Metal negative electrode for secondary battery and method for manufacturing same

Abstract

Provided are: a metal negative electrode for a secondary battery capable of simplifying a manufacturing process and suppressing dendrite growing toward a positive electrode; and a method for manufacturing the same.　In a metal negative electrode 3 for a secondary battery 1 which can be charged and discharged, a conical micro-projection shape 3a, which is continuously reduced in diameter toward a projection end 3aa, is integrally formed on a surface H. The micro-projection shape 3a is composed of a cone or a pyramid in which the projection end 3aa is acute or curved, and a portion in which the ratio of a dimension a of the bottom surface to a dimension b of the height direction is 1:3 or more occupies 50% or more of the entirety of the micro-projection shape 3a.

Description

Metallic negative electrode for secondary battery and method of manufacturing same

[0001] The present invention relates to a metal negative electrode for a chargeable and dischargeable secondary battery and a method for producing the same.

[0002] For electric vehicles (EVs), such as electric cars and electric motorcycles, extending the driving range is considered an important issue, and to solve this issue, it is necessary to increase the capacity and voltage of the secondary batteries installed in the vehicles. Various problems arise when attempting to increase the capacity and voltage of such secondary batteries, one of which is the problem of dendrites occurring in the metal negative electrodes of secondary batteries.

[0003] These dendrites consist of needle-like protrusions that develop on the metal negative electrode as the secondary battery is charged and discharged, and grow with repeated charging and discharging. When these dendrites grow, they may penetrate the separator, causing their tips to come into contact with the positive electrode and cause a short circuit. Therefore, as disclosed in Patent Document 1, for example, a technique has been disclosed for suppressing dendrite growth by fusing particles onto the surface of the metal negative electrode to form protrusions in advance.

[0004] Japanese Patent Application Laid-Open No. 2022-187895

[0005] However, in the above-mentioned conventional technology, protrusions are formed by fusing particles onto the surface of the metal negative electrode of a lithium-ion secondary battery, which requires particles separate from the base material, complicating the manufacturing process and increasing manufacturing costs, and is not sufficient to suppress dendrites growing toward the positive electrode.As a result, if the secondary battery is repeatedly charged and discharged over a long period of time, the tips of the grown dendrites may come into contact with the positive electrode, causing a short circuit.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a metal negative electrode for a secondary battery that can simplify the manufacturing process and suppress dendrites from growing toward the positive electrode, and a manufacturing method thereof.

[0007] The invention as set forth in claim 1 is characterized in that a metal negative electrode for a chargeable and dischargeable secondary battery has minute cone-shaped protrusions formed integrally on the surface, the diameter of which decreases continuously toward the tip.

[0008] A second aspect of the present invention is a metal negative electrode for a secondary battery according to the first aspect, wherein the minute protrusions have a shape of a cone or pyramid with an acute or curved tip.

[0009] The invention described in claim 3 is characterized in that, in the metal negative electrode of the secondary battery described in claim 2, the ratio of the bottom dimension to the height dimension of the microprotrusion shapes is 1:3 or more, and these microprotrusion shapes account for 50% or more of the total microprotrusion shapes.

[0010] The invention described in claim 4 is characterized in that, in the metal negative electrode of the secondary battery described in claim 1, the surface including the microprotrusion shapes has a surface area at least twice as large as that of a flat surface on which the microprotrusion shapes are not formed.

[0011] The invention of claim 5 is characterized in that the metal negative electrode of the secondary battery of claim 1 is made of a material selected from the group consisting of zinc, copper, lithium, magnesium, sodium, potassium, calcium, and alloys containing these metals as components.

[0012] The invention described in claim 6 is a method for manufacturing a metal negative electrode of a chargeable and dischargeable secondary battery, characterized in that cone-shaped micro-protrusions whose diameter continuously decreases toward the tip are integrally formed on the surface by surface processing including dry surface processing in which the surface is irradiated with particle beams such as neutral atoms or neutral molecules, or quantum beams such as laser light or ion beams, or plasma, wet surface processing including chemical etching with a solution, and dry and wet film formation methods.

[0013] According to the present invention, minute cone-shaped protrusions whose diameter continuously decreases toward the tip are integrally formed on the surface, which simplifies the manufacturing process and suppresses dendrites from growing toward the positive electrode.

[0014] Schematic diagram showing a secondary battery to which a metal negative electrode according to an embodiment of the present invention is applied. Schematic diagram showing a metal negative electrode of the secondary battery. Schematic diagram showing a microprojection shape formed on the metal negative electrode (before dendrite formation). Schematic diagram showing a state in which dendrites have been formed on the microprojection shape. Micrograph showing the microprojection shape (before dendrite formation). Enlarged micrograph showing the microprojection shape (before dendrite formation). Micrograph showing the microprojection shape (after dendrite formation). Enlarged micrograph showing the microprojection shape (after dendrite formation). A metal negative electrode (with roughening treatment) on which a microprojection shape according to the present invention is formed and a metal negative electrode (untreated) on which a microprojection shape is not formed are shown. Graph showing the relationship between the compared capacity and potential. Graph showing the experimental results when a metal negative electrode (roughened) on which a microprojection shape according to the present invention is formed is charged and discharged. Graph showing the experimental results when a metal negative electrode (untreated) on which a microprojection shape is not formed is repeatedly charged and discharged. Schematic diagram showing a microprojection shape (conical shape with a curved tip) according to another embodiment of the present invention. Schematic diagram showing a microprojection shape (pyramidal shape with an acute tip) according to another embodiment of the present invention. Schematic diagram showing a microprojection shape (pyramidal shape with a curved tip) according to another embodiment of the present invention. Schematic diagram showing a microprojection shape (conical shape with a truncated tip) according to another embodiment of the present invention.

[0015] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The metal negative electrode according to this embodiment is made of a metal plate-shaped member applied to a chargeable and dischargeable secondary battery. As shown in Figure 1, the applicable secondary battery 1 is configured to include a metal positive electrode 2, a metal negative electrode 3, a separator 4, a positive electrode-side current collector 5, a negative electrode-side current collector 6, and a case C containing an alkaline electrolyte (alkaline electrolyte E).

[0016] The metal positive electrode 2 is made of a sheet-like member (nickel foil) made of nickel (Ni), and is attached to the surface of the positive electrode-side current collector 5, and is configured to be attached to the case C in a state facing the metal negative electrode 3 and the separator 4. The separator 4 is made of, for example, nonwoven fabric, and is configured to be attached to a position in the case C between the metal positive electrode 2 and the metal negative electrode 3.

[0017] The metal negative electrode 3 is made of a sheet-like member (zinc foil) made of zinc (Zn), and is attached to the surface of the negative electrode-side current collector 6, and is also configured to be attached to the case C in a state facing the metal positive electrode 2 and the separator 4. In addition to zinc, the metal negative electrode 3 may be made of a material selected from the group consisting of copper (Cu), lithium (Li), magnesium (Mg), sodium (Na), potassium (K), calcium (Ca), and any of those metals or alloys containing these metals as components.

[0018] The secondary battery 1 can be charged or discharged by connecting a power source such as a battery or a load such as a motor between the positive electrode side current collector 5 in contact with the metal positive electrode 2 and the negative electrode side current collector 6 in contact with the metal negative electrode 3. In other words, the secondary battery 1 can convert electrical energy into chemical energy and store (charge) it by passing a current in the opposite direction to that during discharge.

[0019] 2 and 3, the metal negative electrode 3 according to this embodiment has cone-shaped microprotrusions 3a whose diameters continuously decrease toward the tips 3aa integrally formed on the surface H. Specifically, the metal negative electrode 3 according to this embodiment has a plurality of microprotrusions 3a formed on its surface H, each of which has a tip 3aa that is an acute-angled cone, and those having a ratio of the dimension a (diameter dimension) of the bottom surface 3ab to the height dimension b (dimension from the tip 3aa to the bottom surface 3ab) of 1:3 account for 50% or more of the total microprotrusions 3a.

[0020] Such microprojection shapes 3a can be obtained, for example, by dry surface processing (sputtering processing) in which surface H is irradiated with particle beams such as neutral atoms or neutral molecules or quantum beams such as laser light, and a large number of microprojection shapes 3a are formed in groups, as shown in the micrographs of Figures 5 and 6. Furthermore, surface H including microprojection shapes 3a of metal negative electrode 3 according to this embodiment has a surface area at least twice as large as that of a plane on which microprojection shapes 3a are not formed.

[0021] When the secondary battery 1 according to this embodiment is charged or discharged, dendrites D are generated on the side surfaces 3ac of the minute protrusions 3a, as shown in Fig. 4. Specifically, zinc (Zn) of the metal negative electrode dissolves in the electrolyte (such as alkaline electrolyte E), and Zn+4OH － →Zn(OH) ４ ２－ +2e － When the secondary battery 1 is charged, the electrical energy used during charging further converts Zn(OH) ４ ２－ →ZnO+H ２ O + 2OH － This chemical reaction occurs, resulting in the precipitation of ZnO, known as dendritic seeds (see Figures 7 and 8).

[0022] When the secondary battery 1 is discharged, the dendrite D is composed of ZnO+H ２ O + 2OH － →Zn(OH) ４ ２－ The ZnO formed on the surface of the metal negative electrode dissolves due to a chemical reaction (see FIGS. 5 and 6). However, as the secondary battery 1 is repeatedly charged and discharged, the ZnO continues to precipitate and gradually grows locally.

[0023] As dendrites D grow in this way, there is a possibility that their tips will break through separator 4 and reach metal positive electrode 2, and if the tips of dendrites D come into contact with metal positive electrode 2, a short circuit will occur, resulting in a loss of function of secondary battery 1. Furthermore, since dendrites D deposited in this embodiment contain insulating oxide (ZnO), they may adversely affect the conductivity of metal negative electrode 3 compared to the initial surface state (Zn) which has excellent conductivity, and may degrade battery performance.

[0024] In contrast, the metal negative electrode 3 according to this embodiment has microprojections 3a formed on its surface H, and the microprojections 3a are pyramidal (conical) extending in a direction α perpendicular to the surface H, as shown in Fig. 4 , so that dendrites D generated on the side surface 3ac grow in a direction shifted by a predetermined angle from the perpendicular direction α. ​​On the other hand, the separator 4 and metal positive electrode 2 of the secondary battery 1 are attached in the direction α perpendicular to the surface H of the metal negative electrode 3, so that it is difficult for the tip of the grown dendrite D to reach the separator 4 and the metal positive electrode 2, thereby suppressing a short circuit in the secondary battery 1.

[0025] However, since the metal negative electrode 3 has a plurality of microprojections 3a formed on its surface H, the surface area is increased, and even if dendrites D containing insulating oxides (ZnO) are generated, deterioration of the conductivity of the metal negative electrode 3 can be relatively suppressed. Furthermore, since the microprojections 3a according to this embodiment are formed on the surface H of the metal negative electrode 3 by surface processing (ion beam irradiation processing (also referred to as sputtering processing)), the surface is formed more directly, making it possible to eliminate the need for materials separate from the base material or particles resulting from processing, and simplifying the manufacturing process and materials.

[0026] Although the minute protrusion shape 3a according to this embodiment has a conical shape with an acute-angled tip 3aa as shown in Fig. 3, it may also have a conical shape with a curved tip 3aa as shown in Fig. 12, a pyramidal shape with an acute-angled tip 3aa as shown in Fig. 13, or a pyramidal shape with a curved tip 3aa as shown in Fig. 14. Furthermore, it may also have a cut-off shape in which the conical tip 3aa is notched to form a flat surface (pointed tip surface) as shown in Fig. 15.

[0027] The micro-protrusion shape 3a of the metal negative electrode 3 in this embodiment is formed by dry surface processing in which the surface is irradiated with particle beams such as neutral atoms or neutral molecules, or quantum beams such as laser light or ion beams, or plasma, but is not limited to this. A cone-shaped micro-protrusion shape 3a whose diameter continuously decreases toward the tip 3aa may be integrally formed on the surface H by wet surface processing including chemical etching with a solution, and surface processing including dry and wet film formation methods.

[0028] Next, experimental results showing the technical advantages of the metal negative electrode 3 according to this embodiment will be described. An example (unevenness treatment) in which the surface of a zinc foil (Zn) measuring 1 cm in length, 1 cm in width, and 50 μm in thickness was subjected to sputtering to form micro-projection shapes 3a, and a comparative example (untreated) in which the surface of a zinc foil (Zn) measuring 1 cm in length, 1 cm in width, and 50 μm in thickness was left flat without any processing were prepared. ２ The chemical reaction Zn(OH) that occurs when charging the secondary battery 1 by passing a current of ４ ２－ →ZnO+H ２ O + 2OH － The relationship between the voltage and the current was observed.

[0029] As a result, as shown in Figure 9, the voltage required for the chemical reaction was higher in the Example than in the Comparative Example. In other words, the overvoltage for the generation of ZnO, which is a dendrite species, was lower in the Example. This is thought to be because the Example had a larger surface area due to the unevenness treatment formed compared to the Comparative Example, resulting in a smaller overvoltage per unit area. Therefore, it was found that the Example had lower resistance on the electrode surface than the Comparative Example, and it was found that the load applied when current was passed through the metal negative electrode 3 could be reduced.

[0030] Next, an experiment was conducted in which the Example and the Comparative Example were repeatedly discharged and charged, and the voltage was measured each time. As a result, as shown in Figure 10, the Example was able to measure a predetermined voltage even after about 200 discharge and charge cycles, whereas the Comparative Example was no longer able to measure a predetermined voltage after about 10 discharge and charge cycles, as shown in Figure 11. This indicates that the Example has a significantly improved battery life compared to the Comparative Example.

[0031] According to this embodiment, the pyramidal microprotrusion shapes 3a, whose diameters continuously decrease toward the tips 3aa, are integrally formed on the surface H, which eliminates the need for separate particles or the like in the manufacturing process, simplifies the manufacturing process, and suppresses dendrites D from growing toward the metal positive electrode 2. In particular, the microprotrusion shapes 3a are formed in the shape of acute-angled or curved cones or pyramids, which reliably suppresses dendrites D from growing toward the metal positive electrode 2.

[0032] Furthermore, in the microprojection shapes 3a according to this embodiment, those having a ratio of the dimension a of the bottom surface 3ab to the dimension b in the height direction of 1:3 or more account for 50% or more of the entire microprojection shapes 3a, and therefore it is possible to improve the surface area while reliably suppressing dendrites D growing toward the metal positive electrode 2. Furthermore, by making the surface area of ​​the surface H including the microprojection shapes 3a at least twice that of a plane on which the microprojection shapes 3a are not formed as in this embodiment, it is possible to further improve the surface area.

[0033] Furthermore, the metal negative electrode 3 according to this embodiment is made of a material selected from the group consisting of zinc, copper, lithium, magnesium, sodium, potassium, and calcium, and alloys containing these metals as components, and therefore can be applied to a general-purpose secondary battery 1. Furthermore, by surface processing including dry surface processing in which the surface H is irradiated with particle beams such as neutral atoms or neutral molecules or quantum beams such as laser light, wet surface processing including chemical etching with a solution, and dry and wet film formation methods, cone-shaped microprojection shapes 3a whose diameter continuously decreases toward the tip 3aa are integrally formed on the surface, so that the surface of the base material can be deformed to form the microprojection shapes 3a, and separate particles, etc., can be eliminated during the manufacturing process.

[0034] Although the present embodiment has been described above, the present invention is not limited to this, and for example, the applicable secondary battery 1 is not limited to rocking chair type secondary batteries such as zinc-air secondary batteries, zinc-nickel secondary batteries, zinc-silver secondary batteries, lithium-air secondary batteries, lithium-sulfur secondary batteries, magnesium-air secondary batteries, sodium-sulfur secondary batteries, potassium secondary batteries, calcium secondary batteries, and polyvalent ion secondary batteries, but may also be any one of systems that do not utilize a redox reaction in the positive electrode, such as a lithium ion capacitor.

[0035] The present invention can also be applied to devices with different external shapes or devices with additional functions, provided that they have the same gist as the present invention.

[0036] REFERENCE SIGNS LIST 1 Secondary battery 2 Metal positive electrode 3 Metal negative electrode 3a Microprojection shape 3aa Tip 3ab Bottom surface 3ac Side surface 4 Separator 5 Positive electrode side current collector 6 Negative electrode side current collector C Case H Surface D Dendrite E Electrolyte

Claims

1. A metal negative electrode for a rechargeable and dischargeable secondary battery, characterized in that minute pyramidal protrusions whose diameter continuously decreases toward their tips are integrally formed on the surface.

2. The metallic negative electrode of a secondary battery according to claim 1, wherein the shape of said minute protrusions is a cone or pyramid with an acute or curved tip.

3. The metallic negative electrode of a secondary battery as described in claim 2, characterized in that the ratio of the bottom dimension to the height direction dimension of said microprotrusions is 1:3 or more in 50% or more of the total number of microprotrusions.

4. The metallic negative electrode of a secondary battery according to claim 1, characterized in that the surface including said minute protrusions has a surface area at least twice as large as a flat surface on which said minute protrusions are not formed.

5. The metallic negative electrode of the secondary battery according to claim 1, characterized in that it is made of a material selected from the group consisting of zinc, copper, lithium, magnesium, sodium, potassium and calcium, and alloys containing these metals as components.

6. A method for manufacturing a metal negative electrode of a rechargeable and dischargeable secondary battery, comprising the steps of: forming, integrally on the surface, cone-shaped micro-projections whose diameter continuously decreases toward their tips, by surface processing including dry surface processing in which the surface is irradiated with particle beams such as neutral atoms or neutral molecules, or quantum beams such as laser light or ion beams, or plasma; wet surface processing including chemical etching with a solution; and dry and wet film formation methods.

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