JP6783717B2 - Non-aqueous secondary battery - Google Patents

Description

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

In non-aqueous secondary batteries such as lithium ion secondary batteries, it has been studied to add an additive to the non-aqueous electrolytic solution to improve the battery performance. As a prior art related to this, in Patent Document 1, a lithium salt compound composed of a lithium cation having a long-chain ether compound (glime) as a ligand and a difluorophosphate anion is added to a non-aqueous electrolytic solution. It is described that this can improve the high temperature durability characteristics of the battery.

International Publication 2016/189769

According to Patent Document 1, the solubility in a non-aqueous electrolytic solution can be improved by coordinating a glyme with lithium difluorophosphate (LiPO ₂ F ₂ ) to form the above lithium salt compound. Therefore, more LiPO ₂ F ₂ can be contained in the non-aqueous electrolytic solution, and the effect of improving the high temperature durability characteristics can be suitably exhibited.

However, according to the study by the present inventors, in a battery containing a graphite-based material in the negative electrode and the lithium salt compound in the non-aqueous electrolyte solution, the lithium salt compound is layered between graphite together with a charge carrier during charging and discharging. May be co-inserted into. Therefore, the layered structure of graphite is distorted, and the battery resistance may increase or the durability characteristics may decrease. Therefore, in a battery containing a graphite-based material in the negative electrode and the lithium salt compound in the non-aqueous electrolytic solution, it is required to better balance the reduction of battery resistance and the improvement of durability characteristics.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a non-aqueous secondary battery having improved battery performance.

INDUSTRIAL APPLICABILITY The present invention provides a non-aqueous secondary battery including a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolytic solution. The negative electrode active material contains amorphous coated graphite in which the ratio of the BET specific surface area (unit: m ² / g) to the average particle size (unit: μm) is 0.15 or less. The non-aqueous electrolyte solution contains a lithium salt compound composed of a lithium cation having 2,5,8,11-te-laoxadodecane as a ligand and a difluorophosphate anion. The content of the lithium salt compound in terms of lithium difluorophosphate is 0.1 × 10 ^-3 mol or more per 1 m ² of the BET specific surface area of the positive electrode active material.

By setting the content of the lithium salt compound in terms of lithium difluorophosphate to a predetermined value or more, a high-quality (low resistance and high durability) film can be provided on the surfaces of the positive electrode and / or the negative electrode. Further, by including the amorphous coated graphite in the negative electrode, it is possible to preferably suppress the co-insertion of the lithium salt compound between the layers of graphite. As a result, in the non-aqueous secondary battery, the battery resistance can be suppressed to a low level and the durability characteristics can be improved.

In the present specification, the "average particle diameter" in the volume-based particle size distribution measured by laser diffraction and light scattering method means a particle diameter corresponding to cumulative 50% (D ₅₀ particle size). The unit is μm. Further, in the present specification, the "BET specific surface area" means a value obtained by analyzing the surface area measured by the nitrogen adsorption method by the BET method. The unit is m ² / g.

In a preferred embodiment, the content of the lithium salt compound in terms of lithium difluorophosphate is 0.17 × 10 ^-3 mol or less per 1 m ² of the BET specific surface area of the positive electrode active material. As a result, it is possible to better prevent the lithium salt compound from being co-inserted between the layers of graphite, and to realize even better battery performance.

In a preferred embodiment, the content of the lithium salt compound in terms of lithium difluorophosphate is 0.15 × 10 ^-3 mol or more per 1 m ² of the BET specific surface area of the positive electrode active material. As a result, the resistance caused by the positive electrode can be suppressed to a lower level.

In a preferred embodiment, the amorphous coated graphite has a BET specific surface area ratio of 0.1 or more to the average particle size. As a result, it is possible to better prevent the lithium salt compound from being co-inserted between the layers of graphite, and to realize even better battery performance.

FIG. 1 is a vertical cross-sectional view schematically showing a non-aqueous secondary battery according to an embodiment. FIG. 2 is a graph showing the relationship between the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material and the IV resistance at 15% SOC. FIG. 3 is a graph showing the relationship between the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material and the IV resistance at 50% SOC. FIG. 4 is a graph showing the relationship between the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material and the cycle characteristics (capacity retention rate).

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings as appropriate. Matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention can be grasped as design matters of those skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art. Further, in the following drawings, members / parts having the same action may be described with the same reference numerals, and duplicate description may be omitted or simplified. Further, the dimensional relations (length, width, thickness, etc.) in each drawing do not necessarily reflect the actual dimensional relations.

FIG. 1 is a vertical cross-sectional view schematically showing the internal structure of the non-aqueous secondary battery 100 according to the embodiment. The non-aqueous secondary battery 100 includes an electrode body 40 and a non-aqueous electrolytic solution (not shown) housed in a battery case 50.

The battery case 50 includes a flat rectangular parallelepiped (square) battery case body 52 having an opening at the upper end, and a lid plate 54 that closes the opening. The material of the battery case 50 is not particularly limited, but is, for example, a lightweight metal such as aluminum. The outer shape of the battery case 50 may be cylindrical, coin-shaped, or the like, or may be a bag made of a laminated film. The lid 54 is provided with a liquid injection port (not shown). A positive electrode terminal 70 and a negative electrode terminal 72 for external connection protrude from the lid plate 54.

The electrode body 40 has a band-shaped positive electrode sheet 10, a band-shaped negative electrode sheet 20, and a band-shaped separator sheet 30. The electrode body 40 is a so-called wound electrode body in which a positive electrode sheet 10 and a negative electrode sheet 20 are stacked with a separator sheet 30 interposed therebetween and wound in the longitudinal direction. The appearance of the electrode body 40 is a flat shape. The electrode body 40 has a substantially rounded rectangular shape in a cross section orthogonal to the winding axis. However, the electrode body 40 may be a so-called laminated electrode body in which a rectangular positive electrode sheet and a rectangular negative electrode sheet are laminated with a rectangular separator sheet interposed therebetween.

The positive electrode sheet 10 includes a band-shaped positive electrode current collector and a positive electrode active material layer fixed to the surface thereof. As the positive electrode current collector, a conductive member made of a metal having good conductivity (for example, aluminum) is suitable. The positive electrode active material layer is formed on the surface of the positive electrode current collector along the longitudinal direction of the positive electrode current collector. A positive electrode active material layer non-formed portion 12n on which a positive electrode active material layer is not formed is provided at one end (left side in FIG. 1) in the width direction of the positive electrode current collector. The positive electrode sheet 10 is electrically connected to the positive electrode terminal 70 via a positive electrode current collector plate 12c attached to the positive electrode active material layer non-forming portion 12n.

The positive electrode active material layer contains a positive electrode active material. As the positive electrode active material, one or two or more kinds of various materials known to be usable for the positive electrode of the battery can be used without particular limitation. As one preferable example, a lithium transition metal composite oxide such as a lithium nickel-manganese composite oxide, a lithium nickel cobalt alumina composite oxide, and a lithium nickel manganese cobalt composite oxide can be mentioned.

The positive electrode active material layer may contain components other than the positive electrode active material, such as a conductive material and a binder. Examples of the conductive material include carbon black such as acetylene black and Ketjen black, and carbon materials such as activated carbon and graphite. Examples of the binder include vinyl halide resins such as polyvinylidene fluoride (PVdF) and polyalkylene oxides such as polyethylene oxide (PEO).

The density of the positive electrode active material layer is not particularly limited, but is approximately 1.5 g / cm ³ or more, typically 2 g / cm ³ or more, for example 2.5 g / cm ³ or more, and approximately 4 g / cm ³ or less. Typically, it is 3.5 g / cm ³ or less, for example, 3 g / cm ³ or less. In such a high-density positive electrode, it is necessary to contain a large amount of LiPO ₂ F ₂ in the non-aqueous electrolytic solution from the viewpoint of forming a sufficient amount of film on the surface thereof. Therefore, it is particularly effective to apply the techniques disclosed herein.

A film is typically formed on the surface of the positive electrode active material layer. This film contains a component derived from a lithium salt compound contained in the non-aqueous electrolytic solution at the time of construction of the non-aqueous secondary battery 100, for example, in the case of LiPO ₂ F ₂ , PO ₂ F ₂ ⁻ . As a result, a high-quality film in which low resistance and high durability are balanced at a high level is realized. That is, LiPO ₂ F ₂ has a large intramolecular charge bias and a strong force to attract charge carriers. As a result, the charge carrier concentration on the surface of the positive electrode active material layer can be increased, and the resistance caused by the positive electrode can be suppressed low. Further, LiPO ₂ F ₂ is firmly bonded to the active site of the positive electrode active material layer and has excellent stability. As a result, the durability characteristics can be improved. Further, the elution of metal elements from the positive electrode active material can be suppressed, and the structural stability of the positive electrode active material can be improved.

The negative electrode sheet 20 includes a band-shaped negative electrode current collector and a negative electrode active material layer fixed to the surface thereof. As the negative electrode current collector, a conductive material made of a metal having good conductivity (for example, copper) is suitable. The negative electrode active material layer is formed on the surface of the negative electrode current collector along the longitudinal direction of the negative electrode current collector. A negative electrode active material layer non-formed portion 22n on which a negative electrode active material layer is not formed is provided at one end (right side in FIG. 1) in the width direction of the negative electrode current collector. The negative electrode sheet 20 is electrically connected to the negative electrode terminal 72 via a negative electrode current collector plate 22c attached to the negative electrode active material layer non-forming portion 22n.

The negative electrode active material layer contains a negative electrode active material. The negative electrode active material contains amorphous coated graphite. The negative electrode active material may be composed of only amorphous coated graphite, and in addition to the amorphous coated graphite, one or two materials known to be usable for the negative electrode of the battery may be used. You may use more than seeds together. The amorphous coated graphite preferably occupies approximately 50% by mass or more, preferably 80% by mass or more, more preferably 95% by mass or more, and particularly substantially 100% by mass of the entire negative electrode active material.

Amorphous coated graphite is in the form of particles. Amorphous coated graphite has a graphite particle as a core and an amorphous layer formed on the surface thereof and containing amorphous carbon. The amorphous coated graphite has a value of the ratio of the BET specific surface area to the average particle size (BET specific surface area / average particle size) of 0.15 or less. In general, the higher the amorphous property, the larger the BET specific surface area per unit particle size, and the larger the value of the above ratio. From this, the value of the above ratio is one index indicating the thickness of the amorphous layer of the amorphous coated graphite. According to the study by the present inventors, since graphite has high surface crystallinity, the lithium salt compound in the non-aqueous electrolytic solution is likely to be co-inserted between the layers of graphite together with the charge carrier. By satisfying the value of the above ratio, the surface crystallinity of graphite can be lowered, and the co-insertion of the lithium salt compound can be preferably suppressed. The lower limit of the above ratio is not particularly limited, but is preferably about 0.05 or more, for example 0.10 or more, from the viewpoint of better improving the battery performance (for example, energy density).

The shape of the graphite particles that form the core of the amorphous coated graphite is not particularly limited. In one preferred example, the scaly graphite is stressed to form a spherical shape, which is a substantially spherical shape. In other words, the core graphite particles may be so-called spherical graphite. Generally, in graphite, a portion having high reaction activity called an edge surface is developed. In spherical graphite, this highly reactive edge surface is folded and folded. This makes it possible to better prevent the lithium salt compound from being co-inserted between the graphite layers. In addition, the contact between the edge surface and the non-aqueous electrolytic solution can be better suppressed. Furthermore, the sphericality reduces the orientation of graphite, and the conductivity in the negative electrode active material layer can be homogenized. Therefore, by using spherical graphite, the resistance caused by the negative electrode can be suitably reduced and the durability characteristics can be improved better.
In addition, in this specification, "substantially spherical shape" is a term including spherical shape, rugby ball shape, polygonal shape and the like, and for example, the average aspect ratio (in the smallest rectangle circumscribing particles, the minor axis direction) The ratio of the length in the major axis direction to the length of) is approximately 1 to 2, for example, 1 to 1.5.

The average particle size of the amorphous coated graphite is not particularly limited, but is, for example, approximately 1 μm or more, typically 5 μm or more, preferably 10 μm or more, and approximately 50 μm or less, typically 30 μm or less, preferably 30 μm or less. It is 20 μm or less. The BET specific surface area of the amorphous coated graphite is not particularly limited, but is, for example, approximately 1 m ² / g or more, preferably 2 m ² / g or more, for example 2.5 m ² / g or more, and is approximately 10 m ^2. It is / g or less, typically 5 m ² / g or less, preferably 4 m ² / g or less, for example 3 m ² / g or less. By satisfying one or two of the above properties, excellent battery performance (for example, high input / output characteristics and high durability characteristics) can be better exhibited.

The negative electrode active material layer may contain components other than the negative electrode active material, such as a thickener, a binder, a dispersant, and a conductive material. Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC) and methyl cellulose (MC). Examples of the binder include rubbers such as styrene-butadiene rubber (SBR) and vinyl halide resins such as polyvinylidene fluoride (PVdF). Examples of the conductive material include amorphous carbon materials such as carbon black and activated carbon.

The ratio of the negative electrode active material to the total solid content of the negative electrode active material layer is not particularly limited, but is approximately 50% by mass or more, typically 90% by mass or more, for example 95% by mass or more, and is approximately 99.8. It is preferably 99.5% by mass or less, typically 99.5% by mass or less, for example, 99% by mass or less. The density of the negative electrode active material layer is not particularly limited, but is generally 0.5 g / cm ³ or more, for example 1 g / cm ³ or more, and approximately 2 g / cm ³ or less, for example 1.5 g / cm ³ or less. Good.

A film is typically formed on the surface of the negative electrode active material layer. This coating, components derived from the lithium salt compound was contained in the nonaqueous electrolyte during the construction of the non-aqueous secondary battery 100, typically degradation products LiPO ₂ F _2, for example PO ₂ F ₂ ^- and include I'm out. As a result, a high-quality film in which low resistance and high durability are balanced at a high level is realized.

The separator sheet 30 is arranged between the positive electrode active material layer of the positive electrode sheet 10 and the negative electrode active material layer of the negative electrode sheet 20. The separator sheet 30 insulates the positive electrode active material layer of the positive electrode sheet 10 and the negative electrode active material layer of the negative electrode sheet 20. The separator sheet 30 is made of a strip-shaped resin base material. Examples of the resin base material include sheets (films) made of polyolefins such as polyethylene (PE) and polypropylene (PP), and resins such as polyester, polyamide and cellulose. The separator sheet 30 may have a single-layer structure, or may have a structure in which two or more types of porous resin sheets having different materials and properties (thickness, porosity, etc.) are laminated. Further, the separator sheet 30 may be provided with a heat resistant layer (HRL layer) on its surface.

The non-aqueous electrolyte typically contains a non-aqueous solvent, a supporting salt, and a predetermined lithium salt compound. The non-aqueous electrolyte solution is always liquid within the operating temperature range of the non-aqueous secondary battery 100 (typically, the temperature range of -20 to + 60 ° C., for example, −10 to 50 ° C.).

The non-aqueous solvent is not particularly limited, and examples thereof include organic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones. Of these, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are preferable.

The supporting salt dissociates in a non-aqueous solvent to produce a charge carrier. Examples of the supporting salt include lithium salt, sodium salt, magnesium salt and the like. Of these, lithium salts such as LiPF ₆ and LiBF ₄ are preferable. The supporting salt is typically a compound having a higher solubility in a non-aqueous solvent than the lithium salt compound described later. The solubility of the supporting salt (value at 25 ° C.; the same applies hereinafter) is typically preferably 0.5 mol / L or more, for example 0.8 mol / L or more. The concentration of the supporting salt is, for example, preferably 0.8 to 1.3 mol / L.

The lithium salt compound is 2,5,8,11-te-laoxadodecane (also referred to as methyl triglime. CH _3- O- (CH ₂ CH _2- O) _3- CH ₃ ; hereinafter abbreviated as "TOD". complexed with lithium cations as ligands present) be, difluorophosphate anions (PO 2 _F 2 _-. ^and), and a. The lithium salt compound can function as a so-called film-forming agent.
The lithium salt compound is represented by the following chemical formula ((1): [Li ₂ (TOD)] ²⁺ [(PO ₂ F ₂ ) ⁻ ] ₂ ).

The single (uncomplexed) LiPO ₂ F ₂ has a low solubility in a non-aqueous solvent such as carbonates, which is about 0.11 mol / L. Therefore, when the density of the positive electrode is as high as 2 g / cm ³ or more, even if a single LiPO ₂ F ₂ is contained in the non-aqueous electrolyte solution at a concentration up to the solubility, the film per unit area of the positive electrode The amount of formation becomes small, and it is difficult to obtain the effect of improving battery performance. On the other hand, according to the study by the present inventors, the solubility of the lithium salt compound (1) in a non-aqueous solvent is about three times that of LiPO ₂ F ₂ alone based on lithium difluorophosphate. It is 0.30 mol / L. In this way, the content of LiPO ₂ F ₂ in the non-aqueous electrolytic solution can be increased by complexing. As a result, even in a high-capacity type battery, a sufficient amount of film can be formed on the surface of the positive electrode and / or the negative electrode.

In the present embodiment, the lithium salt compound of (1) above is added according to the BET specific surface area of the positive electrode active material. Specifically, the lithium salt compound is prepared so that the content of the lithium salt compound in terms of lithium difluorophosphate is 0.1 × 10 ^-3 mol or more per 1 m ² of the BET specific surface area of the positive electrode active material. Has been added. As a result, a sufficient amount of film can be formed on the surface of the positive electrode. Then, the resistance caused by the positive electrode can be reduced and the durability characteristics can be improved. From the above viewpoint, it is more preferable that the content of the lithium salt compound in terms of lithium difluorophosphate is 0.15 × 10 ^-3 mol or more per 1 m ² of the BET specific surface area of the positive electrode active material. The upper limit of the content of the lithium salt compound in terms of lithium difluorophosphate is not particularly limited, but is approximately 0.25 × 10 per 1 m ² of the BET specific surface area of the positive electrode active material in consideration of solubility. It may be ^-3 mol or less, typically 0.2 × 10 ^-3 mol or less, for example 0.17 × 10 ^-3 mol or less.

The concentration of the lithium salt compound in (1) in terms of lithium difluorophosphate is not particularly limited, but is typically lower than the concentration of the supporting salt, and is approximately 0.05 to 0.5 mol / mol /. It can be L, for example 0.2-0.3 mol / L. Since the lithium salt compound of the above (1) contains _two LiPO ₂ F ₂ structural portions in one molecule, the concentration of the lithium salt compound of the above (1) in terms of lithium difluorophosphate is as described above. It is a value obtained by doubling the concentration of [Li ₂ (TOD)] ₂ ⁺ [(PO ₂ F ₂ ) ⁻ ] ₂ itself in (1).

The concentration of the lithium salt compound in terms of lithium difluorophosphate can be measured by ion chromatography (IC: Ion Chromatography). The lithium salt compound contained in the non-aqueous electrolytic solution at the time of battery construction may be in a state in which a part or all of the lithium salt compound is attached to the surface of the positive electrode and / or the negative electrode after charging / discharging.

The non-aqueous electrolytic solution may be composed of the above-mentioned three components of the non-aqueous solvent, the supporting salt and the lithium salt compound, or may additionally contain components other than the above. Examples of the additive component include a dispersant, a gelling agent, a film forming agent (excluding the lithium salt compound of (1) above), an overcharge gas generating agent, and the like.

As described above, in the non-aqueous secondary battery 100 of the present embodiment, the negative electrode contains amorphous coated graphite whose surface crystallinity is suppressed, and the non-aqueous electrolytic solution contains the lithium of the above (1). It contains a salt compound in a content of a predetermined value or more. In the non-aqueous secondary battery 100, the lithium salt compound forms a high-quality (low resistance and high durability) film containing a component derived from the lithium salt compound on the surface of the positive electrode and the negative electrode. Further, in the non-aqueous secondary battery 100, by including the amorphous coated graphite having the above properties in the negative electrode, the co-insertion of the lithium salt compound into the graphite layers is suppressed. Combined with these effects, in the non-aqueous secondary battery 100, the battery resistance can be suppressed to a low level and the durability characteristics can be improved.

The non-aqueous secondary battery 100 is superior in input / output characteristics and durability characteristics as compared with conventional products. Further, in the non-aqueous secondary battery 100, even if the charge / discharge cycle is repeated, gas is less generated and the battery is less likely to swell. Therefore, the technique disclosed herein can be particularly preferably applied to a high energy density type non-aqueous secondary battery 100 in which the remaining space in the battery case 50 is reduced. The non-aqueous secondary battery 100 can be used for various purposes, and by taking advantage of such properties, it can be suitably used as, for example, a drive power source mounted on a vehicle. The type of vehicle is not particularly limited, and examples thereof include a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), an electric vehicle (EV), an electric truck, an electric scooter, an electrically assisted bicycle, an electric wheelchair, and an electric railway. ..

Hereinafter, some examples of the present invention will be described, but the present invention is not intended to be limited to those shown in such specific examples.

[Construction of non-aqueous secondary batteries]
First, the mass ratio of lithium nickel cobalt alumina composite oxide (NCA) as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder has a mass ratio of NCA: AB :. Weighed so that PVdF = 94: 3.5: 2.5 and kneaded with N-methylpyrrolidone (NMP) while adjusting the viscosity to prepare a positive electrode slurry. This positive electrode slurry is applied to an aluminum foil (positive electrode current collector) having a thickness of 12 μm, dried, and then rolled to form a positive electrode active material layer (electrode density 2.9 g / cm ³ ) on the positive electrode current collector. A positive electrode having a positive electrode was prepared.

Next, as the negative electrode active material, amorphous coated graphite having the properties (BET specific surface area / average particle size) shown in Table 1 was prepared. Specifically, sphericalized natural graphite was mixed with coal tar pitch, which is a carbon precursor, at a predetermined ratio, carbonized at a firing temperature of 1000 ° C., and then pulverized and classified. As a result, a particulate negative electrode active material having an amorphous layer on the surface of spherical natural graphite was obtained. Amorphous coated graphite (C) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the dispersant have a mass ratio of C: SBR: CMC =. Weighed so that the ratio was 99: 0.5: 0.5, and kneaded with ion-exchanged water while adjusting the viscosity to prepare a negative electrode slurry. This negative electrode slurry is applied to a copper foil (negative electrode current collector) having a thickness of 8 μm, dried, and then rolled to form a negative electrode active material layer (electrode density 1.4 g / cm ³ ) on the negative electrode current collector. A negative electrode having a negative electrode was prepared.

An electrode body was prepared by winding the positive electrode and the negative electrode prepared above with a separator. As the separator, one having a three-layer structure (thickness 20 μm) in which polypropylene (PP) was laminated on both sides of polyethylene (PE) was used.

Next, 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 EC: DMC: EMC = 30:40:30. The non-aqueous electrolyte solution of Reference Example 1 was prepared by dissolving at a concentration of 1.1 mol / L. Further, in Reference Examples 2 and 3 and Examples 1 to 10, the content of lithium difluorophosphate (LiPO ₂ F ₂ equivalent concentration / positive electrode active material BET) per 1 m ² of BET specific surface area of the positive electrode active material is shown in Table 1. LiPO ₂ F ₂ (Reference Examples 2 and 3) or [Li ₂ (TOD)] ²⁺ [(PO ₂ F ₂ ) ⁻ ] with respect to the non-aqueous electrolyte solution of Reference Example 1 so as to have the values shown. ₂ (Examples 1 to 10) was added to prepare a non-aqueous electrolyte solution.

Then, the electrode body was housed in an aluminum battery case, and the opening of the battery case and the lid were welded together. Next, the non-aqueous electrolytic solution was injected from the electrolytic solution injection port provided on the lid. Next, the internal pressure sensor was fixed to the cell injection port and sealed with a sealing material. In this way, lithium ion secondary batteries (Reference Examples 1 to 3 and Examples 1 to 10) were constructed.

[Activation treatment]
Each of the above-constructed batteries was activated. Specifically, in an environment of 25 ° C., constant current charging was performed at a charging rate of 1 / 3C until the battery voltage reached 4.1V, and then constant voltage charging was performed until the charging rate reached 2 / 100C (conditioning). processing). Then, the battery after the conditioning treatment was left in a constant temperature bath at a temperature of 60 ° C. for 24 hours (aging treatment).

[Measurement of initial capacity]
The activated battery is discharged at a constant current at a discharge rate of 1 / 3C in an environment of 25 ° C. until the battery voltage reaches 3.0V, and then in the voltage range of 3.0V to 4.1V. , The initial capacity was measured according to the following procedures 1 and 2.
(Procedure 1) After constant current charging at a charging rate of 1 / 3C until the battery voltage reaches 4.1V, constant voltage charging is performed until the charging rate reaches 1 / 100C.
(Procedure 2) After constant current discharge at a discharge rate of 1 / 3C until the battery voltage reaches 3.0V, constant voltage discharge is performed until the discharge rate reaches 1 / 100C.
Then, the constant current constant voltage discharge capacity in step 2 was set as the initial capacity.

[Measurement of IV resistance]
Next, the IV resistance in the state of SOC 15% and SOC 50% was measured in an environment of 25 ° C. Specifically, in an environment of 25 ° C., the SOC of the battery was adjusted to 15% or 50%, and then the battery was left in a constant temperature bath at 0 ° C. for 3 hours. After confirming that the surface temperature of the battery was 0 ± 0.5 ° C., the battery was discharged at a discharge rate of 10 C for 5 seconds, and the amount of voltage drop was measured. The IV resistance (mΩ) was calculated by dividing this voltage drop by the value of the discharge current. The results are shown in Table 1.

[Measurement of capacity retention rate]
Next, the battery was housed in a constant temperature bath at a temperature of 60 ° C., and charging / discharging was performed for 500 cycles at a charging / discharging rate of 2C in a SOC range of 0 to 100%. Then, the battery capacity after the high temperature cycle test was measured in the same manner as the initial capacity. Then, the battery capacity after the high temperature cycle test was divided by the initial capacity to calculate the capacity retention rate (%). The results are shown in Table 1.

[Measurement of gas generation]
In addition, before and after the high temperature cycle test, the gauge pressure of the internal pressure sensor arranged at the cell injection port was read to determine the internal pressure increase value. Then, the gas generation amount (cm ³ / Ah) was calculated from the internal pressure increase value and the remaining space in the battery based on the equation of state. The results are shown in Table 1.

Reference Example 1 is a test example using a non-aqueous electrolyte solution that does not contain LiPO ₂ F ₂ . The results will be considered with reference to the evaluation results of the battery of Reference Example 1.

FIG. 2 is a graph showing the relationship between the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material and the IV resistance at 15% SOC.
As shown in Table 1 and FIG. 2, from the results of Reference Examples 2 and 3 and Examples 1 to 5, the IV resistance at 15% SOC is the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material. It tended to decrease as it became larger. It is considered that this is because as the content of LiPO ₂ F ₂ in the non-aqueous electrolyte solution increased, the amount of the film formed on the surface of the positive electrode increased, and the IV resistance caused by the positive electrode could be reduced.

FIG. 3 is a graph showing the relationship between the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material and the IV resistance at 50% SOC.
As shown in Table 1 and FIG. 3, from the results of Reference Examples 2 and 3 and Examples 1 to 5, the IV resistance at 50% SOC is the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material. Regardless, the values were almost the same. It is considered that this is because the influence of the IV resistance caused by the negative electrode is large at 50% SOC, and the effect of reducing the IV resistance caused by the positive electrode is relatively small.

FIG. 4 is a graph showing the relationship between the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material and the cycle characteristics (capacity retention rate).
As shown in Table 1 and FIG. 4, from the results of Reference Examples 2 and 3 and Examples 1 to 5, the durability characteristics became higher as the content of LiPO ₂ F ₂ per unit BET specific surface area of the positive electrode active material increased. It tended to improve. That is, it was confirmed that the amount of gas generated was reduced along with the improvement of the capacity retention rate. The effect of improving the durability characteristics reached a plateau when the content of Li ₂ PO ₂ F ₂ per positive electrode BET was 0.1 × 10 ^-3 mol / m ² .

Examples 6 to 10 are test examples in which the surface crystallinity (value of the ratio of BET specific surface area / average particle size) of amorphous coated graphite was changed. From the results of Examples 6 to 10, in Examples 8 to 10 in which the value of the above ratio exceeds 0.15, that is, the surface crystallinity is high, the battery resistance (IV resistance) at 15% SOC increases and (capacity retention rate). ) Was observed to decrease. On the other hand, in Examples 6 and 7, in which the value of the above ratio is 0.15 or less, that is, the surface crystallinity is suppressed, the battery resistance can be suppressed low and the durability characteristics can be improved. ..

From these results, the negative electrode contains amorphous coated graphite having the above ratio value of 0.15 or less, and the non-aqueous electrolyte solution contains [Li ₂ (TOD)] ²⁺ [(PO ₂ F ₂ ) ⁻ ]. ₂ by adding, by increasing the LiPO ₂ F ₂ content in the nonaqueous electrolytic solution to a predetermined value or more, it was shown that it is possible to improve the battery performance. Such results support the technical significance of the techniques disclosed herein.

Although the present invention has been described in detail above, the above-described embodiments and examples are merely examples, and the inventions disclosed here include various modifications and modifications of the above-mentioned specific examples.

10 Positive electrode sheet 20 Negative electrode sheet 30 Separator sheet 40 Winding electrode body 50 Battery case 100 Non-aqueous secondary battery

Claims (4)

A positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolytic solution are provided.
The negative active material has an average particle diameter (D ₅₀ particle size. Units of volume criteria based on laser diffraction light scattering method, [mu] m.) Value based on the BET specific surface area (nitrogen adsorption method for. Units, m ² / Contains amorphous coated graphite having a ratio of g.) Of 0.15 or less.
The non-aqueous electrolyte solution contains a lithium salt compound composed of a lithium cation having 2,5,8,11-te-laoxadodecane as a ligand and a difluorophosphate anion.
A non-aqueous secondary battery in which the content of the lithium salt compound in terms of lithium difluorophosphate is 0.1 × 10 ^-3 mol or more per 1 m ² of the BET specific surface area of the positive electrode active material. The non-aqueous secondary battery according to claim 1, wherein the content of the lithium salt compound in terms of lithium difluorophosphate is 0.17 × 10 ^-3 mol or less per 1 m ² of the BET specific surface area of the positive electrode active material. .. The non-aqueous battery according to claim 1 or 2, wherein the content of the lithium salt compound in terms of lithium difluorophosphate is 0.15 × 10 ^-3 mol or more per 1 m ² of the BET specific surface area of the positive electrode active material. Next battery. The non-aqueous secondary battery according to any one of claims 1 to 3, wherein the amorphous coated graphite has a ratio of the BET specific surface area to the average particle diameter of 0.1 or more.

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