JP6658069B2 - Lithium battery - Google Patents

Description

  The present invention relates to lithium batteries.

  In recent years, lithium batteries have been put to practical use as secondary batteries having high output and high energy density. Research and development have been actively conducted with the aim of further increasing energy density.

  However, in the case of a lithium battery using lithium metal for the negative electrode, during the dissolution and deposition of lithium metal during charging and discharging, the formation of dendrites of lithium metal on the negative electrode and the reaction between the lithium metal and the electrolyte occur. There is a problem that efficiency is poor and charge / discharge cycle characteristics are inferior. In order to solve such a problem, for example, Patent Literature 1 discloses a lithium battery in which a metal current collector is used as a negative electrode, lithium metal is deposited on the surface of the current collector during charging, and the lithium metal is dissolved during discharging, A technique for suppressing lithium dendrite by smoothing the surface roughness of the current collector has been proposed.

Patent No. 4049506

  However, even with the above-described lithium battery of the related art, a short-circuit failure may occur inside the battery during repeated charging and discharging. This is considered to be because the voltage concentration caused by the unevenness of the current collector surface has been moved to the current collector end portion by the flattening of the current collector surface. The lithium to be doped uniformly in the negative electrode, or lithium to be doped in the negative electrode, is deposited and grown in the form of dendrites as metallic lithium at the end of the negative electrode current collector. Contact, causing a short circuit inside the battery.

  The present invention has been made in order to solve the above-described conventional problems, and a foreign substance such as a precipitate grown from an end portion of a negative electrode current collector crosses a separator from a negative electrode current collector and comes into contact with a positive electrode. It is an object of the present invention to provide a lithium battery capable of effectively preventing short-circuit failure of a lithium battery.

  The lithium battery according to the present invention is a lithium battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode current collector of the negative electrode during charging, and the lithium metal is dissolved during discharging. The ratio of the average surface roughness of the outer peripheral portion and the central portion on at least one main surface of the negative electrode current collector ((average surface roughness a of the outer peripheral portion a) / (average surface roughness b of the central portion)) is a / B <1.

  With this configuration, the current is easily concentrated at the center of the main surface of the negative electrode current collector, and even if lithium dendrite is generated, it is preferentially generated at the center of the current, thereby growing from the end of the negative electrode current collector. It is possible to provide a lithium battery in which a short circuit inside the battery due to the contact of the lithium metal over the separator and the contact with the positive electrode is prevented.

  The negative electrode current collector has a rectangular shape or a square shape, and the outer peripheral portion extends from the periphery on the main surface of the negative electrode current collector toward the center to a length of ４ of each side. Preferably, it is a region.

  With this configuration, it is possible to provide a lithium battery in which a short circuit inside the battery is further prevented.

  It is preferable that the ratio of the surface roughness satisfies a / b ≦ 0.9.

  With this configuration, it is possible to provide a lithium battery in which a short circuit inside the battery is further prevented.

  It is preferable that the average surface roughness b at the center is 0.1 μm ≦ b ≦ 1 μm.

  With such a configuration, the generation of dendritic lithium metal on the negative electrode current collector itself is suppressed, and a lithium battery in which a short circuit inside the battery is further prevented can be provided.

  In the negative electrode current collector, the ratio of the average surface roughness of the outer peripheral portion and the central portion on both main surfaces ((average surface roughness a of the outer peripheral portion a) / (average surface roughness b of the central portion)) is a / B <1 is preferable.

  With this configuration, it is possible to provide a lithium battery in which a short circuit inside the battery is further prevented.

  According to the present invention, a foreign substance such as a precipitate grown from an end portion of a negative electrode current collector effectively prevents a short circuit failure inside the battery due to the separator crossing the separator from the negative electrode current collector and contacting the positive electrode. A lithium battery can be provided.

It is sectional drawing which shows the conceptual structure of a lithium battery. It is the front view seen from the main surface of the negative electrode current collector.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiments. The components described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the components described below can be appropriately combined.

<Lithium battery>
FIG. 1 shows a lithium battery 100 according to the present embodiment. Here, FIG. 1 shows a pre-charged state in which charging and discharging have been performed a plurality of times. As shown in FIG. 1, a lithium battery 100 according to the present embodiment includes a plate-shaped negative electrode 20 and a plate-shaped positive electrode 10 facing each other, and a plate-shaped A power generating element 30 having a separator 18, a non-aqueous electrolyte containing lithium ions, a case 50 for accommodating them in a sealed state, and one end electrically connected to the negative electrode 20 and the other It has a negative electrode lead 62 having an end protruding outside the case, and a positive electrode lead 60 having one end electrically connected to the positive electrode 10 and the other end protruding outside the case.

  The negative electrode 20 has a negative electrode current collector 22 and a lithium metal layer 24 formed on the negative electrode current collector 22. Further, the positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the lithium metal layer 24 and the positive electrode active material layer 14.

<Negative electrode>
The negative electrode 20 includes a negative electrode current collector 22. As the negative electrode current collector 22, a metal plate and a metal foil made of copper, nickel, iron, and stainless steel, a metal plate and a metal foil in which another conductive material is coated with these metals or alloys, and the like can be used.

(Negative electrode current collector)
FIG. 2 is a front view of the negative electrode current collector 22 as viewed from the main surface, and includes an outer peripheral portion 70 and a central portion 72 on the main surface.
The negative electrode current collector 22 according to the present embodiment has a ratio of the average surface roughness of the outer peripheral portion and the central portion on at least one main surface ((average surface roughness a of the outer peripheral portion) / (average surface roughness of the central portion). B)) is such that a / b <1.

  The current tends to concentrate at the center of the main surface of the negative electrode current collector 22, and even if lithium dendrite is generated, it is preferentially generated at the center of the current, thereby growing from the end of the negative electrode current collector. It is possible to prevent a short circuit inside the battery due to the lithium metal getting over the separator and coming into contact with the positive electrode.

  The surface roughness is defined by the Japanese Industrial Standards (JIS B 0601-1994), and the surface roughness Ra is measured at 10 points within a visual field in a predetermined range, and the average value is calculated as the surface roughness in the visual field. And The surface roughness of the negative electrode current collector surface is determined in ten different visual fields, and the average value of all the visual fields is defined as the average surface roughness of the negative electrode current collector. The surface roughness can be measured by, for example, a stylus type surface roughness meter, an atomic force microscope (AFM), a laser scanning microscope, or the like. Among them, an atomic force microscope (AFM) is preferable.

  The negative electrode current collector 22 has a rectangular shape or a square shape, and the outer peripheral portion has a length of a quarter of the length of each side from the periphery on the main surface of the negative electrode current collector toward the center. It is preferable that this is the region up to this point.

  As shown in FIG. 2, the lengths from the periphery of each side toward the center are L12, L14, L22, and L24, and the outer periphery is defined as L12, L14, L22, L24 from the edge of each side of the negative electrode current collector. Up to the area 70. The region in which the outer peripheral portion extends from the periphery on the main surface of the negative electrode current collector to the center and extends to a quarter of the length of each side is L12 = L14 = L10 / 4 and L22 = L24 = This is an area up to a length satisfying L20 / 4.

  It is preferable that the ratio of the average surface roughness between the outer peripheral portion and the central portion on at least one main surface of the negative electrode current collector satisfies a / b ≦ 0.9.

  It is preferable that the average surface roughness b at the center is 0.1 μm ≦ b ≦ 1 μm.

  The negative electrode current collector has a ratio of the average surface roughness of the outer peripheral portion and the central portion on both main surfaces ((average outer surface roughness a of the outer peripheral portion) / (average surface roughness of the central portion b)). It is preferable that a / b <1.

  There is no particular limitation on making the negative electrode current collector 22 have a predetermined surface roughness, and examples thereof include a plating method, a vapor deposition method, an etching method, and a polishing method. Further, in order to process only a predetermined place, it is possible to perform processing by masking a part which is not to be processed with a surface protection tape, a protective film or the like.

<Lithium metal layer>
The lithium metal layer 24 is formed by reducing lithium ions in the nonaqueous electrolyte impregnated in the separator 18 and depositing on the surface of the negative electrode current collector 22 during charging. This lithium metal layer is oxidized at the time of discharge and is dissolved again in the non-aqueous electrolyte as lithium ions. In the lithium battery of the present invention, the lithium metal layer thus deposited on the negative electrode current collector 22 serves as a negative electrode active material of a general lithium battery.

<Positive electrode>
The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 provided on the positive electrode current collector. As the positive electrode current collector 12, an aluminum foil or the like can be used.
The positive electrode active material layer 14 includes a positive electrode active material, a binder, and a conductive auxiliary added as needed.

(Positive electrode active material)
The positive electrode active material performs occlusion and release of lithium ions, elimination and insertion (intercalation) of lithium ions, or doping and undoping of lithium ions and a counter anion of the lithium ions (for example, ClO ₄ ⁻ ). There is no particular limitation as long as it can reversibly proceed, and a known electrode active material can be used. For example, lithium cobalt oxide (LiCoO _2), lithium nickel oxide (LiNiO _2), lithium manganese spinel (LiMn ₂ O _4), and the general _{formula: LiNi x Co y Mn z MaO} 2 (x + y + z + a = 1,0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Oxide, lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (where M is at least one element selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO) ), And a composite metal oxide such as lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

(Binder)
The binder is not particularly limited as long as the above active material can be bound to the current collector, and a known binder can be used. For example, fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), and styrene-butadiene rubber (SB)
R) and a water-soluble polymer (carboxymethylcellulose, polyvinyl alcohol, sodium polyacrylate, dextrin, gluten, etc.).

(Conduction aid)
The conductive assistant is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive assistant can be used. For example, carbon blacks, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO can be used.

(Separator)
The separator 18 is not particularly limited as long as it is formed of an electrically insulating porous body. For example, a single-layered body of a film made of polyethylene, polypropylene or polyolefin, a laminated body or a stretched film of a mixture of the above resins, or a woven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester and polypropylene Alternatively, a nonwoven fabric is an example of the separator.

(Non-aqueous electrolyte)
Although not shown, the solvent constituting the non-aqueous electrolyte is not particularly limited as long as it can be used for a lithium battery.For example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate Examples thereof include carbonate, sulfolane, dimethoxyethane, tetrahydrofuran and dioxolan, and these can be used alone or as a mixture of a plurality of components.

The solute constituting the nonaqueous electrolyte is not particularly limited as long as it is a solute that can be used for a lithium battery. For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , and LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ), (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiCF ₃ (CF ₂ ) ₃ SO _{3 and the} like. These can be used alone or as a mixture of a plurality of components.

  The non-aqueous electrolyte is not a liquid, and may be a gel electrolyte obtained by adding a gelling agent to the non-aqueous electrolyte. Further, instead of the non-aqueous electrolyte, a solid electrolyte (a solid polymer electrolyte or an electrolyte made of an ion-conductive inorganic material) may be used.

(Case)
The case 50 hermetically seals the laminate 30 and the non-aqueous electrolyte. Case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside, entry of moisture and the like from the outside into lithium battery 100, and the like. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is preferably polyethylene, polypropylene, or the like.

(Lead)
The leads 60 and 62 are formed from a conductive material such as aluminum or nickel.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment.

(Example 1)
(Negative electrode)
As the negative electrode current collector, a copper (Cu) metal foil having a side length of 25 mm, a thickness of 1 mm, and an average surface roughness of 2.0 μm was used. Subsequently, the ratio of the average surface roughness of the outer peripheral portion and the central portion on at least one main surface of the negative electrode current collector ((average surface roughness a of the outer peripheral portion) / (average surface roughness b of the central portion)) Was processed with emery paper so that the value was 0.9. Processing was performed using emery paper having a particle size of # 3000 up to a surface roughness of 1 μm, and # 4000 when the surface roughness was 1 μm or less. The region of the outer peripheral portion was a region extending from the periphery of each side of the negative electrode current collector to the center and extending to a quarter of the length of each side. When processing the outer peripheral portion and the central portion, a surface protection tape (manufactured by 3M, surface protection tape # 330) was attached to a portion not to be processed.

(Measurement of average surface roughness of negative electrode)
The surface roughness Ra of the negative electrode current collector was measured using an atomic force microscope (AFM) (manufactured by Digital Instruments, model number: MMAFM-2). The measurement was performed with one field of view in a 50 × 50 μm range, and the surface roughness Ra was determined in ten different fields of view for one sample, and the average value was defined as the average surface roughness.
The results are shown in Table 1 as the average surface roughness a of the outer peripheral portion of the negative electrode current collector, the average surface roughness b of the central portion of the negative electrode current collector, and the ratio a / b of the average surface roughness.
Note that the ratio of the length of each side of the negative electrode current collector to the distance from the periphery of each side to the center is shown in Table 1 as the outer peripheral portion of the negative electrode current collector and as the outer peripheral portion of the negative electrode current collector. In other words, 1/4 in Table 1 means that the lengths of L12 and L14 in FIG. 2 are each 1/4 of L10, and the lengths of L22 and L24 are each L20. Means 1/4 of

(Positive electrode)
LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used as a positive electrode active material. LiNi _1/3 Mn _1/3 Co _1/3 O ₂ and polyvinylidene fluoride (PVDF) solution were added to carbon black and graphite as conductive assistants and mixed to prepare a coating material. This paint was applied to an aluminum foil (thickness: 20 μm) as a current collector, dried at 80 ° C., and rolled to obtain a positive electrode active material layer.

(Creating batteries)
The above positive electrode, negative electrode and separator (microporous film made of polyolefin) were cut into predetermined dimensions. The positive electrode, the negative electrode, and the separator were laminated in this order. When laminating, a small amount of hot melt adhesive (ethylene-methacrylic acid copolymer, EMAA) was applied and fixed so that the positive electrode, the negative electrode, and the separator did not shift. Aluminum foil (width 4 mm, length 40 mm, thickness 80 μm) and nickel foil (width 4 mm, length 40 mm, thickness 80 μm) were ultrasonically welded to the positive electrode and the negative electrode, respectively, as external lead terminals. A polypropylene (PP) grafted with maleic anhydride was wound around the external lead-out terminal and was thermally bonded. This is for improving the sealing property between the external terminal and the exterior body. Subsequently, an aluminum laminate sheet having a structure of PET (12 μm) / Al (40 μm) / PP (50 μm) was prepared as a battery outer package for enclosing a laminate in which a positive electrode, a negative electrode, and a separator were laminated (PET: polyethylene terephthalate) , PP is: polypropylene). The value in parentheses indicates the thickness (unit: μm) of each layer. At this time, the bag was made so that the PP was inside. The laminate is put in this package, and LiPF ₆ is dissolved in an electrolytic solution (a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 30: 70 vol%) to a concentration of 1 M). Was added in an appropriate amount, and the outer package was vacuum-sealed to produce a lithium battery.

(Examples 2 to 9)
Except that the ratio of the initial average surface roughness of the negative electrode current collector to the average surface roughness of the outer peripheral portion and the central portion on one main surface of the negative electrode current collector was a value shown in Table 1. A lithium battery was manufactured in the same manner as in Example 1.
(Example 10)
Implemented except that the ratio of the initial average surface roughness of the negative electrode current collector to the average surface roughness of the outer peripheral portion and the central portion on both main surfaces of the negative electrode current collector was a value shown in Table 1. A lithium battery was produced in the same manner as in Example 1.
(Examples 11 to 16)
A lithium battery was fabricated in the same manner as in Example 1, except that the outer peripheral region on at least one main surface of the negative electrode current collector was processed as shown in Table 1.

(Comparative Example 1)
The entire surface of the negative electrode current collector was processed using emery paper having a particle size of # 3000 so that the surface roughness was 1.5 μm. A lithium battery was produced in the same manner as in Example 1 except that the negative electrode current collector was used.

(Evaluation of short-circuit prevention effect)
Next, the evaluation test of the short-circuit prevention effect using the lithium batteries of the examples and the comparative examples thus manufactured and the test results will be described.

  As test conditions, 50 lithium batteries of each of the examples and comparative examples were prepared, and charge / discharge tests were performed on the prepared batteries under the following conditions. After constant-current charging to 4.2 V at 1 C, constant-voltage charging at 4.2 V until the current became 1/20 C, and then discharging to 3.0 V at 1 C were repeated 300 cycles at 25 ° C. In the process of charging / discharging in this way, the case where the charging potential did not reach 1.0 V was defined as the occurrence of an internal short circuit in the battery, and the number of occurrences was examined. The 1C current was calculated based on the nominal capacity of the battery when the discharge capacity per gram of the positive electrode active material was 190 mAh. The nC current is a current obtained by multiplying the 1C current by n.

  The evaluation results of the short-circuit prevention effects of the examples and the comparative examples are shown in Table 1 as the number of internal short-circuit occurrences.

  From Table 1, it became clear that the batteries of the examples had less internal short-circuits than the batteries of the comparative examples.

  Further, since the number of internal short-circuit occurrences of the batteries of Examples 1 to 3 was smaller than that of the batteries of Examples 4 and 5, the average surface roughness b of the negative electrode current collector center was 0.1 μm ≦ b ≦ 1 μm. It became clear that short-circuit failure could be further prevented. Since the batteries of Examples 6 and 7 had a smaller number of internal short circuits than the batteries of Examples 8 and 9, the ratio of the average surface roughness of the outer peripheral portion and the central portion on at least one main surface of the negative electrode current collector was reduced. When ((average surface roughness a at the outer peripheral portion a) / (average surface roughness b at the central portion)) is a / b ≦ 0.9, it has become clear that short circuit failure can be further prevented. Since the number of occurrences of internal short circuits in the batteries of Examples 10 to 13 was smaller than that of the batteries of Examples 14 to 16, the outer peripheral portion on at least one main surface of the negative electrode current collector was on the main surface of the negative electrode current collector. It has been clarified that short-circuit failure can be prevented in a region extending from the periphery to the center up to a quarter of the length of each side. In the batteries of Examples 14 to 16, the current tends to concentrate at the center of the main surface of the current collector. However, since the area of the outer peripheral portion is wider than 1/4, It is considered that a short circuit inside the battery occurred due to dendrite generated at the center portion due to excessive concentration of current.

  In the battery of Comparative Example 1, an internal short circuit occurred in 50 of the 50 batteries. The reason why the internal short circuit occurs as described above is that lithium metal grown from the end of the negative electrode current collector crosses the separator from the current collector on the negative electrode side and comes into contact with the positive electrode.

  As described above, the lithium battery according to the present invention has a short-circuit failure inside the battery caused by foreign substances such as precipitates grown from the end portion of the negative electrode current collector crossing the separator from the current collector on the negative electrode side and contacting the positive electrode. It is possible to prevent it effectively. Providing a battery in which a short circuit failure is prevented in this way greatly contributes particularly to the field of electronics.

Reference Signs List 10 positive electrode 12 positive electrode current collector 14 positive electrode active material layer 18 separator 20 negative electrode 22 negative electrode current collector 30 laminate 50 case 52 metal foil 54 polymer film 60 positive electrode lead 62 negative electrode lead 100 lithium battery

Claims (5)

A positive electrode, a negative electrode comprising a negative electrode current collector, and a nonaqueous electrolyte, wherein the lithium metal is precipitated on the anode of the negative electrode current collector on during charging, a lithium battery wherein lithium metal is dissolved during discharge The ratio of the average surface roughness of the outer peripheral portion and the central portion on at least one main surface of the negative electrode current collector ((average surface roughness a of the outer peripheral portion) / (average surface roughness b of the central portion)) is , A / b <1. The negative electrode current collector has a rectangular shape or a square shape, and the outer peripheral portion extends from a periphery on a main surface of the negative electrode current collector toward the center to a length of a quarter of a length of each side. The lithium battery according to claim 1, wherein:   The lithium battery according to claim 1, wherein the ratio of the surface roughness is a / b ≦ 0.9.   The lithium battery according to any one of claims 1 to 3, wherein an average surface roughness b of the central portion satisfies 0.1 µm ≤ b ≤ 1 µm.   The negative electrode current collector has a ratio of the average surface roughness of the outer peripheral portion and the central portion on both main surfaces ((average surface roughness a of the outer peripheral portion) / (average surface roughness b of the central portion)). The lithium battery according to claim 1, wherein a / b <1.

Images