JPH05166510A - Secondary lithium battery - Google Patents

Abstract

PURPOSE:To provide a secondary lithium battery possessing a high operation voltage and high energy density and capable of lengthening its cycle life. CONSTITUTION:A negative electrode material is used which is of lithium or contains lithium, and positive electrode material is used which is expressed by a general formula LixCrMnO4 (where, 0<x<=2).

Description

Detailed Description of the Invention

[0001]

The present invention has a high operating voltage,
The present invention relates to a lithium secondary battery having a large charge / discharge capacity and a long cycle life.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries using lithium as a negative electrode material have been attracting attention as batteries having high energy density (high operating voltage and large charge / discharge capacity). As a positive electrode material for such a lithium secondary battery, for example, titanium, molybdenum, copper, niobium, vanadium, manganese, chromium, nickel, oxides of metals such as cobalt, sulfides, selenides, etc. have been proposed. Among them, development using lithium-nickel-cobalt composite oxides such as LiCoO ₂ and LiNiO ₂ , which have excellent cycle characteristics, is under way.

[0003]

However, in recent years, lithium secondary batteries have not been improved in terms of cycle life and safety, but have not used chemically active lithium in the metal state as the negative electrode. In many cases, a method of holding lithium in an ionized state is used, such as using a material occluded by intercalation.

However, the potential of the negative electrode material is nobler than that of the conventionally used lithium metal electrode, and therefore the operating voltage of the battery, which is a combination of the positive electrode and the negative electrode, becomes low. .. Therefore, it is desirable to provide a positive electrode material for a lithium secondary battery, which has excellent cycle characteristics and can output a higher operating voltage. As such, for example, a spinel-containing composite oxide represented by LiMn ₂ O ₄ is known.

However, it is difficult for LiMn ₂ O ₄ to obtain a charge / discharge capacity close to the theoretical value, and the cycle life is not excellent.

As one of the causes of this, when synthesizing LiMn ₂ O ₄ , a method of synthesizing by mixing starting materials such as hydroxides, oxides or carbonates of Li and Mn and firing at several hundred degrees However, since it is difficult to obtain a stable LiMn ₂ O ₄ spinel crystal skeleton by this method, the crystal structure is easily broken when charge and discharge are repeated, and good cycle characteristics cannot be obtained. ..

As another cause, when LiMn ₂ O ₄ discharges, Li enters into the LiMn ₂ O ₄ lattice.
The electron conductivity of ₂ O ₄ drops sharply, and when it is recharged, that is, during the reaction of extracting Li from the LiMn ₂ O ₄ lattice, the electron conductivity is lowered and electron transfer is not possible. This is probably because the extraction of Li from the LiMn ₂ O ₄ lattice was incomplete.

This tendency becomes remarkable especially when high rate charging is required, and there is a drawback that the operating voltage is lowered because sufficient charging is not possible and the cycle characteristics are rapidly deteriorated.

The present invention aims to provide a highly economical secondary battery by using a positive electrode material which has a high energy density at a high operating voltage, a long cycle life, and can be easily synthesized industrially. To aim.

[0010]

[Means for Solving the Problems] In order to achieve the above object, the present inventors have conducted various studies on positive electrode materials for lithium secondary batteries, and as a result, have found that a specific lithium-containing composite represented by the general formula Li _x CrMnO ₄ It has been found that when an oxide is used as a positive electrode material, it has a long cycle life and a high operating voltage. That is, to obtain a high operating voltage and a long cycle life, Li _x C
It is characterized in that x in the general formula rMnO ₄ is constituted by 0 <x ≦ 2.

In the present invention, when _x of Li _x CrMnO ₄ is 0, the absolute amount of lithium migrating between the positive electrode and the negative electrode is insufficient, and sufficient capacity cannot be exhibited. On the contrary, when it exceeds 2, charging efficiency is increased. It becomes low and neither is suitable. Of these, x is preferably 1.0 to 1.5 because a stable spinel crystal skeleton is formed and a sufficient discharge capacity and electron conductivity when Li is inserted can be secured. And more preferably, x is 1.05 to 1.3, particularly 1.1 to 1.2.

In practice, in order to manufacture such a positive electrode material, the starting materials are prepared so as to have a predetermined stoichiometric ratio, and the starting materials are fired at 700 to 950 ° C. in an oxidizing atmosphere. In the synthesis, oxides, hydroxides, carbonates, nitrates or the like of metal elements forming the general formula of Li _x CrMnO _{4 can} be used as starting materials. Further, it is desirable that the particle size of the starting material is as small as possible. This is because the solid phase reaction is performed during firing, and the smaller the particle size of the starting material, the faster the reaction proceeds uniformly. The average particle size of the starting material is preferably 30 μm or less, more preferably 15 μm or less.

The firing conditions for the synthesis are as follows:
It is desirable that the temperature be 700 to 950 ° C, preferably 850 to 900 ° C. The time is desirably 15 to 30 hours, preferably 20 to 25 hours.

In order to make the composition uniform during synthesis, a step is taken out of the firing furnace after 1 to 4 hours, preferably 2 to 3 hours after the initial firing, and the fired product is crushed and stirred, and then returned to the firing furnace. It is desirable to put in.

At the end of firing, if the rate of temperature decrease when lowering the temperature from the firing temperature is too fast, a stable spinel crystal skeleton cannot be obtained and cycle characteristics are hindered. Therefore, the rate of temperature decrease is preferably as low as possible. The temperature lowering rate at the end of firing is 50 ° C to 120 ° C per hour, preferably per hour
It is desirable that the temperature be 80 ° C to 100 ° C. However, these above conditions are not particularly limited.

On the other hand, as the negative electrode material, for example, lithium metal or a negative electrode material containing lithium, lithium alloy such as Li-Al alloy, alloy of low melting point metal such as Pb, Bi, Sn and Li and lithium is included. It is possible to use an organic electroconductive substance, an intercalation compound such as an organic fired body, a metal oxide, a sulfide, or a selenide containing lithium that operates at a base potential lower than that of the positive electrode material, but is not limited thereto. is not.

Any electrolyte can be used as long as lithium ions can move, but solid electrolytes such as polyethylene oxide in which LiClO ₄ is dissolved or inorganic lithium solid electrolyte dispersed in a resin can be used. A non-aqueous solvent electrolyte prepared by dissolving a lithium salt in an organic solvent such as an ester or an ether, for example, a 1: 1 mixed solvent of propylene carbonate and dimethoxyethane in which 1 mol of lithium perchlorate is dissolved. Can be used.

[0018]

The positive electrode material of the present invention composed of the general formula Li _x CrMnO ₄ (where 0 <x ≦ 2) is in a discharged state at the time of synthesis and is a negative electrode material that may contain lithium or lithium. By combining and charging and extracting Li in the positive electrode material from the crystal lattice, a charged state is achieved. It is considered that the reaction on the positive electrode material side at this time proceeds as shown in the following equation.

Li _x CrMnO ₄ → Li _xy CrMnO ₄ + yLi ⁺ + ye ⁻ ··· (1) Here, Li ^{+ in} the right term of the formula (1) shifts to the negative electrode material side. The battery thus constructed has a high energy density at a high operating voltage and a long cycle life.

[0020]

EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[Example 1] _{x in the} general formula Li _x CrMnO ₄ is
Li ₂ CO ₃ as a starting material to 0.12 mol
0.2 mol of MnCO ₃ and 0.1 mol of Cr ₂ O ₃ were weighed and mixed well, and the mixture was mixed in an oxidizing atmosphere at 900 ° C in a firing furnace for 2
After firing for an hour, take it out once, pulverize and mix the fired product,
After again firing at 900 ° C. for 20 hours in a firing furnace in an oxidizing atmosphere, the temperature was lowered at a rate of 100 ° C. per hour until the temperature reached 100 ° C., and the fired product was taken out and ground in a mortar.

10 parts by weight of Ketjen black as a conductive agent and 5 parts by weight of an ethylene-propylene copolymer resin as a binder were added to 85 parts by weight of this pulverized product, and this was dissolved in xylene to form a slurry. This slurry has a thickness of 30μ
m evenly applied to aluminum foil, then apply this at about 80 ℃
After drying with a roller press, the coating thickness is about 100.
The positive electrode was rolled to a size of μm and cut into a size of 20 mm × 20 mm.

On the other hand, as the negative electrode, a metal lithium foil having a thickness of 300 μm was cut into a size of 25 mm × 25 mm, and a nickel lead was pressure-bonded to an end of the foil. As shown in Figure 1,
A positive electrode terminal 5 and a negative electrode terminal 4 were filled with a beaker cell having a sufficient amount of electrolyte with a negative-polarity 1 and a positive-electrode 3 coating surfaces facing each other and a polypropylene microporous film 2 having a thickness of 25 μm interposed as a separator therebetween. A charging / discharging test was conducted in an argon atmosphere at 25 ° C. with a discharge power supply connected.

A mixed solvent of propylene carbonate and dimethoxyethane having a volume ratio of 1: 1 in which 1 mol / l of LiClO ₄ was dissolved was used as an electrolyte. Charge and discharge conditions are constant current and current density of 1mA / cm ² , charging voltage is up to 4.5V, discharging voltage is 2.5V.
It was done by controlling the potential up to.

FIG. 2 shows the discharge curve at the fifth cycle of charge / discharge. Table 1 shows the average operating voltage at 5,10,25,50 cycles, and Table 2 shows the discharge capacity maintenance rate when the fifth cycle is the initial value.

[Example 2] _{x in the} general formula of Li _x CrMnO ₄ is
Li ₂ CO ₃ as a starting material to 0.05 mol
Except that 0.2 mol of MnCO ₃ and 0.1 mol of Cr ₂ O ₃ were weighed,
All were carried out in the same manner as in Example 1, and the charge / discharge test was also carried out in the same manner. Table 1 shows the average operating voltage at 5,10,25,50 cycles, and Table 2 shows the discharge capacity maintenance rate when the fifth cycle is the initial value.

[Example 3] _{x in the} general formula of Li _x CrMnO ₄ is
Li ₂ CO ₃ as a starting material in an amount of 0.1 mol and M
Except that 0.2 mol of nCO ₃ and 0.1 mol of Cr ₂ O ₃ were weighed,
All were carried out in the same manner as in Example 1, and the charge / discharge test was also carried out in the same manner. Table 1 shows the average operating voltage at 5,10,25,50 cycles, and Table 2 shows the discharge capacity maintenance rate when the fifth cycle is the initial value.

[Example 4] _{x in the} general formula of Li _x CrMnO ₄ is
1.8 mol of Li ₂ CO ₃ as starting material and M
Except that 0.2 mol of nCO ₃ and 0.1 mol of Cr ₂ O ₃ were weighed,
All were carried out in the same manner as in Example 1, and the charge / discharge test was also carried out in the same manner. Table 1 shows the average operating voltage at 5,10,25,50 cycles, and Table 2 shows the discharge capacity maintenance rate when the fifth cycle is the initial value.

[Comparative Example] To synthesize LiMn ₂ O ₄ , Li ₂ C
After 0.1 mol of O ₃ and 0.4 mol of MnO ₂ were weighed and mixed well, the mixture was baked in an oxidizing atmosphere in a baking furnace at 800 ° C. for 20 hours, and the baked product was taken out and ground in a mortar. The same procedure as in Example 1 was carried out except that this pulverized product was used.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

As is clear from the results of these Examples and Comparative Examples, the secondary battery to which the present invention is applied can exhibit a high operating voltage and has excellent cycle characteristics as compared with the conventional ones. Become.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of test batteries used in Examples to which the present invention is applied and Comparative Examples.

FIG. 2 is a characteristic diagram showing discharge curves at the 5th cycle of Example 1 and Comparative Example to which the present invention is applied.

[Explanation of symbols]

 1 Negative electrode 2 Microporous film 3 Positive electrode 4 Negative electrode terminal 5 Positive electrode terminal

Claims (1)

[Claims] 1. A lithium secondary battery comprising a positive electrode material represented by the general formula Li _x CrMnO ₄ (where 0 <x ≦ 2).