carbon as a negative electrode active material instead of graphite, and adopt a PC solvent as a non-aqueous electrolyte solvent. . However, as shown in FIG. 4, low crystalline carbon and non-graphitizable carbon have a large slope of the charge curve and discharge curve compared to the graphite material. Somewhat low. Here, in FIG. 4, the discharge curve and discharge curve of non-graphitizable carbon are represented by X ₁ and X ₂ , respectively, and the discharge curve and discharge curve of the graphite material are represented by Y ₁ and Y ₂ , respectively.
In addition, since the current efficiency is low, the irreversible capacity increases and the battery capacity decreases. Furthermore, since low-crystalline carbon and non-graphitizable carbon have a low true density, there is a problem that the capacity density per unit volume is low.

  Recently, several attempts have been made to use a non-aqueous electrolyte containing PC as a main solvent in a battery using a graphite-based negative electrode. For example, it has been proposed to suppress decomposition of PC by coating the surface of graphite particles with amorphous carbon (see Patent Document 1). In addition, a proposal to use a mixed solvent of PC and EC and limit the PC ratio to less than 10% by volume (see Patent Document 2), using a non-aqueous electrolyte containing PC as a main solvent, added to the non-aqueous electrolyte A proposal to add an agent (see Patent Document 3) has been made.

On the other hand, in order to improve battery characteristics, it has been proposed to use a graphite material having a c-axis direction crystallite thickness Lc of 60 nm or more and less than 100 nm as a negative electrode active material (see Patent Document 4).
JP 2002-241117 A Japanese Patent Laid-Open No. 9-120838 JP-A-6-13107 JP 2000-260480 A

Thus, satisfactory output performance cannot be obtained in an extremely low temperature range of 0 ° C. or lower, particularly about −30 ° C. In particular, it is a big problem in batteries that require high output such as for HEV. In a high temperature environment, the constituent components of the nonaqueous electrolyte are decomposed and gas is easily generated. Therefore, when it is stored for a long time in a high temperature environment, the battery characteristics are deteriorated, and the battery is swollen, so that the durability is lowered.
These things depend on the degree of graphitization of the carbon material and the interaction between the carbon material and the solvent constituting the nonaqueous electrolyte. Since the conventional negative electrode using graphite as an active material is generally combined with a non-aqueous electrolyte containing EC as a main component of a non-aqueous solvent, the above-described problems occur. That is, when graphite is used as a negative electrode active material and a nonaqueous electrolyte containing EC as a main component of a nonaqueous solvent is used, it is difficult to achieve both low-temperature output performance and long-term durability performance of the battery.

  Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery that has a long life and high output even in a low temperature environment.

The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode including a lithium-containing composite oxide, a negative electrode including a carbon material capable of inserting and extracting lithium, and a non-aqueous electrolyte, and the carbon material is graphitizable carbon. In the wide-angle X-ray diffraction pattern of graphitizable carbon material, which is measured using CuKα rays, including the material, it belongs to the (100) plane of the peak intensity I (101) attributed to the (101) plane The ratio to peak intensity I (100) is given by the following formula:
0.5 ≦ I (101) / I (100) ≦ 0.7
And the crystallite thickness Lc (004) in the c-axis direction of the graphitizable carbon material is 20 nm or more and less than 60 nm. The non-aqueous electrolyte includes a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. The non-aqueous solvent includes propylene carbonate (PC) and ethylene carbonate (EC), and propylene carbonate and ethylene carbonate occupy 40% by volume or more and 80% by volume or less of the non-aqueous solvent. Furthermore, the ratio of propylene carbonate in the total of propylene carbonate and ethylene carbonate is 60% by volume or more and 90% by volume or less.

Then, in the above KiHisui electrolyte secondary batteries, non-aqueous solvent further comprises at least one linear carbonate selected from dimethyl carbonate and ethyl methyl carbonate, 20% by volume ratio of the chain carbonate is non-aqueous solvent It is preferable that it is 60 volume% or less.

  When graphitizable carbon having the above physical properties is used as the negative electrode active material, charging and discharging are possible even in a battery using a non-aqueous solvent containing propylene carbonate as a main solvent if it contains a small amount of ethylene carbonate. Is possible. Moreover, by making propylene carbonate a main solvent, it becomes possible to reduce the ratio of low-viscosity solvents other than propylene carbonate and ethylene carbonate as a result. For this reason, it becomes possible to provide a nonaqueous electrolyte secondary battery that is excellent in long-term durability while ensuring low-temperature performance. Furthermore, the graphitizable carbon used in the present invention has a graphite hexagonal network planar structure. Further, the graphitized carbon of the graphitizable carbon is small and the edge surface ratio is small. Therefore, despite the fact that the non-aqueous solvent mainly contains propylene carbonate, it becomes easy to form a coating film composed of a decomposition product of ethylene carbonate on the edge surface of the graphitizable carbon, and the intercalation reaction of lithium ions is facilitated. Promoted. Therefore, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery that has high output even in a low temperature environment and excellent long-term durability.

  That is, according to the present invention, since the non-aqueous electrolyte contains propylene carbonate, it is possible to prevent the decomposition of propylene carbonate, etc. A battery can be provided.

The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode including a lithium-containing composite oxide, a negative electrode including a carbon material capable of inserting and extracting lithium, and a non-aqueous electrolyte. Here, the carbon material includes a graphitizable carbon material, and in a wide-angle X-ray diffraction pattern measured using CuKα rays, the peak intensity attributed to the (101) plane of the graphitizable carbon material. The ratio of I (101) to the peak intensity I (100) attributed to the (100) plane is given by the following formula:
0.5 ≦ I (101) / I (100) ≦ 0.7
Meet. Further, the crystallite thickness Lc (004) in the c-axis direction of the graphitizable carbon material is 20 nm or more and 60 nm or less.
Non-aqueous electrolyte contains a solute dissolved in a nonaqueous solvent and a nonaqueous solvent, the nonaqueous solvent propylene carbonate, et Chirenkabone Doo and at least one linear carbonate selected from dimethyl carbonate and ethyl methyl carbonate And including. Here, propylene carbonate and ethylene carbonate occupy 40% by volume or more and 80% by volume or less of the nonaqueous solvent. Moreover, the ratio of the propylene carbonate which occupies for the sum total of propylene carbonate and ethylene carbonate is 60 volume% or more and 90 volume% or less.

The positive electrode, for example, can be composed of the positive electrode current collector and the positive electrode material mixture layer carried on each side thereof. Similarly, a negative electrode can be comprised from the negative electrode collector and the negative mix layer carry | supported by the both surfaces, for example. The positive electrode mixture layer can be composed of, for example, a positive electrode active material, a conductive material, and a binder. The negative electrode mixture layer can be composed of, for example, a negative electrode active material and a binder.

As described above, a lithium-containing composite oxide is used as the positive electrode active material. Examples of the lithium-containing composite oxide include LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ having a spinel structure. In order to improve the cycle life characteristics, a part of the transition metal contained in the lithium-containing composite oxide may be substituted with another element. For example, LiNiO _{2 in} which a part of Ni element is substituted with Co or another element (Al, Mn, Ti, etc.) can be used as the positive electrode active material.

  As described above, as the negative electrode active material, a carbon material capable of inserting and extracting lithium is used. Such carbon materials include graphitizable carbon materials. In the present invention, an easily graphitizable carbon material is different from low crystalline carbon and non-graphitizable carbon such as coke having almost no graphite layer structure, and has a certain degree of graphite hexagonal network plane structure, in the process of graphitization. A carbon material in a state.

  As a parameter indicating the degree of graphitization of a carbon material, generally, a value of a spacing between (002) planes or a crystallite size value by powder X-ray diffraction is used. In the present invention, as a parameter representing the degree of graphitization, the ratio of the peak intensity I (101) attributed to the (101) plane to the peak intensity I (100) attributed to the (100) plane (hereinafter referred to as the peak). Intensity ratio I (101) / I (100)) and c-axis direction crystallite thickness Lc (004). Here, the peak intensity ratio I (101) / I (100) is a ratio between the height of the peak intensity I (101) and the height of the peak intensity I (100).

The graphitizable carbon material used in the present invention has a peak intensity ratio I (101) / I (100) in the wide-angle X-ray diffraction pattern measured using CuKα rays, and the following formula:
0.5 ≦ I (101) / I (100) ≦ 0.7
Meet. Furthermore, the thickness Lc (004) of the crystallite in the c-axis direction of the graphitizable carbon material is 20 nm or more and less than 60 nm. The peak intensity ratio I (101) / I (100) is more preferably 0.6 or more and 0.8 or less.

As described above, the non-aqueous electrolyte includes a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. In the present invention, the non-aqueous solvent contains propylene carbonate, and d Ji Ren carbonate, at least one linear carbonate selected from dimethyl carbonate and ethyl methyl carbonate, and. Here, the ratio of PC and EC in the non-aqueous solvent is 40% by volume to 80% by volume. Moreover, the ratio of PC to the sum total of PC and EC is 60 volume%-90 volume%. Thus, in the present invention, it is important that PC is the main solvent in the non-aqueous solvent.

By using the graphitizable carbon material as described above, for example, since decomposition of PC is suppressed even under high temperature storage, a non-aqueous solvent containing PC as a main solvent can be used. Moreover, since PC is the main solvent, the low temperature characteristics of the battery can be improved.
The graphitizable carbon material used in the present invention has a graphite hexagonal plane structure, a small crystallite size, and a small edge surface ratio. Furthermore, EC is also contained in the nonaqueous electrolyte as a nonaqueous solvent. Therefore, although the non-aqueous solvent mainly contains PC, it is easy to form a film made of a decomposition product of EC on the edge surface of the graphitizable carbon material. Lithium ions are solvated with an organic solvent such as PC, and when lithium ions intercalate between graphite layers (edge portions), repulsion occurs and PC may be decomposed. By forming a film made of an EC decomposition product on the edge surface of the graphitizable carbon material, lithium ions are desolvated and only lithium ions are intercalated into the carbon material. For this reason, the intercalation reaction of lithium ions is promoted.
Therefore, it is possible to obtain a battery having a long life and high output even in a low temperature environment.

When the peak intensity ratio I (101) / I (100) is less than 0.5, the carbon material is included in the class of low crystalline carbon materials having almost no graphite structure rather than being graphitized. It is. Such a carbon material can be used together with a non-aqueous electrolyte using PC as a main solvent. However, such a low crystalline carbon material has a large irreversible capacity, a small battery capacity, and poor high-rate discharge performance. On the other hand, when the peak intensity ratio I (101) / I (100) exceeds 0.7 , the carbon material is in a state of crystallinity called so-called graphite. For this reason, when a non-aqueous electrolyte mainly composed of PC is used, gas generation accompanying the decomposition of PC becomes remarkable. In this case, if the ratio of PC is reduced in order to suppress gas generation, the low temperature characteristics become insufficient, and the high temperature storage performance also deteriorates.

  The crystallite thickness Lc (004) in the c-axis direction is 20 nm or more and 60 nm or less from the viewpoint of charge acceptance and capacity.

  In the graphitizable carbon material, lithium ions are stored between layers by an intercalation reaction. However, the capacity density of the graphitizable carbon material does not reach 372 Ah / kg, which is the theoretical capacity density of graphite, but is about 240 Ah / kg to 270 Ah / kg.

When the proportion of PC and EC in the non-aqueous solvent is less than 40% by volume, the other over 60% is mainly a low melting point solvent such as a chain carbonate. However, these low-melting-point solvents generally have a low dielectric constant and tend to lack electrochemical stability, and are not suitable for extending the life. On the other hand, when the proportion of PC and EC in the non-aqueous solvent exceeds 80% by volume, the viscosity of the non-aqueous electrolyte increases and sufficient low-temperature performance cannot be obtained.
When the ratio of PC to the total of PC and EC is less than 60% by volume, the influence of EC becomes large, the amount of gas generation increases, and the life performance decreases. On the other hand, when the proportion of PC exceeds 90% by volume, the PC is decomposed and gas is generated during the initial charge.

In the present invention, the nonaqueous solvent contains chain carbonates such as dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in addition to PC and EC . At this time , the proportion of the chain carbonate in the nonaqueous solvent is preferably 20% by volume to 60% by volume. Thus, since the non-aqueous solvent further contains a chain carbonate, the viscosity of the electrolytic solution can be lowered, so that the low temperature characteristics and the output characteristics can be further improved .

  In addition, as a electrically conductive material, a binder, etc., a well-known thing can be used without specifically limiting. Examples of the positive electrode binder and the negative electrode binder include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), fluorine, and the like. Vinylidene chloride-hexafluoropropylene copolymer and the like are used.

  Examples of the conductive material include graphites, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and other carbon blacks, carbon fibers, metal fibers, and the like.

  For the positive electrode current collector, for example, a sheet made of stainless steel, aluminum, titanium, or the like is used. For the negative electrode current collector, for example, a sheet made of stainless steel, nickel, copper, or the like is used. The thicknesses of the positive electrode current collector and the negative electrode current collector are not particularly limited, but are preferably 1 to 500 μm.

  Hereinafter, the present invention will be described based on examples. The present invention is not limited to these examples.

(Preparation of positive electrode)
As the positive electrode active material, a lithium nickel composite oxide represented by a composition formula LiNi _0.55 Co _0.3 Al _0.15 O ₂ was used. This positive electrode active material was produced as follows.
A saturated aqueous solution was prepared by adding Co sulfate and Al sulfate to NiSO ₄ aqueous solution at a predetermined ratio.
Next, nickel hydroxide was obtained by a precipitation method.
First, while stirring the obtained saturated aqueous solution, a sodium hydroxide aqueous solution was slowly added dropwise to neutralize the saturated aqueous solution to obtain a precipitate. The resulting precipitate was filtered, washed with water, and dried at 80 ° C. to obtain ternary nickel hydroxide Ni _0.55 Co _0.3 Al _0.15 (OH) ₂ . The average particle diameter of the obtained nickel hydroxide was about 10 μm.

Next, the obtained ternary nickel hydroxide and lithium hydroxide monohydrate were mixed. At this time, the sum of the number of moles of Ni, Co, and Al was made equal to the number of moles of Li. Next, the obtained mixture was heat-treated at 800 ° C. for 10 hours in dry air to obtain a lithium nickel composite oxide represented by a composition formula LiNi _0.55 Co _0.3 Al _0.15 O ₂ .

  The obtained lithium nickel composite oxide was confirmed by powder X-ray diffraction method to have a single-phase hexagonal layered structure and that Co and Al were dissolved.

  Next, the lithium nickel composite oxide was pulverized and classified to obtain a positive electrode active material powder.

  100 parts by weight of the positive electrode active material and 5 parts by weight of acetylene black as a conductive material were mixed. An N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF) as a binder was added to the obtained mixture and kneaded to obtain a positive electrode mixture paste. The amount of PVdF was 5 parts by weight per 100 parts by weight of the active material.

Next, the positive electrode mixture paste was applied to both surfaces of an aluminum foil as a positive electrode current collector, dried, and rolled to obtain a positive electrode having a thickness of 0.075 mm, a mixture width of 100 mm, and a length of 2800 mm. Here, the total thickness of the positive electrode mixture layer formed on both surfaces of the current collector was 0.055 mm (55 μm).
The positive electrode was heat-treated (dried) at 110 ° C. for 10 hours in the atmosphere.

(Preparation of negative electrode)
A negative electrode active material was prepared as follows.
The blocky coke produced in the heat treatment process of the anisotropic pitch was heat-treated at 2000 ° C. in an argon atmosphere to obtain the target graphitizable carbon material powder. The average particle diameter of the obtained graphitizable carbon material powder was about 9 μm.

The obtained graphitizable carbon material was analyzed by an X-ray diffraction method.
The interplanar spacing (d ₀₀₂ ) of the graphitizable carbon material (d ₀₀₂ ) and the crystallite thickness Lc (004) in the c-axis direction are known as Gakushin methods using high-purity silicon powder as an internal standard substance. Calculated by the method. As a result, the spacing (d ₀₀₂ ) was 0.340 nm and the thickness Lc was 30 nm.

  Further, the obtained graphitizable carbon material was measured by a wide angle X-ray diffraction method using CuΚα rays. As a result, a diffraction peak attributed to the (100) plane was observed around 2θ of 42 °, and a slight diffraction peak attributed to the (101) plane was observed near 44 °. The peak intensity ratio I (101) / I (100) after removing the background was 0.70.

The negative electrode was produced in substantially the same manner as the positive electrode.
An NMP solution of PVdF was kneaded with 100 parts by weight of the carbon powder to obtain a negative electrode mixture paste. Here, the amount of PVdF was 8 parts by weight per 100 parts by weight of the carbon powder.

  Next, the negative electrode mixture paste was coated on both sides of a copper foil as a negative electrode current collector, dried and rolled to obtain a negative electrode having a thickness of 0.110 mm, a mixture width of 105 mm, and a length of 3000 mm. The total thickness of the negative electrode mixture layers formed on both surfaces of the negative electrode current collector was 95 μm.

(Battery assembly)
Using the positive electrode and the negative electrode obtained as described above, a square battery as shown in FIG. 1 was produced.
The obtained positive electrode and negative electrode were wound in an ellipsoidal shape through a separator made of a polyethylene microporous film having a thickness of 0.027 mm and a width of 108 mm, the center part was compressed, and the flat electrode group 1 was Configured. An aluminum positive electrode lead 2 and a nickel negative electrode lead 3 were welded to the positive electrode and the negative electrode, respectively.

  Next, an insulating ring (not shown) made of polyethylene resin was attached to the upper part of the electrode plate group 1 and accommodated in a rectangular battery case 4 having a width of 80 mm, a thickness of 12 mm, and a height of 120 mm. The other end of the positive electrode lead 2 was spot welded to the aluminum sealing plate 5. The other end of the negative electrode lead 3 was spot welded to the lower part of the nickel negative electrode terminal 6 in the center of the sealing plate 5. The sealing plate 5 and the negative electrode terminal 6 are insulated by an insulating material 7.

Next, the opening end of the battery case 4 and the peripheral edge of the sealing plate 5 were laser welded, and then a predetermined amount of nonaqueous electrolyte was injected from the inlet provided in the sealing plate. Finally, the inlet was closed with an aluminum plug 8 and sealed by laser welding to complete the battery. At this time, the negative electrode was designed to have a capacity density of about 200 Ah / kg in a fully charged state. The capacity density of the negative electrode is calculated by dividing the fully charged battery capacity by the weight of the carbon material contained in the negative electrode mixture layer portion facing the positive electrode mixture layer. The fully charged state means a state where the battery is charged up to a predetermined charging upper limit voltage.
As the non-aqueous electrolyte, a solution in which 1.0 mol / l LiPF ₆ was dissolved in a solvent in which PC, EC and dimethyl carbonate (DMC) were mixed at a volume ratio of 40:20:40 was used.

  The battery thus obtained was used as the battery of Example 1.

Comparative Example 1

  Graphite-based carbon material powder was obtained by subjecting the massive coke produced in the heat treatment process of anisotropic pitch to heat treatment at 2800 ° C. in an argon atmosphere. This carbon material was used as a negative electrode active material. The average particle size was about 9 μm.

In the same manner as in Example 1, the graphitized carbon material was analyzed. As a result, the (002) plane spacing (d ₀₀₂ ) was 0.335 nm and Lc (004) was 120 nm. The peak intensity ratio I (101) / I (100) after removing the background was 1.80.

  A battery was fabricated in the same manner as in Example 1 except that the above negative electrode active material was used. The obtained battery was used as the battery of Comparative Example 1.

Comparative Example 2

  A battery was fabricated in the same manner as in Example 1, except that commercially available non-graphitizable carbon (Carbotron P manufactured by Kureha Chemical Industry Co., Ltd., average particle size 10 μm) was used as the negative electrode active material. The obtained battery was used as the battery of Comparative Example 2.

The non-graphitizable carbon was analyzed in the same manner as in Example 1. As a result, the surface interval (d ₀₀₂ ) of the 002 plane was 0.380 nm, and Lc (004) was 10 nm. In addition, the peak which can be assigned to (101) plane and (100) plane was not recognized, and it was suggested that the said non-graphitizable carbon is a turbostratic structure which hardly has a graphite structure.

Comparative Example 3

  A battery was fabricated in the same manner as in Example 1 except that the ratio of PC, EC, and DMC contained in the nonaqueous solvent was 20:40:40 by volume ratio. The obtained battery was used as the battery of Comparative Example 3.

(Evaluation)
The batteries of these Examples and Comparative Examples were repeatedly charged and discharged for 3 cycles under the conditions of a constant current of 1.8 A, a charge upper limit voltage of 4.2 V, and a discharge lower limit voltage of 2.5 V in a 25 ° C. environment. The discharge capacity at the third cycle is shown in Table 1.

Next, in order to evaluate the output characteristics of these batteries, a current-voltage characteristic test was performed according to the following procedure.
First, each battery was charged at a predetermined constant current value so as to be in a 50% state of charge (SOC) in a 25 ° C. environment. Next, the charged battery was left in a 0 ° C. environment for 6 hours or more.

  Next, with respect to the battery after standing, the discharge pulse and the charge pulse as shown in FIG. 2 are repeated in an environment of 0 ° C., and each discharge pulse is applied to measure the battery voltage after 10 seconds. Plotted against.

  Next, as shown in FIG. 3, the least square method is used to obtain a straight line that best represents the measured value, and the straight line is extrapolated to the discharge lower limit voltage of 2.5 V, and the predicted current value at 2.5 V is obtained. I (A) was determined. The output (W) when the battery voltage was 2.5 V was calculated from (W) = I (A) × 2.5 (V).

  Table 1 shows the discharge capacity at 25 ° C. and the output (W) at 0 ° C. However, in the battery of Comparative Example 1, the behavior seen as a gas generation reaction during the initial charge was noticeable, and the battery case was greatly swollen. Further, only a capacity of 1 Ah or less was obtained by the subsequent discharge. For this reason, subsequent tests were stopped.

From the results of Table 1, it can be seen that the battery of Example 1 using non-graphite carbon which is graphitized as a negative electrode active material and using a non-aqueous electrolyte containing PC as a main solvent gives high output in a low temperature environment. Recognize. On the other hand, the battery of Comparative Example 1 using a carbon material that has been graphitized by heat treatment at 2800 ° C. as the negative electrode active material uses an electrolyte mainly composed of PC. It was impossible to function.
The battery of Comparative Example 2 using non-graphitizable carbon as the negative electrode active material had a low temperature output of 200 W and a low discharge capacity of 4.9 Ah. This is presumably due to a decrease in battery capacity due to the large irreversible capacity of non-graphitizable carbon.

  Moreover, since the battery of Comparative Example 3 using the same negative electrode carbon material as in Example 1 uses a nonaqueous electrolyte having a higher EC ratio than PC, the low-temperature output was reduced.

  From these results, a battery having a high capacity and a high output at a low temperature can be obtained by combining a graphitizable carbon negative electrode in the process of graphitization with a nonaqueous electrolyte containing PC as a main solvent, as in the battery of Example 1. It can be seen that

As the positive electrode active material, a lithium nickel composite oxide represented by a composition formula LiNi _0.4 Co _0.3 Mn _0.3 O ₂ was used. This positive electrode active material was produced by a precipitation method.
A saturated aqueous solution was prepared by adding Co sulfate and Al sulfate to NiSO ₄ aqueous solution at a predetermined ratio. Next, while stirring the obtained saturated aqueous solution, a sodium hydroxide aqueous solution was slowly added dropwise to neutralize the saturated aqueous solution to obtain a precipitate. The resulting precipitate was filtered, washed with water, and dried at 80 ° C. to obtain ternary nickel hydroxide Ni _0.4 Co _0.3 Mn _0.3 (OH) ₂ . The average particle diameter of the obtained nickel hydroxide was about 9 μm.

Next, the obtained ternary nickel hydroxide and lithium hydroxide monohydrate were mixed. At this time, the sum of the number of moles of Ni, Co, and Al was made equal to the number of moles of Li. Next, the obtained mixture was heat-treated in dry air at 850 ° C. for 10 hours to obtain a lithium nickel composite oxide represented by a composition formula LiNi _0.4 Co _0.3 Mn _0.3 O ₂ .

  The obtained lithium nickel composite oxide was confirmed to have a single-phase hexagonal layered structure and to have Co and Mn dissolved by powder X-ray diffraction.

  Subsequently, the lithium nickel composite oxide was pulverized and classified to obtain a positive electrode active material powder, and a positive electrode was produced in the same manner as in Example 1.

  Negative electrode active materials A to G were obtained by changing the heat treatment temperature of the massive coke produced in the heat treatment process of the anisotropic pitch used in Example 1 as shown in Table 2. Also in this example, the peak intensity ratios I (101) / I (100) and Lc (004) were used to evaluate the degree of graphitization of the obtained negative electrode active material.

Negative electrodes A to G were produced in the same manner as Example 1 using the obtained negative electrode active materials A to G. Using negative electrodes A to G, batteries A to G were produced in the same manner as in Example 1. Further, for the obtained batteries A to G, the discharge capacity and the output at 0 ° C. were measured in the same manner as in Example 1. The obtained results are shown in Table 2 together with the results of the battery of Example 1. Table 2 also shows the heat treatment temperature, the peak intensity ratio, and the value of Lc (004). The batteries A, B, and G are comparative examples, and the batteries E and F are reference examples .

As shown in Table 2, from the viewpoint of the discharge capacity and output, the most favorable embodiment 1, The battery F Celsius to batteries also showed good good results. Therefore, the carbon material that has been moderately graphitized, that is, the value of I (101) / I (100) is 0.5 to 1. It can be seen that it is important to use a negative electrode in the range of 0, further 0.5 to 0.7 , and Lc (004) of 20 nm to 60 nm. Further, the A value of 0.7 I (101) / I (100 ), it can be seen that particularly excellent in output characteristics.

  On the other hand, in the batteries A and B using the carbon material having a low degree of graphitization, the capacity reduction accompanying the large irreversible capacity of the negative electrode was observed, and the output value was small. In the battery G, side reactions such as gas generation occurred, and as a result, the discharge capacity and the output value were small.

In the battery of Example 1, in the same manner as in Example 1, except that the ratio of PC, EC, and DMC constituting the nonaqueous solvent was changed as shown in Table 3, the batteries H to O were Produced. Here, the batteries H, I, K and O are comparative examples . Table 3 shows the ratio of PC to the total of PC and EC (hereinafter also referred to as PC ratio).

  These batteries H to O were charged and discharged at 25 ° C. for 3 cycles in the same manner as in Example 1, and the discharge capacity at the third cycle was set as the initial capacity.

  Next, in the same manner as in Example 1, an output test at 0 ° C. was performed at 50% SOC.

  Furthermore, the battery was charged again to SOC 80% in a 25 ° C. environment and stored in a 60 ° C. environment for 28 days. The battery thickness before storage and the battery thickness after storage were measured, and the difference in battery thickness before and after storage was defined as battery swelling.

  The battery after storage was charged and discharged three times between 2.5 V and 4.2 V at a constant current of 1.8 A under a 25 ° C. environment, and the third discharge capacity was defined as the recovery capacity. The capacity recovery rate was determined by {(recovery capacity) / (initial capacity)} × 100 (%).

  Table 3 shows the output at 0 ° C., battery swelling and capacity recovery rate. However, since the side reaction which seemed to generate | occur | produce gas at the time of the first charge occurred remarkably and the battery I was difficult to charge / discharge, the subsequent test was stopped. This is considered due to the fact that the non-aqueous electrolyte does not contain EC at all.

  As shown in Table 3, the batteries H and K exhibited significant battery swelling due to storage, a low capacity recovery rate, and poor durability. This is considered to be caused by gas generation due to decomposition of the nonaqueous electrolyte during storage.

  Gas is generated in the battery H because the proportion of PC and EC in the non-aqueous solvent is as small as 30% by volume and the proportion of DMC is as dominant as 70% by volume, so the stability of the non-aqueous electrolyte is low. It is thought to be due to On the other hand, Battery J exhibited good output characteristics and storage characteristics. For this reason, it is considered that the sum of PC and EC needs to occupy at least 40% by volume of the non-aqueous solvent.

  In the battery K, the total proportion of PC and EC in the non-aqueous solvent is 50% by volume, but the proportion of PC in the total of PC and EC is as small as 50% by volume. In this case, it is considered that gas generation was promoted because the instability of EC was not sufficiently eliminated by PC.

  The batteries L to N all have good storage characteristics. Therefore, the proportion of PC and EC in the non-aqueous solvent is 40% by volume or more and 80% by volume or less, and the proportion of PC in the total of PC and EC needs to be 60% by volume to 90% by volume.

  Battery O was inferior in output characteristics. This is presumably because the ratio of PC and EC in the non-aqueous solvent is dominant at 90% by volume, so that the viscosity of the non-aqueous electrolyte is increased and the low-temperature output characteristics are deteriorated.

  From the above, the proportion of PC and EC in the non-aqueous solvent is 40% by volume to 80% by volume, and the proportion of PC in the total of PC and EC is 60% by volume to 90% by volume. Thus, it is possible to provide a non-aqueous electrolyte secondary battery that provides a high output in a low temperature environment and has a long life.

  As the positive electrode active material, any lithium-containing composite oxide such as lithium manganese composite oxide and lithium cobalt composite oxide can be used in addition to the lithium nickel composite oxide used in the above examples. Moreover, even if it is an oxide which does not contain lithium, the oxide which can contain lithium previously by chemical or electrochemical operation can also be used.

  Moreover, in the said Example, a nonaqueous solvent consists of DMC which is a chain carbonate of PC, EC, and a low-viscosity solvent. Instead of the DMC, conventionally known solvents such as chain carbonates such as diethyl carbonate and ethyl methyl carbonate, aliphatic carboxylic acid esters such as methyl propionate and ethyl propionate can be used. In addition, a mixed solvent obtained by mixing a solvent having 4V class oxidation-reduction potential and PC and EC can also be used as the non-aqueous solvent.

As the solute dissolved in the non-aqueous solvent, conventionally known solutes such as LiBF ₄ and LiClO ₄ can be used in addition to LiPF ₆ .

As the separator, a separator made of a polyethylene microporous film was used, but it is not particularly limited as long as it is a microporous thin film having a large ion permeability and a predetermined mechanical strength and having an insulating property. be able to. Examples of such a separator include sheets made of olefin polymers such as polypropylene or glass fibers, nonwoven fabrics, and woven fabrics. Moreover, generally the thickness of a separator is 10-300 micrometers.
Even when the non-aqueous electrolyte secondary battery is configured by using the gel-like polymer carrying the non-aqueous electrolyte instead of the separator, the same effect as described above can be obtained.

  Furthermore, in the above embodiment, the present invention has been described using a rectangular battery using a flat electrode group, but a plurality of cylindrical batteries or thin electrodes stacked in a cylindrical case by winding the electrodes in a spiral shape are stacked. A similar effect can be obtained by using a rectangular battery housed in a rectangular battery case.

  The nonaqueous electrolyte secondary battery of the present invention is expected to be used as a secondary battery for assisting an electric motor such as a hybrid electric vehicle or a fuel cell vehicle that requires high output and long life under a low temperature environment. It can also be used as a driving power source having high output such as an electric tool.

The perspective view which notched a part of the produced square type nonaqueous electrolyte secondary battery produced in the Example is shown. It is a figure which shows the test procedure of the electric current-voltage characteristic test done in the Example. It is a figure explaining how to obtain the current value when the battery voltage is 2.5V. It is a graph which shows an example of the charge curve and discharge curve of a non-graphitizable carbon material and a graphite material.

DESCRIPTION OF SYMBOLS 1 Electrode plate group 2 Positive electrode lead 3 Negative electrode lead 4 Battery case 5 Sealing plate 6 Negative electrode terminal 7 Insulation material 8 Sealing

Claims (1)

A positive electrode including a lithium-containing composite oxide, a negative electrode including a carbon material capable of inserting and extracting lithium, and a non-aqueous electrolyte,
The carbon material includes an easily graphitizable carbon material,
In the wide-angle X-ray diffraction pattern of the graphitizable carbon material measured using CuKα rays, the peak intensity I attributed to the (100) plane of the peak intensity I attributed to the (101) plane (101) The ratio to (100) is:
0.5 ≦ I (101) / I (100) ≦ 0.7
The filling,
The crystallite thickness Lc (004) in the c-axis direction of the graphitizable carbon material is 20 nm or more and 60 nm or less,
The non-aqueous electrolyte includes a non-aqueous solvent and a solute dissolved in the non-aqueous solvent,
The non-aqueous solvents include propylene carbonate, and d Chirenkabone doo, and at least one linear carbonate selected from dimethyl carbonate and ethyl methyl carbonate, and,
The propylene carbonate and the ethylene carbonate occupy 40% by volume or more and 80% by volume or less of the non-aqueous solvent, and the chain carbonate occupies 20% by volume or more and 60% by volume or less of the non-aqueous solvent,
The nonaqueous electrolyte secondary battery in which the proportion of the propylene carbonate in the total of the propylene carbonate and the ethylene carbonate is 60% by volume or more and 90% by volume or less.

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