JP2008171661A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery capable of quick charging. <P>SOLUTION: In the lithium-ion secondary battery provided with a negative electrode made of a negative electrode active material capable of inserting/desorbing lithium ion, a positive electrode made of a positive electrode active material capable of inserting/desorbing lithium ion, a porous plastic sheet arranged between the negative electrode and the positive electrode for insulating each electrode from the other, and nonaqueous electrolyte solution containing lithium ion, a ratio A/C of a volume A of lithium which the negative electrode can insert/desorb to a volume C of lithium which the cathode can insert/desorb is to be set at: 1.1≤A/C≤1.6. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery suitable for rapid charging and having good cycle characteristics.

  Lithium ion secondary batteries have higher energy density than other secondary batteries such as nickel cadmium batteries and nickel metal hydride batteries, so they are used in mobile phones, laptop computers, video cameras, etc. Spread as a power source. Currently, research and development of lithium-ion batteries with various performances are being carried out according to specifications, from small batteries for portable devices to large batteries such as hybrid cars and electric cars. In particular, portable devices such as mobile phones, digital cameras, and digital audio devices that are convenient to carry and can be used outdoors are required to have smaller and higher capacity batteries and rapid charging characteristics. .

A lithium ion secondary battery includes a positive electrode composed of a lithium-containing transition metal oxide such as lithium cobaltate, lithium nickelate, and lithium manganate, and a negative electrode composed of a material capable of inserting and removing lithium ions such as carbon. And a non-aqueous electrolytic solution in which a lithium salt such as LiPF ₆ is dissolved in an organic solvent.

  The lithium ion secondary battery is charged by detaching lithium ions from the positive electrode and inserting the lithium ion into the negative electrode. Lithium ion migration occurs inside the actual battery. If the movement amount of the lithium ions is larger than the insertion amount into the negative electrode, lithium metal is deposited on the negative electrode. When lithium metal is deposited, the form is often dendritic. If this deposited metal grows from above the negative electrode and breaks through the separator that insulates the positive electrode from the negative electrode and reaches the positive electrode, an internal short circuit may occur, leading to a decrease in battery voltage and eventually heat generation.

  In order not to cause such a problem, that is, to prevent lithium metal from being deposited on the negative electrode, as in Patent Documents 1 to 3, the amount of lithium ions that can be removed from the positive electrode by the amount of lithium ions that can be accepted by the negative electrode It is designed to be more than that.

  Further, from the viewpoint of increasing the energy density, a device has been devised so that the amount of lithium ions that can be accepted by the negative electrode is substantially the same as the amount of lithium ions desorbed from the positive electrode.

  Thus, when the amount of lithium ions that can be received by the negative electrode is larger than the amount of lithium ions desorbed from the positive electrode, theoretically, no lithium metal is deposited on the negative electrode.

Japanese Patent No. 2734822 Japanese Patent No. 3028582 Japanese Patent No. 3413656

  Due to the increasing demand for rapid charging, when performing rapid charging, it is necessary to flow a large current from the power source of the charging source into the lithium ion secondary battery, but the speed of the lithium ions inserted into the negative electrode is higher than the charging speed. As a result, the electrons on the negative electrode react with lithium ions, and lithium metal is deposited on the negative electrode, which adversely affects battery characteristics and safety. For this reason, quick charging has not been possible until now.

  The present invention has been made in view of the above problems, and provides a lithium ion secondary battery in which the deposition of lithium metal is suppressed by improving the acceptability of the negative electrode even when rapid charging is performed, and the cycle characteristics are good. For the purpose.

  As a result of various studies to achieve the above object, the present inventors have determined that the ratio A / C between the lithium capacity A of the opposing negative electrode and the lithium capacity C of the positive electrode is 1.1 ≦ A / C ≦ 1. .6, it was found that rapid charging was possible without precipitating lithium metal on the negative electrode, and the present invention was completed. Even in the prior art, the lithium capacity A of the negative electrode is set large by about 5%, but this was sufficient because it considered only the completion of charging, but it was insufficient for rapid charging. It is necessary to increase the ratio significantly.

  The present invention is arranged between a negative electrode made of a negative electrode active material capable of inserting and removing lithium ions, a positive electrode made of a positive electrode active material capable of inserting and removing lithium ions, and between the positive electrode and the negative electrode, In a lithium ion secondary battery comprising a separator that insulates between electrodes and a non-aqueous electrolyte containing lithium ions, a lithium capacity A in which the negative electrode can be inserted and removed, and the positive electrode can be inserted and removed The ratio A / C to the lithium capacity C is set to 1.1 ≦ A / C ≦ 1.6.

  When the lithium capacity A of the negative electrode is sufficiently larger than the lithium capacity C of the positive electrode, the acceptability is increased. When 1.1 ≦ A / C ≦ 1.6, the lithium metal is deposited even when rapid charging is performed. Is suppressed, and convenience is improved. That is, according to the present invention, it is possible to provide a lithium ion secondary battery that can be rapidly charged and also has good cycle characteristics.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The lithium ion secondary battery of the present embodiment includes a positive electrode, a negative electrode, and a separator. As the positive electrode active material, lithium-containing transition metal oxides such as lithium cobaltate, lithium nickelate and lithium manganate are suitable. The negative electrode is preferably an electrode made of graphite or amorphous carbon capable of inserting and removing lithium ions and mixed with a binder appropriately selected according to characteristics important for the battery. Furthermore, the ratio A / C between the lithium capacity A in which the negative electrode can be inserted / removed and the lithium capacity C in which the positive electrode can be inserted / removed is set to 1.1 ≦ A / C ≦ 1.6. When A / C is lower than 1.1, it is not suitable for rapid charging. When A / C is higher than 1.6, the rapid charging characteristics are excellent, but the energy density of the battery is low. It is no longer practical.

The lithium cobaltate may be a general LiCoO ₂ having a plateau near 4 V at the metallic lithium counter electrode, improving the thermal stability so that the crystal structure does not become unstable even when the amount of lithium ion desorption increases. It is desirable to add Mg, Al, Zr or the like to the surface, or insert or substitute at the Co site.

Lithium nickelate has a plateau in the vicinity of 4 V at the metallic lithium counter electrode, and LiNi _1-x Co _x O ₂ in which Ni sites are partially substituted with Co in order to improve thermal stability and cycle characteristics. LiNi _1-xy Co _x Al _y O ₂ into which Al is inserted is desirable.

Lithium manganate composition formula _{Li 1 + x Mn 2-xy} M y O 4-z (0.03 ≦ x ≦ 0.16,0 ≦ y ≦ 0.1 with plateau 4V near lithium metal counter electrode, −0.1 ≦ z ≦ 0.1, and M = Mg, Al, Ti, Co, or Ni is preferably selected.

The particle shape of these Li-containing transition metal oxides is not particularly limited, such as a lump, a sphere, or a plate, and the particle size and specific surface area are appropriately selected in consideration of the positive electrode film thickness, the positive electrode density, the binder type, and the like. However, in order to keep the energy density high, it is desirable that the density of the positive electrode in the portion from which the current collector metal foil is removed is 2.8 × 10 ⁻⁶ g / m ³ or more. Moreover, it is desirable that the weight ratio of the positive electrode active material in the positive electrode mixture composed of the positive electrode active material, the binder, the conductivity imparting agent, etc. is 80% or more.

For lithium _{manganate, Li 1 + x Mn 2-} xy M y O 4-z (0.03 ≦ x ≦ 0.16,0 ≦ y ≦ 0.1, -0.1 ≦ z ≦ 0.1 , M = Mg, Al, Ti, Co, Ni), as a starting material used for the synthesis of Li ₂ CO ₃ , LiOH, Li ₂ O, Li ₂ SO ₄ or the like as a Li source. However, the maximum particle size is preferably 2 μm or less in order to improve the reactivity with the Mn source and the crystallinity of the synthesized lithium manganate. MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ , MnOOH, MnCO ₃ , Mn (NO ₃ ) ₂ or the like can be used as the Mn source, but the maximum particle size is preferably 30 μm or less. Among them, Li ₂ CO ₃ is used as the Li source and MnO ₂ , Mn ₂ O _3, or Mn ₃ O ₄ is used as the Mn source from the viewpoint of cost, ease of handling, and ease of obtaining an active material with high filling properties. Is particularly preferred.

Similarly, in the case of lithium nickelate, NiO ₂ , Ni ₂ O ₃ , Ni ₃ O ₄ , NiOOH, NiCO ₃ , Ni (NO ₃ ) _2, etc. can be used as the Ni source. 30 μm or less is desirable. Further, from the viewpoint of cost, ease of handling, and ease of obtaining an active material with high filling properties, Li ₂ CO ₃ is particularly preferable as the Li source, and NiO ₂ , Ni ₂ O ₃ or Ni ₃ O ₄ is particularly preferable as the Ni source. .

Similarly, in the case of lithium cobaltate, CoO ₂ , Co ₂ O ₃ , Co ₃ O ₄ , CoOOH, CoCO ₃ , Co (NO ₃ ) _2, etc. can be used as the Co source. 30 μm or less is desirable. Further, from the viewpoint of cost, ease of handling, and ease of obtaining an active material with high filling properties, Li ₂ CO ₃ is particularly preferable as the Li source, and CoO ₂ , Co ₂ O ₃ or Co ₃ O ₄ is particularly preferable as the Co source. .

Hereinafter, a method for synthesizing the Li-containing transition metal oxide will be described. The above starting materials are appropriately selected, and weighed and mixed so that a predetermined metal composition ratio is obtained. At this time, in order to improve the reactivity of the Li source with the Mn source, the Ni source, and the Co source, and to avoid the remaining of different phases of Mn ₂ O ₃ , Ni ₂ O ₃ , and Co ₂ O ₃ , the Li source The maximum particle size of the raw material is preferably 2 μm or less, and the maximum particle size of the raw materials of the Mn source, Ni source, and Co source is preferably 30 μm or less. Mixing may be performed by selecting an apparatus such as a ball mill, a V-type mixer, a cutter mixer, or a shaker. The obtained mixed powder is fired in an atmosphere having an oxygen concentration in air or higher in a temperature range of 600 ° C to 950 ° C.

  This lithium manganate, lithium nickelate, and lithium cobaltate are mixed with a binder and a conductivity imparting agent such as acetylene black or carbon to form an electrode. The binder may be a commonly used resin binder, and polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like can be used. The current collector metal foil is preferably an Al foil.

  The negative electrode uses graphite or amorphous carbon that can insert and desorb lithium ions, and attach importance to batteries such as rate characteristics, output characteristics, low-temperature discharge characteristics, pulse discharge characteristics, energy density, weight reduction, and miniaturization. It mixes with the binder suitably selected according to the characteristic, and it is set as an electrode. As the binder, commonly used polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like can be used, and a rubber-based binder can also be used. As the current collector metal foil, a Cu foil is preferable.

  For the separator, a porous plastic film having a three-layer structure of polypropylene or polypropylene, polyethylene, and polypropylene can be used. The thickness is not particularly limited, but is preferably 10 μm to 30 μm in consideration of rate characteristics, battery energy density, and mechanical strength.

  As the solvent for the non-aqueous electrolyte, those commonly used may be used. For example, carbonates, ethers, ketones and the like can be used. Preferably, at least one kind from ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), etc. as the high dielectric constant solvent, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate as the low viscosity solvent At least one selected from (EMC), esters and the like, and a mixed solution thereof is used. EC + DEC, EC + EMC, EC + DMC, PC + DEC, PC + EMC, PC + DMC, PC + EC + DEC, etc. are preferable. good. Furthermore, a trace amount of additives may be added for the purpose of improving water consumption, oxidation resistance, safety, and the like.

As the supporting salt, LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) N, Li (C ₂ F ₅ SO ₂ ) ₂ N or the like is used, but a system containing LiPF ₆ is preferable. The concentration of the supporting salt is preferably 0.8 mol / L to 1.5 mol / L, more preferably 0.9 mol / L to 1.2 mol / L.

  Next, a basic manufacturing procedure of the lithium ion secondary battery in the example of the present invention will be described.

  FIG. 1 is a perspective view of a laminate inside the battery of the present invention. However, for the sake of explanation, the thickness direction is emphasized. As the positive electrode active material 4, for example, lithium manganate is used, and this and a conductivity-imparting agent are dry-mixed, and uniformly dispersed in N-methyl-2-pyrrolidone (NMP) in which PVdF as a binder is dissolved. Was made. The slurry was applied on an aluminum metal foil 5 having a thickness of 20 μm, and NMP was evaporated to prepare a positive electrode sheet. The solid content ratio in the positive electrode was wt%, and lithium manganate: conductivity imparting agent: PVdF = 90: 6: 4. The positive electrode sheet was formed to have a width of 55 mm and a height of 100 mm, and the aluminum metal foil 5 of the active material uncoated portion 6 was formed to have a width of 10 mm and a height of 15 mm for the electrode part.

  A negative electrode active material 7 is uniformly dispersed in N-methyl-2-pyrrolidone (NMP) in which PVdF as a binder is dissolved to prepare a slurry, and the slurry is applied onto a copper foil 8 having a thickness of 15 μm. Then, the negative electrode sheet was produced by evaporating NMP. The solid content ratio in the negative electrode was set to graphite: PVdF = 90: 10 by weight%. The negative electrode sheet was formed to have a width of 59 mm and a height of 104 mm, and the copper foil 8 of the active material uncoated portion 6 was formed to have a width of 10 mm and a height of 15 mm for the electrode part.

The positive electrode sheet and the negative electrode sheet thus prepared were stacked in a plurality of layers via a porous membrane separator 9 having a three-layer structure of polypropylene or polypropylene / polyethylene / polypropylene having a thickness of 25 μm to prepare a laminate. At that time, the active material uncoated portions 6 of the positive electrode and the negative electrode were collected for the positive electrode or the negative electrode, and an external current extraction tab of aluminum was used for the positive electrode and nickel was applied to the negative electrode by ultrasonic welding. The obtained laminate was heat-sealed using a laminate film embossed in accordance with the shape of the laminate on one side and a flat laminate film on the other. In the electrolytic solution, 1 mol / L LiPF ₆ was used as a supporting salt, and ethylene carbonate (EC): diethyl carbonate (DEC) = 30: 70 (volume%) was used as a solvent.

  The manufactured lithium ion secondary battery is actually 0.3 C (C is a time rate. 1 C is a current value at which charging or discharging is completed in 1 hour. 0.2 C is completed in 5 hours in order to function as a battery. The battery was charged with a constant current and a constant voltage up to 4.2 V for 10 hours.

(Example 1)
FIG. 2 is a schematic external view of a laminate cell. FIG. 2 shows a laminated film-covered lithium ion secondary battery 1 (hereinafter, laminated cell 1) produced using lithium manganate as the positive electrode. The laminate cell 1 is provided with a positive terminal 2 and a negative terminal 3 protruding. The active material coating amount of the aluminum metal foil used for the positive electrode sheet and the copper foil used for the negative electrode sheet so that the ratio A / C of the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode of the laminate cell 1 is 1.1. The active material application amount was adjusted and applied, and NMP was evaporated by drying to produce a laminate cell 1. The A / C value was changed by setting the coating weight of the lithium capacity C to 1 and the coating weight of the lithium capacity A to 1.1. In the following examples, the change in the A / C value was compared by changing the coating weight of the negative electrode while keeping the coating weight of the positive electrode constant.

(Example 2)
The laminate cell 1 was produced such that the ratio A / C between the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode was 1.6. The rest of the configuration was the same as in Example 1.

(Example 3)
A mixed positive electrode of lithium manganate and lithium nickelate (lithium manganate: lithium nickelate = 80: 20 wt%) is used as the positive electrode, and the ratio A / C between the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode is 1 A laminate cell 1 was produced so that the ratio was 1. The rest of the configuration was the same as in Example 1.

Example 4
A mixed positive electrode of lithium manganate and lithium nickelate (lithium manganate: lithium nickelate = 80: 20 wt%) is used as the positive electrode, and the ratio A / C between the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode is 1 A laminate cell 1 was produced so that the thickness was 6. The rest of the configuration was the same as in Example 1.

(Example 5)
Laminate cell 1 was produced using lithium cobaltate as the positive electrode and the ratio A / C of lithium capacity A of the negative electrode to lithium capacity C of the positive electrode being 1.1. The rest of the configuration was the same as in Example 1.

(Example 6)
Laminate cell 1 was produced using lithium cobaltate as the positive electrode and the ratio A / C of lithium capacity A of the negative electrode to lithium capacity C of the positive electrode being 1.6. The rest of the configuration was the same as in Example 1.

(Comparative Example 1)
A laminate cell was prepared using lithium manganate as the positive electrode and the ratio A / C between the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode being 1.05. Otherwise, the same procedure as in Example 1 was performed.

(Comparative Example 2)
A mixed positive electrode of lithium manganate and lithium nickelate (lithium manganate: lithium nickelate = 80: 20 wt%) is used as the positive electrode, and the ratio A / C between the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode is 1 A laminate cell was prepared so that the thickness was .05. Other than that was carried out similarly to Example 3.

(Comparative Example 3)
A laminate cell was prepared using lithium cobalt oxide as the positive electrode and the ratio A / C of the lithium capacity A of the negative electrode to the lithium capacity C of the positive electrode being 1.05. Other than that was carried out similarly to Example 5.

(Quick charge test)
These laminate cells were subjected to a quick charge test of 4.2 V at a current value of 7C. Here, constant charge charging is performed at the beginning, but after the charge voltage reaches 4.2V, the apparatus is set to perform constant voltage charge at a charge voltage of 4.2V. FIG. 3 shows the results of the quick charge test. The results of the charging time and charging ratio of Example 1, Example 2, and Comparative Example 1 are shown in FIG. It can be seen that Example 2 in which the ratio A / C between the lithium capacity A of the negative electrode and the lithium capacity C of the positive electrode is 1.6 is very excellent in quick charge characteristics. In addition, A / C = 1.1 in Example 1 reaches 90% in 20 minutes. On the other hand, it can be seen that Comparative Example 1 is not suitable for rapid charging.

  Similarly, experiments were performed on Examples 3 to 6 and Comparative Examples 2 to 3, and Table 1 shows times until the charging ratio reaches 90%.

  From the results of the quick charge test shown in Table 1, the rapid charge is excellent when the ratio A / C between the lithium capacity A of the opposing negative electrode and the lithium capacity C of the positive electrode is 1.1 ≦ A / C ≦ 1.6. It became clear. In A / C = 1.05 of Comparative Examples 1 to 3, since the negative electrode acceptability is low, the battery voltage quickly reaches 4.2 V, so the constant voltage region becomes long and charging takes time. It was also found that A / C = 1.05 is not suitable for rapid charging, and that A / C is preferably 1.1 or more. In addition, when A / C exceeds 1.6, although quick charge characteristics are excellent, the energy density of the battery becomes low, which is not practical. As described above, various embodiments have been described. However, the present invention is not limited to these embodiments, and various modifications can be made without departing from the gist of the present invention.

The perspective view of the laminated body in the battery inside of this invention. Schematic external view of laminate cell. Rapid charge test results.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laminate cell 2 Positive electrode terminal 3 Negative electrode terminal 4 Positive electrode active material 5 Aluminum metal foil 6 Active material uncoated part 7 Negative electrode active material 8 Copper foil 9 Porous membrane separator

Claims (1)

  A negative electrode made of a negative electrode active material capable of inserting / extracting lithium ions, a positive electrode made of a positive electrode active material capable of inserting / extracting lithium ions, and disposed between the positive electrode and the negative electrode to insulate the electrodes from each other In a lithium ion secondary battery comprising a separator that performs the above operation and a non-aqueous electrolyte solution containing lithium ions, a lithium capacity A into which the negative electrode can be inserted and removed, and a lithium capacity C into which the positive electrode can be inserted and removed A / C ratio is set to 1.1 ≦ A / C ≦ 1.6.

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