KR19990076064A - Lithium Secondary Battery Using Carbon Composite Electrode - Google Patents

Abstract

In the present invention, a composite negative electrode is prepared by mixing two negative electrode materials having different insertion / release rates of lithium, and a lithium ion having improved charge and discharge life by manufacturing a battery by combining this with a positive electrode manufactured using lithium manganese oxide. Provide a secondary battery.

Description

Lithium Secondary Battery Using Carbon Composite Electrode

The present invention relates to a lithium secondary battery using a lithium manganese oxide positive electrode, and more particularly, to a method of improving the charge and discharge life of a lithium secondary battery using a lithium manganese oxide positive electrode by improving the carbonaceous negative electrode.

Recently, as the penetration of notebook computers and cordless phones has been expanded, the demand for high performance secondary batteries with high energy density has exploded. Therefore, to meet this, a lithium secondary battery employing a cathode material capable of reversible intercalation between a carbon anode and lithium has now emerged.

As the material of the carbonaceous anode in a lithium ion battery, graphite materials heat-treated at a high temperature of about 2500 ° C. and hard carbons having a low graphitization degree are mainly used. As a cathode active material, lithium transition metal oxides (LiMO ₂ ) are used. Materials having structures (M1, M2 = Ni, Co, Mn) of (M = Co, Ni, Mn) and lithium (transition metal 1, transition metal 2) oxides (Li (M1, M2) O ₂ ) are mainly used. It is becoming. Among them, lithium cobalt oxide having excellent capacity characteristics, high rate discharge characteristics, and long-term charge / discharge characteristics is currently used mainly.

However, since this material contains a rare element cobalt, there is a limit to the supply amount, and also because the price of cobalt is expensive, it causes a rise in battery raw material costs. In addition, a conventional lithium secondary battery using lithium cobalt oxide has been recognized as an inferior safety material in that overcharging may cause overheating, ignition and explosion of the battery. To improve this, spinel-structured lithium manganese oxide, which has satisfactory properties in terms of cost and safety, is currently being actively studied.

When the lithium manganese oxide having the spinel structure is applied to the positive electrode, the safety problem due to overcharging is greatly improved, while the reversibility of the positive electrode is rapidly deteriorated when the high rate discharge is repeated. This is thought to occur because the degree of lithium insertion of the lithium manganese oxide is locally uneven at high rate discharge, and this nonuniformity is caused by the low conductivity of the lithium manganese oxide, the nonuniformity of the electrode, and the difference in the particle size of the active material itself. More specifically, if the reversible capacity of the positive electrode decreases due to repetition of charge and discharge, _x representing the insertion amount of lithium in the structure of lithium manganese oxide represented by the formula Li _x Mn ₂ O ₄ when local nonuniformity in the electrode exists. It occurs when it is 1 or more, and when the potential of the positive electrode active material is lowered to less than about 3 V based on lithium metal. In this case, when the amount of lithium insertion x is 1 or more, irreversible change of the crystal structure occurs and an irrecoverable capacity loss occurs.

Currently, researches to improve the long-term charge / discharge life of a positive electrode using lithium manganese oxide are mostly focused on stabilizing the structure of lithium manganese oxide by adding other elements, and representative examples thereof are US Patent No. 5370949 and Japanese Patent Laid-Open No. 7-272765. Japanese Unexamined Patent Application Publication No. 5-166510, Japanese Unexamined Patent Publication No. 7-272765, and Japanese Unexamined Patent Publication No. 4-289662. However, this method somewhat improves the structure deterioration caused by the normal charge / discharge of lithium manganese oxide and the structure deterioration due to excessive lithium insertion, but does not fundamentally prevent the insertion of lithium into the low potential region of lithium manganese oxide. There is a limit. In addition, an increase in the amount of inert elements also has a defect indicating a decrease in capacity.

Therefore, in order to solve this problem, the present invention has devised a composite anode in which the discharge reaction rate is divided into two stages. Such a negative electrode may be manufactured by mixing two negative electrode materials having different insertion / discharge rates of lithium, for example, and when a battery is formed using a lithium manganese oxide positive electrode, a capacity decrease that occurs during repeated high rate discharges is rapidly increased. Can be mitigated.

The present invention relates to a lithium secondary battery employing a composite negative electrode having a structure in which the discharge reaction rate can be divided into two stages, and more particularly, to a lithium secondary battery using lithium manganese oxide as a positive electrode material. At this time, some of the lithium ions inserted into the negative electrode is released from a material having a high discharge reaction rate, and a part of the total capacity is utilized. In this case, the amount of lithium inserted into the positive electrode active material is locally biased to the small active material. Subsequently, the subsequent discharge proceeds in a material having a relatively low discharge reaction rate, and thus, under constant current, the overvoltage caused by the negative electrode rapidly increases to reach the battery termination voltage before the lithium insertion amount in some positive electrode materials becomes excessively high.

The advantage of such a structure is clearly shown through comparison with the case of using a negative electrode consisting of only one type of active material. First, when the negative electrode is made of only a material exhibiting a fast discharge reaction, the insertion of lithium may be excessively progressed in a portion of the positive electrode and the positive electrode active material particles having a small size during high rate discharge. This phenomenon is particularly aggravated when the electrode structure is uneven and when the particle size distribution of the positive electrode active material is wide. As such, excessive insertion of lithium locally leads to structural deformation of lithium manganese oxide, thereby inhibiting reversible insertion / release of lithium, leading to a drastic reduction in battery capacity due to repeated charge and discharge. On the other hand, when the negative electrode is made of only a material exhibiting a relatively slow discharge reaction compared to the positive electrode, the battery voltage is drastically reduced due to the overvoltage induced in the negative electrode from the beginning during high rate discharge, and thus the capacity of the battery is drastically reduced.

In contrast, in the composite negative electrode implemented in the present invention, most of the battery capacity is exhibited even at high rate discharge, and at the same time, non-uniformity of lithium insertion amount in the positive electrode active material which may occur at the end of discharge is controlled, thus high rate discharge characteristics and long-term charge / discharge It has the characteristic that the characteristic is improved in balance.

A composite anode having the above characteristics can be basically realized in the following configuration: at least two or more carbon materials having significantly different diffusion abilities of lithium ions in the material; Or lithium alloy powder with a high discharge potential and slow lithium ion release reaction with a carbon material having a large diffusion rate of lithium ions.

The composite negative electrode is formed by dispersing a mixture of the active materials as described above in a binder solution composed of a polyvinylidene fluoride (PVdF) binder and an N-methylpyrrolidinone (NMP) solvent, and then applying a copper current collector. Can be. The composite negative electrode plate may be wound in a cylindrical shape together with a lithium manganese oxide positive electrode plate with a battery separator interposed therebetween to form a jelly roll, and then injected with an electrolyte solution using a non-aqueous solvent to form a battery having a standard specification.

As a specific example of the mixing between the above carbon materials, a flake-like artificial layer having a large layer structure and having a graphite lamination thickness constant (Lc) measured by X-ray scattering method having a value of 100 nanometer or more Mesocarbon microbeads (MCMB), or mesoface pitch, obtained by processing graphite or natural graphite powder into a series of materials having a slow lithium ion diffusion and by processing a meso face pitch with a graphite lamination thickness constant (Lc) value of less than 100 nanometers. Carbon fiber (MPCF) can be used as a material of a rapid diffusion of lithium ions in the material. In contrast, it is also effective to use two materials having similar graphite structures but having different insertion / release rates of lithium due to the difference in average particle size. Examples thereof include a mixture of a meso carbon microbead (MCMB) having an average particle size of 15 or more, preferably 20 to 30, more preferably, a standard particle size of 25 microns (μ) and an average particle size of 10 or less. And use. At this time, the rate of lithium ion diffusion is evaluated by the results obtained through conventional electrochemical experiments, especially impedance measurement (J. Electrochemical Society, 142, 1995, p371-378; Solid State Ionics 92, 1996, p91-97) It is appropriate to choose a substance that exhibits a difference of two or more times. Alternatively, the graphite lamination thickness constant Lc of the crystal lattice may be selected to be 100 nanometers (nm) or more and the following. When the two or more carbon materials having different release rates of lithium ions are mixed using such a criterion, the present invention is not limited to the above-described materials.

Second, an example of a mixture of a carbon material having a high diffusion rate of lithium ions and a lithium alloy powder having a relatively high discharge potential and a slow lithium ion emission reaction is an alloy powder having a size of about 100 mesh or less, which is electrochemically formed with lithium. Reactive diffusion of lithium ions into the material from a material capable of forming a lithium alloy upon reaction and a mesocarbon microbead (MCMB) or mesoface pitch carbon fiber (MPCF) obtained through the processing of the aforementioned mesophase pitch. It is mentioned to use as a substance. Examples of the alloy powder include aluminum, tin, or an alloy powder of antimony and lithium, and the present invention is not limited to the above-mentioned materials. At this time, the metal powder is preferably 100 mesh or less, and particularly preferably about 300 mesh powder.

In the case of using the composite cathode, a material having a fast discharge rate of 30 to 95% by weight and a material having a low discharge rate are suitably configured at a ratio of 5 to 70% by weight. Harmonious When the compounding ratio of a material having a fast discharge rate is 95% or more, a rapid capacity decrease occurs at high charge / discharge over a long period, and when the compounding ratio of a material having a slow discharge rate is 70% or more, it is used for a high rate discharge. A severe decrease in dose is caused.

When the composite negative electrode formed as described above is assembled with a lithium manganese oxide positive electrode to construct a battery, a battery separator, an electrolyte solution, and other battery assembly parts use a conventional material used for forming a lithium ion secondary battery. Moreover, the form and specification of a battery jelly roll can also use a normal battery specification as it is.

Hereinafter, the effects of the present invention will be described in detail through examples. However, the present invention is not limited to the following examples.

Example 1

Synthesized spinel structure of lithium manganese oxide is 82% by weight, acetylene black of Chevron, USA 10%, and the binder is made of Atochem Co., Ltd., China 761 8% to make a positive electrode coating, and on a 20 micron thick aluminum foil It was applied to prepare a positive electrode. At this time, the size of the active material used is less than 10 microns on average, and a large amount of powder has a particle size of 1 micron or less. 90% by weight of MCMB10-28 from Osaka Gas, Japan, and 10% by weight of Atochem Co., Ltd. were used as a negative electrode to make a negative electrode paint. The negative electrode was then coated on 10 micron thick copper foil. . Celgard 2400 ^TM was used as a separator, and lithium hexafluorophosphate was dissolved in an ethylene carbonate and diethylene carbonate mixed solvent as an electrolyte. A battery of a normal 18650 standard was assembled using the electrode, the separator, and the electrolyte solution, and charging and discharging were repeated under the following conditions. At this time, the remaining capacity after charging and discharging 100 times and 200 times under each condition was confirmed by charging and discharging under the condition 1, which is shown in Table 1.

Charge current Discharge current Final voltage Initial capacity 100 times 200 times Condition 1 400 mA 240 mA 3.0-4.2 V 1200 92% 86% Condition 2 400 mA 600 mA 3.0-4.2 V 1200 91% 84% Condition 3 400 mA 1200 mA 3.0-4.2 V 1200 88% 78% Condition 4 400 mA 2400 mA 3.0-4.2 V 1200 83% 70%

Example 2

A positive electrode prepared in the same manner as in Example 1 was used, and a negative electrode was prepared in the same manner as in Example 1. However, this time, Osa Japan used a mixture of MCMB6-28 from Gas and SFG75 from Tim Cal, Switzerland. Using the assembled battery, the initial capacity and the discharge capacity at 1C and 2C discharge rates were measured, and long-term charge and discharge experiments were performed using the condition 3 of Example 1. The results of this experiment are summarized in Table 2 below. Battery capacity is expressed as a percentage of the initial rated capacity (C / 5 discharge rate). Eggs without experimental results are marked with x.

MCMB6-28: SFG75 Discharge capacity (1C) Discharge capacity (2C) 100 times 200 times 300 times 100: 0 97% 90% 88% 80% x 95: 5 96% 89% 91% 87% x 90: 10 96% 89% 92% 89% 85% 80: 20 95% 88% 92% 90% x 60: 40 94% 87% 93% 90% 87% 40: 60 94% 87% 93% 89% x 30: 70 92% 85% 92% 88% 84% 20: 80 89% 78% 92% 87% x 0: 100 86% 70% 91% 85% x

Example 3

Experiments were carried out in the same manner as in Example 2, but this time, Osa used MPCF of Petoka Co., Ltd. instead of MCMB of gas. Table 3 below summarizes the experimental results.

MPCF: SFG75 Discharge capacity (1C) Discharge capacity (2C) 100 times 200 times 300 times 100: 0 98% 91% 87% 78% x 90: 10 98% 90% 90% 87% 84% 60: 40 95% 87% 92% 90% 87% 30: 70 92% 85% 91% 88% 85% 20: 80 88% 81% 91% 87% x 0: 100 86% 78% 91% 85% x

Example 4

In Example 2 325 mesh tin powder was used instead of SFG75. Evaluation was performed in the same manner, and the results are summarized in Table 4 below.

MCMB10-28: Sn Powder Discharge capacity (1C) Discharge capacity (2C) 50 times 100 times 200 times 100: 0 97% 90% 91% 86% 82% 95: 5 96% 89% 91% 87% 84% 90: 10 95% 86% 92% 90% 86% 80: 20 93% 83% 92% 90% 86% 60: 40 92% 80% 91% 88% 83% 40: 60 89% 76% 89% 85% 80%

A composite negative electrode was prepared by mixing two negative electrode materials having different insertion / release rates of lithium, and a positive electrode was manufactured using lithium manganese oxide to prepare a battery, thereby improving the charge and discharge life of the lithium ion secondary battery.

Claims (8)

a) a cathode comprising a lithium manganese oxide having a spinel structure as an active material; b) a battery separator; c) an organic electrolyte containing a non-aqueous solvent; And d) composite anodes containing at least two active materials with different lithium discharge rates An organic electrolyte lithium secondary battery comprising a. The method of claim 1, The lithium secondary battery, wherein the composite negative electrode comprises graphitized carbon having a graphite lamination thickness constant (Lc) of a crystal lattice of 100 nanometers or more and graphitized carbon having a lamination thickness constant (Lc) of less than 100 nanometers. The method of claim 2, The lithium secondary battery, wherein the graphitized carbon having an Lc value of less than 100 nanometers is meso carbon mite beads (MCMB) or meso face pitch carbon fiber (MPCF). The method of claim 1, The lithium secondary battery, wherein the composite negative electrode contains 30-95 parts by weight of a carbon material having a high lithium discharge rate and 5-70 parts by weight of a carbon material having a low lithium discharge rate. The method of claim 1, At least one active material in the composite anode is an organic electrolyte lithium secondary battery comprising an alloy powder having a size of less than 100 mesh. The method of claim 5, A lithium secondary battery, wherein the lithium alloy is aluminum, tin, or an alloy of antimony and lithium. The method of claim 5, The lithium secondary battery whose alloy powder is 300 mesh or less. The method of claim 5, The composite negative electrode contains a lithium alloy powder of 5-70 parts by weight and a carbon material in the proportion of 30-95 parts by weight.