JP7228113B2 - 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 secondary batteries have been used as portable power sources for personal computers, mobile terminals, etc., and for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). It is suitable for use as a power source for electric appliances.

In a non-aqueous electrolyte secondary battery, a technique of adding trilithium phosphate (Li ₃ PO ₄ ) to a positive electrode active material layer is known (see, for example, Patent Documents 1 and 2). Li ₃ PO ₄ has a function of capturing acid (for example, hydrogen fluoride (HF)) generated by decomposition of the non-aqueous electrolyte, and components reacting with the acid form a film on the surface of the negative electrode active material. Patent Document 1 describes that this function can improve the durability of the non-aqueous electrolyte secondary battery, and Patent Document 2 describes that heat generation during overcharging can be suppressed. Are listed.

Japanese Patent Application Laid-Open No. 2014-103098 JP 2018-98141 A

However, as a result of extensive studies by the present inventors, it was found that in a conventional non-aqueous electrolyte secondary battery in which Li ₃ PO ₄ was added to the positive electrode active material layer, the performance of suppressing heat generation during overcharge and deterioration during high-temperature storage It was found that there is room for improvement in terms of resistance.

Therefore, the present invention provides a non-aqueous electrolyte secondary battery in which Li ₃ PO ₄ is added to a positive electrode active material layer, which is excellent in both heat generation suppression performance during overcharge and deterioration resistance during high-temperature storage. An object of the present invention is to provide an electrolyte secondary battery.

A non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material, Li ₃ PO ₄ , a conductive material, and a binder . The positive electrode active material is a lithium-nickel-manganese-cobalt-based composite oxide having a layered structure, and the content of the positive electrode active material in the positive electrode active material layer is 70% by mass or more. The conductive material is acetylene black, and the content of the conductive material in the positive electrode active material layer is 3% by mass or more and 13% by mass or less. The binder is polyvinylidene fluoride, and the content of the binder in the positive electrode active material layer is 2% by mass or more and 10% by mass or less. The negative electrode has a negative electrode active material layer containing a negative electrode active material, and the negative electrode active material is natural graphite. When the particle diameter at which the cumulative value in the volume frequency particle size distribution measurement by the laser diffraction scattering method is 10% is D10, the particle diameter at which 50% is D50, and the particle diameter at which 90% is D90, the following formula (1) is 0.28 or more and 0.4 or less.
[{D90 (μm) of positive electrode active material − D10 (μm) of positive electrode active material}/D50 (μm) of positive electrode active material]/{specific surface area of Li ₃ PO ₄ (m ² /g)×Li ₃ PO ₄ D50 (μm) of × content ratio of Li ₃ PO ₄ with respect to positive electrode active material (% by mass)} (1)
According to such a configuration, the non-aqueous electrolyte secondary battery in which Li ₃ PO ₄ is added to the positive electrode active material layer has both heat generation suppression performance during overcharge and deterioration resistance during high temperature storage. An excellent non-aqueous electrolyte secondary battery can be provided.

1 is a cross-sectional view schematically showing the internal structure of a lithium secondary battery according to one embodiment of the present invention; FIG. 1 is a schematic diagram showing the configuration of a wound electrode assembly of a lithium secondary battery according to one embodiment of the present invention; FIG. 4 is a graph showing the relationship between the above formula (1) and the calorific value (relative value) studied in Examples. 4 is a graph showing the relationship between the above formula (1) and the capacity retention rate (relative value) during high-temperature storage, studied in Examples.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments according to the present invention will be described with reference to the drawings. Matters other than those specifically mentioned in this specification that are necessary for the implementation of the present invention (for example, the general configuration and manufacturing process of non-aqueous electrolyte secondary batteries that do not characterize the present invention ) can be grasped as a design matter of a person skilled in the art based on the prior art 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 following drawings, members and portions having the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

In this specification, the term "secondary battery" generally refers to an electricity storage device that can be repeatedly charged and discharged, and includes so-called storage batteries and electricity storage elements such as electric double layer capacitors.
In addition, a "non-aqueous electrolyte secondary battery" includes a non-aqueous electrolyte (typically, a non-aqueous electrolyte containing a supporting electrolyte in a non-aqueous solvent) and uses lithium ions as charge carriers. , a secondary battery in which charging and discharging are realized by the movement of electric charges accompanying lithium ions between the positive and negative electrodes.

Hereinafter, the present invention will be described in detail by taking as an example a flat prismatic non-aqueous electrolyte secondary battery having a flat wound electrode assembly and a flat battery case. It is not intended to be limited to what has been described.

The lithium secondary battery 100 shown in FIG. 1 is constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (that is, an outer container) 30. It is a sealed battery that is 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 above a predetermined level. there is Further, the battery case 30 is provided with an injection port (not shown) for injecting a non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive collector plate 42a. The negative terminal 44 is electrically connected to the negative collector plate 44a. As the material of the battery case 30, for example, a metal material such as aluminum that is lightweight and has good thermal conductivity is used.

As shown in FIGS. 1 and 2, the wound electrode body 20 has a positive electrode active material layer 54 formed on one side or both sides (here, both sides) of a long positive electrode current collector 52 along the longitudinal direction. A positive electrode sheet 50 and a negative electrode sheet 60 having a negative electrode active material layer 64 formed on one side or both sides (here, both sides) of a long negative electrode current collector 62 along the longitudinal direction are two long sheets. It has a form in which the separator sheet 70 is interposed and wound in the longitudinal direction. The positive electrode active material layer non-forming portions 52a (i.e., positive electrode active a portion where the positive electrode current collector 52 is exposed without the material layer 54 being formed) and a negative electrode active material layer non-formation portion 62a (that is, a portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed). A positive collector plate 42a and a negative collector plate 44a are respectively joined to the .

Examples of the positive electrode current collector 52 forming the positive electrode sheet 50 include aluminum foil.
The positive electrode active material layer 54 contains a positive electrode active material and Li ₃ PO ₄ .
As the positive electrode active material, a material capable of intercalating and deintercalating lithium ions is used, and one or more of the materials conventionally used in lithium secondary batteries (for example, oxides with a layered structure and oxides with a spinel structure) are used. It can be used without any particular limitation. Examples of positive electrode active materials include lithium-nickel-based composite oxides, lithium-cobalt-based composite oxides, lithium-manganese-based composite oxides, and lithium-nickel-manganese-based composite oxides (eg, LiNi _0.5 Mn _1.5 O ₄ ). , lithium-nickel-manganese-cobalt-based composite oxides (eg, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) and other lithium-containing transition metal oxides. As the positive electrode active material, one having an operating potential of less than 4.3 V (vs. Li/Li ⁺ ) is preferable, and a lithium-nickel-manganese-cobalt-based composite oxide having a layered structure (in particular, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) is more preferred. The content of the positive electrode active material is preferably 70% by mass or more in the positive electrode active material layer 54 (that is, with respect to the total mass of the positive electrode active material layer 54).

The positive electrode active material layer 54 may contain components other than the positive electrode active material and Li ₃ PO ₄ . Examples thereof include conductive materials, binders, and the like.
Carbon black such as acetylene black (AB) and other carbon materials (eg, graphite) can be suitably used as the conductive material. The content of the conductive material in the positive electrode active material layer 54 is preferably 1% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 13% by mass or less.
As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used. The content of the binder in the positive electrode active material layer 54 is preferably 1% by mass or more and 15% by mass or less, more preferably 2% by mass or more and 10% by mass or less.

It is considered that the wider the particle size distribution of the positive electrode active material is, the more the positive electrode active material with low structural stability is mixed in, thereby lowering the thermal stability and increasing the amount of heat generated during overcharge. Therefore, it is considered important to pay attention to the particle size distribution of the positive electrode active material in consideration of the amount of heat generated from the positive electrode active material. On the other hand, in order to exhibit the heat generation suppressing effect of Li ₃ PO ₄ to a higher degree, it is considered important to pay attention to the physical properties of Li ₃ PO ₄ as particles and the addition amount. On the other hand, it is also important to consider the balance with resistance to capacity deterioration during high-temperature storage.

Therefore, in the present embodiment, the relationship between the particle size distribution of the positive electrode active material and the specific surface area, particle size and mixing ratio of Li ₃ PO ₄ is defined. That is, when the particle diameter at which the cumulative value in the volume frequency particle size distribution measurement by the laser diffraction scattering method is 10% is D10, the particle diameter at which the cumulative value is 50% is D50, and the particle diameter at which the cumulative value is 90% is D90, the following formula ( The value of the ratio represented by 1) is 0.28 or more and 0.4 or less.
[{D90 (μm) of positive electrode active material − D10 (μm) of positive electrode active material}/D50 (μm) of positive electrode active material]/{specific surface area of Li ₃ PO ₄ (m ² /g)×Li ₃ PO ₄ D50 (μm) of × content ratio of Li ₃ PO ₄ with respect to positive electrode active material (% by mass)} (1)

As demonstrated by the experimental results described later, when the value of the ratio represented by the above formula (1) is 0.28 or more and 0.4 or less, excellent heat generation suppression performance during overcharging and excellent It is also possible to achieve both resistance to deterioration during storage at high temperatures.
Specifically, when the value of the ratio represented by the above formula (1) is less than 0.28, the resistance to deterioration during high-temperature storage decreases. On the other hand, when the value of the ratio represented by the above formula (1) exceeds 0.4, the heat generation suppressing performance during overcharge deteriorates.

The volume frequency particle size distribution measurement by the laser diffraction scattering method can be performed using a known particle size distribution measuring device based on the laser diffraction scattering method.
The specific surface area of Li ₃ PO ₄ can be obtained by measuring according to a known method. For example, it can be obtained by a physical gas adsorption method.

Examples of the negative electrode current collector 62 forming the negative electrode sheet 60 include copper foil.
The negative electrode active material layer 64 contains a negative electrode active material.
Carbon materials such as graphite, hard carbon, and soft carbon can be used as the negative electrode active material. Graphite may be natural graphite, artificial graphite, or amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

The negative electrode active material layer 64 may contain components other than the active material, such as binders and thickeners. As the binder, for example, styrene-butadiene rubber (SBR) or the like can be used. As a thickening agent, for example, carboxymethyl cellulose (CMC) or the like can be used.

The content of the negative electrode active material in the negative electrode active material layer is preferably 90% by mass or more, more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.5% by mass or more and 2% by mass or less.

Examples of the separator 70 include porous sheets (films) made of resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated 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). A heat-resistant layer (HRL) may be provided on the surface of the separator 70 .

A non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting electrolyte.
As the non-aqueous solvent, organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in electrolytes of general lithium secondary batteries, can be used without particular limitation. can. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), Examples include monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC) and the like. Such non-aqueous solvents can be used singly or in combination of two or more.
Lithium salts such as LiPF ₆ , LiBF ₄ and LiClO ₄ (preferably LiPF ₆ ) can be suitably used as the supporting salt. 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 contains components other than the components described above, such as gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; may contain various additives.

The lithium secondary battery 100 configured as described above suppresses heat generation during overcharging and suppresses capacity deterioration during high-temperature storage.

The lithium secondary battery 100 configured as described above can be used for various purposes. Suitable applications include drive power supplies mounted in vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). The lithium secondary battery 100 can also typically be used in the form of an assembled battery in which a plurality of batteries are connected in series and/or in parallel.

As an example, the prismatic lithium ion secondary battery 100 including the flattened wound electrode body 20 has been described. However, the non-aqueous electrolyte secondary battery disclosed herein can also be configured as a lithium ion secondary battery including a laminated electrode assembly. The non-aqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery or a laminated lithium ion secondary battery. Moreover, the non-aqueous electrolyte secondary battery disclosed herein can also be configured as a non-aqueous electrolyte secondary battery other than the lithium-ion secondary battery.

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.

<Production of lithium-ion secondary battery for evaluation>
A dispersing machine was used to obtain a paste in which acetylene black (AB), PVdF and N-methylpyrrolidone (NMP) as conductive materials were mixed. After adding a mixed powder of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material and Li ₃ PO ₄ to this paste, the solid content was uniformly dispersed, A slurry for forming a positive electrode active material layer was prepared. The slurry for forming the positive electrode active material was prepared so that LNCM:Li ₃ PO ₄ :AB:PVdF=90−x:x:8:2 (mass ratio). This slurry was applied in strips on both sides of a long aluminum foil having a thickness of 15 μm, dried, and then pressed to prepare a positive electrode sheet.
In addition, natural graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were combined in a ratio of C:SBR:CMC=98:1:1. A slurry for forming a negative electrode active material layer was prepared by mixing with ion-exchanged water at a mass ratio. This slurry was coated on both sides of a long copper foil having a thickness of 10 μm in a strip shape, dried, and then pressed to prepare a negative electrode sheet.
As a separator sheet, two porous polyolefin sheets having a thickness of 20 μm and having a three-layer structure of PP/PE/PP were prepared.
The prepared positive electrode sheet, negative electrode sheet, and two prepared separator sheets were superimposed and wound to prepare a wound electrode assembly. At this time, a separator was interposed between the positive electrode sheet and the negative electrode sheet. An electrode terminal was attached to each of the positive electrode sheet and the negative electrode sheet, and these were housed in a battery case having a liquid inlet.
Subsequently, a non-aqueous electrolyte was injected from the liquid inlet of the battery case, and the liquid inlet was airtightly sealed. The non-aqueous electrolyte contains _LiPF6 as a supporting salt in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3. A solution dissolved at a concentration of 1.0 mol/L was used.
In this way, _a lithium ion secondary battery for evaluation is produced, using positive _electrode materials with different particle size distributions; using Li ₃ PO ₄ with different particle sizes and specific surface areas; Several lithium-ion secondary batteries for evaluation having different ratio values represented by the following formula (1) were produced by performing at least one method of changing.
[{D90 (μm) of positive electrode active material+D10 (μm) of positive electrode active material}/D50 (μm) of positive electrode active material]/{specific surface area of Li ₃ PO ₄ (m ² /g)×Li ₃ PO ₄ D50 (μm) of × content ratio of Li ₃ PO ₄ with respect to positive electrode active material (% by mass)} (1)

<Evaluation of calorific value during overcharging>
Each evaluation lithium secondary battery prepared above was subjected to constant current charging to 4.10 V at a current value of 0.3 C as an initial charge, and then constant current discharge to 3.00 V at a current value of 0.3. After that, a thermocouple was attached to the battery case of each evaluation lithium secondary battery to measure the temperature. After that, the battery was charged to 5.1 V and the temperature was measured. Then, the temperature difference (that is, the amount of temperature rise) before and after charging was obtained. As an index of the amount of heat generated, a ratio of measured values of temperature difference of each lithium secondary battery for evaluation was obtained with respect to a reference temperature difference of 100. The results are shown in FIG.

<Evaluation of resistance to capacity deterioration during high-temperature storage>
Each lithium ion secondary battery for evaluation produced as described above was placed in a constant temperature bath at 25°C. Each lithium ion secondary battery for evaluation was subjected to constant current charging to 4.10 V at a current value of 0.3 C as an initial charge, and then constant current discharge to 3.00 V at a current value of 0.3. Next, after constant current charging to 4.10 V with a current value of 0.2 C, constant voltage charging was performed until the current value became 1/50 C, and the battery was in a fully charged state. After that, constant current discharge was performed at a current value of 0.2C to 3.00V. The discharge capacity at this time was measured and taken as the initial capacity.
Each of the lithium ion secondary batteries for evaluation was charged at a current value of 0.3 C until the SOC reached 100%, and then stored in a constant temperature bath at 60° C. for one month. The discharge capacity of each lithium-ion secondary battery for evaluation was measured by the same method as described above, and the discharge capacity at this time was determined as the battery capacity after high-temperature storage. The capacity retention rate (%) was calculated as (battery capacity after high temperature storage/initial capacity)×100. The ratio of the measured values of the capacity retention rate of each lithium secondary battery for evaluation was determined when the reference value of the capacity retention rate was set to 100. The results are shown in FIG.

From FIG. 3, it can be seen that when the value of the ratio represented by the above formula (1) is 0.4 or less, the heat generation suppressing performance during overcharge is excellent. Further, from FIG. 4, it can be seen that when the value of the ratio represented by the above formula (1) is 0.28 or more, the deterioration resistance during high-temperature storage is excellent.
Therefore, when the value of the ratio represented by the above formula (1) is 0.28 or more and 0.4 or less, excellent heat generation suppression performance during overcharge and excellent deterioration resistance during high temperature storage are achieved. I know that they can be compatible.
That is, according to the non-aqueous electrolyte secondary battery disclosed herein, it can be seen that a non-aqueous electrolyte secondary battery that is excellent in both heat generation suppression performance during charging and deterioration resistance during high-temperature storage is provided. .

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.

20 Wound electrode assembly 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 positive electrode current collector 52a positive electrode active material layer non-formed 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-formation portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 lithium secondary battery

Claims (1)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode has a positive electrode active material layer,
The positive electrode active material layer contains _a positive electrode active material, _Li3PO4 , a conductive material, and a binder,
The positive electrode active material is a lithium-nickel-manganese-cobalt-based composite oxide having a layered structure, and the content of the positive electrode active material in the positive electrode active material layer is 70% by mass or more,
The conductive material is acetylene black, and the content of the conductive material in the positive electrode active material layer is 3% by mass or more and 13% by mass or less ,
The binder is polyvinylidene fluoride, and the content of the binder in the positive electrode active material layer is 2% by mass or more and 10% by mass or less ,
The negative electrode has a negative electrode active material layer containing a negative electrode active material,
The negative electrode active material is natural graphite,
When the particle diameter at which the cumulative value in the volume frequency particle size distribution measurement by the laser diffraction scattering method is 10% is D10, the particle diameter at which 50% is D50, and the particle diameter at which 90% is D90, the following formula (1) The value of the ratio represented by is 0.28 or more and 0.4 or less,
Non-aqueous electrolyte secondary battery.
[{D90 (μm) of positive electrode active material − D10 (μm) of positive electrode active material}/D50 (μm) of positive electrode active material]/{specific surface area of Li ₃ PO ₄ (m ² /g)×Li ₃ PO ₄ D50 (μm) of × content ratio of Li ₃ PO ₄ with respect to positive electrode active material (% by mass)} (1)

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