Negative electrode material for secondary battery, negative electrode for secondary battery, and secondary battery

Abstract

This negative electrode material for a secondary battery is made of an aluminum alloy. The negative electrode material for a secondary battery comprises aluminum, element X that forms an alloy with the aluminum, and inevitable impurity elements. The element X is one or more selected from the group consisting of Mn, Zn, Cr and Cu. The content of the element X to the total amount of the negative electrode material for a secondary battery exceeds 0.5 mass%, but is no greater than 4 mass%.

Description

Negative electrode material for secondary battery, negative electrode for secondary battery and secondary battery

[0001] The present invention relates to a negative electrode material for a secondary battery, a negative electrode for a secondary battery, and a secondary battery. This application claims priority based on Japanese Patent Application No. 2023-174611, filed on October 6, 2023, the contents of which are incorporated herein by reference.

[0002] Regarding the negative electrodes that constitute secondary batteries, metal materials have been attracting attention as materials with a larger theoretical capacity than graphite, which is a conventional negative electrode material.

[0003] As an example of an anode formed from a metal material, Patent Document 1 discloses an anode formed by alloying an aluminum alloy having a Vickers hardness of 60 or less with lithium metal.

[0004] Japanese Patent Application Publication No. 4-206262

[0005] Among battery characteristics, cycle characteristics are an indicator of a battery's long life. Furthermore, improving the initial charge / discharge efficiency is an important battery characteristic for achieving high energy density. Therefore, improving these characteristics is an important issue. The conventional technology described in Patent Document 1 has the problem of being unable to sufficiently achieve both of these performance characteristics.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode material for a secondary battery that can improve the cycle characteristics and initial charge / discharge efficiency of the secondary battery.

[0007] Another object of the present invention is to provide a secondary battery negative electrode and a secondary battery having the above-mentioned secondary battery negative electrode material.

[0008] The present invention includes the following [1] to [6]. [1] An anode material for a secondary battery made of an aluminum alloy, the anode material for a secondary battery comprising aluminum, an element X that forms an alloy with the aluminum, and an impurity element, the element X being at least one element selected from the group consisting of Mn and Cr, and the content of the element X relative to the total amount of the anode material for a secondary battery being 0.1% by mass or more and 8% by mass or less. [2] The anode material for a secondary battery according to [1], wherein the aluminum is high-purity aluminum having a purity of 99.9% by mass or more. [3] The anode material for a secondary battery according to [1] or [2], wherein Ia and Ib, obtained by X-ray diffraction measurement using CuKα as a radiation source and with a diffraction angle 2θ measurement range of 10° to 90°, satisfy the following formula (X): [Ia / Ib]≦0.3 (X) (Ia is the sum of the integral values ​​of peaks derived from non-aluminum phases obtained by the X-ray diffraction measurement, and Ib is the sum of the integral values ​​of peaks derived from aluminum phases.) [4] The negative electrode material for secondary batteries according to [1] or [2] contains precipitated particles, and in a binarized image of a backscattered electron image obtained by observing the surface of the negative electrode material for secondary batteries under a scanning electron microscope, when the sum of the areas of white parts corresponding to the areas where the precipitated particles are present is A and the area of ​​black parts other than the areas of A is B, the proportion of A to the sum of A and B is 10% or less. [5] A negative electrode for secondary batteries comprising the negative electrode material for secondary batteries according to [1] or [2]. [6] A secondary battery having the negative electrode for secondary batteries according to [5].

[0009] According to the present invention, it is possible to provide a negative electrode material for a secondary battery that can improve the cycle characteristics and initial charge / discharge efficiency of the secondary battery. Furthermore, it is possible to provide a negative electrode for a secondary battery and a secondary battery that include such a negative electrode material for a secondary battery.

[0010] Fig. 1 is a schematic diagram showing an example of a lithium secondary battery. Fig. 2 is a schematic diagram for explaining a negative electrode material which is a substituted solid solution, which is one aspect of the present invention. Fig. 3 is a cross-sectional view of a negative electrode of a lithium secondary battery using the negative electrode material of Example 5 after 5 cycles. Fig. 4 is a cross-sectional view of a negative electrode of a lithium secondary battery using the negative electrode material of Comparative Example 5 after 5 cycles.

[0011] <Negative electrode material for secondary batteries, negative electrode for secondary batteries> The negative electrode material for secondary batteries of this embodiment is made of an aluminum alloy. The negative electrode material for secondary batteries of this embodiment is made of aluminum, an element X that forms an alloy with aluminum, and an impurity element. Hereinafter, the "negative electrode material for secondary batteries" may be referred to as the "negative electrode material."

[0012] The element X is an element that forms an alloy with aluminum, and specifically is one or more elements selected from the group consisting of Mn and Cr. The element X is an element that is intentionally added to the negative electrode material.

[0013] The content of element X relative to the total amount of the negative electrode material is 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and even more preferably 0.7% by mass or more. The content of element X relative to the total amount of the negative electrode material is 8% by mass or less, preferably 6% by mass or less, more preferably 4% by mass or less, and even more preferably 2% by mass or less.

[0014] The content of element X relative to the total amount of the negative electrode material is, for example, 0.1% by mass to 8% by mass, preferably 0.5% by mass to 6% by mass, more preferably 0.6% by mass to 4% by mass, and even more preferably 0.7% by mass to 2% by mass.

[0015] When the content of element X relative to the total amount of the negative electrode material is equal to or greater than the above-mentioned lower limit, the hardness of the negative electrode material is improved, and therefore the cycle characteristics are less likely to deteriorate. Through research by the present inventors, it has been found that a negative electrode material containing element X has a higher hardness than aluminum. In the negative electrode material of this embodiment, the portion involved in charging and discharging of the negative electrode material functions as an active material, and the excess negative electrode material (remainder) not involved in charging and discharging may serve as a current collector. The portion functioning as the active material undergoes volumetric changes with the absorption and desorption of lithium during charging and discharging, while the remainder functioning as a current collector does not undergo volumetric changes. As a result, the volume change during charging and discharging generates internal stress in the remainder functioning as a current collector. The resulting internal stress may cause the remainder to bend. However, in the negative electrode material of this embodiment, the hardness is improved by the inclusion of element X, and therefore the current collector is less likely to become wavy or wrinkled even after repeated charging and discharging, making it stable and less likely to deteriorate in cycle characteristics.

[0016] Furthermore, in a negative electrode material containing a different element, the different element may precipitate during charging and discharging of a lithium secondary battery. When the different element is element X, the precipitates, Mn or Cr, and compounds containing these elements are unlikely to react with lithium. Therefore, even if precipitates are formed during repeated charging and discharging of a negative electrode material containing element X, the lithium ions that can contribute to charging and discharging are unlikely to decrease, and therefore the cycle characteristics are unlikely to deteriorate.

[0017] When the content of element X relative to the total amount of the negative electrode material is equal to or greater than the above lower limit, the hardness is improved and the cycle characteristics are less likely to deteriorate. When the content of element X relative to the total amount of the negative electrode material is equal to or less than the above upper limit, the negative electrode material does not become too hard and excessive precipitates are less likely to form. This makes it possible to improve the cycle characteristics and initial charge / discharge efficiency of the secondary battery. Furthermore, when the content of element X relative to the total amount of the negative electrode material is equal to or less than the above upper limit, the phenomenon in which precipitates containing element X trap lithium ions is less likely to occur, and lithium ions are more likely to diffuse, making it easier to improve the initial charge / discharge efficiency.

[0018] (Composition Analysis of Negative Electrode Material) The composition of the negative electrode material is measured using an ICP (inductively coupled plasma) emission spectrometer (for example, SPS5000 manufactured by Seiko Instruments Inc.).

[0019] The negative electrode material is an alloy of element X and aluminum, and is preferably a solid solution in which element X is dissolved in aluminum. Furthermore, this solid solution is preferably a substitutional solid solution in which a part of the crystal lattice of the metal that constitutes aluminum is substituted with element X.

[0020] The crystal structure of aluminum is a face-centered cubic lattice structure. In one embodiment of the present invention, the negative electrode material is a substitution solid solution in which some of the aluminum atoms in the face-centered cubic lattice structure are substituted with element X, as shown in Figure 2. When the negative electrode material is a substitution solid solution in which aluminum atoms are substituted with element X, the hardness of the aluminum is improved by solid solution strengthening.

[0021] Furthermore, if element X does not dissolve in aluminum and precipitates, the precipitate traps lithium ions due to the Zener-pinning effect, which tends to inhibit the diffusion of lithium ions. Therefore, if the negative electrode material is a substitutional solid solution, lithium ions are not trapped in the precipitate and therefore tend to diffuse more easily, resulting in a further improvement in the initial charge-discharge efficiency.

[0022] The following method can be used to confirm that the negative electrode material is a substitutional solid solution. Specifically, when the following conditions (i) and (ii) or (i) and (iii) are met, it can be determined that element X is dissolved in aluminum. (i) The presence of element X can be confirmed by ICP analysis. (ii) In the diffraction pattern of the negative electrode material measured by X-ray diffraction (XRD), no peaks attributable to element X alone or to compounds containing element X can be confirmed. (iii) The lattice constant of aluminum determined from the diffraction pattern of the negative electrode material measured by X-ray diffraction (XRD) is shifted.

[0023] The aluminum used for the negative electrode material may be aluminum with a purity of 99.9% by mass or more, high-purity aluminum with a purity of 99.99% by mass or more, etc. High-purity aluminum can be produced by appropriately employing a known method for increasing the purity of aluminum, such as a segregation method, a three-layer electrolysis method, a zone melting refining method, or an ultra-high vacuum melting method. The purity of aluminum mentioned above is the aluminum purity of the remainder after excluding element X from the entire negative electrode material.

[0024] In one aspect of the present invention, the aluminum is preferably high-purity aluminum having a purity of 99.9 mass% or more, and more preferably high-purity aluminum having a purity of 99.99 mass% or more. That is, one aspect of the present invention is a negative electrode material for a secondary battery made of an aluminum alloy, which comprises high-purity aluminum having a purity of 99.99 mass% or more, element X, and an impurity element, and the content of element X relative to the total amount of the negative electrode material for a secondary battery is more than 0.5 mass% and not more than 4 mass%.

[0025] When high-purity aluminum having a purity of 99.99% by mass or more is compared with low-purity aluminum having a purity of less than 99.99% by mass, the high-purity aluminum has a larger amount of element X in solid solution than the low-purity aluminum. Therefore, when high-purity aluminum is selected, element X is less likely to be consumed by precipitates and is more likely to be consumed in the formation of a substitutional solid solution. Therefore, when high-purity aluminum is selected, the Zener-pinning effect is less likely to occur, lithium ions are more likely to diffuse, and the initial charge-discharge efficiency is further improved.

[0026] Aluminum may contain unavoidable impurities (unavoidable impurities). Examples of the unavoidable impurities include manufacturing residues that are inevitably mixed in during the refining process. The impurity elements contained in the negative electrode material are elements that make up the unavoidable impurities, and specific examples include Fe and Cu.

[0027] In one embodiment of the present invention, the content of either or both of Fe and Cu contained as unavoidable impurities in the negative electrode material is preferably 300 ppm by mass or less, more preferably 100 ppm by mass or less, and even more preferably 50 ppm by mass or less, relative to the total mass of the negative electrode material. Also, in one embodiment of the present invention, the content of either or both of Fe and Cu contained in the negative electrode material may be 0 ppm by mass, relative to the total mass of the negative electrode material.

[0028] In one embodiment of the present invention, when the negative electrode material contains both Fe and Cu, the total amount of Fe and Cu relative to the total mass of the negative electrode material is preferably 300 ppm by mass or less, more preferably 100 ppm by mass or less, and even more preferably 50 ppm by mass or less. Note that, with regard to inevitable impurities, this does not preclude the inclusion of an amount of inevitable impurities below the detection limit by the analytical method described in this specification.

[0029] In one embodiment of the present invention, the Vickers hardness of the negative electrode material is preferably 30 HV or more and 60 HV or less, more preferably 35 HV or more and 55 HV or less, even more preferably 40 HV or more and 55 HV or less, and particularly preferably 45 HV or more and 55 HV or less.

[0030] The Vickers hardness is measured by the following method using a micro Vickers hardness tester (HV0.05).

[0031] (Method for Measuring Vickers Hardness) The Vickers hardness is a value measured in accordance with JIS Z2244:2009 "Vickers Hardness Test - Test Method." To measure the Vickers hardness, a square pyramidal diamond indenter is pressed into the surface of a test piece of a negative electrode material, and after the test force is released, the Vickers hardness is calculated from the diagonal length of the indentation remaining on the surface.

[0032] The above standard stipulates that the hardness symbol be changed depending on the test force. In this embodiment, the micro Vickers hardness HV0.05 is measured when the test force is 0.05 kgf (=0.4903 N).

[0033] It is presumed that when the Vickers hardness is equal to or less than the upper limit, distortion of the crystalline structure can be alleviated when aluminum absorbs lithium, and the crystalline structure can be maintained. Therefore, a secondary battery negative electrode manufactured from the negative electrode material can easily maintain its discharge capacity even when used in, for example, a lithium secondary battery and repeatedly charged and discharged.

[0034] In one embodiment of the negative electrode material, in a diffraction pattern obtained by measuring the surface of the negative electrode material by X-ray diffraction (XRD), the ratio of Ia to Ib (Ia / Ib) defined below preferably satisfies formula (X): [Ia / Ib]≦0.3 (X)

[0035] The ratio (Ia / Ib) is more preferably 0.2 or less, further preferably 0.15 or less, and particularly preferably 0.1 or less.

[0036] [Method for Measuring Ia and Ib] "X-ray diffraction measurement" is performed as follows. Using CuKα as a radiation source and setting the measurement range of the diffraction angle 2θ to 10° or more and 90° or less, an XRD pattern of the measurement target is obtained using an X-ray diffractometer (e.g., Ultima IV, manufactured by Rigaku Corporation). The measurement target is the surface of a foil punched to φ15 from the negative electrode material.

[0037] The obtained diffraction peaks are fitted to calculate the integrated intensity of each peak. These peaks are then divided into A: peaks derived from non-aluminum phases and B: peaks derived from aluminum phases, and the sum of the integrated values ​​of each of the peaks A and B is calculated. Ia is the sum of the integrated values ​​of the peaks derived from non-aluminum phases, and Ib is the sum of the integrated values ​​of the peaks derived from aluminum phases. The fitting of the diffraction peaks is performed using analysis software (e.g., Image J).

[0038] In one embodiment of the present invention, the IACS conductivity of the negative electrode material is preferably 50% IACS or less, more preferably 40% IACS or less, and even more preferably 35% IACS or less. IACS conductivity is an abbreviation for International Annealed Copper Standard and indicates the international annealed copper wire standard.

[0039] In one embodiment of the present invention, the negative electrode material may contain precipitated particles. In this specification, precipitated particles are particles containing element X. In this case, the area ratio of precipitated particles obtained by observing the surface of the negative electrode material is preferably 10% or less. The area ratio of precipitated particles can be measured by the following method.

[0040] [Method for Measuring the Area Ratio of Precipitated Particles] First, the surface of the negative electrode material is observed with a scanning electron microscope, and a backscattered electron image is taken at a measurement magnification of 500 times and an accumulation time of 160 seconds.

[0041] The obtained image (backscattered electron image) is binarized and analyzed using quantitative analysis software TRI / 3D-BON-FCS (manufactured by Ratoc System Engineering).

[0042] Specifically, the image is converted to two shades using Auto-LW, a function of the quantitative analysis software, and regions where precipitate particles are present, displayed in white, are distinguished from regions where precipitate particles are not present, displayed in black. The amount of backscattered electrons generated varies depending on the elements that make up the sample, with the tendency for the amount generated to increase as the atomic number increases. Regions where precipitate particles are present contain a large amount of element X, while black regions contain a large amount of Al. Therefore, in the image (backscattered electron image), precipitate particle regions that contain a large amount of element X, a heavy element, appear white, while Al regions that contain Al, a light element, appear black.

[0043] In the binary image thus obtained, the areas corresponding to the areas where precipitate particles are present on the surface of the negative electrode material are white, and the areas where precipitate particles are not present are black. When the total area of ​​the white areas is A and the total area of ​​the black areas is B, the ratio of A to the sum of A and B is preferably 10% or less, more preferably 8% or less, even more preferably 6% or less, even more preferably 4% or less, and particularly preferably 2% or less.

[0044] When the ratio of A to the sum of A and B is equal to or less than the upper limit, the Zener-pinning effect is unlikely to occur, lithium ions are easily diffused, and the initial charge-discharge efficiency is further improved.

[0045] The lower limit of the ratio of A to the sum of A and B is, for example, 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more. The ratio of A to the sum of A and B is, for example, 0.1% to 10%, 0.2% to 8%, 0.3% to 6%, 0.4% to 4%, or 0.5% to 2%.

[0046] <<Method for Producing Negative Electrode Material>> The method for producing a negative electrode material preferably includes a casting step and a rolling step of an alloy.

[0047] (Casting Step) When casting, for example, a predetermined amount of element X is added to high-purity aluminum, and the aluminum and metal are melted at approximately 680° C. to 800° C. to obtain a molten alloy. When obtaining the molten alloy, element X is added in such a proportion that the content of element X relative to the total amount of the negative electrode material is more than 0.5 mass % and not more than 4 mass %.

[0048] The molten alloy is preferably subjected to a cleaning treatment (e.g., vacuum treatment of molten aluminum) to remove gases and non-metallic inclusions. The vacuum treatment is performed, for example, at a temperature of 700°C to 800°C for 1 hour to 10 hours at a vacuum level of 0.1 Pa to 100 Pa.

[0049] The molten alloy can be purified by blowing in flux, inert gas, or chlorine gas. The molten alloy purified by vacuum treatment or the like is usually cast into a mold to obtain an ingot.

[0050] The mold is made of iron or graphite heated to 50° C. or higher and 200° C. or lower. The negative electrode material of this embodiment can be cast by pouring a molten alloy at 680° C. or higher and 800° C. or lower into the mold. Alternatively, an ingot can be obtained by commonly used semi-continuous casting.

[0051] (Rolling process) In the rolling process of the ingot, for example, hot rolling (first rolling) and cold rolling (secondary rolling) are performed to process the ingot into a plate material (rolled material). Hot rolling is performed, for example, at a temperature of 350°C to 550°C and at a rolling rate of 2% to 30% per rolling, repeatedly until the ingot reaches a target thickness.

[0052] After hot rolling, intermediate annealing is usually performed before cold rolling. In the intermediate annealing, for example, the hot-rolled sheet material may be heated to 350°C or higher and 550°C or lower and cooled immediately after the temperature rise, or may be held for 1 hour to 5 hours and then cooled. If the cooling step is performed by rapid cooling, the non-aluminum phase tends to become smaller. On the other hand, if the cooling is performed at a slow cooling rate, the metal particles constituting the non-aluminum phase tend to grow. The cooling step may be adjusted appropriately depending on the desired size of the non-aluminum phase. The cooling treatment softens the material, resulting in a state suitable for cold rolling.

[0053] Cold rolling is performed repeatedly at a temperature below the recrystallization temperature of aluminum, with a reduction ratio per rolling of 1% to 20%, until the aluminum ingot reaches a target thickness. The cold rolling temperature may be from room temperature to 80°C.

[0054] After cold rolling, the rolled material may be subjected to the above-mentioned annealing treatment in which the rolled material is heated at 100°C or higher and 350°C or lower. The heat treatment after cold rolling is usually performed in air, but may also be performed in a nitrogen atmosphere or a vacuum atmosphere. The heat treatment can soften work-hardened sheet material and may also adjust various physical properties by controlling the crystal structure.

[0055] In this embodiment, it is preferable to perform heat treatment after cold rolling. By performing heat treatment, a non-aluminum phase can be grown. By adjusting the heat treatment temperature and time, physical properties such as strength and electrical conductivity can be adjusted to desired ranges.

[0056] The negative electrode material and the negative electrode for a non-aqueous electrolyte battery having the above-described configuration can improve the cycle characteristics of a non-aqueous electrolyte secondary battery.

[0057] 1 is a schematic diagram of a lithium secondary battery, which is an example of a secondary battery. The lithium secondary battery (secondary battery) 10 has a positive electrode 2, a negative electrode 3, and a non-aqueous electrolyte (electrolyte) 6.

[0058] The cylindrical lithium secondary battery 10 is manufactured as follows: First, as shown in Fig. 1 , a pair of strip-shaped separators 1, a strip-shaped positive electrode 2 having a positive electrode lead 21 at one end, and a strip-shaped negative electrode 3 having a negative electrode lead 31 at one end are stacked in this order: separator 1, positive electrode 2, separator 1, negative electrode 3, and then wound to form an electrode group 4.

[0059] Next, the electrode group 4 and an insulator (not shown) are housed in the battery can 5, the bottom of the can is sealed, the electrode group 4 is impregnated with an electrolyte solution 6, and the electrolyte is disposed between the positive electrode 2 and the negative electrode 3. Furthermore, the top of the battery can 5 is sealed with a top insulator 7 and a sealing member 8, whereby a lithium secondary battery 10 can be manufactured.

[0060] The shape of the electrode group 4 can be, for example, a columnar shape such that the cross-sectional shape when the electrode group 4 is cut perpendicular to the winding axis is a circle, an ellipse, a rectangle, or a rectangle with rounded corners.

[0061] The shape of a lithium secondary battery having such an electrode group 4 can be any shape specified in IEC 60086, which is a standard for batteries established by the International Electrotechnical Commission (IEC), or JIS C 8500. Examples of shapes include a cylindrical shape and a rectangular shape.

[0062] Furthermore, the lithium secondary battery is not limited to the above-mentioned wound type configuration, and may be a stacked type configuration in which a stacked structure of a positive electrode, a separator, a negative electrode, and a separator is repeatedly stacked. Examples of stacked lithium secondary batteries include so-called coin type batteries, button type batteries, and paper type (or sheet type) batteries.

[0063] The separator 1, the positive electrode 2, and the negative electrode lead 31 can be made of materials known for use in lithium secondary batteries.

[0064] In one embodiment of the present invention, the positive electrode capacity of the secondary battery is preferably 150 mAh / g or more. The positive electrode capacity is the charge capacity of the positive electrode when charging is performed until the positive electrode potential reaches 4.3 V with metallic lithium as the reference (counter electrode).

[0065] The negative electrode 3 can be made of the negative electrode material of the present embodiment described above.

[0066] The electrolyte solution 6 is a non-aqueous electrolyte solution. Examples of organic solvents that can be used in the electrolyte solution include carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane. Examples of organic solvents that can be used include ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of organic solvents include esters such as methyl formate, methyl acetate, propyl propionate, and γ-butyrolactone, nitriles such as acetonitrile and butyronitrile, amides such as N,N-dimethylformamide and N,N-dimethylacetamide, carbamates such as 3-methyl-2-oxazolidone, and sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone. Furthermore, the above organic solvents may also be used in which a fluoro group has been introduced (an organic solvent in which one or more hydrogen atoms have been substituted with a fluorine atom).

[0067] As the organic solvent, it is preferable to use a mixture of two or more of these. Among them, a mixed solvent containing a carbonate is preferable, and a mixed solvent of a cyclic carbonate and an acyclic carbonate and a mixed solvent of a cyclic carbonate and an ether are more preferable. As a mixed solvent of a cyclic carbonate and an acyclic carbonate, a mixed solvent containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is preferable. An electrolyte solution using such a mixed solvent has many advantages, such as a wide operating temperature range, resistance to deterioration even when charged and discharged at a high current rate, and resistance to deterioration even when used for a long time.

[0068] The organic solvent may be an acyclic carbonate that does not contain a cyclic carbonate. That is, the organic solvent is preferably a solvent consisting only of an acyclic carbonate. By using a solvent consisting only of an acyclic carbonate, the decomposition product SEI (Solid Electrolyte Interface) on the negative electrode, which is derived from the cyclic carbonate, is less likely to be formed. As the acyclic carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferred.

[0069] The electrolyte contained in the electrolytic solution is LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiBF ４ , LiCF ３ SO ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) (COCF ３ ), Li(C ４ F ９ SO ３ ), LiC(SO ２ CF ３ ) ３ , Li ２ B １０ Cl １０ , LiBOB (wherein BOB is bis(oxalato)borate), LiFSI (wherein FSI is bis(fluorosulfonyl)imide), lithium salts of lower aliphatic carboxylic acids, LiAlCl ４ The electrolyte may be a fluorine-containing lithium salt such as LiPF ６ , LiAsF ６ , LiSbF ６ , LiBF ４ , LiCF ３ SO ３ , LiN(SO ２CF ３ ) ２ and LiC(SO ２ CF ３ ) ３ It is preferable to use a material containing at least one selected from the group consisting of:

[0070] Additionally, the electrolyte may contain borate-based salt additives such as lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiOdFB), tris(trimethylsilyl)borate (TMSB), and the like.

[0071] According to the lithium secondary battery 10 described above, the initial charge / discharge efficiency and cycle characteristics can be improved.

[0072] While the preferred embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to these examples. The shapes and combinations of the components shown in the above examples are merely examples, and various modifications can be made based on design requirements, etc., without departing from the spirit of the present invention.

[0073] Next, the present invention will be described in more detail with reference to examples.

[0074] [Analysis of Composition of Negative Electrode Material] The composition of the negative electrode material was measured by the method described above in (Analysis of Composition of Negative Electrode Material).

[0075] [Measurement of Vickers Hardness] The Vickers hardness was measured by the method described above (Method for measuring Vickers hardness).

[0076] [Method for measuring Ia and Ib] Ia and Ib were measured by the method described in the above [Method for measuring Ia and Ib], and the ratio Ia / Ib was calculated.

[0077] [Method for measuring the area ratio of precipitated particles] The area ratio of precipitated particles was measured by the method described above in [Method for measuring the area ratio of precipitated particles].

[0078] [Measurement of IACS Conductivity] The conductivity (EC: % IACS) of the negative electrode material was measured by a four-terminal method in accordance with JIS H0505 (2018).

[0079] Example 1 Fabrication of Negative Electrode High-purity aluminum (purity: 99.99% or more) and Mn, which is element X, were melted at 750°C to obtain an aluminum-manganese molten metal. Next, the aluminum-manganese molten metal was subjected to a vacuum treatment by being held at a temperature of 720°C for 2 hours under conditions of a vacuum degree of 50 Pa. The aluminum-manganese molten metal after the vacuum treatment was cast in a cast iron mold (22 mm x 150 mm x 200 mm) at 150°C to obtain an ingot.

[0080] Rolling was performed under the following conditions. Both sides of the ingot were machined by 2 mm, and then cold-rolled from a thickness of 18 mm at a processing rate of 99.9%. The thickness of the obtained rolled material was 100 μm. A high-purity aluminum-manganese alloy foil (thickness: 100 μm) with an aluminum purity of 99.99% and a manganese content of 2% by mass was used as the negative electrode material, and was cut into a disk shape with a diameter of 14 mm to produce a negative electrode.

[0081] [Preparation of Positive Electrode] The positive electrode was prepared by the following method. ０．８ Co ０．１５ Al ０．０５ O ２ The conductive material (acetylene black) and the binder (PVdF) are mixed in a LiNi ０．８ Co ０．１５ Al ０．０５ O ２ The cathode active material was mixed in a ratio of 92:5:3 (mass ratio) with 70 parts by mass of N-methyl-2-pyrrolidone to prepare an electrode mixture paste for the positive electrode. The obtained electrode mixture paste was applied to a 15 μm-thick aluminum foil current collector by a doctor blade method. The applied electrode mixture was dried at 60°C for 2 hours and then further vacuum-dried at 150°C for 10 hours to volatilize the N-methyl-2-pyrrolidone. The coating weight of the cathode active material after drying was 17.2 mg / cm. ２ (Discharge capacity 3mAh / cm ２ The obtained laminate of the electrode mixture layer and the current collector was rolled, and then cut into a disk shape with a diameter of 14 mm. ０．８ Co ０．１５ Al ０．０５ O ２ A positive electrode was produced, which was a laminate of a positive electrode mixture layer made of the material and a current collector.

[0082] [Preparation of Electrolyte Solution 1] A mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of EC:DMC=50:50 was added to the electrolyte solution. ４ Electrolyte solution 1 (1 mol / L LiBF ４ EC:DMC (50:50) was prepared.

[0083] [Production of Lithium Secondary Battery] A polyethylene porous separator was placed between the negative electrode and the positive electrode, and the resulting mixture was housed in a battery case (standard 2032). The above-mentioned electrolyte solution was poured into the battery case, and the battery case was sealed to produce a coin-shaped lithium secondary battery having a diameter of 20 mm and a thickness of 3.2 mm.

[0084] [Charge / Discharge Evaluation: Initial Charging / Discharging] The coin-type lithium secondary battery was left at room temperature for 10 hours to allow the separator to be sufficiently impregnated with the electrolyte. Next, the battery was charged at a constant current of 0.6 mA (0.2 C) to 3.9 V (Li absorption into Al), followed by a constant voltage charge at 3.9 V for 5 hours at room temperature. The battery was then discharged at a constant current of 0.6 mA (0.2 C) to 2.75 V (Li desorption from Al), for the initial charging / discharging. The discharge capacity was measured, and the resulting value was designated the "initial discharge capacity" (mAh), i.e., the discharge capacity at the first cycle.

[0085] The initial efficiency was calculated using the initial discharge capacity value and the initial charge capacity value according to the following formula: Initial efficiency (%) = Initial discharge capacity (mAh) / Initial charge capacity (mAh) × 100

[0086] [Evaluation of cycle characteristics: number of cycles at 80% retention rate] After the initial charge / discharge, charging at 0.6 mA and discharging at 0.6 mA were repeated under the same conditions as the initial charge / discharge. The discharge capacity at the fifth cycle test was defined as 100%, and the number of cycles (n times) at which the discharge capacity reached 80% after the sixth cycle was defined as the lifespan, and the number of cycles for life was calculated.

[0087] 80% (number of life cycles) = nth discharge capacity / 5th discharge capacity × 100

[0088] <Examples 2 to 6, Comparative Examples 1 to 5> Anode materials were produced and secondary batteries were fabricated in the same manner as in Example 1, except that the aluminum purity and the type and content of element X were changed to the conditions shown in Table 1 below.

[0089] Table 1 below shows the type and content of element X, aluminum purity, Vickers hardness of the negative electrode material, IACS conductivity, Ia / Ib, area ratio of precipitated particles, cycle characteristics, and initial charge / discharge efficiency.

[0090]

[0091] FIG. 3 shows a cross-sectional view of the negative electrode of a lithium secondary battery using the negative electrode material of Example 5 after 5 cycles, and FIG. 4 shows a cross-sectional view of the negative electrode of a lithium secondary battery using the negative electrode material of Comparative Example 5 after 5 cycles.

[0092] In Fig. 3, the precipitated particles are indicated by the reference numeral 20. As shown in Fig. 3, it can be seen that a small amount of precipitated particles is present in the negative electrode material of Example 5. It can be seen that after five cycles, cracks occurred in both the horizontal and vertical directions in Comparative Example 5, compared to Example 5.

[0093] 3 and 4, it was confirmed that the anode material of Example 5 had improved hardness due to the inclusion of element X, was stable even after repeated charging and discharging, and was less likely to experience a decrease in cycle characteristics. Furthermore, it was confirmed that the anode material of Example 5 produced only a small amount of precipitated particles, was less likely to trap lithium ions due to precipitates containing element X, and facilitated the diffusion of lithium ions, thereby improving the initial charge-discharge efficiency.

[0094] Example 7 Preparation of Electrolyte Solution 2 and Battery Evaluation LiBF was used as the lithium salt of Electrolyte Solution 1. ４ LiPF ６ Electrolyte 2 (1 mol / L LiPF) was prepared in the same manner as electrolyte 1, except that ６ An EC:DMC (50:50) mixture was prepared, and the cycle characteristics and initial charge-discharge efficiency were calculated under the same conditions as in Example 1.

[0095] Example 8 Preparation of Electrolyte 3 and Battery Evaluation LiBF was used as the lithium salt of Electrolyte 1. ４ LiN(SO ２ CF３ ) ２ Electrolyte solution 3 (1 mol / L LiTFSI EC:DMC (50:50)) was prepared in the same manner as in Electrolyte Solution 1, except that LiTFSI (hereinafter referred to as LiTFSI) was used. The cycle characteristics and initial charge-discharge efficiency were calculated under the same conditions as in Example 1.

[0096] Example 9 Preparation of Electrolyte 4 and Battery Evaluation LiBF was used as the lithium salt of Electrolyte 1. ４ LiN(SO ２ C ２ F ５ ) ２ Electrolyte solution 4 (1 mol / L LiBETI EC:DMC (50:50)) was prepared in the same manner as in Electrolyte Solution 1, except that (hereinafter referred to as LiBETI) was used. Cycle characteristics and initial charge / discharge efficiency were calculated under the same conditions as in Example 1.

[0097] Example 10 Preparation of Electrolyte 5 and Battery Evaluation LiBF was used as the lithium salt of Electrolyte 1. ４ LiClO ４ Electrolyte 5 (1 mol / L LiClO) was prepared in the same manner as electrolyte 1, except that ４ EC:DMC (50:50) was prepared.

[0098] Example 11 Preparation of Electrolyte Solution 6 and Battery Evaluation LiBF was used as the lithium salt of Electrolyte Solution 1. ４ LiTFSI and LiPF ６ Electrolyte 6 (1.0 mol / L LiTFSI + LiPF) was prepared in the same manner as electrolyte 1, except that the molar ratio of the two was 80:20. ６ An EC:DMC (50:50) mixture was prepared, and the cycle characteristics and initial charge-discharge efficiency were calculated under the same conditions as in Example 1.

[0099] Example 12 Preparation of Electrolyte 7 and Battery Evaluation LiBF was used as the lithium salt of Electrolyte 1. ４ LiTFSI and LiPF ６ Electrolyte solution 7 (1.0 mol / L LiTFSI + LiBF) was prepared in the same manner as electrolyte solution 1, except that the molar ratio of the two was 80:20. ４ An EC:DMC (50:50) mixture was prepared, and the cycle characteristics and initial charge-discharge efficiency were calculated under the same conditions as in Example 1.

[0100] Example 13 Preparation of Electrolyte Solution 8 and Battery Evaluation Electrolyte solution 8 (1 mol / L LiBF ４ A PC:DMC (50:50) battery was prepared, and the cycle characteristics and initial charge-discharge efficiency were calculated under the same conditions as in Example 1.

[0101] Example 14 Preparation of Electrolyte 9 and Battery Evaluation The EC of Electrolyte 1 was replaced with PC, LiBF ４ LiPF ６ Electrolyte 9 (1 mol / L LiPF) was prepared in the same manner as electrolyte 1, except that ６ A PC:DMC (50:50) battery was prepared, and the cycle characteristics and initial charge-discharge efficiency were calculated under the same conditions as in Example 1.

[0102] Example 15 Preparation of Electrolyte Solution 10 and Battery Evaluation Electrolyte solution 10 (1 mol / L LiBF ４ A DMC is prepared, and the cycle characteristics and initial charge-discharge efficiency are calculated under the same conditions as in Example 1.

[0103] Example 16 Preparation of Electrolyte Solution 11 and Battery Evaluation Electrolyte solution 11 (1 mol / L LiBF ４ An EMC is prepared, and the cycle characteristics and initial charge / discharge efficiency are calculated under the same conditions as in Example 1.

[0104] Example 17 Preparation of Electrolyte Solution 12 and Battery Evaluation Electrolyte solution 12 (1 mol / L LiBF ) was prepared in the same manner as in Electrolyte Solution 1, except that all of the EC and DMC in Electrolyte Solution 1 were replaced with DEC. ４ A DEC is prepared, and the cycle characteristics and initial charge / discharge efficiency are calculated under the same conditions as in Example 1.

[0105] Example 18 Preparation of Electrolyte 13 and Battery Evaluation LiBF ４ LiPF ６ Electrolyte 13 (1 mol / L LiPF) was prepared in the same manner as electrolyte 10, except that ６A DMC is prepared, and the cycle characteristics and initial charge-discharge efficiency are calculated under the same conditions as in Example 1.

[0106] Example 19 Preparation of Electrolyte 14 and Battery Evaluation LiBF ４ LiPF ６ Electrolyte 14 (1 mol / L LiPF) was prepared in the same manner as electrolyte 11, except that ６ An EMC is prepared, and the cycle characteristics and initial charge / discharge efficiency are calculated under the same conditions as in Example 1.

[0107] Example 20 Preparation of Electrolyte 15 and Battery Evaluation Electrolyte 12: LiBF ４ LiPF ６ Electrolyte 15 (1 mol / L LiPF) was prepared in the same manner as electrolyte 12, except that ６ A DEC is prepared, and the cycle characteristics and initial charge / discharge efficiency are calculated under the same conditions as in Example 1.

[0108] 3... Negative electrode, 6... Electrolyte, 6... Non-aqueous electrolyte (electrolyte), 10... Secondary battery (lithium secondary battery)

Claims

1. A secondary battery negative electrode material made of an aluminum alloy, the secondary battery negative electrode material comprising aluminum, an element X that forms an alloy with the aluminum, and an impurity element, the element X being at least one element selected from the group consisting of Mn and Cr, and a content of the element X relative to a total amount of the secondary battery negative electrode material being greater than 0.5 mass% and not more than 4 mass%.

2. The negative electrode material for secondary batteries according to claim 1, wherein the aluminum is high-purity aluminum having a purity of 99.9 mass% or more.

3. The negative electrode material for secondary batteries according to claim 1 or 2, wherein Ia and Ib obtained by X-ray diffraction measurement using CuKα as a radiation source and with a measurement range of diffraction angle 2θ of 10° to 90° satisfy the following formula (X): [Ia / Ib]≦0.3 (X) (Ia is the sum of the integral values ​​of peaks derived from non-aluminum phases obtained by the X-ray diffraction measurement, and Ib is the sum of the integral values ​​of peaks derived from aluminum phases).

4. The negative electrode material for a secondary battery according to claim 1 or 2, wherein the negative electrode material for a secondary battery contains precipitated particles, and in a binary image of a backscattered electron image obtained by observing the surface of the negative electrode material for a secondary battery with a scanning electron microscope, when a sum of areas of white parts corresponding to the areas where the precipitated particles are present is A and an area of ​​black parts other than the areas of A is B, the ratio of A to the sum of A and B is 10% or less.

5. A negative electrode for a secondary battery comprising the negative electrode material for a secondary battery according to claim 1 or 2.

6. A secondary battery having the negative electrode for secondary batteries according to claim 5.

Images