JP5205863B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a long-life and safer non-aqueous electrolyte secondary battery.

In recent years, many portable electronic devices such as camera-type VTRs, mobile phones, laptop computers and the like have appeared, and their size and weight have been reduced. As portable power sources for these electronic devices, research and development for improving the energy density of batteries, particularly secondary batteries, are being actively promoted. Batteries using non-aqueous electrolytes, especially lithium ion secondary batteries, are expected to have higher energy density than lead batteries and nickel cadmium batteries, which are conventional aqueous electrolyte secondary batteries. The market is growing and the market is growing significantly.
In particular, since the characteristics of lithium ion secondary batteries such as light weight and high energy density are suitable for use in electric vehicles and hybrid electric vehicles, studies aiming to increase the size and output of the batteries have become active.

In a non-aqueous secondary battery represented by a lithium ion secondary battery, an oxide positive electrode such as LiCoO ₂ , LiNiO ₂ , or LiMn ₂ O ₄ is generally used as a positive electrode active material. This is because a high capacity, a high voltage can be obtained, and a high filling property can be obtained, which is advantageous for reducing the size and weight of portable devices. However, these positive electrodes start releasing oxygen at 200 to 300 ° C. when heated in a charged state. In addition, since a flammable organic electrolyte is used as the electrolyte, there is a risk that the battery may ignite when the oxygen release starts. Therefore, when the oxide positive electrode is used, it is not easy to ensure safety particularly in a large battery.
In contrast, A. K. It has been shown that the positive electrode material having an olivine structure reported by Padhi et al. Does not release oxygen even when the temperature exceeds 350 ° C. and is extremely excellent in safety (see Non-Patent Document 1).
J. et al. Electrocem. Soc. , Vol 144, p. 1188

In this olivine-based positive electrode, charge / discharge proceeds in a two-layer coexistence state of LiFePO ₄ and FePO ₄ , so that the potential flatness is very high. For this reason, when performing constant current / constant voltage charging, which is a charging method of a normal Li ion battery, there is a feature that charging is performed almost in a constant current charging state. As a result, compared with conventional positive electrode materials (LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, etc.), when charging at the same charging rate, the charging time can be shortened, but charging at a high current value is relatively Since it continues for a long time, the movement of Li ions in the negative electrode does not follow, the Li ion concentration at the negative electrode / electrolyte interface increases, and there arises a problem that Li metal tends to precipitate on the negative electrode. As a result, Li metal is deposited when the positive electrode / negative electrode capacity ratio (a / b) generally used in Li ion batteries using conventional positive electrode materials is in the range of 0.9 <(a / b) <1, There was a problem that the cycle life was not sufficient. Further, in the case of a conventional positive electrode material, if a / b <0.8, the positive electrode side potential reaches a high potential of 4.2 V or more, and the structural instability increases and the decomposition of the electrolytic solution easily proceeds. Cycle characteristics are not sufficient. Moreover, since the ratio of the negative electrode material to the internal volume of the battery is increased and disadvantageous for increasing the capacity of the battery, it has not been studied so far.
The present invention has been made in view of such problems, and an object thereof is to provide a non-aqueous electrolyte secondary battery that can improve cycle characteristics while using an olivine-based positive electrode.

As a result of intensive studies, the present invention has been found that the above-mentioned problems can be solved by optimizing the positive electrode / negative electrode capacity ratio.
That is, the present invention provides the following non-aqueous electrolyte secondary battery.
A nonaqueous electrolyte secondary battery comprising a positive electrode containing LiFePO4, a negative electrode containing a negative electrode active material that can be doped and dedoped with lithium, and a nonaqueous electrolyte solution,
When the discharge capacity per unit area of the positive electrode is a (mAh / cm ² ) and the discharge capacity per unit area of the negative electrode is b (mAh / cm ² ), the relationship between a and b is 0. 65 ≦ (a / b) ≦ 0. 7 5 non-aqueous electrolyte secondary battery which is a.

  According to the non-aqueous electrolyte secondary battery of the present invention, it is possible to reduce Li metal deposition and improve cycle characteristics by optimizing the positive electrode / negative electrode capacity ratio.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a cross-sectional structure of a secondary battery according to an embodiment of the present invention. This secondary battery is a so-called lithium ion secondary battery in which lithium is used as an electrode reactant and the capacity of the negative electrode is represented by a capacity component due to insertion and extraction of lithium. This secondary battery is called a so-called cylindrical type, and is a winding in which a pair of strip-like positive electrode 21 and strip-like negative electrode 22 are wound through a separator 23 inside a substantially hollow cylindrical battery can 11. A rotating electrode body 20 is provided. The battery can 11 is made of, for example, iron plated with nickel, and has one end closed and the other end open. Inside the battery can 11, a pair of insulating plates 12 and 13 are arranged perpendicular to the winding peripheral surface so as to sandwich the winding electrode body 20.

  At the open end of the battery can 11, a battery lid 14, a safety valve mechanism 15 provided inside the battery lid 14, and a heat sensitive resistance element (PTC element) 16 are interposed via a gasket 17. It is attached by caulking, and the inside of the battery can 11 is sealed. The battery lid 14 is made of, for example, the same material as the battery can 11. The safety valve mechanism 15 constitutes a current cutoff sealing body together with the heat sensitive resistance element 16. The safety valve mechanism 15 is electrically connected to the battery lid 14 via the heat sensitive resistance element 16, and when the internal pressure of the battery exceeds a certain level due to an internal short circuit or external heating, the disc plate 15A (power The lead-out plate is inverted to disconnect the electrical connection between the battery lid 14 and the wound electrode body 20. When the temperature rises, the heat sensitive resistance element 16 limits the current by increasing the resistance value and prevents abnormal heat generation due to a large current. The gasket 17 is made of, for example, an insulating material, and asphalt is applied to the surface.

  For example, a center pin 24 is inserted in the center of the wound electrode body 20. A positive electrode lead 25 made of aluminum or the like is connected to the positive electrode 21 of the spirally wound electrode body 20, and a negative electrode lead 26 made of nickel or the like is connected to the negative electrode 22. The positive electrode lead 25 is electrically connected to the battery lid 14 by being welded to the safety valve mechanism 15, and the negative electrode lead 26 is welded to and electrically connected to the battery can 11.

<Positive electrode>
FIG. 2 shows an enlarged part of the spirally wound electrode body 20 shown in FIG. As shown in FIG. 2, the positive electrode 21 has, for example, a structure in which a positive electrode active material layer 21B is provided on both surfaces of a positive electrode current collector 21A having a pair of opposed surfaces. Although not shown, the positive electrode active material layer 21B may be provided only on one surface of the positive electrode current collector 21A. The positive electrode current collector 21A is made of, for example, a metal foil such as an aluminum foil.

The positive electrode active material layer 21B contains LiFePO ₄ as a positive electrode active material. LiFePO ₄ is a lithium compound having an olivine type crystal structure. Furthermore, a conductive additive such as graphite, a binder such as polyvinylidene fluoride, and the like may be included as necessary. The content of the conductive auxiliary agent is preferably in the range of 1.5 to 10% by mass. Moreover, it is preferable that content of a binder exists in the range of 3-10 mass%.

  Moreover, it is preferable that the positive electrode active material layer 21B contains fibrous carbon. The content of fibrous carbon is preferably in the range of 1 to 5 mass% with respect to the positive electrode. By containing fibrous carbon within the above range, the internal resistance of the battery can be reduced. As the fibrous carbon, those having an average fiber diameter of 100 to 200 nm and an average fiber length of 5 to 20 μm are desirable. Specifically, vapor grown carbon fiber (VGCF), carbon nanotubes and the like can be preferably used.

<Negative electrode>
The negative electrode 22 has, for example, a structure in which a negative electrode active material layer 22B is provided on both surfaces of a negative electrode current collector 22A having a pair of opposed surfaces. Although not shown, the negative electrode active material layer 22B may be provided only on one surface of the negative electrode current collector 22A. The anode current collector 22A is made of, for example, a metal foil such as a copper foil.

  The negative electrode active material layer 22B includes one or more types of negative electrode materials that can be doped and dedoped with lithium as a negative electrode active material, and a binder similar to the positive electrode active material layer 21B as necessary. It is comprised including.

  Examples of such negative electrode active materials include carbon materials such as non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, or activated carbon. Is mentioned. Among these, examples of coke include pitch coke, needle coke, and petroleum coke. An organic polymer compound fired body refers to a carbonized material obtained by firing a polymer material such as phenol resin or furan resin at an appropriate temperature, and part of it is non-graphitizable carbon or graphitizable carbon. Some are classified as: These carbon materials are preferable because the change in crystal structure that occurs during charge and discharge is very small, a high charge and discharge capacity can be obtained, and good cycle characteristics can be obtained. In particular, graphite is preferable because it has a high electrochemical equivalent and can provide a high energy density. Further, non-graphitizable carbon is preferable because excellent characteristics can be obtained. Furthermore, those having a low charge / discharge potential, specifically, those having a charge / discharge potential close to that of lithium metal are preferable because a high energy density of the battery can be easily realized.

  The negative electrode active material layer 22B preferably further contains fibrous carbon. The content of fibrous carbon is preferably in the range of 0.5 to 3% by mass with respect to the negative electrode. By containing fibrous carbon within the above range, the internal resistance of the battery can be reduced. As fibrous carbon, the thing similar to what is contained in a positive electrode can be used.

  Examples of the negative electrode active material also include a material containing at least one of a metal element and a metalloid element that can be doped and dedoped with lithium as a constituent element. This is because a high energy density can be obtained by using such a material. In particular, the use with a carbon material is more preferable because a high energy density can be obtained and excellent cycle characteristics can be obtained. This negative electrode material may be a single element or an alloy or a compound of a metal element or a metalloid element, or may have at least a part of one or more of these phases. Note that alloys include those containing one or more metal elements and one or more metalloid elements in addition to those composed of two or more metal elements, and may contain non-metallic elements. Some alloys have a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or a mixture of two or more of them.

  Examples of the metal element or metalloid element include magnesium, boron, aluminum, gallium, indium, silicon, germanium, tin, lead, bismuth, cadmium, silver, zinc, hafnium, zirconium, yttrium, palladium, or platinum. . These may be crystalline or amorphous.

  Among the above metal elements or metalloid elements, those containing a group 4B metal element or metalloid element in the short-period type periodic table as a constituent element are preferable, and those containing at least one of silicon and tin as a constituent element are particularly preferable. . This is because silicon and tin have a large ability to occlude and release lithium, and a high energy density can be obtained.

In the non-aqueous electrolyte secondary battery of the present invention, it is essential that the discharge capacity ratio between the positive electrode and the negative electrode is within a specific range. That is, when the discharge capacity per unit area of the positive electrode is a (mAh / cm ² ) and the discharge capacity per unit area of the negative electrode is b (mAh / cm ² ), the relationship between a and b is 0.5 ≦ ( a / b) ≦ 0.85, preferably 0.65 ≦ (a / b) ≦ 0.8. Here, the discharge capacity a per unit area of the positive electrode is a coin in which one side of the positive electrode is peeled off, cut into a certain area (π × 7.5 mm × 7.5 mm), and then Li metal as a counter electrode. Type battery with a constant current of 0.1 mA and an upper limit voltage of 3.7 V, a constant current and constant voltage charge with a final current value of 0.05 mA, and a constant current discharge with a constant current of 0.1 mA and a final voltage of 2.0 V twice. Repeatedly, the value of the second discharge capacity. In addition, the discharge capacity b per unit area of the negative electrode is determined by peeling off one side of the negative electrode, cutting it out to a certain area (π × 7.5 mm × 7.5 mm), and then using a lithium battery as a counter electrode. , A constant current of 0.1 mA, a lower limit voltage of 0 V, a constant current and a constant voltage of 0.05 mA, and a constant current discharge of 0.1 mA and a final voltage of 2.0 V were repeated twice. The value of the discharge capacity.

  If the discharge capacity ratio of the electrode is within the above range, it is possible to prevent the lithium metal from being deposited, so that the cycle characteristics can be improved. When (a / b) <0.5, the amount of the negative electrode active material not involved in the battery reaction in the battery increases, the resistance of the battery increases, and a problem arises that high output cannot be obtained. It is not preferable. Further, in the case of a positive electrode active material having an olivine structure as in the present invention, the structure becomes unstable even when the potential reaches a higher potential than the normal positive electrode side with (a / b) <0.8. In addition, since the battery voltage usually has an upper limit of 3.6 to 3.8 V, decomposition of the electrolytic solution is not promoted.

<Separator>
The separator 23 separates the positive electrode 21 and the negative electrode 22 and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. The separator 23 is made of, for example, a porous film made of synthetic resin or ceramic, including polyethylene and at least one of polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, Al ₂ O ₃ , and SiO _2. Is preferred. Thereby, the oxidative decomposition of the separator physically contacting with the positive electrode during continuous charging can be suppressed, and a rapid current rise can be prevented. The separator may be a porous film formed by mixing at least one of polyethylene, polypropylene, and polytetrafluoroethylene, and Al ₂ O ₃ , polyvinylidene fluoride, and SiO ₂ may be applied to the surface of the porous film. Good. Further, the porous film may be laminated. A porous membrane made of polyolefin is preferable because it is excellent in short-circuit prevention effect and can improve battery safety by a shutdown effect.

<Non-aqueous electrolyte>
The nonaqueous electrolytic solution includes a nonaqueous solvent and an electrolyte salt dissolved in the solvent. As the non-aqueous solvent, cyclic carbonates such as ethylene carbonate or propylene carbonate can be used, and it is preferable to use one of ethylene carbonate and propylene carbonate, particularly a mixture of both. This is because the cycle characteristics can be improved.

  Further, as the non-aqueous solvent, it is preferable to use a mixture of chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, or methyl propyl carbonate in addition to these cyclic carbonates. This is because high ionic conductivity can be obtained.

  Furthermore, the non-aqueous solvent preferably contains 2,4-difluoroanisole or vinylene carbonate. This is because 2,4-difluoroanisole can improve discharge capacity, and vinylene carbonate can improve cycle characteristics. Therefore, it is preferable to use a mixture of these because the discharge capacity and cycle characteristics can be improved.

  Besides these, non-aqueous solvents include butylene carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1, 3-dioxolane, methyl acetate, methyl propionate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropironitrile, N, N-dimethylformamide, N-methylpyrrolidinone, N-methyloxazolidinone, N, N -Dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethyl sulfoxide or trimethyl phosphate.

  A compound obtained by substituting at least a part of hydrogen in these non-aqueous solvents with fluorine may be preferable because the reversibility of the electrode reaction may be improved depending on the type of electrode to be combined.

As electrolyte salt, lithium salt is mentioned, for example, 1 type may be used independently, and 2 or more types may be mixed and used for it. Lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ , LiCl, difluoro [oxolato-O, O ′] lithium borate, lithium bisoxalate borate, or LiBr. Among them, LiPF ₆ is preferable because it can obtain high ion conductivity and can improve cycle characteristics.

<Manufacturing method>
The secondary battery of the present invention can be manufactured, for example, as follows.

  First, the positive electrode can be manufactured as follows. For example, the positive electrode active material described above, a conductive agent, and a binder are mixed to prepare a positive electrode mixture, and the positive electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to form a paste. A positive electrode mixture slurry is prepared. Next, the positive electrode mixture slurry is applied to the positive electrode current collector 21A, the solvent is dried, and the positive electrode active material layer 21B is formed by compression molding using a roll press or the like, and the positive electrode 21 is manufactured.

  Moreover, a negative electrode can be manufactured as follows. For example, the above-described negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and the negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to obtain a paste-like negative electrode mixture Make a slurry. Next, the negative electrode mixture slurry is applied to the negative electrode current collector 22A, the solvent is dried, and the negative electrode active material layer 22B is formed by compression molding with a roll press machine or the like, and the negative electrode 22 is manufactured.

  Here, in order for the relationship between the discharge capacity a per unit area of the positive electrode and the discharge capacity b per unit area of the negative electrode to be 0.5 ≦ (a / b) ≦ 0.85, each electrode This can be done easily by adjusting the coating amount.

  Subsequently, the positive electrode lead 25 is attached to the positive electrode current collector 21A by welding or the like, and the negative electrode lead 26 is attached to the negative electrode current collector 22A by welding or the like. After that, the positive electrode 21 and the negative electrode 22 are wound through the separator 23, and the tip of the positive electrode lead 25 is welded to the safety valve mechanism 15, and the tip of the negative electrode lead 26 is welded to the battery can 11. The positive electrode 21 and the negative electrode 22 are sandwiched between a pair of insulating plates 12 and 13 and stored in the battery can 11. After the positive electrode 21 and the negative electrode 22 are accommodated in the battery can 11, the electrolytic solution is injected into the battery can 11 and impregnated in the separator 23. After that, the battery lid 14, the safety valve mechanism 15, and the heat sensitive resistance element 16 are fixed to the opening end of the battery can 11 by caulking through the gasket 17. Thereby, the secondary battery shown in FIG. 1 is formed.

  In the secondary battery, when charged, for example, lithium ions are released from the positive electrode active material layer 21B and inserted into the negative electrode active material layer 22B through the electrolytic solution. In addition, when discharging is performed, for example, lithium ions are released from the negative electrode active material layer 22B and inserted into the positive electrode active material layer 21B through the electrolytic solution.

  As mentioned above, although this invention was demonstrated by embodiment, this invention is not limited to the said embodiment, A various deformation | transformation is possible. For example, in the above embodiment, a secondary battery having a winding structure has been described, but the present invention can be similarly applied to a secondary battery having a structure in which a positive electrode and a negative electrode are folded or stacked. . In addition, the present invention can also be applied to a secondary battery such as a so-called coin type, button type, square type or laminated film type.

  Moreover, although the case where the electrolytic solution is used has been described in the above embodiment, the present invention can also be applied to the case where another electrolyte is used. Examples of other electrolytes include so-called gel electrolytes in which an electrolytic solution is held in a polymer compound.

  Hereinafter, the present invention will be specifically described based on Examples and Comparative Examples. The present invention is not limited to the embodiments described below.

<Discharge capacity per unit area of electrode>
For the discharge capacity a per unit area of the positive electrode and the discharge capacity b per unit area of the negative electrode, a / b is calculated. In the batteries of the following examples and comparative examples, a / b has a predetermined value. In order to determine the coating amount of the mixture and the negative electrode mixture, a coin-type battery was produced.
First, 85 parts by weight of a lithium phosphate compound having an olivine structure represented by LiFePO ₄ coated with carbon, 10 parts by weight of polyvinylidene fluoride, 5 parts by weight of artificial graphite, and N-methylpyrrolidone outside the amount are kneaded, A positive electrode mixture paint was obtained. This was applied to one side of a 15 μm thick aluminum foil, dried, and then pressed to form a positive electrode. 90 parts by weight of artificial graphite, 10 parts by weight of polyvinylidene fluoride, and N-methylpyrrolidone outside the amount were kneaded to obtain a negative electrode mixture paint. This was applied to one side of a 15 μm thick copper foil, dried and then pressed to form a negative electrode.
This positive and negative electrode is punched into a circle with a diameter of 15 mm, the counter electrode is sandwiched between a Li microporous film and a polypropylene microporous film with a thickness of 25 μm, and the electrolyte is ethylene carbonate (EC) and dimethyl carbonate (DMC) A coin-type battery was manufactured using 1 mol / l LiPF ₆ dissolved in an equal volume mixed solvent. Using this coin type battery, the positive electrode has a constant current of 0.1 mA and an upper limit voltage of 3.7 V, a constant current and constant voltage charge with a final current value of 0.05 mA, and a final current of 2.0 mA with a constant current of 0.1 mA. The constant current discharge was repeated twice, and the discharge capacity value for the second time was defined as a. The discharge capacity b per unit area of the negative electrode is a constant current of 0.1 mA, a lower limit voltage of 0 V, a constant current constant voltage charge of a final current value of 0.05 mA, and a final voltage of 2.0 V at a constant current of 0.1 mA. This constant current discharge was repeated twice, and the discharge capacity value for the second time. Thereby, a / b can be calculated.

<Example 1>
For both the positive and negative electrodes, a positive electrode and a negative electrode were prepared in the same manner as that used in the coin type battery except that the positive electrode mixture and the negative electrode mixture were applied to both surfaces of the metal foil. The coating amount of the positive electrode mixture and the negative electrode mixture was adjusted to be a / b = 0.85. Then, the positive electrode and the negative electrode are wound with a 25 μm-thick polypropylene microporous film sandwiched between them, and a non-aqueous electrolyte similar to that used in the coin-type battery is placed in a metal case with a diameter of 18 mm and a height of 65 mm. A 18650 size cylindrical battery with a capacity of 1 Ah was produced.

<Example 2>
A cylindrical battery was produced in the same manner as in Example 1 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.80.

<Example 3>
A cylindrical battery was fabricated in the same manner as in Example 1 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.65.

<Example 4>
A cylindrical battery was produced in the same manner as in Example 1 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.50.

<Example 5>
85 parts by weight of an olivine positive electrode coated with carbon, 10 parts by weight of polyvinylidene fluoride, 4 parts by weight of artificial graphite, 1 part by weight of fibrous carbon material, and N-methylpyrrolidone outside the amount are mixed to obtain a positive electrode mixture paint. It was. This was coated on both sides of a 15 μm thick aluminum foil, dried and then pressed to form a positive electrode. 89 parts by weight of artificial graphite, 10 parts by weight of polyvinylidene fluoride, 1 part by weight of a fibrous carbon material, and N-methylpyrrolidone outside the amount were kneaded to obtain a negative electrode mixture paint. This was applied to both sides of a 15 μm thick copper foil, dried and then pressed to form a negative electrode. As the fibrous carbon, both positive and negative electrodes having an average fiber diameter of 150 nm and an average fiber length of 10 μm were used. Here, the application amount of the positive electrode mixture and the negative electrode mixture was adjusted so that a / b = 0.85, and a cylindrical battery was fabricated in the same manner as in Example 1 except that.

<Example 6>
A cylindrical battery was produced in the same manner as in Example 5 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.75.

<Comparative Example 1>
A cylindrical battery was fabricated in the same manner as in Example 1 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.90.

<Comparative example 2>
A cylindrical battery was fabricated in the same manner as in Example 1 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.45.

<Comparative Example 3>
A cylindrical battery was fabricated in the same manner as in Example 5 except that the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.45.

<Comparative example 4>
Implementation was performed except that the lithium phosphate compound having an olivine structure represented by carbon-coated LiFePO ₄ was LiCoO ₂ and the coating amounts of the positive electrode mixture and the negative electrode mixture were adjusted so that a / b = 0.50. A cylindrical battery was produced in the same manner as in Example 1.

<Evaluation method 1>
In Examples 1 to 6 and Comparative Examples 1 to 4, a cycle test was performed in which charging was repeated at a constant current and a constant voltage of 1A_3.6V, an end current value of 0.1A, and discharging was repeated at a constant current of 3A_2.0V. The rate of change of the discharge capacity with the number of cycles when the initial discharge capacity was 100% was calculated as the capacity maintenance rate. The results of Examples 1 to 4 and Comparative Examples 1, 2 and 4 in which the carbon material of the electrode is only graphite are shown in Table 1, and the results of Examples 5 and 6 and Comparative Example 3 in which the carbon material is graphite and fibrous carbon. Are shown in Table 2. Moreover, the result of Table 1 was made into the graph and was shown in FIG. The result of Table 2 and the result of Example 1 are combined into a graph and shown in FIG. Furthermore, among the results in Tables 1 and 2, the capacity retention rate after 500 cycles with respect to a / b is shown as a graph in FIG.

<Evaluation method 2>
In Examples 1 to 6 and Comparative Examples 1 to 3, after charging was performed at a constant current constant voltage of 1A_3.6V and a final current value of 0.1 A, the battery voltage when discharged at 35 A for 5 seconds was measured. The case where the voltage at this time does not fall below 2.0 V was taken as an example. The results of Examples 1 to 4 and Comparative Examples 1 and 2 where the carbon material of the electrode is only graphite are shown in Table 1, and the results of Examples 5 and 6 and Comparative Example 3 where the carbon material is graphite and fibrous carbon. Are shown in Table 2.

From the results shown in Table 1 and FIG. 3, when comparing Example 1 with Comparative Example 1, Example 1 in which a / b = 0.85 was maintained in comparison with Comparative Example 1 with a / b = 0.90. The rate has improved dramatically. From this, it was found that the upper limit value of a / b was 0.85.
Further, when Examples 1 to 4 and Comparative Example 2 are compared, a decrease in capacity retention rate is suppressed as a / b decreases, but Comparative Example 2 in which a / b = 0.45 is evaluated. Since the battery voltage when discharged for 5 seconds at 35 A tested in Method 2 was below 2.0 V, an undesirable result was obtained. Below 2.0V, the resistance of the battery increases and the required amount of current cannot flow. In contrast, in Example 3 where a / b = 0.50, the battery voltage exceeded 2.0V. From this, it was found that the lower limit value of a / b was 0.50.

  Comparative Example 4 uses a lithium compound that does not have an olivine structure as the positive electrode active material, but a / b = 0.50, which is within the scope of the present invention, the capacity retention rate is It did not improve. Compared to Example 4 where a / b was 0.50 and the same value, the capacity retention rate was significantly reduced. From this, it was found that even when the discharge capacity ratio of the present invention was applied to a lithium compound having no olivine structure, the cycle characteristics were not improved.

Next, from the results of Table 2 and FIG. 4, Example 5 in which the carbon material is graphite and fibrous carbon is the same value as Example 1 in which the carbon material is only graphite and a / b = 0.85. However, the capacity retention rate of Example 5 was improved. From this, it was found that the capacity retention rate was further improved by containing fibrous carbon in the carbon material of the electrode. Further, in Example 3 including fibrous carbon and a / b = 0.75, the capacity retention rate was further improved.
On the other hand, Comparative Example 3 in which a / b = 0.45 was not preferable because the battery voltage when discharged at 35 A for 5 seconds was less than 2.0 V. It has been found that even if fibrous carbon is contained, a preferable battery voltage cannot be obtained when a / b is less than 0.50.

  From FIG. 5, when a / b exceeded 0.85, the capacity retention rate after 500 cycles rapidly decreased. From this, it was confirmed again that the upper limit of a / b is 0.85. Furthermore, it was found that if a / b is 0.80 or less, a high capacity retention rate can be maintained regardless of the presence or absence of fibrous carbon.

It is sectional drawing showing the structure of the secondary battery which concerns on one embodiment of this invention. It is sectional drawing which expands and represents a part of winding electrode body in the secondary battery shown in FIG. It is a graph which shows the change of the capacity | capacitance maintenance factor accompanying the cycle number of Examples 1-4 and the cylindrical secondary battery of Comparative Examples 1, 2, and 4. FIG. It is a graph which shows the change of the capacity | capacitance maintenance factor accompanying the cycle number of the cylindrical secondary battery of Example 5, 6 and the comparative example 3. FIG. It is a graph which shows the change of the capacity | capacitance maintenance factor after 500 cycles with respect to a / b.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Battery can, 12, 13 ... Insulation board, 14 ... Battery cover, 15 ... Safety valve mechanism, 15A ... Disc board, 16 ... Heat sensitive resistance element, 17 ... Gasket, 20 ... Winding electrode body, 21 ... Positive electrode, 21A ... positive electrode current collector, 21B ... positive electrode active material layer, 22 ... negative electrode, 22A ... negative electrode current collector, 22B ... negative electrode active material layer, 23 ... separator, 24 ... center pin, 25 ... positive electrode lead, 26 ... negative electrode lead

Claims (2)

A non-aqueous electrolyte secondary battery comprising a positive electrode containing LiFePO ₄ , a negative electrode containing a negative electrode active material that can be doped and dedoped with lithium, and a non-aqueous electrolyte,
When the discharge capacity per unit area of the positive electrode is a (mAh / cm ² ) and the discharge capacity per unit area of the negative electrode is b (mAh / cm ² ), the relationship between a and b is 0. 65 ≦ (a / b) ≦ 0. 7 5 non-aqueous electrolyte secondary battery which is a.   The nonaqueous electrolyte secondary battery according to claim 1, wherein at least one of the positive electrode and the negative electrode further contains fibrous carbon.

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