JP5658058B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  In order to adopt a lithium ion secondary battery as a plug-in hybrid vehicle battery, it is necessary to realize low cost, low volume, light weight and high output while maintaining high safety. For this reason, the positive electrode material is required to have a large capacity and high safety.

  In Patent Document 1, for the purpose of suppressing gas generation inside the battery, a heat treatment was performed by adhering a molybdate compound to composite oxide particles containing lithium, nickel, cobalt, and aluminum. A positive electrode active material having a carbonate ion content of 0.15 wt% or less is disclosed.

  Patent Document 2 also discloses the same technique as Patent Document 1.

  Patent Document 3 discloses lithium cobalt oxide or nickel acid to which at least one element selected from B, Bi, Mo, P, Cr, V, and W is added for the purpose of reducing internal resistance. Lithium is disclosed as a positive electrode active material. doing.

JP 2010-55778 A JP 2010-40383 A JP 2000-113884 A

  In Patent Documents 1 and 2, the amount of molybdenum is mixed and heated in an amount of 2 atomic% or less (2 mol%) or less with respect to the composite oxide. In this case, since molybdenum is unevenly distributed on the surface of the composite oxide, the thermal stability of the charged state cannot be improved.

  In Patent Document 3, the amount of additive element is 0.1 wt. % Or 10 wt. %, And the thermal stability of the charged state cannot be improved.

  Thus, even if it uses the technique of patent documents 1-3, it cannot be said that it is enough in order to achieve the capacity | capacitance and safety which are requested | required of the battery for plug-in hybrid vehicles.

  The objective of this invention is providing the positive electrode active material for lithium ion secondary batteries which further improved discharge capacity and safety | security.

In the present invention, composition formula Li _{1.1 + x} Ni _a M ¹ _b M ² _c O ₂ (wherein M ¹ includes at least ^one of Mo and W, and M ² is Mn. −0.07 ≦ x ≦ 0.1, 0.90 ≦ a ≦ 0.98, 0.02 ≦ b ≦ 0.06, and 0.00 ≦ c ≦ 0.06).

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material for lithium ion secondary batteries which achieves the discharge capacity and safety which are requested | required of the battery for plug-in hybrid vehicles can be provided.

It is a graph which shows the result of the differential scanning calorimetry of the mixture of the positive electrode and electrolyte solution of a charging state. It is sectional drawing which shows the structure of a lithium ion secondary battery.

  The present invention relates to a positive electrode active material for a lithium ion secondary battery having a large capacity and high safety, and a lithium ion secondary battery using the same.

In order to employ a lithium ion secondary battery as a plug-in hybrid vehicle battery, it is required to have a large capacity and high safety. In the lithium ion secondary battery, these characteristics are closely related to the properties of the positive electrode active material. In order to obtain a high capacity in the layered positive electrode active material represented by the composition formula LiMO ₂ (M is a transition metal), it is necessary to increase the Ni content in the transition metal.

  However, the positive electrode active material having a high Ni content (high Ni content) has a drawback that the structural stability in the charged state is low and the thermal stability in the charged state is low. That is, when the temperature of the battery rises due to an internal short circuit or the like, oxygen is released from the positive electrode at a relatively low temperature, and this oxygen reacts with the electrolytic solution to cause a reaction with rapid heat generation.

  By using a positive electrode active material having an appropriate composition, the present inventor can increase the capacity, suppress the maximum heat generation when the temperature is raised in the charged state, and improve the thermal stability in the charged state. I found. Specifically, a predetermined ratio of Mo, W, or Mn is added to a positive electrode active material having a high Ni content.

  Hereinafter, a positive electrode active material according to an embodiment of the present invention, and a positive electrode material, a positive electrode mixture, a positive electrode, and a lithium ion secondary battery using the same will be described.

The positive electrode active material has a composition formula of Li _{1.1 + x} Ni _a M ¹ _b M ² _c O ₂ (wherein M ¹ includes at least ^one of Mo and W, and M ² is Mn. −0. 07 ≦ x ≦ 0.1, 0.90 ≦ a ≦ 0.98, 0.02 ≦ b ≦ 0.06, and 0.00 ≦ c ≦ 0.06).

  The positive electrode active material preferably satisfies 0.94 ≦ a ≦ 0.98.

The content of ^{M 1} of the transition metal contained in the positive electrode active material is preferably 3~5mol%. That is, it is preferable that 0.03 ≦ b ≦ 0.05.

The positive electrode active material preferably satisfies −0.04 ≦ x ≦ 0.05.

  The positive electrode active material can be used for a positive electrode material, a positive electrode mixture, a positive electrode, and a lithium ion secondary battery. Here, the positive electrode material is a mixture of a positive electrode active material and an additive. The positive electrode mixture includes a positive electrode active material and a binder. The positive electrode mixture preferably contains a conductive additive. The positive electrode is obtained by coating a positive electrode current collector plate with a positive electrode mixture.

  The lithium ion secondary battery includes a positive electrode and a negative electrode capable of occluding and releasing lithium, a separator sandwiched between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The positive electrode includes a positive electrode composite containing the positive electrode active material. Contains agents.

  The positive electrode active material can significantly reduce the maximum calorific value when mixed with an electrolyte and heated, compared to a high Ni-containing positive electrode active material not containing Mo, W or Mn, Safety in a high temperature state when applied to a lithium ion secondary battery can be improved.

  In the above composition formula, when x <−0.07, since the amount of Li present in the Li layer is small, the layered crystal structure cannot be maintained. On the other hand, in the case of 0.1 <x, the amount of transition metal in the composite oxide decreases, so that the battery capacity decreases.

  In the above composition formula, when a <0.9, the content is reduced because the content of Ni mainly contributing to the charge / discharge reaction is small. On the other hand, in the case of a> 0.98, the content of other elements is reduced, so that the thermal stability is lowered.

  In the above composition formula, when b <0.02, the effect of improving the thermal stability is small. When b> 0.06, the crystal structure becomes unstable.

  In the above composition formula, when c> 0.06, the content of Ni mainly contributing to the charge / discharge reaction is decreased, and the capacity is decreased.

(Preparation of positive electrode active material)
As raw materials not containing lithium, nickel oxide, manganese dioxide, molybdenum oxide and tungsten oxide were appropriately used, weighed so as to have a predetermined atomic ratio, and then added with pure water and mixed to form a slurry.

  The slurry was pulverized with a zirconia bead mill until the average particle size of the particles contained in the slurry became 0.2 μm. In this slurry, a polyvinyl alcohol (PVA) solution was converted to a solid content ratio of 1 wt. % And mixed for another hour, dried by a spray dryer and granulated.

Lithium hydroxide and lithium carbonate were added to the granulated particulate material so that the Li: (NiM ¹ M ² ) ratio was 1.00 to 1.25: 1.

  Next, the obtained particulate material (powder) was fired at 750 to 950 ° C. for 10 hours to obtain a material having a layered structure crystal. Thereafter, this material was crushed to obtain a particulate positive electrode active material.

  Further, coarse particles having a particle size of 30 μm or more were removed from the positive electrode active material by classification, and then used for production of an electrode.

  The method for producing the positive electrode active material is not limited to the above method, and other methods such as a coprecipitation method may be used.

  Table 1 shows the composition ratio of the positive electrode active material produced by the above method.

(Prototype battery)
The positive electrode active material 1 and the carbon-based conductive agent were weighed to a mass ratio of 85: 10.7 and mixed using a mortar. The mixed material of the positive electrode active material and the conductive agent and the binder dissolved in N-methyl-2-pyrrolidone (NMP) are mixed in a mass ratio of 95.7: 4. 3 was mixed uniformly to make a slurry.

The slurry was applied on an aluminum current collector foil having a thickness of 20 μm, dried at 120 ° C., and compression-molded with a press so that the electrode density was 2.7 g / cm ³ . Thereafter, it was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

Using the produced positive electrode and the negative electrode formed of metallic lithium, 1.0 mol of LiPF ₆ was mixed in a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 2. A prototype battery was produced using the non-aqueous electrolyte dissolved in 1 liter / liter.

  Next, the following tests were performed using the prototype battery described above.

(Charge / discharge test)
Charging / discharging from the upper limit voltage of 4.3 V to the lower limit voltage of 2.7 V at 0.1 C was repeated three times for initialization. Furthermore, charging / discharging from the upper limit voltage 4.3V to the lower limit voltage 2.7V was performed at 0.1 C, and the discharge capacity was measured.

(Differential scanning calorimetry)
After charging at a constant current / constant voltage to 4.3 V, the electrode is taken out from the prototype battery, washed with DMC, punched into a disk shape with a diameter of 3.5 mm, placed in a sample pan, and 1 μl (microliter) of the electrolyte solution. In addition, it was sealed and used as a sample.

  The heat generation behavior when this sample was heated from room temperature to 400 ° C. at 5 ° C./min was examined.

[Examples 2 to 20, Comparative Examples 1 to 10]
A prototype battery was prepared in the same manner as in Example 1 except that the positive electrode active material shown in Table 1 was used, and a charge / discharge test and differential scanning calorimetry were performed.

  In Examples 1-20 and Comparative Examples 1-10, the value obtained by dividing the obtained discharge capacity by the discharge capacity of Comparative Example 1 was taken as the capacity ratio. In these Examples and Comparative Examples, the maximum calorific value ratio was obtained by dividing the obtained maximum calorific value (maximum calorific value) by the maximum calorific value of Comparative Example 1.

  Table 2 summarizes the capacity ratio and the maximum heat generation ratio for Examples 1 to 8 and Comparative Examples 1 to 3.

  In Table 2, Examples 1-7 are positive electrode active materials which do not contain Mn but contain Mo, and change the composition of Li in the range of 103-120. In Example 8, Mo in the composition of the positive electrode active material used in Example 1 was replaced with W.

  In Comparative Example 1, Ni is less than in the example, Co and Mn are included, and Mo is not included. Comparative Example 2 has less Li than Example 2. Comparative Example 3 has more Li than Example 7.

  From Table 2, it can be seen that in Examples 1 to 8, the discharge capacity is larger and the maximum heat generation amount is smaller than in Comparative Example 1.

  The reason why the discharge capacity is larger than that of Comparative Example 1 is considered to be that the content of Li contained in the positive electrode active material of each Example is within an appropriate range. In addition, the reason why the maximum heat generation amount is smaller than that of Comparative Example 1 is considered to be because Mo or W, which is an element contributing to the thermal stability of the charged state, is contained in an appropriate amount of 4%. On the other hand, in Comparative Example 1, the amount of Ni is small and Mo is not included.

  On the other hand, in Comparative Examples 2 and 3, it was impossible to achieve both improvement of the discharge capacity and reduction of the maximum heat generation amount. In Comparative Example 2, it is considered that the capacity is reduced because the Li content is small. In Comparative Example 3, it is considered that the capacity was lowered because the Li content was too large to be taken into the crystal and could not be involved in charge / discharge.

  Table 3 summarizes the capacity ratio and the maximum heat generation ratio for Examples 9 to 16 and Comparative Examples 4 to 5.

  In Table 3, Example 9 is a composition obtained by reducing Ni in the composition of Example 1 and adding Mn. In Example 10, among the compositions of Example 2, Ni was reduced and Mn was added. Examples 11-15 change the composition of Li in the range of 106-120 among the compositions of Example 10. FIG. In Example 16, Mo in the composition of Example 9 is replaced with W.

  Comparative Example 4 has less Li than Example 10. Comparative Example 5 has more Li than Example 15.

  From Table 3, it can be seen that in Examples 9 to 16, the discharge capacity is larger and the maximum heat generation amount is smaller than that of Comparative Example 1.

  The reason why the discharge capacity shows a large value is considered to be that the content of Li contained in the positive electrode active material of each example is within an appropriate range. Further, the reason why the maximum heat generation amount is small is considered to be that 4% of Mo or W, which is an element contributing to the thermal stability of the charged state, is included.

  On the other hand, in Comparative Examples 4 and 5, it was impossible to achieve both improvement of the discharge capacity and reduction of the maximum heat generation amount. In Comparative Example 4, it is considered that the discharge capacity is reduced because the Li content is small. In Comparative Example 5, it is considered that the discharge capacity was reduced because the Li content was too high to be taken into the crystal and could not be involved in charge / discharge.

  Table 4 summarizes the capacity ratio and the maximum calorific value ratio for Examples 17 to 18 and Comparative Examples 6 to 7.

  In Table 4, Example 17 is the composition of Example 1 in which Ni is increased and Mo is decreased. Example 18 is a composition obtained by reducing Ni and increasing Mo in the composition of Example 1.

  In Comparative Example 6, Ni was further increased and Mo was reduced compared to Example 17. In Comparative Example 7, Ni is further reduced and Mo is increased as compared with Example 18.

  From Table 4, it can be seen that in Examples 17 and 18, the discharge capacity is larger and the maximum heat generation amount is smaller than in Comparative Example 1.

  The reason why the discharge capacity shows a large value is considered to be because the Ni content is large among the transition metals contained in the positive electrode active material selected in each example. Further, the reason why the maximum heat generation amount is small is considered to be that 2 to 6% of Mo which is an element contributing to the thermal stability of the charged state is contained.

  On the other hand, in Comparative Examples 6 and 7, it was impossible to achieve both improvement of the discharge capacity and reduction of the maximum heat generation amount. In Comparative Example 6, Mo, which is an element that contributes to the thermal stability of the charged state, was as low as 1%, and thus the discharge capacity was large, but the thermal stability of the charged state was not sufficiently obtained. In Comparative Example 7, the discharge capacity was reduced because Mo was too much as 8%.

  Table 5 summarizes the capacity ratio and the maximum heat generation ratio for Examples 19 to 20 and Comparative Examples 8 to 10.

  In Table 5, Example 19 is a composition obtained by increasing Ni and decreasing Mo in the composition of Example 9. In Example 20, among the compositions of Example 9, Mn was reduced and Mo was increased.

  In Comparative Example 8, Ni was further increased and Mo was reduced compared to Example 19. In Comparative Example 9, Ni was reduced and Mo was increased compared to Example 9.

  From Table 5, it can be seen that in Examples 19 and 20, the discharge capacity is larger and the maximum heat generation amount is smaller than in Comparative Example 1. The reason why the discharge capacity shows a large value is considered to be because the Ni content is large among the transition metals contained in the positive electrode active material selected in each example. Further, the reason why the maximum heat generation amount is small is considered to be that 2 to 6% of Mo which is an element contributing to the thermal stability of the charged state is contained.

  On the other hand, in Comparative Examples 8 to 10, it was impossible to achieve both improvement of the discharge capacity and reduction of the maximum heat generation amount. In Comparative Example 8, Mo, which is an element that contributes to the thermal stability of the charged state, is as low as 1%, and thus the discharge capacity is large, but the thermal stability of the charged state cannot be obtained sufficiently. In Comparative Example 9, it is considered that the discharge capacity is reduced because Mo is too much as 8%. Further, in Comparative Example 10, it is considered that the discharge capacity was decreased because Mn was too much as 8%.

  FIG. 1 shows the results of differential scanning calorimetry (DSC) of Example 1 and Comparative Example 1. The horizontal axis represents temperature, and the vertical axis represents heat flow per unit mass.

  In this figure, the temperature dependence of the heat flow is smaller in Example 1 than in Comparative Example 1. On the other hand, Comparative Example 1 has a steep peak near 290 ° C.

  From this result, it can be seen that Example 1 has high thermal stability in the charged state.

  FIG. 2 is a cross-sectional view showing a lithium ion secondary battery.

  In this figure, a positive electrode plate 3 (also referred to as a positive electrode) in which a positive electrode material is applied on both surfaces of a current collector and a negative electrode plate 4 (also referred to as a negative electrode) in which a negative electrode material is applied on both surfaces of the current collector are directly shown. The electrode group is formed by arranging and winding the separator 5 between the positive electrode plate 3 and the negative electrode plate 4 so as not to contact each other. This electrode group is inserted into a battery can 9 made of SUS. The separator 5 is formed of a microporous polypropylene film.

In the battery can 9, 1.0 mol / liter of LiPF ₆ was dissolved in a non-aqueous electrolyte (for example, a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1: 2. Stuff). The battery can 9 is sealed using a sealing lid portion 8 to which a packing 10 is attached. The lead piece 6 of the positive electrode plate 3 is electrically connected to the sealing lid portion 8. The lead piece 7 of the negative electrode plate 4 is electrically connected to the bottom of the battery can 9. An insulating plate 11 is disposed between the sealing lid 8 and the electrode group and between the bottom of the battery can 9 and the electrode group.

  As described above, by using the material shown in the present embodiment as the positive electrode material of the lithium ion secondary battery, high capacity, high output and high safety, which are the performance required for the plug-in hybrid vehicle battery, are achieved. can do.

  The present invention is particularly applicable to a positive electrode material for a lithium ion secondary battery. It can also be used for lithium ion secondary batteries for plug-in hybrid vehicles.

  3: positive electrode plate, 4: negative electrode plate, 5: separator, 6, 7: lead piece, 8: sealing lid, 9: battery can, 10: packing, 11: insulating plate.

Claims (7)

Composition formula Li _{1.1 + x} Ni _a M ¹ _b M ² _c O ₂ (wherein M ¹ includes at least ^one of Mo and W, and M ² is Mn. −0.07 ≦ x ≦ 0. 1. 0.90 ≦ a ≦ 0.98, 0.02 ≦ b <0.05, 0.00 ≦ c ≦ 0.06)).   The positive electrode active material according to claim 1, wherein 0.94 ≦ a ≦ 0.98.   The positive electrode active material according to claim 1, wherein −0.04 ≦ x ≦ 0.05. It is 0.03 <= b < 0.05, The positive electrode active material as described in any one of Claims 1-3 characterized by the above-mentioned.   A positive electrode mixture comprising the positive electrode active material according to any one of claims 1 to 4 and a binder.   A positive electrode comprising the positive electrode mixture according to claim 5 and a positive electrode current collector plate coated with the positive electrode mixture.   A positive electrode and a negative electrode capable of occluding and releasing lithium, a separator sandwiched between the positive electrode and the negative electrode, and a nonaqueous electrolyte, wherein the positive electrode includes the positive electrode mixture according to claim 5. Lithium ion secondary battery.

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