JP2000003709A - Positive electrode material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the positive electrode material for lithium secondary battery stabilized in the high temperature condition and capable of preventing the lowering of capacity in the case of quick charging and discharging. SOLUTION: This positive electrode material for lithium secondary battery has a compound powder, composed of the oxide powder having the spinel structure expressed with the composition formula Li1+xMn2-xO4-x (0<=x<=0.3333, -0.1<=y<=0.2) and a LiV2O4 layer formed on the surface of the oxide powder. With this composition, rapid charging and discharging performance is improved, and elution of the manganese ion is restricted, and high-temperature stability of the battery is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode material for a lithium secondary battery having a spinel structure that can be used for a lithium secondary battery.

[0002]

2. Description of the Related Art Recently, lithium secondary batteries have attracted attention because of their high charge / discharge voltage and large charge / discharge capacity. As the positive electrode material, LiCoO ₂ having an ordered layered rock salt structure has been used. But Li
CoO ₂ is being replaced by LiMn ₂ O ₄ having a spinel structure in terms of resources and price. Particularly, in order to improve the cycle durability, L with a slight excess of lithium is used.
Attention has been paid to i _{1 + x} Mn _2-x O ₄ ( _{x to} 0.03) (for example, Y. Gao and JR Dahn, J. Electrochem. Soc., 14).
3,100 (1996)). It is considered that the cycle durability is improved by partially replacing the manganese site with lithium, thereby reducing the change in the crystal lattice due to charge and discharge, that is, desorption and insertion of lithium ions.

[0003]

SUMMARY OF THE INVENTION LiMn _{2 of a} normal composition
The battery performance of a lithium secondary battery using Li _{1 + x} Mn _2-x O ₄ containing O ₄ or Li in excess as a positive electrode active material is expressed by L
Compared with the battery performance of a lithium secondary battery using iCoO ₂ as a positive electrode active material, the former lithium secondary battery has the following two important problems; a rapid decrease in capacity during rapid charge and discharge.

When the temperature is maintained at 60 ° C. or more in the filled state, self-standing is remarkable. About the cause of LiMn ₂ O ₄ and Li _{1 + x} Mn
The resistivity of _2-x O ₄ is about 50 kΩcm at room temperature,
This is considered to be 1-2 orders of magnitude greater than oO ₂ . For this reason, in rapid charge and discharge, electrons cannot be sufficiently exchanged between the metal current collector and the positive electrode active material, so that the capacity is significantly reduced.

[0005] Since LiMn ₂ O ₄ and Li _{1 + x} Mn _2-x O ₄ are inexpensive, a decrease in capacity during rapid charging and discharging has been accepted. Lithium secondary batteries have been mainly used as power supplies for small electronic devices that do not require much current. For this reason, the reduction in capacity did not become a particularly serious problem. However, in view of future use as an energy source for electric vehicles; a. Sufficient capacity cannot be secured during rapid acceleration.

B. Kinetic energy cannot be regenerated as electric energy during deceleration. c. It cannot be charged in the same time as gasoline refueling, and it takes several hours to fully charge. There is a serious problem. It is believed that the main cause is that manganese ions are eluted into the electrolyte. Particularly at high temperatures, the amount of manganese eluted increases, which promotes formation of an undesired film on the negative electrode surface. Therefore, when a fully charged battery is stored at 60 ° C. for about 10 days, it is almost completely discharged. Also, even if the battery is returned to room temperature and recharged, only less than half of the initial capacity can be charged. Depending on the conditions of use, 6
Leave at 0 ° C. for more than 10 days. Alternatively, in severe operating conditions, the battery temperature is expected to be higher.
In such a case, the loss of the battery capacity significantly impairs the reliability of the electric vehicle.

An object of the present invention is to provide a positive electrode material for a lithium secondary battery which has a small capacity reduction even in rapid charging and discharging and is stable even at a high temperature.

[0008]

The positive electrode material for a lithium secondary battery according to the present invention has a composition formula of Li _{1 + x} Mn _2-x O _4-y (where 0 ≦ x ≦ 0.3333, −0.1 .Ltoreq.y.ltoreq.0.2), which is characterized by having a composite powder composed of an oxide powder having a spinel structure represented by the formula: and a LIV ₂ O ₄ layer formed on the surface thereof.

In charging and discharging a lithium secondary battery, lithium ions reciprocate between a positive electrode and a negative electrode. Further, the exchange rate of lithium ions at the interface between the electrolyte and the positive electrode and the negative electrode of the lithium secondary battery can be considerably increased by optimizing the electrode configuration. Therefore, in order to prevent the battery capacity from being deteriorated even by rapid charge / discharge, electrons corresponding to the entry and exit of lithium ions must be quickly exchanged between the metal current collector and the positive electrode active material.

However, the active material Li _{1 + x} Mn _2-x O
_4-y is a semiconductor whose resistivity is 50 kΩcm at room temperature.
Therefore, the upper limit of the electron exchange rate is determined by a time constant caused by the capacitance (capacitance) of the active material and the electrode. In the present invention, in order to significantly lower the resistivity of the active material, a LiV ₂ O ₄ layer having metallic conductivity is provided on the surface of the Li _{1 + x} Mn _2-x O _4-y powder of the active material. . With this LiV ₂ O ₄ layer, the resistivity and capacitance (capacitance) of the electrode are greatly reduced, and the rapid charge / discharge performance is improved.

Here, LiV_TwoO_FourThe layer is Li_{1 + x}Mn_2-x
O_4-yIt has the same spinel structure as
The conductivity is also large. Therefore, LiV_TwoO_FourLayer is Li_{1 + x}Mn
_2-xO _4-yEven if present on the powder surface, electron and ion conduction
It is not hindered in both directions. Where LiV_TwoO_FourIs Li_{1 + x}M
n_2-xO_4-yHas the same spinel structure as
Since they are almost the same, Li_{1 + x}Mn_2-xO_4-yOn the surface of
Easy to grow. Also, when X exceeds 0.3333,
It is not preferable because a spinel structure compound cannot be synthesized.

It is necessary that Y is in the range of -0.1≤y≤0.2 in order to maintain the spinel structure. Furthermore, since the LiV ₂ O ₄ layer exists on the surface of the active material, elution of manganese ions in the active material into the electrolytic solution is suppressed, and the high-temperature stability of the battery can be significantly improved. LiV ₂ O ₄ does not need to be formed in a layer on the entire surface of the active material.
_Since direct contact between _{1 + x} Mn _2-x O _4-y and the electrolytic solution can be prevented, elution of Mn can be substantially suppressed.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The composite powder of the present invention can be produced by a solid phase reaction method, a spray combustion method or the like. In the solid-phase reaction method, for example, lithium carbonate and manganese dioxide are wet-mixed at a predetermined molar ratio, formed into pellets, and then fired in an oxidizing atmosphere to obtain Li _{1 + x} Mn _2-x O _2.
4-y particles are obtained. In order to form a LiV ₂ O ₄ layer on the surface of the Li _{1 + x} Mn _2-x O _4-y particles, a solution prepared by dissolving vanadium pentoxide in an aqueous solution of lithium hydroxide is prepared, and the solution described above is added to the solution. Impregnated with the particles synthesized in the above, further dried, and then calcined at a temperature of about 500 to 850 ° C. in a nitrogen atmosphere. The spray combustion method is, for example, adding the above Li _{1 + x} Mn _2-x O _4-y particles to a solution of vanadium pentoxide in an aqueous solution of lithium hydroxide,
An O / W emulsion is formed by adding an emulsifier, kerosene and the like thereto. By spray-burning this emulsion, Li _{1 + x} Mn _2-x O _4-y particles
A composite fine powder having a V ₂ O ₄ layer can be synthesized.

[0014]

The present invention will be specifically described below with reference to examples. (Li _{1 + x} Mn _2-x O _4-y Particle: Production of Active Material) Li: Mn = 1.03: 1. Lithium carbonate (Li ₂ CO ₃ ) and manganese dioxide (MnO ₂ ) in molar ratio. The mixture was mixed well to give a ratio of 97. Mixing using ethanol as a solvent,
I went with a planetary ball mill. After drying the mixed particles,
It was press-formed into a pellet and kept at 650 ° C. for 8 hours in an oxygen stream. Thereafter, the furnace was cooled to room temperature at 1 ° C./min.

The pellets are sufficiently pulverized to obtain Li _1.03 M
An n _1.97 O ₄ sample was obtained. This sample was confirmed to have a spinel structure by X-ray diffraction. This sample was prepared as Comparative Example C
It was set to 1. (Example 1) Li _{1 + x} Mn _2-x O _4-y
Formation of V ₂ O ₄ Layer (Solid State Reaction Method) 2 g of vanadium pentoxide (V ₂ O ₅ ) was dissolved in 200 ml of 1N nitric acid aqueous solution. 0.8 g of lithium nitrate (LiNO ₃ ) was added to this solution.
Is dissolved, and an aqueous ammonia solution is further added to add p to the solution.
H was adjusted to 7-8. This solution was impregnated with 10 g of the above prepared comparative example C1 for 1 hour and then at 200 ° C. for 1 hour.
Dried for 0 hours. The obtained powder was kept at 700 ° C. for 8 hours in a nitrogen stream. Thereafter, the furnace was cooled at room temperature at 1 ° C./min. This was designated as sample 1 of this example.

Example 2 3.6 g of vanadium pentoxide (V ₂ O ₅ ) was dissolved in 200 ml of 0.1N aqueous solution of lithium hydroxide (LiOH). Add a small amount of L
The pH of the solution was adjusted to 7-8 by adding iOH. This solution was impregnated with 10 g of the above-mentioned Comparative Example C1 for 1 hour, and then dried at 200 ° C. for 10 hours. The obtained powder is 700
C. for 8 hours in a nitrogen stream. Then to room temperature 1
The furnace was cooled at a rate of ° C / min. This was designated as Example 2. Example 3 (Spray Combustion Method) LiOH and V ₂ O ₅ in a molar ratio of Li: V =
The aqueous solution dissolved at a ratio of 1: 2 was adjusted to have a lithium ion concentration of 0.1 mol / liter. A small amount of LiOH was added to the solution to bring the solution pH to 7
Adjusted to ~ 8. The solution has a particle size of about 1 μm.
i _1.03 Mn _1.97 O ₄ fine particles per liter.
9 g was added and mixed and dispersed. Next, in a raw material aqueous solution in which the fine particles were dispersed, glycerin fatty acid ester as an emulsifier was suspended in kerosene, and an emulsion was prepared using an ultrasonic homogenizer.

This emulsion was sprayed from an atomizer and burned in an oxygen stream. The oxygen supply was adjusted so that the combustion atmosphere was slightly reduced. After recovering the oxide powder synthesized by combustion, it was dried in the air at 300 ° C. This was designated as Example 3. (Example 4) The sample of Example 3 was kept in a nitrogen stream at a temperature of 700 ° C for 8 hours, and then cooled in a furnace at room temperature at 1 ° C / minute. This was designated as Example 4.

(Comparative Example C2) A sample of Example 3 was heated at 900 ° C.
, And kept in a nitrogen stream for 8 hours, and then cooled in a furnace at a rate of 1 ° C./min to room temperature. This was designated as Comparative Example C2. (Evaluation of Battery) Next, lithium secondary batteries using each of Examples 1 to 4 and Comparative Examples C1 and C2 as positive electrode materials were assembled using test cells, and the characteristics of the lithium secondary batteries were evaluated. did.

In this lithium secondary battery, the positive electrode was manufactured as follows. First, Examples 1 to
4, 70% by weight of one of Comparative Examples C1 and C2,
25 wt% of carbon as a conductive agent and 5 wt% of trifluoroethylene (TrFE) as a binder were mixed well. About 10 mg of this mixed powder was pressed against a SUS mesh wire net at 0.1 ton / cm ² to form a positive electrode.

The negative electrode is made of a metal Li having a thickness of 0.4 mm.
One foil was used. A polypropylene nonwoven fabric was used for the separator provided between the positive electrode and the negative electrode. Further, the electrolyte in the lithium secondary battery is 1N LiPF ₆
A solution was employed, and the solvent was a 1: 1 mixture of ethylene carbonate and diethyl carbonate. Next, charge / discharge conditions for comparing the rapid discharge performance of the lithium secondary battery will be described.

[0021] Each lithium secondary battery is 1 m up to 4.5V.
The battery was charged at a constant current of A / cm ² . After the voltage reached 4.5 V, charging was further performed at a constant voltage of 4.5 V. The total of the above charging times was 2 hours. Next, after the completion of the charging, the discharging was started. Discharge current density 1mA / cm ²
At a constant current of 3.5 V until the voltage reached 3.5 V. Immediately after that, charging was started again. The above was regarded as one cycle, and the same charge / discharge was performed in three cycles. Similarly, after charging, the battery was discharged until the voltage reached 3.5 V while the discharge current density was sequentially changed from ² mA / cm ² to 20 mA / cm ² . Charge / discharge was performed three cycles at each discharge current density.

Next, the high-temperature durability of this lithium secondary battery
Will be described. Each Lichi
Battery in a high-temperature bath at 60 ° C and up to 4.5V.
At 1mA / cm ^TwoThe battery was charged at a constant current of Then this charge
After the termination, the discharge was started. Discharge current density 1mA / cm^Two
At a constant current of 3.5 V until the voltage reached 3.5 V. Immediately after
Charging was started again. The above was regarded as one cycle.

FIG. 1 shows the relationship between the discharge current density and the discharge capacity of a lithium secondary battery using Examples 1 to 4 and Comparative Examples C1 and C2 as positive electrodes. When compared with Comparative Example C1,
It can be seen that in the lithium secondary batteries using Examples 1 to 4, the decrease in the discharge capacity at a high current density is extremely small.
On the other hand, the result of Comparative Example C2 hardly differs from the result of Comparative Example C1. This is because the post heat treatment temperature was as high as 900 ° C., so that V diffused uniformly into the particles and LiV ₂ O ₄ on the active material surface.
This is probably because the film disappeared.

FIG. 2 shows the relationship between the charge current density and the discharge capacity of the lithium secondary batteries using Examples 1 to 4 and Comparative Examples C1 and C2 as positive electrodes. When compared with Comparative Example C1,
It can be seen that in the lithium secondary battery using Examples 1-4, the decrease in the charge capacity at a high current density is extremely small.
On the other hand, as in the case of FIG. 1, the result of Comparative Example C2 was hardly different from the result of Comparative Example C1.

Next, FIG. 3 shows the relationship between the discharge capacity and the number of cycles at 60 ° C. of the lithium secondary batteries using Examples 1 to 4 and Comparative Examples C1 and C2 as positive electrodes. As compared with Comparative Example C1, it can be understood that the cycle durability at 60 ° C. is significantly improved in the lithium secondary batteries using the present example samples 1 to 4. On the other hand, the result of Comparative Example C2 is Comparative Example C
There was almost no difference from the result of 1.

Although the detailed description has been omitted, the distribution of V on the particle surface may be changed continuously. That the composition when expressed as _{Li V 2-x Mn x O} 4, be continuously varied from 2 x is toward the center from the particle surface to 0, almost the same effects as those described above can get.

[0027]

According to the present invention, it is necessary to rapidly exchange electrons corresponding to the entrance and exit of Li ions between the metal current collector and the active material so that the battery capacity is not deteriorated even by rapid charge and discharge. In the present invention, the electrical resistivity of the active material is significantly reduced by forming a metallic conductive LiV ₂ O ₄ layer on the surface of the active material Li _{1 + x} Mn _2-x O _4-y. , Rapid charge and discharge performance could be improved.

Further, since the LiV ₂ O ₄ layer exists on the surface of the active material, elution of manganese ions was suppressed, and the high-temperature stability of the battery was improved.

[Brief description of the drawings]

FIG. 1 shows Examples 1 to 4 and Comparative Examples C1 and C2.
4 is a graph showing a relationship between a discharge current density and a discharge capacity of a lithium secondary battery having a positive electrode as a positive electrode.

FIG. 2 shows Examples 1 to 4 and Comparative Examples C1 and C2.
4 is a graph showing a relationship between a discharge current density and a discharge capacity of a lithium secondary battery having a positive electrode as a positive electrode.

FIG. 3 shows Examples 1-4 and Comparative Examples C1 and C2.
4 is a graph showing the relationship between the discharge capacity at 60 ° C. and the number of cycles of a lithium secondary battery having a positive electrode as a positive electrode.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuma Takatori 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory, Inc. (72) Inventor Naoyoshi Watanabe Nagakute-cho, Aichi-gun, Aichi Prefecture Inside 41 Toyota Chuo R & D Co., Ltd., No. 41, Chuchu Yokomichi (72) Inventor Tatsuo Noritake No. 41, Toyota Chuo R & D Laboratories Co., Ltd. 41 F-term in Toyota Central R & D Laboratories Co., Ltd. 41 No. 41, Chuchu-shi, Nagakute-cho, Aichi-gun, Aichi-gun 5H003 AA00 AA01 BB05 BC01 BC05 BC06 BD03 5H014 AA02 EE10 HH01

Claims (1)

[Claims] A composition formula Li _{1 + x} Mn _2-x O _4-y (where 0
.Ltoreq.x.ltoreq.0.3333, -0.1.ltoreq.y.ltoreq.0.2) and an oxide powder having a spinel structure and L formed on the surface thereof.
A positive electrode material for a lithium secondary battery, comprising a composite powder composed of an iV ₂ O ₄ layer.