JP6883265B2 - Lithium secondary battery - Google Patents

Description

The present invention relates to a lithium secondary battery.

Patent Document 1 discloses a lithium secondary battery including a positive electrode containing a positive electrode active material, a negative electrode containing a graphite-based negative electrode active material, and a non-aqueous electrolyte solution.

Japanese Unexamined Patent Publication No. 2015-103332 Japanese Unexamined Patent Publication No. 2017-098240 Japanese Unexamined Patent Publication No. 2017-098241 JP-A-2017-130443

For the above-mentioned lithium secondary batteries, it is required to improve the battery performance in a wide range of temperature environments. Specifically, it is required that the capacity deterioration is small even when exposed to a high temperature environment for a long time and that excellent input characteristics are exhibited even in a low temperature environment.

The present invention has been made in view of the above points, and an object of the present invention is to provide a lithium secondary battery having excellent high temperature storage characteristics and low temperature input characteristics.

INDUSTRIAL APPLICABILITY The present invention provides a lithium secondary battery including a positive electrode containing a positive electrode active material, a negative electrode containing a graphite-based negative electrode active material, and a non-aqueous electrolyte solution. The graphite-based negative electrode active material has a BET specific surface area of 3.8 m ² / g or more and 4.8 m ² / g or less based on the nitrogen adsorption method. At least one of the positive electrode, the negative electrode and the non-aqueous electrolytic solution contains a naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof. The ratio of the naphthyl group-containing polymerizable unsaturated monomer or its derivative to the graphite-based negative electrode active material is 3.21 × 10 ^-4 or more and 3.51 × 10 ^-4 or less on a mass basis.

With the above configuration, it becomes difficult for a high resistance film to be formed on the surface of the negative electrode in a high temperature environment. As a result, the battery resistance can be suppressed to a low level, and the high temperature storage characteristics can be improved. Further, with the above configuration, at least the liquid retention property of the negative electrode is enhanced. As a result, a good conductive path can be formed in the battery even in a low temperature environment, and the low temperature input characteristics can be improved. Combined with the above effects, it is possible to realize a lithium secondary battery having excellent high temperature storage characteristics and low temperature input characteristics.

Hereinafter, preferred embodiments of the lithium secondary battery disclosed herein will be described. 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. In this specification, when the numerical range is described as A to B (where A and B are arbitrary numerical values), it means A or more and B or less.

The lithium secondary battery of the present embodiment includes a positive electrode, a negative electrode, and a non-aqueous electrolytic solution. Hereinafter, each component will be described in order.

The positive electrode contains at least the positive electrode active material. The positive electrode typically further comprises trilithium _{phosphate (Li 3 PO 4).} The positive electrode typically includes a positive electrode current collector and a positive electrode active material layer that is fixed onto the positive electrode current collector and contains a positive electrode active material. As the positive electrode current collector, a metal foil such as an aluminum foil is suitable.

The positive electrode active material may be any material that can reversibly occlude and release lithium ions, which are charge carriers. Preferable examples of the positive electrode active material are lithium transition metals such as lithium nickel-containing composite oxide, lithium cobalt-containing composite oxide, lithium nickel cobalt-containing composite oxide, lithium manganese-containing composite oxide, and lithium nickel cobalt manganese-containing composite oxide. Composite oxides can be mentioned. Of these, a layered structure lithium nickel cobalt manganese-containing composite oxide is preferable.

Li ₃ PO ₄ for example (a) coats the surface of the positive electrode active material; (b) suppresses oxidative decomposition of the non-aqueous electrolyte solution (for example, hydrolysis of the supporting salt that may be contained in the non-aqueous electrolyte solution); (C) By capturing or consuming fluorine-containing supporting salt (for example, fluorine-containing lithium salt) that can be contained in the non-aqueous electrolyte solution and hydrofluoric acid (HF) produced by hydrolysis of the fluorine-containing solvent, the non-aqueous electrolyte solution can be used. It has at least one of the following actions of relaxing the acidity (pH); (d) forming a stable film containing phosphate ions on the surface of the opposite negative electrode to improve overcharge resistance. By _{such an action, Li 3} PO ₄ has an effect of suppressing elution of metal elements from the positive electrode active material and improving battery characteristics during normal use, such as cycle characteristics and high temperature storage characteristics.

The positive electrode may contain one or more optional components, if necessary, in addition to the above-mentioned positive electrode active material and Li ₃ PO _4. Examples of optional components include dispersants, binders, conductive aids, thickeners, pH adjusters and the like. Examples of the dispersant include a naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof, which will be described later. Examples of the binder include vinyl halide resins such as polyvinylidene fluoride (PVdF) and polyalkylene oxides such as polyethylene oxide (PEO). Examples of the conductive auxiliary agent include carbon materials such as carbon black (typically acetylene black), activated carbon, graphite, and carbon fiber. Examples of the pH adjuster include acidic substances such as phosphoric acid.

The negative electrode contains at least a graphite-based negative electrode active material. The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector and containing a graphite-based negative electrode active material. As the negative electrode current collector, a metal foil such as a copper foil is suitable.

The graphite-based negative electrode active material may be any graphite-based material that can reversibly occlude and release lithium ions, which are charge carriers. Preferable examples of the graphite-based negative electrode active material include carbon materials such as natural graphite, artificial graphite, and amorphous coated graphite. In the present specification, the term "graphite-based negative electrode active material" means that the proportion of graphite in the entire negative electrode active material is approximately 50% by mass or more, typically 80% by mass or more, for example, 90. A material that is mass% or more. The graphite-based negative electrode active material is typically in the form of particles.

In the present embodiment, the BET specific surface area of the graphite-based negative electrode active material (the BET specific surface area measured by the constant volume adsorption method using nitrogen gas (so-called nitrogen gas adsorption method) is analyzed by the BET method. The same shall apply hereinafter. ) Is 3.8 to 4.8 m ² / g. By setting the BET specific surface area to a predetermined value or more, the liquid retention property of the non-aqueous electrolytic solution is improved at the negative electrode. As a result, the reaction resistance can be reduced and excellent input characteristics can be realized even in a low temperature environment. From the viewpoint of improving the low temperature input characteristics, the BET specific surface area may be 4 m ² / g or more. Further, by setting the BET specific surface area to a predetermined value or less, decomposition of the non-aqueous electrolytic solution can be suppressed at the negative electrode. As a result, the formation of a high resistance film that contributes to capacity deterioration can be suppressed, and the battery capacity can be maintained even in a high temperature environment. From the viewpoint of improving the high temperature storage property, the BET specific surface area may be 4.5 m ² / g or less.

Although not particularly limited, the ratio of the negative electrode active material to the entire negative electrode active material layer (100% by mass) is approximately 50 to 99 mass from the viewpoint of exerting the effect of the technology disclosed herein at a high level. %, For example, 90 to 98% by mass.

The negative electrode may contain one or more optional components in addition to the above-mentioned graphite-based negative electrode active material, if necessary. Examples of optional components include dispersants, binders, thickeners and the like. Examples of the dispersant include a naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof, which will be described later, and celluloses such as carboxymethyl cellulose (CMC). Examples of the binder include rubbers such as styrene-butadiene rubber (SBR).

The non-aqueous electrolyte solution exhibits a liquid state at room temperature (25 ° C.). The non-aqueous electrolyte typically contains a supporting salt and a non-aqueous solvent. The supporting salt dissociates in a solvent to produce a charge carrier (lithium ion). Examples of the supporting salt include fluorine-containing lithium salts such as _{LiPF 6} and LiBF _4. Examples of the non-aqueous solvent include non-fluorine or fluorine-containing carbonate. As a preferable example of carbonate, cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), monofluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl-2,2 , 2-Trifluoroethyl carbonate (MTFEC) and other chain carbonates. The non-aqueous electrolyte solution may contain, if necessary, one or more optional components in addition to the above-mentioned supporting salt and non-aqueous solvent. Examples of the optional component include a naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof, which will be described later.

In the present embodiment, at least one of the positive electrode, the negative electrode, and the non-aqueous electrolytic solution contains a naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof. The naphthyl group-containing polymerizable unsaturated monomer or its derivative has an excellent affinity with a non-aqueous electrolytic solution due to the structure described later. Due to this material property, the naphthyl group-containing polymerizable unsaturated monomer or its derivative added in the battery dissolves in the non-aqueous electrolyte solution and adheres to the surface of the positive electrode / negative electrode (positive electrode / negative electrode active material). A naphthyl group-containing polymerizable unsaturated monomer or derivative thereof is typically a material that is not electrolyzed (stable) within the SOC (State of Charge) range in which the battery is normally used.

As described in Patent Documents 2 to 4, the naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof is a hydrocarbon having a naphthalene ring and a derivative thereof. As the naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof, a compound represented by the following formula (1) is preferable.

In the above formula (1), R represents a hydrogen atom or a methyl group, and X and Y may be the same or different, and a hydrogen atom, an alkyl group, an alkoxy group, an alkoxycarbonyloxy group, a phosphoryloxy group, It indicates any one of a hydroxyl group, a sulfonic acid group, a carboxyl group, an amino group, a nitro group, a halogen atom, an aryloxy group, an alkylthio group or an arylthio group. When W is present, W is an organic group or single bond with 1 to 20 carbon, nitrogen and / or oxygen atoms.

Examples of the polymerizable unsaturated monomer having a naphthyl group or a derivative thereof include vinylnaphthalene, naphthyl (meth) acrylate, naphthylalkyl (meth) acrylate, and derivatives thereof. These can be used alone or in combination of two or more. Of these, naphthyl (meth) acrylate represented by the following formula (2) or a derivative thereof is preferable.

In the above formula (2), R represents a hydrogen atom or a methyl group, and X and Y may be the same or different, and a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a sulfonic acid group, a carboxyl group, Indicates either an alkoxycarbonyloxy group, a phosphoryloxy group, an amino group, a nitro group, a halogen atom, an aryloxy group, an alkylthio group or an arylthio group.

Examples of the naphthyl (meth) acrylate or a derivative thereof include 1-naphthyl (meth) acrylate, 2-naphthyl (meth) acrylate, and derivatives thereof, and these may be used alone or in combination of two or more. Can be used in combination. Of these, 4-substituted-1-naphthyl (meth) acrylate represented by the following formula (3) is preferable.

In the above formula (3), R represents a hydrogen atom or a methyl group, and Z represents a hydroxyl group or an alkoxy group having 1 to 8 carbon atoms. When Z, which is a substituent in the above formula (3), is an alkoxy group, the number of carbon atoms of the alkoxy group is usually 1 to 8, preferably 1 to 4, and more preferably 1 to 2. Especially preferably 1.

Examples of the 4-substituted-1-naphthyl (meth) acrylate include 4-methyl-1-naphthyl (meth) acrylate, 4-ethyl-1-naphthyl (meth) acrylate, and 4-methoxy-1-naphthyl (meth) acrylate. Acrylate, 4-ethoxy-1-naphthyl (meth) acrylate, 4-hydroxy-1-naphthyl (meth) acrylate, 2-methoxy-4-hydroxy-1-naphthyl (meth) acrylate, 2-ethoxy-4-hydroxy- 1-naphthyl (meth) acrylate, 2-hydroxy-4-methoxy-1-naphthyl (meth) acrylate, 2-hydroxy-4-ethoxy-1-naphthyl (meth) acrylate, 4-methoxycarbonyloxy-1-naphthyl ( Examples thereof include meth) acrylate, 4-phenoxycarbonyloxy-1-naphthyl (meth) acrylate, 4-phosphoryloxy-1-naphthyl (meth) acrylate, or derivatives thereof, and these may be used alone or in combination of two. The above can be used in combination.

The term "derivative" as used herein refers to a small portion (or a plurality of portions) in the molecule of a naphthyl group-containing polymerizable unsaturated monomer by introducing a functional group, substituting an atom, or other chemical reaction. It is a general compound obtained by changing. For example, naphthalene contains one or more functional groups such as an alkyl group, an alkoxy group, a hydroxyl group, a sulfonic acid group, a carboxyl group, an amino group, a nitro group, a halogen atom, an aryloxy group, an alkylthio group and an arylthio group. The introduced compound is a naphthalene derivative.

The naphthyl group-containing polymerizable unsaturated monomer or its derivative often has a weight average molecular weight (molecular weight in terms of standard polystyrene measured using a gel permeation chromatograph (GPC)) of 1,000 to 100,000. , 3,000 to 50,000, more preferably.

In the present embodiment, the ratio (x / y) of the mass (x) of the naphthyl group-containing polymerizable unsaturated monomer or its derivative to the mass (y) of the graphite-based negative electrode active material is 3.21 × 10 ^{-4 to} 3. It is .51 × 10 ^-4 . By setting the above x / y in a predetermined range, the effect of adding the graphite-based negative electrode active material and the naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof is appropriately exhibited. As a result, it is possible to suppress capacity deterioration in a high temperature environment and improve input characteristics in a low temperature environment. For example, when the above x / y is set to a predetermined value or more, the effect of adding the naphthyl group-containing polymerizable unsaturated monomer or its derivative is appropriately exhibited, and the liquid retention property of the negative electrode is improved. This makes it possible to form a good conductive path on the negative electrode and reduce the reaction resistance. From the viewpoint of improving the low temperature input characteristic, the x / y may be 3.3 × 10 ^-4 or more, for example, 3.36 × 10 ^-4 or more.

The lithium secondary battery of the present embodiment can exhibit excellent battery performance in a wide range of temperature environments due to the above-described configuration. Specifically, even if it is exposed to a high temperature environment for a long time, the capacity deterioration is small, and excellent storage characteristics can be realized. Further, the reaction resistance can be suppressed to a low level even in a low temperature environment, and excellent input characteristics can be realized. Taking advantage of the above characteristics, the lithium secondary battery of the present embodiment is preferably used as a power source for driving a motor mounted on a vehicle such as a plug-in hybrid vehicle, a hybrid vehicle, or an electric vehicle. Can be done.

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

<Preparation of positive electrode>
First, lithium nickel cobalt manganese-containing composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) as a positive electrode active material, trilithium phosphate (LPO, Li ₃ PO ₄ ), and as a dispersant. Naftyl group-containing polymerizable unsaturated monomer, polyvinylidene fluoride (PVdF) as a binder, acetylene black (AB) as a conductive material, and N-methyl-2-pyrrolidone (NMP) as an organic solvent. Was kneaded to prepare a positive electrode paste. At this time, the ratio (x / y) of the mass (x) of the naphthyl group-containing polymerizable unsaturated monomer to the mass (y) of the graphite-based negative electrode active material described later is the mass as shown in Tables 1 and 2 below. The standard was changed in the range of 0 (no additives) to 3.81 × 10 ^-4. Then, the positive electrode paste was applied to the surface of the aluminum foil and dried to prepare a positive electrode.

<Manufacturing of negative electrode>
First, natural graphite (BET specific surface area: 2.8 to 5.3 m ² / g) as a graphite-based negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxylmethyl cellulose as a thickener (BET). CMC) and ion-exchanged water as a solvent were mixed to prepare a negative electrode paste. Then, the negative electrode paste was applied to the surface of the copper foil and dried to prepare a negative electrode.

<Construction of lithium secondary battery>
Next, the prepared positive electrode and the prepared negative electrode were laminated via a resin separator to prepare an electrode body. _{Further, LiPF 6} as a lithium salt is added to a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) as a non-aqueous electrolytic solution. A dissolved product was prepared. Then, the electrode body and the non-aqueous electrolytic solution were housed in a battery case to construct a 4V class lithium secondary battery.

<Initial charge / discharge>
Next, each of the lithium secondary batteries constructed above is charged with a constant current at a rate of 0.2 C until the next charge / discharge operation: the battery voltage reaches 4.1 V in a temperature environment of 25 ° C. Constant voltage charge until current reaches 0.01C rate; constant current discharge (CC discharge) at 0.2C rate until battery voltage reaches 3.0V, then until current reaches 0.01C rate Constant voltage discharge (CV discharge) was performed; and the CCCV discharge capacity at this time was used as the initial capacity.

<Evaluation of capacity storage characteristics at 75 ° C>
The lithium secondary battery after the initial charge and discharge was installed in a constant temperature bath at 75 ° C. Next, the charge / discharge cycle of the lithium secondary battery was repeated for 100 days in a temperature environment of 75 ° C. The charge / discharge condition for one cycle was to charge / discharge between SOC 20 and 80% at a current rate of 2C. Next, the battery capacity (CCCV discharge capacity) after the charge / discharge cycle was measured in the same manner as the above initial capacity. Next, the capacity deterioration rate (%) was obtained by subtracting the battery capacity after the charge / discharge cycle from the initial capacity and dividing by the initial capacity. Then, the capacity deterioration slope (−% / √day) with respect to the charge / discharge cycle time (√day) was calculated from the root law. The results are shown in Table 1. The values in Table 1 are shown in bold, with the capacitance deterioration slope being suppressed to 1.70 (−% / √day) or less as a good product.

As shown in Table 1, the BET specific surface area of the graphite-based negative electrode active material is 4.8 m ² / g or less, and the x / y ratio is 3.21 × 10 ^-4 or more. Compared with the case where the BET specific surface area and / or the x / y ratio of the negative electrode active material is out of the above range, the capacitance deterioration can be suppressed to be small, and relatively high high temperature storage characteristics can be realized. The reason for this is considered to be that the formation of a high resistance film that contributes to capacity deterioration was suppressed by supporting a predetermined amount of the naphthyl group-containing polymerizable unsaturated monomer on the surface of the graphite-based negative electrode active material. ..

<Evaluation of input characteristics at -10 ° C and 56% SOC>
The lithium secondary battery after the initial charge and discharge was adjusted to a state of SOC 56% in a temperature environment of 25 ° C. Next, the lithium secondary battery adjusted to SOC 56% was subjected to constant current discharge at a rate of 10 C for 10 seconds. Then, the input value (W) was calculated from the product of the voltage value and the current value 10 seconds after the start of discharge. The results are shown in Table 2. The values in Table 2 show that the graphite-based negative electrode active material has a BET specific surface area of 2.8 m ² / g and an x / y ratio of 0 (that is, no naphthyl group-containing polymerizable unsaturated monomer is added). It is expressed as a relative ratio (%) when the input value of the test example is 100.0 (reference). In addition, those whose relative ratio is improved to 120% or more are regarded as good products, and the results are shown in bold.

As shown in Table 2, by setting the BET specific surface area of the graphite-based negative electrode active material to 3.8 m ² / g or more and setting the x / y ratio within a predetermined range, for example, the graphite-based negative electrode active material can be used. Compared with the case where the BET specific surface area is 3.3 m ² / g or less, a relatively high input value can be realized. The reason for this is that by setting the BET specific surface area of the graphite-based negative electrode active material to a predetermined value or more and further containing a naphthyl group-containing polymerizable unsaturated monomer, the liquid retention property of the negative electrode is improved and the reaction on the surface of the active material is improved. It is possible that the resistance has decreased.

From the above, the BET specific surface area of the graphite-based negative electrode active material is set to 3.8 to 4.8 m ² / g, and the ratio of the naphthyl group-containing polymerizable unsaturated monomer or its derivative to the graphite-based negative electrode active material is set. ^{By setting 3.21 × 10 -4 to} 3.51 × 10 ^{-4 on the} basis of mass, it is possible to realize a lithium secondary battery having both high temperature storage characteristics and low temperature input characteristics. Such results show the significance of the technology disclosed herein.

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

Claims (1)

A positive electrode containing a positive electrode active material, a negative electrode containing a graphite-based negative electrode active material, and a non-aqueous electrolyte solution are provided.
The graphite-based negative electrode active material has a BET specific surface area of 3.8 m ² / g or more and 4.8 m ² / g or less based on the nitrogen adsorption method.
At least one of the positive electrode, the negative electrode and the non-aqueous electrolytic solution contains a naphthyl group-containing polymerizable unsaturated monomer or a derivative thereof.
The ratio of the naphthyl group-containing polymerizable unsaturated monomer or its derivative to the graphite-based negative electrode active material is 3.21 × 10 ^-4 or more and 3.51 × 10 ^-4 or less on a mass basis. battery.