JP2016115656A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of suppressing inhomogeneity of a salt concentration at an electrode.SOLUTION: A lithium ion secondary battery 1 includes at least a pair of electrodes 30, 40. The electrodes 30, 40 include: metal foils 32, 42; active material layers 34, 44, for covering the metal foils 32, 42; and protective layers 36, 46, for covering the active material layers 34, 44. In the active material layers 34, 44 and the protective layers 36, 46, an electrolyte containing a lithium salt is impregnated. Rates of porosity of the protective layers 36, 46 are greater than or equal to 56% and less than or equal to 84%.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a lithium ion secondary battery.

  For example, a lithium ion secondary battery described in Patent Document 1 below includes a plurality of electrodes including a metal foil, an active material layer covering the metal foil, and a protective layer covering the active material layer. The protective layer suppresses a short circuit between the electrodes due to dendrite growth.

JP 2010-225544 A

  In a lithium ion secondary battery, an electrode is impregnated with an electrolytic solution in which a lithium salt is dissolved, and charging and discharging are performed by movement of lithium ions. In the conventional lithium ion secondary battery, the electrolyte solution does not uniformly impregnate the electrode, and the concentration (spot) of the lithium salt in the electrode (salt concentration) may be uneven. That is, the salt concentration at the electrode may be non-uniform. Here, the salt concentration is, for example, the number of moles of lithium salt per unit mass of the electrode (unit: mol / g). If the salt concentration is uneven, the internal resistance of the lithium ion secondary battery increases and the output may decrease.

  Then, an object of this invention is to provide the lithium ion secondary battery which can suppress the nonuniformity of the salt concentration in an electrode.

  A lithium ion secondary battery according to one aspect of the present invention is a lithium ion secondary battery including at least a pair of electrodes, and the electrodes cover a metal foil, an active material layer covering the metal foil, and an active material layer. The active material layer and the protective layer are impregnated with an electrolytic solution containing a lithium salt, and the porosity of the protective layer is 56% to 84%.

  In this lithium ion secondary battery, the electrolyte is easily held in the voids in the protective layer. That is, since the voids are formed in the protective layer, the entire electrode is easily impregnated with the electrolyte. As a result, salt concentration unevenness is suppressed. The higher the porosity of the protective layer, the easier the electrolyte is retained in the electrode. However, if the porosity is too high, it becomes difficult to suppress short-circuiting between the electrodes due to dendrites or the like by the protective layer. Therefore, the porosity of the protective layer is 84% or less in order to suppress both the unevenness of the salt concentration and the short circuit. On the other hand, the lower the porosity of the protective layer, the harder the electrolyte is held on the electrode. Moreover, since parts other than a space | gap of a protective layer act as an insulator, internal resistance rises and output falls, so that the porosity of a protective layer is low. Therefore, the porosity of the protective layer is 56% or more in order to suppress both the salt concentration unevenness and the internal resistance.

  In the lithium ion secondary battery according to one aspect of the present invention, the porosity of the protective layer may be higher than the porosity of the active material layer. In this case, the entire electrolyte is more easily impregnated with the electrolyte, and uneven salt concentration is easily suppressed.

  In the lithium ion secondary battery according to one aspect of the present invention, the protective layer may include ceramic particles. In this case, it is easy to make the porosity of the protective layer within a suitable range.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery which can suppress the nonuniformity of the salt concentration in an electrode can be provided.

FIG. 1 is an exploded perspective view of a lithium ion secondary battery according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of the lithium ion secondary battery shown in FIG. FIG. 3 is an enlarged view of FIG. FIG. 4 is a schematic front view of the negative electrode. FIG. 5 is a graph showing the LiPF ₆ concentration in each region of the negative electrode included in each of the lithium ion secondary batteries of Examples and Comparative Examples of the present invention.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In addition, the same code | symbol is attached | subjected about the same or equivalent component. The present invention is not limited to the following embodiments.

  As shown in FIGS. 1 and 2, the lithium ion secondary battery 1 includes a case 10 and an electrode assembly 20. The case 10 is, for example, a substantially rectangular parallelepiped housing. The case 10 includes a main body portion 12 that is open on one side and a lid portion 14 that closes the opening of the main body portion 12.

  The electrode assembly 20 is accommodated in the case 10. The electrode assembly 20 includes a plurality of positive electrodes 30 and a plurality of negative electrodes 40, and a separator 50 disposed between the pair of positive electrodes 30 and the negative electrode 40. The separator 50 is a microporous film, for example, and holds an electrolytic solution. The electrolytic solution is, for example, an organic solvent-based or non-aqueous electrolytic solution. The space inside the case 10 may be filled with an electrolytic solution.

  As shown in FIG. 3, the positive electrode 30 includes a rectangular positive metal foil 32, a positive electrode active material layer 34 that covers the entire surface of the positive electrode metal foil 32, and an entire surface of the positive electrode active material layer 34. And a protective layer 36 for covering. However, the positive electrode active material layer 34 may cover only a part of the surface of the positive electrode metal foil 32. The positive electrode active material layer 34 covers both surfaces of the positive electrode metal foil 32. However, the positive electrode active material layer 34 may cover only one surface of the positive electrode metal foil 32. Further, the protective layer 36 may cover only a part of the surface of the positive electrode active material layer 34. The protective layer 36 covers the respective surfaces of the positive electrode active material layer 34 provided on both surfaces of the positive electrode metal foil 32. However, the protective layer 36 may cover only one surface of the positive electrode active material layers 34 provided on both surfaces of the positive electrode metal foil 32.

  The negative electrode 40 includes a rectangular negative electrode metal foil 42, a negative electrode active material layer 44 that covers the entire surface of the negative electrode metal foil 42, and a protective layer 46 that covers the entire surface of the negative electrode active material layer 44. . However, the negative electrode active material layer 44 may cover only a part of the surface of the negative electrode metal foil 42. The negative electrode active material layer 44 covers both surfaces of the negative electrode metal foil 42. However, the negative electrode active material layer 44 may cover only one surface of the negative electrode metal foil 42. Further, the protective layer 46 may cover only a part of the surface of the negative electrode active material layer 44. The protective layer 46 covers the respective surfaces of the negative electrode active material layer 44 provided on both surfaces of the negative electrode metal foil 42. The protective layer 46 may cover only one surface of the negative electrode active material layer 44 provided on both surfaces of the negative electrode metal foil 42.

  The separator 50 is located between the positive electrode 30 and the negative electrode 40 and prevents contact between both electrodes (current short circuit). Lithium ions pass through the separator 50. Each of the protective layer 36 of the positive electrode 30 and the protective layer 46 of the negative electrode 40 faces the separator 50.

  At least a part of the electrolytic solution is impregnated in the inside (void) of the positive electrode active material layer 34, the protective layer 36, the negative electrode active material layer 44, the protective layer 46, and the separator 50.

  The end of the positive electrode metal foil 32 is a tab portion 32t (see FIG. 1). The positive electrode active material layer 34 and the protective layer 36 are not formed on the tab portion 32t. The tab portion 32t is electrically connected to the positive electrode lead 37. The positive lead 37 is electrically connected to the positive terminal 38. The positive terminal 38 passes through the lid portion 14 of the case 10 and extends over the inside and outside of the case 10. That is, the positive electrode 30 is electrically connected to the positive electrode terminal 38 via the positive electrode lead 37. The lid 14 and the positive terminal 38 are insulated by an annular insulating member 60.

  The end of the negative electrode metal foil 42 is a tab portion 42t. The negative electrode active material layer 44 and the protective layer 46 are not formed on the tab portion 42t. The tab portion 42t is electrically connected to the negative electrode lead 47. The negative electrode lead 47 is electrically connected to the negative electrode terminal 48. The negative terminal 48 passes through the lid portion 14 of the case 10 and extends over the inside and the outside of the case 10. That is, the negative electrode 40 is electrically connected to the negative electrode terminal 48 via the negative electrode lead 47. The lid 14 and the negative electrode terminal 48 are insulated by an annular insulating member 60.

  The positive electrode metal foil 32 may include at least one selected from the group consisting of aluminum, copper, stainless steel, titanium, and nickel, for example. The thickness of the positive electrode metal foil 32 may be, for example, 15 to 20 μm.

The positive electrode active material layer 34 includes a particulate positive electrode active material. The positive electrode active material is, for example, lithium nickel cobalt manganese oxide (LiNi _x Co _y Mn _z M _a O 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.)), Lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese Spinel (LiMn ₂ O ₄ ), lithium vanadium compound (LiV ₂ O ₅ ), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), or olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, It may be one or more elements or VO selected from Mg, Nb, Ti, Al, B, Nb, Cu, Zn, Mo, Ca, Sr, W, and Zr.) .

The porosity of the positive electrode active material layer 34 may be 27 to 42%, for example. The porosity of the positive electrode active material layer 34 can be represented by the following formula 1, for example.
ε1 = {1- (M1 / Va1) / (M1 / Vt1)} × 100 (1)
ε1 is the porosity of the positive electrode active material layer 34. M1 is the mass (unit: g) of the positive electrode active material layer 34 as a whole. Va1 is an apparent volume (unit: cm ³ ) of the positive electrode active material layer 34 and includes a space between materials (solids) constituting the positive electrode active material layer 34. That is, Va1 includes the volume of the void formed in the positive electrode active material layer 34. Vt1 is the volume (unit: cm ³ ) of the substance (solid) itself constituting the positive electrode active material layer 34, and does not include voids between the substances (solid) constituting the positive electrode active material layer 34. M1 / Va1 is the apparent density (unit: g / cm ³ ) of the positive electrode active material layer 34. M1 / Vt1 is the true density (unit: g / cm ³ ) of the positive electrode active material layer 34. The volume of the electrolytic solution impregnated in the positive electrode active material layer 34 is not included in Va1 or Vt1. The mass of the electrolytic solution impregnated in the positive electrode active material layer 34 is not included in M1. The unit of volume is not limited to cm ³ .

  The positive electrode active material layer 34 may include a conductive additive. The conductive aid may be, for example, carbon-based particles such as graphite, carbon black, acetylene black, ketjen black (registered trademark), vapor grown carbon fiber (VGCF) and the like. The content of the conductive additive in the positive electrode active material layer 34 may be, for example, 1 to 30 parts by mass with respect to 100 parts by mass of the positive electrode active material 11.

  The positive electrode active material layer 34 may include a binder. The binder binds the substances included in the positive electrode active material layer 34 together, and binds the entire positive electrode active material to the surface of the positive electrode metal foil 32. The binder is, for example, a fluorine-containing resin such as polyvinylidene fluoride, polytetrafluoroethylene, or fluororubber, a thermoplastic resin such as polypropylene or polyethylene, an imide resin such as polyimide or polyamideimide, or an alkoxysilanol group-containing resin. It's okay. The binder content in the positive electrode active material layer 34 may be, for example, 1 to 30 parts by mass with respect to 100 parts by mass of the positive electrode active material.

  The thickness of the positive electrode active material layer 34 may be 40 to 70 μm, for example. However, the dimension of the positive electrode active material layer 34 is not limited to the above range.

A void is formed in the protective layer 36. The protective layer 36 may be formed from a ceramic material, for example. For example, the protective layer 36 may include a plurality (a large number) of ceramic particles, and voids may be formed between the ceramic particles. The particulate positive electrode active material and ceramic particles may be uniformly dispersed in the positive electrode active material layer 34. The ceramic particles may be, for example, particles containing an inorganic oxide or particles made of an inorganic oxide. The ceramic particles may be porous. The inorganic oxide should just be a substance which does not inhibit a battery reaction. The battery reaction is, for example, a reaction related to occlusion and release (intercalation) of lithium or lithium ions (Li ⁺ ) in a positive electrode active material, or lithium or lithium ions and a counter anion (electrolyte in an electrolytic solution). is there. Inorganic oxides include, for example, Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , Sc ₂ O ₃ , Y ₂ O ₃ , Bi ₂ O ₃ , La ₂ O ₃ , CeO ₂ , Nd ₂ O ₃ and Sm. It may be at least one selected from the group consisting of ₂ O ₃ . The protective layer 36 may contain the same binder as that of the positive electrode active material layer 34. The binder binds the ceramic particles. As a result, the shape of the protective layer 36 and the voids therein are easily maintained.

The porosity of the protective layer 36 is 56% or more and 84% or less. The porosity of the protective layer 36 can be expressed by the following formula 2, for example.
ε2 = {1− (M2 / Va2) / (M2 / Vt2)} × 100 (2)
ε2 is the porosity of the protective layer 36. M2 is the mass of the entire protective layer 36 (unit: g). Va2 is an apparent volume (unit: cm ³ ) of the protective layer 36 and includes a space between substances (solids) constituting the protective layer 36. That is, Va2 includes the volume of the void formed in the protective layer 36. Vt2 is the volume (unit: cm ³ ) of the substance (solid) constituting the protective layer 36 itself and does not include voids between the substances (solid) constituting the protective layer 36. M2 / Va2 is the apparent density (unit: g / cm ³ ) of the protective layer 36. M2 / Vt2 is the true density (unit: g / cm ³ ) of the protective layer 36. The volume of the electrolytic solution impregnated in the protective layer 36 is not included in Va2 or Vt2. The mass of the electrolytic solution impregnated in the protective layer 36 is not included in M2. The unit of volume is not limited to cm ³ .

For example, when the protective layer 36 is made of alumina, the true density of the protective layer 36 is about 4.0 g / cm ³ , similar to the true density of alumina itself. True density, and porosity is calculated from an apparent density of 0.65 g / cm ³ of 4.0 g / cm ³ is 0.8375%. True density, and porosity is calculated from an apparent density of 1.21 g / cm ³ of 4.0 g / cm ³ is 0.6975%. True density, and porosity is calculated from an apparent density of 1.77 g / cm ³ of 4.0 g / cm ³ is 0.5575%. In addition, when the porosity of the protective layer 36 is 0.5575%, it is not included in the scope of the present invention.

  In the lithium ion secondary battery 1, the electrolytic solution is easily held in the voids in the protective layer 36. That is, the formation of voids in the protective layer 36 makes it easy for the electrolytic solution to uniformly impregnate the entire positive electrode 30. As a result, salt concentration unevenness is suppressed. The higher the porosity of the protective layer 36, the easier the electrolytic solution is held in the positive electrode 30, but if the porosity is too high, the protective layer 36 suppresses short circuit between the positive electrode 30 and the negative electrode 40 caused by dendrites or the like. Becomes difficult. Therefore, the porosity of the protective layer 36 is 84% or less. On the other hand, as the porosity of the protective layer 36 is lower, the electrolytic solution is less likely to be held on the positive electrode 30. In addition, since portions other than the voids in the protective layer 36 act as insulators, the lower the porosity of the protective layer 36, the higher the internal resistance and the lower the output. Therefore, the porosity of the protective layer 36 is 56% or more in order to suppress both the salt concentration unevenness and the internal resistance. The same effect as that of the protective layer 36 of the positive electrode 30 can be obtained in the protective layer 46 of the negative electrode 40.

  The porosity of the protective layer 36 may be higher than the porosity of the positive electrode active material layer 34. In this case, the electrolyte solution is more easily impregnated in the entire positive electrode 30, and uneven salt concentration is easily suppressed.

  The thickness of the protective layer 36 may be 1 to 15 μm, for example. The thicker the protective layer 36, the easier the electrolyte solution is held in the electrode, and the salt concentration unevenness in the positive electrode is suppressed. However, the thicker the protective layer 36, the higher the internal resistance of the battery. Therefore, in order to suppress unevenness in salt concentration and internal resistance, the thickness of the protective layer 36 is preferably within the above range. However, the thickness of the protective layer 36 is not limited to the above range.

  The formation method of the positive electrode 30 may be as follows, for example. A slurry containing a positive electrode active material, a conductive additive, a binder, and a solvent is applied to the surface of the positive electrode metal foil 32 to form a coating film on the surface of the positive electrode metal foil 32. This coating film is a precursor of the positive electrode active material layer 34. After the coating film is dried, the coating film and the positive electrode metal foil 32 are pressed to obtain the positive electrode metal foil 32 whose surface is covered with the positive electrode active material layer 34. Subsequently, a slurry containing ceramic particles and a binder is applied to the surface of the positive electrode active material layer 34 to form a coating film on the surface of the positive electrode active material layer 34. This coating film is a precursor of the protective layer 36. By drying this coating film, the protective layer 36 is formed on the positive electrode active material layer 34, and the positive electrode 30 is obtained. When forming the protective layer 36, it is not necessary to press. The solvent for the slurry may be, for example, N-methyl-2-pyrrolidone (NMP), methanol, or methyl isobutyl ketone (MIBK).

  The higher the solid content of the slurry used to form the protective layer 36, the higher the density of the protective layer 36 and the lower the porosity of the protective layer 36. On the other hand, the lower the solid content of the slurry, the lower the density of the protective layer 36 and the higher the porosity of the protective layer 36. Therefore, the porosity of the protective layer 36 can be controlled by the solid content rate of the slurry. For example, if the solid content of the slurry is 40% by weight, the porosity of the protective layer 36 is 35%. If the solid content is 37% by weight, the porosity is 56%. If the solid content is 35% by weight, the porosity is 70%. If the solid content is 33% by weight, the porosity is 84%. In order to set the porosity of the protective layer 36 to 56% or more and 84% or less, for example, the solid content of the slurry may be 33% by weight or more and 37% by weight or less.

  The negative electrode active material layer 44 may be the same as the positive electrode active material layer 34 except that it contains a negative electrode active material instead of the positive electrode active material.

  The negative electrode active material may be, for example, a carbon-based material that absorbs and releases lithium, an element that forms an alloy with lithium, a compound that includes an element that forms an alloy with lithium, or a polymer material.

  The carbon-based material may be, for example, non-graphitizable carbon, coke, graphite, glassy carbon, a fired body of an organic polymer compound, carbon fiber, activated carbon, or carbon black. The fired body of an organic polymer compound refers to a material obtained by firing and carbonizing a polymer such as phenols or furans.

  Elements that alloy with lithium include, for example, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, 1n, Si, Ge, It may be at least one selected from the group consisting of Sn, Pb, Sb and Bi.

Examples of the compound containing an element that forms an alloy with lithium include ZnLiAl, AlSb, SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , _{CrSi 2, Cu 5 Si, FeSi} 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2, SiC, Si 3 N 4, Si 2 N 2 O, SiO v (0 <v ≦ 2, Or 0.5 ≦ v ≦ 1.5), SnO _w (0 <w ≦ 2), SnSiO ₃ , tin alloy (for example, Cu—Sn alloy or Co—Sn alloy), LiSiO, and LiSnO. It may be at least one kind.

  The polymeric material can be, for example, polyacetylene or polypyrrole.

  The protective layer 46 of the negative electrode 40 may be the same as the protective layer 36 of the positive electrode 30.

  The negative electrode metal foil 42 may contain copper, for example.

  The manufacturing method of the negative electrode 40 may be the same as that of the positive electrode 30 except that the composition of the active material to be used is different.

  The separator 50 may be, for example, a porous film made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramics.

The electrolytic solution may include an electrolyte and a solvent that dissolves the electrolyte. The electrolyte contained in the electrolytic solution may be a lithium salt such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO _3, or LiN (CF ₃ SO ₂ ) ₂ . The solvent of the electrolytic solution may be, for example, a cyclic ester, a chain ester, or an ether. The cyclic ester may be, for example, ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, or gamma valerolactone. The chain esters may be, for example, methyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, or acetic acid alkyl ester. The ether may be, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, or 1,2-dibutoxyethane. The concentration of the electrolyte in the electrolytic solution may be, for example, 0.5 to 1.7 mol / L. The electrolytic solution may contain a gelling agent.

  The case 10 may be made of resin or metal, for example.

  The present invention is not limited to the above embodiment. For example, in the above embodiment, the protective layer is formed on both the positive electrode 30 and the negative electrode 40, but the protective layer may be formed on only one of the positive electrode 30 and the negative electrode 40. Further, only the porosity of one protective layer of the positive electrode 30 and the negative electrode 40 may be 56% or more and 84% or less.

  Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples.

(Example)
[Production of positive electrode]
A slurry obtained by mixing a positive electrode active material, a conductive additive, a binder, and a solvent was applied onto a rectangular aluminum foil to form a rectangular coating film. This coating film was heated and dried. The dried coating film was pressed with a roll press to form a positive electrode active material layer covering the surface of the aluminum foil. Through the above steps, a positive electrode having an aluminum foil and a positive electrode active material layer was obtained.

LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (so-called NCM523) was used as the positive electrode active material. As the conductive assistant, scaly graphite was used. As the binder, polyvinylidene fluoride was used. As a solvent, N-methyl-2-pyrrolidone (NMP) was used.

[Production of negative electrode]
A slurry obtained by mixing graphite powder, carboxymethyl cellulose, styrene butadiene rubber, and ion-exchanged water was applied onto a rectangular copper foil to form a rectangular coating film. After drying this coating film, the negative electrode active material layer which covers the surface of copper foil was formed by pressing a coating film with a roll press. A slurry containing ceramic particles, the binder, and the solvent was applied onto the negative electrode active material layer to form a rectangular coating film. By heating and drying this coating film, a protective layer covering the surface of the negative electrode active material layer was formed. Through the above steps, a negative electrode having a copper foil, a negative electrode active material layer, and a protective layer was obtained. Alumina was used as the ceramic particles. The thickness of the protective layer was 6 μm. The porosity of the protective layer was adjusted to 70%.

[Assembly of lithium ion secondary battery]
Leads were connected to the tab portions of the positive electrode and the negative electrode, respectively. The positive electrode active material layer and the negative electrode protective layer were opposed to each other, and a separator was interposed between the positive electrode active material layer and the negative electrode protective layer. Subsequently, the positive electrode, the negative electrode, and the separator were stacked to produce a laminate (electrode assembly). A polypropylene porous membrane was used as the separator. The electrode assembly was accommodated in the case. After injecting the electrolyte into the case, the case was sealed. Through the above steps, a lithium ion secondary battery was obtained.

As a solvent for the electrolytic solution, a mixed solution of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate was used. LiPF ₆ was used as the electrolyte (lithium salt) for the electrolytic solution.

[Evaluation of lithium ion secondary battery]
As shown in FIG. 4, the negative electrode of the lithium ion secondary battery was virtually divided into regions 1 to 9. Each area | region 1-9 is the same rectangular shape. The LiPF ₆ concentration (mmol) in each region 1-9 was measured. An ICP (Inductively Coupled Plasma) emission spectrometer was used for the measurement of the LiPF ₆ concentration. By dividing the LiPF ₆ concentration in each region 1 to 9 by the mass of the negative electrode, the LiPF ₆ concentration (mmol / g) per unit mass in each region was calculated. The values of LiPF ₆ concentration in each region 1 to 9, and their standard deviation and average value are shown in Table 1 below.

(Comparative Examples 1 and 2)
Except that the porosity of the protective layer of the negative electrode was adjusted to 0% in Comparative Example 1 and 35% in Comparative Example 2, lithium ion secondary batteries of Comparative Examples 1 and 2 were prepared in the same manner as in the Example. . The lithium ion secondary batteries of Comparative Examples 1 and 2 were evaluated in the same manner as in the examples. The evaluation results of the lithium ion secondary battery of each comparative example are shown in Table 1 below.

FIG. 5 shows the LiPF ₆ concentration in each region 1 to 9 of each lithium ion secondary battery described in Table 1. In FIG. 5, black triangle marks indicate examples, black square marks indicate comparative example 1, and white circular marks indicate comparative example 2.

  From the results for the examples shown in Table 1 and FIG. 5, it was confirmed that when the porosity of the protective layer was 70%, the standard deviation was small and the salt concentration unevenness was small. From the results for Comparative Example 2, it was confirmed that when the porosity of the protective layer was 35%, the standard deviation was larger and the salt concentration unevenness was larger than when the porosity was 70%. From the results for Comparative Example 3, it was confirmed that when the porosity of the protective layer was 0%, the standard deviation was larger and the salt concentration unevenness was even greater than when the porosity was 35%. That is, it was confirmed that the effect of suppressing the unevenness of the salt concentration increases as the porosity of the protective layer increases.

DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery, 10 ... Case, 30 ... Positive electrode (electrode), 32 ... Metal foil for positive electrodes, 34 ... Positive electrode active material layer, 36 ... Protective layer, 40 ... Negative electrode (electrode), 42 ... Metal for negative electrodes Foil, 44 ... negative electrode active material layer, 46 ... protective layer, 50 ... separator.

Claims (3)

A lithium ion secondary battery comprising at least a pair of electrodes,
The electrode has a metal foil, an active material layer covering the metal foil, and a protective layer covering the active material layer,
The active material layer and the protective layer are impregnated with an electrolyte solution containing a lithium salt,
The lithium ion secondary battery whose porosity of the said protective layer is 56% or more and 84% or less.   The lithium ion secondary battery according to claim 1, wherein a porosity of the protective layer is higher than a porosity of the active material layer.   The lithium ion secondary battery according to claim 1, wherein the protective layer includes ceramic particles.

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