JP3262019B2 - Method for producing negative electrode material for lithium ion secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a method for producing a negative electrode material for a secondary battery, and more particularly to a method for producing a negative electrode material for a lithium battery having a high discharge capacity and excellent cycle characteristics.

[0002]

2. Description of the Related Art With the spread of portable small electric and electronic devices, new types of secondary batteries such as Ni-hydrogen batteries and lithium ion batteries have been actively developed.

[0003] Among such secondary batteries, a lithium ion battery is a battery using lithium as a negative electrode active material and a non-aqueous solvent as an electrolyte. Since lithium is a very base metal, a high voltage can be obtained and a battery having a high energy density can be obtained. Therefore, a lithium ion battery is widely used as a primary battery.

Therefore, it is expected that a lithium ion battery will be used as a secondary battery. However, when metal lithium is applied to a secondary battery, the repetition of charge and discharge causes lithium to grow in a dendrite shape from the negative electrode, and penetrates the separator, which is an insulator, to short-circuit with the positive electrode. There was a drawback that the life was short.

As one means for solving the problem of the secondary battery using lithium metal as the negative electrode active material, a carbonaceous material capable of inserting and extracting lithium ions is used.
It has been proposed to use (eg, natural graphite, artificial graphite, petroleum coke, fired resin, carbon fiber, pyrolytic carbon, carbon black, mesophase spherules, bulk mesophase, etc.) as the negative electrode material. For example, JP-A-57-2080
No. 79, JP-A-4-115458, JP-A-5-234584, and JP-A-5-234584
-307958.

In a lithium ion secondary battery comprising a negative electrode made of this carbonaceous material, the reaction at the negative electrode during charging and discharging is such that lithium ions (Li ⁺ ) only enter and exit between carbon (graphite) layers. That is, during charging, electrons are sent to the carbonaceous material of the negative electrode, carbon is negatively charged, and lithium ions occluded in the positive electrode are desorbed and occluded in the negatively charged carbonaceous material of the negative electrode (intercalation). Is done. vice versa,
During discharge, lithium ions occluded in the carbonaceous material of the negative electrode are desorbed (deintercalated) and occluded in the positive electrode. By using such a mechanism, precipitation of metallic lithium on the negative electrode can be prevented, and the problem of deterioration of the negative electrode due to precipitation of dendrite can be avoided.

However, in the lithium secondary battery using the carbonaceous material for the negative electrode as described above, the discharge capacity is small from the beginning due to the small amount of occlusion and release of lithium ions at the negative electrode, or the occlusion of lithium ions. For a large amount of highly crystalline graphite material, even if the initial discharge capacity is high, the deterioration of the carbonaceous structure (graphite structure) and the decomposition of the solvent in the non-aqueous electrolyte gradually occur due to repeated charge and discharge. Coulomb efficiency of the first cycle after happening [(discharge capacity / charge capacity)
× 100 <%>] is extremely reduced, so that an extra amount of electricity is consumed.

A method has been proposed in which an intermetallic compound is used as a lithium ion host material instead of the carbonaceous material. These intermetallic compounds include FeSi ₂ , YSi ₂ , MoSi _2, etc., and by using this, the discharge capacity is increased as the theoretical capacity of the graphite-based carbon material exceeds 372 mAh / g, which is caused by the reaction of the electrolyte solution As the irreversible capacity decreases, 1
The coulomb efficiency of the cycle improved. JP-A-7-2402
See No. 01 and JP-A-5-159780.

[0009]

The first object of the present invention is to avoid the problem of negative electrode deterioration due to the precipitation of dendrite, and to achieve a discharge capacity equal to or higher than that of the conventional intermetallic compound material. And to develop a negative electrode material that can exhibit coulomb efficiency.

By the way, when an intermetallic compound having a composition such as FeSi ₂ , YSi ₂ , or MoSi ₂ is applied to a lithium ion secondary battery, it is charged at 1/10 C (a charging method for fully charging in 10 hours). -Compared to the charge / discharge capacity at 1 / 10C discharge, 2C charge (charging method to fully charge in 0.5 hours)-Charge / discharge capacity at 2C discharge is greatly reduced The problem remains. It is considered that this is because lithium ions intrude from the surface of the intermetallic compound and undergo internal diffusion during the charge reaction, but the diffusion rate is low, which causes a problem in the rate characteristics.

[0011] Among the performances of the lithium ion secondary battery, although the discharge capacity and the cycle characteristics are the same as or better than those of other batteries, the rate characteristics (large current characteristics) are Ni- It is far from aqueous secondary batteries such as hydrogen batteries and Ni-Cd batteries, and it can be said that improving this rate characteristic is necessary to make lithium ion secondary batteries more excellent.

Accordingly, a further object of the present invention is to provide a method for producing a negative electrode material for a lithium ion secondary battery which has the same discharge capacity and Coulomb efficiency in the first cycle as the conventional intermetallic compound, but also has excellent rate characteristics. It is to provide.

[0013]

The present inventors have SUMMARY OF THE INVENTION As a result of repeated extensive studies to solve the above problem, in the formula, a compound represented by AB _x, A is Fe, Mn, Co, Mo, Cr , Nb, V,
B and at least one element selected from Cu and W
Of those composed of at least one element selected from Si, C, Ge, Sn, Pb, Al, and P as an essential element, 0.5
When producing a material satisfying ≦ X ≦ 3, it is particularly preferable to solidify from a molten state at a cooling rate of 100 ° C./sec or more and 5000 ° C./sec or less to crystallize fine crystal grains of 50 μm or less. Then, if necessary, it was found that a negative electrode for a lithium ion secondary battery having excellent rate characteristics can be obtained by performing heat treatment, and the present invention was completed. Also, casting method such solidification rate can be realized also found that atomizing method, rotational electrode method and the like are suitable.

That is, according to the present invention, Fe , Mn , Co, Mo,
Cr, Nb, V, Cu and W silicides are excellent. These have sites that absorb and store lithium ions. In order to further promote rapid diffusion of lithium ions, a microstructure formed by rapid solidification is preferable. As above
When the solidification method is performed to have a cooling rate of 100 ° C / sec or more and 5000 ° C / sec or less , the crystal grain size of the obtained alloy becomes extremely small, resulting in a rapidly solidified structure having fine crystal grains of 50μm or less. . Normally, diffusion of atoms in an intermetallic compound is dominated by grain boundary diffusion as compared with intragranular diffusion. Lithium ions also follow this, and grain boundary diffusion is performed at the center, so the diffusion rate increases as the crystal grain size decreases, and when this is applied as a negative electrode material of a lithium ion secondary battery,
The rate characteristics are improved.

Here, the gist of the present invention is as follows. (1) The alloy represented by the chemical formula: AB _x
Rapid cooling and solidification at a cooling rate of
Lithium ion 2 with an average crystal grain size of 50 μm or less
Method for producing negative electrode material for secondary battery. Where A is Fe , Mn,
One or more elements selected from the group consisting of Co, Mo, Cr, Nb, V, Cu, and W, B is Si or a part of Si
The, C, Ge, Sn, Pb , those that have been substituted Al, and at least one element chosen from the group consisting of P, and
0.5 ≦ X ≦ 3.

(2) A heat treatment is performed after the quenching / solidification.
1. Method for producing negative electrode material for lithium ion secondary battery according to 1
Law.

[0017]

[0018]

[0019]

[0020]

Thus, according to the present invention, the discharge capacity and 1
Although the coulomb efficiency at the cycle is the same as the conventional one, a negative electrode material for a lithium ion secondary battery having improved rate characteristics can be obtained.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. First, the reason for limiting the alloy of the present invention as described above will be described.

The alloys utilized in the present invention has the formula: A compound of crystalline denoted by AB _x. Here, site A is F
One or more elements of e , Mn , Co, Mo, Cr, Nb, V, Cu and W, and the element whose B site is Si are basically used. Further, the Si site of the silicide is changed to C, Ge, Sn, Pb, Al,
It may be a compound partially substituted with one or more kinds of P.

As described above, the silicide is a compound having a site for storing lithium ions and capable of operating as a negative electrode material. At this time, the X value of AB _x is set to 0.5 ≦ X ≦ 3. When X is less than 0.5 or greater than 3, when used as a negative electrode material for a lithium ion battery, the number of sites for storing Li ions is reduced and the discharge capacity is reduced. The tendency is that the elements occupying the A site are Fe , Mn, Co, Mo, Cr,
The same applies to Nb, V, Cu, or W alone or a substitutional solid solution composed of the above elements.

The element occupying the B site is Si or S
This tendency is the same for the substitutional solid solutions of i, C, Ge, Sn, Pb, Al, and P. When these elements form a substitution-type solid solution, the composition ratio thereof is not particularly limited.
The ratio of Si, C, Ge, Sn, Pb, Al, and P is preferably M, where Si: M = 1: 0.2 to 1: 0.

[0026] Thus, AB _X compounds according to the present invention the composition is as follows. Chemical formula: AB _{x wherein} A is one or more elements selected from the group consisting of Fe , Mn, Co, Mo, Cr, Nb, V, Cu, and W;
B is a portion of Si or Si, C, Ge, Sn, Pb, Al, and selected from the group consisting of P 1 kind or at least two elements
Substituted , and 0.5 ≦ X ≦ 3.

[0027]

According to the present onset bright, negative electrode material consisting of the AB _x compounds rapidly solidified structure, but is used as having a further if necessary thermally treated tissues, Li ions by the microstructure This is to increase the diffusion speed of the. Further , the structure has a crystal grain size of 50 μm or less.

Next, a method for producing a negative electrode material for a lithium ion secondary battery according to the present invention will be described. The negative electrode metal material for a lithium ion secondary battery according to the present invention is obtained by first melting an appropriate supply source of each alloying element substantially simultaneously in a melting furnace or the like, and then changing the melted state to 100 ° C./sec or more. It is manufactured by quenching and solidifying at a cooling rate of.

The melting at this time is performed by arc melting, plasma melting, high frequency induction heating, in an inert gas or in a vacuum.
It can be performed by an appropriate method such as resistance heating.
Rapid cooling and solidification at a cooling rate of 100 ° C / sec or more include, for example, gas atomizing, oil atomizing, water atomizing, rotating electrode methods, casting on a rotating drum, casting into a mold that provides the effect of rapid solidification by water cooling, etc. It can be performed by appropriate means. There is no particular limitation as long as a cooling rate of 100 ° C./sec or more and 5000 ° C./sec or less can be realized.

The cooling rate during quenching and solidification from the melt is 100
If the average crystal grain size is less than 50 ° C./sec, the crystal grain size becomes large when the average crystal grain size exceeds 50 μm, and the diffusion rate of lithium ions becomes small. It becomes difficult to obtain materials for use. The cooling rate at the time of solidification is preferably 1 × 10 ³ ° C./sec or more.

Since the negative electrode intermetallic compound material for a lithium ion secondary battery of the present invention is produced by rapid solidification as described above, distortion due to rapid cooling may occur. As a result, the crystal lattice is in a distorted state, and there is also a distortion at the site where lithium is stored between the lattices, so that lithium cannot be occluded sufficiently and a predetermined charge capacity may not be obtained. This lattice strain can be removed by heat treatment (strain relief annealing) in an inert gas or vacuum.

This heat treatment can be performed by heating at a temperature lower than the solidus of the intermetallic compound. Above this temperature, the quenched intermetallic compound is melted and then slowly cooled, and the quenching effect cannot be obtained. This temperature varies depending on the alloy composition, but can be easily obtained using a thermal analyzer such as DTA. Considering that a temperature of 500 ° C. or higher is necessary in a practical state for releasing strain, a preferable temperature for these heat treatments is 500 ° C.
° C to the solidus temperature. The longer the time, the better, but preferably 4 to 10 hours from the economical point of view. The production of the negative electrode from the negative electrode intermetallic compound material for a lithium ion secondary battery of the present invention can be performed by a method well known to those skilled in the art.

For example, if necessary, the intermetallic compound of the present invention is pulverized in an inert atmosphere to obtain a powder, and the obtained powder is mixed with a binder such as PVDF, PMMA, or PTFE, and then mixed with a binder such as NMP or DMF. After dissolving
If necessary, the mixture is sufficiently stirred using a homogenizer, glass beads or the like to form a paste. This paste is applied to a support such as a copper foil having an electrolyzed surface, dried, and then pressed to produce a negative electrode.

The weight ratio of the binder to be mixed is preferably about 5 to 10% by weight from the viewpoint of the mechanical strength of the negative electrode and the electrode characteristics. Further, the support is not particularly limited to a copper foil, and may be any of a thin foil of copper, stainless steel, nickel, or the like, a net-shaped sheet, a punching plate, and the like.

In producing such a negative electrode, the maximum particle size of the powder determines the thickness of the electrode. The thinner the electrode, the better, and the total area of the battery active material contained in the battery can be increased. Thus, the powder is preferably 100 μm or less. If the powder is too fine, the reaction area increases and it can contribute to the rate characteristics.On the other hand, if the powder is too fine, the properties of the powder surface change due to oxidation and the like, making it difficult for lithium ions to penetrate. Exert. Therefore, the preferred powder particle size is 5-100
μm, more preferably 10 to 50 μm.

The intermetallic compound of the present invention is lithium ion 2
It is used as a negative electrode for a secondary battery. The shape of the lithium ion secondary battery is not particularly limited, and may be a wound battery type, a square type, a coin type, a sheet type, and the like. , Positive electrode, separator, electrolyte,
A battery having a configuration including an electrolyte can be used without any problem.

Incidentally, the intermetallic compound according to the present invention can be used in an appropriate combination with a positive electrode active material and an organic solvent-based electrolyte. However, these organic solvent-based electrolytes and positive electrode active materials are used in lithium secondary batteries. This is not particularly limited as long as it can be normally used.

As the positive electrode active material, for example, a lithium-containing transition metal oxide LiM (1) _1-X M (2) _X O ₂ (where 0 X is 0 ≦
X is a numerical value in the range of 1 or less, wherein M (1) and M (2) represent transition metals, and Ba, Co, Ni, Mn, Cr, Ti, V, Fe, Zn,
Al, In, Sn, Sc, at least one of Y) or LiM (1) _2y M (2) _y O ₄ (where Y is a numerical value in the range of 0 ≦ Y ≦ 1; M (1) and M (2) represent transition metals Ba, C
o, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, Sc, Y
, Transition metal chalcogenides, vanadium oxides (V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O ₈ , et
c.) and its Li compound, niobium oxide and its Li compound, a conjugated polymer using an organic conductive substance, a Chevrel phase compound, or activated carbon or activated carbon fiber.

The preparation of the positive electrode can be performed by a method well known to those skilled in the art. As with the negative electrode, a binder such as PVDF or PTFE is used, but since the positive electrode material does not have conductivity, it is mixed with a conductive material such as acetylene black or Ketjen black, and then a solvent such as NMP is added to paste. Shape. It is bonded to a support of an electrode active material such as an aluminum foil by a doctor blade method, a roll molding method, or the like, and pressed to form an electrode.

The organic solvent in the organic solvent-based electrolyte is not particularly limited. For example, propylene carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propionate, methyl acetate, methyl formate , 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyllactam, 1,3-dioxolan, 4-methyl-1,3-dioxolan, anisole, N,
N-diformamide, diethyl ether, sulfolane,
Methylsulfolane, acetonitrile, chloronitrile,
Propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, methyl acetate, methyl formate,
Trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene,
Tetrahydrofuran, 2-methyltetrahydrofuran,
A single solvent or a mixed solvent of two or more compounds such as a compound containing an aromatic ring such as dimethyl sulfoxide, ethylene glycol, thiophene and pyrrole can be used.

As the electrolyte, any of those conventionally known can be used, for example, LiClO ₄ , LiBF ₄ , LiPF
_{6, LiAsF 6, LiB (C} 6 H 5), LiCF 3 SO 3, LiCH 3 SO 3, Li (CF 3
SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , Li (CF ₂ SO ₂ ) ₂ , LiCl, LiBr, LiI
One or a mixture of two or more of these lithium salts can be used.

On the other hand, the separator plays a role as an insulator provided between the positive electrode and the negative electrode. In addition, since it greatly contributes to the retention of the electrolyte, polypropylene,
A porous material such as polyethylene, a mixed cloth of both, or a glass filter is used.

[0044]

EXAMPLES Next, specific working effects of the present invention will be described with reference to examples. Hereinafter, unless otherwise specified,% means% by weight.

Example 1 First, an alloy having a composition included in the present invention and several alloys were produced as comparative examples. Tables 1 and 2 show the composition of the alloy and the sample preparation method. Sample preparation by the atomization method is as follows. After preparing an alloy into a predetermined component using the gas atomization method (cooling rate, about 5.0 × 10 ³ ° C / sec), heat-treating for removing strain is performed, and the obtained powder is removed. Classification was performed using a sieve at 45 to 15 μm.

Charge / discharge test / rate characteristic test (investigation of charge / discharge capacity) 10% of polyvinylidene fluoride as a binder was added to the thus-prepared metal powder for a lithium ion secondary battery, and N-methylpyrrolidone was graphite. 10% for powder
By adding, the above-mentioned polyvinylidene fluoride was dissolved. This was kneaded for about 15 minutes to produce a substantially uniform slurry. This was applied to an electrolytic copper foil having a thickness of 30 μm by a doctor blade method, and punched out using a punch to a size of 13 mm to obtain a negative electrode.

Next, in order to evaluate the electrode characteristics of a single electrode of the above electrode, a so-called triode cell using lithium metal for the counter electrode and the reference electrode was used. The distance between the lugine tube and the negative electrode was set to within 1 mm, and the current density characteristics (rate characteristics) were taken into consideration so as not to affect the measurement. A 1: 1 mixed solvent of ethylene carbonate and dimethylethane was used as an electrolyte, and LiPF ₆ (lithium phosphorus hexafluoride) was used as a supporting electrolyte, and 1 mol was dissolved in the electrolyte. The measurement was performed at 25 ° C., and an atmosphere such as a glove box was maintained in an inert atmosphere, and the dew point of the atmosphere was about −70 ° C.

<Measurement of Charge Capacity / Discharge Capacity> Using the above-mentioned three-electrode cell, the performance as a negative electrode material for a lithium secondary battery was evaluated. First, at 1/10 C charge (conditions for full charge in 10 hours), charge the reference electrode until the potential of the working electrode (negative electrode) becomes 0 V, and charge at the same current value (1/10 C
The discharge was performed up to 1.5 V. The charge capacity in the first cycle at this time was evaluated as an initial capacity value. A discharge capacity of 800 mAh / cc (cc: negative electrode volume) or more obtained in this evaluation was regarded as a pass. The charge / discharge capacity is shown in mAh / cc in view of the need for battery miniaturization.

<Coulomb Efficiency (Charge / Discharge Efficiency) Test> Coulomb efficiency was determined using the value of the first cycle of the above charge / discharge capacity. Coulomb efficiency = discharge capacity / charge capacity × 100 (%) From this, a coulomb efficiency of 90% or more was judged to be acceptable.

<Rate Characteristic Test> At 2C (conditions under which the battery is fully charged in 0.5 hours), the reference electrode is charged until the potential of the working electrode (negative electrode) becomes 0 V, and (1 / Ten
(At C). If the rate characteristics are excellent,
The discharge capacity at this time does not largely change from that at 1/10. In this study, the discharge capacity (2C) / discharge capacity (1 / 10C) x 100 (%) is 90% or more compared to that of the conventional intermetallic compound (eg, FeSi ₂ ). Passed.

Table 1 shows the test results. Indeed, formula, a compound represented by AB _x, A is Fe, Mn, Co, Mo,
One or more elements selected from the group consisting of Cr, Nb, V, Cu, and W, and B has Si as an essential element, and Si,
Of those composed of one or more elements selected from the group consisting of C, Ge, Sn, Pb, Al, and P, 0.1.
An intermetallic compound such that 5 ≤ X ≤ 3, from the molten state
It can be seen that those solidified at a cooling rate of 100 ° C./sec or more and 5000 ° C./sec or less and further subjected to heat treatment have a large discharge capacity and excellent rate characteristics.

[0052]

[Table 1]

[0053]

[0054] Example 2 In this Example, Example 1 was repeated in order to see the influence of the presence or absence of heat treatment. Powder or ribbon ingot obtained by rotating electrode method, after pulverized by impact grinding, the resulting powder was classified with a 45～15Myuemu. As a comparative example, a normal dissolution method was performed. Measure the distance between the secondary arms of the ingot dendrite obtained by casting the Al-4% Cu alloy, and use a mold that is assumed to have a cooling rate of 30 ° C / sec. An ingot was prepared by pouring into a mold, and pulverized and classified in the same manner as in the ingot.

The heat treatment was performed by heating at 950 ° C. × 10 hours in an Ar gas atmosphere. The results are summarized in Table 2, and as can be seen, according to the present invention, results equivalent to or better than those of the conventional intermetallic compound were obtained.

[0056]

[Table 2]

[0057]

As is apparent from the above description, when the metal material for a lithium ion secondary battery according to the present invention is used, the performance of discharge capacity and Coulomb efficiency can be maintained, and the lithium ion having excellent rate characteristics can be maintained. A secondary battery can be obtained. Therefore, a lithium ion secondary battery having the advantages of a Ni-hydrogen battery and a Ni-Cd battery can be obtained.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kazutaka Asabe 4-5-33 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries, Ltd. (56) References JP-A-10-294112 (JP, A) JP 10-223221 (JP, A) JP 10-125317 (JP, A) JP 10-162823 (JP, A) JP 7-130358 (JP, A) JP 7-249410 (JP JP-A 8-153538 (JP, A) JP-A 8-153517 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/04 H01M 4/02 H01M 4/58 H01M 10/40

Claims (2)

(57) [Claims] 1. An alloy represented by the chemical formula: AB _x is rapidly cooled and solidified from a molten state at a cooling rate of 100 ° C./sec or more and 5000 ° C./sec or less, and lithium ions having an average crystal grain size of 50 μm or less are obtained. A method for producing a negative electrode material for a secondary battery. However, A
Is one or more elements selected from the group consisting of Fe, Mn, Co, Mo, Cr, Nb, V, Cu, and W, B is Si or a part of Si, C, Ge, Substituted with one or more elements selected from the group consisting of Sn, Pb, Al, and P, and 0.5 ≦ X ≦ 3. 2. A heat treatment after the quenching and solidification.
The method for producing a negative electrode material for a lithium ion secondary battery according to the above.