WO2016178509A1

WO2016178509A1 - Negative electrode active material for lithium secondary battery

Info

Publication number: WO2016178509A1
Application number: PCT/KR2016/004697
Authority: WO
Inventors: 공문규; 성재욱
Original assignee: 공문규; 성재욱
Priority date: 2015-05-06
Filing date: 2016-05-04
Publication date: 2016-11-10
Also published as: KR101762773B1; KR20160131208A

Abstract

The present invention provides a negative electrode active material for a lithium secondary battery. The negative electrode active material is a silicon-containing alloy composed of chemical formula 1 of Si_xM_yN_z, where 30≤x≤60, 1≤y≤20, 30≤z≤60, x+y+z = 100; x, y, and z each represent atomic%; and M and N each represent a transition metal, and is characterized by having a porous structure which can be obtained by rapidly solidifying the alloy in a molten state and is composed of solid active silicon fine particles and an inactive intermetallic compound which is distributed around the silicon fine particles and may have a solid region and an amorphous region, wherein some portions of the solid region distributed around the fine particles are removed.

Description

Anode Active Material for Lithium Secondary Battery

The present invention relates to a negative electrode active material for a lithium secondary battery, and more particularly, to a negative electrode active material for a lithium secondary battery having a high charge and discharge capacity and excellent capacity retention.

Lithium metal is used as a negative electrode active material of a conventional lithium battery. However, when a lithium metal is used, a carbon-based material is used as a negative electrode active material instead of lithium metal because a short circuit of the battery occurs due to the formation of a dendrite. .

Examples of the carbon-based active material include crystalline carbon such as graphite and artificial graphite, and amorphous carbon such as soft carbon and hard carbon. However, although the amorphous carbon has a large capacity, there is a problem in that irreversibility is large in the charging and discharging process. Graphite is typically used as the crystalline carbon, and has a theoretical limit capacity of 372 mAh / g, which has a high capacity, and is used as a negative electrode active material.

However, even if the graphite or carbon-based active material has a rather high theoretical capacity, it is only about 380 mAh / g, and there is a problem in that the above-described negative electrode cannot be used in the development of a high capacity lithium battery in the future.

In order to improve such a problem, the currently active research is a negative electrode active material of the metal-based or intermetallic compounds. For example, lithium batteries using metals or semimetals such as aluminum, germanium, silicon, tin, zinc, and lead as negative electrode active materials have been studied. Such a material has a high energy density and high energy density, and can absorb and release more lithium ions than a negative electrode active material using a carbon-based material, thereby manufacturing a battery having a high capacity and a high energy density. Pure silicon, for example, is known to have a high theoretical capacity of 4017 mAh / g.

However, when compared with carbon-based materials, the cycle characteristics are deteriorated, which is still an obstacle to practical use. When the silicon is used as a lithium occlusion and release material as a negative electrode active material, it may be difficult to change the volume between the active materials due to the volume change during the charge and discharge process. This is because conductivity decreases or a phenomenon in which the negative electrode active material is peeled off from the negative electrode current collector occurs. That is, the silicon and the like contained in the negative electrode active material expands to about 300 to 400% by occluding lithium by charging, and when the lithium is discharged, the inorganic particles contract.

In order to solve this problem, Japanese Laid-Open Patent Publication No. 2006-286312 discloses a lithium secondary battery using a silicon thin film formed by a dry process as a negative electrode active material. In patent document 1, the silicon thin film has the structure isolate | separated into a columnar phase according to the short circuit formed in the thickness direction. Patent document 1 attempts to mitigate expansion and contraction of silicon by the physical shape of a silicon thin film, but productivity of a thin film falls significantly, and therefore it is difficult to apply it to actual battery mass production.

Accordingly, there is a demand for developing a material that can be used in a secondary battery as a structure of a composite particle composed of active silicon microparticles and an inert intermetallic compound distributed around the active silicon microparticles.

In order to solve the above problems, the present invention is to provide a negative electrode active material for a lithium secondary battery that the electrical change does not occur because the volume change is small during charging and discharging.

In another aspect, the present invention is to provide an alloy composition composition method for suppressing the generation of the silicon coarse region when producing a silicon negative electrode active material using a quench solidification process.

In addition, the present invention is to provide a porous structure through some etching process of the matrix structure other than silicon through an additional etching process.

In order to achieve the above object, the present invention is an alloy containing silicon (Si), the alloy is made of the following formula (1),

[Formula 1]

Si _x M _y N _z

Where 30≤x≤60, 1≤y≤20, 30≤z≤60, x + y + z = 100, x, y, z are atomic%

M and N are transition metals

In an inert intermetallic compound obtained by quenching and solidifying the alloy in a molten state, an inert intermetallic compound in which a solid region and an amorphous region distributed around the solid state active silicon (Si) microparticles and the silicon (Si) microparticles can be mixed, It provides a negative electrode active material for a lithium secondary battery having a porous feature to remove some of the solid region distributed around the fine particles.

In another aspect, the present invention provides a negative electrode active material for a lithium secondary battery to suppress the volume growth of the solid phase when the inert intermetallic compound changes the state of the silicon alloy from the high-temperature liquid state to a solid phase.

In another aspect, the present invention provides a negative electrode active material for a lithium secondary battery wherein the transition metal is one or more selected from the group consisting of Al, Ni, Cu, Ti and Fe.

In another aspect, the present invention provides a negative electrode active material for a lithium secondary battery that the inert intermetallic compound may include Al ₃ Ni, NiSi ₂ and Al phase.

In another aspect, the present invention provides a negative electrode active material for a lithium secondary battery is further coated with a metal element or a carbon-based material on the porous structure surface.

In another aspect, the present invention provides a negative electrode active material for a lithium secondary battery, characterized in that the ratio of the X-Ray Dirrractometer (XRD) peak area of the silicon (Si) microparticles and the silicon compound (SiM) phase is 1 to 1.

The negative electrode active material significantly improves the performance and stability of the battery, and has excellent capacity and cycle characteristics of the lithium secondary battery including the negative electrode active material.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of a melt-spinning process, which is a type of quench solidification process.

Figure 2 is a schematic diagram showing the state change process of the liquid phase to a solid state through the quench solidification process of the silicon alloy.

3 is a result drawing of a transmission electron microscope (TEM) of the silicon alloy of Example 1;

4 is a XRD analysis results of Examples 1 to 3.

5, 7, 9 and 11 are enlarged SEM photographs of the negative electrode active materials prepared in Examples 1 to 3 and Comparative Example 1.

6, 8, 10 and 12 is a view showing the result after the coin-shaped secondary battery using a negative electrode active material prepared in Examples 1 to 3 and Comparative Example 1, and subjected to charge and discharge evaluation.

(10: RF coil 20: nozzle 30: inert gas 40: cooling wheel 50: silicon crystal core 60: liquid metal 70: inert intermetallic compound)

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. First, it should be noted that in the drawings, the same components or parts denote the same reference numerals as much as possible. In describing the present invention, detailed descriptions of related well-known functions or configurations are omitted in order not to obscure the subject matter of the present invention.

As used herein, the terms “about”, “substantially”, and the like, are used at, or in close proximity to, numerical values when manufacturing and material tolerances inherent in the meanings indicated are intended to aid the understanding of the invention. Accurate or absolute figures are used to assist in the prevention of unfair use by unscrupulous infringers.

The unit "%" used in the present specification means "atomic%" unless otherwise specified.

The present invention relates to a negative electrode active material for a lithium secondary battery and a lithium secondary battery comprising the same, wherein the negative electrode active material for a lithium secondary battery includes active silicon (Si) fine particles and an inert intermetallic compound distributed around the active fine particles. It is a porous structure.

The present invention suppresses the occurrence of the silicon coarse region when the molten silicon alloy is made into a solid phase by using a quench solidification process to produce the negative electrode active material. In addition, by applying an additional etching process it can effectively suppress the expansion problem of the silicon-based negative electrode active material due to the lithium insertion.

Therefore, at least two kinds of intermetallic compounds should be formed in a composite structure made of silicon and an intermetallic compound. Some of the two metals are to facilitate the selective removal even in weakly acidic material. In addition, the metal remaining by the etching may serve to maintain electrical conductivity between the silicon particles.

Recently, silicon has attracted attention as a negative electrode active material of a lithium secondary battery that requires continuous high capacity, but has a disadvantage in that its life is sharply reduced due to severe volume expansion during charge and discharge. In order to solve this disadvantage, attention has been paid to composite particles composed of active silicon microparticles and inert intermetallic compounds distributed around the active silicon microparticles.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of a melt-spinning process, which is a type of quench solidification process. Usually, after dissolving the silicon alloy using the RF coil 10, the liquid silicon alloy is sent to the cooling wheel 40 through the nozzle 20 using the inert gas 30.

In the case of the melt-spinning process, which is known to have the highest cooling rate during the quench solidification process, phase transformation by rapid cooling is possible at a rate of 10 ⁴ to 10 ⁶ K / sec. The high temperature metal solution coated in a wide and thin direction in the rotational direction in contact with the cooling wheel 40 rotating at high speed is deprived of heat by the cooling wheel 40 having excellent thermal conductivity, so that the phase transformation occurs from the high temperature metal solution to the metal plate material at room temperature. . Since phase transformation from the liquid phase to the solid phase is terminated in such a short time, the phase transformation is terminated by forming a plurality of solid phase nuclei rather than growing solid phase nuclei. That is, by minimizing the phase transformation time to increase the nucleus of the solid phase it is possible to finely form the size of the silicon and intermetallic compound.

However, even when the quenching and solidification process is used, there is a region in which coarse growth occurs after the solid silicon nucleus is formed. This area occurs larger away from the surface in contact with the cooling wheel and occurs differently depending on the composition of the alloy. In particular, the greater the melting point of silicon and the intermetallic compound, the greater the extent and size of the coarsening region. Such non-uniformity of the microstructure affects the life of the secondary battery, and in particular, as the charge and discharge are repeated, cracks in the tissue due to cracks may occur during repeated contraction and expansion.

The present invention is quench solidified using a silicon (Si) alloy composed of the following formula.

[Formula 1]

Si _x M _y N _z

M and N are transition metals

Meanwhile, the transition metal may be one or more selected from the group consisting of Al, Ni, Cu, Ti, and Fe.

The greater the melting point of the silicon and the intermetallic compound in the quench solidification process, the greater the extent and size of the coarsening region, thereby minimizing the coarsening region in the compound of the chemical composition range.

Figure 2 is a schematic diagram showing the state change process of the liquid phase to a solid state through the quench solidification process of the silicon alloy. (a) is a schematic diagram showing the process of the phase change from the liquid phase to the solid phase of the conventional silicon alloy (b) is a schematic diagram of the phase change of the silicon alloy of the present invention composition. (a) After the silicon solid core 50 is formed in the metal solution 60, the silicon core grows. At this time, a solid core of an intermetallic compound having a lower melting point than silicon is formed. After that, the nucleus of silicon and the nucleus of the intermetallic compound continue to grow, and the phase transformation is terminated when both liquid phases change into solid phase.

(b) is a schematic diagram of solid-state transformation in the presence of silicon and two or more intermetallic compounds. Initially, the solid core 50 of the high melting point silicon is formed and starts to grow. Thereafter, a plurality of nuclei 70 of two or more intermetallic compounds are formed, and the nucleus 70 of the intermetallic compound suppresses the growth of the solid silicon 50. Silicon whose phase transformation from liquid phase to solid phase is suppressed forms new solid silicon rather than growth of solid silicon to complete phase transformation.

Since the number of nuclei of the silicon solid phase increases in this way, it is possible to obtain a very dense microstructure. In addition, the formed intermetallic compound may help the nucleus of the silicon solid phase to be easily formed. When the solid silicon nucleus is formed on the surface of the intermetallic compound present in the liquid phase 60, it serves to reduce the surface energy, thereby helping to form a silicon solid phase nucleus, thereby achieving a fine tissue formation .

As a result, the present invention, which is rapidly solidified, is composed of a large number of solid state active silicon (Si) microparticles with minimized coarsening region and an inert intermetallic compound distributed around the silicon (Si) microparticles. The crystalline region and the amorphous region exist in a mixed state.

It is known that it is impossible to solve the problem of volume expansion due to the insertion of lithium, which is a characteristic of the silicon anode active material, even when the microstructure nonuniformity is overcome. In order to solve such a problem, when a porous structure is introduced, cycle characteristics of charge and discharge may be greatly improved.

After a uniform alloy ribbon with controlled microstructure is obtained, some of the inert intermetallic compounds are removed through an etching process to form a porous structure.

In the case of conventional porous silicon, a porous structure is formed by directly etching silicon using hydrofluoric acid, but in the present invention, a method of etching an intermetallic compound other than silicon is selected to enable commercial etching process. In addition, in the present invention, the intermetallic oxide may be composed of Al ₃ Ni so as not to be removed in an etching process as opposed to Al and NiSi ₂ , which are easily etched to easily form a porous structure using a common acid.

The Al ₃ Ni is an intermetallic compound that cannot be etched in a weak acid and is not removed even in various etching processes. Through such an etching process, a porous silicon alloy can be manufactured.

Hereinafter, the present invention will be described in detail by examples. However, the following examples are illustrative of the present invention, and the content of the present invention is not limited by the examples.

Example 1

A silicon (Si) alloy was prepared by an arc dissolving method under argon gas to have a composition of Si 50%, Ni 9.5%, and Al 40.5%, and the prepared Si alloy was formed of Ni ₃ and Al ₃ Ni and NiSi using a melt spinning method. _A silicon (Si) alloy was prepared such that silicon (Si) microparticles were located between matrix phases (inactive intermetallic compounds) composed of ₂ and Al. At this time, the quenching speed (rotational speed of the cooling wheel) was set to 40 m / sec by a melt-spinning process.

The method applied in the present invention is not limited to the above method, and in addition to the melt spinning method, if a sufficient quenching speed can be obtained, the fine powder production technique (gas atomizer method, centrifugal gas atomizer method, plasma method) as described above can be obtained. It can also be manufactured by the atomizer method, the rotating electrode method, the mechanical etching method and the like.

In the silicon alloy, an inactive intermetallic compound was etched around silicon particles to prepare a negative active material for a lithium secondary battery of a silicon alloy having a porous structure.

Example 2

A silicon (Si) alloy was formed in the same manner as in Example 1 except that the composition was made of Si 40%, Ni 11.4%, and Al 48.6%.

.

Example 3

The silicon (Si) alloy was formed in the same manner as in Example 1 except that the composition was made of Si 30%, Ni 13.3%, and Al 56.7%.

Comparative Example 1

The silicon (Si) alloy was carried out in the same manner as in Example 1 except that the composition was made of Si 60%, Ni 15%, and Al 25%.

◎ SEM / XRD analysis

As a result of the transmission electron microscope (TEM) of the silicon alloy of Example 1 in Figure 3 it can be seen that the coarse region is formed in a uniform microstructure in the rapid cooling process of the silicon particles, Figure 4 is Example 1 to XRD analysis of 3 shows that the degree of generation of peaks is very similar, and the analysis results show the formation of intermetallic compounds of Al ₃ Ni, NiSi ₂ and Al.

◎ SEM analysis

Scanning Electron Microscopy (SEM) analysis was performed on the prepared anode active material. 5, 7, 9 and 11 are enlarged SEM photographs of the negative electrode active materials of Examples 1 to 3 and Comparative Example 1.

Examples of the anode active material before etching (a) can be confirmed that there is no coarse region and the silicon crystal of a uniform fine structure, the cathode active material after the etching (b) can be confirmed that the empty space is formed around the silicon particles. .

On the other hand, Comparative Example 1 can observe the microstructure (a) according to the thickness of the quench solidified plate and the silicon coarse region (b) as the distance from the cooling wheel.

◎ Charge and discharge capacity

Coin-shaped secondary batteries were prepared using the negative electrode active materials prepared in Examples 1 to 3 and Comparative Example 1, and after the charge and discharge evaluations, the results are shown in FIGS. 6, 8, 10, and 12. In the manufacture of the coin-shaped electrode plate, the mixing ratio of the active material, the conductive agent (Super P-based conductive agent), and the binder (PI-based binder) is 77: 15: 2: 6 (active material: additive: conductive agent: binder). It was prepared as possible.

Example 1 it can be seen that the charge before etching (a) has a maximum capacity of 900mAh / g, and as a result of the continuous charge and discharge, the capacity is gradually reduced to fall below 250mAh / g. However, after the etching (b) it can be seen that the charge capacity of 1500 ~ 1800mAh / g.

In Examples 2 and 3, as in Example 1, it can be seen that the charging capacity after etching is increased by at least two times, and Example 1, which has the largest content of silicon (Si), is larger than other examples. .

However, in Comparative Example 1, the charge capacity was about 1445.5 mAh / g, similar to the charge capacity after etching in Example 2, but it can be seen that the capacity decreases rapidly after several charge and discharge cycles.

The present invention described above is not limited to the above-described embodiment and the accompanying drawings, and various substitutions, modifications, and changes are possible within the scope without departing from the technical spirit of the present invention. It will be evident to those who have knowledge of.

Claims

As an alloy containing silicon (Si),

The alloy is made of the formula 1,

[Formula 1]

Si x M y N z

(Where 30 ≦ x ≦ 60, 1 ≦ y ≦ 20, 30 ≦ z ≦ 60, x + y + z = 100, x, y and z are atomic% and M and N are transition metals, respectively)

Obtained by quenching and solidifying the alloy in a molten state,

In an inert intermetallic compound in which solid and amorphous regions distributed around solid active silicon (Si) microparticles and the silicon (Si) microparticles can be mixed,

A negative electrode active material for a lithium secondary battery having a porous feature to remove some of the solid region distributed around the fine particles.
The method of claim 1,

The inert intermetallic compound is a negative electrode active material for a lithium secondary battery that generates a large number of fine silicon particles by inhibiting the volume growth of the solid phase when the silicon alloy changes from a high temperature liquid phase to a solid phase.
The method of claim 1,

The transition metal is an anode active material for a lithium secondary battery selected from the group consisting of Al, Ni, Cu, Ti and Fe.
The method of claim 1,

The inert intermetallic compound is an anode active material for a lithium secondary battery that may include Al 3 Ni, NiSi 2 and Al phase.
The method of claim 1,

The anode active material for a lithium secondary battery is further coated with a metal element or a carbon-based material, the porous structure surface.
The method of claim 1,

The silicon (Si) microparticles and the silicon compound (SiM) phase is a negative active material for a lithium secondary battery having a ratio of XRD (X-Ray Dirrractometer) peak area 1 to 1 to 2.