JP2016100329A - Negative electrode active material for secondary battery, and secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for a lithium secondary battery which is small in volume change when charged and discharged, and hard to cause electrical insulation.SOLUTION: The present invention provides a negative electrode active material for a secondary battery with an improved expansion coefficient. The negative electrode active material comprises: an alloy represented by SixTiyFezAlu (where "x", "y", "z" and "u" are represented in atom% and given as follows. x=1-(y+z+u), y is 0.09 to 0.14, z is 0.09 to 0.14, and u is over 0.01 to less than 0.2). The negative electrode active material is 70-150% in expansion coefficient after 50 cycles. The degree of amorphization of a matrix like region in the alloy is in a range of 25% or larger. The contents of Si, Ti, Fe and Al are represented in atom% (at%) as follows. Si is 60-70%; Ti is 9-14%; Fe is 9-14%; and Al is in a range between 1 and 20% exclusive.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a negative electrode active material for a secondary battery and a secondary battery using the same.

  Conventionally, lithium metal was used as the negative electrode active material of the lithium battery. However, when lithium metal is used, there is a risk of explosion due to the formation of dendrites, so there is a risk of explosion. Carbon-related materials are often used as negative electrode active materials.

  Examples of the carbon 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 that the irreversibility is large in the process of charging and discharging. Graphite is typically used as the crystalline carbon, and has a theoretical limit capacity of 372 mAh / g, a high capacity, and is used as a negative electrode active material.

  However, such graphite and carbon-enhanced materials are only about 380 mAh / g even if the theoretical capacity is somewhat high, and the above-described negative electrode cannot be used in the development of high-capacity lithium batteries in the future. There is.

  In order to improve such a problem, a material that is being actively studied is a metal-related or intermetallic compound-based negative electrode active material. For example, a lithium battery using a metal or a semimetal such as aluminum, germanium, silicon, tin, zinc, or lead as a negative electrode active material has been studied. Such a material has a high energy density while having a high capacity, can absorb and release more lithium ions than a negative electrode active material using a carbon-related material, and has a high capacity and a high energy density. Can be manufactured. For example, pure silicon is known to have a high theoretical capacity of 4017 mAh / g.

  However, when compared with the carbon-related material, the cycle characteristics are still deteriorated, and it is still an obstacle to practical use. This is because the silicon or the like is used as a negative electrode active material as a lithium storage and release material. In this case, a phenomenon occurs in which the conductivity between the active materials is reduced due to a change in volume during the charge / discharge process or the negative electrode active material is peeled off from the negative electrode current collector. In other words, when the silicon contained in the negative electrode active material absorbs lithium by charging and expands to a volume of about 300 to 400% and discharges, the inorganic particles shrink when lithium is released. become.

  If such a charge / discharge cycle is repeated, electrical insulation may occur due to cracks in the negative electrode active material, and the lifetime is drastically reduced, which is problematic for use in lithium batteries. In order to improve such problems, research has been conducted to use nano-sized particles as silicon particles, or to make silicon have porosity so as to have a buffering effect against volume change.

  Korean Patent No. 2004-0063802 relates to a negative electrode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery, and a method for causing the metal to spring after alloying other metals such as silicon and nickel. Korean Patent No. 2004-0082876 relates to a method for producing porous silicon and nano-sized silicon particles and application to a negative electrode material for a lithium secondary battery. And a silicon precursor such as silicon dioxide are disclosed, and after heat treatment, a technique of squirting with an acid has been disclosed.

  In the above patents, the initial capacity retention rate may be improved by the buffer effect due to the porous structure, but only the porous silicon particles having poor conductivity are used. There is a problem that the transmission efficiency decreases and the initial efficiency and capacity maintenance characteristics deteriorate.

Korean Published Patent No. 2004-0063802 Korean Published Patent No. 2004-0082876

  Accordingly, the present invention has been devised to solve the above-described problems, and its purpose is to provide a negative electrode active material for a lithium secondary battery that is less likely to cause electrical insulation with little volume change during charge and discharge. Is to provide.

  Another object of the present invention is to provide a negative electrode active material for lithium secondary excellent in initial efficiency and capacity maintenance characteristics.

  Another object of the present invention is to provide a negative electrode active material optimized in consideration of the degree of amorphization in battery design.

  In order to achieve the above object, an embodiment of the present invention provides an anode active material for a secondary battery in which the degree of amorphization of the matrix-like fine crystal region in the alloy is 25% or more as an alloy having the following chemical formula: It is characterized by doing.

  SixTiyFezAlu (where x, y, z, u are atomic%, x: 1- (y + z + u), y: 0.09 to 0.14, z: 0.09 to 0.14, u: 0.01 Exceeding less than 0.2)

  In the secondary battery negative electrode active material, the expansion coefficient after 50 cycles is preferably in the range of 70 to 150%.

  In the secondary battery negative electrode active material, it is preferable that Al is in the range of 5 to 19% by atomic% (at%).

  In the secondary battery negative electrode active material, it is most preferable that Al is in the range of 10 to 19% by atomic% (at%).

  The negative electrode active material for a secondary battery preferably has Ti and Fe in a range of 9 to 12.5% in atomic% (at%).

  Moreover, it is preferable that the said negative electrode active material for secondary batteries has the range of 2: 1 to 1: 2 of Ti and Fe.

  Moreover, it is preferable that the said negative electrode active material for secondary batteries has the range of the ratio of Ti and Fe 1: 1.

  The secondary battery negative electrode active material preferably has a discharge capacity after 50 cycles of 90% or more compared to the initial discharge capacity.

  Moreover, it is preferable that the said negative electrode active material for secondary batteries is 98% or more of the efficiency after 50 cycles.

  Further, according to another embodiment of the present invention, the negative electrode has an expansion coefficient after 70 cycles of 70 to 150%, an alloy having the following chemical formula, and an amorphous matrix-like fine crystal region in the alloy. The degree of conversion is in the range of 25% or more, and in atomic% (at%), Si: 60 to 70%, Ti: 9 to 14%, Fe: 9 to 14%, Al: 5 to 19% A secondary battery comprising the negative electrode active material is provided.

  SixTiyFezAlu (where x, y, z, u are atomic% (at%), x: 1- (y + z + u), y: 0.09 to 0.14, z: 0.09 to 0.14, u : 0.05-0.19)

  As described above, according to the embodiment of the present invention, a negative electrode active material for a lithium secondary battery is obtained that has little change in volume during charge and discharge, does not generate electrical insulation well, and has excellent initial efficiency and capacity maintenance characteristics. There is an effect that can be.

  In addition, according to the embodiment of the present invention, it is possible to provide the degree of amorphization of the negative electrode active material optimized in the battery design through the measurement of the expansion coefficient after 50 cycles.

  In addition, according to the embodiment of the present invention, it is possible to provide a negative electrode active material that is optimized in consideration of the degree of amorphization in battery design.

It is the organization photograph which measured the expansion characteristic after 50 cycles in the negative electrode active material by a comparative example. It is the organization photograph which measured the expansion characteristic after 50 cycles in the negative electrode active material by a comparative example. It is the organization photograph which measured the expansion characteristic after 50 cycles in the negative electrode active material by a comparative example. FIG. 4 is a photograph of an organization in which an expansion property after 50 cycles was measured in a negative electrode active material according to an example of the present invention. 3 is a graph illustrating measurement of the degree of amorphization of a negative electrode active material according to an embodiment of the present invention.

  Specific details of other embodiments are included in the detailed description of the invention and the drawings. Advantages and features of the present invention and methods for performing them will be apparent with reference to the embodiments and the like described in detail below in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be embodied in various different forms, and the following description is connected to other parts. In this case, this includes not only the case of being directly connected but also the case of being connected with another medium in between. In the drawings, parts not related to the present invention are omitted for clarity of explanation of the present invention, and similar parts are denoted by the same reference numerals throughout the specification. Hereinafter, the present invention will be described with reference to the accompanying drawings.

  Embodiments of the present invention provide a negative electrode active material for a secondary battery having an improved expansion coefficient and a secondary battery including the same. In particular, in the example of the present invention, a negative electrode active material in which the degree of amorphization of the matrix-like fine crystal region in the alloy is 25% or more can be obtained in the negative electrode active material for secondary batteries.

  In general, when studying silicon-based negative electrode active materials, the initial plate thickness after the formation process is the initial plate thickness at the time of full charge (electrode plate thickness before electrolyte injection). Measure how much it has increased compared to. In other words, the coefficient of expansion after one cycle is measured. Here, a change in volume generated while lithium is occluded in the negative electrode active material appears.

  However, in the embodiment of the present invention, the thickness after repeated charging and discharging is measured for 50 cycles instead of the one cycle, and the expansion coefficient after 50 cycles compared with the initial electrode plate thickness is measured. did. Through the measurement of the expansion coefficient after 50 cycles, the volume of the lithium due to occlusion and release, and the generation of SEI (Solid Electrolyte Interface or Interface) layer that accumulates while the electrolytic solution decomposes due to side reactions that occur on the active material surface. Can be monitored.

  When a coin half cell is manufactured and the characteristics of a silicon-related negative electrode material are evaluated, the lithium metal electrode used as a counter electrode generally starts to deteriorate after 50 cycles and affects the results. Therefore, in the embodiment of the present invention, after evaluating the 50 cycle life, the coin cell is disassembled and the change in the electrode plate thickness is measured, so that not only the initial electrode plate expansion due to simple lithium occlusion but also the subsequent 50 cycles. Taking into account the expansion of the electrode plate due to the growth of the side reaction layer, it was used as an index for evaluating the performance of the negative electrode active material. Therefore, in the example of the present invention, it was found that the change in the expansion coefficient after 50 cycles was a technically meaningful index for performance evaluation, and the optimum component range could be derived therefrom.

  Usually, in the case of graphite, a very stable SEI layer is generated in the initial formation charging stage, and after the initial charging stage, the change in the volume of the electrode plate occurs at a level of 20% or less. It shows a tendency that the SEI layer in the charging stage is maintained as it is. However, since the volume of the electrode plate is large in the silicon-related negative electrode active material, the SEI layer that was generated on the active material surface in the early stage moves away when the active material shrinks, and the new active material surface is exposed to the electrolyte. In the next expansion, the phenomenon that a new SEI layer is generated on the surface is continuously repeated to develop a side reaction layer that is a very thick SEI layer.

  The side reaction layer accumulated on the surface of the active material acts as a low antibody in the secondary battery to obstruct lithium migration, and the electrolyte is consumed to form the side reaction layer. It can cause problems that reduce battery life. In addition, the increase in the electrode plate thickness due to the development of the side reaction layer causes physical deformation of the battery jelly roll, causing the current to concentrate on the electrode plate of a certain area and causing the battery to deteriorate rapidly. It can be caused.

  In the case of an existing silicon alloy material, the mechanism (Matrix) inside the active material is repeated as it is repeatedly charged and discharged, and only the silicon portion contracts and expands, so that a crack may occur between the mechanism and silicon. is there. In such a case, it was discovered that a side reaction layer of the electrolytic solution was generated inside the active material while the electrolytic solution penetrated into the cracks, and the active material was dispersed. A sudden expansion of is observed.

  Such a phenomenon is a phenomenon that cannot be found when measuring the electrode plate thickness after one cycle. Even when a material having an excellent initial expansion coefficient is actually applied to a battery, This suggests that the material may cause various problems such as increase in resistance and depletion of electrolyte. Therefore, the electrode plate expansion after 50 cycles presented in this example is a very useful evaluation index in the evaluation of expansion, contraction, and side reaction phenomenon of the active material during the development of the silicon negative electrode active material.

  In the embodiment of the present invention, the magnitude of the expansion coefficient after 50 cycles is examined based on the composition of the metal compound for negative electrode active material used in the embodiment of the present invention, and the optimal expansion coefficient in accordance with the change in the composition. It came to derive the range.

  On the other hand, in the embodiment of the present invention, there is a microcrystalline region on the alloy matrix, which facilitates lithium diffusion. The ratio of the presence of such fine crystal regions can be shown through the degree of amorphization, and the formation of the amorphous region in a matrix form suppresses the volume expansion when the secondary battery is charged. be able to.

  The present invention is characterized in that the degree of amorphization of the matrix fine crystal region is 25% or more. When the degree of amorphization is formed within the above range, lithium diffusion becomes very easy. And when it is in the range of the degree of amorphization, the expansion rate after 50 cycles is similarly excellent, and therefore when used as a negative electrode active material, the volume expansion during charging is It turns out that it is suppressed.

  In an embodiment of the present invention, the degree of amorphization is preferably 25% or more in the range of the rotation angle 2θ = 20 ° to 100 ° of the XRD pattern of the alloy. Within the range of the degree of amorphization, volume expansion is suppressed and electrical insulation often occurs.

  The calculation of the degree of amorphization used in the present invention is as follows, and the degree of amorphization can be obtained as shown in FIG.

  Amorphization degree (%) = ((total area−crystallization area)) ÷ total area)

  In the embodiment of the present invention, a high degree of amorphization means that there are many fine crystal regions. Therefore, during charging, lithium ions are accumulated through a buffering action in the fine crystal regions, thereby causing a volume expansion factor. The effect that can be suppressed can be obtained.

Moreover, in the Example of this invention, the expansion coefficient after 50 cycles has the range of 70 to 150%, and provides the negative electrode active material for secondary batteries which consists of a following formula.
SixTiyFezAlu (1)
(Where x, y, z, u are atomic% (at%), x: 1- (y + z + u), y: 0.09 to 0.14, z: 0.09 to 0.14, u: 0.01 over less than 0.19)

  In this example, the Si has a range of 60 to 70% in atomic% (at%), and Ti and Fe have a range of 9 to 14%. On the other hand, the Al has a range of more than 1% and less than 20%, preferably 5 to 19%.

Ti and Fe contained in the alloy combine with Si to form an intermetallic compound called Si ₂ TiFe. Therefore, when the contents of Ti and Fe are 14 at%, respectively, a phenomenon that 28 at% or more of Si is consumed to form an intermetallic compound and the capacity per g of the active material is reduced. In this case, 1000 mAh / In order to obtain a capacity of g or more, the content of Si to be added must be very high.

  In general, when a large amount of Si, which is a semimetal, is contained, the viscosity of the hot water at the time of melting tends to deteriorate the workability of rapid solidification, so the Si content is maintained within 70% as much as possible. For this reason, it is preferable that the content of Ti and Fe does not exceed 14%. In the examples of the present invention, it was derived that it is preferable to reduce the content of Ti and Fe to 14% or less in the process of deriving optimum alloy components in relation to the expansion rate.

  Al can also have a range of greater than 1% and less than 20% in at%. When Al is contained in an amount of about 1%, the expansion after 50 cycles is severely caused and a phenomenon in which the active material is scattered appears, which is not preferable. Moreover, when Al is 20%, the discharge capacity due to the change in the Si: Matrix volume fraction decreases, which is not preferable. In the examples of the present invention, it is derived that when having a range of 5 to 19% in at%, it is possible to have the most preferable expansion rate range, and it is found that the discharge capacity does not decrease within this range. It was. Most preferably, Al is 10 to 19%, and in this range, the most preferable range of 50 cycle expansion rate can be obtained, and the discharge capacity does not decrease.

  In addition, the method for producing the negative electrode active material of the present invention is not particularly limited. For example, various fine powder production techniques known in this field (gas atomizer method, centrifugal gas atomizer method, plasma atomizer method, rotating electrode) Method, mechanical alloying method, etc.).

  In the present invention, for example, Si and the components constituting the matrix are mixed, the mixture is melted by an arc melting method or the like, and then applied to a single roll rapid solidification method in which the melt is sprayed onto a rotating copper roll. A substance can be produced. However, the method applied in the present invention is not limited to the above method, and the above-described fine powder production technique (gas atomizer) can be used as long as a sufficient rapid cooling rate can be obtained in addition to the single roll rapid solidification method. Method, centrifugal gas atomizer method, plasma atomizer method, rotating electrode method, mechanical alloying method, etc.).

In addition, a secondary battery can be manufactured using the negative electrode active material according to an embodiment of the present invention, and the positive electrode of the secondary battery can include a lithium intercalation compound. In addition, inorganic sulfur (S8, Elemental Sulfur) and yellow compound (Sulfur Compound) can also be used. As the yellow compound, Li ₂ Sn (n ≧ 1), catholyte ( Examples include Li ₂ Sn (n ≧ 1), organic sulfur compound or carbon-sulfur polymer (C ₂ Sf) n: f = 2.5 to 50, n ≧ 2) dissolved in Catholyte). .

Similarly, the type of electrolyte contained in the secondary battery of the present invention is not particularly limited, and general means known in this field can be employed. In one example of the present invention, the electrolytic solution may include a non-aqueous organic solvent and a lithium salt. The lithium salt can be dissolved in an organic solvent to act as a lithium ion supply source in the battery, and promote the movement of lithium ions between the positive electrode and the negative electrode. Examples of lithium salts that can be used in the present invention include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₃ , Li (CF ₃ SO ₂ ) ₂ N , LiC ₄ F ₉ SO ₃ , LiCl ₄ , LiAlO ₄ , LiAlCl ₄ , LiN (CxF _{2x + 1} SO ₂ ) (CyF _{2y + 1} SO ₂ ) (where x and y are natural numbers), LiCl, Lil, and For example, one or two or more of lithium bisoxalate borate or the like may be included as a supporting electrolytic salt. The concentration of the lithium salt in the electrolyte can be changed depending on the application, and is usually used within the range of 0.1M to 2.0M.

  The organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Examples thereof include benzene, toluene, fluorobenzene, and 1,2-difluorobenzene. 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, fluorotoluene, 1,2-difluoro Rotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3- Dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene, 1,2-diiodotoluene, 1,3-diiodotoluene 1,4-diiodotoluene, 1,2,3-triiodotoluene, 1,2,4-triiodotoluene, R-CN (wherein R is a straight chain having 2 to 50 carbon atoms, branched Or a hydrocarbon group having a cyclic structure, which may include a double bond, an aromatic ring or an ether bond), dimethylformamide, dimethyl acetate, xylene, cyclohexane, Trahydrofuran, 2-methyltetrahydrofuran, cyclohexanone, ethyl alcohol, isopropyl alcohol, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, propylene carbonate, methyl propionate, ethyl propionate, methyl acetate, ethyl acetate , Propyl acetate, dimethoxyethane, 1,3-dioxolane, diglyme, tetraglyme, ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane, valerolactone, decanolide or mevalolacton. However, the present invention is not limited to this.

  The secondary battery of the present invention can additionally contain normal elements such as a separator, can, battery case or gasket in addition to the above elements, and the specific types thereof are not particularly limited as well. In addition, the secondary battery of the present invention includes the above-described elements, and can be manufactured in a usual manner and shape in this field. Examples of shapes that the secondary battery of the present invention can have include a cylindrical shape, a square shape, a coin shape, and a pouch shape, but are not limited thereto.

Hereinafter, the present invention will be described in more detail with reference to examples.
In this embodiment, the Si has an atomic% (at%) range of 60 to 70%, and Ti and Fe have a range of 9 to 14%. On the other hand, the Al has a range of more than 1% and less than 20%, preferably 5 to 19%. Most preferably, it is 10 to 19% of range.

  Table 1 below is a table showing the composition ranges of Examples and Comparative Examples of the present invention. On the other hand, Table 2 below relates to the evaluation of the negative electrode active material based on the composition of Table 1, and in particular, 1 CY-charge / discharge amount, 1 CY-efficiency, 1 CY-electrode capacity, and examples and comparative examples. The figure shows 50 CY-discharge capacity, 50 CY-efficiency, 50 CY-capacity retention rate, 50 CY-expansion rate, and degree of amorphization (%). The technical meaning of each item in Table 2 will be described later.

  In the Example of this invention, charging / discharging was repeated 50 times and this was measured. The charging / discharging method was performed in accordance with a charging / discharging method for lithium secondary electron active materials generally known in this field.

  First, in the case of Examples 1 to 5 of the present invention, Al is carried out with a composition in the range of 5% to 19% at%, Comparative Example 1 is not added Al, and Comparative Example 2 is Al. Is shown when 1% is added. Comparative Example 3 shows a case where 20% Al is added.

On the other hand, in the case of Ti and Fe, it combines with Si to form an intermetallic compound called Si ₂ TiFe. Therefore, when the content of Ti and Fe is high, a phenomenon appears in which the capacity per g of the active material is reduced by forming an intermetallic compound with Si, and in this case, a capacity of 1000 mAh / g or more will be obtained. If so, the Si content to be introduced must be very high. In general, when a large amount of Si, which is a metalloid, is contained, the viscosity of molten hot water tends to be high and the rapid solidification workability tends to deteriorate. Therefore, the Si content is maintained within 70% as much as possible. Better. Therefore, it is preferable that the content of Ti and Fe does not exceed 14% in consideration of forming an intermetallic compound with Si.

  By examining Table 1 and Table 2 below, it is preferable to reduce the Ti and Fe contents to 14% or less in the process of deriving optimum alloy components in relation to the expansion rate in the examples of the present invention. Was derived.

  Further, Al preferably has a range of more than 1% and less than 20% in at%. If Al is contained in an amount of about 1%, expansion will occur severely after 50 cycles. In this case, a phenomenon in which the active material is scattered appears, which is not preferable. In addition, when Al is 20%, the discharge capacity due to the change in the Si: Martrix volume fraction rapidly decreases, which is not preferable.

  In the embodiment of the present invention, it is derived that when the atomic% (at%) is in the range of 5 to 19%, it is possible to have the most preferable expansion rate range, and also within this range, the discharge capacity is reduced. I found that there was no. Most preferably, Al is 10 to 19%, and in this range, the most preferable range of 50 cycle expansion rate can be obtained, and furthermore, the discharge capacity does not decrease.

  By examining Table 2 below and Examples 1 to 5 of the present invention, it can be confirmed that the performance of the active material can be improved by adding Al. In particular, it can be seen that when Al is added, the discharge capacity, reversible efficiency, and expansion characteristics are significantly improved. On the other hand, in the case of Comparative Example 1 in which Al was not added, the 50-cycle expansion characteristics showed a value exceeding 200%. Further, in the case of Comparative Example 2, the 50-cycle expansion characteristics exceed 200% as in the case of adding 1% Al. On the other hand, in the case of Comparative Example 3 in which 20% Al is added, the 50 cycle expansion is 40.2%, which is very low. In this case, the discharge capacity is significantly reduced, so that the performance of the secondary battery negative electrode active material is improved. There is a problem that the effect is rather low.

  Therefore, by examining Tables 1 and 2 below in the examples of the present invention, it can be seen that the discharge capacity, reversible efficiency, and expansion characteristics due to the addition of Al are significantly improved in the negative electrode active material. Further, at this time, the addition amount of Al exceeds at least 1% in at%, but it is understood that optimum performance is exhibited in the range of less than 20%. In the case of Comparative Examples 1 and 2, it can be seen that the degree of amorphization (%) appears to be less than 25%. Therefore, in the examples of the present invention, the preferred amorphization degree within the component range of Al is at least 25. It turns out that it is more than%.

  1A, 1B, 1C, and 2 are organization photographs showing the expansion coefficient characteristics after 50 cycles for Comparative Example 2 and Example 5, respectively. In FIGS. 1A, 1B, and 1C, the portion that forms a light-colored particle shape is Matrix, and the dark-colored background portion is Si, but in the initial stage before the life test, Matrix is well gathered as in FIG. 1C. However, 50 cycles of charge / discharge were repeated, and it was confirmed that bright colored particles forming Matrix were scattered while the volume of the Si portion was increased.

  In the case of FIG. 1C, despite the state after the same 50 cycles, Matrix is well gathered without scattering from each other regardless of the contraction and expansion of silicon. The phenomenon that the active material Matrix is scattered causes a rapid increase in the expansion value after 50 cycles. When Al is added to 1% or less as in Comparative Examples 1 and 2, the expansion after 50 cycles appears very severe at 200% or more, but in the case of Example 5 where no scattering phenomenon of the active material is observed, 50 cycles. It can be seen that the later expansion rate is about 78%, which is very excellent, and the life characteristics are also very excellent.

  First, the active material evaluation in the examples of the present invention was performed by manufacturing an electrode plate having the following composition. The silicon alloy active material was evaluated by producing an electrode plate having a composition of conductive additives (carbon black series): binder (organic type, PAI binder) of 86.6%: 3.4%: 10%. Then, a slurry dispersed in an NMP solvent is prepared and coated on a copper foil current collector by a doctor blade method, dried in an oven at 110 ° C., and heat-treated at 210 ° C. in an Ar atmosphere for 1 hour to form a binder. Cured.

A coin cell was made from the electrode plate made by the above method using lithium metal as a counter electrode, and a chemical conversion step was performed under the following conditions.
Charge (lithium insertion): 0.1C, 0.005V, 0.05C cut-off
Discharge (lithium release): 0.1C, 1.5V cut-off

After the chemical conversion step, a cycle test was performed under the following conditions.
Charging: 0.5C, 0.01V, 0.05C cut-off
Discharge: 0.5C, 1.0V cut-off

  In Table 2 above, 1 CY-charge (mAh / g) is the chemical charge capacity per gram of active material, and the amount of charge in the charge stage during the chemical conversion process, which is the first charge stage after assembly of the coin cell, is measured. It is the value divided by the weight of the active material contained in the coin cell electrode plate.

  1 CY-discharge (mAh / g) is a conversion discharge capacity per gram of active material, and is included in the coin cell electrode plate by measuring the amount of charge in the discharge stage during the conversion process, which is the first discharge stage after assembling the coin cell. It is a value divided by the weight of the active material. The capacity per g in the present embodiment means a 0.1C conversion discharge capacity which is the discharge capacity measured at this time.

  The 1CY-efficiency is a percentage obtained by dividing the discharge capacity by the charge capacity in the chemical conversion process, which is the first charge / discharge process. Generally, graphite has a high initial efficiency of 94%, silicon alloys have an initial efficiency of 80 to 90%, and silicon oxide (SiOx) has an initial efficiency value of a maximum of 70%. .

  The reason why the initial efficiency of any substance is less than 100% is that the first input lithium is trapped irreversibly during charging during the chemical conversion process, or it is consumed by side reactions such as SEI formation. This is because, if the initial efficiency is low, a loss that requires additional addition of the negative electrode active material and the positive electrode active material is caused. Therefore, high initial efficiency is important in battery design.

  The silicon alloy used in the examples of the present invention has an initial efficiency value of 85%, and the conductive additive and binder also consume lithium in an initial irreversible manner, so that the substantial active material itself The initial efficiency value of is about 90%.

  The 50 CY-discharge is the discharge capacity per gram of active material in 50 cycles, and activates the amount of charge measured at the time of discharge in the 50th cycle including the chemical conversion step during the cycle test that proceeds at 0.5 C after the chemical conversion step. It is the value divided by the material weight. When the active material is deteriorated during the cycle test, it is indicated by a numerical value lower than the initial discharge capacity. When there is little deterioration, the active material is indicated by a numerical value similar to the initial discharge capacity.

  The 50CY-efficiency is the ratio of the charge amount relative to the charge amount in 50 cycles expressed in%. A higher 50CY-efficiency means less lithium loss due to side reactions and other deterioration in the cycle. Generally, when 50 CY-efficiency is 99.5% or more, it is judged as a very good value, and since the cannon for assembling coin cells cannot be ignored in the laboratory environment, to decide.

  The 50 CY-maintenance is the ratio of the discharge capacity in the 50th cycle, expressed as a percentage, based on the discharge capacity of the first cycle when the 0.5C cycle is performed thereafter, excluding the cycle that was advanced during the chemical conversion process. It is.

  It can be seen that the higher the 50CY-maintenance ratio is, the closer the slope of the battery life is to the horizontal. When the 50CY-maintenance ratio is 90% or less, the deterioration occurred during the cycle and the discharge capacity decreased. means. In some embodiments, the 50 CY-maintenance rate may also exceed 100%, which shows silicon particles that are additionally activated at the same time that there is little degradation during life. It is judged.

  The 50CY-expansion is the percentage increase in thickness after 50 cycles compared to the thickness of the initial electrode plate. The measurement method of 50CY-expansion will be described in detail as follows.

  First, the thickness of the initial current collector is measured. Thereafter, the thickness of the circularly cut electrode plate is measured using a micrometer for assembly into a coin cell, and then the thickness of the current collector is calculated by subtracting the thickness of the current collector.

  In succession, after completion of the 50-cycle test, the coin cell is disassembled from the dry room and only the negative electrode plate is separated, and then the electrolyte remaining on the electrode plate is washed using a DEC solution and dried to a micrometer. The thickness of the active material after the cycle is calculated by subtracting the thickness of the current collector by measuring the thickness using. That is, 50 CY-expansion is expressed as a percentage by dividing the thickness of the active material increased after 50 cycles compared with the thickness of the initial active material by the thickness of the initial active material.

Tables 3 and 4 below show the 1 CY-charge / discharge amount, 1 CY-efficiency, 1 CY-electrode plate of the example of the present invention and the comparative example for the experiment for confirming the ratio range of Ti and Fe. The capacity, 50CY-discharge capacity, 50CY-efficiency, and 50CY-capacity maintenance ratio are shown. The technical meaning of each item in Table 4 is as described above.
Table 3 shows the composition range of the example of the present invention and the comparative example in order to confirm the ratio range of Ti and Fe. At% of other substances excluding Ti and Fe was fixed and proceeded, and only the ratio of Ti and Fe was changed and the experiment was conducted.

  According to the following Table 3, the ratio of Ti and Fe is preferably in the range of 2: 1 to 1: 2, and more preferably 1: 1. In Examples 7 to 9, in which the ratio of Ti and Fe is maintained in the range of 2: 1 to 1: 2, the capacity maintenance ratio is shown to be higher than 90%, and the ratio is 1: 1. Example 8 showed the highest capacity retention at 96.4%. On the other hand, in the case of Comparative Example 4 and Comparative Example 5 formed in a range in which the ratio of Ti and Fe is out of 2: 1 to 1: 2, the capacity retention rate is low as 51.2% and 81.3%. It was. Therefore, in the embodiments of the present invention, the ratio of Ti and Fe is maintained within the range of 2: 1 to 1: 2 in order to maximize battery performance, but is most preferably controlled at a ratio of 1: 1. .

As described above, those having ordinary knowledge in the technical field to which the present invention pertains can implement the present invention in other specific forms without changing the technical idea or essential features thereof. You will understand that. Therefore, it should be understood that the embodiments and the like described above are illustrative from all aspects and are not limiting. The scope of the present invention is indicated by the scope of claims described below rather than the above detailed description, and all modifications or variations derived from the meaning and scope of the scope of claims and the equivalent concept thereof are included. It should be construed as included in the scope of the present invention.

Claims (12)

A negative electrode active material for a secondary battery, characterized in that the degree of amorphization of the matrix-like fine crystal region in the alloy is 25% or more as an alloy having the following chemical formula.
SixTiyFezAlu (where x, y, z, u are atomic%, x: 1- (y + z + u), y: 0.09 to 0.14, z: 0.09 to 0.14, u: 0.01 Exceeding less than 0.2)   2. The negative electrode active material for a secondary battery according to claim 1, wherein an expansion coefficient after 50 cycles of the negative electrode active material is in a range of 70 to 150%.   2. The negative electrode active material for a secondary battery according to claim 1, wherein the negative electrode active material for the secondary battery has an atomic% (at%) of Al in the range of 5 to 19%.   The negative electrode active material for a secondary battery according to claim 3, wherein the negative electrode active material for the secondary battery has an atomic percentage (at%) of Al in the range of 10 to 19%.   2. The negative electrode active material for a secondary battery according to claim 1, wherein the negative electrode active material for the secondary battery has an atomic% (at%) of Ti and Fe in a range of 9 to 12.5%, respectively. .   2. The negative electrode active material for secondary battery according to claim 1, wherein the secondary battery negative electrode active material has a ratio of Ti and Fe of 2: 1 to 1: 2.   The secondary battery negative electrode active material according to claim 6, wherein the secondary battery negative electrode active material has a ratio of Ti to Fe of 1: 1.   The negative electrode active material for a secondary battery according to claim 1, wherein the secondary battery negative electrode active material has a discharge capacity after 50 cycles of 90% or more compared to the initial discharge capacity.   The negative electrode active material for a secondary battery according to claim 1, wherein the secondary battery negative electrode active material has an efficiency of 98% or more after 50 cycles.   A secondary battery comprising a negative electrode including any one of the negative electrode active materials according to any one of Items 1 to 9, a positive electrode, and an electrolyte. The negative electrode has an expansion coefficient of 70 to 150% after 50 cycles,
An alloy having the following chemical formula, the amorphous degree of the matrix-like fine crystal region in the alloy has a range of 25% or more,
A negative electrode active material having a range of atomic percent (at%) of Si: 60 to 70%, Ti: 9 to 14%, Fe: 9 to 14%, Al: 5 to 19%. Item 11. The secondary battery according to Item 10.
SixTiyFezAlu (where x, y, z, u are atomic% (at%), x: 1- (y + z + u), y: 0.09 to 0.14, z: 0.09 to 0.14, u : 0.05-0.19) The secondary battery according to claim 11, comprising a negative electrode having a ratio of Ti to Fe in a range of 2: 1 to 1: 2.