KR100681465B1 - Electrode active material with high capacity and good cycleability and lithium secondary cell comprising the same - Google Patents

Abstract

The present invention provides a novel compound for a lithium secondary battery electrode active material represented by the following general formula (I), a preparation method thereof, an electrode including the compound, and a lithium secondary battery using the same.

Na _x M _y Si _z O _α (Ⅰ)

(In the above, M is a metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn, and 0 <x <1, 0 <y <2, 0 <z <1, 0 <α <2) .)

The compound represented by Chemical Formula 1 of the present invention not only improves the electronic conductivity due to the co-precipitated metal atom (M) but also suppresses the deterioration of the electrode due to charging and discharging, thereby providing a lithium secondary battery having high capacity and long life characteristics. Can be.

Electrode active material, coprecipitation, lithium secondary battery, charge and discharge capacity, lifespan characteristics

Description

ELECTRODE ACTIVE MATERIAL WITH HIGH CAPACITY AND GOOD CYCLEABILITY AND LITHIUM SECONDARY CELL COMPRISING THE SAME

FIG. 1A is an electron microscope (SEM) photograph of the compound (Na _0.15 Ni _0.5 Si _0.45 O _0.7 ) particles prepared in Example 2, and FIGS. 1B, 1C, and 1D are angles of Na, Si, and Ni in the compound component. EDX analysis.

Figure 2 is an XRD analysis of the compound (Na _0.14 Cu _0.4 Si _0.45 O _0.7 ) particles prepared in Example 1.

3 is a comparison using the coin-type lithium secondary half cells (Example 9 to 12) using the compounds prepared in Examples 1 to 4 as the electrode active material and pure silicon and silicon monooxide as the electrode active material, respectively It is a figure which shows the efficiency change (life characteristic) according to the charge / discharge cycle of the coin type lithium secondary half battery of Examples 3-4.

The present invention relates to a novel compound for an electrode active material having high charge and discharge capacity and long life characteristics, an electrode including the same, and a lithium secondary battery having the same.

Recently, electronic devices such as mobile phones, video cameras, computers, etc. are becoming more portable and miniaturized. In addition to these situations, the battery, which is a main power supply device in these devices, is also miniaturized. In addition, the power of these devices is diversified, so the power consumed is on the rise. One battery that meets these requirements is a lithium ion secondary battery, which is high in capacity and exhibits high voltage.

Conventional lithium ion secondary batteries use a carbonaceous material such as graphite as a negative electrode active material, but the maximum theoretical capacity of the lithium ion secondary battery at this time is 372 mAh / g, which is a limitation in manufacturing a battery having high charge and discharge capacity and long life characteristics. There was.

Various studies have been conducted to solve the above problems. Japanese Patent Application Laid-open No. Hei 10-284056 proposes to realize high charge and discharge capacity using silicon oxide capable of forming a metal alloy with lithium as a negative electrode active material and graphite as a conductive agent. However, the low electronic conductivity of silicon has led to problems such as reduced battery capacity and reduced battery life characteristics.

In addition, Japanese Patent Application Laid-Open No. 2003-223892A discloses a technique for improving high discharge capacity and long life characteristics of a battery by mixing and milling a fine powder of a silicon compound, graphite, and a conductive agent, followed by firing. However, in the above method, since the silicon compound is integrated with graphite and the conductive agent, there is a possibility of improving the battery capacity or improving the life characteristics by reducing the electrical resistance, but a large volume generated during charging and discharging of the silicon compound Due to the change, the silicon compound is desorbed from the graphite and the conductive agent, which causes a problem of degrading the performance of the battery.

The present inventors have analyzed the problems of the prior art, by using a metal precursor compound containing a transition metal such as copper, cobalt, nickel or manganese and a compound having an amorphous structure formed by the coprecipitation of the silicon precursor compound as a negative electrode active material of a lithium secondary battery In use, it was found that the battery exhibits high charge and discharge capacity and long life characteristics.

Accordingly, an object of the present invention is to provide a novel compound for an electrode active material having a high capacity and long life and a method for producing the same.

In addition, another object of the present invention is to provide an electrode including the compound and a lithium secondary battery using the same.

The present invention provides a novel compound represented by Formula 1 below, which may be used as an electrode active material for lithium secondary batteries, preferably as a negative electrode active material.

Na _x M _y Si _z O _α (Ⅰ)

In the above,

M is a metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn, and 0 <x <1, 0 <y <2, 0 <z <1, 0 <α <2.

In addition, an electrode including a compound represented by Chemical Formula 1 and a lithium secondary battery using the same are provided.

In addition, the present invention comprises the steps of a) dissolving a metal precursor compound and a silicon precursor compound comprising a transition metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn in a solvent, respectively; b) mixing, coprecipitation and drying a metal solution comprising a transition metal prepared in step a) and a solution comprising silicon; c) providing a method for preparing a compound represented by Chemical Formula 1 including the step of heat-treating the co-precipitate of step b).

Hereinafter, the present invention will be described in detail.

One of the elemental components of the novel compound for electrode active material according to the present invention is preferably silicon (Si) capable of occluding and releasing lithium. This is because it can exhibit a large capacity compared to the carbon material such as conventional graphite. Although silicon has advantages such as large capacity, silicon has a problem of degradation of the electrode due to low electron conductivity and severe volume change, thereby limiting its application as a negative electrode active material.

Accordingly, the present invention produces a new compound represented by Chemical Formula 1 by coprecipitating a metal compound having excellent conductivity with a precursor compound including silicon, and thus using the electrode active material for a lithium secondary battery as an electrode active material for improving electron conductivity and charging and discharging. It can maintain a high charge and discharge capacity without degradation of the active material and improve the life characteristics at room temperature.                     

Therefore, it is preferable that the other one of the elemental components of the novel compound for electrode active material according to the present invention has excellent conductivity. Since silicon is a semiconductor having a resistivity of 2.3 × 10 ⁵ cm, the conductivity can be improved due to the metal having excellent conductivity. Also preferred are transition metals. This is because, in the heat treatment of the formed coprecipitation, metals other than transition metals are not easily reduced. Non-limiting examples of the conductive metal having excellent conductivity include Cu, Ni, Co, Al, Sn, Zn, Cr, Fe, or Mn.

In addition, sodium, which is another elementary component of the novel compound for electrode active materials of the present invention, is attributable to sodium silicate (Na ₂ SiO ₃ ), which is a silicon precursor compound. Therefore, the case where Na in the general formula (1) is replaced by another element is also within the equivalent scope of the present invention.

As such, the composite oxide of Chemical Formula 1 including transition metals such as Cu, Ni, Co, Al, Sn, or Mn having excellent conductivity, silicon, and sodium components may be present in amorphous, crystalline, or mixed form thereof, particularly amorphous. This is preferable. Because the amorphous structure has no long range regularity of the lattice, it can absorb part of the expansion itself upon lattice expansion, and can provide many diffusion paths of lithium into the electrode.

The novel compounds represented by the general formula (1) of the present invention are not only amorphous but also electrochemically react with lithium to form a metal oxide network inert without forming lithium solid solution or intermetallic compound, thereby effectively inhibiting the volume expansion of silicon. Can be. In addition, silicon itself is in amorphous form, so the volume expansion is not large compared to crystalline.                     

The compound of Formula 1 may be prepared by the coprecipitation method as follows. For example, first, a solution is prepared by dissolving a metal precursor compound including a transition metal such as Cu, Co, Ni, Al, Sn or Mn and a precursor compound including silicon in a solvent.

As the precursor compound containing silicon, any water-soluble compound can be used, and a compound having a pH in the range of 12 to 14 is particularly preferable. This is because the coprecipitation reaction using the pH difference with other reactants can be easily performed. An example of a silicon precursor compound having a pH ranging from 12 to 14 and water soluble is sodium silicate (Na ₂ SiO ₃ ).

As a compound containing a transition metal such as Cu, Ni, Co, Al, Sn, or Mn, a water-soluble compound including the metal may be used, and non-limiting examples thereof include alkoxides and nitrates containing each metal. And acetates, halides, hydroxides, oxides, carbonates, oxalates, sulfates, or salts containing the above elements, and compounds which can be ionized. In particular, metal superoxide or metal chloride whose pH is about 4-6 weak acid is preferable. This is because the coprecipitation reaction can proceed easily due to the pH difference with the precursor compound including silicon having a pH in the range of 12 to 14, preferably sodium silicate having a pH of 14. Specific examples of metal oxides or metal chlorides including Cu, Ni, Co, Al, Sn, or Mn include (CH ₃ COO) ₂ Cu, (CH ₃ COO) ₂ Co, (CH ₃ COO) ₂ Mn, ( CH ₃ COO) ₂ Ni, (CH ₃ COO) ₂ Al, (CH ₃ COO) ₂ Sn, SnCl ₄ , SnCl ₂ or mixtures thereof.

The solvent may be a polar solvent such as water or lower alcohol. The concentration of the solution containing the silicon and the solution containing the transition metal is preferably 0.1 to 1M each. If it is less than 0.1M, the electrode material having a high charge / discharge capacity cannot be manufactured, and if it is more than 1M, silicon and transition metal do not co-precipitate, respectively, which causes degradation of life characteristics.

The prepared metal solution containing a transition metal such as Cu, Co, Ni, Al, Sn or Mn and a solution containing silicon are stirred and mixed at an appropriate concentration ratio.

When an aqueous solution containing a transition metal having a pH of 4 to 6 and an aqueous solution containing silicon having a pH of 12 to 14 are mixed, a sudden increase in pH causes coprecipitation with a composite hydroxide containing silicon and a transition metal. In this case, the pH difference between the two solutions is preferably about 6 to 10, as described above, and other pH adjusting or precipitating agents may be used.

The molar concentration ratio of the solution containing the transition metal and the solution containing silicon is preferably 1: 0.5 to 1. When the molar concentration ratio is less than 1: 0.5, the high charge / discharge capacity of the battery cannot be obtained, and when the molar ratio is greater than 1: 1, the electronic conductivity is deteriorated and the lifespan characteristics are deteriorated. In addition, the stirring time is preferably 0.1 to 1 hour, the molar concentration ratio and the stirring time can be adjusted within the above range to suit the purpose.

The co-precipitated material is dried in a temperature range of 65 to 80 ° C. for 24 to 48 hours and then subjected to a heat treatment step.

The heat treatment time is preferably 6 to 18 hours, the heat treatment temperature is preferably in the range of 500 to 800 ℃. When the heat treatment temperature is less than 500 ° C, transition metals such as Cu, Ni, Co, Al, Sn, Mn, etc. are not reduced in the coprecipitation, so that the life characteristics due to the decrease in the electronic conductivity of the electrode active material are lowered, and may exceed 800 ° C In this case, the active material is crystallized, and thus it is impossible to implement high charge and discharge capacity.

In addition, the heat treatment is preferably performed in a reducing atmosphere in which an inert gas and hydrogen gas are mixed, and in this case, the mixing ratio of hydrogen gas in the mixed gas is preferably 1 to 10%.

By carrying out the heat treatment in a reducing atmosphere, the transition metal present in the hydroxide form is reduced to a metal state, and the compounds in the hydroxide form become amorphous oxide forms, thereby forming a novel transition metal, sodium and A composite oxide of silicon is made. At this time, the reduction of the transition metal can be confirmed through XRD, and also the oxidation number of the compound can be determined through ICP analysis.

The novel compound of Formula 1 thus prepared may be used alone as an electrode active material, preferably as a negative electrode active material, or may be used by coating on the surface of a general electrode active material.

The compound of Formula 1 may be prepared by preparing an electrode slurry using a positive electrode active material, preferably a negative electrode active material as an electrode material, and coating the prepared electrode slurry on a current collector. As a manufacturing method of such an electrode, the conventional method known in the art can be used, and is not particularly limited.

In addition, the present invention a) a positive electrode comprising a positive electrode active material capable of occluding and releasing lithium; b) a negative electrode comprising a compound represented by Formula 1; It provides a lithium secondary battery comprising c) a separator and d) an electrolyte. The lithium secondary battery of the present invention may be prepared by inserting a porous separator between a positive electrode and a negative electrode by a conventional method known in the art and adding a nonaqueous electrolyte.

As a positive electrode active material for manufacturing the electrode of this invention, it is preferable to use a lithium containing transition metal oxide. For example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), LiNi _{1- Y} Co _Y O ₂ , LiCo _{1- Y} Mn _Y O ₂ , LiNi _1-Y Mn _Y O ₂ (where 0 ≦ Y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _2-z Ni _z O ₄ , LiMn _{2- z} Co _z O ₄ (here , 0 <Z <2), at least one selected from the group consisting of LiCoPO ₄ and LiFePO ₄ can be used, of which LiCoO ₂ is more preferred.

As the separator of the present invention, it is preferable to use a porous separator. 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 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 one or more selected from the group consisting of diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and methyl propyl carbonate (MPC). In addition, the non-aqueous electrolyte of the lithium secondary battery of the present invention includes a lithium salt together with the carbonate compound, and specific examples thereof include LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiN (CF ₃ SO ₂ ) It is preferable that at least one selected from the group consisting of _two be selected.

The external shape of the lithium secondary battery according to the present invention is preferably a cylindrical, square or pouch type of cans.

The present invention will be described in more detail with reference to the following examples and comparative examples. However, an Example is for illustrating this invention and is not limited only to these.

Example  Na _0.14 Cu _0.4 Si _0.45 O _0.7  Produce

Copper acetate and sodium silicate were dissolved in water to prepare an aqueous solution of 0.4 M concentration, respectively, and then mixed at a molar ratio of 1: 1. The mixture was stirred for 30 minutes and then dried in an oven at 65 ° C. for 24 hours. The co-precipitated material was heat-treated at 500 ° C. for 12 hours under a mixed gas in which hydrogen was mixed at a volume ratio of 4% by argon to Na _0.14 Cu _0.4 Si _0.45 O _0.7 was prepared.

Example  2. Na _0.15 Ni _0.5 Si _0.45 O _0.7 Produce

Na _0.15 Ni _0.5 Si _0.45 O _0.7 was prepared in the same manner as in Example 1, except that nickel acetate was used instead of copper _acetate .

Example  3. Na _0.15 Co _0.5 Si _0.45 O _0.7 Produce

Na _0.15 Co _0.5 Si _0.45 O _0.7 was prepared in the same manner as in Example 1, except that cobalt superoxide was used instead of copper _superoxide .

Example  4. Na _0.15 Mn _0.55 Si _0.45 O _0.7 Produce

Na _0.15 Mn _0.55 Si _0.45 O _0.7 was prepared in the same manner as in Example 1, except that manganese superoxide was used instead of copper _superoxide .

Example  5-8. Electrode manufacturing

Each compound of Examples 1 to 4 was used as an electrode active material, and the slurry was dispersed in an N-methyl pyrrolidinone (NMP) solvent with 10% acetylene black (conductive material) and 10% PVdF (binder) relative to the active material. Got it. After the slurry was coated on a copper foil and heated, the NMP solvent was evaporated to dryness, and pressed to apply a pressure of about 500 kg / cm ² to prepare an electrode.

Comparative example  1-2. Electrode manufacturing

The electrodes of Comparative Examples 1 and 2 were prepared by the same method as Example 5, except that pure silicon and silicon monooxide were used as electrode active materials, respectively.

Example  9-12. Fabrication of Lithium Secondary Half Battery

Coin type half cells were manufactured using the electrodes of Examples 5 to 8 as the positive electrode and using lithium metal as the negative electrode. The electrolyte used was a solution in which 1 mol of LiPF ₆ was dissolved in a solvent in which EC (ethylene carbonate) and EMC (ethylmethyl carbonate) were mixed at a volume ratio of 1: 2.

Comparative example  3-4. Lithium Secondary Half Battery Manufacture

A half cell was manufactured by the same method as Example 9, except that the pure silicon electrode of Comparative Example 1 and the silicon monooxide electrode of Comparative Example 2 were used as the positive electrode, respectively.

Experimental Example  1. Analysis of Compound

In order to evaluate the components and the surface of the compound represented by Formula 1 prepared according to the present invention, the following experiment was performed.

1-1. Inductively Coupled Plasma Analysis

As a result of confirming the composition of the compound of Example 1 by ICP analysis, 5.78 wt% of sodium, 45.65 wt% of copper, and 22.64 wt% of silicon were calculated by calculation of the oxidation value of oxygen. As a result of determining the molar ratio of each material, it was found that Na _0.14 Cu _0.4 Si _0.45 O _0.7 .

1-2. SEM  & EDX  analysis

The sample particle | grains of Example 2 were used for the sample.

As a result of analysis by electron microscope (SEM) and EDX (Energy Dispersive X-ray spectrometer), Na _0.15 Ni _0.5 Si _0.45 O _0.7 compound of the present invention can be confirmed that the presence of nickel and silicon in nanometer units by coprecipitation method In particular, it was found that Na, Si, and Ni components were all uniformly mixed (see FIGS. 1B, 1C, and 1D).

1-3. X-ray diffraction analysis XRD X-ray Diffraction

The sample particle | grains of Example 1 were used for the sample.

As a result of measurement using XRD, it was confirmed that Na _0.14 Cu _0.4 Si _0.45 O _0.7 compound formed by coprecipitation of silicon and transition metal according to the present invention not only SiO peak but also pure copper peak (see FIG. 2).

Experimental Example  1. Performance Evaluation of Lithium Secondary Battery

Performance evaluation of the lithium secondary battery using the compound of Formula 1 prepared according to the present invention as an electrode active material was performed as follows.

The lithium secondary batteries of Examples 9 to 12 using each of the novel compounds prepared in Examples 1 to 4, and the lithium secondary batteries of Comparative Examples 3 and 4 using pure silicon electrodes and silicon monooxide electrodes, respectively. The charge and discharge voltage range of each battery was 0.005 to 1.5V, and was charged and discharged at 0.1C / 0.1C. Efficiency according to charge and discharge was expressed as a percentage by dividing the discharge capacity in each cycle by the charge capacity, the results are shown in Table 1 below.

As a result of confirming the initial discharge capacity, the lithium secondary battery using the compound represented by the formula (1) of the present invention as an electrode active material showed a significantly higher capacity than the theoretical capacity of 372mAh / g of the carbon material, such as conventional graphite. In addition, as a result of checking the charge and discharge efficiency in the 50th cycle, it was confirmed that the lithium secondary battery of the present invention had higher charge and discharge efficiency and longer life characteristics than the cells of Comparative Examples 3 and 4 using silicon or silicon monooxide. (See Table 1 and FIG. 3).

battery Initial discharge capacity (mAh / g) Charge / discharge efficiency at 50th cycle (%) Example 9 800 99 Example 10 750 98 Example 11 700 98 Example 12 700 98 Comparative Example 3 3000 30 Comparative Example 4 1000 50

The novel compound represented by Chemical Formula 1 of the present invention not only improves the electronic conductivity due to the co-precipitated metal atoms, but also prevents the degeneration of the electrode due to the charge and discharge, thereby improving the charge / discharge capacity and life characteristics of the battery.

Claims (11)

Compound represented by the following general formula (I): Na _x M _y Si _z O _α (Ⅰ) In the above, M is a metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn, and 0 <x <1, 0 <y <2, 0 <z <1, 0 <α <2. The compound of claim 1, wherein the compound is in an amorphous structure. An electrode active material comprising the compound of claim 1 or 2. a) a positive electrode comprising a positive electrode active material capable of occluding and releasing lithium; b) a negative electrode comprising the compound of claim 1 or 2; c) membrane; And d) electrolyte Lithium secondary battery comprising a. a) dissolving a metal precursor compound and a silicon precursor compound comprising a transition metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn in a solvent, respectively; b) mixing, coprecipitation and drying the solution comprising the transition metal prepared in step a) and the solution comprising silicon; c) heat-treating the coprecipitates of step b) Method for preparing a compound represented by the following general formula (I) of claim 1 or claim 2 comprising: Na _x M _y Si _z O _α (Ⅰ) In the above, M is a metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn, and 0 <x <1, 0 <y <2, 0 <z <1, 0 <α <2. The method of claim 5, wherein the metal precursor compound comprising a transition metal selected from the group consisting of Cu, Co, Ni, Al, Sn, and Mn is a compound having a pH in the range of 4-6. The method of claim 5, wherein the silicon precursor compound is a compound having a pH in the range of 12-14. The method of claim 7, wherein the silicon precursor compound is sodium silicate (Na ₂ SiO ₃ ). The method according to claim 5, wherein the molar concentration ratio of the metal precursor compound solution and the silicon precursor compound solution including the transition metal selected from the group consisting of Cu, Co, Ni, Al, Sn and Mn is 1: 0.5 to 1. The method according to claim 5, wherein the heat treatment step c) is performed under a mixed gas of inert gas and hydrogen containing hydrogen in a volume ratio of 1 to 10%. The method of claim 5, wherein the heat treatment temperature is in the range of 500 to 800 ° C. 7.

Images