阅读说明：本技术 锂二次电池 (Lithium secondary battery ) 是由 原田朋宏 于 2019-07-05 设计创作，主要内容包括：一种锂二次电池,其具备正极、负极、锂离子传导性的非水电解质、以及配置于所述正极与所述负极之间的分隔件,所述正极具备正极集电体和包含正极活性物质的正极合剂层,所述正极活性物质包含含有锂和过渡金属的复合氧化物,所述正极和所述负极所具有的每单位面积的锂的合计量M_(Li)与所述正极所具有的每单位面积的过渡金属量M_(TM)的摩尔比：M_(Li)/M_(TM)为1.1以下,所述负极具备负极集电体和层叠于所述负极集电体上的多个绝缘性的多孔膜。(A lithium secondary battery comprising a positive electrode, a negative electrode, a lithium ion conductive nonaqueous electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material containing a composite oxide containing lithium and a transition metal, and the positive electrode and the negative electrode have a total amount M of lithium per unit area Li And the amount M of the transition metal per unit area of the positive electrode TM The molar ratio of (A): m Li /M TM The negative electrode is 1.1 or less, and comprises a negative electrode current collector and a plurality of insulating porous films laminated on the negative electrode current collector.)

1. A lithium secondary battery comprising a positive electrode, a negative electrode, a lithium ion conductive nonaqueous electrolyte, and a separator disposed between the positive electrode and the negative electrode,

the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material,

the positive electrode active material includes a composite oxide containing lithium and a transition metal,

a total amount M of lithium per unit area of the positive electrode and the negative electrode_LiAnd the amount M of the transition metal per unit area of the positive electrode_TMThe molar ratio of (A): m_Li/M_TMThe content of the organic acid is below 1.1,

the negative electrode includes a negative electrode current collector and a plurality of insulating porous films laminated on the negative electrode current collector.

2. The lithium secondary battery according to claim 1, wherein the plurality of porous films include a 1 st porous film and a 2 nd porous film disposed between the 1 st porous film and the negative electrode collector,

the thickness of the 1 st porous membrane is equal to or greater than the thickness of the 2 nd porous membrane.

3. The lithium secondary battery according to claim 2, wherein the thickness of the 1 st porous film is 0.1 μm or more and 2 μm or less,

the thickness of the 2 nd porous film is 0.1 to 1 μm.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein each of the plurality of porous films has an opposing region opposing the positive electrode mixture layer,

at least one of the plurality of porous films has a non-opposing region that does not oppose the positive electrode mixture layer,

at least a part of the non-opposing region is in contact with the anode current collector in a charged state.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein at least one of the plurality of porous films contains a polymer having a unit derived from vinylidene fluoride.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein at least one of the plurality of porous films comprises at least one of a lithium salt and an ambient temperature molten salt.

7. The lithium secondary battery according to any one of claims 1 to 6, wherein the nonaqueous electrolyte contains at least one of fluoroethylene carbonate and vinylene carbonate.

8. The lithium secondary battery according to any one of claims 1 to 7, wherein the nonaqueous electrolyte contains lithium ions and anions,

the anion includes at least one of an imide-based anion and a boron-containing oxalate-based anion.

9. The lithium secondary battery according to any one of claims 1 to 8, wherein the imide-based anion is at least one of a bis (fluorosulfonyl) imide anion and a bis (trifluoromethanesulfonyl) imide anion.

Technical Field

The present invention relates to an improvement of a lithium secondary battery.

Background

Nonaqueous electrolyte secondary batteries are used for ICT applications such as personal computers and smart phones, vehicle applications, electric power storage applications, and the like. In such applications, a nonaqueous electrolyte secondary battery is required to have a further high capacity. As a high-capacity nonaqueous electrolyte secondary battery, a lithium ion battery is known. The capacity of a lithium ion battery can be increased by using an alloy active material such as graphite and a silicon compound in combination as a negative electrode active material. However, the capacity of lithium ion batteries has been increasing to the limit.

Promising nonaqueous electrolyte secondary batteries having a high capacity exceeding lithium ion batteries are lithium secondary batteries (lithium metal secondary batteries). In a lithium secondary battery, lithium metal is precipitated on a negative electrode during charging, and the lithium metal is dissolved in a nonaqueous electrolyte during discharging. Lithium secondary batteries may have lithium metal precipitated in dendrite form on the negative electrode during charging. Further, the specific surface area of the negative electrode increases with the formation of dendrites, and side reactions may increase. Therefore, the discharge capacity and the cycle characteristics are easily degraded.

Patent document 1 teaches: in order to suppress the growth of dendrites, a protective film containing a lithium ion-conductive polymer material is provided on a negative electrode containing lithium.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2015/030230

Disclosure of Invention

The method of patent document 1 has the following cases: during charging, lithium metal is precipitated between the protective film and the separator. Further, since the protective film as described above is easily deteriorated when the charge and discharge cycle increases, lithium may be precipitated in a dendrite form to puncture the protective film.

In view of the above, one aspect of the present invention relates to a lithium secondary battery including a positive electrode, a negative electrode, a lithium ion conductive nonaqueous electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode includes a positive electrode collector and a positive electrode mixture layer including a positive electrode active material, the positive electrode active material includes a composite oxide including lithium and a transition metal, and a total amount M of lithium per unit area included in the positive electrode and the negative electrode_LiAnd the amount M of the transition metal per unit area of the positive electrode_TMThe molar ratio of (A): m_Li/M_TMThe negative electrode is 1.1 or less, and includes a negative electrode current collector and a plurality of insulating porous films laminated on the negative electrode current collector.

According to the lithium secondary battery of the present invention, precipitation of lithium in a dendrite form is suppressed.

Drawings

Fig. 1 is a cross-sectional view schematically showing a negative electrode immediately after assembly of a battery according to an embodiment of the present invention.

Fig. 2 is a cross-sectional view schematically showing a negative electrode in a charged state according to an embodiment of the present invention.

Fig. 3 is a longitudinal sectional view schematically showing a lithium secondary battery according to an embodiment of the present invention.

Fig. 4A is a view showing an SEM image (magnification 2500 times) after charging of the cross section of the negative electrode produced in example 1.

Fig. 4B is an enlarged view showing a part of the SEM image of fig. 4A in an enlarged manner.

Fig. 5 is a view showing an SEM image (magnification 2500 times) after charging of the cross section of the negative electrode produced in comparative example 1.

Detailed Description

The lithium secondary battery of the present embodiment includes a positive electrode, a negative electrode, a lithium ion conductive nonaqueous electrolyte, and a separator disposed between the positive electrode and the negative electrode. The positive electrode includes a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material. The positive electrode active material includes a composite oxide containing lithium and a transition metal. The negative electrode includes a negative electrode current collector and a plurality of insulating porous films laminated on the negative electrode current collector.

Here, the total amount M of lithium per unit area that the positive electrode and the negative electrode have_LiAnd the amount M of transition metal per unit area of the positive electrode_TMThe molar ratio of (A): m_Li/M_TMIs 1.1 or less. That is, the negative electrode has substantially no lithium metal capable of discharging immediately after assembly. In other words, the negative electrode may not include the negative electrode active material layer, but includes the negative electrode current collector. This increases the volumetric energy density of the battery. In the fully discharged state, the molar ratio: m_Li/M_TMIs also 1.1 or less.

The fully discharged state of the lithium secondary battery refers to: when the rated capacity of the battery is C, the battery is discharged to a State of Charge (SOC: State of Charge) of 0.05 xC or less. For example, the discharge is performed at a constant current of 0.05C until the lower limit voltage. The lower limit voltage is, for example, 2.5V to 3.0V.

In a lithium secondary battery, lithium metal is deposited on the surface of a negative electrode current collector by charging. More specifically, lithium ions contained in the nonaqueous electrolyte accept electrons on the negative electrode current collector by charging to become lithium metal, and are deposited on the surface of the negative electrode current collector. According to the negative electrode of the present embodiment, the deposition position of lithium metal can be controlled between the surface of the negative electrode current collector and the porous film. Thus, dendrite is suppressed. This also makes it easier to suppress expansion of the negative electrode due to deposition of lithium metal.

The reason why the deposition position of lithium metal is controlled is not determined, but is presumed as follows. The plurality of porous films provided in the negative electrode are formed so as to be in contact with each other. Each of these porous films has through holes penetrating through its main surface. Therefore, lithium ions contained in the nonaqueous electrolyte reach the surface of the negative electrode current collector through the through-holes provided in the respective porous films, and receive electrons. However, in the present embodiment, since there are at least two porous membranes, the positions of the through-holes of the respective porous membranes are often shifted, and there are almost no through-holes that penetrate all the porous membranes at the same time. Therefore, even if lithium metal generated by receiving electrons from the negative electrode current collector enters the through-holes of the porous membrane (for example, the 2 nd porous membrane described later) disposed on the negative electrode current collector side, it cannot enter the through-holes of the porous membranes (for example, the 1 st porous membrane and the 3 rd porous membrane described later) disposed on the separator side. This suppresses deposition of lithium metal on the surface of the outermost porous film facing the separator.

Further, each porous film is insulating. Therefore, only the lithium ions reaching the surface of the negative electrode current collector accept electrons, and lithium metal is deposited therein. This also suppresses deposition of lithium metal in the through-hole of the porous film disposed on the negative electrode current collector side. As a result, the deposition position of lithium metal is controlled between the surface of the negative electrode current collector and the porous film disposed on the negative electrode current collector side.

In addition, since there are a plurality of porous films, the moving distance of lithium ions becomes long, and lithium ions are easily diffused in the surface direction of the negative electrode current collector. Therefore, concentration of deposition sites of lithium metal is alleviated, and the lithium metal is easily deposited in a bulk form in a dispersed manner.

The negative electrode is generally larger than the positive electrode, and a part of the main surface of the negative electrode current collector does not face the positive electrode mixture layer. Lithium metal is easily deposited on the portion of the negative electrode current collector facing the positive electrode mixture layer. Therefore, each porous film may have a region (facing region) facing the positive electrode mixture layer.

In view of enhancing the effect of suppressing dendrites, at least one of the porous films may have a non-facing region not facing the positive electrode mixture layer in addition to the facing region. The at least one porous film may be formed on the entire main surface of the negative electrode current collector facing the positive electrode.

In the case where at least one porous film has an opposing region and a non-opposing region, the contact between the non-opposing region of either porous film and the negative electrode current collector is maintained also in a charged state. Therefore, the distance between the region of the porous membrane facing the negative electrode current collector and the negative electrode current collector is limited, and lithium metal is precipitated when the porous membrane is moderately pressed. The reason why the growth of the dendrite is suppressed by the squeezing is not determined, but is presumed as follows. When lithium metal is precipitated, the growth of lithium metal is limited in the thickness (Z-axis) direction of the negative electrode current collector, but is not limited at all in the plane (XY-plane) direction of the negative electrode current collector. Therefore, as compared with the case where lithium metal can grow freely in three dimensions along the XY plane direction and the Z axis direction, lithium ions are less likely to diffuse in the Z axis direction, and the tendency to suppress the growth of dendrites is enhanced. In addition, the growth of lithium metal in the Z-axis direction is physically hindered.

Fig. 1 is a cross-sectional view schematically showing a negative electrode immediately after the battery of the present embodiment is assembled. Fig. 2 is a cross-sectional view schematically showing the negative electrode in the charged state of the present embodiment. In fig. 2, the non-facing regions are disposed at both ends of the negative electrode current collector 21. The illustrated example shows the developed state of the negative electrode. In addition, the through-holes are omitted.

The negative electrode 20 includes a negative electrode current collector 21, a 1 st porous membrane 22A disposed on the separator side, and a 2 nd porous membrane 22B disposed between the 1 st porous membrane 22A and the negative electrode current collector 21. As shown in fig. 1, the 1 st porous membrane 22A is in contact with the 2 nd porous membrane 22B, and the 2 nd porous membrane 22B is in contact with the negative electrode collector 21.

When the lithium secondary battery is charged, as shown in fig. 2, lithium metal (Li) is deposited between the negative electrode current collector 21 and the 2 nd porous membrane 22B. However, the contact between the negative electrode current collector 21 and the non-opposing region of the 2 nd porous film 22B is maintained. Therefore, as described above, the distance between the opposing regions of the negative electrode current collector 21 and the 2 nd porous film 22B is limited, and lithium metal Li is precipitated under moderate compression. Even in the charged state, the 1 st porous membrane 22A and the 2 nd porous membrane 22B are kept in contact with each other.

The structure of the lithium secondary battery will be specifically described below.

(cathode)

The negative electrode is an electrode from which lithium metal is precipitated during charging. The lithium metal deposited on the surface of the negative electrode current collector is dissolved in the nonaqueous electrolyte as lithium ions by discharge. The precipitated lithium metal comes from lithium ions in the nonaqueous electrolyte. The lithium ions contained in the nonaqueous electrolyte may be lithium ions derived from a lithium salt added to the nonaqueous electrolyte, may be lithium ions supplied from the positive electrode active material by charging, or may be both of them. The lithium salt may be a lithium salt derived from a raw material (see below) used for producing the porous film.

The negative electrode includes a negative electrode current collector and a plurality of porous films. The porous membrane is insulating and has at least one through hole penetrating from the separator-side main surface to the negative electrode current collector-side main surface. The porous film is disposed on at least one main surface of the negative electrode current collector. An interface can be recognized at the boundary of each porous membrane, but it is difficult to separate these. However, the porous membrane and the separator can be separated.

The insulating material constituting the porous film is not particularly limited, and examples thereof include: fluororesins such as Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), a copolymer of vinylidene fluoride and hexafluoropropylene (VdF-HFP), and a copolymer of vinylidene fluoride and trifluoroethylene (VdF-TrFE); polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, urethane resins, polycarbonate resins, epoxy resins, and the like. Among them, from the viewpoint of swelling property with respect to the nonaqueous electrolyte, the protective film preferably contains a unit having a unit derived from vinylidene fluoride (— CH)₂-CF₂-) of a fluorine resin. Examples of such insulating materials include PVdF, VdF-HFP, VdF-TrFE, and the like.

The diameter of the through hole is not particularly limited as long as lithium ions can pass through the through hole. The average diameter of the through holes may be, for example, in the range of 100nm to 1000nm, or in the range of 200nm to 800 nm. The relationship in size between the through-holes (1 st through-holes) of the porous membrane (1 st porous membrane) disposed on the separator side and the through-holes (2 nd through-holes) of the porous membrane (2 nd porous membrane) disposed on the negative electrode collector side is not particularly limited, and the average diameter of the 2 nd through-holes may be smaller than the average diameter of the 1 st through-holes from the viewpoint of easy control of the deposition position of lithium metal.

The average diameter of the through-holes is: an average value of through-holes contained in a region of 2mm square when the main surface of the porous membrane is observed from the normal direction of the main surface of the porous membrane. The average diameter of the through-hole can be calculated from the cross section of the negative electrode in the thickness direction. For example, arbitrary 10 through holes are selected from a cross section having a length of 10mm in the thickness direction of the negative electrode, and the average value of the widths thereof is determined as the average diameter of the through holes. The width of the through hole is a length in a direction perpendicular to the thickness direction of the negative electrode. For measuring the width of the through hole, a Scanning Electron Microscope (SEM) can be used.

The thickness of the porous film is not particularly limited. However, from the viewpoint of increasing the capacity, it is desirable that the thickness of the entire porous film is not excessively large. On the other hand, the porous film is desired to have a thickness to a degree that can withstand expansion and contraction of the negative electrode accompanying charge and discharge. The thickness of the entire porous film may be, for example, 0.1 μm or more and 5 μm or less, or 0.5 μm or more and 2.5 μm or less. The thickness of the entire porous membrane is the sum of the thicknesses of the 1 st porous membrane and the 2 nd porous membrane on one principal surface side of the negative electrode current collector and the thickness of the 3 rd porous membrane described later.

The thickness of the 1 st porous membrane (1 st thickness) may be equal to or greater than the thickness of the 2 nd porous membrane (2 nd thickness). In this case, even if expansion and contraction of the negative electrode repeatedly occur, the 1 st porous film disposed on the separator side is less likely to be damaged. Therefore, the lithium metal is prevented from entering the 1 st porous film and precipitating in a dendrite form. The 1 st thickness may be 1.1 times or more, 1.3 times or more, and 1.5 times or more of the 2 nd thickness.

The 1 st thickness may be, for example, 0.1 μm or more and 2 μm or less, and may be 0.5 μm or more and 1 μm or less. The thickness 2 may be, for example, 0.1 μm or more and 1 μm or less, and may be 0.1 μm or more and 0.7 μm or less.

The thickness of the porous film can be determined as an average value of the thickness at any 5 points in the above cross section of the negative electrode. The thickness of the porous film is the length of the porous film in the thickness direction of the negative electrode.

An insulating 3 rd porous film may be disposed between the 1 st porous film and the 2 nd porous film. The 3 rd porous membrane similarly includes at least one through hole penetrating from the main surface on the separator side to the main surface on the negative electrode current collector side.

The 3 rd porous film may have two or more layers. The 3 rd porous film is formed of, for example, the insulating material. The thickness of the 3 rd porous film (3 rd thickness) is not particularly limited. The thickness of the No. 3 film may be appropriately set so that the thickness of the entire porous film is, for example, in the above range.

The negative electrode includes a negative electrode current collector made of a metal material that does not react with lithium metal.

Examples of the metal material constituting the negative electrode current collector include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements. As the alloy, a copper alloy, stainless steel (SUS), or the like is preferable. As the metal material, copper and/or a copper alloy is preferable from the viewpoint of conductivity. The copper content in the negative electrode current collector is preferably 50 mass% or more, and may be 80 mass% or more. The metal material is in the form of foil, for example. The thickness of the negative electrode current collector is not particularly limited, and is, for example, 5 to 20 μm.

[ method for producing porous film ]

The porous film is formed by applying a raw material liquid for a porous film on at least one main surface of the negative electrode current collector. The negative electrode is produced, for example, by a method including the steps of: a first step of preparing a negative electrode current collector; a 2 nd step of applying a starting solution (2 nd starting solution) for the 2 nd porous membrane to at least one main surface of the negative electrode current collector and then drying the solution to form a 2 nd porous membrane; and a 3 rd step of applying the raw material solution (1 st raw material solution) for the 1 st porous film to the main surface of the negative electrode collector on which the 2 nd porous film is formed, and then drying the applied raw material solution to form the 1 st porous film.

In the case of forming the 3 rd porous film, the 4 th step is performed after the 2 nd step and before the 3 rd step, and the 4 th step is performed by applying a raw material liquid (3 rd raw material liquid) of the 3 rd porous film on the surface of the 2 nd porous film and then drying the applied raw material liquid to form the 3 rd porous film. In this case, in the 3 rd step, the 1 st raw material liquid is applied to the main surface of the negative electrode current collector through the 2 nd porous film and the 3 rd porous film.

Each raw material liquid contains, for example, the insulating material and a solvent. As the solvent, a good solvent having high compatibility with the insulating material used and a poor solvent having low solubility with the insulating material used can be used. Alternatively, at least one of a lithium salt and an ambient temperature molten salt (hereinafter, may be simply referred to as a salt) may be used together with a good solvent having high compatibility with the insulating material used.

When a good solvent and a poor solvent are used, a region including the insulating material and the good solvent and a region including the poor solvent are separated in the coating film. Due to the separation, the region containing the poor solvent is arranged to be interposed between the regions containing the insulating material. Subsequently, the solvent is removed by drying, thereby forming through holes and non-through holes between the regions including the insulating material. For example, the pore diameter, porosity, and the like of the obtained porous film are controlled by the kind of the solvent and the mass ratio of the good solvent to the poor solvent.

When a good solvent and a salt are used, a region containing an insulating material, a good solvent, and a salt and having a high concentration of the insulating material is separated from a region containing an insulating material and a good solvent and having a relatively low concentration of the insulating material in the coating film. Then, the solvent is removed by drying, thereby forming through holes and forming non-through holes so as to correspond to regions where the concentration of the insulating material is relatively low. At this time, the salt remains in the porous film. For example, the pore size, porosity, and the like of the obtained porous film are controlled by the kind and concentration of the salt.

The good solvent and the poor solvent can be appropriately selected depending on the insulating material used. When the good solvent and the poor solvent are used in combination, the boiling points of the good solvent and the poor solvent may differ by 10 ℃ or more, for example, from the viewpoint of easy pore formation. Examples of the solvent include N-methyl-2-pyrrolidone (NMP).

As the lithium salt, for example, a known lithium salt used for a nonaqueous electrolyte of a lithium secondary battery can be used. Specifically, as the anion of the lithium salt, BF is mentioned₄ ^-、ClO₄ ^-、PF₆ ^-、CF₃SO₃ ^-、CF₃CO₂ ^-Anions of oxalates, anions of imides described later as anions of ambient temperature molten salts, and the like. Of the oxalate typeThe anion may contain boron and/or phosphorus. The anion of the oxalate salt may be the anion of an oxalate complex. Examples of the anion of the oxalate salt include difluorooxalate Borate (BF)₂(C₂O₄)^-) Bispyrioxalato borate (B (C)₂O₄)₂ ^-)、B(CN)₂(C₂O₄)^-、PF₄(C₂O₄)^-、PF₂(C₂O₄)₂ ^-And the like. The lithium salt may be used alone or in combination of two or more.

The normal temperature molten salt is a salt which is liquid at normal temperature (20 ℃ to 40 ℃), and is also called an ionic liquid.

The ambient temperature molten salt may be a salt of an organic onium cation and an imide anion, from the viewpoint of easy pore formation. The ambient temperature molten salt may be used alone or in combination of two or more.

Examples of the organic onium cation include a nitrogen-containing onium cation such as an aliphatic amine, alicyclic amine, or aromatic amine-derived cation (for example, a quaternary ammonium cation), and an organic onium cation having a nitrogen-containing heterocycle (that is, a cyclic amine-derived cation); a sulfonium-containing cation; phosphonium-containing cations (e.g., quaternary phosphonium cations, etc.), and the like. The organic onium cation may have a functional group such as a hydroxyl group (-OH) or a silanol group (-Si-OH), and may have a divalent group (e.g., -SiO-) derived from silanol or the like.

Among them, from the viewpoint of heat resistance, a cation having a nitrogen-containing heterocycle is preferable. Examples of the nitrogen-containing heterocyclic skeleton include pyrrolidine, imidazoline, imidazole, pyridine, and piperidine. Specific examples thereof include N-methyl-N-propylpiperidinium cation (N-methyl-N-propylpiperidinium cation), N, N, N-trimethyl-N-propylammonium cation (N, N, N-trimethyl-N-propylpyrrolidinium cation), and 1-methyl-1-propylpyrrolidinium cation (1-methyl-1-propylpyrrolidinium cation).

As the imide anion, N (SO) may be mentioned₂C_mF_2m+1)(SO₂C_nF_2n+1) (m and n are each independentlyAn integer of 0 or more. ) And the like. m and n can be 0-3, 0, 1 or 2 respectively. The imide anion may be bis (trifluoromethanesulfonimide) anion (N (SO)₂CF₃)₂ ^-、TFSI^-) Bis (perfluoroethanesulfonyl) imide anion (N (SO)₂C₂F₅)₂ ^-) Bis (fluorosulfonyl) imide anion (N (SO)₂F)₂ ^-). In particular, bis (fluorosulfonyl) imide anion, TFSI^-。

The concentration of the salt in the raw material solution is not particularly limited, and may be appropriately set in consideration of the pore diameter, porosity, and the like of the porous film to be formed. For example, the salt concentration may be 0.5 to 30% by mass, and may be 1 to 10% by mass.

The concentration of the insulating material in the raw material liquid is not particularly limited, and may be appropriately set in consideration of the thickness of the porous film to be formed. The concentration of the insulating material may be, for example, 3 to 35 mass%, or 5 to 15 mass%.

[ Positive electrode ]

The positive electrode includes a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material. The positive electrode mixture layer may be formed as follows: the positive electrode current collector is formed by applying a positive electrode slurry, in which a positive electrode mixture containing a positive electrode active material, a binder, a conductive agent, and the like is dispersed in a dispersion medium, to the surface of a positive electrode current collector and drying the positive electrode slurry. The dried coating film may be rolled as necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector, or may be formed on both surfaces.

As the positive electrode active material, a composite oxide containing lithium and a transition metal is used. Molar ratio of lithium to transition metal contained in the composite oxide: the lithium/transition metal is, for example, 0.9 to 1.1.

As such a positive electrode active material, a layered rock salt type composite oxide is exemplified. Specifically, the positive electrode active material includes, for example, Li_aCoO₂、Li_aNiO₂、Li_aMnO₂、Li_aCo_bNi_1-bO₂、Li_aCo_bM_1-bO_c、Li_aNi_bM_1-bO_c、LiMPO₄(M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B.). Here, a is more than 0 and less than or equal to 1.1, b is more than or equal to 0 and less than or equal to 0.9, and c is more than or equal to 2 and less than or equal to 2.3. The value a indicating the molar ratio of lithium is a value immediately after the active material is produced, and increases and decreases with charge and discharge.

Among them, a layered rock salt type composite oxide containing a nickel element is preferable. Such a composite oxide is represented by Li_aNi_xM_1-xO₂(M is at least one selected from the group consisting of Mn, Co and Al, 0 < a.ltoreq.1.1, 0.3. ltoreq. x.ltoreq.1). From the viewpoint of high capacity, it is more preferable that 0.85. ltoreq. x.ltoreq.1 is satisfied. In addition, from the viewpoint of stability of the crystal structure, a lithium-nickel-cobalt-aluminum composite oxide (NCA) containing Co and Al as M is further preferable: li_aNi_xCo_yAl_zO₂(a is more than 0 and less than or equal to 1.1, x is more than or equal to 0.85 and less than 1, y is more than 0 and less than 0.15, z is more than 0 and less than or equal to 0.1, and x + y + z is 1). Specific examples of NCA include LiNi_0.8Co_0.15Al_0.05O₂、LiNi_0.8Co_0.18Al_0.02O₂、LiNi_0.9Co_0.05Al_0.05O₂And the like.

Examples of the binder include: resin materials such as PTFE, PVdF and other fluororesins; polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aromatic polyamide resins; polyimide resins such as polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymers; vinyl resins such as PAN and polyvinyl acetate; polyvinylpyrrolidone; polyether sulfone; rubber-like materials such as styrene-butadiene copolymer rubber (SBR). These may be used alone or in combination of two or more.

Examples of the conductive agent include: natural graphite, artificial graphite, and other graphite; carbon blacks such as acetylene black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and organic conductive materials such as phenylene derivatives. These may be used alone or in combination of two or more.

The shape and thickness of the positive electrode collector may be selected according to the shape and range of the negative electrode collector, respectively. Examples of the material of the positive electrode current collector include stainless steel, aluminum (Al), aluminum alloy, and titanium.

[ non-aqueous electrolyte ]

As the nonaqueous electrolyte, a nonaqueous electrolyte having lithium ion conductivity is used. Such a nonaqueous electrolyte contains a nonaqueous solvent and lithium ions and anions dissolved in the nonaqueous solvent. The nonaqueous electrolyte may be in a liquid state or a gel state.

The liquid nonaqueous electrolyte may be prepared by dissolving a lithium salt in a nonaqueous solvent. The lithium salt generates lithium ions and anions when dissolved in the nonaqueous solvent, but may contain a lithium salt that is not dissociated in the nonaqueous electrolyte.

The gel-like nonaqueous electrolyte contains a liquid nonaqueous electrolyte and a matrix polymer. As the matrix polymer, for example, a polymer material gelled by absorbing a nonaqueous solvent is used. Examples of such polymer materials include fluororesins, acrylic resins, and/or polyether resins.

As the lithium salt, a known lithium salt used in a nonaqueous electrolyte of a lithium secondary battery can be used. Specifically, a compound exemplified as a lithium salt used for forming the porous film can be cited. The nonaqueous electrolyte may contain one of these lithium salts, or may contain two or more of them.

The nonaqueous electrolyte may contain at least one of an imide-based anion and a boron-containing oxalate-based anion, from the viewpoint of further suppressing precipitation of lithium metal in a dendritic form. The anion of the boron-containing oxalate is particularly preferred. The anion of the boron-containing oxalate may be used in combination with other anions. The other anion may be PF₆ ^-And/or anions of the imide type.

The concentration of the lithium salt in the nonaqueous electrolyte may be, for example, 0.5mol/L to 3.5 mol/L. The concentration of the lithium salt is the sum of the concentration of the dissociated lithium salt and the concentration of the undissociated lithium salt. The concentration of the anion in the nonaqueous electrolyte may be set to 0.5mol/L to 3.5 mol/L.

Examples of the nonaqueous solvent include esters, ethers, nitriles, amides, and halogen substitutes thereof. The nonaqueous electrolyte may contain one of these nonaqueous solvents, or may contain two or more of these nonaqueous solvents. Examples of the halogen substituent include fluoride.

Examples of the ester include a carbonate and a carboxylate. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate (FEC). Examples of the chain carbonate include dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), and diethyl carbonate. Examples of the cyclic carboxylic acid ester include γ -butyrolactone and γ -valerolactone. Examples of the chain carboxylic acid ester include ethyl acetate, methyl propionate, and methyl fluoropropionate.

Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ether include 1, 3-dioxolane, 4-methyl-1, 3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether include 1, 2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methylphenyl ether, benzyl ether, diphenyl ether, benzyl ether, 1, 2-diethoxyethane, and diethylene glycol dimethyl ether.

Examples of the nitrile include acetonitrile, propionitrile, and benzonitrile. Examples of the amide include dimethylformamide and dimethylacetamide.

Among them, vinylene carbonate and fluoroethylene carbonate are preferable from the viewpoint of charge-discharge efficiency. The use of these nonaqueous solvents sometimes facilitates the formation of dendrites. However, according to the present embodiment, lithium metal is precipitated between the surface of the negative electrode current collector and the 2 nd porous membrane when pressed by the porous membrane, and thus dendrite is suppressed.

[ separator ]

A separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous film, woven fabric, nonwoven fabric, or the like can be used. As the material of the separator, polyolefin such as polypropylene or polyethylene is preferable.

[ lithium Secondary Battery ]

Fig. 3 is a longitudinal sectional view of an example of a cylindrical lithium secondary battery according to an embodiment of the present invention.

The lithium secondary battery 100 is a wound battery including a wound electrode group 40 and a non-aqueous electrolyte, not shown. The wound electrode group 40 includes a strip-shaped positive electrode 10, a strip-shaped negative electrode 20, and a separator 30. The positive electrode 10 is connected to a positive electrode lead 13, and the negative electrode 20 is connected to a negative electrode lead 23.

One end of the positive electrode lead 13 in the longitudinal direction is connected to the positive electrode 10, and the other end is connected to the sealing plate 90. The sealing plate 90 includes a positive electrode terminal 15. One end of the negative electrode lead 23 is connected to the negative electrode 20, and the other end is connected to the bottom of the battery case 70 serving as a negative electrode terminal. The battery case 70 is a bottomed cylindrical battery can, one end in the longitudinal direction of which is open, and the bottom of the other end serves as a negative electrode terminal. The battery case (battery can) 70 is made of metal, and is formed of iron, for example. The inner surface of the battery case 70 made of iron is usually plated with nickel. Upper and lower insulating rings 80 and 60 made of resin are disposed above and below the wound electrode group 40, respectively.

The illustrated example describes a cylindrical lithium secondary battery including a wound electrode group, but the present embodiment is not limited to this case. The shape of the lithium secondary battery can be appropriately selected from various shapes such as a cylindrical shape, a coin shape, a square shape, a plate shape, and a flat shape according to the use and the like. The form of the electrode group is not particularly limited, and may be a laminate type.

In addition, as for the configuration of the lithium secondary battery other than the negative electrode, a known configuration may be used without particular limitation.

The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[ example 1]

(1) Production of positive electrode

Mixing lithium nickel composite oxide (LiNi)_0.8Co_0.18Al_0.02O₂) Acetylene black and PVdF in a ratio of 95: 2.5: 2.5, NMP was added, and then stirred by a mixer (manufactured by PRIMIX Corporation, t.k.hivis MIX), thereby preparing a positive electrode slurry. Then, a positive electrode slurry was applied to the surface of the Al foil, the coating film was dried, and the resultant was rolled to prepare an Al foil having a density of 3.6g/cm formed on both surfaces thereof³The positive electrode of the positive electrode mixture layer.

(2) Production of negative electrode

The electrolytic copper foil (10 μm thick) was cut into a prescribed electrode size. A raw material solution containing PVdF (concentration: 8 mass%), LiTFSI (concentration: 1 mass%), and NMP was prepared.

The raw material solution was applied to both main surfaces of an electrolytic copper foil and then hot-air dried to form a 2 nd porous film having a 2 nd thickness of 0.5. mu.m. The above-mentioned raw material liquid was applied to the surfaces of the two 2 nd porous films in the same manner, and then hot air-dried to form 1 st porous films having a thickness of 0.5 μm.

The obtained cross section of the negative electrode in the thickness direction was observed by SEM to confirm that a plurality of 1 st through holes having an average diameter of 800nm were formed in the 1 st porous film. Similarly, it was confirmed that a plurality of 2 nd through holes having an average diameter of 800nm were formed in the 2 nd porous film.

(3) Preparation of non-aqueous electrolyte

FEC, EMC and DMC were written in FEC: EMC: DMC 20: 5: 75 by volume. The obtained mixed solvent is added with lithium difluoro oxalate borate to reach 0.3mol/L, LiPF₆The dissolution was carried out to 1.0mol/L, thereby preparing a nonaqueous electrolyte.

(4) Manufacture of batteries

An Al electrode tab was attached to the positive electrode obtained above. The negative electrode obtained above was provided with a Ni electrode tab. The positive electrode and the negative electrode were wound in a spiral shape with a polyethylene film (separator) interposed therebetween in an inert atmosphere, thereby producing a wound electrode body. The obtained electrode assembly was housed in a bag-shaped outer case formed of a laminate sheet having an Al layer, the nonaqueous electrolyte was injected, and the outer case was sealed, thereby producing a lithium secondary battery T1.

[ example 2]

A lithium secondary battery T2 was produced in the same manner as in example 1, except that the 1 st thickness of the 1 st porous film was set to 1 μm in the production (2) of the negative electrode.

[ example 3]

A lithium secondary battery T3 was produced in the same manner as in example 1 except that a 3 rd porous film (3 rd thickness: 1 μm: 0.5 μm × 2) composed of two layers was formed between the 2 nd porous film and the 1 st porous film in the production (2) of the negative electrode.

Comparative example 1

A lithium secondary battery R1 was produced in the same manner as in example 1, except that the same raw material solution was used to form a porous film having a thickness of 2 μm in the production (2) of the negative electrode.

The obtained cross section of the negative electrode in the thickness direction was observed by SEM to confirm that a large number of through holes having an average diameter of 800nm were formed in the formed porous film.

Comparative example 2

A lithium secondary battery R2 was produced in the same manner as in example 1, except that a raw material solution not containing LiTFSI was used in the production (2) of the negative electrode.

The obtained negative electrode was observed for a cross section in the thickness direction by SEM, and as a result, both the films were formed as non-porous films having no through-hole.

[ evaluation ]

The obtained batteries T1 to T3, R1 and R2 were subjected to a charge-discharge test.

In the charge/discharge test, the battery was charged in a thermostatic bath at 25 ℃ under the following conditions, and then left for 20 minutes, and then discharged under the following conditions. The charge and discharge were performed as 1 cycle, and a charge and discharge test was performed for 50 cycles.

(charging) constant current charging was performed at a current of 20mA until the battery voltage reached 4.1V, and then constant voltage charging was performed at a voltage of 4.1V until the current value reached 2 mA.

(discharge) constant current discharge was performed at a current of 20mA until the cell voltage reached 3.0V.

(a) Deposition site of lithium metal and (b) damage of negative electrode

After 50 cycles of charge and discharge, the battery was disassembled and the negative electrode was taken out. The negative electrode was cut in the thickness direction, and the cross section was observed with a Scanning Electron Microscope (SEM). The evaluation results are shown in table 1. Fig. 4A shows an SEM image (magnification 2500 times) of a cross section of the negative electrode taken out of the cell T1. Fig. 4B is an enlarged view of a part of the SEM image of fig. 4A. In fig. 4B, the interface between the 1 st porous membrane 22A and the 2 nd porous membrane 22B is shown by a broken line for convenience. According to fig. 4A and 4B, lithium metal (Li) is precipitated between the anode current collector 21 and the 2 nd porous film 22B. Fig. 5 shows an SEM image (magnification 2500 times) of a cross section of the negative electrode taken out of the cell R1. According to fig. 5, lithium metal (Li) is deposited on a main surface (a main surface on the separator side not shown) of the porous film 22 that does not face the negative electrode current collector 21.

(c) Capacity retention rate

The value obtained by dividing the discharge capacity at the 50 th cycle by the discharge capacity at the 1 st cycle was defined as a capacity retention rate (%). The evaluation results are shown in table 1.

[ Table 1]

As shown in table 1, in all of the batteries T1 to T3, the deposition position of lithium metal was controlled between the negative electrode current collector and the 2 nd porous membrane, and the capacity retention rate was high. On the other hand, in the batteries R1 and R2, the porous film or the non-porous film was damaged, and lithium metal was precipitated between each film and the separator. Part of the precipitated lithium metal is in the form of dendrite. The capacity retention rate is lower than that of batteries T1 to T3.

Industrial applicability

The lithium secondary battery of the present invention has excellent discharge capacity and cycle characteristics, and therefore can be used for electronic devices such as mobile phones, smart phones, tablet terminals, electric vehicles including hybrid vehicles and plug-in hybrid vehicles, household storage batteries combined with solar cells, and the like.

Description of the reference numerals

10 positive electrode

13 positive electrode lead

15 positive terminal

20 negative electrode

21 negative electrode current collector

22A 1 st porous Membrane

22B No. 2 porous Membrane

23 cathode lead

30 divider

40 coiled electrode group

60 lower insulating ring

70 Battery case

80 upper insulating ring

90 sealing plate

100 lithium secondary battery

22 porous membrane