JP2019215972A - Non-aqueous lithium secondary battery - Google Patents

Abstract

To provide a non-aqueous lithium secondary battery having LiPOadded to a positive electrode active material layer, in which a rise in battery temperature at the time of overcharge is suppressed and low-temperature performance is excellent.SOLUTION: A non-aqueous lithium 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 and LiPO. The negative electrode has a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material. The content of LiPOis 1 mass% or more and 5 mass% or less with respect to the positive electrode active material. The CO2 adsorption amount of the negative electrode active material is 0.05 mL/g or more and 3 mL/g or less.SELECTED DRAWING: Figure 1

Description

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

  2. Description of the Related Art In recent years, non-aqueous lithium secondary batteries such as lithium secondary batteries using a non-aqueous electrolyte have been used in portable power supplies such as personal computers and portable terminals, electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). ) Is suitably used for a vehicle driving power supply.

With the spread of nonaqueous lithium secondary batteries, higher performance is desired. Therefore, various techniques have been developed to improve the performance of non-aqueous lithium secondary batteries. For example, Patent Literature 1 discloses a technique of adding Li ₃ PO ₄ to a positive electrode active material layer. Patent Literature 2 discloses a technique that focuses on the specific surface area of a carbon material used as a negative electrode active material.

JP 2015-103332 A Japanese Patent No. 6287187

However, as a result of diligent studies by the present inventors, in a conventional non-aqueous lithium secondary battery in which Li ₃ PO ₄ is added to the positive electrode active material layer, improvement in battery temperature rise during overcharge and improvement in low-temperature characteristics have been improved. I found that there was room.

Therefore, the present invention relates to a non-aqueous lithium secondary battery in which Li ₃ PO ₄ is added to a positive electrode active material layer, in which an increase in battery temperature during overcharging is suppressed and the non-aqueous lithium secondary battery has excellent low-temperature performance. It is intended to provide a secondary battery.

As a result of intensive studies, the present inventor has found that by properly managing the amount of Li ₃ PO ₄ contained in the positive electrode active material layer and the amount of CO ₂ adsorbed on the negative electrode active material, it is possible to improve the overcharge with Li ₃ PO _4. It has been found that a good film formation state and excellent low-temperature performance of the battery due to the CO ₂ adsorption amount can be compatible.
That is, the nonaqueous lithium secondary battery disclosed herein includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode has a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material and Li ₃ PO ₄ . The negative electrode has a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material. The content of Li ₃ PO ₄ is 1% by mass or more and 5% by mass or less based on the positive electrode active material. The CO2 adsorption amount of the negative electrode active material is 0.05 mL / g or more and 3 mL / g or less.
According to such a configuration, a non-aqueous lithium secondary battery in which an increase in battery temperature during overcharge is suppressed and which has excellent low-temperature performance (particularly, low battery resistance at low temperatures) is provided.

1 is a cross-sectional view schematically illustrating an internal structure of a lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating a configuration of a wound electrode body of the lithium secondary battery according to one embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, matters other than matters specifically mentioned in the present specification and necessary for carrying out the present invention (for example, a general configuration and a manufacturing process of a non-aqueous lithium secondary battery not characterizing the present invention) Can be understood as a design matter of a person skilled in the art based on the prior art in the field. 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.

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 addition, a “non-aqueous lithium secondary battery” includes a non-aqueous electrolyte (typically, a non-aqueous electrolyte containing a supporting electrolyte in a non-aqueous solvent), uses lithium ions as a charge carrier, and connects between a positive electrode and a negative electrode. Refers to a secondary battery in which charge and discharge are realized by transfer of electric charge accompanying lithium ions.

  Hereinafter, the present invention will be described in detail by taking a flat rectangular non-aqueous lithium secondary battery having a flat wound electrode body and a flat battery case as an example, but the present invention is described in such an embodiment. It is not intended to be limited to those described.

  The lithium secondary battery 100 shown in FIG. 1 is constructed by housing a flat wound electrode body 20 and a nonaqueous electrolyte (not shown) in a flat rectangular battery case (that is, an outer container) 30. A sealed lithium secondary battery 100. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode 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 exceeds a predetermined level. I have. Further, the battery case 30 is provided with an inlet (not shown) for injecting the non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive current collector 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector 44a. As a material of the battery case 30, for example, a metal material having a light weight and good heat conductivity such as aluminum is used.

  As shown in FIGS. 1 and 2, the wound electrode body 20 has a cathode active material layer 54 formed on one or both sides (here, both sides) of a long cathode current collector 52 along the longitudinal direction. The positive electrode sheet 50 and the negative electrode sheet 60 in which the negative electrode active material layer 64 is formed on one side or both sides (here, both sides) of the long negative electrode current collector 62 along the longitudinal direction are two long sheets. And is wound in the longitudinal direction with the separator sheet 70 interposed therebetween. In addition, the positive electrode active material layer non-formed portion 52a (that is, the positive electrode active portion) formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axis direction (that is, the sheet width direction orthogonal to the longitudinal direction). A portion where the positive electrode current collector 52 is exposed without forming the material layer 54) and a portion 62a where the negative electrode active material layer is not formed (that is, a portion where the negative electrode current collector 62 is exposed without forming the negative electrode active material layer 64). Are connected to 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 an 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 occluding and releasing lithium ions is used, and one or more of materials conventionally used in lithium secondary batteries (eg, oxides having a layered structure and oxides having a spinel structure) are used. It can be used without particular limitation. Examples of the positive electrode active material include a lithium nickel-based composite oxide, a lithium cobalt-based composite oxide, a lithium manganese-based composite oxide, and a lithium nickel manganese-based composite oxide (eg, LiNi _0.5 Mn _1.5 O ₄ ). And a lithium-containing transition metal oxide such as lithium nickel manganese cobalt-based composite oxide (eg, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ). Among them, a lithium nickel manganese cobalt-based composite oxide (particularly, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) is preferable. 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, based on the total mass of the positive electrode active material layer 54).
The content of Li ₃ PO ₄ will be described later.

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

Examples of the negative electrode current collector 62 constituting the negative electrode sheet 60 include, for example, copper foil.
The negative electrode active material layer 64 contains 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 graphite may be natural graphite or artificial graphite, and may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

  The negative electrode active material layer 64 may include components other than the active material, such as a binder and a thickener. As the binder, for example, styrene butadiene rubber (SBR) or the like can be used. As the thickener, 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, and 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 from 0.1% by mass to 8% by mass, and more preferably from 0.5% by mass to 3% by mass. The content of the thickener in the negative electrode active material layer is preferably from 0.3% by mass to 3% by mass, and more preferably from 0.5% by mass to 2% by mass.

As described above, by appropriately managing the amount of Li ₃ PO ₄ contained in the positive electrode active material layer 54 and the amount of CO ₂ adsorbed on the negative electrode active material, good film formation during overcharging with Li ₃ PO ₄ The state and the excellent low-temperature performance of the battery due to the CO ₂ adsorption amount can be compatible.
Therefore, in the present invention, the content of Li ₃ PO ₄ is 1% by mass or more and 5% by mass or less, preferably 2% by mass or more and 4% by mass or less based on the positive electrode active material.
Further, the CO ₂ adsorption amount of the negative electrode active material is 0.05 mL / g (cc / g) or more and 3 mL / g (cc / g) or less, and preferably 0.2 mL / g or more and 2 mL / g or less.
To explain this in more detail, Li ₃ PO ₄ has a function of decomposing at the time of overcharge and forming a film on the negative electrode active material, thereby suppressing a rise in battery temperature at the time of overcharge. On the other hand, the CO ₂ adsorption amount of the negative electrode active material indicates the surface state of the negative electrode active material. If the amount of CO ₂ adsorbed is large, the reaction area of the negative electrode active material on which a film is to be formed at the time of overcharging becomes large, and if the content of Li ₃ PO ₄ is small, the overcharge performance deteriorates. However, when the CO ₂ adsorption amount is large, the reaction area of the negative electrode active material is large, so that the low temperature characteristics are improved. If the amount of adsorbed CO ₂ is small, the reaction area of the negative electrode active material on which a film is to be formed at the time of overcharging becomes small. Therefore, even if the content of Li ₃ PO ₄ is small, the function of suppressing the temperature rise during overcharging is sufficient. Be demonstrated. However, when the CO ₂ adsorption amount is small, the reaction area of the negative electrode active material is small, so that the low-temperature characteristics deteriorate.
Therefore, when the content of Li ₃ PO _{4 and} the amount of adsorbed CO ₂ are within the above ranges, it is possible to suppress the rise in the battery temperature at the time of overcharging and achieve both low-temperature characteristics.

The CO ₂ adsorption amount of the negative electrode active material can be adjusted by controlling firing conditions (eg, firing temperature, firing time, and the like) when producing the carbon material used as the negative electrode active material. When amorphous carbon-coated graphite is used as the negative electrode active material, the adjustment can be performed by controlling the amount of the amorphous carbon-coated graphite in addition to controlling the firing conditions.
In addition, the CO ₂ adsorption amount of the negative electrode active material can be measured using a gas adsorption amount measuring device by a constant solution method.

  Examples of the separator 70 include a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or 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 a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt.
As the non-aqueous solvent, various types of organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones used in general lithium secondary battery electrolytes can be used without particular limitation. it can. As specific examples, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), Monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC) and the like are exemplified. Such nonaqueous solvents can be used alone or in an appropriate combination of two or more.
As the supporting salt, for example, a lithium salt (preferably LiPF ₆ ) such as LiPF ₆ , LiBF ₄ , and LiClO ₄ can be suitably used. The concentration of the supporting salt is preferably from 0.7 mol / L to 1.3 mol / L.

  In addition, as long as the effect of the present invention is not significantly impaired, the non-aqueous electrolyte may include components other than the components described above, for example, a gas generating agent such as biphenyl (BP) and cyclohexylbenzene (CHB); a thickening agent; Various additives may be included.

  The lithium secondary battery 100 configured as described above suppresses a rise in battery temperature during overcharge and has excellent low-temperature performance. The lithium secondary battery 100 has particularly low battery resistance at low temperatures.

  The lithium secondary battery 100 configured as described above can be used for various applications. Suitable applications include a drive power supply mounted on vehicles such as an electric vehicle (EV), a hybrid vehicle (HV), and a plug-in hybrid vehicle (PHV). The lithium secondary battery 100 can also be used in the form of a battery pack typically formed by connecting a plurality of batteries in series and / or in parallel.

  The rectangular lithium secondary battery 100 including the flat wound electrode body 20 has been described as an example. However, the non-aqueous lithium secondary battery disclosed herein can also be configured as a lithium secondary battery including a stacked electrode body. Further, the nonaqueous lithium secondary battery disclosed herein can also be configured as a cylindrical lithium secondary battery.

  Hereinafter, examples according to the present invention will be described, but the present invention is not intended to be limited to those shown in the examples.

<Production of lithium secondary batteries A1 to A5 and B1 to B4 for evaluation>
Using a disperser, a paste in which acetylene black (AB) as a conductive material, PVdF, and N-methylpyrrolidone (NMP) were mixed was obtained. 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 is added to this paste, and then the solid content is uniformly dispersed. A slurry for forming a positive electrode active material layer was prepared. The slurry for forming a positive electrode active material was prepared such that LNCM: Li ₃ PO ₄ : AB: PVdF = 90-x: x: 8: 2 (mass ratio) (x is a value shown in Table 1). ). This slurry was applied in a strip shape on both sides of a long aluminum foil having a thickness of 15 μm, dried, and then pressed to produce a positive electrode sheet.
Further, amorphous carbon-coated graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed with C: SBR: CMC = 98: The mixture was mixed with ion-exchanged water at a mass ratio of 1: 1 to prepare a slurry for forming a negative electrode active material layer. This slurry was applied to both sides of a long copper foil having a thickness of 10 μm in a strip shape, dried, and then pressed to produce a negative electrode sheet. The amorphous carbon-coated graphite used had the CO ₂ adsorption amount shown in Table 1. This CO ₂ adsorption amount was measured by the following method.
In addition, two 20 μm thick porous polyolefin sheets having a three-layer structure of PP / PE / PP were prepared as separator sheets.
The prepared positive electrode sheet, negative electrode sheet, and two prepared separator sheets were overlapped and wound to produce a wound electrode body. At this time, a separator was interposed between the positive electrode sheet and the negative electrode sheet. Electrode terminals were attached to the positive electrode sheet and the negative electrode sheet, respectively, and were accommodated 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 hermetically sealed. In the non-aqueous electrolyte, LiPF ₆ as a supporting salt was added to 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 0.0 mol / L was used.
Thus, evaluation lithium secondary batteries A1 to A5 and B1 to B4 were produced.

<Measurement of CO ₂ adsorption amount of negative electrode active material>
As a pretreatment, the amorphous carbon-coated graphite was subjected to degassing at 350 ° C. under reduced pressure, and then heat-treated at 150 ° C. This was the constant溶法using Shimadzu "TriStar II3020", measuring a temperature of about -80 ° C., it was measured CO ₂ adsorption amount under the conditions of pressure range 0.3Atm～1atm.

<Battery temperature measurement>
Initial charge / discharge treatment was performed on each of the evaluation lithium secondary batteries prepared above. Thereafter, a thermocouple was attached to the battery case of each evaluation lithium secondary battery, and the temperature was measured. Thereafter, the battery was charged to 5.1 V, and the temperature was measured. Then, a temperature difference (that is, a temperature rise amount) before and after charging was obtained. In each of the lithium secondary battery systems having different Li ₃ PO ₄ contents, the temperature rise amount of each evaluation lithium secondary battery was determined based on the temperature rise amount of lithium secondary battery A5 (reference value: 100%). Was calculated (%). The results are shown in Table 1.

<Low temperature resistance evaluation>
In a temperature environment of −35 ° C., each lithium secondary battery for evaluation adjusted to SOC 27% was discharged at a rate of 10 C for 2 seconds, and a resistance value was obtained from a discharge curve at that time. Using the resistance value of the lithium secondary battery A5 as a reference (reference value: 100%), the ratio (%) of the resistance value of each evaluation lithium secondary battery was calculated. The results are shown in Table 1.

From Table 1, the content of Li ₃ PO ₄ is 1% by mass or more and 5% by mass or less based on the positive electrode active material, and the CO 2 adsorption amount of the negative electrode active material is 0.05 mL / g or more and 3 mL / g or less. In some cases, it can be seen that the rise in battery temperature when the voltage rises is greatly suppressed, and in addition, the resistance at low temperatures is low.
Therefore, according to the non-aqueous lithium secondary battery disclosed herein, it is understood that the rise in battery temperature during overcharge is suppressed and the low-temperature performance is excellent.

  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.

Reference Signs List 20 wound electrode body 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-formed portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 lithium secondary battery

Claims (1)

A positive electrode, a negative electrode, and a non-aqueous electrolyte, a non-aqueous lithium secondary battery including:
The positive electrode has a positive electrode active material layer,
The positive electrode active material layer contains a positive electrode active material and Li ₃ PO ₄ ,
The negative electrode has a negative electrode active material layer,
The negative electrode active material layer contains a negative electrode active material,
The content of Li ₃ PO ₄ is 1% by mass or more and 5% by mass or less based on the positive electrode active material,
The CO2 adsorption amount of the negative electrode active material is 0.05 mL / g or more and 3 mL / g or less.
Non-aqueous lithium secondary battery characterized by the above-mentioned.

Images