Negative electrode material, negative electrode for non-aqueous electrolyte batteries and non-aqueous electrolyte secondary batteries

Abstract

To provide a negative electrode material for a non-aqueous electrolyte secondary battery, a negative electrode for a non-aqueous electrolyte battery, and a non-aqueous electrolyte secondary battery using the same, which can improve the cycle characteristics of the non-aqueous electrolyte secondary battery. [Solution] The negative electrode material consists of aluminum, element M1 which has a solid solubility limit of 1.5% by mass or more in aluminum, and element M2 which has a solid solubility limit of less than 1.5% by mass in aluminum, wherein the aluminum purity of the remainder after removing elements M1 and M2 from the entire negative electrode material is 99% by mass or more, element M1 exists in the metallic crystal of aluminum in the negative electrode material by substituting aluminum atoms, and element M2 forms a non-aluminum phase with a length of 10 nm to 10 μm in a cross-sectional image obtained under the following conditions. (conditions) The negative electrode material is rolled to a thickness of 50 μm before charging and discharging, and the resulting rolled material is cut perpendicular to the rolling direction. An elemental mapping image is created from the resulting cross-section using SEM-EDX to obtain a cross-sectional image.

Description

Technical Field

[0001] The present invention relates to a negative electrode material, a negative electrode for a non-aqueous electrolyte battery, and a non-aqueous electrolyte secondary battery.

Background Art

[0002] Rechargeable secondary batteries include a secondary battery using a non-aqueous electrolyte as an electrolyte (hereinafter, non-aqueous electrolyte secondary battery) and a secondary battery using a solid electrolyte (all-solid secondary battery). Among these, non-aqueous electrolyte secondary batteries have already been put into practical use not only for small power sources such as mobile phones and notebook computers but also for medium or large power sources such as automotive applications and power storage applications.

[0003] Regarding non-aqueous electrolyte secondary batteries, in order to improve performance, studies on electrodes and active materials contained in the electrodes have been underway. For example, regarding lithium secondary batteries among non-aqueous electrolyte secondary batteries, studies have been conducted to improve battery performance by using a material having a larger theoretical capacity than graphite, which is a conventional negative electrode material, for the negative electrode constituting the lithium secondary battery. As such a material, similar to graphite, for example, metal materials capable of occluding and releasing lithium ions have been attracting attention.

[0004] As an example of a negative electrode formed from a metal material, for example, Non-Patent Document 1 describes using an aluminum alloy obtained by adding iron (Fe) to aluminum (Al) as a negative electrode material for a lithium ion secondary battery.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] As the application fields of non-aqueous electrolyte secondary batteries expand, there is a need for further improvement in their cycle characteristics. In the negative electrode described in Non-Patent Document 1 above, the charge-discharge efficiency, which was initially around 80%, decreased to below 60% after 10 charge-discharge cycles, indicating that there is room for improvement in cycle characteristics.

[0007] Furthermore, anode materials with improved cycle characteristics are in demand not only for lithium-ion secondary batteries but also for other non-aqueous electrolyte secondary batteries.

[0008] "Cycle characteristics" are evaluated by the discharge capacity retention rate when charging and discharging are repeated. A high discharge capacity retention rate when a non-aqueous electrolyte secondary battery is repeatedly charged and discharged is considered to indicate "good cycle characteristics."

[0009] The present invention has been made in view of the above circumstances, and aims to provide a negative electrode material for a non-aqueous electrolyte secondary battery that can improve the cycle characteristics of the non-aqueous electrolyte secondary battery. Furthermore, the present invention also aims to provide a negative electrode for a non-aqueous electrolyte battery using such a negative electrode material, and a non-aqueous electrolyte secondary battery using the same. [Means for solving the problem]

[0010] To solve the above problems, one aspect of the present invention includes the following aspects.

[0011] [1] A negative electrode material made of an aluminum alloy, wherein the negative electrode material comprises aluminum, element M1 having a solid solubility limit of 1.5% by mass or more in aluminum, and element M2 having a solid solubility limit of less than 1.5% by mass in aluminum, wherein element M1 is one or more selected from the group consisting of Si, Ge and Mn, and element M2 is one or more selected from the group consisting of B, Ti, V, Cr, Fe, Co, Ni, Sr, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, Pb, La, Ce, Pr and Nd, the aluminum purity of the remainder after removing elements M1 and M2 from the entire negative electrode material is 99% by mass or more, element M1 exists in the metallic crystal of aluminum in the negative electrode material by substituting aluminum atoms, and element M2 forms a non-aluminum phase with a length of 10 nm or more and 10 μm or less in a cross-sectional image obtained under the following conditions. (Conditions) The negative electrode material is rolled to a thickness of 50 μm before charging and discharging, and the resulting rolled material is cut perpendicular to the rolling direction. An elemental mapping image is created from the obtained cross-section using SEM-EDX, and this is used as the cross-sectional image.

[0012] [2] The negative electrode material according to [1], wherein the total content of elements M1 and M2 in the negative electrode material is 0.5% by mass or more and 8% by mass or less.

[0013] [3] The negative electrode material according to [1] or [2], wherein the Vickers hardness is 10 HV or more and 70 HV or less.

[0014] [4] A negative electrode material according to any one of [1] to [3], obtained by heating a rolled material comprising aluminum, element M1, and element M2, wherein the aluminum content in the remainder after removing element M1 and element M2 from the whole of the rolled material is 99% by mass or more, at a temperature of 100°C to 350°C.

[0015] A negative electrode for a non-aqueous electrolyte battery, comprising the negative electrode material described in any one of items [5], [1], to [4].

[0016] A non-aqueous electrolyte secondary battery having a negative electrode for a non-aqueous electrolyte battery as described in [[6]][[5]].

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a negative electrode material for a non-aqueous electrolyte secondary battery that can improve the cycle characteristics of the non-aqueous electrolyte secondary battery. Further, it is possible to provide a negative electrode for a non-aqueous electrolyte battery using such a negative electrode material, and a non-aqueous electrolyte secondary battery using the same.

Brief Description of the Drawings

[0018] [Figure 1] FIG. 1 is a cross-sectional view of an aluminum negative electrode of a lithium-ion secondary battery. [Figure 2] FIG. 2 is a cross-sectional view of an aluminum negative electrode of a lithium-ion secondary battery. [Figure 3] FIG. 3 is a schematic diagram showing an example of a non-aqueous electrolyte secondary battery.

Modes for Carrying Out the Invention

[0019] 《Negative Electrode Material, Negative Electrode for Non-Aqueous Electrolyte Battery》 The negative electrode material of the present embodiment is a negative electrode material made of an aluminum alloy. Further, the negative electrode for a non-aqueous electrolyte battery of the present embodiment is made of the negative electrode material described below.

[0020] The negative electrode material has the following characteristics. Requirement (1): It consists of aluminum, an element M1 whose solid solubility limit with respect to aluminum is 1.5% by mass or more, and an element M2 whose solid solubility limit with respect to aluminum is less than 1.5% by mass. Requirement (2): The aluminum purity of the remainder obtained by removing element M1 and element M2 from the entire negative electrode material is 99% by mass or more. Requirement (3): Element M1 exists by substituting aluminum atoms in the metal crystal of aluminum in the negative electrode material. Requirement (4): Element M2 forms a non-aluminum phase having a length of 10 nm or more and 10 μm or less in the cross-sectional image of the negative electrode material obtained under the following conditions. (conditions) The negative electrode material is rolled to a thickness of 50 μm before charging and discharging, and the resulting rolled material is cut perpendicular to the rolling direction. An elemental mapping image is created from the resulting cross-section using SEM-EDX to obtain a cross-sectional image.

[0021] [Requirement (1)] The negative electrode material comprises aluminum, element M1, and element M2.

[0022] For the negative electrode material, aluminum with a purity of 99.9% by mass or higher, or high-purity aluminum with a purity of 99.99% by mass or higher, can be used. High-purity aluminum can be manufactured by appropriately employing known methods for increasing the purity of aluminum, such as the segregation method, the three-layer electrolysis method, the band melting refining method, and the ultra-high vacuum solubility method.

[0023] Aluminum may contain unavoidable impurities. These unavoidable impurities include manufacturing residues that are inevitably mixed in during the refining process. Specifically, these include iron and copper. The copper content in high-purity aluminum 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 high-purity aluminum. Alternatively, the copper content in high-purity aluminum may be 0 ppm by mass, relative to the total mass of the high-purity aluminum.

[0024] The difference between element M1 and element M2 lies in their solid solubility limits (solid solubility limit concentration, mass%) in aluminum. Specifically, element M1 is an element whose solid solubility limit in aluminum is 1.5 mass% or more, while element M2 is an element whose solid solubility limit in aluminum is less than 1.5 mass%.

[0025] Element M1 is one or more selected from the group consisting of Si, Ge, and Mn. Element M1 is preferably Si, Ge, or both, with Si being more preferred.

[0026] Element M2 is one or more selected from the group consisting of B, Ti, V, Cr, Fe, Co, Ni, Sr, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, Pb, La, Ce, Pr, and Nd. Preferably, element M2 is one or more selected from the group consisting of B, Ti, V, Cr, Fe, Co, Ni, Sr, Y, Zr, Sn, and Ce; more preferably, one or more selected from the group consisting of Fe, Ni, Ce, and Co; and even more preferably, one selected from the group consisting of Fe, Ni, Ce, and Co.

[0027] Whether an element A corresponds to element M1 or element M2 can be determined by using the literature value for the solid solubility limit of element A in aluminum.

[0028] Furthermore, after preparing an aluminum alloy by adding 1.5 mass% of element A to aluminum, if the resulting aluminum alloy satisfies the following conditions (i) and (ii), or (i) and (iii), it can be determined that element A is in solid solution in aluminum. That is, the solid solubility limit of element A in aluminum can be determined to be 1.5 mass% or more, and element A can be determined to correspond to element M1. (i) The presence of element M1 can be confirmed by the above ICP analysis. (ii) No peaks attributable to element M1 or compounds containing element M1 can be observed in the diffraction pattern of the negative electrode material measured by X-ray diffraction (XRD). (iii) The lattice constant of aluminum, as determined from the diffraction pattern of the negative electrode material measured by X-ray diffraction (XRD), is shifted.

[0029] [Requirement (2)] In the negative electrode material, the aluminum purity (aluminum content) of the remainder after removing elements M1 and M2 is 99% by mass or higher. That is, the negative electrode material has an aluminum purity of 99% by mass or higher after removing the intentionally added elements M1 and M2. This aluminum purity is preferably 99.9% by mass or higher.

[0030] The composition of the negative electrode material is measured using, for example, a solid-state emission spectrometer (e.g., Thermo ARL-4460) or an ICP (inductively coupled plasma) emission spectrometer (e.g., Seiko Instruments Inc. SPS5000).

[0031] The aluminum purity described above is determined by using the above apparatus to determine the overall composition of the negative electrode material, then removing the elements corresponding to M1 and M2 from the obtained composition, determining the aluminum content (mol%) of the remainder, and converting it to mass%.

[0032] [Requirement (3)] Element M1 exists by forming a substitutional solid solution with the aluminum in the negative electrode material, substituting for aluminum atoms in the aluminum metallic crystal.

[0033] Whether element M1 is substituting for aluminum atoms in the aluminum metal crystal can be easily evaluated by confirming that conditions (i) and (ii), or (i) and (iii), described in [Requirement (1)] above are met.

[0034] [Requirement (4)] In the cross-sectional image obtained under the above conditions, element M2 forms a non-aluminum phase with a length of 10 nm or more and 10 μm or less. The size (length) of the non-aluminum phase is preferably 100 nm or more, and more preferably 500 nm or more. Furthermore, the size of the non-aluminum phase is preferably 5 μm or less, and more preferably 4 μm or less.

[0035] The upper and lower limits for the size of the non-aluminum phase can be any combination. The size of the non-aluminum phase may be between 100 nm and 5 μm, between 500 nm and 5 μm, or between 500 nm and 4 μm.

[0036] The following provides a detailed explanation of the method for measuring the size of the non-aluminum phase.

[0037] (Method for measuring the size of the non-aluminum phase) First, the negative electrode material before charging and discharging is rolled to a thickness of 50 μm, and the resulting rolled material is cut perpendicular to the rolling direction. Specifically, the rolled material is first cut using a diamond cutter. Then, the cross-section is further processed using a focused ion beam processing machine to create the cross-section. <Processing conditions> Argon ion milling system: IB-19520CCP (manufactured by JEOL Ltd.) Acceleration voltage: 6kV Atmosphere: Atmosphere Temperature: -100℃

[0038] The obtained cross-section was then scanned using the following scanning electron microscope (with energy-dispersive X-ray analyzer) to obtain scanning electron microscope images (SEM and SEM-EDX images) under the following conditions. <Measurement conditions> Scanning electron microscope: JSM-5500 (manufactured by JEOL Ltd.) Energy-dispersive X-ray fluorescence analyzer: X-MaxN (manufactured by Oxford Instruments) Acceleration voltage: 10kV (during SEM observation), 10kV (during SEM-EDX) Magnification: 1000 to 10000 times

[0039] The elemental mapping image to be created is the cross-sectional image shown above. The elemental mapping image confirms that the non-aluminum phase contains element M2.

[0040] Next, for the non-aluminum phase included in the cross-sectional image, we assume the smallest rectangle that circumscribes the non-aluminum phase and measure the length of the longest side of that rectangle.

[0041] If the number of non-aluminum phases in the cross-sectional image is 20 or more at a magnification of 1000x, measure the length of all non-aluminum phases according to the above definition, and determine the "size (length) of the non-aluminum phase" by taking the arithmetic mean of the obtained lengths. If the length of the non-aluminum phase is small and difficult to measure, increase the magnification to determine the length.

[0042] If the number of non-aluminum phases in the cross-sectional image is less than 20, a cross-sectional image of a different field of view is created, and the same measurement is performed for non-aluminum phases whose total number in multiple cross-sectional images is 20 or more, in order to determine the "size of the non-aluminum phases."

[0043] The size of the non-aluminum phase can be controlled by the cooling rate during aluminum casting, as described later. A faster cooling rate during aluminum casting tends to result in a smaller non-aluminum phase.

[0044] The inventors hypothesized that elements M1 and M2, finely dispersed in the structure as described in requirements (3) and (4), function as fillers supporting the structure of the negative electrode material, thereby suppressing damage to the negative electrode during repeated charging and discharging. It is known that with conventional aluminum negative electrodes and aluminum alloy negative electrodes, when used as the negative electrode in non-aqueous electrolyte secondary batteries, repeated charging and discharging causes gradual and microscopic disintegration from the ion exchange surface, resulting in damage to the negative electrode.

[0045] In contrast, in the negative electrode made of the negative electrode material of this embodiment, it is thought that element M1, which penetrates into the aluminum metal crystal, and element M2, which is deposited between the metal crystals as a larger mass than element M1, cooperate to support the metal crystal. As a result, in the negative electrode made of the negative electrode material of this embodiment, damage to the negative electrode can be suppressed by charging and discharging.

[0046] In particular, if the negative electrode material is manufactured by annealing (heat treating) rolled aluminum alloy, the following effects can be expected.

[0047] Figures 1 and 2 are cross-sectional views of the aluminum negative electrode of a conventional lithium-ion secondary battery, and are magnified photographs showing the negative electrode after charging. Figure 1 shows the negative electrode 50A made of annealed negative electrode material, and Figure 2 shows the negative electrode 50B made of unannealed negative electrode material.

[0048] The negative electrodes shown in Figures 1 and 2 are manufactured from a conventional negative electrode material in which 1% by mass of Si is added as element M1 to high-purity aluminum with a purity of 99.99% by mass or higher, but no element M2 is added. In a lithium-ion secondary battery, the negative electrodes 50A and 50B have their surface 50a facing the positive electrode, and lithium ions are inserted and removed from the surface 50a side.

[0049] The enlarged images in Figures 1 and 2 were taken using the following method. For coin-type batteries manufactured using the method described later in "(Method for measuring initial efficiency and discharge capacity retention rate), (1) Manufacturing of lithium secondary batteries," five charge and discharge cycles were performed, with each cycle counting as one.

[0050] Next, the discharged coin-type battery was disassembled in an argon glove box under an inert atmosphere, and the removed negative electrode was washed with anhydrous dimethyl carbonate. After drying the obtained negative electrode, it was cut parallel to the thickness direction with a cutter blade, and the cut surface was further processed with a focused ion beam. The obtained cross-section was observed with an SEM. The conditions for focused ion beam processing and SEM observation were the same as those described above (Method for measuring the size of the non-aluminum phase).

[0051] As shown in Figure 1, in the negative electrode 50A, a Li-Al alloy layer 51 grows from the surface 50a in the thickness direction of the negative electrode 50A (normal direction to the surface 50a). The symbol 50x indicates a portion of the negative electrode 50A where the Li-Al alloy layer 51 has not been formed, and where the original composition of the negative electrode 50A remains. In the Li-Al alloy layer 51, the blackened areas (symbol A) indicate where Li has been inserted (forming a Li-Al alloy).

[0052] As is clear from the photograph in Figure 1, in the negative electrode 50A after annealing, Li penetrates in the thickness direction of the negative electrode 50A, and the region indicated by symbol A forms a striped pattern in the thickness direction of the negative electrode 50A. In other words, in the negative electrode 50A, the structure is subdivided in the width direction of the negative electrode 50A (same direction as the surface 50a).

[0053] In contrast, as shown in Figure 2, in the unannealed negative electrode 50B, the portion where Li is inserted in the Li-Al alloy layer 51 (indicated by A) is formed in the width direction of the negative electrode 50B (in the same direction as the surface 50a).

[0054] Due to these differences, the 50A negative electrode tends to have higher initial charge-discharge efficiency than the 50B negative electrode because lithium is easily inserted and removed. On the other hand, the 50A negative electrode tends to have lower cycle characteristics than the 50B negative electrode because its microstructure, which is subdivided in the width direction, is prone to breakdown after repeated charge-discharge cycles.

[0055] In the anode (anode material) after such annealing treatment, if the aforementioned element M2 is further added to form the anode material of this embodiment, a non-aluminum phase of element M2 is formed within the anode (requirement (4) above). The non-aluminum phase is thought to function in binding together the microstructure that has been subdivided in the width direction of the anode, and an improvement in cycle characteristics can be expected.

[0056] On the other hand, in the case of the negative electrode material of this embodiment that has undergone annealing treatment, it is considered that the effect of Li penetrating in the thickness direction of the negative electrode and Li being easily inserted and removed is maintained.

[0057] As a result, in the negative electrode material of this embodiment, when annealed, it becomes easier to achieve both improved cycle characteristics and high initial charge / discharge efficiency.

[0058] The annealed anode material described above can be manufactured, for example, using a rolled material composed of aluminum, element M1, and element M2 as the anode material precursor. The anode material precursor (rolled material) has an aluminum content of 99% by mass or more in the remainder after removing elements M1 and M2 from the entire rolled material.

[0059] The annealed negative electrode material is obtained by heating the above-mentioned rolled material at a temperature of 100°C to 550°C. The method for manufacturing the annealed negative electrode material includes a step of heating the above-mentioned rolled material at a temperature of 100°C to 350°C. The annealing temperature may be 150°C to 350°C, or 200°C to 350°C.

[0060] Cycle characteristics are evaluated by measuring the discharge capacity retention rate using the following method.

[0061] (Method for measuring initial efficiency and discharge capacity retention rate) (1) Manufacturing of lithium secondary batteries First, as the object to be measured, a 50 μm thick aluminum negative electrode is cut into a φ16 mm disc shape.

[0062] Next, LiCoO2 is formed into a φ14mm disc shape to serve as the counter electrode (positive electrode).

[0063] Next, an electrolyte solution is prepared by dissolving LiBF4 in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of EC:DMC = 50:50 to a concentration of 1.0 mol / L.

[0064] A porous polyethylene separator is placed between the aluminum negative electrode and the counter electrode, and then housed in a battery case (standard 2032). The electrolyte is poured into the battery case, and the battery case is sealed to manufacture a coin-type (full-cell) lithium secondary battery with a diameter of 20 mm and a thickness of 3.2 mm.

[0065] (2) Measurement of initial efficiency For the manufactured lithium secondary batteries, both charging and discharging were performed at a test temperature of 25°C with a current setting of 0.6 mA / cm² over the positive electrode area. 2(0.2CA) is used, and charging and discharging are performed with a maximum charging voltage of 4.2V and a minimum discharging voltage of 3.0V. The discharge capacity is measured, and the charging capacity is measured, and the obtained value is defined as the "initial charging capacity" (mAh). The discharge capacity is measured again, and the obtained value is defined as the "initial discharging capacity" (mAh), i.e., the discharge capacity of the first cycle.

[0066] Using the values ​​of the initial discharge capacity and the initial charge capacity, the initial efficiency is calculated using the following formula. Initial efficiency (%) = Initial discharge capacity (mAh) ÷ Initial charge capacity (mAh) × 100

[0067] (3) Measurement of discharge capacity maintenance rate After the initial charge and discharge cycle, the charging and discharging process is repeated under the same conditions as the initial charge and discharge cycle. The discharge capacity is measured in each cycle, and the discharge capacity retention rate is calculated as the ratio to the initial discharge capacity.

[0068] Regarding the discharge capacity retention rate, the maximum number of cycles at which the initial discharge capacity remains at 80% or more is determined and evaluated. A larger "maximum number of cycles" indicates better cycle characteristics.

[0069] (composition) In the negative electrode material, the total content of elements M1 and M2 relative to the entire negative electrode material is preferably 0.5% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less, and even more preferably 0.5% by mass or more and 3% by mass or less.

[0070] Furthermore, in the negative electrode material, the content of element M1 relative to the total negative electrode material is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.5% by mass or more. In addition, the content of element M1 is preferably 5% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less. The upper and lower limits of the content of element M1 can be arbitrarily combined.

[0071] Furthermore, in the negative electrode material, the content of element M2 relative to the total negative electrode material is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more. In addition, the content of element M2 is preferably less than 8% by mass, more preferably 5% by mass or less, and even more preferably 3% by mass or less, as this facilitates rolling. The upper and lower limits of the element M2 content can be arbitrarily combined.

[0072] (Vickers hardness) Furthermore, the Vickers hardness of the negative electrode material is preferably between 10 HV and 70 HV. More preferably, the Vickers hardness of the negative electrode material is between 20 HV and 70 HV, and even more preferably between 30 HV and 70 HV.

[0073] The Vickers hardness used is the Vickers hardness (HV0.05) measured using a micro-Vickers hardness tester by the following method.

[0074] (Method for measuring Vickers hardness) Vickers hardness is a value measured according to JIS Z2244:2009 "Vickers hardness test - Test method". To measure Vickers hardness, a square pyramidal diamond indenter is pressed into the surface of the test piece against the negative electrode material, and after releasing the test force, the value is calculated from the diagonal length of the indentation remaining on the surface.

[0075] The above standard stipulates that the hardness symbol should 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).

[0076] If the Vickers hardness is below the above upper limit, it is presumed that the strain on the crystalline structure can be relieved when aluminum absorbs lithium, and the crystalline structure can be maintained. For this reason, anodes for non-aqueous electrolyte batteries manufactured from anode material tend to maintain their discharge capacity even when used in lithium secondary batteries and subjected to repeated charging and discharging.

[0077] [Method for manufacturing negative electrode material] The above-described manufacturing method preferably comprises an alloy casting step and a rolling step.

[0078] (Casting process) When casting, for example, predetermined amounts of elements M1 and M2 are added to high-purity aluminum, and it is melted at a temperature of approximately 680°C to 800°C to obtain a molten alloy of aluminum and metal.

[0079] It is preferable to clean the molten alloy by removing gases and nonmetallic inclusions (for example, vacuum treatment of molten aluminum). Vacuum treatment is carried out, for example, at a temperature of 700°C to 800°C for 1 to 10 hours and at a vacuum level of 0.1 Pa to 100 Pa.

[0080] To purify the molten alloy, processes such as blowing in flux, inert gas, or chlorine gas can be used. The molten alloy, after being purified by vacuum treatment or other methods, is usually cast in a mold to obtain an ingot.

[0081] The mold is made of iron or graphite heated to between 50°C and 200°C. The negative electrode material of this embodiment can be cast by pouring molten alloy at between 680°C and 800°C into the mold. Alternatively, an ingot can be obtained by the commonly used semi-continuous casting method.

[0082] (Rolling process) In the rolling process of ingots, for example, hot rolling (primary rolling) and cold rolling (secondary rolling) are performed to process the ingot into a sheet material (rolled material). Hot rolling is performed repeatedly, for example, at a temperature of 350°C to 550°C and with a processing rate of 2% to 30% per rolling pass, until the ingot reaches the desired thickness.

[0083] After hot rolling, an intermediate annealing treatment is usually performed before cold rolling. The intermediate annealing treatment may involve, for example, heating the hot-rolled sheet material to 350°C to 550°C, then immediately allowing it to cool, or holding it for 1 to 5 hours before allowing it to cool. Rapid cooling tends to reduce the size of the non-aluminum phase. On the other hand, slow cooling allows the metal particles constituting the non-aluminum phase to grow more easily. The cooling process can be adjusted as appropriate according to the desired size of the non-aluminum phase. The cooling treatment softens the material, resulting in a state favorable for cold rolling.

[0084] Cold rolling is a process in which an aluminum ingot is repeatedly rolled at a temperature below the recrystallization temperature of aluminum, with a reduction rate of 1% to 20% per roll, until the desired thickness is achieved. The temperature for cold rolling should be between room temperature and 80°C.

[0085] After cold rolling, the rolled material may be subjected to the aforementioned annealing treatment, which involves heating it to a temperature between 100°C and 350°C. While the heat treatment after cold rolling is usually performed in air, it may also be carried out in a nitrogen atmosphere or a vacuum atmosphere. Heat treatment can soften work-hardened sheet materials, and in some cases, it can also be used to control the crystalline structure and adjust various physical properties.

[0086] 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 conductivity can be adjusted to a desired range.

[0087] The negative electrode material and negative electrode for non-aqueous electrolyte batteries with the above configuration can improve the cycle characteristics of non-aqueous electrolyte secondary batteries.

[0088] 《Nonaqueous electrolyte secondary battery》

[0089] Figure 3 is a schematic diagram showing an example of a non-aqueous electrolyte secondary battery. The lithium secondary battery (non-aqueous electrolyte secondary battery) 10 has a positive electrode 2, an aluminum negative electrode (negative electrode) 3, and a non-aqueous electrolyte (electrolyte) 6.

[0090] A cylindrical lithium secondary battery 10 is manufactured as follows. First, as shown in Figure 3, 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 the order of separator 1, positive electrode 2, separator 1, negative electrode 3, and wound to form an electrode group 4.

[0091] Next, the electrode group 4 and an insulator (not shown) are placed in the battery can 5, the bottom of the can is sealed, the electrode group 4 is impregnated with electrolyte 6, and the electrolyte is placed 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 body 8 to manufacture the lithium secondary battery 10.

[0092] As for the shape of the electrode group 4, for example, a columnar shape can be given 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.

[0093] Furthermore, the shape of the lithium secondary battery having such electrode group 4 can be one of those specified in IEC60086 or JIS C 8500, which are battery standards established by the International Electrotechnical Commission (IEC). For example, cylindrical or prismatic shapes can be used.

[0094] Furthermore, lithium secondary batteries are not limited to the wound configuration described above; they may also have a stacked configuration in which a stacked structure of positive electrode, separator, negative electrode, separator is repeatedly stacked. Examples of stacked lithium secondary batteries include so-called coin-type batteries, button-type batteries, or paper-type (or sheet-type) batteries.

[0095] The separator 1, positive electrode 2, and negative electrode lead 31 can be made from materials known as components of a lithium secondary battery.

[0096] The negative electrode 3 can be the aluminum negative electrode of this embodiment described above. That is, the negative electrode 3 has a negative electrode body made of aluminum, and in the lithiumization reaction measured under the measurement conditions described above, the lithiumization capacity is 1 mAh / cm². 2 The resistance value at that time is 6.5 kΩ·cm 2 The following aluminum anodes can be used.

[0097] Furthermore, the negative electrode 3 comprises a negative electrode body made of aluminum and a coating formed on the surface of the negative electrode body, and an aluminum negative electrode can be used in which the coating is made of one of the materials selected from the group consisting of gold, carbon, and alumina hydrate.

[0098] Electrolyte 6 is a non-aqueous electrolyte. Examples of organic solvents included in the electrolyte are: 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; Ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran; 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; Sulfur-containing compounds such as sulfolanes, dimethyl sulfoxides, and 1,3-propanesaltones; The above organic solvents with an additional fluorogroup introduced (one or more hydrogen atoms in the organic solvent are replaced with fluorine atoms). You can use it.

[0099] It is preferable to use a mixture of two or more of these organic solvents. Among these, a mixed solvent containing carbonates is preferred, and a mixed solvent of cyclic carbonates and acyclic carbonates, and a mixed solvent of cyclic carbonates and ethers are even more preferred. As a mixed solvent of cyclic carbonates and acyclic carbonates, a mixed solvent containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is preferred. Electrolytes using such mixed solvents have many advantages, including a wide operating temperature range, resistance to degradation even when charging and discharging at high current rates, and resistance to degradation even after prolonged use.

[0100] The electrolytes contained in the electrolyte solution include LiClO4, LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(COCF3), Li(C4F9SO3), LiC(SO2CF3)3, and Li2B 10 Cl 10 Examples of lithium salts include LiBOB (where BOB is bis(oxalato)borate), LiFSI (where FSI is bis(fluorosulfonyl)imide), lithium salts of lower aliphatic carboxylates, and LiAlCl4, and mixtures of two or more of these may be used. In particular, it is preferable to use an electrolyte that contains at least one selected from the group consisting of fluorine-containing LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(SO2CF3)2, and LiC(SO2CF3)3.

[0101] Furthermore, the inventors' investigations revealed that, in addition to its lithium-ion conductivity as an electrolyte, LiBF4 has the effect of suppressing the formation of the aforementioned film X compared to other electrolytes such as LiPF6. When LiBF4 is present in the electrolyte, BF3 is released from LiBF4 in the electrolyte. For example, when ethylene carbonate (EC) is used as the solvent for the electrolyte, it is thought that BF3 protects the lone pair of electrons of EC and suppresses the decomposition of the electrolyte.

[0102] According to the lithium secondary battery 10 described above, the cycle characteristics can be improved.

[0103] Preferred embodiments of the present invention have been described above with reference to the attached drawings, but the present invention is not limited to these examples. The shapes and combinations of the constituent members shown in the above examples are merely examples, and can be modified in various ways based on design requirements, etc., without departing from the spirit of the present invention.

[0104] Furthermore, the present invention also encompasses the following aspects.

[0105] <1> A negative electrode material made of an aluminum alloy, wherein the negative electrode material consists of aluminum, element M1 having a solid solubility limit of 1.5% by mass or more in aluminum, and element M2 having a solid solubility limit of less than 1.5% by mass in aluminum, wherein element M1 is one or more selected from the group consisting of Si, Ge, and Mn, and element M2 is one or more selected from the group consisting of B, Ti, V, Cr, Fe, Co, Ni, Sr, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, Pb, La, Ce, Pr, and Nd, the aluminum purity of the remainder after removing elements M1 and M2 from the entire negative electrode material is 99% by mass or more, element M1 exists in the aluminum metallic crystal of the negative electrode material by substituting aluminum atoms, and element M2 forms a non-aluminum phase with a length of 500 nm to 5 μm in a cross-sectional image obtained under the following conditions. (Conditions) The negative electrode material is rolled to a thickness of 50 μm before charging and discharging, and the resulting rolled material is cut perpendicular to the rolling direction. An elemental mapping image is created from the obtained cross-section using SEM-EDX, and this is used as the cross-sectional image.

[0106] <2> The total content of elements M1 and M2 in the negative electrode material is 0.5% by mass or more and 3% by mass or less. <1> The negative electrode material described above.

[0107] <3> The Vickers hardness is between 30 HV and 70 HV. <1> or <2> The negative electrode material described above.

[0108] <4> <1> from <3> A negative electrode for a non-aqueous electrolyte battery, comprising the negative electrode material described in any one of the items.

[0109] <5> <4> A non-aqueous electrolyte secondary battery having a negative electrode for a non-aqueous electrolyte battery as described above. [Examples]

[0110] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0111] In this embodiment, the following measurements were performed as appropriate.

[0112] (Size measurement of the non-aluminum phase) The size of the non-aluminum phase was measured using the method described above (Method for measuring the size of the non-aluminum phase).

[0113] (Initial efficiency, discharge capacity retention rate) The initial efficiency and discharge capacity retention rate were measured according to the method described above in (Measurement Method for Initial Efficiency and Discharge Capacity Retention Rate).

[0114] (Vickers hardness) Vickers hardness was measured according to the method described above (Method for measuring Vickers hardness).

[0115] <Example 1> [Fabrication of the negative electrode] By heating and holding high-purity aluminum (purity: 99.99% by mass or higher), high-purity chemically produced silicon (purity: 99.999% by mass or higher), and iron (purity: 99.9% by mass) at 760°C, an aluminum-silicon-iron alloy molten metal with a silicon content of 0.5% by mass and an iron content of 1% by mass was obtained.

[0116] Next, the molten alloy was purified by holding it at a temperature of 740°C for 2 hours under a vacuum of 50 Pa. The molten alloy was cast in a cast iron mold (22mm x 150mm x 200mm) dried at 150°C to obtain an ingot.

[0117] The rolling process was carried out under the following conditions: After machining both sides of the ingot by 2 mm, cold rolling was performed from a thickness of 18 mm with a machining rate of 99.6%. The thickness of the resulting rolled material was 50 μm.

[0118] The rolled material was heat-treated at 350°C for 3 hours to obtain the anode material of Example 1. The size of the non-aluminum phase in the obtained anode material was 2.8 μm.

[0119] The obtained negative electrode material was cut into a φ16 mm disc shape to obtain the negative electrode of Example 1.

[0120] [Fabrication of the positive electrode] Lithium cobalt oxide is used as the positive electrode active material (product name: Cellseed, manufactured by Nippon Chemical Industrial Co., Ltd.). 90 parts by mass of an average particle size (D50) of 10 μm, 5 parts by mass of polyvinylidene fluoride (manufactured by Kureha Corporation) as a binder, and 5 parts by mass of acetylene black (product name Denka Black, manufactured by Denka Co., Ltd.) as a conductive material were mixed, and then 70 parts by mass of N-methyl-2-pyrrolidone were added to prepare the electrode mixture for the positive electrode.

[0121] The obtained electrode mixture was coated onto a 15 μm thick aluminum foil current collector using the doctor blade method. The coated electrode mixture was dried at 60°C for 2 hours, and then vacuum-dried at 150°C for 10 hours to volatilize N-methyl-2-pyrrolidone. The coating amount of positive electrode active material after drying was 21.5 mg / cm². 2 That was the case.

[0122] After rolling the resulting electrode mixture layer and current collector laminate, it was cut into a φ14 mm disc shape to produce a positive electrode, which is a laminate of a positive electrode mixture layer made of lithium cobalt oxide and a current collector.

[0123] [Preparation of electrolyte solution] An electrolyte solution was prepared by dissolving LiBF4 at a concentration of 1 mol / L in a mixed solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of EC:DMC = 50:50.

[0124] [Manufacturing of lithium-ion batteries] A porous polyethylene separator was placed between the negative electrode and the positive electrode as described above, housed in a battery case (standard 2032), and the electrolyte was poured in. The battery case was then sealed to produce a coin-shaped lithium secondary battery (a lithium secondary battery including the negative electrode of Example 1) with a diameter of 20 mm and a thickness of 3.2 mm.

[0125] <Example 2> Except for using nickel (99.9% purity) instead of iron, the negative electrode material and negative electrode of Example 2 were obtained in the same manner as in Example 1, with a silicon content of 0.5 mass% and a nickel content of 1 mass%. A lithium secondary battery was fabricated using the negative electrode of Example 2. The size of the non-aluminum phase in the obtained negative electrode material was 2.2 μm.

[0126] <Example 3> Except for using cobalt (99.9% purity) instead of iron, the negative electrode material and negative electrode of Example 3 were obtained in the same manner as in Example 1, with a silicon content of 0.5 mass% and a cobalt content of 1 mass%. A lithium secondary battery was fabricated using the negative electrode of Example 3. The size of the non-aluminum phase in the obtained negative electrode material was 2.1 μm.

[0127] <Example 4> Except for using cerium (99.9% purity) instead of iron, the negative electrode material and negative electrode of Example 4 were obtained in the same manner as in Example 1, with a silicon content of 0.5 mass% and a cerium content of 1 mass%. A lithium secondary battery was fabricated using the negative electrode of Example 4. The size of the non-aluminum phase in the obtained negative electrode material was 3.5 μm.

[0128] <Comparative Example 1> A negative electrode material and negative electrode of Comparative Example 1, with a silicon content of 1% by mass, were obtained in the same manner as in Example 1, except that iron was not used and the amount of silicon was changed. A lithium secondary battery was fabricated using the negative electrode of Comparative Example 1. A non-aluminum phase consisting of Si was confirmed in the obtained negative electrode material.

[0129] <Comparative Example 2> A negative electrode material and negative electrode for Comparative Example 2 were obtained in the same manner as in Example 1, except that iron was not used and the amount of silicon was changed, with a silicon content of 0.5 mass%. A lithium secondary battery was fabricated using the negative electrode of Comparative Example 2. No non-aluminum phase consisting of Si was found in the obtained negative electrode material.

[0130] <Comparative Example 3> The negative electrode material and negative electrode of Comparative Example 3 were obtained in the same manner as in Example 1, except that only aluminum was used instead of iron and silicon. A lithium secondary battery was fabricated using the negative electrode of Comparative Example 2. No non-aluminum phase was detected in the obtained negative electrode material.

[0131] The evaluation results are shown in Table 1. In the table, the "%" in the M1 and M2 columns represents "mass%".

[0132] [Table 1]

[0133] The evaluation results showed that the negative electrodes of Examples 1 to 4 all exhibited higher discharge capacity retention rates and superior cycle characteristics compared to the comparative example. Furthermore, considering that the anode using Al-Fe alloy described in the aforementioned prior art documents saw its charge-discharge efficiency drop to below 60% after 10 charge-discharge cycles, the anodes of Examples 1-4 are considered to have superior cycle characteristics compared to Al-Fe alloy.

[0134] From these results, it has been confirmed that the present invention is useful. [Explanation of Symbols]

[0135] 3...Negative electrode, 6...Electrolyte, 6...Non-aqueous electrolyte (electrolyte), 10...Lithium secondary battery (non-aqueous electrolyte secondary battery)

Claims

1. A negative electrode material made of an aluminum alloy, The negative electrode material consists of aluminum, element M1 whose solid solubility limit in aluminum is 1.5% by mass or more, and element M2 whose solid solubility limit in aluminum is less than 1.5% by mass. The element M1 is one or more selected from the group consisting of Si, Ge, and Mn. The element M2 is one or more selected from the group consisting of B, Ti, V, Cr, Fe, Co, Ni, Sr, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, Pb, La, Ce, Pr, and Nd. The aluminum purity of the remainder of the negative electrode material after removing elements M1 and M2 is 99% by mass or more. The element M1 exists in the aluminum metal crystal of the negative electrode material, substituting for aluminum atoms. The element M2 is a negative electrode material that forms a non-aluminum phase with a length of 10 nm to 10 μm in a cross-sectional image obtained under the following conditions. (conditions) The negative electrode material is rolled to a thickness of 50 μm before charging and discharging, and the resulting rolled material is cut perpendicular to the rolling direction. An elemental mapping image is created of the resulting cross-section using SEM-EDX, and this is used as the cross-sectional image.

2. The negative electrode material according to claim 1, wherein the total content of elements M1 and M2 in the negative electrode material is 0.5% by mass or more and 8% by mass or less.

3. The negative electrode material according to claim 1 or 2, wherein the Vickers hardness is 10 HV or more and 70 HV or less.

4. A negative electrode material according to claim 1 or 2, wherein a rolled material comprising aluminum, element M1, and element M2, wherein the aluminum content in the remainder after removing elements M1 and M2 from the entire rolled material is 99% by mass or more, is obtained by heating the negative electrode material precursor at 100°C to 350°C.

5. A negative electrode for a non-aqueous electrolyte battery comprising the negative electrode material according to claim 1 or 2.

6. A non-aqueous electrolyte secondary battery having a negative electrode for a non-aqueous electrolyte battery as described in claim 5.

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