JP4081676B2 - Anode material for non-aqueous electrolyte secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a negative electrode material for non-aqueous electrolyte secondary battery using the silicon composite which is useful as a negative electrode active material for a lithium ion secondary battery.
[0002]
[Prior art]
In recent years, with the remarkable development of portable electronic devices, communication devices, etc., secondary batteries with high energy density are strongly demanded from the viewpoints of economy and downsizing and weight reduction of devices. Conventionally, as a measure for increasing the capacity of this type of secondary battery, for example, a method of using an oxide such as V, Si, B, Zr, Sn, or a composite oxide thereof as a negative electrode material (for example, Patent Document 1: Kaihei 5-174818, Patent Document 2: Japanese Patent Laid-Open No. 6-60867, and a method of applying a melt-quenched metal oxide as a negative electrode material (for example, see Patent Document 3: Japanese Patent Laid-Open No. 10-294112) Further, a method using silicon oxide as a negative electrode material (for example, Patent Document 4: Japanese Patent No. 2997741), a method using Si ₂ N ₂ O and Ge ₂ N ₂ O as a negative electrode material (for example, Patent Document 5: Japanese Patent Laid-Open No. 11-133). -102705) and the like are known.
[0003]
However, in the above conventional method, although the charge / discharge capacity is increased and the energy density is increased, the cycleability is insufficient, or the required characteristics of the market are still insufficient, and are not always satisfactory. However, further improvement in energy density has been desired.
In particular, in Japanese Patent No. 2997741 (Patent Document 4), silicon oxide is used as a negative electrode material for a lithium ion secondary battery to obtain a high-capacity electrode. There is room for improvement because the irreversible capacity at the time of discharge is large and the cycle performance has not reached the practical level.
[0004]
[Patent Document 1]
JP-A-5-174818 [Patent Document 2]
Japanese Patent Laid-Open No. 6-60867 [Patent Document 3]
JP-A-10-294112 [Patent Document 4]
Japanese Patent No. 2999741 [Patent Document 5]
Japanese Patent Laid-Open No. 11-102705
[Problems to be solved by the invention]
This invention is made in view of the said situation, and provides the negative electrode material for nonaqueous electrolyte secondary batteries using the silicon composite which enables manufacture of the negative electrode of a lithium ion secondary battery with higher cycle property. With the goal.
[0006]
Means for Solving the Problem and Embodiment of the Invention
As a result of intensive studies to achieve the above object, the present inventor has found a silicon composite that is effective as an activator for a non-aqueous electrolyte secondary battery negative electrode with higher cycleability.
In other words, the development of electrode materials having a large charge / discharge capacity is extremely important, and research and development are being conducted in various places. Under such circumstances, silicon and amorphous silicon oxide (SiO _x ) as negative electrode active materials for lithium ion secondary batteries are of great interest because of their large capacity, but were repeatedly charged and discharged. Since the deterioration at the time is large, that is, the cycle property is inferior, and the initial efficiency is particularly low, it has not been put into practical use except for a few.
[0007]
The present inventors have studied from the structure itself about the cause of a sudden decrease in charge / discharge capacity after many times of charge / discharge when this silicon oxide (SiO _x ) is used as a negative electrode active material for a lithium ion secondary battery. As a result of analysis, it was found that a large volume change occurs due to insertion and extraction of a large amount of lithium, and this is accompanied by particle destruction.
Therefore, based on these facts, as a result of intensive studies on a structure that is stable with respect to volume changes associated with insertion and extraction of lithium, silicon microcrystals or fine particles are converted into an inert and strong substance such as silicon dioxide. Dispersion can solve the above problems as a negative electrode active material for a lithium ion secondary battery, stably have a large capacity charge / discharge capacity, and greatly improve charge cycle performance and efficiency. I found what I could do. Therefore, it has been found that it is effective to finely disperse silicon microcrystals and / or fine particles in a silicon compound, for example, silicon dioxide, and the present invention has been made.
[0008]
Accordingly, the present invention provides a negative electrode material for a non-aqueous electrolyte secondary battery using the following silicon composite.
(1) crystallites of silicon particles having a dispersed structure in silicon-based compounds (excluding those obtained by coating the surface of particles with carbon) non-aqueous electrolyte secondary battery using the silicofluoride-containing complex Negative electrode material .
(2) Particles having a structure in which silicon microcrystals are dispersed in a silicon-based compound (excluding those obtained by coating the surface of particles with carbon) and a mixture of a silicon composite and a conductive agent, A negative electrode material for a non-aqueous electrolyte secondary battery using a mixture having a conductive agent content of 1 to 60% by weight in the mixture.
( 3 ) The negative electrode material for a nonaqueous electrolyte secondary battery according to (1) or (2), which has an average particle size of 0.01 to 30 μm and a BET specific surface area of 0.5 to ²⁰ m ² / g.
( 4 ) The size of the silicon microcrystal is 1 to 500 nm, and the silicon-based compound is silicon dioxide. (1) To the nonaqueous electrolyte secondary battery according to any one of (3), Negative electrode material .
( 5 ) In the X-ray diffraction of the silicon composite, a diffraction peak attributed to Si (111) is observed, and the silicon crystal size determined by the Scherrer method based on the half width of the diffraction line is wherein the 1~500nm those of (1) to (4) a non-aqueous electrolyte secondary battery negative electrode material according to any one of.
( 6 ) Any one of (1) to (5) , wherein the silicon composite is obtained by disproportionating silicon oxide in an inert gas atmosphere in a temperature range of 900 to 1400 ° C. Negative electrode material for non-aqueous electrolyte secondary battery .
( 7 ) A silicon oxide powder represented by the general formula SiO _x (1.0 ≦ x <1.6) having an average particle diameter of 0.01 to 30 μm and a BET specific surface area of 0.1 to 30 m ² / g. Oh Ru (6) negative electrode material for a non-aqueous electrolyte secondary battery as claimed.
[0009]
Hereinafter, the present invention will be described in more detail.
The present invention, when used as a negative electrode active material for a lithium ion secondary battery, is expected from the fact that the charge / discharge capacity is several times that of the current mainstream graphite-based materials, but it is repeatedly The present invention relates to a silicon composite in which the cycle performance and efficiency of a silicon-based material in which performance degradation due to charge / discharge of silicon is a major bottleneck has been improved. The silicon composite has silicon microcrystals, preferably silicon dioxide. It has a structure dispersed in.
[0010]
In this case, the silicon composite of the present invention preferably has the following properties.
i. In X-ray diffraction (Cu-Kα) using copper as the counter-cathode, a diffraction peak attributed to Si (111) centered around 2θ = 28.4 ° is observed, and based on the broadening of the diffraction line The particle diameter of the silicon crystal determined by the Scherrer equation is preferably 1 to 500 nm, more preferably 2 to 200 nm, and still more preferably 2 to 20 nm. If the size of the silicon fine particles is smaller than 1 nm, the charge / discharge capacity may be reduced. Conversely, if the silicon fine particle is larger than 500 nm, the expansion / contraction during charge / discharge increases, and the cycle performance may decrease. The size of the silicon fine particles can be measured by a transmission electron micrograph.
ii. In solid-state NMR ( ²⁹ Si-DDMAS) measurement, there is a peak characteristic of Si diamond crystals in the vicinity of −84 ppm, along with a broad silicon dioxide peak whose spectrum is centered around −110 ppm. This spectrum is completely different from ordinary silicon oxide (SiO _x : x = 1.0 + α), and the structure itself is clearly different. Further, it is confirmed by transmission electron microscope that silicon crystals are dispersed in amorphous silicon dioxide.
[0011]
The dispersion amount of silicon fine particles in the silicon / silicon dioxide dispersion is preferably about 2 to 36% by weight, particularly about 10 to 30% by weight. If the amount of dispersed silicon is less than 2% by weight, the charge / discharge capacity may be reduced, and conversely if it exceeds 36% by weight, the cycle performance may be inferior.
[0012]
The average particle diameter of the silicon composite powder of the present invention is 0.01 μm or more, more preferably 0.1 μm or more, still more preferably 0.2 μm or more, particularly preferably 0.3 μm or more, and the upper limit is 30 μm or less, more preferably Is preferably 20 μm or less, more preferably 10 μm or less. If the average particle size is too small, the bulk density will be too small, and the charge / discharge capacity per unit volume will decrease. Conversely, if the average particle size is too large, it will be difficult to produce an electrode film, There is a risk of peeling. The average particle diameter is a value measured as a weight average value D ₅₀ (that is, a particle diameter or a median diameter when the cumulative weight is 50%) in particle size distribution measurement by a laser light diffraction method.
[0013]
The BET specific surface area of the silicon composite powder of the present invention is preferably 0.5 to ²⁰ m ² / g, particularly 1 to 10 m ² / g. When the BET specific surface area is less than 0.5 m ² / g, the surface activity is reduced, the binding force of the binder during electrode production is reduced, and as a result, the cycle performance when charging and discharging are repeated may be reduced. On the other hand, if the BET specific surface area is larger than 20 m ² / g, the amount of absorption of the solvent becomes large at the time of producing the electrode, and a large amount of the binder may be added to maintain the binding property. The cycle performance may be reduced, and the cycle performance may be reduced. The BET specific surface area is a value measured by the BET one-point method which is measured by the N ₂ gas adsorption amount.
[0014]
The silicon composite of the present invention can be used as it is as an end product without coating the particle surface with other substances, and the particle surface is not particularly coated with carbon. The silicon composite of which the surface of the present invention is not coated with other substances such as carbon is repeatedly charged and discharged under relatively mild conditions that are less than the full reversible capacity of, for example, a button-type lithium secondary battery. In such applications, it is particularly useful as a negative electrode material having excellent cycle characteristics.
[0015]
Next, the manufacturing method of the silicon composite in this invention is demonstrated.
The silicon composite powder of the present invention is a particle having a structure in which silicon microcrystals are dispersed in a silicon-based compound, and preferably has an average particle diameter of about 0.01 to 30 μm. Although not particularly limited, for example, the following method can be suitably employed.
A method in which silicon oxide powder represented by the general formula SiO _x (1.0 ≦ x <1.6) is subjected to heat treatment in an inert gas atmosphere at a temperature range of 900 to 1400 ° C. to disproportionate.
[0016]
In the present invention, silicon oxide is a general term for amorphous silicon oxide obtained by cooling and precipitating silicon monoxide gas generated by heating a mixture of silicon dioxide and metal silicon. The silicon oxide powder used in the present invention is represented by the general formula SiO _x , and the average particle diameter is 0.01 μm or more, more preferably 0.1 μm or more, still more preferably 0.5 μm or more, and the upper limit is 30 μm or less. Preferably it is 20 micrometers or less. The BET specific surface area is 0.1 m ² / g or more, more preferably 0.2 m ² / g or more, and the upper limit is preferably 30 m ² / g or less, more preferably 20 m ² / g or less. The range of x is desirably 1.0 ≦ x <1.6, more preferably 1.0 ≦ x ≦ 1.3, and still more preferably 1.0 ≦ x ≦ 1.2. When the average particle diameter and BET specific surface area of silicon oxide powder are outside the above ranges, a silicon composite powder having a desired average particle diameter and BET specific surface area cannot be obtained, and production of SiO _x powder having a value of _x smaller than 1.0 When the value of x is 1.6 or more, the proportion of inactive SiO ₂ is large when heat treatment is performed and the disproportionation reaction is performed, and it is used as a lithium ion secondary battery. In such a case, the charge / discharge capacity may be reduced.
[0017]
On the other hand, in disproportionation of silicon oxide, if the heat treatment temperature is lower than 900 ° C., disproportionation does not proceed at all or it takes an extremely long time to form fine silicon cells (silicon microcrystals), which is efficient. On the other hand, if the temperature is higher than 1400 ° C., the structure of the silicon dioxide portion is advanced and the lithium ion traffic is hindered, so that the function as the lithium ion secondary battery may be deteriorated. More preferably, the heat treatment temperature is 1000 to 1300 ° C, particularly 1100 to 1250 ° C. The treatment time (disproportionation time) can be appropriately controlled in the range of about 10 minutes to 20 hours, particularly about 30 minutes to 12 hours, depending on the disproportionation treatment temperature. For example, at a treatment temperature of 1100 ° C. Is preferably about 5 hours.
[0018]
The disproportionation treatment is not particularly limited as long as a reaction apparatus having a heating mechanism is used in an inert gas atmosphere, and treatment by a continuous method or a batch method is possible, specifically a fluidized bed reaction. A furnace, a rotary furnace, a vertical moving bed reaction furnace, a tunnel furnace, a batch furnace, a rotary kiln, and the like can be appropriately selected according to the purpose. In this case, as the (treatment) gas, an inert gas alone or a mixed gas thereof such as Ar, He, H ₂ , N ₂ or the like can be used.
[0019]
The silicon composite powder obtained in the present invention is used as a negative electrode material (negative electrode active material) to produce a non-aqueous electrolyte secondary battery, particularly a lithium ion secondary battery, having high capacity and excellent cycle characteristics. be able to.
In this case, the obtained lithium ion secondary battery is characterized in that the negative electrode active material is used, and other materials such as positive electrode, negative electrode, electrolyte, separator, and battery shape are not limited. For example, as the positive electrode active material, oxides of transition metals such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , MnO ₂ , TiS ₂ , and MoS ₂ , chalcogen compounds, and the like are used. As the electrolyte, for example, a non-aqueous solution containing a lithium salt such as lithium perchlorate is used, and as the non-aqueous solvent, propylene carbonate, ethylene carbonate, dimethoxyethane, γ-butyrolactone, 2-methyltetrahydrofuran or the like is used alone or 2 Used in combination of more than one type. Various other non-aqueous electrolytes and solid electrolytes can also be used.
[0020]
In addition, when producing a negative electrode using the said silicon composite powder, since silicon composite itself does not have electroconductivity, it is necessary to add electrically conductive agents, such as graphite, to silicon composite powder. Also in this case, the type of the conductive agent is not particularly limited, and any conductive material that does not cause decomposition or alteration in the configured battery may be used. Specifically, Al, Ti, Fe, Ni, Cu, Metal powder and metal fiber such as Zn, Ag, Sn, Si, natural graphite, artificial graphite, various coke powders, mesophase carbon, vapor grown carbon fiber, pitch carbon fiber, PAN carbon fiber, various resin firing Graphite such as a body can be used.
[0021]
Here, the addition amount of the conductive agent is preferably 1 to 60% by weight, particularly 10 to 50% by weight, and particularly preferably 20 to 50% by weight in the mixture of the conductive silicon composite and the conductive agent. If it is less than 1% by weight, it may not be able to withstand expansion / contraction associated with charge / discharge, and if it exceeds 60% by weight, the charge / discharge capacity may be reduced.
[0022]
【Example】
EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated concretely, this invention is not limited to the following Example. In the following examples, “%” indicates wt%, and “gr” indicates gram.
[0023]
[Example]
After 200 g of silicon oxide powder (SiO _x : x = 1.02) having an average particle diameter of 3 μm and a BET specific surface area of 12 m ² / g was placed in a silicon nitride tray, it was left in a processing furnace capable of maintaining an atmosphere. Next, argon gas was introduced, and the inside of the processing furnace was replaced with argon, and then the temperature was increased to 1200 ° C. at a temperature increase rate of 300 ° C./hr while flowing argon at 2 NL / min, and held for 3 hours. After completion of the holding, the temperature was lowered, and after reaching room temperature, the powder was collected. The obtained powder was a powder having an average particle diameter of 3.5 μm and a BET specific surface area of 11 m ² / g. From the X-ray diffraction pattern of Cu—Kα ray of this powder, Si (111) around 2θ = 28.4 ° was obtained. ) And a silicon composite powder having a silicon crystal size of 40 nm dispersed in silicon dioxide determined by the Scherrer method from the half-value width of this diffraction line. It was.
[0024]
[Battery evaluation]
Next, battery evaluation was performed by the following method using the obtained silicon composite powder.
First, artificial graphite (average particle diameter D ₅₀ = 5 μm) was added to the obtained silicon composite to prepare artificial graphite: silicon composite = 50: 50 (weight ratio) to obtain a mixture. Next, 10% polyvinylidene fluoride is added to this mixture, and further N-methylpyrrolidone is added to form a slurry. This slurry is applied to a copper foil having a thickness of 20 μm, dried at 120 ° C. for 1 hour, and then the electrode is attached by a roller press. It was press-molded and finally punched out to 2 cm ² to form a negative electrode.
Here, in order to evaluate the charge / discharge characteristics of the obtained negative electrode, a lithium foil was used as a counter electrode, and lithium hexafluorophosphate was used as a non-aqueous electrolyte with 1/1 (volume) of ethylene carbonate and 1,2-dimethoxyethane. Ratio) A lithium ion secondary battery for evaluation using a non-aqueous electrolyte solution dissolved at a concentration of 1 mol / L in a mixed solution and a polyethylene microporous film having a thickness of 30 μm as a separator was prepared.
[0025]
The prepared lithium ion secondary battery is left at room temperature overnight and then charged with a constant current of 3 mA until the voltage of the test cell reaches 0 V using a secondary battery charge / discharge test device (manufactured by Nagano Co., Ltd.). After reaching 0V, charging was performed by decreasing the current so as to keep the cell voltage at 0V. Then, the charging was terminated when the current value fell below 100 μA. The discharge was performed at a constant current of 3 mA, and when the cell voltage exceeded 2.0 V, the discharge was terminated and the discharge capacity was determined.
The above charging / discharging test was repeated, and the charging / discharging test of the lithium ion secondary battery for evaluation was performed 20 cycles.
As a result, it was confirmed that the lithium ion secondary battery was excellent in cycle performance with initial discharge amount = 670 mAh / g, discharge capacity after 20 cycles = 480 mAh / g, capacity retention after 20 cycles = 71.6%. It was.
[0026]
[Comparative example]
The battery was evaluated in the same manner as in the example except that the silicon oxide powder used in the example was not heat-treated.
From the X-ray diffraction pattern of the silicon oxide powder by Cu-Kα ray, the silicon oxide was an amorphous powder in which no diffraction line attributed to Si (111) near 2θ = 28.4 ° was observed.
As a result of battery evaluation, the initial discharge amount = 700 mAh / g, the discharge capacity after 20 cycles = 220 mAh / g, and the capacity retention rate after 20 cycles = 31.4%. It was the next battery.
[0027]
【The invention's effect】
The silicon composite of the present invention is used as a negative electrode material for a non-aqueous electrolyte secondary battery, gives good cycleability, is simple in its manufacturing method, and can sufficiently withstand production on an industrial scale. .

Claims (7)

Microcrystals of silicon particles having a dispersed structure in silicon-based compounds (excluding those obtained by coating the surface of particles with carbon) negative for a non-aqueous electrolyte secondary battery using the silicofluoride-containing complex electrode material .   Particles having a structure in which silicon microcrystals are dispersed in a silicon-based compound (excluding those in which the surface of the particles is coated with carbon), and a mixture of a silicon composite and a conductive agent, A negative electrode material for a non-aqueous electrolyte secondary battery using a mixture having a conductive agent content of 1 to 60% by weight. The average particle diameter of 0.01 to 30, BET specific surface area of 0.5 to 20 m ² / g and is claim 1 or 2 non-aqueous electrolyte secondary battery negative electrode material according. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the silicon microcrystals have a size of 1 to 500 nm and the silicon compound is silicon dioxide. In the X-ray diffraction of the silicon composite, a diffraction peak attributed to Si (111) is observed, and the silicon crystal size determined by the Scherrer method based on the half width of the diffraction line is 1 to 500 nm. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode material is a non-aqueous electrolyte secondary battery . The non-aqueous electrolyte 2 according to any one of claims 1 to 5 , wherein the silicon composite is obtained by disproportionating silicon oxide in an inert gas atmosphere in a temperature range of 900 to 1400 ° C. Negative electrode material for secondary batteries . Silicon oxide average particle diameter of 0.01 to 30, Ru silicon oxide powder der represented by the general formula SiO _x having a BET specific surface area ^{0.1~30m 2 / g (1.0 ≦ x} <1.6) 請 The negative electrode material for a nonaqueous electrolyte secondary battery according to claim 6 .