WO2012008540A1

WO2012008540A1 - Silicon-alloy negative-electrode material exhibiting high electrical conductivity and manufacturing method therefor

Info

Publication number: WO2012008540A1
Application number: PCT/JP2011/066139
Authority: WO
Inventors: 友紀廣野; 哲朗仮屋; 柳本　勝
Original assignee: 山陽特殊製鋼株式会社
Priority date: 2010-07-16
Filing date: 2011-07-14
Publication date: 2012-01-19
Also published as: JP2012038708A

Abstract

Disclosed is a silicon-alloy negative-electrode material that exhibits high electrical conductivity and both high discharge capacity and a long cycle life. Said negative-electrode material is a powder comprising a composite phase consisting of: a silicon base phase, which is either a silicon phase or a partially-substituted silicon phase in which some silicon has been substituted by one or more elements selected from the group comprising carbon, germanium, tin, lead, aluminum, and phosphorus; and a SixCuy phase comprising an alloy that has the composition SixCuy (x being less than y), i.e. an intermetallic compound of silicon and copper. The silicon base phase comprises particles with a mean diameter of 10 µm or less, and the SixCuy phase encloses at least part of the silicon base phase.

Description

Si-based alloy negative electrode material having excellent electrical conductivity and method for producing the same

Cross-reference of related applications

This application has priority based on Japanese Patent Application No. 2010-161308 filed on July 16, 2010 and priority based on Japanese Patent Application No. 2011-105109 filed on May 10, 2011. And the entire disclosures of which are incorporated herein by reference.

The present invention relates to a Si-based alloy negative electrode material excellent in electrical conductivity of an electricity storage device such as a lithium ion secondary battery or a hybrid capacitor that accompanies movement of lithium ions during charge and discharge, and a method for producing the same.

In recent years, with the widespread use of portable devices, development of high-performance secondary batteries centering on lithium-ion batteries has been actively conducted. Furthermore, lithium-ion secondary batteries as hybrid electric storage devices for automobiles and home use and hybrid capacitors in which the reaction mechanism is applied to the negative electrode are also actively developed. As a negative electrode material for such electricity storage devices, carbonaceous materials such as natural graphite, artificial graphite, and coke that can occlude and release lithium ions are used.

However, because carbonaceous materials insert lithium ions between the C-planes, the theoretical capacity when used for the negative electrode is limited to 372 mAh / g, and the search for new materials to replace carbonaceous materials for the purpose of increasing capacity is required. Has been actively conducted.

On the other hand, Si has attracted attention as a material that can replace carbonaceous materials. The reason is that since Si can form a compound represented by Li ₂₂ Si ₅ and occlude a large amount of lithium, the capacity of the negative electrode can be greatly increased compared to the case where a carbonaceous material is used, As a result, there is a possibility that the storage capacity of the lithium ion secondary battery or the hybrid capacitor can be increased.

However, when Si is used alone as a negative electrode material, the Si phase is pulverized by repeated expansion when alloying with lithium during charging and contraction when dealloying with lithium during discharging. There is a problem that the lifetime of the electricity storage device is extremely short because problems such as the Si phase dropping off from the electrode substrate or the lack of electrical conductivity between the Si phases occur.

In addition, Si has poor electrical conductivity compared to carbonaceous materials and metal-based materials, and the efficient movement of electrons associated with charge / discharge is limited, so the negative electrode material supplements electrical conductivity such as carbonaceous materials. Although used in combination with materials, even in that case, initial charge / discharge characteristics and charge / discharge characteristics with high efficiency are also problems.

As a method for solving the drawbacks when using such a Si phase as a negative electrode, a material in which at least a part of a parent lithium phase such as Si is surrounded by an intermetallic compound of Si and a metal typified by a transition metal, or the like Manufacturing methods have been proposed. For example, Japanese Patent Laid-Open No. 2001-297757 (Patent Document 1) and Japanese Patent Laid-Open No. 10-31804 (Patent Document 2) are known.

As another solution, an electrode in which an active material phase including a Si phase is coated with an electrically conductive material such as Cu that does not alloy with lithium, and a method for manufacturing the same have been proposed. For example, Japanese Unexamined Patent Application Publication No. 2004-228059 (Patent Document 3) and Japanese Unexamined Patent Application Publication No. 2005-44672 (Patent Document 4) are known.

JP 2001-297757 A JP 10-31804 A JP 2004-228059 A JP 2005-44672 A

However, in the above-described method of coating the active material phase with an electrically conductive material such as Cu, it is necessary to coat the active material containing the Si phase with a method such as plating before or after the step of forming the active material on the electrode. In addition, there is a problem that it takes time and labor from an industrial point of view such as control of the coating film thickness.

In addition, a material in which at least a part of a parent lithium phase such as Si is surrounded by an intermetallic compound is an industrially preferable process because a parent lithium phase and an intermetallic compound are formed during a solidification process after melting. In the proposed combination of elements, most of the intermetallic compounds that are in equilibrium with the Si phase become Si-rich compounds that are inferior in electrical conductivity. There was a drawback of poor discharge characteristics. In addition, there is no proposal related to the composition of an intermetallic compound excellent in electrical conductivity that can solve these problems.

As the intermetallic compound that surrounds the Si phase, the present inventors have formed an intermetallic compound with a Cu element, among other intermetallic compounds with the Si phase, to form SiCu ₃ that is particularly excellent in electrical conductivity. I found out. Moreover, the present inventors have found that a composite phase consisting of Si phase and SiCu ₃ alloy, utilizing the large discharge capacity of Si, and, SiCu ₃ with excellent inherent low electrical conductivity Si electrical conductivity It has also been found that both the discharge capacity and the cycle life are improved when the average particle size of the Si phase is 10 μm or less.

Therefore, an object of the present invention is to make it possible to provide a secondary negative electrode material having both excellent discharge capacity and cycle life and excellent electrical conductivity.

According to one aspect of the present invention, a Si-based alloy negative electrode material having excellent electrical conductivity, wherein the negative electrode material is
Si base phase, which is a partially substituted Si phase in which the Si phase or a part of Si is substituted with one or more elements selected from the group consisting of C, Ge, Sn, Pb, Al and P When,
A SixCuy phase made of an alloy having a composition of SixCuy (where x <y), which is an intermetallic compound of Si and Cu;
A powder composed of a composite phase of
There is provided a Si-based alloy negative electrode material in which the Si base phase has a particle shape having an average particle size of 10 μm or less, and at least a part of the Si base phase is surrounded by a SixCuy phase.

According to another aspect of the present invention, there is provided a method for producing a Si-based alloy negative electrode material having excellent electrical conductivity,
Melting a SiCu-based alloy having the overall composition of the negative electrode material to form a molten metal;
A step of quenching the molten metal by a gas atomizing method, a disk atomizing method or a liquid quenching method;
A manufacturing method is provided comprising:

It is a figure which shows the phase diagram of Si-Cu binary system. It is a figure which shows the cross-sectional SEM image of Si-Cu alloy powder.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 shows a phase diagram of the Si—Cu binary system. As shown in this figure, when the Si—Cu alloy melt is cooled, Si precipitates as the primary crystal when the liquidus temperature is reached (eg, 1200 ° C. in the case of Si: 64 atomic% —Cu: 36 atomic%). Begin to. This primary crystal precipitates as a granular crystal if the cooling rate is high as in the liquid quenching method or the atomizing method, and when the temperature reaches the solidus temperature (802 ° C.), a eutectic reaction between Si and SiCu ₃ occurs and solidification is completed. To do. Thus, in the phase diagram on the Si rich side, it is a eutectic reaction between the Si phase and the SiCu ₃ phase, and the Si phase surrounds the SiCu ₃ phase.

On the other hand, combinations of elements for alloying other than Cu and Si include, for example, Fe—Si, Ni—Si, Mn—Si, Co—Si, Cr—Si, Si—W, Mo—Si, Nb—Si, and Si. -Ti, Si-V, etc. are conceivable. However, it will both _{_{_{FeSi 2, NiSi 2, CoSi 2}}} , CrSi 2, WSi 2, MoSi 2, MnSi 2, NbSi 2, TiSi 2, VSi 2 and the Si-rich composition remains than metal elements .

The combination of Si and the transition element described above equilibrates with the Si phase as the only Cu-rich compound (SiCu ₃ ). When the resistance value of this Cu-rich compound (SiCu ₃ ) is examined, it is found that SiCu ₃ : 16.3 × 10 ⁻⁴ Ω · m, similarly FeSi ₂ : 1000 × 10 ⁻⁴ Ω · m, NiSi ₂ : 50 × It can be seen that 10 ⁻⁴ Ω · m, CoSi ₂ : 18 × 10 ⁻⁴ Ω · m and SiCu ₃ have a lower resistance value than other silicide compounds.

There are two factors that have the lowest resistance value of SiCu ₃ , and the first is that SiCu ₃ has a metal-rich composition compared to other silicide compounds. Secondly, when focusing on the transition metal element of the raw material, Cu: 1.73 × 10 ⁻⁴ Ω · m, Fe: 10 × 10 ⁻⁴ Ω · m, Ni: 11.8 × 10 ⁻⁴ Ω · m , Co: 9.71 × 10 ⁻⁴ Ω · m, and simple substance Cu had a very low resistance value compared to other transition metal elements, and was a combination of Si and the transition metal having the lowest resistance value It is.

As can be seen from the above, the combination of Si and transition metal element having the lowest resistance value among the transition metal silicide compounds is Si and Cu. This is because Si, which is a raw material of a transition metal silicide compound, has an extremely low resistance value compared to other simple transition metal elements, and is never obtained by a combination of transition metal elements of Si phase and Si. This is because it is possible to form a metal-rich compound phase (SixCuy (x <y)), for example, an SiCu ₃ phase between Cu and Cu elements. Thus, since the resistance value is the lowest, SiCu ₃ is an Si-rich intermetallic compound (FeSi ₂ , NiSi ₂ , CoSi ₂ , CrSi ₂ , WSi ₂ , MoSi ₂ , MnSi ₂ , NbSi ₂ , TiSi ₂ , It can be seen that the electric conductivity is higher than that of VSi ₂ ).

From the above, it can be seen that only Cu in a combination of Si and a transition metal element precipitates an Si phase and a metal-rich compound (SiCu ₃ ) phase by a eutectic reaction, and this SiCu ₃ is Si— From the Cu binary phase diagram, it is also known that in a Si-rich composition (for example, Si: 64 atomic% -Cu: 36 atomic%), the Si phase surrounds the SiCu ₃ phase. This allows the SiCu ₃ phase to precipitate around the Si phase, which has a much higher electrical conductivity than the combination of Si and other transition metal elements, so that the SiCu ₃ phase plays a role in supplementing the poor electrical conductivity of Si. He will do it.

Further, since the SiCu ₃ phase is not alloyed with lithium, the SiCu ₃ phase itself does not expand or contract even if it is repeatedly charged (lithium enters the negative electrode) -discharge (lithium comes out from the negative electrode). On the contrary, since the SiCu ₃ phase has a lower hardness than Si, it can be a phase that relieves stress due to a large volume expansion and contraction of Si caused by the reaction between Si and lithium.

Further, the average particle size of the Si phase or Si phase having Si as the main phase is 10 μm or less, preferably 8 μm or less, more preferably 5 μm or less, still more preferably 3 μm or less, and most preferably 2 μm or less. This is because if the average particle size is large, the cycle life is reduced. The reaction between Si and lithium occurs at the contact portion of the electrolyte. In the present specification, “average particle diameter” refers to a number-based D50 particle diameter. For large Si particles having a maximum particle size exceeding 10 μm, the reaction with lithium during the initial charging reaction stops only at the surface part of the Si particle in contact with the electrolytic solution, and it takes time until the electrolytic solution penetrates. The internal reaction will not be performed.

Then, it becomes unable to withstand the stress caused by the volume expansion and contraction between the Si surface and the interior caused by the insertion reaction of lithium into the initial Si, the surface Si is cracked, and the Si peels off from the current collector, By becoming an electrically isolated Si island that cannot be removed, those Si cannot be used from the next cycle. At that time, depending on the cracking method, there is a risk of peeling from the current collector while containing unreacted Si or becoming an electrically isolated state where current cannot be collected. Furthermore, when the surface Si disappears and a new unreacted Si surface appears, the above phenomenon is repeated, and the capacity rapidly decreases in the initial few cycles.

From the above, it is known that when the average particle size of the Si phase is large, the reaction with lithium stops only in the surface layer portion of the Si particle in contact with the electrolyte during the initial charging reaction. Therefore, measures are taken to increase the Si surface area with which the electrolytic solution comes into contact in advance by refining the Si phase and increasing the specific surface area of the reacting Si. This increases the reaction rate between the initial lithium and Si, refines it to a size that eliminates unreacted Si, prevents Si from peeling off the current collector, and preventing electrically isolated Si islands that cannot collect current, The rapid decrease in capacity in the initial few cycles due to the repetition of the above-described Si peeling and electrical isolation phenomenon is improved. Therefore, the upper limit is set to 10 μm. The lower the lower limit of the average particle diameter, the better.

Moreover, Si is a main phase, and is a group of phases composed of one or more elements that can reversibly combine and segregate with Li. In this phase, one or two or more elements selected from the group consisting of C, Ge, Sn, Pb, Al, and P, which are such elements, are substituted as a part of Si and partially substituted Si phase may be used. When these elements form a substitutional solid solution, the composition ratio is not particularly limited, but the ratio of C, Ge, Sn, Pb, Al, and P is replaced by Si when Si is 1 when these are M. The total amount of M is preferably less than 0.5, more preferably less than 0.2, even more preferably less than 0.1, and most preferably less than 0.05.

Further, in the SixCuy alloy, which is an alloy of Si and Cu that forms an intermetallic compound, the composition of the SixCuy phase needs to be x <y. For example, FeSi ₂ does not become Fe rich. Since an alloy of Fe element and Si forms a Si-rich compound phase, the electrical conductivity is inferior, and the electrical conductivity between Si phases is prevented from being lowered due to the refinement of Si caused by repeated charge and discharge. Therefore, the composition of the SixCuy phase is set to x <y. Preferably, x = 1 and y = 3.

FIG. 2 shows a cross-sectional SEM image of the Si—Cu alloy powder. As shown in this figure, the black portion is the embedded resin 1, the gray portion is the Si phase 2, and the white portion is the SiCu ₃ phase 3. Focusing particularly on the central Si—Cu particles, the gray Si phase 2 is surrounded by the white SiCu ₃ phase in the portion A inside the particles. However, it can be seen that in the portion B of the particle surface portion, the gray Si phase 2 is exposed on the particle surface. Thus, at least a part of the Si phase is surrounded by the SixCuy phase.

The negative electrode material of the present invention as a whole is one or more selected from the group consisting of 10 to 50% Cu, 0 to 10% C, Ge, Sn, Pb, Al and P in at%. , And the balance Si and inevitable impurities, more preferably selected from the group consisting of 18-36% Cu, 0-3% C, Ge, Sn, Pb, Al and P It has the composition which consists of 1 type (s) or 2 or more types and balance Si and an unavoidable impurity. These preferable overall compositions also apply to the SiCu-based alloy as a starting material for producing the negative electrode material of the present invention.

Hereinafter, the present invention will be specifically described based on examples. A negative electrode material powder having the composition shown in Table 1 was prepared by a liquid quenching method, a gas atomizing method, or a disk atomizing method as described below. For the liquid quenching method, a raw material having a predetermined composition is placed in a quartz tube having pores at the bottom, melted at a high frequency in an Ar atmosphere to form a molten metal, and after the molten metal is discharged on the surface of a rotating copper roll, the copper roll A quenching ribbon was produced by the quenching effect. Thereafter, the produced ribbon was sealed in a zirconia pot container together with zirconia balls in an Ar atmosphere, and powdered by mechanical milling.

For the gas atomization method, a raw material of a predetermined structure is placed in a quartz crucible having pores at the bottom, heated and melted in a high-frequency induction melting furnace in an Ar gas atmosphere, and then gas is injected and discharged in the Ar gas atmosphere, followed by rapid cooling. The desired gas atomized fine powder was obtained by solidification. For the disk atomization method, a raw material having a predetermined structure is placed in a quartz crucible having pores at the bottom, heated and melted in a high-frequency induction melting furnace in an Ar gas atmosphere, and then on a rotating disk (40000 to 60000 r) in an Ar gas atmosphere. P.m.) and then rapidly solidified by cooling to obtain the desired disc atomized fine powder.

In order to evaluate the electrode performance of the negative electrode with a single electrode, a so-called bipolar coin-type cell using lithium metal as a counter electrode was used. First, a negative electrode active material (Si—Cu, etc.), a conductive material (acetylene black), and a binder (polyvinylidene fluoride) are weighed with an electronic balance and mixed with a dispersion (N-methylpyrrolidone). And uniformly applied on the current collector (Cu foil). After coating, the solvent was evaporated by drying under reduced pressure with a vacuum dryer, and then punched into a shape suitable for a coin cell. Similarly, lithium for the counter electrode was punched into a shape suitable for the coin cell.

A 1: 7 mixed solvent of ethylene carbonate and dimethyl carbonate was used, and 1 mol of LiPF ₆ (lithium hexafluorophosphate) was dissolved in the electrolyte as a supporting electrolyte to prepare an electrolyte for use in a lithium ion battery. . Since this electrolyte solution must be handled in an inert atmosphere with dew point control, all the cells were assembled in a glove box with an inert atmosphere. The separator was cut out into a shape suitable for a coin cell, and then held in the electrolyte for several hours under reduced pressure in order to sufficiently permeate the electrolyte into the separator. Thereafter, the negative electrode, separator, and counter electrode lithium punched in the previous step were combined in that order, and the battery was fully filled with the electrolyte.

Using the bipolar cell, the charge capacity and discharge capacity were measured at a temperature of 25 ° C. Charging is performed at a current density of 0.50 mA / cm ² until the potential is equal to that of the metal lithium electrode (0 V), while discharging is performed up to 1.5 V at the same current value (0.50 mA / cm ² ). One cycle of charge-discharge. The charge capacity at the first cycle at this time was evaluated as the initial capacity value. Further, as the cycle life, the discharge capacity of the first cycle was measured, and the discharge capacity of the negative electrode using the negative electrode material was measured. The discharge capacity of the 100th cycle was measured, and used as a measure of the cycle life.

As shown in Table 1 and Table 2, Nos. 1 to 14 are examples of the present invention. 15 to 26 show comparative examples.

Invention Example No. 1 to 14 are powders composed of a composite phase of a SixCuy phase composed of a SixCuy alloy which is an intermetallic compound of Si phase and Si and Cu, and the composition of the SixCuy phase is x <y, and Si is the main phase. Since the average particle size of the Si phase is 10 μm or less, the condition of the present invention is satisfied. Moreover, the discharge capacity of the 1st cycle showed 1000 mAh / g or more. In addition, due to the improvement in electrical conductivity by the intermetallic compound phase SixCuy phase where x <y, the discharge capacity after 100 cycles was 372 mAh / g or more, which is the capacity of the current graphite electrode.

Comparative Example No. Nos. 15 to 16 are examples of the present invention. As in 1-2, it is a powder composed of a composite phase of a SixCuy phase composed of a SixCuy alloy which is an intermetallic compound of Si phase and Si and Cu, and the composition of the SixCuy phase is x <y. The average particle size of the Si phase used as the phase is Comparative Example No. 15 and 12 μm, Comparative Example No. 16 exceeds 20 μm and 10 μm, so the conditions of the present invention are not satisfied. Also, in the charge / discharge capacity, the discharge capacity in the first cycle is the same as that in Comparative Example No. 15 at 1080 mAh / g, Comparative Example No. 16 showed 1090 mAh / g and 1000 mAh / g or more, but the discharge capacity after 100 cycles was Comparative Example No. 15 and 200 mAh / g, Comparative Example No. 16 was 170 mAh / g, which was lower than the current capacity of the graphite electrode, 372 mAh / g.

Comparative Example No. Nos. 17 to 26 are powders composed of a composite phase of a SixCuy phase composed of a SixCuy alloy that is an intermetallic compound of a Si phase and Si and Cu, although the average particle size of the Si phase having Si as a main phase is 10 μm or less. Therefore, the conditions of the present invention are not satisfied. Further, the discharge capacity at the first cycle was less than 1000 mAh / g. Moreover, the electrical conductivity is inferior due to the intermetallic compound phase SixMy phase (M = Ni, Fe, Mn, Co, Cr, W, Mo, Nb, Ti, V) where x> y, and the discharge capacity after 100 cycles is The current capacity of the graphite electrode was less than 372 mAh / g.

As described above, the metal-rich SiCu ₃ phase is excellent in electrical conductivity, and further has an excellent effect of improving both the charge / discharge capacity and the charge / discharge life due to the synergistic effect of making the average particle size of the Si phase 10 μm or less. It plays. In addition, an alloy is melted to form a molten metal, and the molten metal is quenched by a gas atomizing method, a disk atomizing method, or a liquid quenching method, thereby enabling a method for producing a Si-based alloy negative electrode material having excellent electrical conductivity. .

Claims

Si-based alloy negative electrode material excellent in electrical conductivity, the negative electrode material,
Si base phase, which is a partially substituted Si phase in which the Si phase or a part of Si is substituted with one or more elements selected from the group consisting of C, Ge, Sn, Pb, Al and P When,
A SixCuy phase made of an alloy having a composition of SixCuy (where x <y), which is an intermetallic compound of Si and Cu;
A powder composed of a composite phase of
A Si-based alloy negative electrode material, wherein the Si base phase has a particle shape with an average particle size of 10 μm or less, and at least a part of the Si base phase is surrounded by a SixCuy phase.
The negative electrode material according to claim 1, wherein the composition of the SixCuy is SiCu 3 .
The Si base phase is a partially substituted Si phase in which a part of Si is substituted with one or more elements selected from the group consisting of C, Ge, Sn, Pb, Al and P. The negative electrode material according to claim 1.
The negative electrode material according to any one of claims 1 to 3, wherein the initial discharge capacity is 1000 mAh / g or more and the discharge capacity at the 100th cycle is 372 mAh / g or more.
The negative electrode material as a whole is at%,
Cu: 10 to 50%,
One or more selected from the group consisting of C, Ge, Sn, Pb, Al and P: 0 to 10%, and the composition consisting of the balance Si and inevitable impurities The negative electrode material according to one item.
The negative electrode material as a whole is at%,
Cu: 18 to 36%,
One or more selected from the group consisting of C, Ge, Sn, Pb, Al and P: 0 to 3%, and the composition consisting of the balance Si and inevitable impurities The negative electrode material according to one item.
A method for producing a Si-based alloy negative electrode material having excellent electrical conductivity,
Melting a SiCu base alloy having the overall composition of the negative electrode material according to any one of claims 1 to 6 to form a molten metal;
A step of quenching the molten metal by a gas atomizing method, a disk atomizing method or a liquid quenching method;
A manufacturing method comprising:
The SiCu-based alloy is at%,
Cu: 10 to 50%,
The method according to claim 7, comprising one or more selected from the group consisting of C, Ge, Sn, Pb, Al and P: 0 to 10%, and the balance Si and inevitable impurities.
The SiCu-based alloy is at%,
Cu: 18 to 36%,
The method according to claim 7, comprising one or more selected from the group consisting of C, Ge, Sn, Pb, Al and P: 0 to 3%, and the balance Si and inevitable impurities.