JP6895078B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Conventionally, various configurations of a positive electrode and a negative electrode have been studied for the purpose of improving the performance of a lithium ion secondary battery (see Patent Documents 1 and 2). For example, Patent Document 1 discloses a lithium ion secondary battery in which a negative electrode contains a negative electrode active material and a charge storage material that stores a charge carrier at a higher potential than the negative electrode active material.

Japanese Unexamined Patent Publication No. 2013-235565 Japanese Unexamined Patent Publication No. 2012-124181

By the way, the present inventors are studying the addition of an _{inorganic phosphate, specifically trilithium phosphate (Li 3} PO ₄ ), to the positive electrode for the purpose of improving overcharge resistance and the like. However, according to the study by the present inventors, for example, even if _{the amount of Li 3} PO ₄ added to the positive electrode is the same, the battery resistance during normal use and the heat generation temperature during overcharging may vary. .. In addition, the battery resistance during normal use may vary depending on the properties of the negative electrode active material.

The present invention has been made in view of this point, and an object of the present invention is to provide a lithium ion secondary battery having both high input / output characteristics and overcharge resistance.

The present inventors have made various studies on the causes of the above-mentioned variations in battery characteristics. As a result, _{it was found that the specific surface area of Li 3} PO ₄ and the blackness, which is an index indicating cracking or chipping of the negative electrode active material, greatly affect the variation in the battery characteristics. Then, further studies were repeated to define these in an appropriate range, and the present invention was created.

INDUSTRIAL APPLICABILITY The present invention provides a lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode contains a positive electrode active material and Li ₃ PO ₄ . The _Li 3 PO ₄ is, BET specific surface area based on nitrogen adsorption method, is less than 1.3 m ² / g or more 5.89m ² / g. The negative electrode contains a negative electrode active material. The negative electrode active material has a blackness of 24 or less as measured in the following (Procedure 1) to (Procedure 4).
(Procedure 1) Prepare a dispersion liquid containing the above negative electrode active material and an aqueous solvent.
(Procedure 2) The dispersion liquid is centrifuged to recover the supernatant liquid containing the free active material liberated from the negative electrode active material.
(Procedure 3) A sample cell containing the supernatant is placed on an absorptiometer, and the absorbance at each wavelength of 600 nm, 700 nm, 800 nm, and 900 nm is measured.
(Procedure 4) The blackness is calculated by multiplying the sum of the absorbances measured at each wavelength by a predetermined coefficient corresponding to the optical path length of the sample cell.

When _{the specific surface area of Li 3} PO ₄ and the blackness of the negative electrode active material satisfy the above ranges, the battery resistance can be suppressed low. As a result, excellent input / output characteristics can be stably realized even in a low temperature environment, for example. Further, _{the effect of adding Li 3} PO ₄ is appropriately exhibited at the time of overcharging, and the heat generation temperature at the time of overcharging can be suppressed to a low level. As a result, excellent overcharge resistance can be stably realized.

Hereinafter, preferred embodiments of the lithium ion 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 ion 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 typically includes a positive electrode current collector and a positive electrode active material layer having a porous structure fixed on the positive electrode current collector. As the positive electrode current collector, a metal foil such as an aluminum foil is suitable. The positive electrode active material layer contains at least the positive electrode active material and trilithium phosphate (Li ₃ PO ₄ ). The positive electrode is produced, for example, _{by applying a positive electrode paste obtained by kneading a positive electrode active material and Li 3} PO ₄ in an appropriate solvent onto the surface of a positive electrode current collector and drying it.

The positive electrode active material is a material capable of reversibly storing and releasing 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. The ratio of the positive electrode active material to the entire positive electrode active material layer (100% by mass) is generally 50 to 95% by mass, for example, 80 to 90% by mass.

The positive electrode active material is typically in the form of particles. The specific surface area of the positive electrode active material (the surface area measured by the constant volume adsorption method using nitrogen gas and the BET specific surface area analyzed by the BET method; the same applies hereinafter) is not particularly limited, but is typically 0.1 to 1. It is preferably 5 m ² / g, for example 0.5 to ^{3 m 2 / g.}

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 (typically, hydrolysis of the supporting salt contained in the non-aqueous electrolyte). (C) Capturing or consuming hydrofluoric acid (HF) produced by hydrolysis of a fluorine-containing supporting salt (for example, fluorine-containing lithium salt) to alleviate the acidity (pH) of the non-aqueous electrolyte; (d). It forms a stable film on the surface of the opposite negative electrode; it is a material that exerts at least one of the actions. 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, for example, input / output characteristics. Further, at the time of overcharging, the effect of suppressing the rise in battery temperature and improving the overcharge resistance is achieved. _{The ratio of Li 3} PO _{4 to} the entire positive electrode active material layer (100% by mass) is approximately 0.1 to 20% by mass, typically 0.5 to 10% by mass, for example, 1 to 10% by mass. Good.

Li ₃ PO ₄ is typically in the form of particles. In this embodiment, the specific surface area of _{Li 3} PO ₄ ^{is 1.3 to 5.89 m 2} / g. As a result, _{the action of Li 3} PO ₄ is less likely to vary, and _{the effect of adding Li 3} PO ₄ can be stably realized.
That is, by _{setting the specific surface area of Li 3} PO ₄ to a predetermined value or more, _{the particle size of Li 3} PO ₄ can be reduced, and Li ₃ PO ₄ can be widely distributed in the positive electrode active material layer. As a result, the above-mentioned actions (a) to (d) can be more exerted. For example, a stable film can be formed over a wide range on the surface of the facing negative electrode, and heat generation during overcharging can be efficiently suppressed. From this point of view, it is preferable that the specific surface area of _{Li 3} PO ₄ ^{is 1.89 m 2} / g or more, for example, 3.65 m ^{2 / g or more.} Further, by _{setting the specific surface area of Li 3} PO ₄ to a predetermined value or less, for example, the dispersibility of the conductive auxiliary agent in the positive electrode paste can be improved, and the handleability and homogeneity of the positive electrode paste can be further improved. it can. As a result, excellent input / output characteristics can be stably realized. From this point of view, the specific surface area of _{Li 3} PO ₄ ^{is preferably 5.32 m 2} / g or less.

The positive electrode may contain an additional optional component, if necessary, in addition to the above-mentioned positive electrode active material and Li ₃ PO _4. Examples of the optional component include a binder, a conductive auxiliary agent, a thickener, a dispersant, a pH adjuster and the like. 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 typically includes a negative electrode current collector and a negative electrode active material layer having a porous structure fixed on the negative electrode current collector. As the negative electrode current collector, a metal foil such as a copper foil is suitable. The negative electrode active material layer contains at least the negative electrode active material. The negative electrode is produced, for example, by applying a negative electrode paste obtained by kneading a negative electrode active material in an appropriate solvent onto the surface of a negative electrode current collector and drying it.

The negative electrode active material is a material capable of reversibly storing and releasing lithium ions, which are charge carriers. A carbon material can be mentioned as a preferable example of the negative electrode active material. Of these, graphite-based carbon materials such as natural graphite, artificial graphite, and amorphous coated graphite are preferable. Since the graphite-based carbon material has a high orientation of the carbon hexagonal network structure, qualitative variations such as cracks and chips are likely to occur due to the load of stress. Therefore, the application of the techniques disclosed herein is particularly effective. In the present specification, the “graphite-based carbon material” refers to a material in which the proportion of graphite in the carbon material is approximately 50% by mass or more, typically 80% by mass or more. The ratio of the negative electrode active material to the entire negative electrode active material layer (100% by mass) is generally 50 to 99% by mass, for example, 90 to 98% by mass.

The negative electrode active material is typically in the form of particles. In the present embodiment, the blackness of the negative electrode active material is 24 or less. The degree of blackness is an index showing properties such as cracking, peeling, and chipping of the negative electrode active material. By setting the blackness of the negative electrode active material to a predetermined value or less, it is possible to suppress an excessive increase in the reaction active sites on the surface of the negative electrode active material, and it is difficult for the negative electrode to have qualitative variation. As a result, excellent input / output characteristics can be stably exhibited. From this point of view, the blackness of the negative electrode active material is preferably 20 or less. On the other hand, according to the study by the present inventors, by setting the blackness of the negative electrode active material to a predetermined value or more, a stable film can be distributed on the surface of the negative electrode active material, and heat generation during overcharging can be generated. It can be suppressed better. From this point of view, the blackness of the negative electrode active material may be approximately 7 or more, for example, 21 or more. A specific method for measuring the blackness will be described in a test example described later.

The negative electrode may contain an additional optional component, if necessary, in addition to the above-mentioned negative electrode active material. Examples of the optional component include a binder, a thickener, a dispersant and the like. Examples of the binder include rubbers such as styrene-butadiene rubber (SBR). Examples of the dispersant include celluloses such as carboxymethyl cellulose (CMC).

The non-aqueous electrolyte is the same as the conventional one and is not particularly limited. The non-aqueous electrolyte is a non-aqueous electrolyte solution that typically contains a supporting salt and a non-aqueous solvent and exhibits a liquid state at room temperature (25 ° C.). The supporting salt dissociates in a non-aqueous solvent to produce a charge carrier (lithium ion). Examples of the supporting salt include various lithium salts, for example, 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.

In the lithium ion secondary battery of the present embodiment, a good conductive path is formed in the positive and negative electrodes by the above-described configuration, and the battery resistance can be stably suppressed to a low level. Therefore, for example, excellent input / output characteristics can be exhibited even in a low temperature environment where the battery resistance tends to be high. Further, in the lithium ion secondary battery of the present embodiment, heat generation at the time of overcharging can be stably suppressed. Therefore, excellent overcharge resistance can be exhibited.
Taking advantage of the above characteristics, the lithium ion secondary battery of the present embodiment is preferably used, for example, in an application where a high-rate charge / discharge cycle is expected or in an application where high overcharge resistance is required. it can. The lithium ion secondary battery of the present embodiment can be 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.

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, as a positive electrode active material, a layered structure lithium nickel cobalt manganese-containing composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ : NCM) was prepared. _{In addition, trilithium phosphate (Li 3} PO ₄ : LPO) having the specific surface area shown in Table 1 below was prepared. Next, the positive electrode active material (NCM), trilithium phosphate (LPO), polyvinylidene fluoride (PVdF) as a binder, acetylene black (AB) as a conductive auxiliary agent, and N- as a solvent. A positive electrode paste was prepared by mixing with methyl-2-pyrrolidone (NMP). Then, this positive electrode paste was applied to the surface of the aluminum foil and dried to prepare a positive electrode.

<Measurement of blackness of negative electrode active material>
In this test example, a plurality of negative electrode active materials having different states of cracking and chipping were prepared by pulverizing a commercially available negative electrode active material with a predetermined shear stress. Then, the blackness of these was measured according to the following procedures 1 to 4. The results are shown in Table 1.
-Procedure 1: The negative electrode active material and ion-exchanged water were mixed so as to have a mass ratio of 1:10 to prepare a dispersion liquid.
-Procedure 2: The dispersion liquid was centrifuged at a centrifugal force of 10000 G for 30 minutes, and then the supernatant liquid was recovered. The supernatant liquid contains a free active material released from the main body of the negative electrode active material due to cracking or chipping of the negative electrode active material. Next, ion-exchanged water was added to the supernatant and diluted 5-fold by volume.
-Procedure 3: The diluted supernatant was placed in a sample cell for measurement and placed in an absorptiometer. Here, the optical path length is set to 25 mm. Next, the absorbance of the supernatant was measured at each wavelength of 600 nm, 700 nm, 800 nm, and 900 nm.
-Procedure 4: The blackness was calculated by multiplying the sum of the absorbances measured at each of the above wavelengths by a coefficient corresponding to the optical path length of the sample cell ("5" in the case of an optical path length of 25 mm).

<Manufacturing of negative electrode>
First, as the negative electrode active material, natural graphite having a blackness of 7 to 28 was prepared. Next, the negative electrode active material (natural graphite), styrene-butadiene rubber (SBR) as a binder, carboxylmethyl cellulose (CMC) as a thickener, and ion-exchanged water as a solvent are mixed to produce a negative electrode. A paste was prepared. Then, this negative electrode paste was applied to the surface of the copper foil and dried to prepare a negative electrode.

<Construction of lithium-ion secondary battery>
Next, the prepared positive electrode and the prepared negative electrode were laminated via a resin separator to prepare an electrode body. _{Further, as a non-aqueous electrolyte solution, a solution prepared by dissolving LiPF 6} as a lithium salt in a mixed solvent containing a cyclic carbonate and a chain carbonate so as to have a concentration of 1 mol / L was prepared. Then, the electrode body and the non-aqueous electrolytic solution were housed in a battery case to construct a 4V class lithium ion secondary battery.

<Initial charge / discharge>
Next, for each of the lithium ion secondary batteries constructed above, in a temperature environment of 25 ° C., the next charge / discharge operation: (i) Constant current at a rate of 0.2 C until the battery voltage reaches 4.1 V. After charging, constant voltage charge until the current reaches a rate of 0.01C; (ii) constant current discharge at a rate of 0.2C until the battery voltage reaches 3.0V, then a rate of 0.01C. The constant voltage discharge was repeated for a total of 3 cycles.

<I / O measurement at -10 ° C>
Under a temperature environment of 25 ° C., the lithium ion secondary battery after the initial charge / discharge was adjusted to a state of SOC (State of Charge) of 60%. Next, the lithium ion secondary battery was installed in a constant temperature bath at −10 ° C. Next, in a temperature environment of −10 ° C., a battery adjusted to SOC 60% was subjected to constant current charging and constant current discharging for 10 seconds at a rate of 10 C. Next, the −10 ° C. input value (W) and the −10 ° C. output value (W) were calculated from the product of the voltage value and the current value 10 seconds after the start of charging / discharging. The results are shown in Table 1.

<Overcharge test>
A thermocouple was attached to the outer surface of the lithium ion secondary battery and installed in a constant temperature bath at −10 ° C. Next, in a temperature environment of −10 ° C., the lithium ion secondary battery was constantly charged with a constant current until the battery voltage reached 40 V (overcharged state). Then, the maximum battery temperature at this time was recorded as "heat generation temperature (° C.)". The results are shown in Table 1.

As shown in Table 1, by setting the specific surface area of _{Li 3} PO ₄ ^{to 1.3 m 2} / g or more, the heat generation temperature at the time of overcharging can be lowered, specifically, it can be suppressed to 30.2 ° C. or less. It was. In particular, by setting the specific surface area of _{Li 3} PO ₄ ^{to 3.65 m 2} / g or more, it was possible to better suppress heat generation during overcharging.
Further, by setting the specific surface area of _{Li 3} PO ₄ ^{to 5.89 m 2} / g or less and the blackness of the negative electrode active material to 24 or less, _{the specific surface area of Li 3} PO ₄ and / or the black color of the negative electrode active material. Compared with the case where the degree exceeds the above range, the input / output value of −10 ° C. can be relatively improved. In particular, by setting the specific surface area of _{Li 3} PO ₄ ^{to 5.32 m 2} / g or less and the blackness of the negative electrode active material to 20 or less, higher input / output values could be realized.
The above 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, a negative electrode, and a non-aqueous electrolyte are provided.
The positive electrode contains a positive electrode active material and Li ₃ PO ₄ .
The _Li 3 PO ₄ is, BET specific surface area based on nitrogen adsorption method, not more than 1.3 m ² / g or more 5.89m ² / g,
The negative electrode contains a negative electrode active material and contains a negative electrode active material.
The negative electrode active material is a natural graphite, and a lithium ion secondary battery having a blackness of 24 or less as measured in the following (Procedure 1) to (Procedure 4).

(Procedure 1) A dispersion liquid containing the negative electrode active material and an aqueous solvent is prepared.
(Procedure 2) The dispersion liquid is centrifuged to recover the supernatant liquid containing the free active material released from the negative electrode active material.
(Procedure 3) A sample cell containing the supernatant is placed on an absorptiometer, and the absorbance at each wavelength of 600 nm, 700 nm, 800 nm, and 900 nm is measured.
(Procedure 4) The blackness is calculated by multiplying the sum of the absorbances measured at each wavelength by a predetermined coefficient corresponding to the optical path length of the sample cell.