JP2019102189A - Nonaqueous secondary cell - Google Patents

Abstract

To stably provide a nonaqueous secondary cell capable of concurrently satisfying high-output characteristics and overcharge resistance.SOLUTION: The nonaqueous secondary cell includes a positive electrode, a negative electrode and a nonaqueous electrolyte. The positive electrode includes a positive electrode collector and a positive electrode active material layer. The positive electrode active material layer includes a positive electrode active material, LiPOand a binder and a variation (4σ variation) in the content of the LiPOis 1.89% or more to 58.91% or less.SELECTED DRAWING: Figure 1

Description

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

Conventionally, in the non-aqueous secondary battery, an inorganic phosphate is added to the battery in order to improve the battery performance and the overcharge resistance. For example, Patent Document 1 describes that the output characteristics and durability of the battery can be improved by containing trilithium phosphate (Li ₃ PO ₄ ) in a predetermined ratio in the positive electrode.

JP, 2015-103332, A

However, according to the study of the present inventors, even if the content ratio of Li ₃ PO _{4 in} the positive electrode is the same, the output characteristics and the overcharge resistance may vary, and the desired effect may not be obtained. The

  This invention is made in view of this point, The objective is to provide stably the non-aqueous secondary battery which has a high output characteristic and overcharge tolerance.

According to the study of the present inventors, it was found that the variation in the output characteristics and the overcharge resistance described above is due to the difference in the dispersibility of Li ₃ PO ₄ in the positive electrode active material layer.
FIGS. 1A and 1B are schematic views of positive electrode active material layers in which the content ratio of Li ₃ PO ₄ (LPO) is the same and the dispersibility of Li ₃ PO ₄ is different. FIG. 1A shows a case where the dispersibility of Li ₃ PO ₄ in the positive electrode active material layer is good. In FIG. 1A, Li ₃ PO ₄ is homogeneously contained in the entire positive electrode active material layer. On the other hand, FIG. 1 (B) represents the case where the dispersibility of Li ₃ PO ₄ is bad in the positive electrode active material layer. In FIG. 1 (B), Li ₃ PO ₄ is unevenly distributed on one side. Specifically, a large amount of Li ₃ PO ₄ is present on the right side of FIG. 1 (B), and the content ratio of Li ₃ PO ₄ is high. On the left side of FIG. 1 (B), less _Li 3 PO _4, the content of locally _Li 3 PO ₄ is lower.

However, an evaluation index representing such a qualitative difference of the positive electrode active material layer, that is, the dispersibility of Li ₃ PO ₄ has not been clarified in the past. Then, the present inventors considered creating an evaluation index showing the dispersibility of Li ₃ PO ₄ and evaluating the positive electrode active material layer based on this. Then, the present invention was completed after repeated intensive studies.

The present invention provides a non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, Li ₃ PO _4, and a binder, and the variation in the content of the Li ₃ PO ₄ determined in the following (Procedure 1) to (Procedure 4) (4σ variation) is 1.89% or more and 58.91% or less.
(Procedure 1) A total of eight pieces of the positive electrode active material layer are cut out with a size of 40 40 mm.
(Procedure 2) The cut out positive electrode active material layer is immersed in an acid solvent to extract a phosphorus atom in the acid solvent.
(Procedure 3) The above-mentioned acidic solvent is analyzed by inductively coupled plasma emission spectroscopy, and the above-mentioned phosphorus atoms are quantified respectively.
(Procedure 4) The arithmetic mean value and the 4σ value of the content of the Li ₃ PO ₄ are calculated based on the determination result of the phosphorus atom, and the 4σ value is divided by the arithmetic mean value to obtain the Li ₃ Calculate the variance of the content of PO ₄ (4σ variance).

The 4σ variation is one evaluation index that quantitatively represents the dispersibility of Li ₃ PO ₄ . When Li ₃ PO ₄ is dispersed in the positive electrode active material layer so as to satisfy the above-described 4σ variation range, excellent output characteristics can be realized. Further, at the time of overcharging, the effect of the addition of Li ₃ PO ₄ is appropriately exhibited, and the heat generation of the battery can be suppressed. Therefore, excellent overcharge resistance can be realized. Therefore, according to the present invention, it is possible to stably realize a non-aqueous secondary battery stably combining high power characteristics and overcharge resistance.

Are schematic views showing the dispersion of the Li ₃ PO _4, (A) is a case where dispersibility is good, represents (B) is a case where poor dispersibility, respectively. It is a schematic diagram showing the cutting-out method of a positive electrode active material layer based on one Embodiment. It is a graph showing the relation between the variation (4σ variation) of the content of Li ₃ PO ₄ and the 10 second output. Li ₃ is a graph showing the relationship between PO ₄ content of variation and (4 [sigma] variation) and the temperature rise amount at the time of overcharge ([Delta] T).

Hereinafter, preferred embodiments of the non-aqueous secondary battery disclosed herein will be described. The matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be understood as the design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in the present specification and common technical knowledge in the field.
In addition, when the numerical range is described as AB (here, A and B are arbitrary numerical values) in this specification, A or more and B or less are meant.

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

The positive electrode includes a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. As the positive electrode current collector, a metal sheet having good conductivity, for example, an aluminum foil is suitable. The positive electrode active material layer has a porous structure. The positive electrode active material layer contains at least a positive electrode active material, trilithium phosphate (Li ₃ PO ₄ ), and a binder (binder). Such a positive electrode is produced, for example, by applying a positive electrode paste obtained by kneading a positive electrode active material, Li ₃ PO ₄ and a binder in a suitable solvent on the surface of a positive electrode current collector and drying it. Be done.

  The positive electrode active material may be any material that can occlude and release charge carriers reversibly. Preferred examples of the positive electrode active material include lithium transition such as lithium nickel-containing composite oxide, lithium cobalt-containing composite oxide, lithium nickel cobalt-containing composite oxide, lithium manganese-containing composite oxide, lithium nickel cobalt manganese-containing composite oxide, etc. Metal complex oxides can be mentioned. From the viewpoint of high durability, a so-called 4 V class positive electrode active material having an operating potential of 4.2 V or less based on lithium metal is preferable in normal use. A lithium-nickel-cobalt-manganese-containing composite oxide having a layered structure can be mentioned as a preferable example of the 4V class positive electrode active material.

The positive electrode active material is typically in the form of particles. The average particle size of the positive electrode active material (50% by volume particle size based on laser diffraction / light scattering method (D ₅₀ particle size). The same applies hereinafter) is typically 1 to 20 μm, for example, about 3 to 10 μm Good. The specific surface area of the positive electrode active material (BET specific surface area analyzed by BET method surface area measured by fixed volume adsorption method using nitrogen gas. The same applies hereinafter) is typically 0.1 to ⁵ m ² / g, For example, it may be 0.5 to ³ m ² / g.

Li ₃ PO ₄ for example, (a) covers the surface of the positive electrode active material; (b) suppresses the oxidative decomposition of the non-aqueous electrolyte, typically the hydrolysis of the supporting salt contained in the non-aqueous electrolyte; c) capture or consume a fluorine-containing support salt, for example, a hydrofluoric acid (HF) generated by hydrolysis of a fluorine-containing lithium salt to reduce the acidity (pH) of the non-aqueous electrolyte; (d) the opposite negative electrode Forming a stable film on the surface of the at least one material. By such an action, Li ₃ PO ₄ suppresses the elution of the metal element from the positive electrode active material, and exhibits the effect of improving the battery characteristics in normal use, for example, the durability. Further, at the time of overcharging, an increase in the battery temperature is suppressed to exhibit an effect of improving overcharging resistance.

Li ₃ PO ₄ is typically particulate. The average particle size of Li ₃ PO ₄ may be about 0.1 to 30 μm, typically about 1 to 25 μm, for example, about 2 to 10 μm. It is preferable that the average particle size of Li ₃ PO ₄ is typically equivalent to the average particle size of the positive electrode active material (about ± 1 μm) or smaller than the average particle size of the positive electrode active material. Thereby, the above-mentioned action can be exhibited better. The specific surface area of Li ₃ PO ₄ may typically be 0.5 to 30 m ² / g, for example 1 to ²⁰ m ² / g. It is preferable that the specific surface area of Li ₃ PO ₄ is typically equivalent to the specific surface area of the positive electrode active material (about ± 1 m ² / g) or larger than the specific surface area of the positive electrode active material. Thereby, the above-mentioned action can be exhibited better.

  The binder is a component that holds the positive electrode active material layer integrally and fixes the positive electrode active material layer on the positive electrode current collector. Preferred examples of the binder include, for example, halogenated vinyl resins such as polyvinylidene fluoride (PVdF) and polyalkylene oxides such as polyethylene oxide (PEO).

The positive electrode active material layer may contain, in addition to the above-described positive electrode active material, Li ₃ PO _4, and a binder, further optional components as necessary. Examples of optional components include, for example, conductive materials, thickeners, pH adjusters, and the like. Examples of the conductive material include carbon materials such as carbon black such as acetylene black and ketjen black, activated carbon, graphite, and carbon fibers. As a thickener, celluloses, such as carboxymethylcellulose (CMC), are mentioned, for example. Examples of pH adjusters include acidic substances such as phosphoric acid.

The ratio of the positive electrode active material to the entire positive electrode active material layer (100% by mass) may be approximately 50 to 95% by mass, for example, 80 to 90% by mass. The proportion of Li ₃ PO _{4 in} the entire positive electrode active material layer may be approximately 0.1 to 20% by mass, for example, 0.5 to 10% by mass, preferably 1 to 10% by mass. When the ratio of Li ₃ PO ₄ is equal to or higher than the predetermined value, it is possible to better secure the effect of the addition of _Li 3 PO _4. When the ratio of Li ₃ PO ₄ is equal to or less than a predetermined value, it is possible to better reduce the internal resistance of the positive electrode.

In the technology disclosed herein, in the positive electrode active material layer, the variance (4σ variance) of the content of Li ₃ PO ₄ is 1.89 to 58.91%. By setting the above 4σ variance to 1.89% or more, Li ₃ PO ₄ is dispersed in the positive electrode active material layer with a predetermined variance. This can surprisingly improve the output characteristics of the battery. From this point of view, the 4σ variation may be approximately 5% or more, for example, 10% or more. Also, by setting the above-mentioned 4σ variance to 58.91% or less, the variance (localization) of the content of Li ₃ PO ₄ can be suppressed. This allows the effect of addition of Li ₃ PO ₄ is properly exerted, to suppress an increase in battery temperature during overcharge. From this point of view, the 4σ variation may be approximately 50% or less, for example, 40% or less, preferably 20% or less. The 4σ variation can be typically adjusted by the kneading conditions and drying conditions of the positive electrode paste, for example, the solid content of the positive electrode paste, the kneading time, the shear rate, the viscosity, the drying temperature, the drying rate and the like. The calculation method of the 4σ variation will be described in detail in the test example described later.

  The negative electrode is the same as in the prior art and is not particularly limited. The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. As the negative electrode current collector, a metal sheet having good conductivity, for example, a copper foil is suitable. The negative electrode active material layer has a porous structure. The negative electrode active material layer contains at least a negative electrode active material capable of reversibly absorbing and desorbing charge carriers. As a suitable example of a negative electrode active material, carbon materials, such as graphite, are mentioned, for example. The negative electrode active material layer may further contain optional components other than the negative electrode active material, such as a binder and a thickener.

The non-aqueous electrolyte is the same as in the prior art and is not particularly limited. The non-aqueous electrolyte is a non-aqueous electrolyte that typically contains a support salt and a non-aqueous solvent and exhibits a liquid state at room temperature (25 ° C.). The supporting salt dissociates in the non-aqueous solvent to form a charge carrier. As a supporting salt, typically, a lithium salt, for example, a fluorine-containing lithium salt such as LiPF ₆ or LiBF ₄ can be suitably used. As the non-aqueous solvent, for example, non-fluorinated or fluorinated carbonate can be suitably used. Examples of the carbonate include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and monofluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl-2,2, And linear carbonates such as 2-trifluoroethyl carbonate (MTFEC).

In the non-aqueous secondary battery of the present embodiment, when the variance (4σ variance) of the content of Li ₃ PO _{4 in} the positive electrode active material layer satisfies the above range, a good conductive path is formed in the positive electrode active material layer Internal resistance can be kept low. Therefore, excellent battery performance, for example, excellent output characteristics can be exhibited during normal use. Furthermore, at the time of overcharge, a rise in battery temperature can be suppressed, and excellent overcharge resistance can be exhibited.

  The non-aqueous secondary battery of the present embodiment can be preferably used, for example, in applications in which the usage mode in which high rate charge and discharge are repeated is assumed. For example, it can be preferably used as a power source for driving a motor mounted in a vehicle such as a plug-in hybrid vehicle, a hybrid vehicle, an electric vehicle, and the like.

  Hereinafter, although the test example regarding this invention is demonstrated, it is not intending limiting this invention to what is shown to this specific example.

  In this example, 13 types of positive electrode pastes having different kneading conditions were prepared to prepare a positive electrode. And the lithium ion secondary battery was constructed | assembled using the obtained positive electrode, and the performance of the battery was evaluated.

<Fabrication 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 a binder A positive electrode paste was prepared by kneading polyvinylidene fluoride (PVdF) as a mixture, acetylene black (AB) as a conductive material, and N-methyl-2-pyrrolidone (NMP) as an organic solvent. In addition, at the time of kneading | mixing of a positive electrode paste, solid content was adjusted in 50 to 65%, and the viscosity was adjusted in 2000 to 30000 mPa * s. And 13 types of positive electrodes were produced by apply | coating the obtained positive electrode paste on the surface of aluminum foil (positive electrode collector), and making it dry. At the time of drying of the positive electrode paste, the drying temperature was adjusted in the range of 100 to 200 ° C.

<Evaluation of the variation of the positive electrode active material layer>
According to the following procedure, the variance (4σ variance) of the content of Li ₃ PO ₄ in the positive electrode active material layer was evaluated.
Procedure 1: Using a punching punch, a total of eight positive electrode active material layers were punched from the positive electrode with a size of Φ 40 mm. In addition, as shown in FIG. 2, the position which pierce | punched the positive electrode active material layer was made into four places by the application start side of a positive electrode paste, and four places by the application finish side. That is, sampling was performed so as to be line symmetrical with respect to the middle position in the coating direction from the position farthest in the coating direction.
Procedure 2: The positive electrode active material layer punched out with a size of 40 40 mm was immersed in an acidic solvent (for example, sulfuric acid) to extract phosphorus atoms in the acidic solvent.
Procedure 3: The total amount of the above-mentioned acidic solvent was analyzed by inductively coupled plasma-emission spectroscopy (ICP-OES: Inductively Coupled Plasma-Optical Emission Spectroscopy) to quantify each phosphorus atom. Here, as an analyzer, an ICP emission analyzer manufactured by Shimadzu Corporation, model "ICPS-8100" was used.
Procedure 4: First, the content of Li ₃ PO ₄ was calculated based on the results of quantification of the phosphorus atom. The content of Li ₃ PO ₄ was calculated as the ratio (parts by mass) with respect to per unit mass of the positive electrode active material. Next, the arithmetic mean value and the 4σ value of the content of Li ₃ PO ₄ were calculated by using 8 measurement points. Next, a variance (4σ variance) of the content of Li ₃ PO ₄ was calculated by dividing the 4σ value by the arithmetic mean value. The 4σ value is expressed in% as a unit (100%) based on the arithmetic mean value.

<Construction of lithium ion secondary battery>
First, the prepared positive electrode and a negative electrode containing natural graphite as a negative electrode active material were laminated via a resin separator (made of polyolefin resin, shutdown temperature 135 ° C.) to produce an electrode body. Further, as a non-aqueous electrolytic solution, a mixed solvent containing cyclic carbonate and linear carbonate was prepared by dissolving LiPF ₆ as a lithium salt to a concentration of 1 mol / L. And the said electrode body and the said non-aqueous electrolyte were accommodated in the battery case, and 13 types of 4V class lithium ion secondary batteries were constructed | assembled.

<Initial charge and discharge>
Next, for each lithium ion secondary battery, in the temperature environment of 25 ° C., the following charge / discharge operation: after constant current charge at a rate of 0.2 C until the battery voltage becomes 4.1 V, the current becomes Constant-voltage charge to a rate of 0.01 C; constant-current discharge at a rate of 0.2 C until the battery voltage is 3.0 V, and then constant-voltage discharge to a rate of 0.01 C; went.

<Output measurement at 25 ° C, 56% SOC>
Under the temperature environment of 25 ° C., the lithium ion secondary battery prepared above was adjusted to a state of SOC (State of Charge) 56%. Next, constant current discharge was performed for 10 seconds at a rate of 10 C in a temperature environment of 25 ° C. for the battery adjusted to 56% SOC. Then, the output (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 FIG.

As shown in FIG. 3, by setting the variance (4σ variance) of the content of Li ₃ PO ₄ to 1.89% or more, the output can be 1000 W or more, and the 4σ variance is less than 1.89%. Relatively high output could be realized compared to the case. As a reason for this, when the 4σ variation is less than 1.89%, it is considered that as a result of changing the kneading conditions in order to break up the aggregation in the preparation of the positive electrode paste, the migration of the binder occurs.

<Measurement of temperature rise during overcharge>
The thermocouple was attached to the outer surface of the said lithium secondary battery, and it installed in the -10 degreeC thermostat. Next, in a temperature environment of −10 ° C., the lithium secondary battery was subjected to constant current charging until it was overcharged. As a result, the positive electrode and the negative electrode were conducted, and the battery was shut down (SD). Then, the temperature rise amount ΔT (° C.) was measured for 30 seconds after the shutdown. The results are shown in FIG.

As shown in FIG. 4, by setting the variance (4σ variance) of the content of Li ₃ PO ₄ to 58.91% or less, the temperature increase ΔT can be 6.0 ° C. or less, and the 4σ variance is As compared with the case of exceeding 58.91%, the temperature rise could be suppressed relatively small. As the reason for this, it is considered that Li ₃ PO ₄ is unevenly distributed in the positive electrode active material layer when the 4σ variation exceeds 58.91%, and the effect of the addition of Li ₃ PO ₄ is locally reduced. Be

  The present invention has been described in detail above, but the above embodiments and examples are merely examples, and the invention disclosed herein includes various modifications and alterations of the specific example described above.

Claims (1)

A positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode includes a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector,
The positive electrode active material layer is
Containing a positive electrode active material, Li ₃ PO ₄ and a binder,
Following (Step 1) and (Step 4) In the _Li 3 PO ₄ content of variation obtained (4 [sigma] variation) is, at 58.91% or less 1.89% or more, a non-aqueous secondary battery.

(Procedure 1) A total of eight pieces of the positive electrode active material layer are cut out with a size of 40 40 mm.
(Procedure 2) The cut out positive electrode active material layer is immersed in an acidic solvent to extract a phosphorus atom in the acidic solvent.
(Procedure 3) The above-mentioned acidic solvent is analyzed by inductively coupled plasma emission spectroscopy, and the above-mentioned phosphorus atoms are quantified respectively.
(Procedure 4) The arithmetic mean value and the 4σ value of the content of the Li ₃ PO ₄ are calculated based on the determination result of the phosphorus atom, and the Li value is divided by the arithmetic mean value to calculate the Li ₃ Calculate the variance of the content of PO ₄ (4σ variance).

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