JP4029291B2 - Negative electrode material for lithium secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a negative electrode material for a lithium secondary battery and a method for producing the same.

  Lithium secondary batteries such as lithium ion batteries and lithium polymer batteries have a high energy density, and in recent years, their use as main power sources for mobile communication devices, portable electronic devices, and the like is expanding.

  As a negative electrode in such a lithium battery, various carbon materials such as graphite and carbon having a low crystallinity are widely used. However, an electrode made of a carbon material has a low usable current density and an insufficient theoretical capacity. For example, graphite, which is one of the carbon materials, has a theoretical capacity of only 372 mAh / g, and therefore a higher capacity is desired.

  On the other hand, when lithium metal is used as a negative electrode material for a lithium secondary battery, a high theoretical capacity can be obtained, but dendrites are deposited on the negative electrode during charging, reach the positive electrode side by repeated charge and discharge, and cause internal short circuit. There is a major drawback that the phenomenon occurs. In addition, the deposited dendrites have high reaction activity due to their large specific surface area, and an interfacial film consisting of decomposition products of solvents having no electron conductivity is formed on the surface, thereby increasing the internal resistance of the battery. As a result, the charge / discharge efficiency decreases. For these reasons, lithium secondary batteries using lithium metal have the disadvantages of low reliability and short cycle life, and have not yet reached a stage where they are widely put into practical use.

  From such a background, a negative electrode material made of a material other than lithium metal and having a larger discharge capacity than a general-purpose carbon material is desired. For example, elements such as tin, silicon and silver, nitrides and oxides of these elements can occlude lithium by forming an alloy with lithium, and the occlusion amount is much larger than that of carbon. And various types of alloy negative electrodes containing these substances have been proposed.

  However, when these materials are used as negative electrode materials, as the charge / discharge cycle is repeated, large expansion and contraction may occur with the insertion and extraction of lithium, and the electrode itself may collapse.

  As a countermeasure, an attempt has been made to use an alloy composed of a metal that easily stores and releases lithium and a metal that does not store and release lithium as a negative electrode material. According to such an alloy, it can be considered that the presence of a metal that does not occlude / release lithium makes it possible to suppress swelling and miniaturization, and various alloys have been proposed.

For example, Patent Document 1 listed below discloses an alloy having a rapidly solidified structure, and Patent Document 2 listed below discloses that either one of the phases A and B has an average particle size in the matrix of the other phase. An alloy having a structure dispersed in an island shape of 0.05 to 20 μm is disclosed. However, even with these alloy materials, although a large initial discharge capacity can be obtained, expansion and miniaturization are unavoidable while charging and discharging are repeated, and the stage has not yet reached a stage where the reduction in discharge capacity can be sufficiently suppressed. .
JP 2001-297757 A (5th page, FIG. 2) JP 2001-93524 A (page 2)

  The present invention has been made in view of the current state of the prior art, and its main object is to provide a negative electrode material for a lithium secondary battery that can exhibit excellent cycle characteristics while maintaining a high discharge capacity. It is in.

  The inventor has conducted extensive research while paying attention to the current state of the prior art. As a result, when a composite powder containing two or more specific elements and having a nano-order primary particle size is used as a negative electrode material for a lithium secondary battery, expansion and contraction associated with insertion and extraction of lithium are alleviated. Thus, the inventors have found that the electrode can be prevented from deteriorating, and have completed the present invention.

That is, this invention provides the following negative electrode material for lithium secondary batteries, and its manufacturing method.
1. A negative electrode material for a lithium secondary battery comprising a composite powder satisfying the following conditions (1) to (3);
(1) The composite powder is made of Ag, Al, Au, Ca, Cu, Fe, In, Mg, Pd, Pt, Y, Zn, Ti, V, Cr, Mn, Co, Ni, Y, Zr, Nb, A component which is at least one element selected from the group consisting of Mo, Hf, Ta, W and rare earth elements, and B component which is at least one element selected from the group consisting of Ga, Ge, Sb, Si and Sn And an alloy of an A component and a B component,
(2) The ratio of the A component and the B component in the entire composite powder is 30 to 70 atomic% of the A component and 70 to 30 atomic% of the B component, with the total amount of both being 100 atomic%.
(3) 10% or more of the primary particles of the composite powder are in the range of 10 to 500 nm in particle size.
2. 2. The lithium secondary according to item 1, wherein the ratio of the A component and the B component in the entire composite powder is 40 to 60 atomic% of the A component and 60 to 40 atomic% of the B component, with the total amount of both being 100 atomic%. Negative electrode material for batteries.
3. A component is at least one element selected from the group consisting of Ag, Cu, Fe, In and Mg, and B component is at least one element selected from the group consisting of Sb, Si and Sn Item 3. The negative electrode material for a lithium secondary battery according to Item 1 or 2.
4). The combination of the A component and the B component is Ag-Sb, Au-Sb, Cu-Sb, Zn-Sn, Au-Si, Ca-Si, Cu-Si, Y-Si, Zn-Si, Ag-Sn, Au. Item 3. The negative electrode material for a lithium secondary battery according to Item 1 or 2, which is —Sn, Cu—Sn, In—Sn, Mg—Sn, Pd—Sn, Pt—Sn, or Y—Sn.
5. Ag, Al, Au, Ca, Cu, Fe, In, Mg, Pd, Pt, Y, Zn, Ti, V, Cr, Mn, Co, Ni, Y, Zr, Nb, Mo, Hf, Ta, W and A raw material comprising an A component, which is at least one element selected from the group consisting of rare earth elements, and a B component, which is at least one element selected from the group consisting of Ga, Ge, Sb, Si and Sn, is mixed. The method for producing a negative electrode material for a lithium secondary battery according to any one of Items 1 to 4, wherein the composite powder is formed by performing mechanical alloying treatment.

  The negative electrode material for a lithium secondary battery of the present invention includes Ag, Al, Au, Ca, Cu, Fe, In, Mg, Pd, Pt, Y, Zn, Ti, V, Cr, Mn, Co, Ni, Y, A component which is at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta, W and rare earth elements, at least one element selected from the group consisting of Ga, Ge, Sb, Si and Sn A composite powder composed of the B component and the alloy of the A component and the B component is used as an active ingredient. In this composite powder, examples of rare earth elements include La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu.

  In the composite powder, the alloy of the A component and the B component is an alloy phase in which the B component is dissolved in the A component, an intermetallic compound phase of the A component and the B component, and an alloy phase in which the A component is dissolved in the B component. Etc.

  In the composite powder, in particular, the component A is preferably at least one element selected from the group consisting of Ag, Al, Au, Ca, Cu, Fe, In, Mg, Pd, Pt, Y, Zn, and rare earth elements. At least one element selected from the group consisting of Ag, Cu, Fe, In, and Mg is more preferable. The B component is preferably at least one element selected from the group consisting of Sb, Si and Sn.

  Preferred combinations of A component and B component are Ag-Sb, Au-Sb, Cu-Sb, Zn-Sn, Au-Si, Ca-Si, Cu-Si, Y-Si, Zn-Si, Ag-Sn, Each combination includes Au—Sn, Cu—Sn, In—Sn, Mg—Sn, Pd—Sn, Pt—Sn, Y—Sn, and the like.

  The composite powder can exhibit excellent characteristics as described later when the particle size of the primary particles, particularly the particles obtained by alloying the A component and the B component is in the nano-size order. However, when the composite particles are produced by the mechanical alloying method to be described later, primary particles in the order of microns may be included depending on the combination of the component A and the component B, but such micron order primary particles are present. May be acceptable and may contribute to prevention of pulverization and improvement of electrical conductivity. In view of this, the composite powder may have 10% or more primary particles in the range of about 10 to 500 nm, preferably in the range of about 10 to 200 nm, based on the entire composite powder. In this case, in particular, when there are many primary particles in the order of microns, the life of the electrode can be extended. In order to increase the capacity of the electrode, it is preferable that the primary particles are in the range of about 10 to 500 nm (preferably 10 to 200 nm) of 50% or more, preferably 70% or more, more preferably 90% or more. It is.

  When the particle size of the primary particles is within the above-described range, the rearrangement of atoms during lithium occlusion is likely to occur reversibly, and the desired characteristics can be exhibited. The primary particles having a particle size smaller than that described above are not preferable because a long time is required for pulverization, and the primary particles having a particle size larger than that described above are also preferable because the rearrangement of atoms during lithium storage is less likely to occur. Absent. In the present specification, the particle size of the primary particles is a value obtained by observation with a transmission electron microscope.

  By using such a nano-order composite powder as a negative electrode material, the entire material can easily combine with lithium during charging and occlude lithium, and then during discharge, easily leave an irreversible skeleton. Lithium can be released to prevent pulverization.

In the composite powder described above, the ratio of the A component and the B component is about 30 to 70 atomic percent of the A component and about 70 to 30 atomic percent of the B component, where the total amount of the A and B components in the entire composite powder is 100 atomic percent. The component A is preferably about 40 to 60 atom% and the component B is preferably about 60 to 40 atom%. With such a composition, it can be at the time of lithium occlusion, thereby mainly produces lithium ternary compound Li ₂ AB phase of a face-centered cubic structure. In LixAB, lithium can be occluded without an alloy phase separation up to around x = 4, and is effective in suppressing volume change during lithium occlusion.

  The composite powder which is an active ingredient of the negative electrode material of the present invention can be produced by, for example, a mechanical alloying method. In this method, it is possible to easily form fine primary particles having a particle size of about 10 to 200 nm. As a specific method, the target composite powder can be obtained by mixing the raw materials composed of the A component and the B component, performing mechanical alloying treatment, and setting the primary particle diameter to about 10 to 200 nm. . The centrifugal acceleration (input energy) in the mechanical alloying process is preferably about 5 to 20G, and more preferably about 7 to 15G.

  The mechanical alloying process itself may be applied as it is. For example, the target composite powder can be obtained by compounding (partially alloying) the raw material mixture while repeating mixing and adhesion by mechanical joining force. As an apparatus to be used, a mixer, a disperser, a pulverizer and the like generally used in the powder field can be used as they are. Specific examples include a reiki machine, a ball mill, a vibration mill, an agitator mill, and the like. In particular, in order to reduce the stacking of powders mainly composed of battery active materials present between networks, it is necessary to efficiently disperse the powders that are overlapped or aggregated during the compositing operation one by one. Therefore, it is desirable to use a mixer that can give a shearing force. The operating conditions of these devices are not particularly limited.

  The fine primary particles obtained in this way are usually agglomerated to form secondary agglomerates. When an alloy is produced by the mechanical alloying method, the secondary agglomerates are examined by laser diffraction. The maximum particle size is about 38 to 150 μm.

  The particle size of the secondary agglomerates is not particularly limited, but when used as a negative electrode material, 90% or more, preferably 99.9% or more of the secondary agglomerates are in the range of about 1 to 105 μm in particle size. Preferably, it exists in the range of about 1-50 micrometers. Moreover, about the average particle diameter of a secondary aggregate, it is preferable that it is about 5-50 micrometers, and it is more preferable that it is about 5-10 micrometers. When the particle size of the secondary agglomerates is within this range, the electrode can be produced with high accuracy. The particle size of the secondary aggregate is determined by laser diffraction method or scanning electron microscope observation.

  In the negative electrode material composed of the composite powder described above, a ternary compound containing lithium is formed at the first charge (lithium occlusion) and returns to the original composite material at the next discharge (lithium release), but an irreversible compound containing lithium is formed. Some remain. From this, insertion and extraction of lithium is performed through this lithium-containing compound, the irreversible lithium compound formed in the first charge (first time) exists as a skeleton, and in the second and subsequent charge and discharge, It is considered that lithium is occluded in this lithium compound and released. Thereby, it is considered that the volume change due to charging / discharging is mitigated, pulverization is suppressed, deterioration of the electrode is prevented, and the cycle characteristic life is improved.

  Next, the charge / discharge reaction when the composite powder is used as a negative electrode material will be described.

In the negative electrode material of the present invention, as shown in the formula (1), the B component is converted to a Li compound during charging. Further, as shown by the formula (2), the A ₃ B alloy changes from A ₂ LiB to ALi ₂ B as the Li storage amount increases. At the same time, the reaction represented by the formula (3) is also expected to occur. Furthermore, as shown by the formula (4), it is considered that a Li ternary compound containing high Li is formed. Li _x B, especially when x = 4.4, material pulverization occurs due to large volume expansion and electrode characteristics deteriorate, so it is thought that electrode characteristics can be improved by keeping Li storage up to ALi ₂ B. It is done.

xLi + B (Li _x B (x ≦ 4.4) (1)
2Li + A ₃ B (A ₂ LiB + A + Li (ALi ₂ B + 2A (2)
ALiB + A + Li _x B (ALi ₂ B + Li _y B (diffusion reaction) (3)
(y + z) Li + ALi ₂ B (Li _{2 + y} A _1-z B + zLiA (y ≦ 2.4, z ≦ 1) (4)
FIG. 1 is a drawing schematically showing a state in which the internal structure of a composite powder changes based on each of the above reactions when lithium is occluded with respect to a composite powder using A component and B component as raw materials. This drawing schematically shows a process in which a ternary compound containing Li is formed from the reaction between the composite powder and Li.

In the negative electrode material of the present invention, it is important that the lithium occlusion / release process is performed through the ternary compounds A ₂ LiBn and ALi ₂ B.

For example, in a negative electrode using a composite material of Ag / Sn (atomic ratio) = 52/48, the discharge capacity at the second cycle is 570 mAh / g, and the capacity is reduced by about 270 mAh / g as compared to one cycle. This irreversible capacity change is considered to be caused by the AgLi ₂ Sn phase generated by the first charge. Volume change with respect to Ag ₃ Sn (density 9932 g / cm ³ ) by the process of occluding and releasing lithium through Ag ₂ LiSn (density 7920 g / cm ³ ) and AgLi ₂ Sn (density 5630 kg / m ³ ) Are 1.25 times (Ag ₂ LiSn) and 1.76 times (AgLi ₂ Sn), respectively. When Sn (density 7286 kg / m ³ ) occludes Li and becomes Li _4.4 Sn (density 1920 g / cm ³ ), the above ternary compound is formed compared with the volume change of 3.8 times. When done, the volume increase is very small. For this reason, it seems that the capacity | capacitance fall by swelling and refinement | miniaturization of an electrode is suppressed and a cycle life improves. As a result, the composite powder has a high discharge capacity, little deterioration due to charge / discharge, and can achieve both a high discharge capacity and excellent cycle characteristics when used as a negative electrode material for a lithium battery.

FIG. 2 is a transmission electron micrograph showing the internal structure of a lithium occlusion state of an electrode made of AgFeSn (AB type) composite powder. In the figure, dark colors (points 1 and 2) and gray (points 3 and 4) indicate ternary compounds of LiAg ₂ Sn and Li ₂ AgSn, and light colors (5 and 6) indicate LiSn and LiAg compounds. It can be seen that the composite material occludes Li to form a Li ternary compound.

  When the composite powder described above is used as a negative electrode material for a lithium battery, the configuration of the negative electrode for a lithium battery may be the same as that known in the art except that the composite powder is used as a negative electrode material. For example, a negative electrode can be produced by blending a resin-based binder, a conductive additive, etc. as necessary, forming an electrode layer on a known current collector such as a copper foil current collector, and integrating them. it can. When manufacturing a lithium battery using this negative electrode, battery elements (positive electrode, separator, electrolyte, etc.) of known lithium ion batteries are used, and known lithium ions such as rectangular, cylindrical, and coin types are used. What is necessary is just to follow the assembly method of a battery.

  The negative electrode material of the present invention is a material having a large initial discharge capacity and excellent cycle characteristics, and is highly useful as a negative electrode material for lithium secondary batteries such as lithium ion batteries and lithium polymer batteries.

Hereinafter, the present invention will be described in more detail with reference to examples.
Examples 1-50
Metal powder was mixed so that the ratio (atomic%) shown in Table 1 below was obtained, 0.5 part by weight of stearic acid was added as a lubricant to 100 parts by weight of metal powder, and the mixture was put into a planetary ball mill made of Fritche. A composite powder was obtained by performing an alloying treatment. As a result of measuring the particle diameter of the primary particles of the obtained composite powder with a transmission electron microscope, all the particles were in the range of 50 to 200 nm.

  To 85 parts by weight of the composite powder thus obtained, 10 parts by weight of a paste in which polyvinylidene fluoride (PVdF) is dissolved in N-methylpyrrolidone (NMP) and 5 parts by weight of carbon black are added and mixed. A slurry was prepared.

Subsequently, the slurry was placed on an electrolytic copper foil having a thickness of 12 μm and laminated with a doctor blade to form a sheet. The obtained sheet was dried at 80 ° C. for 10 minutes to volatilize and remove NMP, and then the current collector made of electrolytic copper foil and the negative electrode layer made of the composite powder were firmly adhered and bonded by a roll press machine. I let you. This was extracted with a 1 cm ² circular punch and vacuum dried at 120 ° C. for 6 hours to obtain an electrode having a thickness of 10 μm.

1 mol of lithium PF ₆ / ethylene carbonate (EC) + dimethyl carbonate (DMC) (EC: DMC = 1: 2 (volume ratio)) using the above electrode as a negative electrode and metallic lithium as a counter electrode A coin-type model battery (CR2032 type) was produced in a dry box using the solution as an electrolytic solution.

  The negative electrode in this model battery was evaluated by the following method.

First, the model battery was discharged at a constant current of 0.2 mA / cm ² until reaching 0 V, and after 10 minutes of rest, it was charged at a constant current of 0.2 mA / cm ² until reaching 1.0 V. This was made into 1 cycle, charging / discharging was performed repeatedly and the discharge capacity was investigated.

  Table 1 shows the cycle number and discharge capacity of the battery using the composite powder of each example. In Table 2, single metals (Comparative Examples 1 to 8), binary alloys (Comparative Examples 9 to 21) or ternary alloys (Comparative Examples 22 to 24) were used as negative electrode materials as comparative examples. For cases, the number of cycles and the discharge capacity are shown.

As is apparent from the above table, the batteries having the composite powder of each example as the negative electrode have a high initial discharge capacity and a sufficient discharge capacity after 50 cycles.

  FIG. 3 is a graph showing cycle characteristics, which are the relationship between the discharge capacity and the number of cycles, for the battery using the negative electrode material of Example 30 and the battery using the negative electrode material of Comparative Example 8. As is clear from FIG. 3, the battery using the negative electrode material of Example 30 had an initial capacity of 560 mAh / g and maintained a capacity of 370 mAh / g even after 300 cycles. It can be seen that the battery has an excellent cycle life as compared with a battery using a negative electrode material.

  FIG. 4 is a transmission electron micrograph showing the internal structure of the negative electrode material of Example 3. According to FIG. 4, it can be seen that this material is composed of microcrystalline particles of the order of several tens of nm.

Drawing which shows typically the internal structure change of this invention negative electrode material at the time of occluding lithium. The transmission electron micrograph which shows the internal structure of the lithium occlusion state of this invention negative electrode material. The graph which shows cycling characteristics about the battery using the negative electrode material of Example 30, and the battery using the negative electrode material of the comparative example 8. FIG. 4 is a transmission electron micrograph showing the internal structure of the negative electrode material of Example 3. FIG.

Claims (5)

A negative electrode material for a lithium secondary battery comprising a composite powder satisfying the following conditions (1) to (3);
(1) the composite powder, Ag, Al, Au, Ca , Cu, Fe, In, P d, Pt, Y, A component is at least one element selected from the group consisting of Z n及 beauty rare earth elements B component which is at least one element selected from the group consisting of Sb , Si and Sn, and an alloy of A component and B component,
(2) The ratio of the A component and the B component in the entire composite powder is 30 to 70 atomic% of the A component and 70 to 30 atomic% of the B component, with the total amount of both being 100 atomic%.
(3) 10% or more of the primary particles of the composite powder are in the range of 10 to 500 nm in particle size. 2. The lithium secondary according to claim 1, wherein the ratio of the A component and the B component in the entire composite powder is 40 to 60 atomic% of the A component and 60 to 40 atomic% of the B component, with the total amount of both being 100 atomic%. Negative electrode material for batteries. Claim A component, Ag, Cu, at least one element selected from Fe and In or Ranaru group, B component is at least one element selected from the group consisting of Sb, Si and Sn The negative electrode material for lithium secondary batteries according to 1 or 2. The combination of the A component and the B component is Ag-Sb, Au-Sb, Cu-Sb, Zn-Sn, Au-Si, Ca-Si, Cu-Si, Y-Si, Zn-Si, Ag-Sn, Au. The negative electrode material for a lithium secondary battery according to claim 1 or 2, which is -Sn, Cu-Sn, In-Sn , Pd-Sn, Pt-Sn, or Y-Sn. Ag, Al, Au, Ca, Cu, Fe, In, P d, Pt, Y, Z n 及 beauty A component is at least one element selected from the group consisting of rare earth elements, arranged in S b, Si and 5. The composite powder is formed by mixing a raw material composed of a B component which is at least one element selected from the group consisting of Sn and performing mechanical alloying treatment. The manufacturing method of the negative electrode material for lithium secondary batteries as described.