JPH09259863A - Nonaqueous electrolyte secondary battery and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery keeping a high operating voltage and excellent charge and discharge cycle characteristics by providing a specified positive electrode, a negative electrode, and a nonaqueous electrolyte. SOLUTION: This secondary battery has a positive electrode 1 mainly composed of LiMn2 O4 which is a composite oxide of Li and Mn and having P or P oxide added thereto, the adding ratio of P being preferably 0.02-0.1 by mole ratio to Mn, a negative electrode 3, and a nonaqueous electrolyte. A part of Mn is preferably substituted by Co, Ni. The material for the positive electrode active material of this battery is obtained by mixing a Li compound (e.g. Li2 Co3 ), a Mn compound (e.g. Mn3 O4 ), and P or a P compound (e.g. P2 O5 ), and thermally treating the mixture in oxidizing atmosphere, for example, at a high temperature of 650-900%oC to cover the particle surface of LiMn2 O4 which is the composite oxide of Li and Mn with P.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a battery characteristic improvement using a lithium composite oxide as a positive electrode and a method for producing the same.

[0002]

2. Description of the Related Art As electronic devices have become smaller and lighter, there has been an increasing demand for smaller and lighter batteries as power sources. In particular, expectations are high for non-aqueous electrolyte lithium secondary batteries having high voltage and high energy density. In recent years, many attempts have been made to put batteries using transition metal sulfides or oxides as positive electrode active materials for lithium secondary batteries into practical use. U.S. Pat. No. 4,302,51
In No. 8, a battery using LiCoO ₂ as a positive electrode active material was proposed, and in 1992, a cylindrical battery using carbon for the positive electrode and the negative electrode showed 120
It has been reported that the capacity of 70% or more of the initial capacity was retained even after the lapse of 0 cycle, and it is now put into practical use as a 4V class lithium ion secondary battery by each company. Further, attempts are being made to put LiNiO ₂ into practical use as the next positive electrode active material.
However, Co and Ni have a problem that they are scarce in resources and expensive. On the other hand, LiMn ₂ O ₄ (Japanese Patent Publication No. 4-30146) has been proposed as a positive electrode active material using manganese, which has abundant resources and is inexpensive. The discharge potential of this oxide has a two-stage discharge potential of around 4V and around 2.8V. Using this plateau area around 4V,
By repeating charging and discharging in the voltage range of 4.5 V to 3.0 V, high potential and high energy density can be achieved.

In JP-A-5-47383, Li
When CoO ₂ has a high potential of 4 V or higher, it decomposes and adversely affects the battery characteristics. The main active material LiCoO ₂ 1
It is said that by adding a predetermined amount of phosphorus to the moles, a non-aqueous electrolyte secondary battery having excellent cycle characteristics can be obtained.

[0004]

LiMn ₂ O _4, which is a complex oxide of manganese, has the characteristics of high voltage and high energy density, but it has a problem of capacity decrease with charge / discharge cycles, and is practically used. It has not been used as a battery. The present invention solves such a problem, and an object thereof is to provide a secondary battery which maintains a high operating voltage and has excellent charge and discharge characteristics.

[0005]

In order to solve these problems, the present invention is to add phosphorus to a composite oxide of lithium and manganese mainly composed of a positive electrode active material to form particles of the composite oxide. The present invention provides a positive electrode active material having a surface coated with phosphorus, and provides a non-aqueous electrolyte secondary battery having a high voltage and exhibiting excellent charge / discharge cycle characteristics.

LiMn ₂ O ₄ is more stable than LiCoO ₂ at a higher potential, but Li _{1 -X} Mn ₂ O _{4 from which} lithium has been removed has manganese partially eluted into the electrolytic solution.
For this reason, a phenomenon was observed that adversely affects the charge / discharge cycle characteristics of the battery. Therefore, in order to develop a positive electrode active material which maintains a stable state even at a high potential with respect to elution of manganese, the present invention provides a composite oxide LiM of lithium and manganese.
LiMn by adding a predetermined amount of phosphorus to n ₂ O _4
The surface of ₂ O ₄ particles is coated with phosphorus, so that manganese does not elute into the electrolytic solution even at a high potential, and an active material excellent in charge / discharge cycle characteristics can be obtained, particularly in the case of LiCoO ₂ , In order to suppress decomposition at high voltage, a predetermined amount of phosphorus is required for LiCoO _2, whereas in the case of this lithium-manganese composite oxide, the elution of manganese in the compound poses a problem, so the phosphorus added is Was found to be necessary for a predetermined amount of manganese in the compound. That is, it has been found that the phosphorus added is optimal in a molar ratio range of 0.02 to 0.1 with respect to manganese in the compound. LiMn ₂ O ₄ is obtained by mixing a lithium salt such as lithium carbonate and lithium nitrate with a manganese salt such as manganese carbonate and manganese oxide, and heat-synthesizing at a high temperature of 650 to 900 ° C. in an oxidizing atmosphere. During the synthesis, the surface of the particles of LiMn ₂ O ₄ produced by mixing the phosphorus compound in the raw material and heat-treating is coated with phosphorus. By using the positive electrode in which phosphorus is added to LiMn ₂ O ₄ as described above, a non-aqueous electrolyte secondary battery having high voltage and high energy density and excellent charge / discharge characteristics is provided.

[0007]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 shows a sectional view of the cylindrical battery. In the figure, the positive electrode of 1 is LiMn ₂ O ₄ , an aqueous dispersion of carbon black as a conductive material and polytetrafluoroethylene as a binder, weighted to 100:
A mixture of 2.5: 7.5 was applied to both sides of an aluminum foil core material, dried, rolled, cut into a predetermined size, and 2 titanium leads were spot-welded. The mixing ratio of the aqueous dispersion of polytetrafluoroethylene as the binder is calculated by its solid content. For the negative electrode of 3, a carbonaceous material was mixed with a main active material and a styrene-butadiene rubber-based binder (SBR) was mixed at a weight ratio of 95: 3.5.
A copper foil core material is coated on both sides, dried, rolled, and then cut into a predetermined size to spot 4 copper leads. Also in the case of the negative electrode, the mixing ratio of the binder SBR is calculated by its solid content. Reference numeral 5 is a separator made of a polypropylene microporous film, and the positive electrode 1 and the negative electrode 2 are spirally wound via the separator 5 to form an electrode plate group. Insulating plates 6 and 7 made of polypropylene resin are arranged on the upper and lower sides of the electrode plate group respectively, and the insulating plates 6 and 7 are inserted into a case 8 nickel-plated with iron to form a positive electrode lead 2
After spot welding the negative electrode lead 4 to the bottom of the case 8 on the titanium sealing plate 10, the electrolytic solution is injected, and the battery is sealed via the gasket 9 to complete the battery. The dimensions of this battery are 17 mm in diameter and 50 mm in height. Reference numeral 11 denotes a positive electrode terminal of the battery, and the battery case also serves as the negative electrode terminal.

The electrolyte used was a mixture of ethylene carbonate and ethylmethyl carbonate in a volume ratio of 1: 3, and lithium hexafluorophosphate dissolved at 1.5 mol / dm ³ .

The positive electrode active material is manganese oxide (Mn
₃ O ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 4: 3.
Diphosphorus pentoxide (P ₂ O ₅ ) was added to the mixture at the ratio of, and heat treatment was performed in oxygen at 800 ° C. for 20 hours to synthesize. The addition ratio of diphosphorus pentoxide was represented by the molar ratio of phosphorus to 1 mol of manganese in the synthesized LiMn ₂ O ₄ , and the five types shown in (Table 1) were examined.

[0011]

[Table 1]

Two cells of each battery were manufactured, and one cell was charged and discharged at a charge and discharge current of 120 mA at 20 ° C., and a charge and discharge test was conducted for 5 cycles under the conditions of a charge end voltage of 4.3 V and a discharge end voltage of 3.0 V. After that, it was stored in a charged state at 80 ° C. for 3 days. After that, the battery was disassembled and the amount of manganese eluted into the electrolytic solution was measured. The result is shown in FIG. The other cell is 20
The charge / discharge current is 120 mA at ℃, and the charge end voltage is 4.3.
A charge / discharge cycle test was performed under the conditions of V and a discharge end voltage of 3.0V. Further, FIG. 3 shows the discharge capacity at the initial charging / discharging (10th cycle) of the batteries A to E and the capacity retention rate of the discharge capacity at the time of 300 cycles.

From FIG. 2, battery A containing no phosphorus was added.
It can be seen that the amount of manganese eluted into the electrolytic solution is very large, but the amount of manganese eluted decreases with the addition of phosphorus. Further, as shown in FIG. 3, the battery A without addition of phosphorus has a large initial discharge capacity, but also has a large decrease in capacity with charge / discharge cycles, and reaches about 40% of the initial capacity at the time of 300 cycles. Battery B containing phosphorus at a molar ratio of 0.01
In the case of, although the capacity retention rate is improved compared to Battery A, 6
It is insufficient at about 0%. This is presumably because the amount of phosphorus added was small, so that the surface of the LiMn ₂ O ₄ particles was not sufficiently covered with phosphorus, and the elution of manganese into the electrolytic solution was still considerable. In the batteries C, D, E, and F in which the added amount of phosphorus was further increased, the initial capacity was smaller than that of the battery A, but the capacity reduction rate was small, and about 300% of the initial capacity was maintained. On the other hand, phosphorus is added in a molar ratio of 0.
Battery F with 15 addition shows a decrease in capacity due to cycling to Battery C.
~ E, but the initial absolute capacity is smaller than the batteries C ~ E. This is because if too much phosphorus is added, LiMn
_It is considered that this is because the phosphorus coating the surface of the ₂ O ₄ particles became too thick and the reaction of LiMn ₂ O ₄ was hindered. As described above, considering the absolute capacity of the battery and the capacity retention rate with charge / discharge cycles, LiMn ₂ O ₄ containing phosphorus in a molar ratio of 0.02 to 0.1 is used as a positive electrode active material. A high-capacity non-aqueous electrolyte secondary battery having excellent discharge cycle characteristics can be obtained.

Example 2 The same battery configuration as in Example 1 was examined by changing the manufacturing method of the negative electrode active material and the positive electrode active material. The negative electrode of FIG. 1 uses a metallic lithium foil, and as a method of manufacturing the positive electrode, a mixture of manganite (MnOOH) and lithium carbonate (Li ₂ CO ₃ ) at a molar ratio of 4: 1 is used. ₃ PO ₄ ) was added and heat treatment was performed in air at 900 ° C. for 20 hours to synthesize. The proportion of phosphoric acid added was the main active material LiMn ₂ O ₄ synthesized.
As shown in (Table 2), 5 types of studies were conducted in a molar ratio of phosphorus to 1 mol of manganese.

[0015]

[Table 2]

The battery evaluation test was conducted in the same manner as in (Example 1).
A charge / discharge cycle test was performed under the conditions of a charge / discharge current of 120 mA, a charge end voltage of 4.3 V, and a discharge end voltage of 3.0 V at 0 ° C. FIG. 4 shows the discharge capacities of the batteries A ′ to F ′ at the initial stage of charge and discharge and the discharge capacity at 300 cycles.
Shown in As shown in FIG. 4, although the manufacturing method such as the starting material of the positive electrode active material and the synthesis conditions and the negative electrode active material were changed, the addition ratio of phosphorus showing a large discharge capacity and good cycle characteristics as in (Example 1). The molar ratio range of 0.02-0.1
It turned out to be. Further, as a method for manufacturing the positive electrode, manganese oxide (MN ₃ O ₄ ) and lithium carbonate (Li ₂ CO ₃ ) are used.
And cobalt carbonate (CoCO ₃ ) were mixed at a molar ratio of 6: 5: 2, diphosphorus pentoxide (P ₂ O ₅ ) was added, and heat treatment was performed in oxygen at 800 ° C. for 20 hours to synthesize manganese. Similar results were obtained in the case of the positive electrode active material in which a part was replaced with cobalt.

Example 3 The same battery configuration as in Example 1 was examined by changing the method of adding phosphorus to the positive electrode active material. The battery A ″ in which phosphorous was not added in Example 1 and the positive electrode active material was manganese oxide (Mn ₃ O ₄ )
And lithium carbonate (Li ₂ CO ₃ ) were mixed at a molar ratio of 4: 3, diphosphorus pentoxide (P ₂ O ₅ ) was added, and the mixture was heat treated in oxygen at 800 ° C. for 20 hours to synthesize. Battery D ″ with a phosphorus / manganese ratio of 0.05, and previously manganese oxide (Mn ₃ O ₄ ) and lithium carbonate (Li ₂ CO ₃ )
Are mixed in a molar ratio of 4: 3, and are mixed in oxygen at 800 ° C. for 2 times.
A battery G in which diphosphorus pentoxide was added at a phosphorus / manganese ratio of 0.05 to LiMn ₂ O ₄ simple substance synthesized by heat treatment for 0 hour was examined. FIG. 5 shows the discharge capacities of the batteries A ″, D ″, and G at the initial stage of charge / discharge (10th cycle) and the discharge capacity at 300 cycles. As described in Example 1, the battery A ″ without addition of phosphorus has a large initial discharge capacity, but the capacity also decreases greatly with charge / discharge cycles, and becomes about 40% of the initial capacity at the time of 300 cycles.
This is a battery D "with phosphorus added, and the initial capacity is battery A"
Although it is smaller, the capacity decrease rate is small, and 80% or more of the initial capacity is maintained at the time of 300 cycles. However, although the battery G added phosphorus at the same ratio as the battery D ″, the initial capacity retention rate at the time of 300 cycles was about 40%, which was almost the same as that of the battery A ″. I couldn't see it. From the above, as a method for synthesizing a positive electrode active material for a non-aqueous electrolyte secondary battery having excellent charge / discharge cycle characteristics, a phosphorus compound is added in advance when LiMn ₂ O ₄ is synthesized, and heat treatment is performed. By doing, LiM
By coating the surface of n ₂ O ₄ particles with phosphorus, a positive electrode active material in which the elution of manganese into the electrolytic solution is suppressed can be obtained.

In the above embodiment, LiMn was used as the positive electrode active material.
_Although only ₂ O ₄ was used, a similar effect can be observed in a compound in which manganese in the compound is replaced with another transition metal such as cobalt or nickel. Also, as the negative electrode, a carbon material,
Although lithium metal is used, a lithium alloy or another material capable of inserting and extracting lithium may be used. Further, as the electrolytic solution, a solution obtained by dissolving lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and ethyl methyl carbonate was used, but the same applies to an electrolytic solution in which a lithium salt is dissolved in another solvent.

[0019]

As described above, according to the present invention, by adding an appropriate amount of phosphorus to the positive electrode active material LiMn ₂ O ₄ , it is possible to obtain a non-aqueous electrolyte lithium secondary battery having excellent charge-discharge cycle characteristics. it can.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery of the present invention.

FIG. 2 is a diagram showing the elution amount of manganese into an electrolytic solution after charging a battery.

FIG. 3 is a diagram showing initial discharge capacities of batteries A to E and capacity retention ratios at 300 cycles.

FIG. 4 is a diagram showing the initial discharge capacity of batteries A ′ to F ′ and the capacity retention ratio at the time of 300 cycles.

FIG. 5 shows initial discharge capacities of batteries A ″, D ″ and G, and 300
Diagram showing capacity retention rate at the time of cycle

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Positive electrode lead plate 3 Negative electrode 4 Negative electrode lead plate 5 Separator 6 Upper insulating plate 7 Lower insulating plate 8 Case 9 Gasket 10 Sealing plate 11 Positive electrode terminal

Claims (5)

[Claims] 1. A non-aqueous electrolytic solution containing LiMn ₂ O ₄ , which is a composite oxide of lithium and manganese, as a main component, and having a positive electrode to which phosphorus or a phosphorus oxide is added, a negative electrode, and a non-aqueous electrolytic solution. Secondary battery. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein part of manganese is replaced by Co and Ni. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the addition ratio of phosphorus is 0.02 to 0.1 in terms of molar ratio with respect to manganese. 4. A lithium compound, a manganese compound, and phosphorus or a phosphorus compound are mixed, and this mixture is heat-treated in an oxidizing atmosphere so that the particle surface of LiMn ₂ O ₄ , which is a composite oxide of lithium and manganese, is changed to phosphorus. A method for producing a coated positive electrode active material for a non-aqueous electrolyte secondary battery. 5. The method for producing a material according to claim 4, wherein a compound of a transition metal other than manganese is mixed in the mixture.