JP2018120824A - Positive electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material which can suppress the capacity deterioration of a secondary battery.SOLUTION: A positive electrode material herein disclosed comprises: a positive electrode active material; and a fluorine resin coating the positive electrode active material. The coverage of the fluorine resin to the positive electrode active material is 96% or more. The fluorine resin is a polytetrafluoroethylene, a tetrafluoroethylene-perfluoro alkyl vinyl ether copolymer, or a polyvinylidene fluoride. If the fluorine resin is the polytetrafluoroethylene, its coating thickness is 0.009 μm or more and 0.199 μm or less. If the fluorine resin is the tetrafluoroethylene-perfluoro alkyl vinyl ether copolymer, the coating thickness is 0.011 μm or more and 0.165 μm or less. If the fluorine resin is the polyvinylidene fluoride, the coating thickness is 0.01 μm or more and 0.098 μm or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode material.

  Secondary batteries such as lithium ion secondary batteries are lighter and have higher energy density than existing batteries, and have recently been used as so-called portable power sources for vehicles and personal computers and power sources for driving vehicles. In particular, lithium-ion secondary batteries that are lightweight and provide high energy density will become increasingly popular as high-output power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). It is expected to do.

  Secondary batteries, in particular lithium ion secondary batteries, are expected to have a longer life (that is, capacity degradation is suppressed over a long period of time) as they become popular. For this reason, various techniques for extending the life of secondary batteries have been developed. For example, Patent Document 1 proposes a technique in which in a nonaqueous electrolyte secondary battery, a fluorine-containing resin layer that suppresses the generation of carbon dioxide from the positive electrode plate is provided on the surface of the positive electrode plate facing the separator. According to the technique described in Patent Document 1, it is possible to suppress the generation of carbon dioxide gas when stored for a long time at a high temperature or when a charge / discharge cycle is repeated at a high temperature. Battery capacity deterioration can be suppressed.

JP 2008-218268 A

  However, the capacity deterioration of the non-aqueous electrolyte secondary battery can also occur when the positive electrode active material and the non-aqueous electrolyte are oxidized to decompose the non-aqueous electrolyte. According to the study by the present inventors, according to the technique described in Patent Document 1, contact between the positive electrode active material and the non-aqueous electrolyte solution is caused by the presence of the fluorine-containing resin layer on the surface of the positive electrode active material layer of the positive electrode plate. Although the oxidation reaction occurring between them can be suppressed to some extent, the positive electrode active material and the non-aqueous electrolyte are in contact with each other inside the positive electrode active material layer, so that they can undergo an oxidation reaction. I found it. That is, it has been found that the decrease in battery capacity caused by the decomposition of the non-aqueous electrolyte due to the oxidation reaction between the positive electrode active material and the non-aqueous electrolyte cannot be sufficiently suppressed.

  Then, an object of this invention is to provide the positive electrode material which can suppress the capacity deterioration of a secondary battery.

The positive electrode material disclosed herein includes a positive electrode active material and a fluororesin that covers the positive electrode active material. The coverage of the fluororesin with respect to the positive electrode active material is 96% or more. The fluororesin is polytetrafluoroethylene, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, or polyvinylidene fluoride. When the fluororesin is polytetrafluoroethylene, the coating thickness is 0.009 μm or more and 0.199 μm or less, and when the fluororesin is a tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer The coating thickness is 0.011 μm or more and 0.165 μm or less, and when the fluororesin is polyvinylidene fluoride, the coating thickness is 0.01 μm or more and 0.098 μm or less.
According to such a configuration, in the secondary battery, direct contact between the positive electrode active material and the non-aqueous electrolyte can be effectively suppressed, and thereby the oxidation reaction between the positive electrode active material and the non-aqueous electrolyte. Can be suppressed. As a result, the capacity deterioration of the secondary battery can be suppressed. That is, according to such a configuration, it is possible to provide a positive electrode material that can suppress the capacity deterioration of the secondary battery.

It is sectional drawing which shows typically the structure of the lithium ion secondary battery constructed | assembled using the positive electrode material which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the winding electrode body of the lithium ion secondary battery constructed | assembled using the positive electrode material which concerns on one Embodiment of this invention.

  Embodiments according to the present invention will be described below with reference to the drawings. Note that matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, a general configuration of a positive electrode material that does not characterize the present invention) are known in the art. It can be grasped as a design matter of those skilled in the art based on the above. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. Moreover, in the following drawings, the same code | symbol is attached | subjected and demonstrated to the member and site | part which show | plays the same effect | action. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

The positive electrode material according to the present embodiment includes a positive electrode active material and a fluororesin that covers the positive electrode active material.
As the positive electrode active material used for the positive electrode material, a positive electrode active material used for a secondary battery, particularly a lithium ion secondary battery, can be suitably used. Examples of the positive electrode active material used for the lithium ion secondary battery include spinel lithium nickel manganese composite oxide, layered lithium composite oxide, and the like. The higher the positive electrode upper limit potential is, the higher the effect of suppressing the degradation of the non-aqueous electrolyte and suppressing the capacity deterioration of the secondary battery. is there. The spinel-based lithium nickel manganese composite oxide refers to an oxide having a spinel crystal structure and containing at least lithium, nickel, manganese, and oxygen as constituent elements. The spinel-based lithium nickel manganese composite oxide is typically Li _a Ni _b Mn _c O ₄ (wherein 1 ≦ a ≦ 1.3, 0.40 ≦ b ≦ 0.56, 1.44 ≦ c ≦ 1.60), and in the spinel composite oxide, a part of Ni, Mn, and O may be substituted with other elements. The spinel-based lithium nickel manganese composite oxide is preferably a spinel-based composite oxide represented by LiNi _0.5 Mn _1.5 O ₄ .

The fluororesin used for the positive electrode material is polytetrafluoroethylene (PTFE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), or polyvinylidene fluoride (PVDF).
In this embodiment, the coverage of the fluororesin with respect to the positive electrode active material is 96% or more. The fluororesin has liquid repellency and exhibits a large contact angle with respect to a non-aqueous electrolyte used in a lithium ion secondary battery. That is, the fluororesin has an electrolyte repellent property. By coating 96% or more of the positive electrode active material with such a fluororesin having an electrolyte repellent property, direct contact between the positive electrode active material and the non-aqueous electrolyte can be suppressed, and thus the positive electrode active material and the non-aqueous electrolyte can be prevented from contacting each other. Oxidation reaction with the water electrolyte can be suppressed. As a result, the capacity deterioration of the secondary battery can be suppressed. In addition, the coverage of the fluororesin with respect to a positive electrode active material can be calculated | required by the elemental analysis by X-ray photoelectron spectroscopy (XPS). In the analysis, it is preferable to use fluorine contained in the fluororesin and a metal element contained in the positive electrode active material. For example, when the positive electrode active material is LiNi _0.5 Mn _1.5 O ₄ , from the detection ratio of Mn and Ni derived from the positive electrode active material and F derived from the fluororesin, the coverage ratio (%) = “ F ”/ (“ F ”+“ Mn ”+“ Ni ”) × 100.

When the fluororesin is polytetrafluoroethylene, the coating thickness is 0.009 μm or more and 0.199 μm or less. In this case, the coating thickness is preferably 0.009 μm or more and 0.151 μm or less.
When the fluororesin is a tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, the coating thickness is 0.011 μm or more and 0.165 μm or less. In this case, the coating thickness is preferably 0.025 μm or more and 0.11 μm or less.
When the fluororesin is polyvinylidene fluoride, the coating thickness is 0.01 μm or more and 0.098 μm or less. In this case, the coating thickness is preferably 0.01 μm or more and 0.057 μm or less.
In any case, if the coating thickness is too small, the electrolyte resin repellency due to the fluororesin cannot be sufficiently exhibited, and the oxidation reaction between the positive electrode active material and the non-aqueous electrolyte is sufficiently suppressed. I can't. In such a coating thickness, for example, a nonaqueous electrolytic solution in which LiPF ₆ is dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 50:50. The contact angle measured by the 2 / θ method is 39 ° or more. On the other hand, if the coating thickness is too large, it becomes difficult to insert and remove ions (particularly lithium ions) serving as charge carriers into the positive electrode active material, and battery characteristics deteriorate. Therefore, when the coating thickness of the fluororesin is within the above range, direct contact between the positive electrode active material and the non-aqueous electrolyte can be effectively suppressed. In the meantime, the oxidation reaction can be effectively suppressed. As a result, the capacity deterioration of the secondary battery can be effectively suppressed.
In this specification, “the coating thickness of the fluororesin” means that when the positive electrode material is subjected to XPS measurement in the depth direction, the fluorine (F) element ratio of the fluororesin is 50% with respect to the outermost surface. The depth (dimension in the depth direction).

  The positive electrode material is prepared by immersing a positive electrode active material in a dispersion in which a fluororesin powder is dispersed in a dispersion medium to adhere the fluororesin powder to the surface of the positive electrode active material, and melting the fluororesin (polytetrafluoroethylene: 327 And tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer: 310 ° C., polyvinylidene fluoride: 177 ° C.). The film thickness of the fluororesin can be controlled by adjusting the amount of the fluororesin powder adhered to the surface of the positive electrode active material, for example, by adjusting the dispersion amount of the fluororesin powder in the dispersion. it can.

The positive electrode material of this embodiment can be used suitably as a positive electrode material of a secondary battery.
Therefore, the configuration of a lithium ion secondary battery 100 as a specific example of a secondary battery (nonaqueous electrolyte secondary battery) using the positive electrode material according to the present embodiment will be described below with reference to the drawings. In the present specification, the “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term including a power storage element such as a so-called storage battery and an electric double layer capacitor.
In the present specification, the “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as a charge carrier and is charged and discharged by movement of charges accompanying the lithium ions between the positive and negative electrodes.

  A lithium ion secondary battery 100 shown in FIG. 1 is constructed by accommodating a flat wound electrode body 20 and a nonaqueous electrolyte (not shown) in a flat rectangular battery case (ie, an outer container) 30. This is a sealed lithium ion secondary battery 100. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or more. It has been. The positive and negative terminals 42 and 44 are electrically connected to the positive and negative current collectors 42a and 44a, respectively. As the material of the battery case 30, for example, a light metal material having good thermal conductivity such as aluminum is used.

  As shown in FIGS. 1 and 2, the wound electrode body 20 has a positive electrode active material layer 54 formed along the longitudinal direction on one side or both sides (here, both sides) of an elongated positive electrode current collector 52. The positive electrode sheet 50 and the negative electrode sheet 60 in which the negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of the long negative electrode current collector 62 are two long shapes. The separator sheet 70 is overlapped and wound in the longitudinal direction. The positive electrode active material layer non-forming portion 52a (that is, the positive electrode) formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20 (referred to as the sheet width direction orthogonal to the longitudinal direction). The portion where the active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer non-formed portion 62a (that is, the portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed). ) Are joined with a positive current collector 42a and a negative current collector 44a, respectively.

  Examples of the positive electrode current collector 52 constituting the positive electrode sheet 50 include aluminum foil. The positive electrode active material layer 54 includes the positive electrode material according to the above-described embodiment, which is a material including a positive electrode active material. The positive electrode active material layer 54 can further include a conductive material, a binder, and the like. As the conductive material, for example, carbon black such as acetylene black (AB) and other (such as graphite) carbon materials can be suitably used. As the binder, for example, polyvinylidene fluoride (PVDF) can be used.

  Examples of the negative electrode current collector 62 constituting the negative electrode sheet 60 include copper foil. The negative electrode active material layer 64 includes a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon can be used. The negative electrode active material layer 64 can further include a binder, a thickener, and the like. As the binder, for example, styrene butadiene rubber (SBR) can be used. As the thickener, for example, carboxymethyl cellulose (CMC) can be used.

  As the separator 70, various microporous sheets similar to those conventionally used in lithium ion secondary batteries can be used. For example, a microporous resin made of a resin such as polyethylene (PE) or polypropylene (PP). Sheet. Such a microporous resin sheet may have a single-layer structure or a multilayer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 70 may include a heat resistant layer (HRL).

The non-aqueous electrolyte can be the same as that of a conventional lithium ion secondary battery, and typically, an organic solvent (non-aqueous solvent) containing a supporting salt can be used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters and ethers can be used. Of these, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be preferably used. Alternatively, a fluorine-based solvent such as fluorinated carbonate such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), or trifluorodimethyl carbonate (TFDMC) is preferably used. be able to. Such a non-aqueous solvent can be used individually by 1 type or in combination of 2 or more types as appropriate. As the supporting salt, for example, a lithium salt such as LiPF ₆ , LiBF ₄ , or LiClO ₄ can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.
The non-aqueous electrolyte is a component other than the above-described non-aqueous solvent and supporting salt, such as a gas generating agent, a film forming agent, a dispersing agent, and a thickening agent, as long as the effects of the present invention are not significantly impaired. Additives can be included.

  The lithium ion secondary battery 100 can be used for various applications. Suitable applications include drive power sources mounted on vehicles such as plug-in hybrid vehicles (PHV), hybrid vehicles (HV), and electric vehicles (EV). The lithium ion secondary battery 100 can also be used in the form of an assembled battery in which a plurality of lithium ion secondary batteries are electrically connected.

  As described above, a rectangular lithium ion secondary battery including a flat wound electrode body has been described as an example. However, the positive electrode material according to the present embodiment can be used for other types of lithium ion secondary batteries according to a known method. For example, the positive electrode material according to the present embodiment can be used to construct a lithium ion secondary battery including a stacked electrode body. Moreover, the positive electrode material according to the present embodiment can be used to construct a cylindrical lithium ion secondary battery, a laminated lithium ion secondary battery, or the like. Further, the positive electrode material according to the present embodiment can be used for non-aqueous electrolyte secondary batteries other than lithium ion secondary batteries according to a known method.

  EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Preparation of positive electrode material>
[PTFE coated cathode material]
Spinel type LiNi _0.5 Mn _1.5 O ₄ was prepared as a positive electrode active material. This was immersed in a dispersion liquid in which PTFE powder was dispersed in NMP, and PTFE was adhered to the positive electrode active material. This was heat-treated at 320 ° C. to obtain a positive electrode material (PTFE-coated positive electrode material) in which the positive electrode active material was coated with PTFE. In addition, PTFE-coated positive electrode materials having various coating thicknesses were obtained by adjusting the amount of PTFE powder dispersed in the dispersion.

[PFA-coated positive electrode material]
Spinel type LiNi _0.5 Mn _1.5 O ₄ was prepared as a positive electrode active material. This was immersed in a dispersion in which PFA powder was dispersed in NMP, and PFA was adhered to the positive electrode active material. This was heat-treated at 300 ° C. to obtain a positive electrode material (PFA-coated positive electrode material) in which the positive electrode active material was coated with PFA. It should be noted that PFA-coated positive electrode materials having various coating thicknesses were obtained by adjusting the amount of PFA powder dispersed in the dispersion.

[PVDF coated cathode material]
Spinel type LiNi _0.5 Mn _1.5 O ₄ was prepared as a positive electrode active material. This was immersed in a dispersion liquid in which PVDF powder was dispersed in NMP, and PVDF was adhered to the positive electrode active material. This was heat-treated at 170 ° C. to obtain a positive electrode material (PVDF-coated positive electrode material) in which the positive electrode active material was coated with PVDF. In addition, PVDF-coated positive electrode materials having various coating thicknesses were obtained by adjusting the amount of PVDF powder dispersed in the dispersion.

[Comparative cathode material]
Spinel-type LiNi _0.5 Mn _1.5 O ₄ was used as a positive electrode material without being coated with a fluororesin.

<Measurement of coverage>
XPS measurement was performed on the prepared positive electrode material. From the detection ratio of Mn and Ni derived from the positive electrode active material and F derived from the fluororesin, the coverage was calculated as coverage (%) = “F” / (“F” + “Mn” + “Ni”) × 100. . The results are shown in Table 1.

<Measurement of coating thickness>
XPS measurement was performed on the prepared positive electrode material. From the XPS measurement result in the depth direction, the depth until the element ratio of F decreases to 50% with respect to the outermost surface (the depth direction from the outermost surface to where the element ratio of F decreases to 50% Dimension) was determined and this was taken as the coating thickness. The results are shown in Table 1 as “Coating Thickness”.

<Production of evaluation lithium-ion secondary battery>
A positive electrode material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder are mixed in a mass ratio of positive electrode material: AB: PVDF = 90: 5: 5. N-methylpyrrolidone (NMP) And a slurry for forming a positive electrode active material layer was prepared. This slurry was applied to one side of an aluminum foil, dried, and then pressed to prepare a positive electrode sheet.
Further, a natural graphite carbon material (C) having an average particle diameter of 20 μm as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, C: SBR: Mixing with ion-exchanged water at a mass ratio of CMC = 98: 1: 1 to prepare a slurry for forming a negative electrode active material layer. This slurry was applied to one side of a copper foil, dried, and then pressed to prepare a negative electrode sheet.
In addition, a separator sheet (porous polyolefin sheet) was prepared.
The produced positive electrode sheet and negative electrode sheet were opposed to each other with a separator sheet interposed therebetween to produce an electrode body.
A current collector was attached to the produced electrode body and housed in a laminate case together with a non-aqueous electrolyte solution to obtain a lithium ion secondary battery for evaluation. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 50:50 was used.

<Activation and initial capacity measurement>
Each of the produced lithium ion secondary batteries was placed in a 25 ° C. environment. The initial charge is a constant current-constant voltage method, and after charging at a current value of 1/3 C to 4.9 V, it is charged at a constant voltage until the current value reaches 1/50 C. did. Thereafter, constant current discharge was performed at a current value of 1/3 C, and the discharge capacity at this time was measured to obtain the initial capacity.

<High temperature cycle characteristics evaluation>
Each lithium ion secondary battery was placed in an environment of 60 ° C., and charging / discharging with one cycle of constant current charging at 2C and constant current discharging at 2C was repeated 200 cycles.
The battery capacity after 200 cycles of charge / discharge was determined by the same method as the initial capacity.
As an index of the high-temperature cycle characteristics, the capacity retention rate (%) was obtained from (battery capacity after 200 cycles of charge / discharge / initial capacity) × 100.
And when the capacity maintenance rate of the lithium ion secondary battery using the comparative positive electrode material not coated with the fluororesin is set to 1, other lithium ion secondary batteries (PTFE-coated positive electrode material, PFA-coated positive electrode material, And the ratio of capacity retention ratios of lithium ion secondary batteries using PVDF-coated positive electrode materials, respectively. The results are shown in Table 1.

  According to the results in Table 1, when the positive electrode active material is coated with a fluororesin and the coverage is 96% or more, and the fluororesin is polytetrafluoroethylene, the coating thickness is 0.009 μm or more. When the fluororesin is a tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, the coating thickness is 0.011 μm or more and 0.165 μm or less, and the fluororesin is polyvinylidene fluoride. In some cases, the capacity retention ratio exceeded 1 when the coating thickness was 0.01 μm or more and 0.098 μm or less. That is, the capacity retention rate was improved.

In the production of a positive electrode material on a LiNi _0.5 Mn _1.5 O ₄ film formed by sputtering spinel-type LiNi _0.5 Mn _1.5 O ₄ on a Si substrate and firing at 650 ° C. A fluororesin film was formed by the same method as that for producing a fluororesin film. Then, the contact angle with a non-aqueous electrolyte in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 50:50 was determined by the 2 / θ method. Asked.

As shown in Table 2, it can be seen that the contact angle increases as the film thickness increases in the region where the film thickness is small. Therefore, it turns out that contact with a non-aqueous electrolyte and a positive electrode active material can be prevented effectively when a fluororesin becomes more than a specific film thickness. However, when the results in Table 1 and Table 2 are summarized, it is understood that the combination of the type of fluororesin and the specific film thickness range contributes most to the suppression of capacity deterioration.
Therefore, by using the positive electrode material according to the present embodiment, the direct contact between the positive electrode active material and the non-aqueous electrolyte can be effectively suppressed, whereby the oxidation between the positive electrode active material and the non-aqueous electrolyte is performed. It can be seen that the reaction can be suppressed, and as a result, the capacity deterioration of the secondary battery can be suppressed.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

20 wound electrode body 30 battery case 36 safety valve 42 positive electrode terminal 42a positive electrode current collector plate 44 negative electrode terminal 44a negative electrode current collector plate 50 positive electrode sheet (positive electrode)
52 Positive Current Collector 52a Positive Electrode Active Material Layer Non-Forming Portion 54 Positive Electrode Active Material Layer 60 Negative Electrode Sheet (Negative Electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formed portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 Lithium ion secondary battery

Claims (1)

A positive electrode active material;
A fluororesin that coats the positive electrode active material,
The coverage of the fluororesin with respect to the positive electrode active material is 96% or more,
The fluororesin is polytetrafluoroethylene, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, or polyvinylidene fluoride,
When the fluororesin is polytetrafluoroethylene, the coating thickness is 0.009 μm or more and 0.199 μm or less,
When the fluororesin is a tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, the coating thickness is 0.011 μm or more and 0.165 μm or less,
When the fluororesin is polyvinylidene fluoride, the coating thickness is 0.01 μm or more and 0.098 μm or less.
A positive electrode material characterized by the above.

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