KR20080024597A - Lithium secondary battery and preparation method thereof - Google Patents

Abstract

A lithium secondary battery, and a method for preparing the lithium secondary battery are provided to allow the decrease of battery capacity due to the consumption of lithium to be compensated. A lithium secondary battery comprises a positive electrode; a negative electrode; an electrolyte solution; and a separator, wherein the reversible capacity of the negative electrode within a negative electrode operation voltage range is larger than the reversible capacity of the positive electrode within a positive electrode operation voltage range, and the positive electrode comprises further a second positive electrode active material whose charge/discharge voltage range is lower than the operation voltage range of the positive electrode and is higher than the operation voltage range of the negative electrode other than a first positive electrode active material.

Description

Lithium secondary battery and its manufacturing method {LITHIUM SECONDARY BATTERY AND PREPARATION METHOD THEREOF}

The present invention relates to a lithium secondary battery and a method for manufacturing the same, and more particularly, to a lithium secondary battery and a method for manufacturing the same, by adjusting the reversible capacity ratio of the positive electrode and the negative electrode, thereby preventing the capacity decrease due to the use of the battery. .

Recently, with the development of portable devices such as mobile phones, notebook computers, camcorders, and the like, demand for small secondary batteries such as Ni-MH (Ni-MH) secondary batteries and lithium secondary batteries is increasing. In particular, lithium secondary batteries using lithium and nonaqueous electrolytes have been actively developed due to the high possibility of realizing small, lightweight and high energy density batteries. Generally, a transition metal oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ is used as a cathode material of a lithium secondary battery, and lithium metal or carbon is used as an anode material. This is used, and a lithium secondary battery is comprised using the organic solvent which contains lithium ion as electrolyte between two electrodes.

In the lithium secondary battery, lithium existing in the positive electrode active material during charging is detached and moved to the negative electrode, and inserted into the negative electrode active material, and during discharge, lithium is detached from the negative electrode and moved to the positive electrode, and inserted into the positive electrode active material. Since it uses the electrochemical energy generated in the process, it is also called a rocking-chair type battery.

However, not all lithium initially possessed by the positive electrode active material participates in such a reversible reaction, and some of the lithium participating in the formation of the SEI film of the negative electrode during the initial charging, that is, the battery formation process, participates in the reversible reaction. You will not. In addition, not all of the desorbed lithium is reinserted due to the characteristics of the active material itself. LiCoO ₂ that is generally widely used as a positive electrode active material of a lithium secondary _{battery, LiMn 2 O 4, LiNiO 2} , a composite metal oxide such as _{Li (Ni 1/3 Mn 1} /3 Co 1/3) O 2 is 90 to 97% Although the initial efficiency of the degree, that is, the ratio of the first detached lithium is reinserted into the positive electrode active material, but depending on the material, the initial efficiency varies from 30 to 90%.

This irreversible characteristic is expressed as the irreversible capacity (exactly irreversible efficiency) of the battery, which can be expressed as (charge amount-discharge capacity) / charge capacity. The reversible capacity can be expressed as (1-irreversible capacity), which is the capacity of a battery participating in the reversible reaction, which is related to the amount of lithium inserted and detached during the battery reaction.

In order to be able to use the lithium secondary battery in various fields, characteristics such as high voltage, high capacity, high output, high lifetime, and high safety are required, and research on these is being actively conducted. In particular, in terms of high lifespan characteristics, the focus is on developing a stable active material that does not generate structural changes even by repeated insertion / desorption reaction of lithium.

However, with regard to the life characteristics of a lithium secondary battery, it is not possible to overlook the problem of lithium consumption due to side reaction as one of the causes of the reduction of the reversible capacity caused by continuous use or storage. There may be various forms of side reactions that cause the consumption of lithium by use of the battery, but lithium is consumed in the process of cracking and regenerating the SEI film of the negative electrode.

The present invention is a positive electrode active material in the initial formation process of the battery by designing a battery having a larger reversible capacity than the positive electrode by mixing a second active material having a charge and discharge voltage lower than the operating voltage range of the positive electrode in addition to the general positive electrode active material to the positive electrode It has been found that the amount of lithium in excess of the reversible capacity of is stored in the active site in the negative electrode active material, thereby compensating for the decrease in capacity of the battery due to lithium consumption in the course of continuous use.

Therefore, the present invention has a reversible capacity of the negative electrode in the negative operating voltage range is greater than the reversible capacity of the positive electrode in the positive operating voltage range, and the charge and discharge voltage range is lower than the operating voltage range of the positive electrode in addition to the first positive electrode active material of the positive electrode. It is an object of the present invention to provide a lithium secondary battery, and a method of manufacturing the same, further comprising a second positive electrode active material having a higher charge / discharge voltage range than the operating voltage range of the negative electrode.

The present invention relates to a lithium secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the reversible capacity of the negative electrode in the negative operating voltage range is greater than the reversible capacity of the positive electrode in the positive operating voltage range, and is a primary active material of the positive electrode. In addition to the positive electrode active material, a lithium secondary battery may further include a second positive electrode active material having a lower charge / discharge voltage range than the operating voltage range of the positive electrode and a higher charge / discharge voltage range than the operating voltage range of the negative electrode.

In addition, the present invention comprises the steps of a) designing the mixing ratio of the first positive electrode active material and the second positive electrode active material and the amount of the positive electrode active material and the negative electrode active material such that the reversible capacity of the positive electrode is less than the reversible capacity of the negative electrode; b) preparing a positive electrode active material by mixing a first positive electrode active material, which is the main active material of the positive electrode, and a second positive electrode active material having a charge / discharge voltage range lower than the operating voltage range of the positive electrode and a charge / discharge voltage range higher than the operating voltage range of the negative electrode; ; c) applying the positive electrode active material and the negative electrode active material to the electrode according to the amount designed in step a) to prepare a positive electrode and a negative electrode and to manufacture a battery including the same; d) allowing the battery to be inserted into the negative electrode through a formation process, wherein lithium in a portion exceeding the reversible capacity of the positive electrode is inserted into the negative electrode; It provides a method for producing a lithium secondary battery according to claim 1.

Hereinafter, the present invention will be described in detail.

In the present invention, the irreversible capacity is a value obtained by subtracting the discharge amount from the charge amount of the active material, and the charge amount-discharge amount / charge amount can be expressed as irreversible efficiency. In addition, the reversible capacity is a value obtained by subtracting the irreversible capacity from the filling amount of the active material, and eventually becomes equal to the discharge amount of the active material. Unless otherwise stated, reversible capacity and irreversible capacity mean reversible capacity and irreversible capacity at the time of initial charge and discharge.

In the conventional secondary battery, the capacity relationship between the positive electrode and the negative electrode is as follows.

Lithium that originally existed in the positive electrode active material moves to the negative electrode at the time of initial charge by the formation process, and at this time, part of lithium is consumed to form a solid electrolyte interface (SEI) film on the negative electrode surface, and thus again during discharge. The amount of lithium inserted into the positive electrode is reduced than the initial amount, and the positive electrode has a dead volume where lithium can be inserted and detached, but no lithium is actually present. That is, since the negative electrode of the carbon material forms the SEI film during the initial charging process and consumes lithium, it is common that the irreversible capacity is larger than the positive electrode (that is, the reversible capacity is smaller than the positive electrode).

Due to the continuous charging and discharging of the battery, or storage at high temperature, various side reactions may occur in the lithium battery, and at this time, lithium is consumed and the amount of lithium participating in the reversible chemical reaction of the battery is reduced, and the cathode active material is dead. The volume will increase, resulting in a decrease in battery capacity.

The present invention further comprises a second positive electrode active material as a component of the positive electrode active material, in addition to the general positive electrode active material, a charge and discharge voltage range lower than the operating voltage range of the positive electrode and a charge and discharge voltage range higher than the operating voltage range of the negative electrode. In addition, the reversible capacity of the negative electrode in the negative operating voltage range is characterized in that it is larger than the reversible capacity of the positive electrode in the positive operating voltage range. Therefore, although lithium may be detached from the second positive electrode active material during the initial charging by the formation process, it is not inserted into the second positive electrode active material again during discharge, and such irreversible lithium is consumed in forming the SEI film of the negative electrode, or the positive electrode The portion of lithium exceeding the reversible capacity of may be stored in the active site of the negative electrode. As a result, even when lithium is consumed due to the use and storage of the battery, lithium stored in the active site of the negative electrode can compensate for this, thereby preventing a reduction in capacity due to conventional lithium consumption.

In the present invention, the operating voltage range of the positive electrode and the negative electrode is not particularly limited, but preferably the operating voltage range of the positive electrode may be in the range of 3.5V to 4.3V, the operating voltage range of the negative electrode may be in the range of 0V to 1.5V.

In the present invention, the first positive electrode active material that can be used as the main positive electrode active material may be a material known to those skilled in the art as a lithium transition metal oxide capable of inserting and desorbing lithium, and is not particularly limited, but non-limiting examples thereof include LiCoO ₂ , there is a LiMnO _{2, LiNi 0 .5 Mn 0 .5} O 2, LiMn 2 O 4, LiNiO 2, Li (Mn 1/3 Ni 1/3 Co 1/3) O 2 and a mixture thereof.

In the present invention, the second positive electrode active material included in the positive electrode is not particularly limited as long as the charge and discharge voltage range is lower than the operating voltage range of the positive electrode and the charge and discharge voltage range is higher than the operating voltage range of the negative electrode. Examples include LiFePO ₄ , or LiV ₂ O ₅ .

The LiFePO ₄ is an olivine-structured compound, which is known as a cathode active material having an energy density of about 170 Ah / kg at a discharge voltage of 3.2 V, but is charged to have a discharge voltage range of 3.5 to 4.3 V as in the present invention. In this case, the desorbed lithium is characterized by irreversibility of being not reinserted.

In the present invention, the reversible capacity of the negative electrode is characterized by greater than the reversible capacity of the positive electrode, the present invention is not limited to a specific range as long as the reversible capacity of the negative electrode is larger than the reversible capacity of the positive electrode, preferably the positive operating voltage range The reversible capacity of the anode at can range from 70% to 99.9% of the cathode reversible capacity at the cathode operating voltage range. If it is 70% or less, there may be a problem that the capacity of the battery itself is small, or the volume of the battery is increased because the amount of the negative electrode active material is too much.

In the present invention, the mixing ratio of the second positive electrode active material may be 2 to 50wt% of the total weight of the total positive electrode active material. When the mixing ratio is smaller than the above range, it is difficult to expect the effect according to the present invention, and when the mixing ratio is larger than the above range, the amount of the first positive electrode active material decreases, thereby reducing the battery capacity.

The lithium secondary battery of the present invention is as follows,

a) designing a mixing ratio of the first positive electrode active material and the second positive electrode active material and the amount of the positive electrode active material and the negative electrode active material such that the reversible capacity of the positive electrode is smaller than that of the negative electrode;

b) preparing a positive electrode active material by mixing a first positive electrode active material, which is the main active material of the positive electrode, and a second positive electrode active material having a charge / discharge voltage range lower than the operating voltage range of the positive electrode and a charge / discharge voltage range higher than the operating voltage range of the negative electrode; ;

c) applying the positive electrode active material and the negative electrode active material to the electrode according to the amount designed in step a) to prepare a positive electrode and a negative electrode and to manufacture a battery including the same;

d) allowing the battery to be inserted into the negative electrode through a formation process, wherein lithium in a portion exceeding the reversible capacity of the positive electrode is inserted into the negative electrode;

It may be prepared by a method including, specific examples thereof are as follows.

That is, a positive electrode active material is prepared by mixing lithium transition metal oxides, such as LiCoO ₂ and LiMn ₂ O ₄ , that can be inserted and detached with LiFePO ₄ having a lower charge / discharge voltage range at an appropriate ratio. Carbon materials, such as linked carbon or amorphous carbon, can be used.

Non-limiting examples of negative electrode active materials include lithium metal or lithium alloys, carbon, petroleum coke, activated carbon, graphite, metals, metal oxides, other carbons, and the aforementioned negative electrode active materials ( Quasi-) metal-based components such as Si, Sn, Pb, or a combination of elements thereof, or conventional surface-treated negative electrode active material in the art. Specific examples thereof include carbon such as hardly graphitized carbon and graphite carbon; Li _x Fe ₂ O ₃ (0 ≦ _x ≦ 1), Li _x WO ₂ (0 ≦ _x ≦ 1), Sn _x Me _1-x Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me' Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen, 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , _{Bi 2 O 5, WO 3,} MoO 3, LiCr 3 O 8, LiV 3 O 8, Li having a TiS _2, spinel-like structure _{4/3 Ti 5/3 O} 5 , etc. of _{Li x Ti 5/3-y} L metal oxides such as oxides represented by _y O ₄ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The electrode including the electrode active material may be prepared by a method known to those skilled in the art. For example, the electrode may be used in addition to the active material, a conductive agent for providing electrical conductivity and a binder that enables adhesion between the material and the current collector. have.

1 to 30 wt% of the conductive material and 1 to 10 wt% of the binder were mixed with the electrode active material, and then added to the dispersion solvent and stirred to prepare a paste. Thus, a laminate electrode can be produced.

The conducting agent generally uses carbon black. Products currently marketed as conductive agents include acetylene black series (Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC series (Armak Company ), Vulcan XC-72 (manufactured by Cabot Company), and Super P (manufactured by MMM).

Representative examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) or copolymers thereof, cellulose, and the like, and representative examples of the dispersant are isopropyl alcohol and N-methylpyrrolidone. (NMP), acetone and the like.

The current collector of the metal material is a metal having high conductivity, and any metal can be used as long as the paste of the material is easily adhered and is not reactive in the voltage range of the battery. Representative examples include meshes, foils, and the like, such as aluminum or stainless steel.

At this time, the present invention can be designed by adjusting the mixing ratio and the amount of the first positive electrode active material and the second positive electrode active material so that the reversible capacity of the positive electrode is smaller than the reversible capacity of the negative electrode. However, since the total loading amount of the active material affects the capacity of the electrode, it is preferable to adjust in view of this.

That is, in the present invention, the sum of the charge amount of the first positive electrode active material and the charge amount of the second positive electrode active material is the charge amount of the positive electrode active material, and it is assumed that all of the desorbed lithium moves to the negative electrode.

The remaining amount of the negative electrode except the irreversible capacity of the negative electrode becomes the reversible capacity of the negative electrode, wherein the irreversible capacity of the negative electrode may be determined separately by a coin half cell test. (Charge amount of negative electrode active material-discharge amount of negative electrode active material)

On the other hand, the reversible capacity of the positive electrode is the same as the discharge amount of the first positive electrode active material, because the second positive electrode active material does not charge and discharge in the operating voltage range of the positive electrode.

Therefore, by comparing the reversible capacity of the positive electrode and the negative electrode determined as described above, it is possible to adjust the amount of the active material and the mixing ratio of the first positive electrode active material and the second positive electrode active material to have a larger reversible capacity of the negative electrode than the positive electrode.

The lithium secondary battery of the present invention may be manufactured using a method known in the art, and for example, a separator may be inserted between a positive electrode and a negative electrode and a nonaqueous electrolyte may be added thereto. In addition, the electrode, the separator and the nonaqueous electrolyte and, if necessary, other additives, may be those known in the art.

In addition, a porous separator may be used as a separator in manufacturing a battery of the present invention, and for example, a polypropylene-based, polyethylene-based, or polyolefin-based porous separator may be used, but is not limited thereto.

The nonaqueous electrolyte of the lithium secondary battery that can be used in the present invention may include a cyclic carbonate and / or a linear carbonate. Examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), gamma butyrolactone (GBL), and the like. Examples of the linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), and the like. In addition, the nonaqueous electrolyte of the lithium secondary battery of the present invention contains a lithium salt together with the carbonate compound. Specific examples of lithium salts include LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN (CF ₃ SO ₂ ) ₂ .

The secondary battery of the present invention configured as described above is preferably a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, a lithium ion polymer secondary battery, and the like, but is not limited thereto. no.

Since the secondary battery sealed and assembled in the can or pouch is a cell in an unactivated discharge state, the secondary battery may undergo a formation process of charging and activating a discharge cell. In the formation process, the first charge forms a thin film formed by the reaction between the negative electrode and the electrolyte on the surface of the negative electrode, which is called SEI (Solid Electrolyte interphase), and the physical properties of the film affect the performance of the battery such as cycle characteristics. .

At this time, the present invention has a larger reversible capacity of the negative electrode than the reversible capacity of the positive electrode, and since the positive electrode active material contains a material having a low charge and discharge voltage, lithium in a portion exceeding the reversible capacity of the positive electrode is inserted into the negative electrode to activate the negative electrode. It can be stored in the site, which can later be used up to compensate for the lack of lithium.

The formed cell may undergo an aging period of 2 to 3 weeks, and in aging, the electrolyte may be evenly distributed in the electrode, and also have an impurity detection function such as Ni, Fe, or Cu. Metal impurities in the cells grow during the aging period, causing internal shorts, and thus, detect defective cells.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited thereto.

Example 1

LiMn ₂ O ₄ and LiFePO ₄ were mixed at a weight ratio of 70:30 to prepare a cathode active material. 88 parts by weight of the positive electrode active material was mixed with 6 parts by weight of acetylene black as a conductive agent and 6 parts by weight of PVDF as a binder, and added to NMP (N-methyl-2pyrrolidone) as a dispersion solvent to prepare a positive electrode slurry, followed by Al current collector phase. It was applied to and dried to prepare a positive electrode.

Amorphous carbon is used as the negative electrode active material, and the negative electrode composition ratio is added to the dispersion solvent so that the weight ratio of the active material: conductive agent: binder is 88: 2: 10, the negative electrode slurry is prepared, coated on Cu-foil, and dried. Was prepared.

At this time, the loading amounts of the positive electrode and the negative electrode were adjusted such that the reversible capacity of the positive electrode was 95% of the reversible capacity of the negative electrode.

Ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 1: 2 (volume ratio) solution in which 1 M LiPF ₆ was dissolved was used as the electrolyte, and the lithium secondary battery was prepared using the positive electrode, the negative electrode, the electrolyte, and the separator. The battery was produced.

Comparative Example 1

LiMn ₂ O ₄ without LiFePO ₄ added to the positive electrode active material Only was used, and the battery was manufactured in the same manner as in Example 1 except that the loading amount of the positive electrode and the negative electrode was adjusted so that the reversible capacity of the positive electrode is 115% of the reversible capacity of the negative electrode.

Experimental Example 1

Capacity retention characteristics of the batteries produced in Example 1 and Comparative Example 1 were carried out. In order to induce side reactions in the battery, experiments were carried out for one week and two weeks at 55 ° C in harsh conditions.

The capacity of the 5th cycle was measured by 5A CC / CV charging, 5A end 2.5V discharge, 5cycle charging and discharging before storage, and after 1 week and 2 weeks storage at 55 ° C, the capacity was again measured under the same conditions. . The results are shown in Table 1.

TABLE 1

Capacity characteristics after high temperature storage (%) 55 ℃, 1 week 55 ℃, 2 weeks Comparative Example 1 95.2% 92.5% Example 1 97.7% 97.0%

In the case of Comparative Example 1, the battery capacity was reduced by 2.7%, but in Example 1, only 0.7% was found to show excellent capacity retention characteristics.

The present invention mixes a material having a charge / discharge voltage lower than the positive operating voltage of the positive electrode active material, and designs the battery so that the negative electrode has a larger reversible capacity than the positive electrode, thereby exceeding the reversible capacity of the positive electrode active material during the initial formation of the battery. By allowing lithium to be stored in the active site in the negative electrode active material, it is possible to compensate for the decrease in capacity of the battery due to lithium consumption in the continuous use process.

Claims (12)

In a lithium secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator, the reversible capacity of the negative electrode in the negative operating voltage range is greater than the reversible capacity of the positive electrode in the positive operating voltage range, and is the first positive electrode active material which is the main active material of the positive electrode. In addition, the lithium secondary battery further comprises a second positive electrode active material having a lower charge / discharge voltage range than the operating voltage range of the positive electrode and a higher charge / discharge voltage range than the operating voltage range of the negative electrode. The lithium secondary battery of claim 1, wherein an operating voltage range of the positive electrode is 3.5V to 4.3V. The method of claim 1, wherein the operating voltage range of the negative electrode is a lithium secondary battery, characterized in that 0V ~ 1.5V. The method of claim 1 wherein the first cathode active material is _{LiCoO 2, LiMnO 2, LiNi 0} .5 Mn 0 .5 O 2, LiMn 2 O 4, LiNiO 2, and _{Li (Mn 1/3 Ni 1} /3 Co 1 / ₃ ) A lithium secondary battery characterized by at least one member selected from the group consisting of O ₂ . The lithium secondary battery of claim 1, wherein the second cathode active material is LiFePO ₄ or LiV ₂ O ₅ . The lithium secondary battery according to claim 1, wherein the reversible capacity of the positive electrode in the positive operating voltage range is in the range of 70% to 99.9% of the negative electrode reversible capacity in the negative operating voltage range. The lithium secondary battery of claim 1, wherein a mixing ratio of the second cathode active material is 2 wt% to 50 wt% with respect to the total weight of the entire cathode active material. The lithium secondary battery according to claim 1, wherein the second positive electrode active material does not reversibly charge and discharge when charging and discharging within the operating voltage range of the positive electrode, and thus lithium that is detached during initial charging is not reinserted. The lithium secondary battery according to claim 1, wherein lithium in a portion exceeding the reversible capacity of the positive electrode is stored in the negative electrode. a) designing a mixing ratio of the first positive electrode active material and the second positive electrode active material and the amount of the positive electrode active material and the negative electrode active material such that the reversible capacity of the positive electrode is smaller than that of the negative electrode; b) preparing a positive electrode active material by mixing a first positive electrode active material, which is the main active material of the positive electrode, and a second positive electrode active material having a charge / discharge voltage range lower than the operating voltage range of the positive electrode and a charge / discharge voltage range higher than the operating voltage range of the negative electrode; ; c) applying the positive electrode active material and the negative electrode active material to the electrode according to the amount designed in step a) to prepare a positive electrode and a negative electrode and to manufacture a battery including the same; d) allowing the battery to be inserted into the negative electrode through a formation process, wherein lithium in a portion exceeding the reversible capacity of the positive electrode is inserted into the negative electrode; A method for producing the lithium secondary battery according to claim 1 including. The method according to claim 10, wherein the reversible capacity of the positive electrode in the positive operating voltage range is in the range of 70% to 99.9% of the negative electrode reversible capacity in the negative operating voltage range. The method of claim 10, wherein the mixing ratio of the second cathode active material is 2 wt% to 50 wt% with respect to the total weight of the entire cathode active material.