JP6245954B2 - Negative electrode active material for lithium ion secondary battery - Google Patents

Description

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

Lithium ion secondary batteries that have higher electromotive force and higher energy density than alkaline storage batteries such as nickel / cadmium storage batteries and nickel / hydrogen storage batteries are used in portable small electric / electronic devices that are widely used. Yes. In recent years, as the performance and functionality of these devices have increased, there has been a demand for higher battery capacity, and secondary batteries have been actively developed.
Many studies have been made so far regarding negative electrode active materials for lithium ion secondary batteries. Among them, metallic lithium has attracted attention as a material for the negative electrode active material due to its abundant battery capacity. However, many dendritic lithium deposits on the surface of lithium during charging, causing a decrease in charge / discharge efficiency, resulting in a short circuit with the positive electrode, handling of lithium instability and high reactivity. Because of this problem, it has not been put into practical use.
A carbon-based material has been put to practical use as a negative electrode active material that can replace metallic lithium. The carbon-based material has a smaller expansion / contraction ratio due to charge / discharge than metal lithium or lithium alloy. However, the battery capacity is smaller than that of metallic lithium (theoretical capacity is about 372 mAh / g).
Therefore, silicon and tin are expected as high capacity materials. These materials are characterized by a large battery capacity compared to carbon-based materials, and have been actively studied in recent years. However, the material has a large expansion / contraction ratio due to charge / discharge, and when this material is used as a negative electrode active material, there is a problem that it falls off from the current collector and the life is shortened or the irreversible capacity is large. In addition, although the reversibility after the second cycle is good, the initial reversibility representing the charge / discharge efficiency of the first cycle is poor, and there is a problem that the capacity decreases. In order to solve such problems, attempts have been made to suppress expansion and contraction by alloying silicon or tin with other elements or composite with carbon, thereby reducing the life and irreversible capacity. .

For example, Patent Document 1 proposes a negative electrode active material that includes Si, Al, and other elements and has a matrix that surrounds the periphery of a Si nucleus. The negative electrode active material has a high capacity retention rate. However, it is unclear how much the discharge capacity (mAh / g) is, and since the voltage range is 0.1 to 1.0 V, it is difficult to predict that the discharge capacity is large.
Patent Document 2 proposes a negative electrode material having a structure composed of a Li occlusion phase α such as a Si phase and a phase β made of an intermetallic compound or a solid solution of this element and another element. Although the negative electrode material maintains a cycle characteristic of 90% or more, the discharge capacity is as high as about 1600 mAh / g, which is as low as half or less of the theoretical capacity of 4200 mAh / g of silicon alone. The initial reversibility is unknown.

JP 2009-032644 A JP 2001-297757 A

An object of the present invention is to provide a negative electrode active material and a negative electrode for a lithium ion secondary battery that can be used in a lithium ion secondary battery and have a high discharge capacity and initial reversibility and can exhibit excellent cycle characteristics. There is to do.
Another object of the present invention is to provide a lithium ion secondary battery having a high discharge capacity and an initial reversibility and excellent cycle characteristics.

In order to solve the above problems, the present inventor uses an electrode using a negative electrode active material for a lithium ion secondary battery by a combination of various metal elements as an electrode composition, an electrode capacity, and an initial reversibility representing charge / discharge efficiency in the first cycle. As a result of detailed examination from the viewpoint of rate and cycle characteristics, it has been found that alloy particles containing a specific amount of Si, R element, and A element and having a specific crystal phase can solve the above problems. It came to complete.
According to the present invention, Si and R element (R is at least one selected from the group consisting of rare earth elements including Sc and Y. Preferably, R is at least one selected from the group consisting of Pr, Nd and Gd. shows the seed.) and a element (a is an alkali metal, alkaline earth metal, B, P, Mn, Ag , Sb, Bi, Hf, Co, Cr, C, Mo, Nb, V, Al, Cu, Fe , Ni, W, Ti, Zr, Zn, Sn, Ga, In, and Ta. Preferably, A represents at least one selected from Al and Cu. And the Si content is 30.0 wt% or more and 80.0 wt% or less, the R element content is 10.0 wt% or more and 50.0 wt% or less, and the A element content is 0.01 wt%. % To 30.0% by weight, powder X-ray Negative electrode active material for a lithium ion secondary battery (hereinafter abbreviated as the negative electrode active material of the present invention) containing alloy particles having a Si phase confirmed by folding (XRD) and a phase containing Si, R element and A element Is provided).
According to the present invention, there is also provided a negative electrode for a lithium ion secondary battery (hereinafter sometimes abbreviated as the negative electrode of the present invention) comprising a current collector and an active material layer containing the negative electrode active material of the present invention. The
Furthermore, according to the present invention, there is provided a lithium ion secondary battery (hereinafter sometimes abbreviated as the secondary battery of the present invention) comprising the negative electrode of the present invention, a positive electrode, a separator, and an electrolyte.

  Since the negative electrode active material of the present invention has the above specific composition and crystal phase, by using a negative electrode using this for a lithium ion secondary battery, the secondary battery has a high discharge capacity and initial reversibility, In addition, excellent cycle characteristics can be exhibited.

3 is a diagram showing an X-ray diffraction pattern of negative electrode active material particles according to Example 1. FIG. 2 is a graph showing a charge / discharge curve of a negative electrode obtained in Example 1. FIG. 3 is a graph showing a charge / discharge curve of a negative electrode obtained in Example 2. 6 is a graph showing a charge / discharge curve of a negative electrode obtained in Example 6. 6 is a graph showing a charge / discharge curve of a negative electrode obtained in Comparative Example 1. 5 is a graph showing a charge / discharge curve of a negative electrode obtained in Comparative Example 2. 10 is a graph showing a charge / discharge curve of a negative electrode obtained in Comparative Example 3.

Hereinafter, the present invention will be described in more detail.
The negative electrode active material of the present invention contains Si, a specific R element, and an A element in specific ratios, and includes a Si phase that is confirmed by powder X-ray diffraction (XRD), and a phase that includes Si, R element, and A element. Having alloy particles.

  In the alloy particles, Si is an element capable of occluding and releasing Li. In general, as the Si content increases, the discharge capacity of the lithium ion secondary battery increases, but the cycle life tends to decrease. Therefore, in order to achieve both excellent cycle characteristics and discharge capacity, the Si content is in the range of 30.0% by weight to 80.0% by weight, preferably 35.0% by weight to 76.0%. Control to the range. When the content of Si is less than 30.0% by weight, the charge / discharge capacity is lowered, and a desired capacity may not be obtained. On the other hand, when the Si content exceeds 80.0% by weight, the active material is greatly expanded and contracted during the insertion and extraction of Li, and the cycle characteristics are significantly deteriorated.

  In the alloy particles, the R element represents at least one selected from the group consisting of rare earth elements including Sc and Y. The R element is an element having a large electron donating property and a relatively high density. The R element preferably includes at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Dy and Y, and particularly selected from the group consisting of Sm, Gd and Dy. It is preferable to include at least one kind. The content of R element is controlled in the range of 10.0% by weight to 50.0% by weight, preferably in the range of 15.0% by weight to 50.0%. When the content of R element is less than 10.0% by weight, the phase containing Si, R element and A element confirmed by XRD tends to decrease, and the active material expands / contracts during the insertion / desorption of Li. As a result, the cycle effect of the lithium ion secondary battery may be deteriorated. When the content of the R element exceeds 50.0% by weight, the charge / discharge capacity of the lithium ion secondary battery is lowered, and the desired effect may not be obtained.

In the alloy particles, the element A is alkali metal, alkaline earth metal, B, P, Mn, Ag, Sb, Bi, Hf, Co, Cr, C, Mo, Nb, V, Al, Cu, Fe, Ni, At least one selected from the group consisting of W, Ti, Zr, Zn, Sn, Ga, In and Ta is shown. The element A is an element that contributes to the cycle characteristics of the obtained lithium ion secondary battery. The content of element A is in the range of 0.01 wt% to 30.0 wt%, preferably 0.01 wt% to 25.0 wt%, particularly preferably 1.0 wt% to 25.0 wt%. Control within the following range. When the A element is within the above range, the A phase confirmed by XRD may precipitate. This A phase will be described later, but some may be included. However, if it is excessively contained, the initial reversibility of the lithium ion secondary battery may be deteriorated.
When the content of A element is less than 0.01% by weight, the phase containing Si, R element and A element, which is confirmed by XRD, tends to decrease, and the active material expands / contracts during the insertion / desorption of Li. May be reduced, and the cycle characteristics of the lithium ion secondary battery may be deteriorated. Moreover, the phase containing Si, R element, and A element cannot be obtained, and the initial reversibility of the lithium ion secondary battery may be lowered. When the content of the A element exceeds 30.0% by weight, the A phase is excessively precipitated, and the initial reversibility of the lithium ion secondary battery may be lowered.
The composition of the alloy particles containing Si, R element and A element can be confirmed by quantitative analysis by ICP (Inductively Coupled Plasma) emission spectroscopic analysis.
The alloy particles may contain elements other than Si, R element, and A element as inevitable impurities as long as they are in minute amounts.

  In the alloy particles, both the Si phase and the phase containing Si, R element and A element are confirmed by XRD. The Si phase is a phase that contributes to the charge / discharge capacity in the lithium ion secondary battery, and an increase in the charge / discharge capacity can be expected by increasing the content of the Si phase in the particles. On the other hand, when the content of the Si phase is excessively increased, the alloy particles are finely pulverized and the cycle life of the lithium ion secondary battery may be reduced. The phase containing Si, R element, and A element acts as a matrix of Si phase, and can further relieve the stress generated by the volume change accompanying charge / discharge. Accordingly, there is an advantage that the ratio of the Si phase in the active material can be increased while improving the cycle characteristics of the lithium ion secondary battery but having a large capacity but a large volume change during charge and discharge.

As the phase containing Si, R element and A element, there are various kinds such as RA ₂ Si ₂ phase and RASi phase, and it is preferable that RA ₂ Si ₂ phase is contained therein.
The abundance ratio between the Si phase and the RA ₂ Si ₂ phase is determined by, for example, XRD from the viewpoint of the balance between charge / discharge capacity of the lithium ion secondary battery and improvement of cycle characteristics or suppression of expansion / contraction of the negative electrode active material. The integrated intensity of the strongest peak of Si phase (peak of (111) plane) confirmed near 2θ = 28.4 ± 0.2 ° and RA ₂ confirmed near 2θ = 27.9 ± 0.2 °. The ratio ((RA ₂ Si ₂ phase) / (Si phase)) of the strongest peak of the Si ₂ phase (peak of the (011) plane) to the integrated intensity ((RA ₂ Si ₂ phase) / (Si phase)) is preferably in the range of 0.05 to 0.40. .
The integrated intensity here represents the area of each strongest peak curve obtained from XRD, and the integrated intensity is an X-ray diffraction spectrum using CuKα rays of an X-ray diffractometer (for example, Ultimate IV manufactured by Rigaku Corporation). More demanded.

The alloy particles used for the negative electrode active material of the present invention may contain an A phase depending on the content of the A element. If there is little content of A element, A phase will not precipitate, but if there is much content of A element, A phase will precipitate. When the A phase is contained in a certain amount, excellent battery characteristics can be obtained, but if the A phase is excessively precipitated, the initial reversibility may be deteriorated.
The content of the A phase is the integrated intensity of the highest peak of the Si phase ((111) plane peak) confirmed around 2θ = 28.4 ± 0.2 ° and 2θ = 38.4 ± 0.2 °. It is preferable that the ratio ((A phase) / (Si phase)) of the highest peak of the A phase (peak of the (111) plane) confirmed in the vicinity ((A phase) / (Si phase)) is in the range of 0 or more and 0.20 or less.

The alloy particles used in the negative electrode active material of the present invention include, for example, a strip casting method such as a single roll method, a twin roll method or a disk method, a die casting method, various atomizing methods, a mechanical alloying method (mechanical milling method), an arc Although it can manufacture by a melt | dissolution method, if the alloy particle which has a desired phase is obtained, it will not specifically limit.
The obtained alloy particles can be heat-treated as necessary. The heat treatment can be carried out under an inert atmosphere, usually at 300 to 1200 ° C. for 0.5 to 30 hours.
Further, the obtained alloy particles can be pulverized as necessary. The pulverization method can be performed by using a known pulverizer such as a jet mill, a feather mill, a hammer mill, a ball mill, a stamp mill, an attritor mill, and the like and appropriately changing the pulverization conditions. Moreover, although pulverization using a mortar or the like is possible, it is not particularly limited to these means. By sieving after pulverization as necessary, an alloy powder having a desired particle size can be obtained.

The alloy particles used for the negative electrode active material of the present invention have an average particle diameter D50 of usually 1 to 50 μm, preferably 5 to 30 μm. The shape of the alloy particles is not particularly limited. The average particle diameter D50 can be measured by a laser diffraction / scattering particle size distribution analyzer (for example, product name “MICROTRAC HRA”, model number 9320-X100, manufactured by Nikkiso Co., Ltd.).
The negative electrode active material of the present invention can be used as the negative electrode active material as it is, or can be mixed with other negative electrode active materials.

The negative electrode of the present invention includes a current collector and an active material layer containing the negative electrode active material of the present invention. The active material layer is usually formed on at least one surface of the current collector, and it is preferable that the negative electrode active material of the present invention is present in a substantially uniformly dispersed state throughout the active material layer.
The thickness of the active material layer is usually 0.5 to 40 μm, preferably 0.5 to 30 μm, and more preferably 0.5 to 25 μm. By setting the thickness of the active material layer within this range, the energy density of the lithium ion secondary battery can be sufficiently improved, and the strength of the negative electrode can be increased, and alloy particles in the active material layer can be obtained. Can be effectively prevented from falling off.

  As the current collector, the same one as conventionally used as a current collector of a negative electrode for a lithium ion secondary battery can be used. The current collector is preferably composed of a metal material having a low ability to form a lithium compound. Here, “low ability to form a lithium compound” means that lithium does not form an intermetallic compound or solid solution, or even if formed, the amount of lithium is very small or very unstable. Preferred examples of such a metal material include copper, nickel, and stainless steel. Also, it is possible to use a copper alloy foil represented by a Corson alloy foil. The thickness of the current collector is preferably 9 to 35 μm in consideration of the balance between maintaining the strength of the negative electrode and improving the energy density. In addition, when using copper foil as an electrical power collector, it is preferable to give the rust prevention process using organic compounds, such as a chromate process and a triazole type compound, and an imidazole type compound.

The negative electrode of the present invention is prepared, for example, by dispersing the negative electrode active material of the present invention, a binder and a conductive agent in a solvent to prepare a negative electrode mixture, and coating the mixture on at least one surface of the current collector. It can be manufactured by a method of forming an active material layer by drying.
Examples of the binder include fluorine resins such as polytetrafluoroethylene and polyvinylidene fluoride, polyvinyl acetate, polymethyl methacrylate, ethylene-propylene-diene copolymer, styrene-butadiene copolymer, and acrylonitrile butadiene copolymer. Examples of the polymer include carboxymethyl cellulose.
Examples of the conductive agent include carbonaceous materials such as natural graphite such as flake graphite, artificial graphite, ketjen black, and acetylene black.

The secondary battery of the present invention includes the negative electrode of the present invention, a positive electrode, a separator, and an electrolyte.
The positive electrode is not particularly limited as long as it can be used for a positive electrode of a lithium ion secondary battery, and can be appropriately selected from, for example, known positive electrodes.

  As the separator, it is preferable to use a microporous thin film having high ion permeability, predetermined mechanical strength, and electronic insulation. For example, it is preferable to use a microporous thin film made of a material such as polyethylene, polypropylene, polyphenylene sulfide, polyethylene terephthalate, polyamide, polyimide, etc. Alternatively, a plurality of them may be used in combination. From the viewpoint of manufacturing cost, it is advantageous to use inexpensive polypropylene or the like.

Examples of the electrolyte include a nonaqueous electrolytic solution composed of an organic solvent and a solute dissolved in the organic solvent, and a solid electrolyte. Known electrolytes can be used without particular limitation.
Examples of the organic solvent used for the non-aqueous electrolyte include N-methylpyrrolidone, tetrahydrofuran, ethylene oxide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, dimethylformamide, dimethylacetamide, ethylene carbonate, propylene carbonate, and butylene carbonate. , Dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, formic acid Methyl, methyl acetate, phosphate triester, trimethoxymethane, dioxo Derivatives, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, 1,3-propane sultone, and other aprotic organic solvents. It can be used as a mixed solvent.

Examples of the solute dissolved in the organic solvent include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF _6. , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, lithium tetrachloroborate, lithium tetraphenylborate, and imides. These may be used alone or in combination.

Examples of the electrolyte include polymer electrolytes such as polyethylene oxide, and sulfide electrolytes such as Li ₂ S—SiS ₂ , Li ₂ S—P ₂ S ₅ , and Li ₂ S—B ₂ S ₃ . Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution in the polymer | macromolecule can also be used.

  Examples of the shape of the secondary battery of the present invention include various shapes such as a cylindrical shape, a stacked shape, and a coin shape. The secondary battery of the present invention is housed in the battery case in any shape and connected between the positive electrode and the negative electrode to the positive electrode terminal and the negative electrode terminal using a current collecting lead or the like. And it can manufacture by sealing a battery case.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited to these.
Example 1
(Manufacture of negative electrode active material)
Si, Gd, and Al are weighed so that Si, Gd, which is an R element, and Al, which is an A element, have a mass ratio of Si: Gd: Al = 73.7: 20.92: 5.38. Then, it was melted in an Ar gas atmosphere in a high frequency melting furnace to obtain an alloy melt. Thereafter, the melt was rapidly cooled and solidified by a strip casting method using a single roll casting apparatus equipped with a copper water-cooled roll to obtain an alloy slab. The obtained alloy slab was pulverized by a jet mill, and the pulverized powder was measured by a laser diffraction scattering type particle size distribution measuring device (product name “MICROTRAC HRA”, model number 9320-X100, manufactured by Nikkiso Co., Ltd.). As a result, alloy particles (Si—Gd—Al alloy powder) of a negative electrode active material for a lithium ion secondary battery having an average particle size (D50) of 7.233 μm were obtained. The composition of the obtained alloy particles was analyzed by ICP and found to be Si = 73.7% by weight, Gd = 20.92% by weight, and Al = 5.38% by weight. Further, the alloy particles as the negative electrode active material were subjected to powder X-ray diffraction using an X-ray diffractometer (manufactured by Rigaku Corporation, Ultimate IV). The results are shown in FIG. Than 1, and gdal ₂ Si ₂ phase is at least Si phase and RA ₂ Si ₂ phase was confirmed, but, Al phase is the A phase was not confirmed. The integrated intensity of the strongest peak of Si phase (peak of (111) plane) confirmed near 2θ = 28.4 ± 0.2 ° and GdAl ₂ Si confirmed near 2θ = 27.9 ± 0.2 ° The ratio ((GdAl ₂ Si ₂ phase) / (Si phase)) of the strongest peak of the _two phases (peak of the (011) plane) to the integrated intensity was 0.28. These results are shown in Table 1 together with the results of quantitative analysis and particle size measurement by ICP.

(Manufacture of negative electrode)
The negative electrode active material (Si-Gd-Al alloy powder) 0.5 g obtained above, 0.375 g of the conductive agent (acetylene black), and 0.375 g of the binder (polyvinylidene fluoride) were weighed, and an appropriate amount of N- Methylpyrrolidone was added and well kneaded in a mortar to obtain an electrode paste. The obtained electrode paste was applied to a copper foil by a doctor blade method, dried, and then pressure-formed with a press. Then, it cut | judged to the predetermined dimension and was set as the negative electrode for lithium ion secondary batteries.

(Electrode evaluation)
A charge / discharge test coin cell was constructed using the negative electrode obtained above. In the coin cell, a metal lithium foil as a counter electrode and the negative electrode obtained above as a test electrode were arranged via a separator. Into this, an electrolyte solution in which LiPF ₆ as a supporting electrolyte was dissolved at a concentration of 1M was injected into a 1: 2 mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) to produce a coin cell type battery. Using the manufactured coin cell type battery, the measurement temperature was set to 25 ° C., and the charge condition was CC (constant current) charge until reaching 0.005 V (vs reference electrode) at 0.20 mA / cm ² , and then 0.03 mA. CV (constant voltage) charge was performed until
On the other hand, as a discharge condition, discharge was performed at 0.20 mA / cm ² to 2.0 V (vs reference electrode). Further, (charge-discharge) was set to 1 cycle, and measurement was performed up to 50 cycles. First cycle discharge capacity and initial reversibility (%) representing charge / discharge efficiency of first cycle = (discharge capacity of first cycle) / (charge capacity of first cycle) × 100, and cycle characteristics are evaluated. The capacity retention rate (%) used for this purpose was calculated as: discharge capacity at 50th cycle / discharge capacity at the first cycle × 100. The results are shown in Table 1. The obtained charge / discharge curve of the first cycle is shown in FIG.

Examples 2-10, Comparative Examples 1-4
An alloy particle of a negative electrode active material was obtained in the same manner as in Example 1 except that the blending of the raw materials was changed to be a negative electrode active material having the composition shown in Table 1. About the obtained negative electrode active material, the same measurement and test as Example 1 were performed. Moreover, the negative electrode was produced similarly to Example 1 and electrode evaluation was performed. The results are shown in Table 1. Table 1 shows the results of discharge capacity, initial reversibility rate (%), and capacity retention rate (%) in the first cycle. The charge / discharge polarities of the first cycle of Examples 2 and 6 are shown in FIGS. 3 and 4, and the charge / discharge curves of the first cycle of Comparative Examples 1 to 3 are shown in FIGS.

Claims (7)

Si and R element (R represents at least one selected from the group consisting of Pr, Nd and Gd ) and A element (A represents at least one selected from Al and Cu ), The content is 30.0 wt% or more and 80.0 wt% or less, the R element content is 10.0 wt% or more and 50.0 wt% or less, and the A element content is 0.01 wt% or more and 30.30 wt% or less. A negative electrode active material for a lithium ion secondary battery, comprising 0 wt% or less and alloy particles having a Si phase confirmed by powder X-ray diffraction (XRD) and a phase containing Si, R element and A element. The negative electrode active material according to claim 1, wherein the phase containing Si, R element and A element is an RA ₂ Si ₂ phase. The integrated intensity of the strongest peak of Si phase (peak of (111) plane) confirmed near 2θ = 28.4 ± 0.2 ° and RA ₂ confirmed near 2θ = 27.9 ± 0.2 °. The ratio ((RA ₂ Si ₂ phase) / (Si phase)) of the strongest peak of the Si ₂ phase (peak of the (011) plane) to the integrated intensity ((RA ₂ Si ₂ phase) / (Si phase)) is 0.05 or more and 0.40 or less. Negative electrode active material.   Integrated intensity of the strongest peak of Si phase (peak of (111) plane) confirmed near 2θ = 28.4 ± 0.2 ° and A phase confirmed near 2θ = 28.4 ± 0.2 ° The ratio of the strongest peak (peak of (111) plane) to the integrated intensity ((A phase) / (Si phase)) = 0 or more and 0.20 or less. material.   The negative electrode active material according to claim 1, wherein the A element is Al. The negative electrode for lithium ion secondary batteries provided with the electrical power collector and the active material layer containing the negative electrode active material in any one of Claims 1-5 . A lithium ion secondary battery comprising the negative electrode according to claim 6 , a positive electrode, a separator, and an electrolyte.

Images