JP2017224499A - Negative electrode active material for lithium ion battery and lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for a lithium ion battery which uses Si as an active material and has a high initial discharge capacity and good cycle characteristics.SOLUTION: A negative electrode active material composed of a binary Si-Zr alloy having a matrix phase composed of a Si phase and a Zr silicide phase dispersed in the matrix phase contains Si of 45 to 65 mass%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode active material for a lithium ion battery and a lithium ion battery.

  Lithium ion batteries have the advantage of being able to be miniaturized with high capacity and high voltage, and are widely used as power sources for mobile phones and notebook computers, and in recent years as power sources for power applications such as electric vehicles and hybrid vehicles. High expectations have been gathered and its development is actively underway.

  In this lithium ion battery, lithium ions (hereinafter referred to as Li ions) move between the positive electrode and the negative electrode to perform charging and discharging, and on the negative electrode side, Li ions are occluded in the negative electrode active material during charging, During discharge, Li ions are released from the negative electrode active material.

Conventionally, lithium cobaltate (LiCoO ₂ ) has been generally used as the active material on the positive electrode side, and graphite has been widely used as the negative electrode active material.
However, graphite as the negative electrode active material has a theoretical capacity of only 372 mAh / g, and a higher capacity is desired. Therefore, recently, metal materials such as Si and Sn (theoretical capacity of Si is 4198 mAh / g, the theoretical capacity of Sn is 993 mAh / g), which can be expected to increase in capacity, are actively used as alternative materials for carbon-based negative electrode active materials. It has been studied.

However, since Si and Sn occlude Li ions by an alloying reaction with Li, large volume expansion and contraction occur with the occlusion and release of Li ions.
Therefore, when the negative electrode active material is composed of Si and Sn alone, the cycle is a capacity maintenance characteristic when the Si and Sn particles are cracked or peeled off from the current collector due to the expansion / contraction stress, and charge and discharge are repeated. There has been a problem that the characteristics are poor, and this has been a big barrier to the practical application of a negative electrode active material using Si or the like.

For this reason, as shown in Patent Documents 1 and 2 below, various proposals have been made to alloy Si in a negative electrode active material using Si.
For example, the following Patent Document 1 discloses an invention about a “lithium secondary battery”, in which a binary Si—Ni alloy or a ternary system in which a part of Ni is further substituted with Mn or the like. A point in which an alloy is used as a negative electrode material for a lithium secondary battery, and a structure in which Si crystals are dispersed in an alloy matrix phase in these Si alloys are disclosed.

JP 10-294112 A Special table 2015-524988 gazette

However, the Si alloy described in the above-mentioned patent document has a structure in which the periphery of the Si phase crystallized in an island shape is surrounded by a silicide phase, and although a certain effect is recognized in improving cycle characteristics, the Si phase is recognized. The effect of relaxing the stress generated at the time of expansion / contraction in the surrounding silicide phase was not sufficient, and the Si alloy collapsed as charging and discharging were repeated, and there was still room for improvement.
In view of the above circumstances, the present invention provides a negative electrode active material for lithium ion batteries and a lithium ion battery using Si as an active material, maintaining a high initial discharge capacity and having good cycle characteristics. It was made as a purpose.

  Thus, claim 1 relates to a negative electrode active material for a lithium ion battery, comprising a binary Si—Zr alloy, a matrix phase comprising a Si phase, and a Zr silicide phase dispersed in the matrix phase. And the Si content is 45 to 65% by mass.

  A second aspect of the present invention relates to a lithium ion battery, and has a negative electrode including the negative electrode active material for a lithium ion battery according to the first aspect.

The negative electrode active material of the present invention composed of the binary Si—Zr alloy as described above has a high initial capacity, and also has good capacity maintenance characteristics, that is, cycle characteristics when charging and discharging are repeated.
The negative electrode active material of the present invention has a structure in which a Zr silicide phase is dispersed in a matrix phase composed of a Si phase. It is considered that the Zr silicide phase works to absorb the expansion stress when the Si phase expands, and the cycle characteristics are improved.
Conventionally, in an Si alloy used as a negative electrode active material for a lithium ion battery, an Si phase that causes large volume expansion / contraction due to insertion / extraction of Li ions is dispersed in a silicide phase, and the Si phase is island-shaped. Since the silicide phase has a sea-like structure, stress generated when Si expands is applied to the silicide phase, and the silicide phase and the particles sometimes collapse.
On the other hand, the present invention has a structure in which the sea-island structure is reversed from that of the prior art, that is, the silicide phase is an island shape and the Si phase is a sea shape. By doing so, since most of the Si phase is located on the outermost surface, the stress applied to the silicide phase during the expansion of the Si phase is reduced, and particle collapse can be suppressed. In addition, since the silicide phase arranged in an island shape does not expand, it plays the role of an aggregate that maintains the structure of the particles, and the collapse of the particles can be more effectively suppressed.

  In the present invention, such a sea-island structure is realized by using a binary Si—Zr alloy. In such a Si—Zr alloy, the Si—Zr compound (Zr silicide phase) crystallizes first and then the Si (Si phase) crystallizes in the process of cooling and solidifying the molten alloy. The Si phase is formed in a sea shape.

  The crystallization shape and size of the silicide phase can be changed depending on the cooling rate. The shape of the silicide phase becomes a dendritic shape when the cooling rate is low, but is dispersed and flattened by cooling at a higher rate. In the present invention, by crystallizing the silicide phase into a flat shape, the area in contact with the Si phase per unit volume of the silicide is increased, and the effect of absorbing the expansion of Si is enhanced, so that the cycle characteristics are improved. be able to.

In the negative electrode active material of the present invention, the Si content is 45 to 65% by mass.
If the Si content is less than 45% by mass, the amount of Si is too small and the initial discharge capacity is insufficient. On the other hand, if the Si content exceeds 65%, the amount of silicide phase (Si—Zr compound) produced is relatively reduced, and the capacity retention rate is deteriorated. Therefore, in the present invention, the Si content is set to 45 to 65% by mass. A more desirable Si content is 55 to 60% by mass.

  According to the present invention as described above, it is possible to provide a negative electrode active material for a lithium ion battery and a lithium ion battery that use Si as an active material, retain a high capacity, and have good cycle characteristics.

4 is a microstructural photograph of a negative electrode active material according to Example 3 by a scanning electron microscope (SEM). 6 is a microstructural photograph of a negative electrode active material according to Comparative Example 3 taken with a scanning electron microscope. 5 is a microstructural photograph of a negative electrode active material according to Comparative Example 4 taken with a scanning electron microscope. 6 is a microstructural photograph of a negative electrode active material according to Comparative Example 5 using a scanning electron microscope.

  Next, a negative electrode active material for a lithium ion battery according to an embodiment of the present invention (hereinafter simply referred to as a negative electrode active material), a lithium ion battery using the negative electrode active material as a negative electrode (hereinafter simply referred to as a battery) ) Will be described in detail.

1. This negative electrode active material This negative electrode active material consists of a binary Si-Zr alloy provided with the matrix phase which consists of Si phases, and the Zr silicide phase disperse | distributed in this matrix phase.
The Si phase is a phase mainly containing Si. From the viewpoint of increasing the amount of occlusion of Li and the like, it is preferably made of a single phase of Si. However, inevitable impurities may be contained in the Si phase.

On the other hand, the Zr silicide phase in the present invention is a phase mainly containing Si ₂ Zr, but inevitably other Zr silicide phases (Si ₄ Zr, Si ₃ Zr ₂ , Si ₅ Zr ₄ , SiZr, SiZr _2, etc.). ) May be included. The shape of the Zr silicide phase dispersed in the matrix phase (Si phase) is not particularly limited. However, in consideration of the point that the Zr silicide phase relaxes the stress generated during the expansion and contraction of the Si phase. A flat shape that increases the contact area with the Si phase is desirable.

  The form of the negative electrode active material made of the Si—Zr alloy is not particularly limited. Specifically, forms such as flakes and powders can be exemplified. Preferably, it is in a powder form from the viewpoint of easy application to the production of a negative electrode. Moreover, the negative electrode active material of the present invention may be dispersed in a suitable solvent.

The negative electrode active material of the present invention can be produced by a method that includes a step of rapidly cooling a molten alloy having a predetermined chemical composition to form a quenched alloy.
When the obtained quenched alloy is not in a powder form or when it is desired to reduce the diameter, a step of pulverizing the quenched alloy by an appropriate pulverizing means to form a powder may be added. Moreover, you may add the process of classifying the obtained quenched alloy and adjusting it to a suitable particle size, etc. as needed.

The active material preferably has a particle size (average particle size (d50)) in the range of 1 to 20 μm. The average particle diameter (d50) in the present invention means a volume reference and can be measured using a laser diffraction / scattering type particle size distribution measuring apparatus (Microtrack MT3000).
Even when Si alloy is used as the active material, the active material itself undergoes volume expansion / contraction due to the charge / discharge reaction. Stress is generated in the film. In this case, if the binder cannot withstand the stress, the binder collapses, resulting in peeling of the conductive film from the current collector. As a result, the conductivity in the electrode is lowered, and the charge / discharge cycle characteristics are lowered. To do. However, when the average particle diameter of the active material is set to be 1 to 20 μm fine particles, the contact area with the binder is increased because the active material is miniaturized, and thereby the collapse of the binder is favorably suppressed. As a result, the cycle characteristics can be improved.

  In the manufacturing method described above, the molten alloy specifically includes, for example, each raw material weighed so as to have a predetermined chemical composition, and the measured raw material is melted by an arc furnace, a high frequency induction furnace, a heating furnace, or the like. It can be obtained by dissolving using, for example.

  Specific examples of the method for rapidly cooling the molten alloy include liquids such as roll quenching methods (single roll quenching method, twin roll quenching method, etc.) and atomizing methods (gas atomizing method, water atomizing method, centrifugal atomizing method, etc.). Although a quenching method etc. can be illustrated, it is desirable to use a roll quenching method with a particularly high cooling rate.

Here, when the negative electrode active material of the present invention is produced using a molten alloy containing Si and Zr, specifically, the following method may be used.
That is, when the roll quenching method is applied, a rotating roll that rotates at a peripheral speed of about 10 m / s to 100 m / s is molten alloy that is discharged into a chamber such as a quenching and recovery chamber and continuously flows downward (in a rod shape). (The material is Cu, Fe or the like, and the roll surface may be plated). The molten alloy becomes an alloy material formed into a foil or a piece of foil by being cooled on the roll surface. In this case, the powdered negative electrode active material can be obtained by pulverizing the alloy material by an appropriate pulverizing means such as ball mill, disk mill, coffee mill, mortar pulverization, etc.

On the other hand, when the atomizing method is applied, a gas of N ₂ , Ar, He or the like is sprayed at a high pressure (for example, 1 to 10 MPa) to the molten alloy that is discharged into the spray chamber and continuously flows downward (in a rod shape). And cool the molten metal while crushing it. The cooled molten metal approaches a sphere while freely falling in the spray chamber while being semi-molten, and a powdered negative electrode active material is obtained. Further, high pressure water may be sprayed instead of gas from the viewpoint of improving the cooling effect.

2. This battery This battery is comprised using the negative electrode containing this negative electrode active material.

  The negative electrode has a conductive base material and a conductive film laminated on the surface of the conductive base material. The conductive film contains at least the negative electrode active material described above in the binder. In addition, the conductive film may contain a conductive additive as necessary. In the case of containing a conductive additive, it becomes easy to secure a conductive path for electrons.

  Moreover, the electrically conductive film may contain the aggregate as needed. When the aggregate is contained, it becomes easy to suppress the expansion / contraction of the negative electrode during charge / discharge, and the negative electrode can be prevented from collapsing, so that the cycle characteristics can be further improved.

  The conductive substrate functions as a current collector. Examples of the material include Cu, Cu alloy, Ni, Ni alloy, Fe, and Fe-based alloy. Preferably, Cu or Cu alloy is preferable. Moreover, as a form of a specific electroconductive base material, foil shape, plate shape, etc. can be illustrated. A foil shape is preferable from the viewpoint of reducing the volume of the battery and improving the degree of freedom in shape.

  Examples of the material of the binder include, for example, polyvinylidene fluoride (PVdF) resin, fluorine resin such as polytetrafluoroethylene, polyvinyl alcohol resin, polyimide resin, polyamide resin, polyamideimide resin, styrene butadiene rubber (SBR), and polyacrylic acid. Etc. can be used suitably. These can be used alone or in combination of two or more. Among these, a polyimide resin is particularly preferable because it has high mechanical strength, can withstand the volume expansion of the active material, and prevents the conductive film from being peeled off from the current collector due to the destruction of the binder.

  Examples of the conductive aid include carbon black such as ketjen black, acetylene black and furnace black, graphite, carbon nanotube, fullerene and the like. One or more of these may be used in combination. Of these, ketjen black, acetylene black, and the like can be preferably used from the viewpoint of easily ensuring electron conductivity.

  The content of the conductive auxiliary is preferably 0 to 30 parts by mass, more preferably 4 to 13 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoints of the degree of conductivity improvement, electrode capacity, and the like. It is good to be within the range. The average particle diameter (d50) of the conductive aid is preferably 10 nm to 1 μm, more preferably 20 to 50 nm, from the viewpoints of dispersibility and ease of handling.

  As the above-mentioned aggregate, a material that does not expand or contract during charging / discharging or that has a very small expansion / contraction can be suitably used. For example, graphite, alumina, calcia, zirconia, activated carbon and the like can be exemplified. One or more of these may be used in combination. Of these, graphite and the like can be preferably used from the viewpoints of conductivity, Li activity, and the like.

  The content of the aggregate is preferably 10 to 400 parts by mass, more preferably 43 to 100 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoint of improving cycle characteristics and the like. And good. The average particle diameter of the aggregate is preferably 10 to 50 μm, more preferably 20 to 30 μm, from the viewpoints of functionality as an aggregate and control of the electrode film thickness. The average particle diameter of the aggregate is a value measured using a laser diffraction / scattering particle size distribution measuring apparatus.

  For example, the negative electrode is made into a paste by adding a necessary amount of the negative electrode active material, if necessary, a conductive additive and an aggregate to a binder dissolved in an appropriate solvent, and this is applied to the surface of the conductive substrate. It can be produced by coating, drying, and applying consolidation or heat treatment as necessary.

  When a lithium ion battery is configured using this negative electrode, the positive electrode, electrolyte, separator, etc., which are basic components of the battery other than the negative electrode, are not particularly limited.

Specific examples of the positive electrode include, for example, a material in which a layer containing a positive electrode active material such as LiCoO ₂ , LiNiO ₂ , LiFePO ₄ , LiMnO ₂ is formed on the surface of a current collector such as an aluminum foil. Can do.

  Specific examples of the electrolyte include an electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent. In addition, a polymer in which a lithium salt is dissolved in a polymer, a polymer solid electrolyte in which a polymer is impregnated with the electrolytic solution, and the like can be used.

  Specific examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. These may be contained alone or in combination of two or more.

Specific examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiAsF _{6, and the} like. These may be contained alone or in combination of two or more.

  Other battery components include separators, cans (battery cases), gaskets, and the like. Any of these may be used as long as they are usually employed in lithium ion batteries. A battery can be formed by combining them.

  The battery shape is not particularly limited, and may be any shape such as a cylindrical shape, a square shape, or a coin shape, and can be appropriately selected according to the specific application.

  Hereinafter, the present invention will be described more specifically with reference to examples. Note that% of the alloy composition and the alloy mixing ratio is mass% unless otherwise specified.

1. Production of Negative Electrode Active Material Each raw material was weighed so as to have the alloy composition shown in Table 1. Each weighed raw material was heated and melted using a high frequency induction furnace to obtain a molten alloy.
Each obtained molten alloy was quenched using a single roll quenching method to obtain each quenched alloy ribbon. The roll peripheral speed was 42 m / s and the nozzle distance was 3 mm.
Each of the rapidly cooled alloy ribbons obtained was mechanically pulverized using a mortar to prepare powdery negative electrode active materials.

2. Structure observation of negative electrode active material, etc. Regarding the negative electrode active material according to each example and comparative example, the cross section of the quenched alloy ribbon was observed with a scanning electron microscope (SEM). The observation surface was polished using an ion polishing method. Moreover, the analysis by XRD (X-ray diffraction) was also performed, and the intrinsic peaks of Si and Si compounds described below were confirmed.

As a representative example of a negative electrode active material made of an Si—Zr alloy having a matrix phase made of Si phase and a Zr silicide phase dispersed in the matrix phase in this example, scanning of the negative electrode active material according to Example 3 A scanning electron micrograph is shown in FIG.
2 is a scanning electron of the negative electrode active material of Comparative Example 3 (Si—Fe alloy), FIG. 3 is Comparative Example 4 (Si—Ti alloy), and FIG. 4 is Comparative Example 5 (Si—Cu alloy). A photomicrograph was shown.

In the negative electrode active material according to Comparative Example 3 shown in FIG. 2, the black Si phase extends in a dendritic shape in the drawing, and a gray or gray + white Fe silicide phase is crystallized so as to surround the Si phase.
Next, in the negative electrode active material according to Comparative Example 4 shown in FIG. 3, the black bulk Si phase is dispersed in the drawing, and the Ti silicide phase is crystallized so as to surround the Si phase.
Further, in the negative electrode active material according to Comparative Example 5 shown in FIG. 4, a Cu silicide phase is crystallized so as to surround the black finely dispersed Si phase in the drawing.
As described above, Comparative Examples 3, 4 and 5, that is, the Si alloys other than the Si—Zr alloy, had a sea-island structure in which the Si phase was island-like and the silicide phase was sea-like.

  On the other hand, in the negative electrode active material according to Example 3 shown in FIG. 1, a large number of gray or gray + white flat Zr silicide phases are dispersed in the matrix phase composed of the black Si phase in the figure. Thus, it can be seen that the sea-island relationship is reversed from the above Comparative Examples 3, 4, and 5.

3. 3. Evaluation of negative electrode active material 3.1 Production of coin-type battery for charge / discharge test First, 80 parts by mass of each negative electrode active material powder adjusted to 25 μm or less by classification, 5 parts by mass of acetylene black as a conductive auxiliary agent, and binding 15 parts by mass of polyamic acid as an agent and N-methyl-2-pyrrolidone (NMP) as a solvent were mixed to prepare each paste containing each negative electrode active material.

  Subsequently, each paste was apply | coated to the surface of copper foil (thickness 18 micrometers) used as a negative electrode electrical power collector so that it might be set to 50 micrometers using a doctor blade, and it dried and formed each negative electrode active material layer.

  Next, the copper foil on which each negative electrode active material layer was formed was punched into a disk shape having a diameter of 11 mm to obtain each test electrode.

Next, a Li foil (thickness: 500 μm) was punched out in substantially the same shape as the above test electrode to prepare each counter electrode. In addition, LiPF ₆ was dissolved at a concentration of 1 mol / l in an equal mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) to prepare a nonaqueous electrolytic solution.

  Subsequently, Li foil as a counter electrode and the above-described test negative electrode as a test electrode were arranged via a porous separator containing polypropylene and polyethylene.

  Next, the non-aqueous electrolyte solution was poured into each can, and each negative electrode can and each positive electrode can were caulked and fixed to prepare each coin-type battery.

3.2 with each coin type battery charge and discharge test, a constant current charge-discharge current value 0.2mA performed one cycle, and the discharge capacity and the initial discharge capacity C _0. The second and subsequent cycles, the charge and discharge test was performed at 1 / 5C rate (C rate: the electrode (charging) the electric quantity C ₀ required to discharge in 1 hour (charge) and 1C a current value for discharging (If it is 5C, it will discharge (charge) in 12 minutes, and if it is 1 / 5C, it will discharge in 5 hours.) A value obtained by dividing the capacity (mAh) used at the time of discharge by the amount of active material (g) was defined as each discharge capacity (mAh / g).

Regarding the measured initial discharge capacity C ₀ , 600 (mAh / g) or more was evaluated as “◯”, and less than 600 was evaluated as “×”. The results are shown in Table 1.

  In this example, the cycle characteristics were evaluated by performing the charge / discharge cycle 50 times. Then, the capacity retention ratio (discharge capacity after 50 cycles / initial discharge capacity (discharge capacity at the first cycle) × 100) was determined from each obtained discharge capacity. When the capacity retention rate is 50% or more, “◯” is evaluated, and when it is less than 50%, “×” is evaluated. The results are also shown in Table 1.

The following can be understood from the results of Table 1 obtained as described above.
Although the comparative example 1 is a negative electrode active material which consists of a Si-Zr alloy, Si amount is excess rather than the upper limit 65 mass% of this invention. For this reason, the initial discharge capacity is good, but the amount of Zr silicide phase is reduced, and the capacity retention rate is evaluated as “x” and the cycle characteristics are poor.
On the other hand, in Comparative Example 2, the Si amount is smaller than the lower limit of 45% by mass of the present invention. Therefore, the capacity retention rate is good, but the initial discharge capacity is low, and the evaluation of the initial discharge capacity is “x”.

  On the other hand, Comparative Examples 3, 4 and 5 satisfy the specified range of the Si amount of the present invention and the initial discharge capacity is good, but the capacity retention rate is evaluated as “x” and the cycle characteristics are poor. In these comparative examples 3, 4 and 5, since the Si phase has an island-like structure and the silicide phase has a sea-like island structure, the stress generated when Si expands is applied to the silicide phase and the particles collapse. It is estimated that the cycle characteristics deteriorated.

  As described above, in each comparative example, the evaluation of either the initial discharge capacity or the capacity maintenance rate is “x”, and these two evaluation items cannot be achieved at the same time. In addition, when the graphite that has been widely used as a negative electrode active material is evaluated based on the above criteria, the capacity retention rate is “◯”, but the initial discharge capacity is low “×”, which is the same as each comparative example. Two evaluation items are not compatible.

  On the other hand, in each example of the present invention, the initial discharge capacity and the capacity maintenance rate are all “◯”, and these evaluation items can be compatible. When the Si phase is in a sea state as in these examples, the majority of the Si phase is located on the outermost surface, so the stress applied to the silicide phase during the Si phase expansion is reduced, and particle collapse is suppressed. Inferred. In addition, since the silicide phase arranged in an island shape does not expand, it is presumed that it plays the role of an aggregate that maintains the structure of the particles and suppresses the collapse of the particles.

  Although the negative electrode active material for a lithium ion battery and the lithium ion battery of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the spirit of the present invention. Modification is possible.

Claims (2)

Made of binary Si-Zr alloy,
A matrix phase composed of Si phase;
A Zr silicide phase dispersed in the matrix phase,
A negative electrode active material for a lithium ion battery, wherein the Si content is 45 to 65% by mass.   It has a negative electrode containing the negative electrode active material for lithium ion batteries of Claim 1, The lithium ion battery characterized by the above-mentioned.