Electrode for lithium secondary battery and lithium secondary battery

Abstract

To provide an electrode for a lithium secondary battery which has a large amount of electrolyte retention, high output, and at the same time, ensures volume density and suppresses a decrease in electronic conductivity. [Solution] The electrode for a lithium secondary battery of the present invention comprises an electrode foil and a composite layer having an active material provided on the electrode foil, the composite layer having an internal layer 54 provided on the electrode foil and a surface layer 55 located on the surface side of the internal layer 54 in the thickness direction of the composite layer, the internal layer 54 having a higher average porosity than the surface layer 55, and the internal layer 54 having a middle layer 542 located midway in the thickness direction of the internal layer 54 and having a lower porosity than both ends.

Description

[Technical field]

[0001] The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery. [Background technology]

[0002] In the field of rechargeable secondary batteries, aqueous batteries such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries have been the mainstream. However, as electrical devices become smaller and lighter, lithium secondary batteries with high energy density have attracted attention, and their research, development, and commercialization have progressed rapidly. On the other hand, in order to address the problems of global warming and depletion of fuel, various automobile manufacturers have developed electric vehicles (EVs) and hybrid electric vehicles (HEVs) that use electric motors to assist part of the drive, and high-capacity, high-output secondary batteries have been required as their power sources. High-voltage non-aqueous lithium secondary batteries are used as power sources that meet such requirements. In particular, prismatic lithium secondary batteries have excellent volumetric efficiency when packed, so there is growing expectation for the development of prismatic lithium secondary batteries for HEVs and EVs.

[0003] Lithium secondary batteries for HEV applications need to supply a large current from the battery to assist motor drive, and so are required to have a low voltage drop during discharge (low internal resistance).

[0004] As a conventional technique, Patent Document 1 discloses a positive electrode body for producing a nonaqueous electrolyte battery that has a large discharge capacity and is unlikely to cause problems such as short circuits between positive and negative electrodes, and a nonaqueous electrolyte battery using this positive electrode body. The positive electrode body of the battery in Patent Document 1 has a two-layer structure having an inner layer and an outer layer. It is said that by making the porosity of the inner layer higher than the porosity of the outer layer, it is possible to suppress internal short circuits in the battery and improve safety.

[0005] Patent Document 2 describes an electrode structure that can obtain high output while increasing the capacity of the battery. The battery electrode in Patent Document 2 has a two-layer structure having an internal layer and an external layer. By making the porosity of the internal layer higher than that of the external layer, it is possible to increase the amount of electrolyte retained in the internal layer, and it is said that it is possible to increase the capacity and output of the battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2010-160987 A [Patent Document 2] JP 2011-9203 A Summary of the Invention [Problem to be solved by the invention]

[0007] The electrodes in Patent Documents 1 and 2 have a two-layer structure with an outer layer with a low porosity and an inner layer with a high porosity. This two-layer structure can prevent internal short circuits and achieve high capacity and high output, but increasing the porosity of the entire inner layer reduces the volume density. In addition, the electronic conductivity of the electrode may decrease.

[0008] Therefore, an object of the present invention is to provide an electrode for a lithium secondary battery that has a large amount of electrolyte retention and high output, while at the same time ensuring volume density and suppressing a decrease in electronic conductivity, and a lithium secondary battery using the same. [Means for solving the problem]

[0009] The inventors have found that by forming a mixture layer on an electrode foil from an internal layer on the electrode foil and a surface layer located on the surface side, and by making the average porosity of the internal layer higher than that of the surface layer, it is possible to form a space for storing an electrolyte solution in the internal layer while ensuring volume density and reaction. In addition, they have found that by providing a middle layer with a low porosity at a midpoint in the thickness direction of the internal layer, it is possible to maintain the mobility of electrons in the internal layer with a high porosity, and have completed the invention.

[0010] That is, the electrode for a lithium secondary battery of the present invention is an electrode for a lithium secondary battery having an electrode foil and a mixture layer having an active material and provided on the electrode foil, the mixture layer having an internal layer provided on the electrode foil and a surface layer located on the surface side of the internal layer in the thickness direction of the mixture layer, the internal layer having a higher average porosity than the surface layer, and the internal layer having a middle layer at a midpoint in the thickness direction of the internal layer and having a lower porosity than both sides.

[0011] The lithium secondary battery of the present invention is characterized by including a battery container, a positive electrode and a negative electrode, each made of the above-described lithium secondary battery electrodes, disposed with a separator therebetween in the battery container, and an electrolyte solution injected into the battery container. This specification includes the disclosure of Japanese Patent Application No. 2019-037399, which is the priority basis of this application. Effect of the Invention

[0012] According to the present invention, the electrolyte can easily permeate the inner layer of the electrode, and a sufficient amount of Li ions can be secured by forming a space in the inner layer to hold the electrolyte. Therefore, the responsiveness of Li ion migration can be improved, the direct current resistance (DCR) can be reduced, and the output of the lithium secondary battery can be increased. In addition, by forming a high-density intermediate layer in the inner layer, the electronic conductivity of the inner layer with low porosity can be maintained. Note that the problems, configurations, and effects other than those described above will be made clear by the following description of the embodiments. [Brief description of the drawings]

[0013] [Figure 1] 1 is an external perspective view showing a lithium secondary battery according to an embodiment of the present invention; [Diagram 2] FIG. 2 is an exploded perspective view of a lithium prismatic secondary battery. [Diagram 3] FIG. 4 is an exploded perspective view showing a state in which a part of the electrode winding group is developed. [Figure 4] FIG. 1 is a cross-sectional view of a positive electrode, which is one embodiment of an electrode for a lithium secondary battery of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present invention will be described in detail below based on the embodiments.

[0015] Fig. 1 is an external perspective view showing one embodiment of a lithium secondary battery of the present invention, specifically a lithium prismatic secondary battery, and Fig. 2 is an exploded perspective view of the lithium prismatic secondary battery.

[0016] The lithium prismatic secondary battery 100 includes a battery can 1 and a battery lid 6. The battery can 1 has side surfaces, including a pair of opposing wide side surfaces 1b having a relatively large area and a pair of opposing narrow side surfaces 1c having a relatively small area, and a bottom surface 1d, and has an opening 1a above the side surfaces.

[0017] The wound group 3 is housed in the battery can 1 via an insulating protective film 2, and the opening 1a of the battery can 1 is sealed by a battery lid 6. The battery lid 6 is a substantially rectangular flat plate, and is welded to close the upper opening 1a of the battery can 1 to seal the battery can 1. The battery lid 6 is provided with a positive electrode external terminal 14 and a negative electrode external terminal 12. The wound group 3 is charged and power is supplied to an external load via the positive electrode external terminal 14 and the negative electrode external terminal 12. A gas exhaust valve 10 is integrally provided on the battery lid 6, and when the pressure inside the battery container rises, the gas exhaust valve 10 is opened to exhaust gas from the inside, and the pressure inside the battery container decreases. This ensures the safety of the lithium prismatic secondary battery 100.

[0018] The wound group 3 is wound in a flat shape, and has a pair of curved portions facing each other with a semicircular cross section, and a flat portion formed continuously between the pair of curved portions. The wound group 3 is inserted into the battery can 1 from one curved portion side so that the winding axis direction is along the width direction of the battery can 1, and the other curved portion side is disposed on the upper opening side.

[0019] The positive electrode foil exposed portion 34c of the wound group 3 is electrically connected to the positive electrode external terminal 14 provided on the battery lid 6 via the positive electrode current collector 44. In addition, the negative electrode foil exposed portion 32c of the wound group 3 is electrically connected to the negative electrode external terminal 12 provided on the battery lid 6 via the negative electrode current collector 24. As a result, power is supplied from the wound group 3 to an external load via the positive electrode current collector 44 and the negative electrode current collector 24, and externally generated power is supplied to the wound group 3 via the positive electrode current collector 44 and the negative electrode current collector 24 for charging.

[0020] In order to electrically insulate the positive electrode current collector 44, the negative electrode current collector 24, and the positive electrode external terminal 14 and the negative electrode external terminal 12 from the battery lid 6, a gasket 5 and an insulating plate 7 are provided on the battery lid 6. After the electrolyte is poured into the battery can 1 through the pouring hole 9, a pouring plug 11 is joined to the battery lid 6 by laser welding to seal the pouring hole 9, thereby hermetically sealing the lithium prismatic secondary battery 100.

[0021] Here, examples of materials for the positive external terminal 14 and the positive current collector plate 44 include aluminum alloys, and examples of materials for the negative external terminal 12 and the negative current collector plate 24 include copper alloys. Examples of materials for the insulating plate 7 and the gasket 5 include insulating resin materials such as polybutylene terephthalate, polyphenylene sulfide, and perfluoroalkoxy fluorine resin.

[0022] The battery lid 6 is provided with a liquid injection hole 9 for injecting an electrolyte into the battery container, and this liquid injection hole 9 is sealed by a liquid injection plug 11 after the electrolyte is injected into the battery container. Here, the electrolyte to be injected into the battery container is, for example, lithium hexafluorophosphate (LiPF 6 A non-aqueous electrolyte solution in which a lithium salt such as the above-mentioned is dissolved can be used.

[0023] The positive electrode external terminal 14 and the negative electrode external terminal 12 have welded joints that are welded to a bus bar or the like. The welded joints have a rectangular parallelepiped block shape that protrudes upward from the battery lid 6, with the lower surface facing the surface of the battery lid 6 and the upper surface parallel to the battery lid 6 at a predetermined height position.

[0024] The positive electrode connection part 14a and the negative electrode connection part 12a have a cylindrical shape that protrudes from the lower surface of the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively, and the tip can be inserted into the positive electrode side through hole 46 and the negative electrode side through hole 26 of the battery lid 6. The positive electrode connection part 14a and the negative electrode connection part 12a penetrate the battery lid 6 and protrude toward the inside of the battery can 1 beyond the positive electrode current collector base 41 and the negative electrode current collector base 21 of the positive electrode current collector 44 and the negative electrode current collector 24, respectively, and the tip is crimped to fix the positive electrode external terminal 14, the negative electrode external terminal 12, the positive electrode current collector 44, and the negative electrode current collector 24 integrally to the battery lid 6. A gasket 5 is interposed between the positive electrode external terminal 14 and the negative electrode external terminal 12 and the battery lid 6, and an insulating plate 7 is interposed between the positive electrode current collector 44, the negative electrode current collector 24, and the battery lid 6.

[0025] The positive current collector plate 44 and the negative current collector plate 24 have a rectangular plate-like positive current collector base 41 and negative current collector base 21 arranged opposite the lower surface of the battery lid 6, and a positive electrode side connection end 42 and a negative electrode side connection end 22 that are bent at the side ends of the positive current collector base 41 and the negative current collector base 21, extend along the wide surface of the battery can 1 toward the bottom side, and are connected in a state of being overlapped opposite the positive electrode foil exposed portion 34c and the negative electrode foil exposed portion 32c of the winding group 3. The positive electrode current collector base 41 and the negative electrode current collector base 21 are respectively formed with a positive electrode side opening hole 43 and a negative electrode side opening hole 23 through which the positive electrode connection portion 14a and the negative electrode connection portion 12a are inserted.

[0026] An insulating protective film 2 is wound around the winding group 3 with a central axis extending in a direction along the flat surface of the winding group 3 and perpendicular to the winding axis direction of the winding group 3. The insulating protective film 2 is made of a single sheet or multiple film members made of a synthetic resin such as PP (polypropylene), and has a length that allows it to be wound around a winding center extending in a direction parallel to the flat surface of the winding group 3 and perpendicular to the winding axis direction.

[0027] 3 is an exploded perspective view showing a state in which a part of the electrode wound group is developed. The wound group 3 is formed by winding a negative electrode 32 and a positive electrode 34 in a flat shape with separators 33, 35 between them. In the wound group 3, the outermost electrode is the negative electrode 32, and the separators 33, 35 are wound further outside the negative electrode 32. The separators 33, 35 serve to insulate the positive electrode 34 from the negative electrode 32.

[0028] The portion of the negative electrode 32 coated with the negative electrode mixture layer 32b is larger in the width direction than the portion of the positive electrode 34 coated with the positive electrode mixture layer 34b, so that the portion coated with the positive electrode mixture layer 34b is always sandwiched between the portions coated with the negative electrode mixture layer 32b. The positive electrode foil exposed portion 34c and the negative electrode foil exposed portion 32c are bundled together at their flat portions and connected by welding or the like. Note that the separators 33 and 35 are wider in the width direction than the portion coated with the negative electrode mixture layer 32b, but are wound in positions where the metal foil surfaces of the ends are exposed at the positive electrode foil exposed portion 34c and the negative electrode foil exposed portion 32c, so this does not interfere with bundling and welding.

[0029] The positive electrode 34 has a positive electrode mixture layer 34b having a positive electrode mixture on both sides of a positive electrode foil 34a, which is a positive electrode current collector, and a positive electrode foil exposed portion 34c, where the positive electrode mixture layer 34b is not formed, is provided at one end of the positive electrode foil 34a in the width direction.

[0030] The negative electrode 32 has a negative electrode mixture layer 32b having a negative electrode mixture on both sides of a negative electrode foil (not shown) which is a negative electrode current collector, and a negative electrode foil exposed portion 32c where the negative electrode mixture layer 32b is not formed is provided at the end on the other width direction side of the negative electrode foil.

[0031] The positive electrode foil exposed portion 34c and the negative electrode foil exposed portion 32c are regions where the metal surface of the electrode foil is exposed, and are wound so as to be located at one side and the other side in the winding axial direction. In addition, the axial core may be, for example, a resin sheet wound around the axial core, the resin sheet having a higher bending rigidity than the positive electrode foil 34a, the negative electrode foil, and the separators 33 and 35.

[0032] FIG. 4 is a cross-sectional view of a positive electrode, which is one embodiment of the electrode for a lithium secondary battery of the present invention. As shown in FIG. 4, the positive electrode mixture layer has an internal layer 54 provided in the positive electrode foil 34a, and a surface layer 55 located on the surface side of the internal layer 54 in the thickness direction of the positive electrode mixture layer. Each layer contains at least two types of active materials having different particle diameters, that is, a positive electrode active material (large) 51 and a positive electrode active material (small) 52, and a conductive assistant 53. Here, the "particle diameter" of the positive electrode active material (large) 51 and the positive electrode active material (small) 52 refers to a particle diameter D50 measured by a laser diffraction / scattering method. It is preferable that the particle diameter D50 of the positive electrode active material (large) 51 is in the range of 10 μm to 15 μm, and the particle diameter D50 of the positive electrode active material (small) 52 is in the range of 4.5 μm to 6.5 μm, but is not limited thereto.

[0033] In this embodiment, the internal layer 54 is composed of three layers, an upper layer 541, a middle layer 542, and a lower layer 543, from the side closer to the surface layer 55. The middle layer 542 provided at a midpoint in the thickness direction of the internal layer 54 has a lower porosity than the upper layer 541 and the lower layer 543 located on both sides of it. Here, the porosity refers to the area ratio of the gap region (void) in the cross section of each layer where the active material particles do not cross the cross section, expressed as a percentage, and can be obtained by observing the cross section of the positive electrode mixture layer with an optical microscope, etc. When obtaining the porosity, a cross section with a sufficiently large area is selected so that at least 100 active material particles appear in the cross section for calculating the porosity. In addition, the cross section to be selected includes the entire thickness direction of the layer for calculating the porosity. By forming the middle layer 542 with a relatively low porosity (high density) at a midpoint in the thickness direction of the internal layer 54, the electronic conductivity of the internal layer 54 can be maintained.

[0034] In this embodiment, the internal layer 54 is configured to have a higher average porosity than the surface layer 55. Here, the average porosity refers to the average value of the porosity of each of the upper layer 541, the middle layer 542, and the lower layer 543 in the case of having a plurality of layers (upper layer 541, middle layer 542, and lower layer 543) like the internal layer 54. In the case of being composed of a single layer like the surface layer 55, the average porosity of the surface layer 55 coincides with the value of the porosity of the surface layer 55. By making the porosity of the internal layer 54 higher than that of the surface layer 55, the electrolyte can easily permeate into the internal layer 54, and a space for holding the electrolyte is formed in the internal layer 54, so that a sufficient amount of Li ions can be secured. Therefore, the responsiveness of Li ion migration can be improved, the direct current resistance (DCR) can be reduced, and the output of the lithium secondary battery can be increased.

[0035] The average porosity of the surface layer 55 is preferably 10% or more and less than 30%, and more preferably 15% or more and less than 25%. On the other hand, the average porosity of the inner layer 54 is preferably 30% or more and 60% or less, and more preferably 35% or more and 55% or less.

[0036] The ratio of the thickness of the middle layer 542 to the total thickness of the internal layer 54 can be appropriately set in consideration of the balance between the electronic conductivity of the internal layer 54 and the amount of electrolyte retained. For example, the ratio of the thickness of the middle layer 542 can be set to 10% or more and 25% or less, more preferably 15% or more and 20% or less.

[0037] Furthermore, the average particle diameter of the positive electrode active material in the internal layer 54 is larger than that of the positive electrode active material in the surface layer 55. When the average particle diameter is large, the gaps between the particles of the positive electrode active material become larger, and the average porosity of the layer increases. Conversely, when the average particle diameter of the positive electrode active material is small, the positive electrode active material is densely packed in the layer, and the average porosity decreases. The average particle diameters of the internal layer 54 and the surface layer 55 refer to the particle diameter D50 at 50% of the cumulative volume of the particles of the positive electrode active material contained in each layer.

[0038] The average particle diameter of the positive electrode active material in the internal layer 54 and the surface layer 55 can be adjusted by changing the relative amount of the positive electrode active material (large) 51 and the positive electrode active material (small) 52 contained in each layer. In this embodiment, the relative amount of the positive electrode active material (small) 52 in the entire positive electrode active material in the surface layer 55 is made larger than the relative amount of the positive electrode active material (small) 52 in the entire positive electrode active material in the internal layer 54, so that the average particle diameter of the positive electrode active material in the internal layer 54 is made larger than the average particle diameter of the positive electrode active material in the surface layer 55.

[0039] In the above embodiment, the case where the average porosity of the internal layer 54 and the surface layer 55 is changed by adjusting the particle diameter of the positive electrode active material contained in the internal layer 54 and the surface layer 55 has been described, but the present invention is not limited thereto. In addition, for example, the particle diameter of the positive electrode active material contained in the internal layer 54 and the surface layer 55 may be made uniform, and instead, the conductive assistant 53 may be composed of two types of conductive assistants, one with a small particle diameter and one with a large particle diameter, and the mixing ratio of the conductive assistant with the small particle diameter and the conductive assistant with the large particle diameter may be changed between the internal layer 54 and the surface layer 55, whereby the average porosity of each layer can be controlled.

[0040] In the above embodiment, the internal layer 54 is composed of three layers, the upper layer 541, the middle layer 542, and the lower layer 543, but the present invention is not limited to this, and the internal layer 54 may be composed of four or more layers. For example, the internal layer 54 may be composed of five layers, including, from the side closer to the surface layer 55, a layer with a high porosity, a layer with a low porosity, a layer with a high porosity, a layer with a low porosity, a layer with a high porosity, and a layer with a high porosity.

[0041] Furthermore, in the above embodiment, the layer structure of the positive electrode mixture layer has been described, but the same can be said for the negative electrode mixture layer. That is, the negative electrode mixture layer may have an internal layer and a surface layer provided on the negative electrode foil, the internal layer may have a higher average porosity than the surface layer, and a middle layer having a lower porosity than both sides may be provided at a midpoint in the thickness direction of the internal layer. EXAMPLES

[0042] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0043] (Example 1-1) <Preparation of negative electrode> Styrene-butadiene rubber as a binder and carboxymethyl cellulose as a thickener were added to natural graphite powder, and pure water was added as a dispersion solvent and kneaded to prepare a negative electrode mixture. This negative electrode mixture was applied to both sides of 8 μm-thick copper foil (negative electrode foil) by slot die coating while leaving the welded parts (negative electrode uncoated parts), and then the negative electrode 32 was produced through drying, pressing, and cutting processes.

[0044] <Preparation of positive electrode> The positive electrode active material is lithium manganese oxide (chemical formula LiMn 2 O 4 Two types of positive electrode active material were used: positive electrode active material (large) 51 (particle diameter 11 μm) and positive electrode active material (small) 52 (particle diameter 5.5 μm). These positive electrode active materials, a conductive assistant, and a PVDF binder were mixed in a predetermined ratio, and NMP was added as a dispersion solvent and kneaded to prepare a positive electrode mixture for forming each layer.

[0045] The upper layer 541, the middle layer 542, and the lower layer 543 constituting the internal layer 54, and the positive electrode mixture for forming the surface layer 55 were simultaneously applied to both sides of an aluminum foil (positive electrode foil) having a thickness of 15 μm by a slot die coating method, and then dried, pressed, and cut to obtain a positive electrode 34. The surface layer 55 with a porosity of 10% and the internal layer 54 with an average porosity of 35% were formed by combining two types of positive electrode active materials, the positive electrode active material (large) 51 and the positive electrode active material (small) 52. The internal layer 54 was composed of the upper layer 541 with a porosity of 48%, the middle layer 542 with a porosity of 10%, and the lower layer 543 with a porosity of 48%. The porosity of each layer is summarized in the table below. In the table below, the average porosity of the internal layer 54 is also described as "porosity" (the same applies to each of the following examples).

[0046] <Preparation of lithium secondary battery> The above positive and negative electrodes were placed in a battery container with a separator between them, and an electrolyte was poured into the battery container to prepare a lithium secondary battery.

[0047] (Examples 1-2, 1-3 and 1-4) A positive electrode having a surface layer 55 and an internal layer 54 composed of an upper layer 541, a middle layer 542, and a lower layer 543 was produced by the method described in Example 1-1. The porosity of the surface layer 55 of Examples 1-2, 1-3, and 1-4 was all 10%. The average porosity of the internal layer 54 of Examples 1-2, 1-3, and 1-4 was 40%, 50%, and 55%, respectively. The porosity of the upper layer 541 of the internal layer of Examples 1-2, 1-3, and 1-4 was 53%, 65%, and 68%, respectively. The porosity of the middle layer 542 of the internal layer of Examples 1-2, 1-3, and 1-4 was 15%, 20%, and 30%, respectively. The porosity of the lower layer 543 of the internal layer of Examples 1-2, 1-3, and 1-4 was 53%, 65%, and 68%, respectively. Using these positive electrodes, a lithium secondary battery was produced by the same method as in Example 1-1.

[0048] (Examples 2-1, 2-2, 2-3 and 2-4) A positive electrode having a surface layer 55 and an internal layer 54 composed of an upper layer 541, a middle layer 542, and a lower layer 543 was produced by the method described in Example 1-1. The porosity of the surface layer 55 of Examples 2-1, 2-2, 2-3, and 2-4 was 20%. The average porosity of the internal layer 54 of Examples 2-1, 2-2, 2-3, and 2-4 was 35%, 40%, 50%, and 55%, respectively. The porosity of the upper layer 541 of the internal layer of Examples 2-1, 2-2, 2-3, and 2-4 was 43%, 48%, 60%, and 68%, respectively. The porosity of the middle layer 542 of the internal layer of Examples 2-1, 2-2, 2-3, and 2-4 was 20%, 25%, 30%, and 30%, respectively. The porosities of the lower layer 543 of the internal layer in Examples 2-1, 2-2, 2-3, and 2-4 were 43%, 48%, 60%, and 68%, respectively. Using these positive electrodes, lithium secondary batteries were fabricated in the same manner as in Example 1-1.

[0049] (Examples 3-1, 3-2, 3-3 and 3-4) A positive electrode having a surface layer 55 and an internal layer 54 composed of an upper layer 541, a middle layer 542, and a lower layer 543 was prepared by the method described in Example 1-1. The porosity of the surface layer 55 of Examples 3-1, 3-2, 3-3, and 3-4 was 30%. The average porosity of the internal layer 54 of Examples 3-1, 3-2, 3-3, and 3-4 was 35%, 40%, 50%, and 55%, respectively. The porosity of the upper layer 541 of the internal layer of Examples 3-1, 3-2, 3-3, and 3-4 was 43%, 48%, 60%, and 68%, respectively. The porosity of the middle layer 542 of the internal layer of Examples 3-1, 3-2, 3-3, and 3-4 was 20%, 25%, 30%, and 30%, respectively. The porosities of the lower layer 543 of the internal layer in Examples 3-1, 3-2, 3-3, and 3-4 were 43%, 48%, 60%, and 68%, respectively. Using these positive electrodes, lithium secondary batteries were fabricated in the same manner as in Example 1-1.

[0050] (Comparative Examples 1-1, 1-2 and 1-3) A positive electrode having a surface layer 55 and an internal layer 54 composed of an upper layer 541, a middle layer 542, and a lower layer 543 was produced by the method described in Example 1-1. The porosity of the surface layer 55 of Comparative Examples 1-1, 1-2, and 1-3 was all 20%. The average porosity of the internal layer 54 of Comparative Examples 1-1, 1-2, and 1-3 was 50%, 20%, and 70%, respectively. The porosity of the upper layer 541 of the internal layer of Comparative Examples 1-1, 1-2, and 1-3 was 45%, 25%, and 70%, respectively. The porosity of the middle layer 542 of the internal layer of Comparative Examples 1-1, 1-2, and 1-3 was 60%, 10%, and 70%, respectively. The porosity of the lower layer 543 of the internal layer of Comparative Examples 1-1, 1-2, and 1-3 was 45%, 25%, and 70%, respectively. Using these positive electrodes, a lithium secondary battery was produced by the same method as in Example 1-1.

[0051] (evaluation) The lithium secondary batteries produced in each of the Examples and Comparative Examples were evaluated for liquid absorption amount and electrical conductivity. The amount of absorbed liquid was evaluated by the weight of the electrolyte impregnated in the positive electrode. Positive electrodes of the same volume were soaked in electrolyte at 25°C for a certain period of time, and their weights were measured. If the weight after absorbing the electrolyte increased by 15% or more compared to before absorbing the electrolyte, it was judged as ◯, and if it was less than 15%, it was judged as ×.

[0052] The conductivity was evaluated based on the unit volume resistance of the positive electrode. For one example or comparative example, in addition to a positive electrode having a surface layer and an internal layer according to that example or comparative example, a positive electrode having a mixture layer composed only of the composition of the internal layer was prepared, and the positive electrodes were impregnated with an electrolyte, and the unit volume resistance of each was measured using Loresta. If the unit volume resistance of the positive electrode having a surface layer and an internal layer was lower than the unit volume resistance of the positive electrode having only the internal layer, it was judged as ◯, and if it was higher, it was judged as ×.

[0053] Tables 1 to 4 show the evaluation results of the porosity of each layer constituting the positive electrode mixture layer, the amount of liquid absorption, and the electrical conductivity in each Example and Comparative Example.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] [Table 4]

[0058] The results in Table 4 reveal the following. First, the positive electrode in Comparative Example 1-1 had a sufficient amount of liquid absorption, but poor electrical conductivity. This result is believed to be due to the fact that the average porosity of the internal layer in Comparative Example 1-1 is high, which improves the liquid absorption, but also increases the gaps, which reduces the paths through which electrons can move, resulting in poor electrical conductivity. The positive electrode in Comparative Example 1-2 had a small amount of liquid absorption, but good electrical conductivity. This result is believed to be due to the fact that the average porosity of the internal layer is low, which reduces the space through which the electrolyte can penetrate, while the electron movement paths are secured to a certain extent, improving electrical conductivity. However, in such an electrode, the electrolyte is easily depleted and the life is short. Furthermore, in the positive electrode in Comparative Example 1-3, the average porosity of the internal layer is large, so the amount of liquid absorption is large, but the gaps are too large, resulting in poor electrical conductivity. Compared to each comparative example, the positive electrodes in Examples 1-1 to 3-4, which have a higher average porosity in the internal layer than in the surface layer and have a high-density layer in the center part of the internal layer, showed excellent liquid absorption and electrical conductivity.

[0059] (Evaluation of liquid absorption and electrical conductivity based on porosity) In the electrode preparation process, it was found that if the porosity is less than 10%, it is difficult to crush by pressing, so the lower limit of the porosity is preferably 10%. In addition, it was found that if the porosity exceeds 60%, the electrical conductivity is deteriorated, so the upper limit of the porosity is preferably 60%. Next, the effect of the porosity on the liquid absorption amount and electrical conductivity was investigated. Electrodes having a single mixture layer with a porosity of 35%, which is the median value of the upper and lower limits, 25%, and 30% were prepared, and the liquid absorption amount and electrical conductivity were evaluated using these electrodes. The evaluation method was the same as in the above-mentioned Examples and Comparative Examples. The evaluation results are shown in Table 5.

[0060] [Table 5]

[0061] As shown in Table 5, when the porosity is less than 30%, the amount of liquid absorption is poor, so it was found that the average porosity of the inner layer is preferably 30% or more. When the porosity exceeds 30%, the conductivity tends to gradually weaken, but good liquid absorption is maintained. From these results, it was revealed that the average porosity of the surface layer is preferably 10% or more and less than 30%, and the average porosity of the inner layer is preferably in the range of 30% or more and 60% or less.

[0062] Although the embodiment of the present invention has been described in detail above, the specific configuration is not limited to this embodiment, and even if there are design changes and the like within the scope that does not deviate from the gist of the present invention, they are included in the present invention. [Explanation of symbols]

[0063] 1 Battery can 1a opening 1b Wide side 1c narrow side 1d Bottom 2. Insulation protection film 3 winding group 5 Gasket 6 Battery cover 7. Insulating plate 9 Filling port 10 Gas exhaust valve 11 Filling valve 12 Negative external terminal 12a Negative electrode connection part 14 Positive external terminal 14a Positive electrode connection 21 Base of negative electrode current collector plate 22 Negative connection end 23 Negative electrode opening hole 24 Negative current collector 26 Negative electrode through hole 32 Negative electrode 32b Negative electrode mixture layer 32c Negative electrode foil exposed part 33 Separator 34 Positive electrode 34a Positive electrode foil 34b Positive electrode mixture layer 34c Positive electrode foil exposed part 35 Separator 41 Positive current collector base 42 Positive connection end 43 Positive electrode opening hole 44 Positive current collector 46 Positive electrode through hole 51 Cathode active material (large) 52 Cathode active material (small) 53 Conductive additives 54 Inner layer 541 Upper layer 542 Middle layer 543 Lower layer 55 Surface layer 100 Lithium prismatic secondary battery All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety.

Claims

1. An electrode for a lithium secondary battery comprising an electrode foil and a mixture layer provided on the electrode foil and having an active material, the mixture layer has an internal layer provided on the electrode foil and a surface layer located on a surface side of the internal layer in a thickness direction of the mixture layer, The inner layer has a higher average porosity than the surface layer, The internal layer has a middle layer at a midpoint in a thickness direction of the internal layer, the middle layer having a porosity different from that of both sides, The porosity of the intermediate layer is higher than the average porosity of the surface layer and is 15% or more and 30% or less; The surface layer, the intermediate layer and the layers on both sides thereof all contain active materials.

2. An electrode for a lithium secondary battery comprising an electrode foil and a mixture layer provided on the electrode foil and having an active material, the mixture layer has an internal layer provided on the electrode foil and a surface layer located on a surface side of the internal layer in a thickness direction of the mixture layer, The inner layer has a higher average porosity than the surface layer, The internal layer has a middle layer at a midpoint in a thickness direction of the internal layer, the middle layer having a porosity different from that of both sides, the porosity of the layer on either side of the intermediate layer that is closer to the surface layer is higher than the average porosity of the surface layer; The porosity of the middle layer is 10% or more and 30% or less, The surface layer, the intermediate layer and the layers on both sides thereof all contain active materials.

3. The average porosity of the surface layer is 10% or more and less than 30%, The average porosity of the inner layer is 30% or more and 60% or less. The electrode for a lithium secondary battery according to claim 1 or 2.

4. 3. The electrode for a lithium secondary battery according to claim 1, wherein an average particle size of the active material in the inner layer is larger than an average particle size of the active material in the surface layer.

5. 5. The electrode for a lithium secondary battery according to claim 4, wherein the internal layer and the surface layer are mixtures containing at least two types of active materials having different particle sizes, and a relative amount of the active material having a smaller particle size in the active material of the surface layer is greater than a relative amount of the active material having a smaller particle size in the active material of the internal layer.

6. 3. The electrode for a lithium secondary battery according to claim 1, wherein a porosity of a layer containing an active material provided on the electrode foil is higher than a porosity of a layer containing an active material located adjacent to the layer on the surface side of the layer.

7. 3. The electrode for a lithium secondary battery according to claim 1, wherein a ratio of a thickness of the intermediate layer to a total thickness of the internal layer is 10% or more and 25% or less.

8. A battery container; A positive electrode and a negative electrode, each of which is made of the lithium secondary battery electrode according to any one of claims 1 to 7, and which are disposed in the battery container with a separator therebetween; An electrolyte solution injected into the battery container; A lithium secondary battery comprising:

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