JP7296042B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery.

In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries and sodium ion secondary batteries have been used as so-called portable power sources for personal computers, portable terminals, etc., and power sources for driving vehicles. In particular, lithium-ion batteries, which are lightweight and provide high energy density, are preferably used as high-output power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV).

A non-aqueous electrolyte secondary battery typically includes an electrode body in which a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer are laminated with a porous separator interposed therebetween; and an electrolyte. In such a secondary battery, charge/discharge is performed by the charge carriers (for example, lithium ions) in the electrolytic solution passing between the electrodes through the pores of the separator. Here, Patent Document 1 discloses a lithium-ion secondary battery separator made of a non-woven fabric including a predetermined fine fiber layer and a fiber reinforcing layer. It is described that this separator has a low internal resistance and is suitable for high rate discharge.

JP 2007-048533 A JP 2013-251259 A

However, separators made of non-woven fabric do not essentially have a so-called shutdown function, which softens or melts when the temperature of the secondary battery rises due to overcharging or the like, and closes the pores included in the separator. Therefore, the use of such a separator in a secondary battery for high-rate charge/discharge applications or applications requiring high safety poses problems in terms of overcharge resistance and safety.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a non-aqueous electrolyte secondary battery with improved high-rate characteristics and overcharge resistance.

The non-aqueous electrolyte secondary battery disclosed herein includes an electrode body formed by winding a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The separator includes a substrate layer having a porous structure, and a particle layer containing resin particles and provided on the surface of the substrate layer. The particle layer includes a diffusion path connecting one end and the other end in a direction along the winding axis, which is the winding axis, and either the base layer or the particle layer is , has a shut-down function that melts or softens when heated to a predetermined temperature or higher to block electrolyte migration.

According to the above configuration, since at least one of the substrate layer and the particle layer has a shutdown function, when the battery temperature rises due to overcharging, the layer quickly shuts down, and the battery chemical reaction of the secondary battery starts at an early stage. can be stopped safely. Further, the base material of the separator is generally manufactured by stretching, and has high electrolyte permeability in the thickness direction, but insufficient electrolyte permeability in the surface direction. On the other hand, the particle layer with the above structure has a diffusion path for electrolyte movement, and when charged and discharged at a high rate, the electrode absorbs and releases electrolyte not only in the thickness direction of the separator but also in the planar direction. can do. This realizes a secondary battery in which an increase in resistance is suppressed even when it is repeatedly charged and discharged at a high rate. In other words, a nonaqueous electrolyte secondary battery with improved high rate characteristics and overcharge resistance is provided.

In Patent Document 2, a coating layer composed of 1 to 50% by weight of fine organic particles of 3 to 10 nm and an emulsion type binder polymer of 0.01 to 0.5 μm is provided on both sides of a porous substrate. is disclosed. It is described that the binder polymer enhances the binding strength of the coating layer in this separator, and enables stable binding between the separator and the electrode. However, Patent Literature 2 does not disclose anything about providing a layer containing organic particles with diffusion paths for movement of charge carriers. In this point, the coating layer of Patent Document 2 and the resin layer disclosed here are clearly distinguished.

1 is a cutaway perspective view schematically showing the configuration of a lithium ion battery according to one embodiment; FIG. FIG. 2 is a partially exploded view illustrating the configuration of a wound electrode body according to one embodiment; It is a cross-sectional schematic diagram explaining the structure of the separator which concerns on one Embodiment. FIG. 2 is a plan view of a main part for explaining the structure of a separator according to one embodiment;

An embodiment of the present invention will be described below with reference to the drawings as appropriate. Matters other than those specifically mentioned in the present specification that are necessary for carrying out the present invention (for example, the configuration and operation of the non-aqueous electrolyte secondary battery that do not characterize the present invention) It can be grasped as a design matter of a person skilled in the art based on the conventional technology in the field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. Further, in the drawings below, members and parts having the same function are given the same reference numerals, and redundant description may be omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each drawing do not necessarily reflect the actual dimensional relationships. Regarding the present technology, the notation “A to B” indicating a numerical range means “A or more and B or less”.

In this technology, "secondary battery" generally refers to a storage device that can be repeatedly charged and discharged, and includes so-called storage batteries such as lithium ion batteries, sodium ion secondary batteries, and lithium polymer batteries, as well as storage elements such as electric double layer capacitors. It is a term to A "non-aqueous electrolyte secondary battery" is a secondary battery that realizes charging and discharging using a non-aqueous electrolyte as a charge carrier. The term "active material" refers to a material that can reversibly occlude and release chemical species that serve as charge carriers in a secondary battery. The present technology will be described below, taking as an example the case where the non-aqueous electrolyte secondary battery is a lithium ion secondary battery.

[Lithium ion secondary battery]
FIG. 1 is a cutaway perspective view showing the configuration of a non-aqueous electrolyte secondary battery (hereinafter simply referred to as "secondary battery", etc.) 1 according to one embodiment. This lithium ion battery 1 is constructed by housing a wound electrode body 20 together with a non-aqueous electrolyte (not shown) in a battery case 10 . FIG. 2 is a partially developed view for explaining the configuration of the wound electrode assembly 20. FIG. FIG. 3 is a diagram schematically showing a partial cross section of the separator, and FIG. 4 is a plan view of the separator. W in the drawing indicates the width direction of the battery case 10 and the wound electrode body 20 , which is the direction that coincides with the winding axis WL of the wound electrode body 20 . The wound electrode assembly 20 includes a positive electrode 30 , a negative electrode 40 and a separator 50 . Each element will be described below.

The positive electrode 30 typically includes a positive electrode current collector 32 and porous positive electrode active material layers 34 formed on both sides thereof. The pores of the positive electrode active material layer 34 are impregnated with a non-aqueous electrolyte. Metal foil such as aluminum foil is preferably used for the positive electrode current collector 32 , for example. The positive electrode current collector 32 has a current collecting portion in which the current collector is exposed without the positive electrode active material layer 34 being provided at one end in the width direction. The positive electrode active material layer 34 contains a granular positive electrode active material. As the positive electrode active material, for example, lithium nickel cobalt manganese composite oxide (eg, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ etc.) capable of reversibly intercalating and deintercalating lithium ions, lithium nickel Lithium transition metals such as composite oxides (e.g. LiNiO2 _etc. ), lithium cobalt composite oxides ( _{e.g. LiCoO2} etc.), lithium nickel manganese composite oxides (e.g. _{LiNi0.5Mn1.5O4 etc.} ) One kind or a combination of two or more kinds of composite oxides is used. The positive electrode active material layer 34 includes, in addition to the positive electrode active material, a conductive material such as acetylene black (AB), an acrylic polymer, polyvinylidene fluoride (PVDF), and styrene-butadiene rubber (SBR) for binding them together. can contain a binder such as In addition to these, the positive electrode active material layer 34 may contain a film forming material such as trilithium phosphate (LPO) for forming a film on the surface of the negative electrode.

The average particle diameter (D ₅₀ ) of the positive electrode active material is not particularly limited, and is typically 0.5 μm or more, preferably 1 μm or more, 2 μm or more, for example 3 μm or more, and for example 20 μm or less, typically 15 μm or less. , preferably 12 μm or less, for example 9 μm or less. The proportion of the positive electrode active material in the entire positive electrode active material layer 34 may be approximately 50% by mass or more, typically 60% by mass or more, such as 70% by mass or more, and typically 95% by mass or less, such as It can be 90% by mass or less. The ratio of the conductive material in the positive electrode active material layer 34 is typically 0.1 parts by mass or more, preferably 1 part by mass or more, for example 3 parts by mass or more with respect to 100 parts by mass of the positive electrode active material. is 15 parts by mass or less, preferably 12 parts by mass or less, for example 10 parts by mass or less. The ratio of the binder in the positive electrode active material layer 34 is typically 0.5 parts by mass or more, preferably 1 part by mass or more, for example 2 parts by mass or more with respect to 100 parts by mass of the positive electrode active material. is 10 parts by mass or less, preferably 8 parts by mass or less, for example 5 parts by mass or less. In addition, the thickness of the positive electrode active material layer 34 after pressing (average thickness of one side; hereinafter the same) is typically 10 μm or more, for example 15 μm or more, and typically 50 μm or less, for example 30 μm or less. can be In addition, although the density of the positive electrode active material layer 34 is not limited to this, it is typically 1.5 g/cm ³ or more, such as 2 g/cm ³ or more, and 3 g/cm ³ or less, such as 2.5 g/cm 3 or less. It can be ³ or less. The compressive elastic modulus of the positive electrode 30 in the thickness direction is typically 20 MPa or more, for example, 30 MPa or more and 100 MPa or less.
As used herein, unless otherwise specified, the term "average particle size" refers to the cumulative 50% particle size ( _D50 ) in the volume-based particle size distribution obtained by the laser diffraction scattering method.

The negative electrode 40 can typically include a negative electrode current collector 42 and porous negative electrode active material layers 44 formed on both sides thereof. The pores of the negative electrode active material layer 44 are impregnated with a non-aqueous electrolyte. Metal foil such as copper foil is preferably used for the negative electrode current collector 42 , for example. The negative electrode current collector 42 has a current collecting portion where the current collector is exposed without the negative electrode active material layer 44 being provided at one end in the width direction. The negative electrode active material layer 44 contains a granular negative electrode active material. Examples of the negative electrode active material include carbon-based materials such as graphite carbon and amorphous carbon, silicon, lithium transition metal oxides, and lithium transition metal nitrides, which are capable of reversibly absorbing and desorbing lithium ions. Combinations of species or more are used. The negative electrode active material layer 44 contains, in addition to the negative electrode active material, binders such as polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR) and thickeners such as carboxymethyl cellulose (CMC) for binding them together. can contain

The average particle diameter (D ₅₀ ) of the negative electrode active material particles is not particularly limited, and may be, for example, 0.5 μm or more, preferably 1 μm or more, and more preferably 5 μm or more. Moreover, it may be 30 μm or less, preferably 20 μm or less, and more preferably 15 μm or less. The proportion of the negative electrode active material in the entire negative electrode active material layer 44 is suitably about 50 mass % or more, preferably 90 mass % to 99 mass %, for example, 95 mass % to 99 mass %. When using a binder, the ratio of the binder in the negative electrode active material layer 44 can be, for example, about 0.1 to 5 parts by mass with respect to 100 parts by mass of the negative electrode active material. .5 to 2 parts by mass is suitable. In addition, in the technology disclosed herein, the compressive elastic modulus in the thickness direction of the negative electrode 40 is higher than that of a negative electrode in a general secondary battery for high-rate applications. That is, the compressive elastic modulus in the thickness direction of the negative electrode 40 is 38 MPa or higher, typically 40 MPa or higher, 50 MPa or higher, or 60 MPa or higher, for example, 70 MPa or higher. The compressive elastic modulus in the thickness direction of the negative electrode 40 is not particularly limited, and may be, for example, 100 MPa or less, 90 MPa or less, or 88 MPa or less. Thereby, a battery with a high volumetric capacity ratio can be realized. The thickness of the negative electrode active material layer 44 after pressing (average thickness of one side; hereinafter the same) is, for example, 20 μm or more, typically 40 μm or more, and for example, 50 μm or more from the viewpoint of high capacity. Good. The average thickness of the negative electrode active material layer 44 may be, for example, 100 μm or less, typically 80 μm or less, for example 65 μm or less. The density of the negative electrode active material layer 44 is not particularly limited, but is, for example, 0.8 g/cm ³ or more, typically 1.0 g/cm ³ or more, and 1.5 g/cm ³ or less, typically may be 1.4 g/cm ³ or less, such as 1.2 g/cm ³ or less.

Separator 50 is a porous member that electrically insulates the positive and negative electrodes and allows charge carrier transfer between the positive and negative electrodes. The pores of the separator 50 are also impregnated with the non-aqueous electrolyte. The separator 50 includes a base layer 52 having a porous structure and a particle layer 54 containing resin particles and provided on the surface of the base layer 52 .

The base material layer 52 is suitably composed of a microporous sheet conventionally used as a resin separator for this type of secondary battery. The base material layer 52 is typically produced by stretching a plasticizer-added film produced by a dry or wet process to form fine voids (pores) through which the electrolyte passes. The pores are formed as pores continuous in the thickness direction of the base material layer 52 or pores penetrating through the substrate layer 52 by both the uniaxial stretching method and the biaxial stretching method. As a result, the base material layer 52 tends to be excellent in electrolyte permeability in the thickness direction, but poor in electrolyte permeability in the plane direction.

The particle layer 54 contains resin particles. The particle layer 54 is typically formed by binding resin particles with a binder and forming a layer on the base material layer 52 . Although the average particle diameter of the resin particles forming the particle layer 54 is not strictly limited, it is, for example, 50 nm or more, typically 100 nmμm or more, and preferably 300 nm or more from the viewpoint of high porosity. The average particle size of the resin particles may be, for example, 1 μm or less, typically 800 nm or less, such as 650 nm or less. As a result, the porosity of the particle layer can be adjusted and the diffusion paths can be formed favorably. As the binder used for the particle layer 54, a resin binder that exhibits suitable binding properties to the resin particles and has resistance to battery chemical reactions can be used without particular limitation. As such a binder, for example, the binder used for the positive electrode active material layer 34 and the negative electrode active material layer 44 (for example, PVdF and acrylic binder) can be used as appropriate. The proportion of the binder in the particle layer 54 can be, for example, about 0.1 to 5 parts by mass with respect to 100 parts by mass of the resin particles, and for example, 0.5 to 2 parts by mass is suitable. is.

This particle layer 54 is also characterized by including diffusion paths 54a. This diffusion path 54 a is a void formed by the absence of continuous resin particles on the substrate layer 52 . The diffusion path 54a is formed to connect one end and the other end of the separator 50 in the direction along the winding axis WL, which is the axis of winding of the wound electrode body 20, which will be described later. With this configuration, the electrolyte can easily move from one end of the wound electrode body 20 to the other end through the diffusion path 54a. In other words, the electrolyte can easily move in the planar direction of the separator 50 . As a result, for example, even when the secondary battery 1 is subjected to high-rate charging and discharging in which a large amount of electric charge is charged and discharged at once, the separator 50 is provided between the positive electrode 30 and the negative electrode 50. Therefore, the electrolyte can move not only in the thickness direction but also in the planar direction. As a result, the electrolyte can move smoothly between the positive electrode 30 and the negative electrode 50, and an increase in resistance during high-rate charging/discharging can be suppressed.

In addition, in the secondary battery 1 that is charged and discharged at a high rate, the positive electrode or the negative electrode repeats large expansion and contraction due to repeated charging and discharging. Since the deformation of the electrode body 20 is restricted by the battery case 10, the non-aqueous electrolyte existing in the voids of the electrode body 20 is pushed out of the electrode body 20, and thereafter cannot be successfully returned to the electrode body 20. can occur. As a result, there is a problem that the salt concentration of the non-aqueous electrolyte in the electrode body 20 becomes uneven and the high rate resistance increases. Here, the separator 50 having the particle layer 54 described above has a diffusion path 54a extending from one end to the other end in the width direction (direction along the winding axis WL). As a result, even when the secondary battery 1 is repeatedly charged and discharged at a high rate, a configuration is realized in which the non-aqueous electrolytic solution easily returns into the electrode body 20 through the diffusion path 54a. As a result, the occurrence of uneven salt concentration in the electrode assembly 20 can be suppressed.

The diffusion paths 54 a are voids formed by the resin particles not continuously present on the substrate layer 52 in the particle layer 54 . The diffusion path 54a may be configured, for example, by intentionally providing a region where no layer of resin particles is formed on the surface of the base material layer 52 . As shown in FIG. 4, the diffusion path 54a may be provided substantially linearly along the width direction of the separator 50, or may be configured by a curved line. Although the width of the diffusion path 54a is not particularly limited, it may be, for example, 0.5 μm or more, 1 μm or more, and 10 μm or less, for example, 5 μm or less. The diffusion path 54a may be provided substantially linearly along the width direction of the separator 50, or may be configured by a curved line. The particle layer 54 may have a striped pattern by regularly providing the diffusion paths 54a.

The total thickness of the separator 50 may be 15 μm or more, preferably 20 μm or more, and may be, for example, 23 μm or more. By setting the total thickness to a predetermined value or more, the mechanical strength (for example, tensile strength and puncture strength) and durability of the separator 50 can be improved. However, an excessively thick separator 50 is not preferable in that it causes a decrease in volumetric capacity and an increase in internal resistance. The total thickness of the separator may be, for example, 30 μm or less, such as 25 μm or less. By setting the total thickness to a predetermined value or less, the ion conductivity can be increased, and the high rate charge/discharge characteristics of the secondary battery 1 can be improved. When the separator is composed of the substrate layer 52 and the particle layer 54, the thickness of the particle layer 54 is preferably approximately 2 μm or more, for example 4 μm or more, and approximately 10 μm or less, for example 8 μm or less. As a result, it is possible to realize good ionic conductivity and ensure properties such as mechanical strength and heat resistance of the separator 50 . As a result, a better balance can be achieved between battery characteristics during normal use and overcharge resistance.

Note that the thickness of the separator 50 and each layer can be actually measured, for example, by observing with an electron microscope (for example, a scanning electron microscope (SEM)). For example, in the separator 50 and each layer, the thickness can be measured at multiple points and the arithmetic average value can be adopted.

Although the porosity of the particle layer 54 is not strictly limited, it is generally 1.1 times or more, preferably 1.2 times or more, for example, 1.25 times or more or 1.3 times the porosity of the base material layer 52. Larger is better. For example, the porosity of the base material layer 52 may be approximately 35% or more, typically 40% or more, for example 45% or more, and approximately 55% or less, typically 50% or less. The porosity (excluding diffusion paths) of the particle layer 54 is approximately 55% or more, typically 60% or more, for example 65% or more, and approximately 75% or less, typically 70% or less. For example, it may be 68% or less. In addition, the porosity of the entire separator 50 is approximately 50% or more, typically 55% or more, for example 60% or more, and approximately 70% or less, typically 68% or less, for example 65% or less. Good. As a result, during normal use of the secondary battery 1, the permeability and moisture retention of the electrolyte are enhanced. As a result, excellent ionic conductivity can be achieved. Moreover, the mechanical strength and durability of the separator 50 can be ensured.
In addition, the value measured by the Archimedes method can be used as the porosity.

One of the base material layer 52 and the particle layer 54 that constitute the separator 50 has a shutdown function that softens or melts when the battery temperature reaches a predetermined high temperature and closes the pores of the separator 50. is preferred. As a result, when the battery temperature rises due to overcharging or the like, the movement of the electrolyte can be restricted at an early stage, and the battery reaction can be stopped safely. Either the substrate layer 52 or the particle layer 54 may be provided with the shutdown function. By forming one of the substrate layer 52 and the particle layer 54 from a resin having a softening point or a melting point at the shutdown temperature (80 to 130° C., for example), the layer can be provided with a shutdown function. A layer having a shutdown function is hereinafter referred to as a shutdown layer. At this time, it is preferable that the layer (which does not have a shutdown function) is made of a resin that does not soften or melt even at a temperature sufficiently higher than the shutdown temperature (for example, 150 to 180° C.).

The resin constituting the shutdown layer is not particularly limited, but a polyolefin resin such as polyethylene (PE) resin, which has durability against the atmosphere caused by the battery chemical reaction, is suitable. The melting point of the polyethylene-based resin is higher than the upper limit of the normal operating temperature of the secondary battery 1, typically 80° C. or higher, for example, 100° C. or higher. Therefore, during normal use of the secondary battery 1 , the porous structure of the separator 50 is maintained, and a good ionic conduction path can be provided between the positive electrode 30 and the negative electrode 40 . The softening point or melting point of the polyethylene resin is generally 130° C. or lower, typically 120° C. or lower, preferably 110° C. or lower, for example 100° C. or lower. As a result, the shutdown function can be quickly activated before the internal temperature of the secondary battery 1 rises significantly. In addition, the melting point of the resin can be measured by general differential scanning calorimetry (DSC).

Moreover, the resin constituting the layer that does not exhibit the shutdown function is not particularly limited, but for example, a resin having a melting point higher than that of the polyethylene-based resin can be used. Hereinafter, a layer that does not exhibit a shutdown function may be referred to as a "high melting point resin layer". Examples of such high melting point resins include polyolefin resins such as polypropylene (PP) resins, polyvinyl chloride resins, polyvinyl acetate resins, polyimide resins, polyamide resins, celluloses, and polyether resins. , polyester-based resins, fluorine-based resins, and the like. Further, for example, when a low-melting-point polyethylene resin such as LLDPE, LDPE, or ethylene-propylene copolymer is used as a polyethylene resin for the PE layer, HDPE or ethylene-vinyl acetate is used as the high-melting-point resin for the high-melting resin layer. A high melting point polyethylene resin such as a copolymer can also be used. From the viewpoint of improving mechanical strength (for example, tensile strength and puncture strength) and chemical stability, the high melting point resin layer preferably contains a polyolefin resin. The term "polyolefin-based resin" as used herein refers to all compounds containing a carbonyl unit in the main chain skeleton. In a preferred example, the high melting point resin layer is a porous PP layer containing polypropylene resin. The polypropylene-based resin is a resin containing propylene as the main monomer component (which is the component that accounts for the largest proportion of the monomer components), preferably a resin in which the copolymerization ratio of propylene exceeds 50% by mass. Examples of polypropylene-based resins include propylene homopolymers and ethylene-propylene copolymers (random copolymers, block copolymers) containing propylene as the main monomer. Polyolefin resin, such as polypropylene (PP) resin, which has durability against the atmosphere caused by battery chemical reaction, is preferable.

The melting point of the high melting point resin is not particularly limited as long as it is higher than that of the resin forming the shutdown layer. The melting point of the high melting point resin is generally 5° C. or higher, typically 10° C. or higher, preferably 20° C. or higher, for example, 30° C. or higher, higher than the melting point of the resin contained in the shutdown layer. This makes it easier to maintain the shape of the high-melting-point resin layer when the shutdown layer functions. Therefore, the short circuit between the positive electrode 10 and the negative electrode 20 can be better suppressed. In addition, by including resins having different melting points in the shutdown layer and the high-melting-point resin layer, two-stage shutdown can be achieved. In other words, the high-melting-point resin layer satisfactorily functions as the second shutdown layer, and higher overcharge resistance can be achieved. The melting point of the high melting point resin is generally 100° C. or higher, typically 110° C. or higher, preferably 130° C. or higher, for example 150° C. or higher. Thereby, the heat resistance of the separator 50 can be improved. The melting point of the high melting point resin is generally 200° C. or lower, typically 180° C. or lower, preferably 170° C. or lower, for example 165° C. or lower. Thereby, the high-melting-point resin layer can preferably exhibit its function as the second shutdown layer.

A preferred example is to provide a shutdown function to the particle layer 54 whose sensitivity to battery heat generation is enhanced by increasing the specific surface area. In this case, the resin particles forming the particle layer 54 are preferably made of a resin (for example, PE) having a shutdown function. According to such a configuration, when the separator 50 reaches the shutdown temperature, the particle layer 54 melts quickly, and the melted particle layer 54 flows into the pores of the base material layer 52 to smoothly block the pores. be able to. Further, the base material layer 52 maintains its shape even at the shutdown temperature, and can suitably suppress a short circuit between the positive electrode 30 and the negative electrode 40 .

As described above, the separator 50 has a higher porosity in the particle layer 54 than in the base material layer 52 . According to studies by the present inventors, the negative electrode active material undergoes a larger volume change due to absorption and release of the electrolyte than the positive electrode active material, and the negative electrode active material layer 44 has a lower density than the positive electrode active material layer 34. It is formed. In other words, the positive electrode active material layer 34 has a lower porosity than the negative electrode active material layer 44, and the amount of the non-aqueous electrolyte that can be impregnated into the voids is reduced. Therefore, although not limited to this, the separator 50 is arranged such that the particle layer 54 having a higher porosity faces the positive electrode 30, so that the electrolyte is suitably supplied to the positive electrode 30, resulting in high-rate charging. The resistance can be kept low even when discharging is performed.

Further, as shown in FIG. 2, the electrode body 20 is formed by stacking a long positive electrode 30 and a long negative electrode 40 in a state insulated from each other by two separators 50, and has an oval cross section about the winding axis WL. It is a so-called wound-type electrode body that is wound into a circle. The width W1 of the positive electrode active material layer 34, the width W2 of the negative electrode active material layer 44, and the width W3 of the separator satisfy the relationship W1<W2<W3. The negative electrode active material layer 44 covers the positive electrode active material layer 34 at both ends in the width direction, and the separator 50 covers the negative electrode active material layer 44 at both ends in the width direction. The particle layer 54 of the separator 50 is provided with a diffusion path 54a extending so as to connect both ends in the width direction.

The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte-supporting salt. The types of non-aqueous solvent and electrolyte-supporting salt are not particularly limited, and may be the same as those used in conventional secondary battery electrolytes. Preferred examples of electrolyte-supporting salts are lithium salts such as LiPF ₆ and LiBF ₄ . Suitable examples of non-aqueous solvents are aprotic solvents such as carbonates, esters and ethers. Among them, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC), and these carbonates containing fluorine It preferably contains one or more fluorinated linear or fluorinated cyclic carbonates. The concentration of the lithium salt in the electrolyte can be, for example, 0.8-1.3 mol/L. The non-aqueous electrolyte may also contain additives such as a film-forming agent and an overcharge inhibitor.

The battery case 10 includes a case body 11 having an opening on one surface and a lid member 12 that seals the opening. The case main body 11 of this example has a flat bottomed prismatic shape. The lid member 12 is configured to airtightly seal the openings formed at the upper ends of the wide and narrow side surfaces of the case body 11 . The lid member 12 is provided with a positive electrode external terminal 38, a negative electrode external terminal 48, an injection hole, and a safety valve. The battery case 10 is preferably made of, but not limited to, metals such as iron, copper, aluminum, titanium, and alloys containing these (eg, steel), high-strength resin, or the like. The positive external terminal 38 and the negative external terminal 48 are connected to a positive current collecting member 38 a and a negative current collecting member 48 a provided inside the case of the lid member 12 . The positive current collecting member 38a and the negative current collecting member 48a are connected to the positive and negative current collecting portions. As a result, electrical energy can be charged into the electrode assembly 20 or extracted from the electrode assembly 20 through the positive electrode external terminal 38 and the negative electrode external terminal 48 .

The lithium ion battery disclosed herein can be used for various purposes, but compared to conventional products, it has a higher volumetric capacity rate, and can be repeatedly charged and discharged at a high rate with low resistance and enhanced overcharge resistance. can be realized. In other words, such excellent battery performance and reliability (including safety such as thermal stability during overcharge) can be achieved at a high level. Therefore, by taking advantage of such characteristics, it can be preferably used in applications requiring high energy density, high input/output density, and applications requiring high reliability. Such uses include, for example, drive power supplies mounted in vehicles such as plug-in hybrid vehicles, hybrid vehicles, and electric vehicles. Such secondary batteries can typically be used in the form of an assembled battery in which a plurality of secondary batteries are connected in series and/or in parallel.

Several examples relating to the present invention are described below, but the present invention is not intended to be limited to those shown in these specific examples.

[Construction of secondary battery for evaluation]
(Example 1)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) powder as a positive electrode active material, trilithium phosphate (LPO) powder, acetylene black (AB) as a conductive material, and polyfluoride as a binder. Vinylidene chloride (PVdF) was first blended at a mass ratio of LNCM: AB: PVdF = 87: 10: 3 and the content of LPO in the positive electrode active material layer was 8.8 mass%, A positive electrode slurry was prepared by dispersing and mixing in N-methylpyrrolidone (NMP). This slurry was applied to both sides of a band-shaped aluminum foil, leaving a non-coated portion along one end in the width direction, and the remaining region was dried, followed by roll pressing to prepare a positive electrode sheet.

Graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at a mass ratio of C:SBR:CMC=98:1:1. A negative electrode paste was prepared by blending and kneading with deionized water. Then, the prepared negative electrode paste is applied to both sides of the strip-shaped copper foil, leaving a non-coated portion along one end in the width direction, and the remaining region is dried, and then roll-pressed to form a negative electrode active material layer. A negative electrode was obtained.

A porous PP sheet made of polypropylene resin having a thickness of 13 μm prepared by stretching is used as a substrate layer, and polyethylene resin particles (PE particles) having an average particle size of about 0.5 μm are coated on one side of the substrate layer. A separator having a two-layer structure (Example 1) was prepared by forming a PE particle layer having a thickness of 7 μm. The PE particle layer was formed by coating the surface of the substrate layer with a PE particle slurry containing PE particles and an acrylic resin binder at a mass ratio of 90:10. The PE particle slurry is applied such that portions coated with the PE particle slurry and portions not coated with the PE particle slurry extend in the width direction and are alternately arranged in the length direction to form stripes. bottom. The porosity of this separator measured by the Archimedes method was 60%.

The prepared positive electrode and negative electrode are layered so as to be insulated from each other in the order of separator-negative electrode-separator-positive electrode from the bottom with two separators interposed therebetween to form a laminate, and then wound to form a wound electrode. constructed the body. At this time, the orientation of the separator was adjusted so that all of the PE particle layers of the separator were in contact with the positive electrode. Next, the prepared electrode body is housed in a battery case, and the electrode body is heated and dried together with the case. was sealed to obtain a secondary battery for evaluation of Example 1. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of EC:EMC:DMC=3:3:4 is supported. A solution obtained by dissolving LiPF ₆ as a salt at a concentration of 1 mol/L was used.

(Example 2)
As a base material layer of the separator, a porous PP sheet made of a polypropylene resin having a thickness of 16 μm and produced by stretching was used. A PE particle layer was formed on one side of the substrate layer in the same manner as in Example 1, except that the thickness was 4 μm. The porosity of this separator measured by the Archimedes method was 57%. A secondary battery for evaluation of Example 2 was prepared in the same manner as in Example 1 except that the separator of Example 2 was used.
(Example 3)
As a base material layer of the separator, a porous PP sheet made of a polypropylene-based resin and having a thickness of 10 μm and produced by stretching was used. A PE particle layer was formed on one side of the substrate layer in the same manner as in Example 1, except that the thickness was 10 μm. The porosity of this separator measured by the Archimedes method was 66%. A secondary battery for evaluation of Example 3 was prepared in the same manner as in Example 1 except that the separator of Example 3 was used.
(Example 4)
A secondary battery for evaluation of Example 4 was prepared in the same manner as in Example 1 except that the same two separators as in Example 1 were used, and the orientation of the separators was adjusted so that the PE particle layers of the separators were both in contact with the negative electrode. bottom.

(Example 5)
Instead of the separator of Example 1, the separator of Example 5 made of non-woven fabric with a thickness of 20 μm was used. The porosity of this separator measured by the Archimedes method was 62%. A secondary battery for evaluation of Example 5 was prepared in the same manner as in Example 1 except that the separator of Example 5 was used.
(Example 6)
Instead of the separator of Example 1, a PP/PE/PP three-layer separator in which both sides of a porous PE sheet made of a polyethylene-based resin and made by stretching are sandwiched between porous PP sheets made of a polypropylene-based resin. was used. The porosity of this separator measured by the Archimedes method was 48%. A secondary battery for evaluation of Example 6 was prepared in the same manner as in Example 1 except that the separator of Example 6 was used.

[conditioning]
The secondary battery for evaluation of each example was subjected to the following conditioning. That is, in an environment of 25 ° C., constant current (CC) charging to 4.1 V at 1 C and constant voltage (CV) charging until the current value becomes 1/50 C, so that the state of charge: SOC) was fully charged (SOC 100%). Then, an initial aging treatment was performed by holding at 60° C. for 24 hours to form a coating on the surface of the negative electrode active material layer. After that, the battery was CC-discharged at a rate of 1 C to 3.0 V, and the state of charge at this time was set to 0% SOC. Note that 1C is a current value that can charge the battery capacity (Ah) predicted from the theoretical capacity of the active material in one hour.

[High rate cycle test]
For the battery of each example after conditioning, the initial IV resistance was first measured. That is, the initial resistance value (IV resistance) was obtained from the slope of the discharge curve when the battery of each example adjusted to SOC 56% in an environment of 25° C. was discharged at 130 A for 10 seconds. Next, the battery whose state of charge has been adjusted to SOC 40% in an environment of 25 ° C. is charged at 125 A for 10 seconds, followed by a rest period of 5 seconds, discharged at 10 A for 125 seconds, and rested for 5 seconds. 2000 cycles of high-rate charge/discharge cycles were performed. Then, the IV resistance after the cycle was measured by the above method, and the resistance increase rate was calculated based on the following formula: Resistance increase rate (%)=Resistance after cycle/Initial resistance×100;

[Evaluation of overcharge resistance]
After the conditioning, the battery of each example was overcharged in a temperature environment of 60°C. That is, the battery of each example adjusted to an SOC of 56% in an environment of 25°C was stabilized in an environment of 60°C, and then charged at constant current (CC) at 190A to 10V. The battery temperature at this time was observed, and the results are shown in the column of "overcharge resistance" in Table 1. In addition, the evaluation of "overcharge resistance" is as follows. It means that the battery temperature at the time exceeds 200°C.

[evaluation]
As shown in Example 5, the separator made of non-woven fabric has excellent electrolyte permeability in both the thickness direction and the surface direction, so that the increase in resistance is suppressed even when charging and discharging are repeatedly performed at a high rate. I found out. However, non-woven fabric separators do not have the ability to shut down when overcharged. Therefore, it was confirmed that the battery temperature easily rises due to overcharging, and the heat generation of the battery exceeds 200°C. Therefore, it can be said that this separator is not suitable for overcharging batteries from the viewpoint of safety.

On the other hand, in the PP/PE/PP three-layer separator of Example 6, the PE layer in the middle exhibits a shutdown function, and when the battery temperature rises due to overcharging, it melts quickly to prevent pores in the PP layer. . As a result, further overcharging is suppressed, the heat generation of the battery is stopped at a low temperature stage of 150° C. or less, and it has been found that the overcharge resistance is excellent. However, the separator of Example 6 is produced by stretching and has poor mobility of the electrolyte in the plane direction. As a result, it was found that the resistance increased significantly when charging and discharging were repeatedly performed at a high rate. Although this separator is excellent in overcharge resistance, it can be said that there is room for further study from the viewpoint of increased resistance during high-rate cycles.

On the other hand, the batteries using the separators of Examples 1 to 4 have excellent overcharge resistance, and the resistance increase rate during high-rate charging and discharging is suppressed lower than the battery of Example 6, and both overcharge resistance and high-rate characteristics are improved. It turned out to be excellent. This is believed to be due to the following reasons.
That is, the separators of Examples 1 to 4 have a shutdown function because the particle layer is composed of PE particles, and can quickly stop charging and discharging when overcharged, thereby suppressing further heat generation. In addition, although the base material layer manufactured by stretching is usually excellent in electrolyte permeability in the thickness direction, it has low electrolyte permeability in the plane direction, and when charging and discharging at a high rate, the electrode can absorb electrolyte from a wide range. storage and release is prevented. However, in the separators of Examples 1 to 4, the particle layer is composed of PE particles, and the diffusion layer is formed along the width direction of the separator. As a result, the mobility of the electrolyte not only in the thickness direction but also in the plane direction of the separator is enhanced, and the electrode can absorb and release the electrolyte not only in the thickness direction but also in the plane direction. As a result, high-rate charging and discharging can be performed with low resistance.

However, as shown in Example 3, when the thickness of the PE particle layer is too large, the porosity becomes too high, and when the PE particle layer shuts down during overcharging, the amount of shrinkage becomes too large, and the resistance tends to be slightly inferior. can be seen. Therefore, when the particle layer has a shutdown function, it can be said that the particle layer should be 50% or less of the thickness of the separator.
Also, from a comparison between Examples 1 and 4, it was found that the increase in resistance during high-rate charging/discharging can be suppressed more effectively when the bead layer is placed in contact with the positive electrode than in the case where the bead layer is placed in contact with the negative electrode. . This is related to the fact that, in general, the positive electrode active material layer having a small volume expansion due to charging and discharging is formed at a higher density than the negative electrode active material layer having a large volume expansion due to charging and discharging. That is, it is difficult for a high-density positive electrode to hold a non-aqueous electrolyte. Therefore, it is considered that by arranging the particle layer with a high porosity on the positive electrode side, a large amount of electrolyte can be more smoothly supplied to the positive electrode active material layer, and an increase in resistance can be effectively suppressed.

From the above results, it was found that by providing a particle layer having a diffusion path in the base material layer of the separator, both high rate characteristics and overcharge resistance can be favorably achieved. It was also found that by arranging this particle layer on the side facing the positive electrode between the positive electrode and the negative electrode, the increase in resistance during high-rate charging/discharging can be effectively reduced. Also, in order to improve the safety during overcharge, it is preferable to set the thickness of the particle layer to about 50% or less, or suppress the porosity of the entire separator to about 66% or less.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

1 Secondary Battery 10 Battery Case 20 Wound Electrode Body 30 Positive Electrode 40 Negative Electrode 50 Separator 52 Base Material Layer 54 Particle Layer 54a Diffusion Path

Claims (1)

an electrode body in which a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode are wound; and a non-aqueous electrolyte,
The separator includes a substrate layer having a porous structure, a particle layer containing resin particles provided on the surface of the substrate layer,
with
The particle layer includes a linear diffusion path having a width of 0.5 μm or more and 10 μm or less connecting one end and the other end in the direction along the winding axis, which is the winding axis,
The base material layer has a porosity of 35% or more and 55% or less, the particle layer has a porosity of 55% or more and 75% or less, and the entire separator has a porosity of 50% or more and 70% or less,
A non-aqueous electrolyte secondary battery, wherein one of the base material layer and the particle layer melts or softens when heated to a predetermined temperature or higher, and has a shutdown function to block movement of the electrolyte.

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