JP2019091571A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize a positive electrode layer of a positive electrode, and to simultaneously realize high capacity and improvement of charge/discharge cycle characteristics. A lithium secondary battery including a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, wherein the positive electrode is a positive electrode current collector made of aluminum or an aluminum alloy having a large number of through holes, and the positive electrode current collector. A positive electrode layer containing a positive electrode active material provided in the body, the negative electrode contains a metal lithium or a lithium alloy as a negative electrode active material, the positive electrode active material contains a lithium-containing metal oxide, the positive electrode, A lithium secondary battery having a discharge capacity of 14 mAh/cm 2 or more, and a mass ratio of the positive electrode active material to the positive electrode current collector per unit area of 4 to 10. [Selection diagram] Figure 2

Description

  The present invention relates to a lithium secondary battery.

  In recent years, lithium secondary batteries are widely used because of their high energy density and the like, and they are mounted as power sources for portable small devices such as mobile phones, digital cameras, notebook computers and the like. In addition, lithium secondary batteries are expected to be developed for large-scale industrial applications such as hybrid vehicles and electric vehicles, or for storage of electric power by natural energy generation such as sunlight and wind power, from the viewpoint of energy resource exhaustion problems and global warming. It is in progress. Lithium secondary batteries are required to have higher density and longer life in order to expand the use of these power sources.

Such a lithium secondary battery transfers and charges lithium ions between the positive electrode and the negative electrode. The positive electrode active material of the lithium secondary battery is currently lithium metal oxide lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium nickelate (LiNiO ₂ ), lithium iron phosphate (LiFePO ₂ ) ₄ ) Metal oxides containing lithium such as ₄ ) have been put to practical use or are being developed for commercialization.

As the negative electrode active material, a carbon material such as graphite or lithium titanium oxide (Li ₄ Ti ₅ O ₁₂ ) is used. A separator for preventing internal short circuit is provided between the positive electrode and the negative electrode containing these active materials. It is intervened. As the separator, a microporous thin film generally made of polyolefin is used.

  As the non-aqueous electrolyte, a non-aqueous electrolyte in which an electrolyte such as a lithium salt is dissolved in a non-aqueous solvent is generally used. Among other non-aqueous electrolytes, gel electrolytes or solid electrolytes are also attracting attention.

  The positive electrode and the negative electrode each include a current collector supporting a positive electrode active material and a negative electrode active material. An aluminum foil is generally used as the positive electrode current collector, and a copper foil is generally used as the negative electrode current collector.

  By the way, metal lithium which is a negative electrode active material is characterized in that the amount of electricity per unit weight is as large as 3.86 Ah / g. For this reason, in order to realize a high capacity lithium secondary battery having the most theoretical energy density, studies using metal lithium as a negative electrode active material are being advanced again.

  In a lithium secondary battery using metal lithium as the negative electrode active material, the capacity of the positive electrode is also high because the capacity of the negative electrode is high. As a method of increasing the capacity of the positive electrode, thickening of the positive electrode layer containing the positive electrode active material formed on the positive electrode current collector can be mentioned (Patent Document 1 and Patent Document 2).

JP, 2011-150986, A JP, 2015-041556, A

  However, when the positive electrode layer is made thick, there is a tendency that cracks occur in the positive electrode layer or the positive electrode layer peels off from the positive electrode current collector. As a result, there is a problem that the capacity and charge-discharge cycle characteristics of the lithium secondary battery are degraded.

  Accordingly, the present invention solves the above-mentioned problems, and provides a lithium secondary battery capable of stabilizing the positive electrode layer of the positive electrode and simultaneously achieving high capacity and improvement of charge / discharge cycle characteristics.

In order to solve the above problems, it is a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode is a positive electrode current collector made of aluminum or an aluminum alloy having a large number of through holes; And a positive electrode layer including a positive electrode active material provided on the positive electrode current collector, the negative electrode including metal lithium or a lithium alloy as a negative electrode active material, and the positive electrode active material including a lithium-containing metal oxide, The lithium secondary battery is characterized in that the positive electrode has a discharge capacity of 14 mAh / cm ² or more, and a mass ratio of the positive electrode active material to the positive electrode current collector per unit area is 4 to 10. Be done.

  According to the present invention, it is possible to provide a lithium secondary battery which can stabilize the positive electrode layer of the positive electrode and can simultaneously realize high capacity and improvement of charge / discharge cycle characteristics.

FIG. 1 is a perspective view showing an example of a laminated lithium secondary battery according to the embodiment. FIG. 2 is a cross-sectional view taken along line II-II of the stacked lithium secondary battery of FIG. FIG. 3 is a diagram in which the current collector capacity is plotted against the evaluation cell capacity in the evaluation cells of Examples 1 to 4 and Comparative Examples 1 to 3.

  Hereinafter, the configuration of the lithium secondary battery according to the embodiment will be described.

<Positive electrode>
The positive electrode includes a positive electrode current collector and a positive electrode layer including a positive electrode active material provided on the positive electrode current collector. The positive electrode layer contains, for example, a positive electrode active material, a conductive material, and a binder.

The positive electrode active material is not particularly limited as long as it is a compound generally used as a positive electrode active material of a lithium secondary battery such as a lithium-containing metal oxide. For example, lithium cobalt complex oxide (eg LiCoO ₂ ), lithium manganese complex oxide (eg LiMnO ₂ , LiMn ₂ O ₄ , LiMn ₂ O ₃ ), lithium nickel complex oxide (eg LiNiO ₂ ), lithium cobalt iron complex oxide objects (e.g. LiCo _0.5 Fe _0.5 O _2), lithium-nickel-cobalt-manganese composite oxide (e.g., _{Li (Ni x Co y Mn 1} -x-y) O 2 (0 <x <1,0 <y <1)), Lithium iron phosphorus complex oxide (for example, LiFePO ₄ ) and the like can be mentioned.

The amount of the positive electrode active material contained in the positive electrode is such that the discharge capacity is 14 mAh / cm ² or more. The amount the amount of the positive electrode active material contained in the positive electrode is preferably discharge capacity is the amount to be ^{14mAh / cm 2 ~22mAh / cm 2} , more preferably the discharge capacity is 14mAh ^/ cm ² ~17mAh ^/ cm 2 It is. When the amount of the positive electrode active material is set to an amount such that the discharge capacity is less than 14 mAh / cm ² , the effect of increasing the energy density of the positive electrode with respect to the conventional positive electrode is reduced. The effect of capacity can not be expected much.

  The positive electrode current collector is aluminum or an aluminum alloy having a large number of through holes. Examples of the positive electrode current collector include metal meshes such as aluminum, porous metals, expanded metals, and punching metals. A preferred positive electrode current collector is a metal porous body of a three-dimensional network structure having a large number of holes three-dimensionally.

  The porosity of the metal porous body having a three-dimensional network structure is preferably 80% to 95%, and more preferably 85% to 95%. By using a metal porous body having a porosity in the above range as a current collector, a large amount of positive electrode active material can be filled in the pores of the metal porous body while maintaining the strength as a positive electrode.

The mass ratio of the positive electrode active material to the positive electrode current collector per unit area of the positive electrode (mass of positive electrode active material / mass of positive electrode current collector) is 4 to 10. If the mass ratio is less than 4, the mass of the positive electrode current collector is increased, and the mass energy density of the lithium secondary battery is reduced. When the mass ratio exceeds 10, the positive electrode layer provided in the positive electrode current collector becomes thick, and a location where the distance between the positive electrode active material contained in the positive electrode layer and the positive electrode current collector becomes long occurs. As a result, the internal resistance in the positive electrode is increased, and the charge / discharge cycle characteristics of the lithium secondary battery are unfavorably deteriorated. The mass ratio is preferably 4 to 7, and more preferably 4 to 5. Mass of the positive electrode current collector per unit area of the positive electrode is, for ^{example, 0.01g / cm 2 ~0.025g / cm 2} . On the other hand, the mass of the positive electrode active material per unit area of the positive electrode is, for ^{example, 0.04g / cm 2 ~0.25g / cm 2} .

  The conductive material is not particularly limited, and known or commercially available ones can be used. For example, carbon black such as acetylene black and ketjen black, carbon nanotubes, carbon fibers, activated carbon, graphite and the like can be mentioned.

  The binding agent is not particularly limited, and known or commercially available ones can be used. For example, polyvinylidene fluoride, polytetrafluoroethylene (PTFE), polyvinyl pyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene butadiene rubber (SBR) , Carboxymethylcellulose, acrylic resin and the like.

  The positive electrode can be produced, for example, by the following method. First, a positive electrode slurry is prepared by dispersing the positive electrode active material, the conductive material, and the binder described above in a solvent. Then, after apply | coating a positive electrode slurry to the one or both surfaces of a positive electrode collector, it dries and a positive electrode is produced by forming a positive electrode layer.

  The solvent is not particularly limited, and solvents generally used in lithium secondary batteries can be used. For example, N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMA) and the like can be mentioned. When polyvinylidene fluoride is used as the binder, N-methyl-2-pyrrolidone (NMP) is preferably used as a solvent.

  Next, an example of a method for producing a metal porous body having a three-dimensional network structure will be described.

  First, an aluminum powder and a support powder for forming a void are mixed at a predetermined volume ratio, and then the mixed powder is compacted. Subsequently, the compact is heat-treated in an inert atmosphere to bond the aluminum powders together. Thereafter, a porous aluminum current collector is produced by removing the support powder.

  The mixed powder may be composited with a metal plate. Compounding is performed by pressing the mixed powder and the metal plate together and heat treating the mixed powder and the metal plate when the mixed powder is filled in a molding die. The composite powder and the metal plate can be combined by using a configuration in which a layer of the mixed powder is provided on one side of the metal plate, a configuration in which the metal plate is sandwiched between the mixed powders, and a configuration in which the mixed powder is sandwiched by the metal plates. Moreover, it can also be set as the multistage structure which consists of repetition of mixed powder and a metal plate. At the time of compounding, particle sizes of aluminum powder and support powder, mixed powders different in mixing ratio, and plural kinds of metal plates may be combined. Examples of the metal plate to be composited with the mixed powder include non-perforated plates and foils, and perforated bodies such as perforated metal meshes, expanded metals and punching metals.

  The aluminum powder can be made of pure aluminum, an aluminum alloy or a mixture thereof. The aluminum powder may be a mixture of pure aluminum powder and additive element powder. As such an additive element, a plurality of elements selected from magnesium, silicon, titanium, iron, nickel, copper, zinc and the like alone or in combination of two or more are suitably used.

  As the support powder, one having a melting point higher than that of the aluminum powder is used. Further, when the mixed powder is composited with a metal plate, one having a melting point higher than the melting point of the lower of the aluminum powder and the metal plate is used. Such a supporting powder is preferably a water-soluble salt, and sodium chloride or potassium chloride is preferable from the viewpoint of easy availability.

  The pressure at the time of pressure molding is preferably 200 MPa or more. By forming with sufficient pressure, the aluminum powders rub against each other, and a strong oxide film on the surface of the aluminum powder that inhibits sintering of the aluminum powders is broken.

  The heat treatment is carried out at a temperature above the melting point of the aluminum powder used and below the melting point of the support powder. When the mixed powder is composited with a metal plate, heat treatment is performed at a temperature above the lower melting point of the aluminum powder and the metal plate and below the melting point of the support powder.

  When sodium chloride or potassium chloride is used as the support powder in the sintered body, the support powder can be removed by elution with water. For example, the support powder can be easily eluted by immersing the sintered body in a sufficient amount of water bath or flowing water bath.

<Negative electrode>
The negative electrode is made of, for example, a negative electrode current collector, and metal lithium or lithium alloy of a negative electrode active material formed on one or both surfaces of the negative electrode current collector. As a lithium alloy, an aluminum lithium system alloy, a silicon lithium system alloy, a tin lithium system alloy etc. can be used, for example.

  The negative electrode current collector is not particularly limited, and any known or commercially available one can be used. For example, a rolled foil made of copper or a copper alloy, an electrolytic foil, or the like can be used.

<Non-aqueous electrolyte>
The non-aqueous electrolyte, when liquid, includes a non-aqueous solvent and an electrolyte.

  The non-aqueous solvent preferably contains cyclic carbonate and chain carbonate as main components. The cyclic carbonate is preferably at least one selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC) and γ-butyrolactone (GBL). The linear carbonate is preferably at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like.

The electrolyte is not particularly limited, and an electrolyte of a lithium salt generally used in a lithium secondary battery can be used. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) (C ₄ F ₄ SO ₂ ), LiN (C _m F _{2m + 1} SO ₂ ) (C _n F _{2n + 1} SO) ₂₎ (m, n is an integer of 1 or _{more), LiC (C p F 2p} + 1 SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r is an integer of 1 or more) , Lithium difluoro (oxalato) borate and the like can be used. These electrolytes may be used alone or in combination of two or more. Further, it is desirable that the electrolyte be dissolved at a concentration of 0.1 to 1.5 mol / L, preferably 0.5 to 1.5 mol / L, with respect to the non-aqueous solvent.

<Separator>
The separator may be, for example, a polyolefin resin such as polyethylene resin or polypropylene resin, a polytetrafluoroethylene resin, a stretched film of cellulose or polyimide, a microporous film or a nonwoven fabric. The stretched film, the microporous membrane or the non-woven fabric may be a single layer or a multilayer structure. The separator is particularly preferably a microporous polyethylene membrane. The thickness of the separator is preferably 10 μm to 30 μm. If the thickness of the separator is less than 10 μm, the lithium secondary battery is likely to be shorted internally, which is not preferable. When the thickness of the separator exceeds 30 μm, the mass energy density of the lithium secondary battery is unfavorably reduced.

  The shape of the lithium secondary battery according to the embodiment is not particularly limited, and examples thereof include coin-type, button-type, sheet-type, laminate-type, cylindrical, square and flat-type.

  Hereinafter, the structure of the lithium secondary battery according to the embodiment will be described with reference to the drawings, taking a stacked lithium secondary battery as an example. FIG. 1 is a perspective view showing an example of a laminated lithium secondary battery, and FIG. 2 is a cross-sectional view taken along the line II-II of FIG.

  The laminated lithium secondary battery 1 includes a bag-like outer package 2 made of a laminate film. In the exterior body 2, a flat electrode group 3 is accommodated. The laminate film has, for example, a structure in which a plurality of (for example, two) plastic films are laminated by sandwiching a metal foil such as aluminum foil between the films. Of the two plastic films, one plastic film is a heat fusible resin film. The exterior body 2 is formed by laminating two laminate films so that their heat-welding resin films face each other, interposing the electrode group 3 between these laminate films, and forming two laminate films around the electrode group 3 The electrode assembly 3 is airtightly stored by heat-sealing and sealing the portions.

  The electrode group 3 has a structure in which a plurality of separators 6 interposed between the positive electrode 4 and the negative electrode 5 and the positive electrode 4 and the negative electrode 5 are laminated such that the negative electrode 5 is positioned in the outermost layer as shown in FIG. The positive electrode 4 is composed of a positive electrode current collector 42 and positive electrode layers 41, 41 formed on both sides of the current collector 42. The negative electrode 5 positioned in the outermost layer is composed of a negative electrode current collector 52 and a negative electrode layer 51 made of metallic lithium formed on the surface of the current collector 52 facing the separator 6. The negative electrode 5 located between the positive electrodes 4 except for the negative electrode 5 located in the outermost layer comprises a negative electrode current collector 52 and negative electrode layers 51 and 51 made of metallic lithium formed on both sides of the current collector 52. It is done.

  The positive electrode 4 has a positive electrode lead 43 in which the positive electrode current collector 42 extends from, for example, the right side surface of the positive electrode layer 41. The respective positive electrode leads 43 are bundled at the front end side in the exterior body 2 and joined together. One end of the positive electrode terminal 7 is joined to the bonding portion of the positive electrode lead 43, and the other end extends to the outside through the sealing portion of the exterior body 2. The negative electrode 5 has a negative electrode lead 53 in which the negative electrode current collector 52 extends from, for example, the left side surface of the negative electrode layer 51. The respective negative electrode leads 53 are bundled at the front end side in the exterior body 2 and joined together. One end of the negative electrode terminal 8 is joined to the bonding portion of the negative electrode lead 53, and the other end extends to the outside through the sealing portion of the exterior body 2.

  As described above, according to the lithium secondary battery according to the present embodiment, the positive electrode layer including the positive electrode active material constituting the positive electrode can be stabilized, and the high capacity and the charge / discharge cycle characteristics can be improved.

  Hereinafter, Examples and Comparative Examples of the present invention will be described in detail.

Example 1
(Production of positive electrode)
First, lithium nickel cobalt manganese complex oxide (Li (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ ) which is a lithium-containing metal oxide as a positive electrode active material, acetylene black and graphite as a conductive material, and a concentration of 12 as a binder Polyvinylidene fluoride dispersed in mass% of N-methyl-2-pyrrolidone (NMP) was respectively added and mixed. This mixture was mixed with lithium nickel cobalt manganese complex oxide: acetylene black: graphite: polyvinylidene fluoride in a mass ratio of 92: 2.5: 2.5: 3. Subsequently, NMP was added as a solvent so that solid content might be 66 mass%, and it stirred and mixed, and prepared positive electrode slurry. As a positive electrode current collector, a porous current collector of aluminum having an average pore diameter of 300 μm, a porosity of 92% and a thickness of 1.0 mm was used. Next, after immersing the positive electrode current collector in the prepared positive electrode slurry, the pressure of the positive electrode slurry is reduced to 0.1 MPa, and the positive electrode slurry is filled in the pores of the positive electrode current collector in an amount such that the discharge capacity is 14 mAh / cm ² . It dried at 80 degreeC for 2 hours. After that, pressing was performed until the electrode density became 2.8 g / cc, and a positive electrode was manufactured by punching a circle having a diameter of 18 mm.

(Fabrication of negative electrode)
A clad foil in which lithium metal having a thickness of 140 μm was bonded to one side of a copper foil having a thickness of 6 μm was used. Thereafter, it was punched into a circle having a diameter of 20 mm to produce a negative electrode.

(Assembly of evaluation cell)
An evaluation cell was assembled using the positive electrode and the negative electrode described above. The negative electrode was laminated on both surfaces of the positive electrode via a microporous polyethylene separator with a thickness of 25 μm and a diameter of 22 μm. The non-aqueous electrolyte was prepared by dissolving 1.0 mol / L of LiPF ₆ in a mixed non-aqueous solvent (volume ratio, EC: EMC = 3: 7) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). . The laminated positive electrode, negative electrode, and separator were enclosed in an outer package made of a laminated film of a bag-like aluminum foil together with a non-aqueous electrolyte to prepare an evaluation cell.

Example 2
(Production of positive electrode)
As a positive electrode current collector, a punching metal of aluminum having an average pore diameter of 300 μm, an opening ratio of 23%, and a thickness of 0.1 mm was used. Next, on the positive electrode current collector, the same positive electrode slurry as in Example 1 was applied by a doctor blade method in an amount such that the discharge capacity was 14 mAh / cm ^2, and dried at 80 ° C. for 2 hours. After that, pressing was performed until the electrode density became 2.8 g / cc, and a positive electrode was manufactured by punching a circle having a diameter of 18 mm.

(Assembly of evaluation cell)
An evaluation cell was produced in the same manner as in Example 1 except that the above-described positive electrode was used.

[Example 3]
(Production of positive electrode)
A positive electrode was produced in the same manner as in Example 1 except that the positive electrode current collector was filled with the positive electrode slurry in an amount such that the discharge capacity was 21 mAh / cm ² .

(Fabrication of negative electrode)
A negative electrode was produced in the same manner as in Example 1 except that a clad foil obtained by bonding lithium metal having a thickness of 213 μm to one side of a copper foil having a thickness of 6 μm was used.

(Assembly of evaluation cell)
An evaluation cell was produced in the same manner as in Example 1 except that the above-described positive electrode and negative electrode were used.

Example 4
(Production of positive electrode)
As a positive electrode current collector, a porous current collector of aluminum having an average pore diameter of 300 μm, a porosity of 95% and a thickness of 1.0 mm was used. A positive electrode was produced in the same manner as in Example 1 except that the positive electrode current collector was filled with the positive electrode slurry in an amount such that the discharge capacity was 21 mAh / cm ² .

(Assembly of evaluation cell)
An evaluation cell was produced in the same manner as in Example 1 except that the above-described positive electrode was used.

Comparative Example 1
(Production of positive electrode)
An aluminum foil with a thickness of 20 μm was used as a positive electrode current collector. Next, the same positive electrode slurry as in Example 1 was applied to the positive electrode current collector by doctor blade method in a discharge capacity of 14 mAh / cm ^2, and dried at 80 ° C. for 2 hours, as in Example 1 The positive electrode was produced by the following method.

(Assembly of evaluation cell)
An evaluation cell was produced in the same manner as in Example 1 except that the above-described positive electrode was used.

Comparative Example 2
(Production of positive electrode)
A positive electrode was produced in the same manner as in Example 1 except that the same positive electrode current collector as in Example 4 was used, and the positive electrode current collector was filled with the positive electrode slurry in an amount such that the discharge capacity was 22 mAh / cm ² . .

(Fabrication of negative electrode)
A negative electrode was produced in the same manner as in Example 1 except that a clad foil in which a lithium metal having a thickness of 224 μm was bonded to one side of a copper foil having a thickness of 6 μm was used.

(Assembly of evaluation cell)
An evaluation cell was produced in the same manner as in Example 1 except that the above-described positive electrode and negative electrode were used.

Comparative Example 3
(Production of positive electrode)
A positive electrode was prepared by the same method as in Example 1, except that a porous current collector of aluminum having an average pore diameter of 300 μm, a porosity of 90% and a thickness of 1.0 mm was used as the positive electrode current collector. Was produced.

(Assembly of evaluation cell)
An evaluation cell was produced in the same manner as in Example 1 except that the above-described positive electrode was used.

[Charge / discharge conditions of charge / discharge cycle test]
The charge / discharge cycle test was performed on the evaluation cells of Examples 1 to 4 and Comparative Examples 1 to 3 described above under the following charge / discharge conditions. Discharge capacity of positive electrode, capacity of evaluation cell, mass of positive electrode active material per unit area (a), mass of positive electrode current collector per unit area (b), of positive electrode active material to positive electrode current collector per unit area The mass ratio (mass of positive electrode active material / mass of positive electrode current collector) (a / b), measurement results of discharge capacity retention in cycles (ii) to (iv) are shown in Table 1 below. In addition, with the cycle, it was set as the charge and discharge after completion of activation at the first time.

(Charge and discharge conditions)
Initial: 0.02C charge to 4.3V, 0.02C discharge to 2.75V (once)
Activation: 0.05C charge to 4.3V, 0.05C discharge to 2.75V (4 times)
cycle:
(I) 0.1C charge to 4.3V, 0.1C discharge to 2.75V (once)
(Ii) 0.1C charge to 4.3V, 0.2C discharge to 2.75V (once)
(Iii) 0.1C charge to 4.3V, 0.5C discharge to 2.75V (once)
(Iv) 0.1C charge to 4.3V, 1.0C discharge to 2.75V (once)
(Discharge capacity maintenance rate)
The discharge capacity retention rate in cycles (ii) to (iv) was calculated by the following equation (1).

Discharge capacity retention rate [%]
= (Discharge capacity in cycles (ii) to (iv) / discharge capacity in cycle (i)) x 100 (1)
Moreover, in the evaluation cells of Examples 1 to 4 and Comparative Examples 1 to 3, the current collector capacity was plotted against the evaluation cell capacity. The results are shown in FIG.

  As is apparent from Table 1 above, in Examples 1 to 4 in which the positive electrode having a mass ratio of the positive electrode active material to the positive electrode current collector per unit area of 4 to 10 was used, in cycles (ii) to (iv) It can be seen that a high discharge capacity retention rate is exhibited. Here, when Example 1 and Example 2 are compared, in Example 1 in which the metal porous body holding the positive electrode active material in the pores as the positive electrode current collector, the positive electrode active material and the positive electrode current collector are used. It can be seen that the reduction in charge / discharge cycle characteristics of the lithium secondary battery can be suppressed because the distance between

  On the other hand, in Comparative Example 1 in which an aluminum foil was used as the positive electrode current collector, cracking and peeling from the positive electrode current collector occurred in the positive electrode layer, and a charge / discharge cycle test could not be performed. As a factor of this, in the positive electrode current collector of aluminum foil, the contact area between the positive electrode current collector and the positive electrode layer is small, and the positive electrode layer can not be held. Moreover, when Example 4 and Comparative Example 2 are compared, in Comparative Example 2 in which the mass ratio of the positive electrode active material to the positive electrode current collector per unit area exceeds 10, discharge capacity maintenance in cycles (iii) and (iv) It can be seen that the rate is greatly reduced. This is because there are many positive electrode layers containing the positive electrode active material with respect to the positive electrode current collector, so there are places where the distance between the positive electrode active material contained in the positive electrode layer and the positive electrode current collector becomes long. It is to increase.

  Also, FIG. 3 which plots the current collector capacity against the evaluation cell capacity shows the amount of current collector required to obtain the battery capacity. In order to increase the energy density of the battery, the lower the value on the vertical axis in FIG. 3, the better. The evaluation cell of Comparative Example 3 can not be expected to improve the energy density because the value on the vertical axis in FIG. 3 is high.

  Therefore, it is understood from the results of Table 1 and FIG. 3 that the evaluation cells of Examples 1 to 4 are excellent in both charge / discharge performance and energy density as compared with the evaluation cells of Comparative Examples 1 to 3.

  DESCRIPTION OF SYMBOLS 1 lithium secondary battery 2 exterior body 3 electrode group 4 positive electrode 5 negative electrode 41 positive electrode layer 42 positive electrode current collector 43 positive electrode lead 51 negative electrode layer 52 negative electrode Current collector, 53: negative electrode lead, 6: separator, 7: positive electrode terminal, 8: negative electrode terminal

Claims (3)

A lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, comprising:
The positive electrode includes a positive electrode current collector made of aluminum or an aluminum alloy having a large number of through holes, and a positive electrode layer including a positive electrode active material provided on the positive electrode current collector.
The negative electrode includes metal lithium or a lithium alloy as a negative electrode active material,
The positive electrode active material includes a lithium-containing metal oxide,
The lithium secondary battery, wherein the positive electrode has a discharge capacity of 14 mAh / cm ² or more, and a mass ratio of the positive electrode active material to the positive electrode current collector per unit area is 4 to 10.   The lithium secondary battery according to claim 1, wherein the positive electrode current collector is a metal mesh, a metal porous body having a three-dimensional network structure, an expanded metal or a punching metal.   The lithium secondary battery according to claim 1, wherein the positive electrode active material is nickel cobalt lithium lithium manganate.

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