JP2611265B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a non-aqueous electrolyte secondary battery,
In particular, the present invention relates to improvement of a positive electrode material of a non-aqueous electrolyte battery using LiMn ₂ O ₄ as a positive electrode active material.

[Summary of the Invention]

The present invention relates to a non-aqueous electrolyte secondary battery using lithium, a lithium alloy or a conductive polymer for a negative electrode and LiMn ₂ O ₄ for a positive electrode, and the amount of graphite added as a conductive agent in the positive electrode. Is specified within a predetermined range, thereby realizing a non-aqueous electrolyte secondary battery having a high capacity and excellent charge / discharge cycle characteristics.

[Conventional technology]

A so-called non-aqueous electrolyte battery using lithium as a negative electrode active material and a non-aqueous electrolyte as an electrolyte is known as a battery having a low self-discharge and excellent storage stability.
It has been widely used as an electronic wristwatch and a power source for various memory backups, which are required to be used for a long time of 10 to 10 years.

By the way, these conventional non-aqueous electrolyte batteries are usually primary batteries, but there are many demands for rechargeable non-aqueous electrolyte secondary batteries as a power source that can be used economically for a long time. Research is ongoing.

Under such circumstances, many studies and proposals have been made on cathode materials for non-aqueous electrolyte secondary batteries. Typical examples are Li for the negative electrode, TiS ₂ , MoS ₂ , and the like for the positive electrode. Nb
Se ₂ , V ₂ O _{5 and the} like have been studied in combination.

However, a secondary battery using such a positive electrode material tends to rapidly decrease in discharge capacity as charge / discharge is repeated, leaving a problem in cycle characteristics.

Furthermore, it is difficult to obtain these positive electrode materials, and the production cost increases if a large-capacity secondary battery is to be produced. Compared with the alkaline storage battery (so-called nickel cadmium battery), which is the mainstream of secondary batteries, Disadvantageous.

Therefore, the applicant of the present application has previously found LiMn ₂ O ₄ synthesized from lithium carbonate or lithium iodide and manganese dioxide as an inexpensive positive electrode material having a high capacity and excellent charge / discharge characteristics. Batteries were proposed in Japanese Patent Application Nos. 61-257479 and 62-107989.

[Problems to be solved by the invention]

The present invention aims at further improvement of the above-mentioned non-aqueous electrolyte battery using LiMn ₂ O ₄ as a positive electrode active material, and particularly aims at further expansion of discharge capacity and improvement of charge / discharge cycle characteristics. is there.

[Means for solving the problem]

The present inventors have previously succeeded in synthesizing LiMn ₂ O ₄ having high capacity and excellent charge / discharge characteristics as a positive electrode active material from lithium carbonate or lithium iodide and inexpensive manganese dioxide. As a result of repeated studies, it was found that graphite was added as a conductive material for the positive electrode material, and that the amount of addition was regulated within a predetermined range to obtain a secondary battery with higher capacity and superior cycle characteristics. succeeded in.

The present invention has been completed based on this finding, and comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte mainly composed of LiMn ₂ O ₄ and graphite, and has a graphite content of 8 to 22.
% By weight.

LiMn ₂ O ₄ used as a positive electrode active material of the non-aqueous electrolyte secondary battery according to the present invention is, for example, lithium carbonate and manganese dioxide in air or in an inert gas atmosphere such as nitrogen.
It can be easily obtained by heating to about 0 ° C. to cause a reaction, or by heating lithium iodide and manganese dioxide to about 300 ° C. in a similar atmosphere or the like to cause a reaction.

In particular, when X-ray diffraction is performed using FeKα rays, if LiMn ₂ O ₄ having a half width of a diffraction peak at a diffraction angle of 46.1 ° of 1.1 ° to 2.1 ° is used as a positive electrode active material,
Excellent charge / discharge characteristics can be obtained.

In the above-mentioned positive electrode active material, graphite is added as a conductive agent.In the present invention, the amount of graphite added is important, and the proportion of graphite in the total amount of LiMn ₂ O ₄ and graphite which is the positive electrode active material is Is set to 8 to 22% by weight.

If the added amount of graphite is less than 8% by weight, the positive electrode active material particles are separated from the graphite particles due to volume shrinkage due to the discharge of the positive electrode active material, and the capacity is deteriorated. Conversely, if the amount of graphite exceeds 22% by weight, the amount of the positive electrode active material that acts substantially decreases, and the capacity also deteriorates.

The amount of graphite added is within the above range,
In particular, it is preferably 8.4 to 21.1% by weight, and 10.5 to 15.
More preferably, it is 8% by weight.

By the way, when LiMn ₂ O ₄ is used as a positive electrode starting material, the positive electrode active material exists as Li _x Mn ₂ O _{4 due to the} deintercalation and intercalation reaction of lithium ions due to charge and discharge, and the value of x Varies in the range of 0.05 ≦ x ≦ 2 according to the following reaction formula.

Here, x represents the value of x before discharging, and y represents the amount of lithium ions that react.

The discharge causes intercalation of lithium ions, and the value of x increases. In a fully discharged state, the value of x reaches x = 2 in average composition.

In addition, lithium ion deintercalation occurs due to charging, and the value of x gradually decreases. In a fully charged state, the value of x reaches 0.05 as an average composition.

In other words, a battery using LiMn ₂ O ₄ as a positive electrode starting material becomes LiMn ₂ O ₄ + Li → 2LiMnO ₂ when it is completely discharged, and LiMnO → Li _0.05 Mn ₂ O ₄ + 1.95Li when it is completely recharged. move on.

However, in actual use, it is rare that the battery is completely charged or completely discharged. In general, the average composition can be represented by the above formula (A).

Therefore, in the present invention, regardless of the state of the positive electrode active material,
The amount of graphite to be added is determined by the weight when converted to LiMn ₂ O ₄ .

On the other hand, the materials used for the negative electrode include metallic lithium,
Lithium alloys (eg, LiAl, liPb, LiSn, LiBi, LiCd, etc.),
Intercalation compound in which lithium ions are mixed in the crystal (for example,
Lithium sandwiched between layers such as TiS ₂ and MoS ₂ . ), Conductive polymers such as polyacetylene, polyaniline, and polythiophene; doping of the above-mentioned conductive polymers with lithium ions; Carbon or the like having poor crystallinity so that it can enter and exit can be used.

As the electrolyte, a non-aqueous electrolyte in which a lithium salt is used as an electrolyte and dissolved in an organic solvent is used. Here, as the organic solvent, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran,
2-methyltetrahydrofuran, 1,3-dioxolane,
A single solvent such as 4-methyl-1,3-dioxolane or a mixture of two or more solvents can be used.

As the electrolyte, LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C
₆ H ₅ ) One of _four or the like or a mixture of two or more of them can be used.

As the shape of the battery, any of conventionally known ones such as a cylindrical type, a coin type, and a button type can be applied.

[Action]

LiMn ₂ O ₄ is used as a positive electrode active material, graphite is added as a conductive agent, and the ratio of graphite in the total amount of LiMn ₂ O ₄ and graphite is set to be 8 to 22% by weight, so that the positive electrode active material is The separation of the positive electrode active material particles and the graphite particles caused by the volume shrinkage due to the discharge is suppressed, the substantial discharge capacity is expanded, and the cycle characteristics are improved.

〔Example〕

Hereinafter, specific examples of the present invention will be described,
It goes without saying that the present invention is not limited to these examples.

It goes without saying that it is not.

Example 1 This example is applied to a cylindrical (so-called jelly roll type) battery using lithium for the negative electrode.

First, 18.5 g of lithium carbonate was added to 86.9 g of commercially available manganese dioxide, and this was mixed well in a mortar. This mixture is then placed on an alumina boat at a temperature of 450 ° C in air,
The firing was performed for one hour.

After cooling, the obtained product was analyzed by X-ray.
An X-ray diffraction chart as shown in the figure was obtained. When this was compared with the X-ray diffraction chart of the substance represented by the chemical formula LiMn ₂ O ₄ on an ASTM card, it was completely matched.

Then, using the obtained LiMn ₂ O ₄ and graphite as a conductive agent, a cylindrical battery as shown in FIG. 2 was prepared.

That is, graphite (KS-15 manufactured by Lonza, average particle size of 7 μm) was added to LiMn ₂ O ₄ obtained by the above-mentioned method as a conductive agent.
m) was added and mixed in the proportions shown in Table 1, and polyvinylidene fluoride as a binder and N-methyl-2-pyrrolidone as a dispersant were added and wet-mixed to prepare a positive electrode paste.

Next, the positive electrode paste was 470 mm long, 26 mm wide, and 0.0 mm thick.
3mm aluminum current collector
A positive electrode plate (1) weighing 5 g and weighing 5.1 g was produced.

On the other hand, a lithium foil having a length of 470 mm, a width of 26 mm and a thickness of 0.07 mm is used as a negative electrode (2), and the positive electrode plate (1) and the negative electrode plate (2) are used.
And a 0.05 mm thick polypropylene separator (3)
, And placed in an iron can (5) plated with nickel and having an inner diameter of 15.3 mm with an insulating plate (4) disposed on both end surfaces. Here, the negative electrode (2) comes into contact with the inner peripheral surface of the iron can (5), and thus the iron can (5) corresponds to the negative electrode can.

Next, a mixed electrolytic solution of propylene carbonate and 1,2-dimethoxyethane in which LiPF ₆ was dissolved at a rate of 1 mol / l was impregnated in the iron can (5), and nickel plating was performed again via a gasket (6). Cover made of damaged iron (7)
And sealed. A lead (8) connected to the positive electrode plate (1) is welded to the inner surface of the lid (7), so that the lid (7) serves as a positive electrode can of the battery.

As described above, cylindrical batteries A with an outer diameter of 15.9 mm and a height of 33 mm
Battery H was assembled.

The batteries A to H were discharged to a final voltage of 2.0 V with a constant resistance of 13 Ω, and then discharged at a final voltage of 3.9 V.
The battery was charged for 9 hours at mA, and the cycle was tested as a cycle life test.

Table 1 shows the capacity deterioration rate at the 20th cycle.
FIG. 3 shows the relationship between the amount of graphite added and the discharge capacity.

 The capacity deterioration rate was obtained according to the following equation.

As a result, Table 1 first revealed that the batteries D to H to which graphite was added at 10.5% by weight or more had a small capacity deterioration rate of 5% or less and exhibited excellent cycle characteristics. On the other hand, 4.2% by weight of graphite or
Batteries A and B to which 6.3% by weight was added show a capacity deterioration rate of 10% or more.

The cause of this capacity deterioration is that the volumetric shrinkage due to the discharge of LiMn ₂ O ₄ causes the separation of the active material particles and the graphite particles and the current collection effect decreases, and this effect appears significantly when the amount of added graphite is small. This is probably due to

On the other hand, from Table 1 and FIG.
Batteries with up to 22% by weight, even at the 20th cycle, 400mAH or more, especially batteries with 10.5-15.8% by weight of graphite added
It was confirmed that D, Battery E, and Battery F had extremely high discharge capacities of 450 mAH or more. It can be said that these cycle characteristics and discharge capacity are extremely excellent as the performance of this type of battery.

In the battery H to which 26.4% by weight of graphite was added, although the capacity deterioration rate was small, the content of the active material in the positive electrode plate (1) was small, and the discharge capacity was reduced, which was not practically preferable. .

Example 2 This example is applied to a coin-type battery using a Li-Al alloy for the negative electrode as shown in FIG.

First, LiMn synthesized by the same method as in the first embodiment
Graphite (KS-15, manufactured by Lonza, average particle diameter 7 μm) was added to ₂ O ₄ as a conductive agent at the ratio shown in Table 2 and mixed.
Further, polyvinylidene fluoride as a binder and N-methyl-2-pyrrolidone as a dispersant were added and wet-mixed to prepare a positive electrode paste.

Next, this positive electrode paste was uniformly applied to both surfaces of an aluminum current collector having a thickness of 0.03 mm to form a positive electrode plate having a thickness of 0.18 mm, and then punched into a diameter of 15.5 mm to obtain a positive electrode (11) having a weight of 0.096 g. .

On the other hand, a commercially available aluminum plate with a thickness of 0.3 mm is 15.5 m in diameter.
m, and the punched aluminum plate is spot-welded to the anode cup (12), and a 15m diameter
m, negative electrode (13)
Was prepared.

A separator (14) is placed on the negative electrode (13), and a plastic gasket (15) is fitted.
Propylene carbonate dissolved in LiPF ₆ and 1,2
Injecting a mixed electrolyte of dimethoxyethane, further inserting the positive electrode (11), covering with a cathode can (16) and sealing the same, coin type batteries I to P having an outer diameter of 20 mm and a thickness of 1.2 mm were respectively obtained. Assembled.

After discharging the batteries I to P to a cutoff voltage of 2.0 V with a constant resistance of 1.5 kΩ, the batteries were charged at a cutoff voltage of 3.1 V.
The battery was charged at 0.5 mA for 21 hours, and a cycle life test was performed using this as one cycle.

Table 2 shows the capacity deterioration rate at the 20th cycle.
FIG. 5 shows the relationship between the added amount of graphite and the discharge capacity. The capacity deterioration rate was obtained in the same manner as in the first embodiment.

From Table 2, it can be seen that even in the coin-type batteries, the batteries L to P to which graphite was particularly added at 10.5% by weight or more had a small capacity deterioration rate and had excellent cycle characteristics.

In addition, from Table 2 and FIG. 5, the discharge capacity can be increased by setting the graphite addition amount to 8% by weight or more, and in particular, the batteries L, M, and the battery having the graphite addition amount of 10.5 to 15.8% by weight can be obtained. In N, it was found that a discharge capacity of 8 mAH or more was secured even in the 20th cycle.

〔The invention's effect〕

As is clear from the above description, in the nonaqueous electrolyte secondary battery of the present invention, LiMn ₂ O ₄ is used as a positive electrode starting material,
Further, since the amount of graphite used as a conductive agent is regulated to a predetermined value, it is possible to suppress the capacity deterioration due to the separation of the positive electrode active material particles and the graphite particles caused by the volume shrinkage due to the discharge of the positive electrode active material. A secondary battery having excellent cycle characteristics and high discharge capacity can be obtained, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing an X-ray diffraction result of synthesized LiMn ₂ O ₄ . FIG. 2 is a partially cutaway side view showing an example of the configuration of a cylindrical battery, and FIG. 3 is a characteristic diagram showing the relationship between the amount of graphite added and the discharge capacity in the cylindrical battery having such a configuration. FIG. 4 is a schematic sectional view showing a configuration example of a coin-type battery, and FIG. 5 is a characteristic diagram showing the relationship between the amount of graphite added and the discharge capacity in the coin-type battery having such a configuration. 1 Positive electrode plate 2 Negative electrode 11 Positive electrode 13 Negative electrode 3, 14 Separator

Claims (1)

(57) [Claims] 1. A positive electrode mainly composed of LiMn ₂ O ₄ and graphite, a negative electrode, and a non-aqueous electrolyte, wherein the proportion of graphite in the total amount of LiMn ₂ O ₄ and graphite is 8 to 22% by weight. Non-aqueous electrolyte secondary battery characterized by the above-mentioned.