JP2017091662A - Negative electrode active material, negative electrode, battery, and method for producing negative electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material excellent in charge/discharge capacity and cycle characteristics.SOLUTION: A negative electrode active material according to the present embodiment contains a Cu-Sn-based alloy including a ζ phase, and therefor the negative electrode active material according to the present embodiment has a high charge/discharge capacity. The negative electrode active material according to the present embodiment includes a ζ phase, and thereby strain accompanied by a volumetric change (expansion and contraction) in absorbing and desorbing lithium ions is relaxed. Accordingly, cycle characteristics of a secondary battery is enhanced.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode active material, a negative electrode and a battery, and a method for producing a negative electrode active material.

  In recent years, small electronic devices such as home video cameras, notebook computers, and smartphones, and hybrid vehicles, plug-in hybrid vehicles, and electric vehicles have been widely used. With the widespread use of these small electronic devices and automobiles, higher capacity, higher cycle characteristics, and compactness of batteries are required.

  A secondary battery represented by a lithium ion battery uses a graphite-based negative electrode active material. However, the graphite-based negative electrode active material has limitations in increasing the capacity and increasing the cycle characteristics.

  Therefore, an alloy-based negative electrode active material having a higher capacity than a graphite-based negative electrode active material has attracted attention. As an alloy-based negative electrode active material, a silicon (Si) -based negative electrode active material and a tin (Sn) -based negative electrode active material are known. Various studies have been made on the above alloy-based negative electrode active material materials for practical use of a more compact and long-life lithium ion battery.

  However, the alloy-based negative electrode active material repeatedly repeats large expansion and contraction during charging and discharging. Therefore, the capacity of the alloy-based negative electrode active material is likely to deteriorate. For example, the volume expansion / contraction rate of graphite accompanying charging is about 12%. On the other hand, the volume expansion / contraction rate of Si alone or Sn accompanying charging is about 400%. For this reason, when the negative electrode plate of Si simple substance or Sn simple substance repeats charging and discharging, remarkable expansion and contraction occur, and cracks and cracks occur in the negative electrode mixture applied to the current collector. As a result, the capacity of the negative electrode is significantly reduced. This is because part of the negative electrode active material is liberated by volume expansion and contraction, and the negative electrode plate loses electronic conductivity.

  International Publication No. WO2013 / 141230 (Patent Document 1) proposes porous silicon particles and porous silicon composite particles suitable for a negative electrode material for a lithium ion battery that achieves high capacity and good cycle characteristics. The porous silicon particles of Patent Document 1 are porous silicon particles having a continuous void formed by joining a plurality of silicon fine particles, and the particle size of the silicon fine particles, the column diameter, or the average x of the column sides is 2 nm to 2 μm. The standard particle size σ of the silicon fine particles, the column diameter, or the column side is 1 to 500 nm, and the ratio (σ / x) of the average x to the standard deviation σ is 0.01 to 0.5. Thus, Patent Document 1 describes that when the silicon fine particles are thermally expanded and contracted during charge and discharge, the silicon fine particles expand so as to fill the voids, so that the negative electrode is hardly cracked.

  In patent document 1, the expansion | swelling shrinkage | contraction change of the silicon particle which arises with occlusion and discharge | release of lithium ion is suppressed by the space | gap which exists beforehand. Thereby, the cycle characteristics of the battery are improved. However, Patent Document 1 only shows the volume maintenance rate after 50 cycles, and even the porous silicon particles disclosed in Patent Document 1 sometimes have low cycle characteristics.

International Publication WO2013 / 141230

Binary Alloy Phase Diagrams, Second Edition, Ed. T. T. B. Massalski, ASM International, Materials Park, Ohio (1990) 2, 1481-1483. J. et al. K. Brandon, et al. , Acta Cryst. (1975). B31, 774-779

  An object of the present invention is to provide a negative electrode active material excellent in charge / discharge capacity and cycle characteristics.

  The negative electrode active material according to the present embodiment contains a Cu—Sn alloy containing a ζ phase.

  The negative electrode active material according to the present embodiment is excellent in charge / discharge capacity and cycle characteristics.

FIG. 1 is an equilibrium diagram of a Cu—Sn alloy. FIG. 2 is a diagram showing an actual measurement profile of X-ray diffraction measurement before and after charging of the negative electrode active material of test number 3 and calculation profiles of ζ phase and ε phase. FIG. 3 is a diagram showing an actual measurement profile of the X-ray diffraction measurement of the negative electrode active material (Test No. 2) of test number 2 and a calculation profile of the ζ phase. FIG. 4 is a diagram showing the cycle characteristics of the coin battery of test number 3. FIG. Figure 5 is a diagram showing a measured profile of the X-ray diffraction measurement of the negative electrode active material (pre-charge), and a calculated profile of the σ phase and D0 ₃ phase Test No. 18. FIG. 6 is a diagram showing an actual measurement profile of X-ray diffraction measurement and a calculation profile of the ε phase of the negative electrode active material of test number 19 (before charging).

  The negative electrode active material according to the present embodiment contains a Cu—Sn alloy containing a ζ phase.

  The negative electrode active material according to the present embodiment contains a Cu—Sn alloy containing a ζ phase. Therefore, the negative electrode active material according to the present embodiment has a high charge / discharge capacity. The negative electrode active material according to the present embodiment includes a ζ phase. Thereby, distortion accompanying volume change (expansion and contraction) at the time of occlusion and release of lithium ions is relieved, and the cycle characteristics of the secondary battery are enhanced.

  Preferably, the Cu-Sn based alloy contains Sn: 21.0 to 24.0 at%, with the balance being Cu and impurities.

  The Cu-Sn alloy may be selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C, instead of a part of Cu, or You may contain 2 or more types.

  In this case, the Cu-Sn alloy has Sn: 21.0 to 24.0 at%, Ti: 2.0 at% or less, V: 2.0 at% or less, Cr: 2.0 at% or less, Mn: 2 0.0 at% or less, Fe: 2.0 at% or less, Co: 2.0 at% or less, Ni: 3.0 at% or less, Zn: 3.0 at% or less, Al: 3.0 at% or less, Si: 3.0 at% % Or less, B: 2.0 at% or less, and C: One or more selected from the group consisting of 2.0 at% or less, and the balance is preferably made of Cu and impurities.

Preferably, the negative electrode active material is in addition to the ζ phase is selected from the group consisting of phase with that contains the site deficiency, [delta] phase F-Cell structure, epsilon phase, eta 'phase, and D0 ₃ structure It has one or more phases.

These δ phase containing site deficiency, epsilon phase, eta 'phase, and any phase having a D0 ₃ structure, the negative electrode active material material, to form a storage site and spreading site metal ions (Li ion) . Therefore, the volume discharge capacity and cycle characteristics of the negative electrode active material are further improved.

  The negative electrode according to the present embodiment includes the negative electrode active material described above. The battery according to the present embodiment includes the above-described negative electrode.

  The manufacturing method of the negative electrode active material according to the present embodiment is a manufacturing method of the above-described negative electrode active material, and includes a manufacturing process of a molten material and a solution heat treatment process. In the manufacturing process of the melting material, the melting material of the raw material is manufactured. In the solution heat treatment step, a solution heat treatment is performed in which the melted material is kept at 550 to 670 ° C. and then rapidly cooled to room temperature or lower.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

[Negative electrode active material]
The negative electrode active material according to the present embodiment contains a Cu—Sn alloy containing a ζ phase. Therefore, the negative electrode active material according to the present embodiment has a high charge / discharge capacity. Here, the Cu—Sn alloy refers to an alloy containing Cu and Sn in total of 90 at% or more.

[Crystal structure of negative electrode active material]
The negative electrode active material according to the present embodiment includes a ζ phase that is a kind of Cu—Sn alloy phase. The ζ phase is one type of equilibrium phase at a high temperature shown in the equilibrium diagram (Non-Patent Document 1) of the Cu—Sn alloy shown in FIG. The ζ phase is known to have a hexagonal crystal structure (
J. et al. K. Brandon, et al. , Acta Cryst. (1975). B31, 774-779 (Non-Patent Document 2)). For the detailed crystal structure of the ζ phase, the space group classification is No. by International Table A The measurement numerical example of the lattice constant is a = 7.332 Å and c = 7.856 、, and the example of atomic coordinates of the constituent elements contained in the unit cell is the structure shown in Table 1.

  The crystal structure of the ζ phase changes when the metal ions are occluded, and returns to the original crystal structure of the ζ phase when the metal ions are released. When the crystal structure of the ζ phase occludes metal ions, 38.0 to 39.0 ° and 49.0 to 51.50 when main diffraction lines of X-ray diffraction are measured using Cu-Kα1 rays. The crystal structure has a peak top in each range of 0 ° at an angle 2θ.

  The crystal structure of the ζ phase changes upon occlusion of metal ions. Therefore, the distortion accompanying the volume change in occlusion of metal ions is relieved. When the strain is alleviated, a decrease in charge / discharge capacity due to breakage of the negative electrode mixture is suppressed even if charge / discharge is repeated. Thereby, the cycle characteristics of the negative electrode active material are enhanced.

  The negative electrode active material according to the present embodiment preferably includes a ζ phase as a main phase. “Main phase” means an alloy phase occupying 50% by volume or more.

The negative electrode active material according to the present embodiment may contain an alloy phase other than the ζ phase that occludes metal ions. Other alloy phases that occlude metal ions are, for example, the ε phase, η ′ phase, and δ phase shown in the phase diagram of FIG. The ε phase and η ′ phase are parallel stable phases at room temperature, and the δ phase is a stable phase at high temperatures. Other alloy phase is further not shown as an equilibrium phase diagram states, including non-equilibrium phase etc. with D0 ₃ structure. The negative electrode active material may have one or more of these alloy phases.

When the other alloy phase described above includes site defects, the charge / discharge capacity and cycle characteristics of the negative electrode active material are improved. Thus, the negative electrode active material material comprises site deficiency, [delta] phase F-Cell structure, epsilon phase, eta 'phase, and D0 ₃ 1 or two or more phases selected structure from the group consisting of phases with It is preferable to have. These alloy phases containing site defects form storage sites and diffusion sites for metal ions such as lithium ions in the negative electrode active material. Therefore, the charge / discharge capacity and cycle characteristics of the negative electrode active material can be further improved. Here, “site deficiency” means that the occupation ratio is less than 1 at a specific atomic site in the crystal structure.

[Method for analyzing crystal structure of negative electrode active material]
The crystal structure of the negative electrode active material can be analyzed, for example, by the following method. X-ray diffraction measurement is performed on the negative electrode active material to obtain actual measurement data of the X-ray diffraction profile. Based on the obtained X-ray diffraction profile, the structure of the phase in the negative electrode active material is analyzed by the Rietveld method. For the analysis by the lead belt method, one of general-purpose analysis software “RIETA-2000” (program name) and “RIETAN-FP” (program name) is used.

  The crystal structure of the negative electrode active material before charging in the battery can be analyzed, for example, by the following method. In a state before charging, the battery is disassembled in a glove box in an argon atmosphere, and the negative electrode is taken out from the battery. The taken-out negative electrode is wrapped in mylar foil. Thereafter, the periphery of the mylar foil is sealed with a thermocompression bonding machine. The negative electrode sealed with Mylar foil is taken out of the glove box.

  Subsequently, the negative electrode is attached to a non-reflective sample plate (a plate cut out so that the specific crystal plane of the silicon single crystal is parallel to the measurement plane) with a hair spray to prepare a measurement sample. A measurement sample is set in an X-ray diffractometer, and X-ray diffraction measurement of the measurement sample is performed to obtain an X-ray diffraction profile. Based on the obtained X-ray diffraction profile, the crystal structure of the negative electrode active material in the negative electrode is specified by the Rietveld method.

  Regarding the crystal structure of the negative electrode active material after charging, the crystal structure can be analyzed by preparing a measurement sample by the above-described method for a fully charged battery and performing X-ray diffraction measurement. About the crystal structure of the negative electrode active material after discharge, the crystal structure can be analyzed by performing X-ray diffraction measurement in the same procedure after the battery after charging is completely discharged.

  The X-ray diffraction measurement for analyzing the crystal structure change accompanying charging / discharging can also be performed by the following method. The coin battery before and after charging is decomposed in an inert gas atmosphere such as argon. The negative electrode active material applied to the negative electrode plate is peeled off from the current collector foil with a spatula or the like, filled in a sample holder for X-ray diffraction, and sealed. An X-ray diffraction measurement is performed on the negative electrode active material while holding the negative electrode active material in the sample holder. In this method, diffraction lines derived from the copper foil or the like of the current collector are removed, and the crystal structure can be easily analyzed.

[Chemical composition of negative electrode active material]
The chemical composition of the negative electrode active material according to the present embodiment is not particularly limited as long as the negative electrode active material contains a Cu—Sn-based alloy containing a ζ phase, but is preferably as follows. Hereinafter, “at%” for the chemical composition means the atomic composition percentage.

  Preferably, the negative electrode active material contains Sn, and the balance is Cu and impurities. The Sn content is preferably 21.0 to 24.0 at%. A more preferable lower limit of the Sn content is 22.0 at%, and a more preferable upper limit of the Sn content is 23.0 at%.

  The negative electrode active material is one or more selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C instead of part of Cu. It may contain.

  In this case, the chemical composition of a preferable negative electrode active material is Sn: 21.0 to 24.0 at%, Ti: 2.0 at% or less, V: 2.0 at% or less, Cr: 2.0 at% or less, Mn : 2.0 at% or less, Fe: 2.0 at% or less, Co: 2.0 at% or less, Ni: 3.0 at% or less, Zn: 3.0 at% or less, Al: 3.0 at% or less, Si: 3 0.0 at% or less, B: 2.0 at% or less, and C: one or more selected from the group consisting of 2.0 at% or less, with the balance being Cu and impurities. Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C are elements that are arbitrarily added as necessary. The more preferable contents of these elements are as follows.

  The upper limit with preferable Ti content is 2.0 at% as above-mentioned. The upper limit with more preferable Ti content is 1.0 at%, More preferably, it is 0.5 at%. When Ti is contained, the preferable lower limit of the Ti content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable V content is 2.0 at% as above-mentioned. The upper limit with more preferable V content is 1.0 at%, More preferably, it is 0.5 at%. When V is contained, the preferable lower limit of the V content is 0.01 at%, more preferably 0.05 at%, and still more preferably 0.1 at%.

  The upper limit with preferable Cr content is 2.0 at% as above-mentioned. The upper limit with more preferable Cr content is 1.0 at%, More preferably, it is 0.5 at%. When Cr is contained, the preferable lower limit of the Cr content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable Mn content is 2.0 at% as above-mentioned. The upper limit with more preferable Mn content is 1.0 at%, More preferably, it is 0.5 at%. When Mn is contained, the preferable lower limit of the Mn content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable Fe content is 2.0 at% as above-mentioned. The upper limit with more preferable Fe content is 1.0 at%, More preferably, it is 0.5 at%. When Fe is contained, the preferable lower limit of the Fe content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable Co content is 2.0 at% as above-mentioned. The upper limit with more preferable Co content is 1.0 at%, More preferably, it is 0.5 at%. When Co is contained, the preferable lower limit of the Co content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable Ni content is 3.0 at% as above-mentioned. The upper limit with more preferable Ni content is 2.0 at%, More preferably, it is 1.0 at%. When Ni is contained, the preferable lower limit of the Ni content is 0.01 at%, more preferably 0.05 at%, and still more preferably 0.1 at%.

  The upper limit with preferable Zn content is 3.0 at% as above-mentioned. The upper limit with more preferable Zn content is 2.0 at%, More preferably, it is 1.0 at%. When Zn is contained, the preferable lower limit of the Zn content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable Al content is 3.0 at% as above-mentioned. The upper limit with more preferable Al content is 2.0 at%, More preferably, it is 1.0 at%. When Al is contained, the preferable lower limit of the Al content is 0.01 at%, more preferably 0.05 at%, and further preferably 0.1 at%.

  The upper limit with preferable Si content is 3.0 at% as above-mentioned. The upper limit with more preferable Si content is 2.0 at%, More preferably, it is 1.0 at%. When Si is contained, the preferable lower limit of the Si content is 0.01 at%, more preferably 0.05 at%, and still more preferably 0.1 at%.

  The upper limit with preferable B content is 2.0 at% as above-mentioned. The upper limit with more preferable B content is 1.0 at%, More preferably, it is 0.5 at%. When B is contained, the preferable lower limit of the B content is 0.01 at%, more preferably 0.05 at%, and still more preferably 0.1 at%.

  The upper limit with preferable C content is 2.0 at% as above-mentioned. The upper limit with more preferable C content is 1.0 at%, More preferably, it is 0.5 at%. When C is contained, the preferable lower limit of the C content is 0.01 at%, more preferably 0.05 at%, and still more preferably 0.1 at%.

[Negative electrode]
The negative electrode active material of this embodiment is applied to a current collector as a negative electrode mixture, and is used as a negative electrode of a nonaqueous electrolyte secondary battery. This negative electrode has a higher charge / discharge capacity (charge / discharge capacity per volume) than a negative electrode made of graphite.

[Expansion coefficient during charging of negative electrode mixture]
The negative electrode mixture expands and contracts with charge and discharge. The smaller the expansion of the negative electrode mixture during charging, the better the cycle characteristics of the secondary battery. When the conventional negative electrode active material using silicon is used, the expansion coefficient of the negative electrode mixture greatly exceeds 30%. However, the negative electrode mixture using the negative electrode active material according to the present embodiment has a small volume expansion after charging. For example, when a laminate cell is produced using a negative electrode mixture, the expansion coefficient is preferably 30% or less, more preferably 20% or less.

The expansion coefficient of the negative electrode mixture can be determined, for example, by the following method. A negative electrode mixture is produced and applied to the current collector. The obtained current collector is made into a laminate cell and charged and discharged. The thickness (A) of the expanded laminate cell after charging and the thickness (B) of the original laminate cell are substituted into the following formula to calculate the expansion rate (%).
Expansion rate (%) = {(A / B) -1} × 100
However, the thickness (B) of the original laminate cell is a measured value in a state where the electrolyte solution is sufficiently infiltrated into the negative electrode plate after the laminate cell is produced and swollen.

  When the behavior of the thickness change changes during the charge / discharge cycle, the analysis is performed after repeating the charge / discharge about 5 to 20 times as necessary. Then, an average value is obtained from a plurality of highly reliable expansion coefficient values.

[battery]
The battery of this embodiment is a nonaqueous electrolyte secondary battery. Nonaqueous electrolyte secondary batteries are metal ion secondary batteries, such as a lithium ion secondary battery, a sodium ion secondary battery, a magnesium ion secondary battery, a calcium ion secondary battery, for example. The battery includes the above-described negative electrode. A battery is provided with the negative electrode of this embodiment, a positive electrode, a separator, and electrolyte solution or electrolyte, for example.

  The shape of the battery may be a cylindrical shape, a square shape, a coin shape, a sheet shape, or the like. The battery of this embodiment may be a battery using a solid electrolyte such as a polymer battery.

[Positive electrode]
The positive electrode of the battery preferably contains a transition metal compound containing a metal ion as an active material. More preferably, the positive electrode contains a lithium (Li) -containing transition metal compound as an active material. The Li-containing transition metal compound is, for example, LiM ₁ -xM′xO ₂ or LiM ₂ yM′O ₄ . Here, in the formula, 0 ≦ x, y ≦ 1, M and M ′ are barium (Ba), cobalt (Co), nickel (Ni), manganese (Mn), chromium (Cr), titanium (Ti), respectively. , Vanadium (V), iron (Fe), zinc (Zn), aluminum (Al), indium (In), tin (Sn), scandium (Sc), and yttrium (Y).

  Other positive electrodes such as transition metal chalcogenides, vanadium oxides and lithium compounds thereof, niobium oxides and lithium compounds thereof, conjugated polymers using organic conductive materials, sheprel phase compounds, activated carbon and activated carbon fibers May be used.

[Electrolyte]
The electrolytic solution is generally a non-aqueous electrolytic solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent. Examples of the lithium salt include 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 and the like. These may be used alone or in combination. The organic solvent is preferably a carbonic acid ester such as propylene carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. Other organic solvents such as carboxylic acid esters and ethers can also be used. These organic solvents may be used alone or in combination.

[Separator]
The separator is installed between the positive electrode and the negative electrode. The separator serves as an insulator. Further, the separator greatly contributes to the retention of the electrolyte. The battery of this embodiment may be provided with a known separator. The separator is, for example, polypropylene, polyethylene, a mixture of both, or a porous body such as a glass filter.

[Method for producing negative electrode active material]
The manufacturing method of the negative electrode active material of this embodiment is demonstrated. The manufacturing method of a negative electrode active material includes a manufacturing process of solution material, a solution heat treatment process, and a crushing process.

[Manufacturing process of melted lumber]
First, the raw material melt is manufactured. For example, a molten metal having the above-described chemical composition is manufactured. The molten metal is manufactured by a normal melting method such as arc melting or resistance heating melting. Next, a molten product is manufactured using the obtained molten metal. The molten material is obtained by making the molten metal into an ingot (bulk alloy) by an ingot-making method. The molten material may be produced by rapidly solidifying the molten metal. Examples of the rapid solidification method include a strip casting method, a melt spin method, a gas atomization method, and an oil atomization method.

[Solution heat treatment process]
Solution heat treatment is performed on the obtained melted material. The solution heat treatment is performed by maintaining the temperature of the melted material in a temperature range where the ζ phase stably exists in an equilibrium state, and then rapidly cooling to a temperature below room temperature. By this solution heat treatment, the high-temperature ζ phase can be obtained as a metastable phase at room temperature. If solidification segregation of the melted material is significant, a homogenization heat treatment may be performed prior to the solution heat treatment. The temperature range of the homogenization heat treatment is higher than the temperature range of the ζ phase and lower than the temperature range of the liquid phase. When performing the homogenization heat treatment, it is preferable to reduce the oxygen concentration in the atmosphere as much as possible by using an inert gas.

  The temperature of the molten material at the time of temperature maintenance is 550-670 degreeC. The temperature holding time is, for example, 1 to 72 hours. The method of rapid cooling to below room temperature is cooling (quenching) by water cooling or oil cooling, for example. By these heat treatments, the high-temperature ζ phase can be obtained as a metastable phase at room temperature. After quenching, so-called sub-zero treatment may be performed by putting it in liquid nitrogen. In this case, decomposition of the obtained ζ phase into an equilibrium phase due to room temperature aging can be suppressed. Thereby, the crystal structure of the negative electrode active material is changed to a stable ζ phase.

  In order to obtain the ζ phase as a metastable phase at room temperature, rapid cooling needs to be performed effectively. In order to perform the rapid cooling effectively, it is necessary to extract the molten material at a sufficiently high speed. In order to remove the molten material at a sufficiently high speed, it is effective to make the molten material small in advance. The size of the melted material is, for example, a granule having a diameter of 50 mm or less, preferably 10 mm or less, more preferably 5 mm or less. The shape of the other melted material is, for example, a plate shape having a thickness of 20 mm or less, preferably 10 mm or less, more preferably 5 mm or less.

[Crushing process]
The heat treatment material (ingot or slab) obtained in the solution heat treatment step is pulverized to produce a negative electrode active material. The pulverization method is, for example, cutting of a heat treatment material, rough pulverization with a hammer mill, mechanical pulverization with a ball mill, an attritor, a disk mill, a jet mill, a pin mill, or the like. The negative electrode active material is adjusted to the required particle size by pulverization. The pulverization of the negative electrode active material material with respect to the coarse powder is preferably performed in an inert gas atmosphere or a dry atmosphere in order to suppress oxidation. In the pulverization step, solution heat treatment may be performed to adjust the ratio of the ζ phase.

  The smaller the average particle size of the negative electrode active material powder, the greater the reaction area of the negative electrode active material and the better the rate characteristics. However, if the average particle diameter of the powder of the negative electrode active material is too small, the properties of the powder surface change due to oxidation or the like, and lithium ions do not easily enter the negative electrode active material. In this case, the rate characteristics and the solubility maintenance rate may be reduced. Therefore, the preferable average particle diameter of the powder of the negative electrode active material is 0.1 to 100 μm, and more preferably 1 to 50 μm.

[Negative Electrode and Battery Manufacturing Method]
The negative electrode using the negative electrode active material according to the present embodiment can be manufactured by a known method. For example, a binder such as polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polytetrafluoroethylene (PTFE), and styrene butadiene rubber (SBR) is mixed with the negative electrode active material powder. In order to impart sufficient conductivity to the negative electrode, carbonaceous material powders such as natural graphite, artificial graphite and acetylene black may be mixed. A binder such as N-methylpyrrolidone (NMP), dimethylformamide (DMF) and water is added to this to dissolve the binder. If necessary, the mixture is stirred using a homogenizer or glass beads to form a negative electrode mixture. This negative electrode mixture is applied to an active material support such as rolled copper foil and electrodeposited copper foil and dried. The obtained dried product is pressed. A negative electrode plate can be manufactured by the above process.

  The binder to be mixed is preferably about 5 to 10% by mass from the viewpoint of mechanical strength of the negative electrode and battery characteristics. The support is not limited to copper foil. The support may be, for example, a thin foil of another metal such as stainless steel or nickel, a net-like sheet punching plate, a mesh knitted with a metal wire, or the like.

  A laminate in which a separator and a metal Li thin plate are sequentially laminated on the negative electrode plate is manufactured. The laminate is placed in a case to manufacture a battery. Through the above steps, the negative electrode and the battery according to the present embodiment can be manufactured.

  Using the negative electrode active material having the chemical composition shown in Table 2, batteries of each test number were manufactured and their characteristics were evaluated.

[Manufacture of negative electrode active material]
In each test number, a raw material having the chemical composition shown in Table 2 was melted at high frequency to produce a molten metal. The obtained molten metal was cast to produce an ingot having a diameter of about 25 mm and a height of about 7 mm. The ingot was cut in half, and the obtained cut piece was subjected to uniform heat treatment at 700 ° C. for 24 hours in an argon atmosphere. After the homogenization heat treatment, it was cooled to room temperature in the furnace and then recovered in the atmosphere. Subsequently, a solution heat treatment was performed on the melted material subjected to the uniform heat treatment. In the solution heat treatment, the melted material was kept at a temperature of 620 ° C. for 20 hours in a vacuum in a heat treatment furnace for quenching, and rapidly cooled with ice water. Furthermore, in order to suppress room temperature aging, the heat treatment material obtained by solution heat treatment was put into liquid nitrogen, held for 1 hour, and then recovered in the room temperature atmosphere. The obtained heat-treated material was pulverized using a rod mill so that the particle size was 45 μm or less to obtain a powder of a negative electrode active material. The obtained negative electrode active material had the chemical composition shown in Table 2.

[Identification of crystal structure]
The crystal structure of the negative electrode active material of Test No. 2, Test No. 3, Test No. 18 and Test No. 19 was specified by the following method. X-ray diffraction measurement was performed on the negative electrode active material to obtain an actual measurement profile. The positions of main diffraction peaks included in the obtained actual measurement profile were specified. Furthermore, using the crystal structure of the ζ phase as an initial structure model, the crystal structure contained in the negative electrode active material was identified from the measured profile using the Rietveld method.

Specifically, X-ray diffraction measurement was performed under the following measurement conditions. The obtained actual measurement profiles are shown in FIGS. 2 (c), 3 (b), 5 (c) and 6 (b).
・ Apparatus: Rigaku SmartLab
・ X-ray tube: Cu-Kα ray ・ X-ray output: 45 kV, 200 mA
-Incident-side monochromator: Johansson element (Cu-Kα2 and Cu-Kβ lines cut)
・ Optical system: Bragg-Bretano geometry
-Incident parallel slit: 5.0 degree
-Incident slit: 1/2 degree
・ Long limit slit: 10.0mm
-Light receiving slit 1: 8.0 mm
・ Reception slit 2: 13.0mm
-Light receiving parallel slit: 5.0 degree
Goniometer: SmartLab goniometer X-ray source-mirror distance: 90.0 mm
-Distance between X-ray source and selected slit: 114.0 mm
-Distance between X-ray source and sample: 300.0 mm
・ Distance between sample and light receiving slit 1: 187.0 mm
・ Distance between sample and receiving slit 2: 300.0mm
-Distance between light receiving slit 1 and light receiving slit 2: 113.0 mm
・ Distance between sample and detector: 331.0mm
・ Detector: D / Tex Ultra
・ Scan range: 10-120 degrees
Scan step: 0.02 degree
-Scan mode: Continuous scan-Scan speed: 0.1 degree / min

  The binary phase diagram of Cu—Sn is known, and at 620 ° C., the Cu-22.5 at% Sn alloy is in the ζ phase. When this ζ phase is quenched at a sufficient cooling rate, almost all of the resulting alloy is the ζ phase. That is, it can be estimated that almost all of the test number 3 is the ζ phase.

  The crystal structure of the ζ phase is known as described above. The crystal structure of the ζ phase is a hexagonal crystal structure, and International Table A No. 173 (P63). The lattice constant and atomic coordinates of the crystal structure of the space group number of the ζ phase are as shown in Table 1. Using the structure model of the space group number of the ζ phase as an initial structure model for Rietveld analysis, a calculated value of an actual measurement profile (hereinafter referred to as a calculation profile) was obtained for test number 3. Rietan-2000 (program name) was used for Rietveld analysis. The calculated profile of the ε phase is shown in FIG. 2A, and the calculated profile of the ζ phase is shown in FIG.

  For the negative electrode active material of Test No. 3, a coin battery described later was prepared and charged and discharged, and then the crystal structure was specified by the same method. Specifically, the coin battery was fully charged (once) and then disassembled in the glove box. The negative electrode active material applied to the negative electrode plate was peeled off from the current collector foil with a spatula and filled into a sample holder. X-ray diffraction measurement was performed under the above-mentioned X-ray diffraction measurement conditions, and an actual measurement profile was obtained. The results are shown in FIG. Further, the coin battery was completely discharged (once), and an actual measurement profile was obtained by the same method. The results are shown in FIG.

[Manufacture of negative electrode for coin battery]
A negative electrode for a coin battery was manufactured using the negative electrode active material of each test number described above. A negative electrode active material, acetylene black (AB) as a conductive additive, styrene butadiene rubber (SBR) (2-fold dilution) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed. The mixing ratio was 75 (1 g): 15 (0.2 g): 10 (0.134 g): 5 (0.067 g) by mass ratio. Distilled water was added to the mixture so that the concentration was 27.2%, and a negative electrode mixture was produced using a kneader.

The manufactured negative electrode mixture was applied onto a copper foil using an applicator (150 μm). The copper foil coated with the negative electrode mixture was dried at 100 ° C. for 20 minutes. The copper foil after drying had a coating film made of a negative electrode active material on the surface. A punching process was performed on the copper foil having the coating film to produce a disc-shaped copper foil having a diameter of 13 mm. The copper foil after punching was pressed with a press pressure of 500 kgf / cm ² to produce a plate-like negative electrode.

[Manufacture of coin-type batteries]
A coin type battery (2016 type) using a Li metal foil as a counter electrode (positive electrode) was manufactured. Specifically, a separator having a diameter of 19 mm was disposed on the negative electrode. Further, a metal Li foil having a diameter of 15 mm was disposed on the separator to form a laminate. The laminate was placed in the case. The outer periphery of the case containing the laminate was pressed with a dedicated caulking machine to produce a coin-type battery (2016 type). The supporting electrolyte is LiPF ₆ and LiPF ₆ : ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC): vinylene carbonate (VC): fluoroethylene carbonate (FEC) = 16: 48: 23: 16: A mixed solvent at 1: 8 (mass ratio) was used as the electrolytic solution.

[Coin battery discharge capacity measurement test]
The discharge capacity was measured for the obtained coin battery of each test number. Constant current doping (insertion of lithium ions into the electrode, lithium ion secondary) with a current value of 0.1 mA (current value of 0.075 mA / cm ² ) until the potential difference becomes 0.005 V with respect to the counter electrode Equivalent to charging the battery). Thereafter, while maintaining 0.005 V, the counter electrode was continuously doped at a constant voltage until 7.5 μA / cm ² was reached. Next, it is dedoped at a current value of 0.1 mA (current value of 0.075 mA / cm ² ) until the potential difference becomes 1.2 V (corresponding to detachment of lithium ions from the electrode and discharge of the lithium ion secondary battery). And the dedoping capacity was measured. Doping and dedoping were repeated 100 times under the same conditions.

  The dedoping capacity corresponds to the discharge capacity when this negative electrode is used as the negative electrode of a lithium ion secondary battery. Therefore, the measured dedoping capacity was defined as the discharge capacity. When the discharge capacity decreased from the start of the measurement test to several cycles, the measurement was repeated several times until the discharge capacity was stabilized to obtain the initial discharge capacity. The discharge capacity at the 100th undoping was defined as the discharge capacity at 100 cycles. The volume density of the negative electrode mixture was calculated from the coating thickness of the negative electrode mixture and the coating mass per unit area of the negative electrode mixture. A discharge capacity per unit volume was obtained by multiplying the discharge capacity by the volume density of the negative electrode mixture. The results are shown in Table 2.

[Calculation of volume maintenance rate]
The obtained initial discharge capacity and the discharge capacity at 100 cycles were substituted into the following equation to calculate the volume retention rate. The results are shown in Table 2.
Volume retention rate (%) = 100 cycle discharge capacity / initial discharge capacity × 100

[Manufacture of negative electrode for laminate cells]
A negative electrode for a laminate cell was manufactured using the negative electrode active material of each test number described above and graphite powder. The graphite powder was prepared as follows. Spherical natural graphite was mixed with 2% by mass of pitch powder and fired at 1000 ° C. in a nitrogen stream to obtain graphite powder. The average particle diameter (D50) of the obtained graphite powder was 20 μm.

The negative electrode active material of each test number described above and graphite powder were mixed to obtain a mixture. The mixing ratio was 20:80 by mass%. Using this mixture as an active material, a negative electrode mixture was produced in the same manner as the negative electrode for coin batteries. The manufactured negative electrode mixture was applied onto a copper foil using an applicator (150 μm). The copper foil coated with the negative electrode mixture was dried at 100 ° C. for 20 minutes. The copper foil after drying had a coating film made of a negative electrode active material on the surface. A 2.5 cm square copper foil was cut out from the copper foil having the coating film. The cut copper foil was pressed at a press pressure of 500 kgf / cm ² to produce a negative electrode.

[Manufacture of positive electrode for laminate cells]
Lithium cobaltate was used as the active material for the positive electrode for the laminate cell. Lithium cobaltate, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were mixed. The mixing ratio was 80 (0.8 g): 10 (0.1 g): 10 (0.1 g) by mass ratio. The concentration of the mixture was adjusted using N-methyl-2-pyrrolidone (NMP), and a positive electrode mixture was produced using a kneader. The obtained positive electrode mixture was applied onto an aluminum foil using an applicator (150 μm). The aluminum foil coated with the positive electrode mixture was dried at 100 ° C. for 20 minutes. The aluminum foil after drying had a coating film made of lithium cobalt oxide on the surface. A 2.3 cm square aluminum foil was cut out from the aluminum foil having the coating film. The cut aluminum foil was pressed at a press pressure of 500 kgf / cm ² to produce a positive electrode.

[Manufacture of laminate cells]
A laminate cell was manufactured using the obtained negative electrode and positive electrode. Specifically, a polyolefin separator was disposed on the negative electrode. Furthermore, the manufactured positive electrode was arrange | positioned on the separator and the laminated body was formed. The laminate was placed in an aluminum laminate sheet. A laminate cell was manufactured using an aluminum tab on the positive electrode side and a nickel tab on the negative electrode side. The supporting electrolyte is LiPF ₆ and LiPF ₆ : ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC): vinylene carbonate (VC): fluoroethylene carbonate (FEC) = 16: 48: 23: 16: A mixed solvent at 1: 8 (mass ratio) was used as the electrolytic solution. The operation until the electrolyte was injected was performed in the atmosphere, and the final assembly for injecting and sealing the electrolyte was performed in a glove box in an argon atmosphere.

[Measurement test of discharge capacity and volume retention ratio of laminate cell]
A discharge capacity measurement test was performed on the obtained laminate cell of each test number. The test method was the same as the test method for coin batteries. Further, the volume retention rate was calculated from the obtained discharge capacity in the same manner as the coin battery. The results are shown in Table 2.

[Thickness measurement test of laminate cell]
A thickness measurement test was performed on the obtained laminate cell of each test number. Specifically, the laminate cell was sandwiched between Teflon (trademark) plates, and further bolted and fixed with a stainless steel reinforcing plate. Using a linear gauge equipped with an optoelectronic transmission type linear encoder, the thickness (A) of the laminate cell after charging and the thickness (B) of the original laminate cell were measured. The obtained numerical value was substituted into the above formula to calculate the expansion rate (%). The results are shown in Table 2. Here, as the thickness (B) of the original laminate cell, the measured value in a state where the electrolyte solution was sufficiently infiltrated into the negative electrode plate after the laminate cell was produced and used was used. As the thickness (A) of the laminate cell after charging, an average value of 5 times in a cycle in which the discharge capacity is stable was used.

[Evaluation results]
FIG. 2 is a diagram showing an actual measurement profile of X-ray diffraction measurement before and after charging of the negative electrode active material of test number 3 and calculation profiles of ζ phase and ε phase. In FIG. 2, (a) is a calculation profile of the ε phase, (b) is a calculation profile of the ζ phase, (c) is an actual measurement profile of the negative electrode active material (before charging), and (d) is after being charged once. (E) shows the measured profile of the negative electrode active material after being discharged once.

  Referring to FIG. 2, the actual measurement profile (c) of the negative electrode active material before charging mainly coincided with the calculated profile (b) of the ζ phase. Furthermore, in the actual measurement profile (c), a weak diffraction peak was observed at the same position as the diffraction peak found in the calculation profile (a) of the ε phase. Therefore, it was confirmed that the crystal structure of the negative electrode active material of Test No. 3 was mainly ζ phase and contained a small amount of ε phase before charging.

  The result of comparing the measured profile (d) in the once charged state and the measured profile (e) in the once discharged state in FIG. 2 will be described. In the measured profile (d) of the charged state, diffraction peaks were observed at an angle 2θ in the respective ranges of 38.0 to 39.0 ° and 49.0 to 51.0 °. In the measured profile (e) in the discharge state, these diffraction peaks disappeared. Further, the intensity of the diffraction peak derived from the ζ phase (indicated by ● in FIG. 2) confirmed in the actual measurement profile (c) before charging was reduced in the actual measurement profile (d) in the charged state. In the measured profile (e) of the discharge state, the intensity of the diffraction peaks derived from these ζ phases increased again. Therefore, it was confirmed that the negative electrode active material of Test No. 3 had a crystal structure mainly of ζ phase before charging, and the crystal structure changed with charging (that is, the ζ phase occludes lithium). It was. Furthermore, it was confirmed that the ratio of the ζ phase of the negative electrode active material increased again by discharge. It was confirmed that the ε phase contained in a small amount in the negative electrode active material before charging exhibits the same behavior.

  FIG. 3 is a diagram showing an actual measurement profile of the X-ray diffraction measurement of the negative electrode active material (Test No. 2) of test number 2 and a calculation profile of the ζ phase. In FIG. 3, (a) shows the calculated profile of the ζ phase, and (b) shows the measured profile of the negative electrode active material (before charging).

  Referring to FIG. 3, the actual measurement profile (b) of the negative electrode active material (before charging) of Test No. 2 substantially coincided with the calculated profile (a) of the ζ phase. Therefore, it was confirmed that the crystal structure of the negative electrode active material of test number 2 was almost ζ phase.

The negative electrode active material of Test No. 1 to Test No. 16 shown in Table 2 contained a Cu—Sn based alloy containing a ζ phase. Therefore, when a coin battery was manufactured using these negative electrode active material materials, the initial discharge capacity was 1500 mAh / cm ³ or more, indicating an excellent discharge capacity. Furthermore, the volume retention rate of these coin batteries was 80% or more, and excellent cycle characteristics were exhibited. When a laminate cell was manufactured using these negative electrode active material materials, the initial discharge capacity was 700 mAh / cm ³ or more, indicating an excellent discharge capacity. Further, these laminate cells had a volume retention rate of 80% or more and an expansion rate of 30% or less, and exhibited excellent cycle characteristics.

  Test Nos. 1 to 4 are examples of Cu—Sn binary negative electrode active material. In particular, the negative electrode active material of Test No. 2 was mostly ζ phase in its crystal structure. Therefore, in test number 2, compared with test number 1, test number 3, and test number 4, both the coin battery and the laminate cell had higher initial discharge capacity and volume retention rate.

FIG. 4 is a diagram showing the cycle characteristics of the coin battery of test number 3. FIG. Referring to FIG. 4, the discharge capacity of the coin battery of test number 3 gradually decreased from the first cycle to the seventh cycle to 1808 mAh / cm ³ and then stabilized. This discharge capacity was about 2.2 times the theoretical value of the discharge capacity of graphite. The discharge capacity of the coin battery of Test No. 3 was 1758 mAh / cm ³ at the 100th cycle, showing a high capacity retention rate of 97% with respect to the discharge capacity at the 7th cycle. Furthermore, after the discharge capacity was stabilized at the 7th cycle, the high discharge capacity was maintained without significantly decreasing the discharge capacity until the 100th cycle.

  Test number 5 to test number 16 contain one type selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B and C in addition to Cu and Sn. It is an example of a negative electrode active material. As shown in Test No. 5 to Test No. 16, even when these elements were contained in the negative electrode active material, the coin battery and the laminate cell exhibited a high discharge capacity and a high volume retention rate.

On the other hand, the test number 17 is an example of a negative electrode active material using only Si. The coin battery manufactured using the negative electrode active material of Test No. 17 had an initial discharge capacity of 1500 mAh / cm ³ or more, but the volume retention rate was 5.3%, which was extremely low. The laminate cell manufactured using the negative electrode active material of Test No. 17 had an initial discharge capacity of 700 mAh / cm ³ or more, but the volume retention rate was 7.5%, which was extremely low. Furthermore, this laminate cell had a high expansion rate of 75.0%. As described above, it is considered that the volume maintenance rate of the coin battery and the laminate cell is reduced because Si has a large volume change due to charge and discharge and the negative electrode is partially damaged.

Test number 18 and test number 19 are examples of negative electrode active material that does not include a ζ phase. Figure 5 is a diagram showing a measured profile of the X-ray diffraction measurement of the negative electrode active material (pre-charge), and a calculated profile of the σ phase and D0 ₃ phase Test No. 18. In FIG. 5, shows the measured profiles of (a) the calculated profile of the σ phase, (b) the calculated profile of D0 ₃ phase, and, (c) the negative electrode active material material (before charging).

Referring to FIG. 5, the pre-charge measured profile (c) was consistent with the calculated profile of the σ phase and D0 ₃ phase. Furthermore, no diffraction peak derived from the ζ phase was observed. Therefore, the ζ phase was not confirmed in the negative electrode active material of test number 18.

Referring to the Cu—Sn binary phase diagram shown in FIG. 1, an alloy (test number 18) containing 19 at% Sn with the balance being Cu and impurities does not form a ζ phase. Therefore, in the binary system, the ζ phase was not formed with the chemical composition of test number 18. Therefore, although the volume retention rate of the coin battery using the negative electrode active material of test number 18 was 80% or more, the initial discharge capacity was less than 1500 mAh / cm ³ . Furthermore, although the volume retention rate of the laminate cell using the negative electrode active material of test number 18 was 80% or more, the initial discharge capacity was less than 700 mAh / cm ³ .

  FIG. 6 is a diagram showing an actual measurement profile of X-ray diffraction measurement and a calculation profile of the ε phase of the negative electrode active material of test number 19 (before charging). In FIG. 6, (a) shows the calculation profile of the ε phase, and (b) shows the actual measurement profile of the negative electrode active material (before charging).

  Referring to FIG. 6, the actual measurement profile (b) before charging coincided with the calculated profile of the ε phase. Furthermore, no diffraction peak derived from the ζ phase was observed. Therefore, no ζ phase was confirmed in the negative electrode active material of test number 19.

Referring to the Cu—Sn binary phase diagram shown in FIG. 1, an alloy (test number 19) containing 25 at% Sn with the balance being Cu and impurities does not form a ζ phase. Therefore, the ζ phase was not formed with the chemical composition of test number 19 in the binary system. Therefore, although the initial discharge capacity of the coin battery using the negative electrode active material of test number 19 was 1500 mAh / cm ³ or more, the volume retention rate was 80% or less. Furthermore, although the initial discharge capacity of the laminate cell using the negative electrode active material of Test No. 19 was 700 mAh / cm ³ or more, the volume retention rate was 80% or less. The expansion rate of the laminate cell using the negative electrode active material of Test No. 19 exceeded 30%.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (8)

  A negative electrode active material containing a Cu—Sn alloy containing a ζ phase. The negative electrode active material according to claim 1,
The Cu-Sn alloy is
Sn: 21.0-24.0 at% is contained,
The balance is a negative electrode active material made of Cu and impurities. The negative electrode active material according to claim 2,
The Cu-Sn based alloy is further replaced with a part of Cu,
A negative electrode active material containing one or more selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, B, and C. The negative electrode active material according to claim 3,
The Cu-Sn alloy is
Sn: 21.0 to 24.0 at%,
Ti: 2.0 at% or less, V: 2.0 at% or less, Cr: 2.0 at% or less, Mn: 2.0 at% or less, Fe: 2.0 at% or less, Co: 2.0 at% or less, Ni: Group consisting of 3.0 at% or less, Zn: 3.0 at% or less, Al: 3.0 at% or less, Si: 3.0 at% or less, B: 2.0 at% or less, and C: 2.0 at% or less 1 type or 2 types or more selected from,
The balance is a negative electrode active material made of Cu and impurities. The negative electrode active material according to any one of claims 1 to 4, wherein in addition to the ζ phase,
It contains the site deficiency, [delta] phase F-Cell structure, having ε-phase, eta 'phase, and D0 ₃ 1 or two or more phases selected structure from the group consisting of phase with the negative electrode active material material.   The negative electrode containing the negative electrode active material of any one of Claims 1-5.   A battery comprising the negative electrode according to claim 6. It is a manufacturing method of the negative electrode active material of any one of Claims 1-5,
A process for producing a raw material melt,
A method for producing a negative electrode active material comprising: a solution heat treatment in which the molten material is held at 550 to 670 ° C. and then rapidly cooled to room temperature or lower.

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