JP2020149794A - Nonaqueous lithium ion secondary battery - Google Patents

Abstract

To provide a nonaqueous lithium ion secondary battery, suppressed in generation of gas in a battery and high in Li precipitation resistance.SOLUTION: According to the present invention, there is provided a nonaqueous lithium ion secondary battery including a positive electrode, a negative electrode 10, and a nonaqueous electrolyte. The negative electrode 10 includes a negative electrode active material 16 and a gas adsorbent 18. The gas adsorbent 18 includes a carbon material 18A and an insulation coat layer 18B disposed on a surface of the carbon material 18A, and has a peak pore diameter in the range of 1 nm or more and 50 nm or less in a pore distribution curve obtained by the BJH method from a nitrogen adsorption isotherm.SELECTED DRAWING: Figure 1

Description

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

In a non-aqueous lithium ion secondary battery, it is required to suppress battery swelling caused by gas generated in the battery in consideration of high temperature storage property and durability. Patent documents 1 to 4 can be mentioned as prior art documents related to this. For example, Patent Document 1 includes a negative electrode having a negative electrode active material layer, a positive electrode having a positive electrode active material layer, and a non-aqueous electrolyte, and the negative electrode active material layer and / or the positive electrode active material layer has a specific surface area of 30 m. A non-aqueous lithium ion secondary battery to which a gas-adsorbed carbon material made of a carbon material of ² / g or more is added is disclosed. According to Patent Document 1, the gas generated in the battery can be adsorbed on a gas-adsorbed carbon material having a high specific surface area to reduce the amount of gas in the battery and suppress the swelling of the battery.

Japanese Unexamined Patent Publication No. 2004-227818 Japanese Unexamined Patent Publication No. 2008-037682 International Publication No. 2015/166839 Japanese Unexamined Patent Publication No. 2016-134267

However, according to the study by the present inventors, when a carbon material having a high specific surface area is used as it is as the gas-adsorbed carbon material as in Patent Document 1, the non-aqueous electrolyte can be decomposed on the surface of the gas-adsorbed carbon material. Therefore, the gas-adsorbed carbon material itself can cause gas generation. Therefore, the technique of Patent Document 1 has a limited effect of reducing the amount of gas in the battery, and the effect is insufficient in a high-capacity non-aqueous lithium ion secondary battery such as an in-vehicle battery. .. Further, according to the study by the present inventors, for example, in high-rate cycle charging / discharging, the gas generated in the battery becomes bubbles and stays between the positive and negative electrodes, resulting in unevenness in the distance between the positive and negative electrodes and Li precipitation resistance. It was also newly found that

The present invention has been made in view of this point, and an object of the present invention is to provide a non-aqueous lithium ion secondary battery in which gas generation in a battery is suppressed and Li precipitation resistance is high.

INDUSTRIAL APPLICABILITY The present invention provides a non-aqueous lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode active material and a gas adsorbent. The gas adsorbent includes a carbon material and an insulating coat layer arranged on the surface of the carbon material, and has a peak pore diameter of 1 nm in a pore distribution curve obtained by the BJH method from a nitrogen adsorption isotherm. It is in the range of 50 nm or less.

In the non-aqueous lithium ion secondary battery having the above configuration, the surface of the gas adsorbent is insulating and the pore diameter is optimized. As a result, the non-aqueous electrolyte is less likely to be decomposed on the surface of the gas adsorbent, and the amount of gas accumulated in the battery can be kept low even in a high temperature environment, for example. Further, in the negative electrode, Li precipitation caused by gas generation can be reduced, and Li precipitation resistance can be improved even in, for example, high-rate cycle charging / discharging.

It is sectional drawing which shows typically the negative electrode which concerns on one Embodiment. It is sectional drawing which shows typically the negative electrode of the comparative example 1. FIG. It is sectional drawing which shows typically the negative electrode of the comparative example 2. 3 is a pore distribution curve of the gas adsorbent according to Example 3 and Comparative Example 3. It is a graph which shows the charge / discharge pattern in a high rate cycle test. It is a graph which shows the charge / discharge pattern at the time of the discharge capacity measurement.

Hereinafter, preferred embodiments of the non-aqueous lithium ion secondary battery disclosed herein will be described. It should be noted that matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention (for example, battery components such as positive electrodes and non-aqueous electrolytes that do not characterize the present invention, and general batteries). Manufacturing process, etc.) can be grasped as a design item of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art. Further, in the present specification, the notation of "A to B" (A and B are arbitrary numbers) indicating the range means "preferably larger than A" and "preferably smaller than B" with the meaning of A or more and B or less. It shall include the meaning of.

Although not shown, the non-aqueous lithium ion secondary battery according to the present embodiment is configured such that an electrode body and a non-aqueous electrolytic solution are housed in an exterior body. The electrode body includes a positive electrode and a negative electrode 10 (see FIG. 1). The electrode body may be, for example, a wound electrode body in which a band-shaped positive electrode and a band-shaped negative electrode are laminated via a band-shaped separator and wound in the longitudinal direction. The wound electrode body may have a flat shape. The exterior body may have a square (rectangular parallelepiped shape) outer shape corresponding to, for example, a flat wound electrode body. The material of the exterior body may be a lightweight metal such as aluminum or a laminated film. The non-aqueous lithium ion secondary battery may be a laminated battery.

The positive electrode may be the same as the conventional one, and is not particularly limited. The positive electrode typically comprises a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. The positive electrode current collector is a conductive member. Examples of the material of the positive electrode current collector include metals having good conductivity such as aluminum and nickel. The positive electrode active material layer essentially contains a positive electrode active material capable of reversibly occluding and releasing charge carriers, and may contain other optional components (for example, a conductive material, a binder, etc.). Examples of the positive electrode active material include lithium transition metal composite oxides such as lithium nickel manganese composite oxide and lithium nickel manganese cobalt composite oxide. Examples of the conductive material include carbon materials such as carbon black (typically acetylene black (AB)). Examples of the binder include vinyl halide resins such as polyvinylidene fluoride (PVdF).

FIG. 1 is a cross-sectional view schematically showing the negative electrode 10. The negative electrode 10 includes a negative electrode current collector 12 and a negative electrode active material layer 14 fixed on the negative electrode current collector 12. The negative electrode current collector 12 is a conductive member. Examples of the material of the negative electrode current collector 12 include metals having good conductivity such as copper and nickel. The negative electrode active material layer 14 contains a negative electrode active material 16 capable of reversibly storing and releasing charge carriers, and a gas adsorbent 18.

The negative electrode active material 16 is not particularly limited, and one or more of the conventionally used ones can be appropriately selected and used. The negative electrode active material may be, for example, a graphite-based carbon material, a lithium transition metal composite oxide such as lithium titanate, a lithium transition metal composite nitride, a silicon compound containing a Si element, or the like. The graphite-based carbon material is a general term for a carbon material composed of only graphite and a carbon material in which graphite accounts for 50% by mass or more. The negative electrode active material 16 is in the form of particles (powder). Although not particularly limited, the average particle size of the negative electrode active material 16 (the particle size corresponding to the cumulative 50% of the volume standard in the particle size distribution based on the laser diffraction / light scattering method; the same applies hereinafter) is approximately the same. It may be 1 to 30 μm, for example 5 to 20 μm. The average particle size of the negative electrode active material 16 may be larger than the average particle size of the gas adsorbent 18.

The gas adsorbent 18 includes a carbon material 18A and an insulating coat layer 18B arranged on the surface of the carbon material 18A. The gas adsorbent 18 may be composed mainly of the carbon material 18A (meaning that it occupies the largest amount on a volume basis. The same shall apply hereinafter). The gas adsorbent 18 may be in the form of particles (powder). The gas adsorbent 18 is formed by coating, for example, a part or all of the surface of a carbon material 18A serving as a core (base material) with an insulating material (child material) having a higher insulating property than the carbon material 18A. The insulating material can be coated by a conventionally known method such as a sputtering method. In the gas adsorbent 18, the insulating coat layer 18B may be formed on more than half of the total surface area. For example, the insulating coat layer 18B may be formed on substantially the entire surface.

Examples of the carbon material 18A include carbon black such as acetylene black (AB) and Ketjen black (KB), and amorphous carbon materials such as activated carbon and carbon fiber. The carbon material 18A may be in the form of particles (powder). The insulating coat layer 18B contains an insulating material. Examples of the insulating material include ceramic materials such as barium titanate (BaTIO ₃ ) and aluminum oxide (alumina, Al ₂ O ₃ ). The insulating material is preferably a dielectric material having a relative permittivity higher than that of the carbon material 18A. Of these, a ferroelectric substance such as barium titanate is preferable. As a result, the reaction activity is increased on the surface of the negative electrode active material 16, and the Li precipitation resistance can be improved at a higher level. The type of element contained in the insulating coat layer 18B is, for example, the surface of the negative electrode active material layer 14 is observed with a scanning electron microscope (SEM), and the obtained observation image is observed by energy dispersive X-ray spectroscopy (EDX). It can be confirmed by analyzing with. Further, the composition of the dielectric material can be confirmed by, for example, a diffraction pattern obtained by an X-ray diffraction method (XRD).

In the technique disclosed herein, the pore distribution of the gas adsorbent 18 is optimized. The gas adsorbent 18 has a peak pore diameter (pore diameter showing the maximum peak in the pore distribution curve obtained by the BJH method from the nitrogen adsorption isotherm obtained by nitrogen gas adsorption measurement; the same applies hereinafter). It is in the range of 50 nm. As a result, the gas in the battery can be effectively captured. From the viewpoint of reducing the amount of gas in the battery at a higher level, the peak pore diameter of the gas adsorbent 18 is preferably in the range of 5 to 26 nm. The gas adsorbent 18 has a typical BET specific surface area (surface area obtained by the BET method (for example, the BET multipoint method) from the nitrogen adsorption isotherm obtained by nitrogen gas adsorption measurement. The same applies hereinafter) of about 30 m ² / g or more. may is 30~2000m ² / g, for example 100~1000m ² / g in specific. By using the gas adsorbent 18 having such a high specific surface area, the effect of the technique disclosed herein can be exhibited at a higher level.

The negative electrode active material layer 14 may contain an optional component (for example, a binder, a thickener, a dispersant, etc.) in addition to the negative electrode active material 16 and the gas adsorbent 18. Examples of the binder include rubbers such as styrene-butadiene rubber (SBR) and fluorinated resins such as polytetrafluoroethylene (PTFE). Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC).

Although not particularly limited, the ratio of the negative electrode active material 16 to the entire negative electrode active material layer 14 is preferably about 80% by mass or more from the viewpoint of achieving high energy density, and is typically 80 to 80 to It is preferably 99% by mass, for example, 85 to 95% by mass. The ratio of the gas adsorbent 18 to the entire negative electrode active material layer 14 is preferably about 1% by mass or more, and typically 1 to 20 from the viewpoint of exerting the effect of the technique disclosed here at a high level. It is preferably mass%, for example 5 to 15 mass%. Further, when the negative electrode active material 16 is 100 parts by mass, the ratio of the gas adsorbent 18 is generally 1 to 20 parts by mass, for example, 5 to 15 parts by mass.

The non-aqueous electrolyte may be the same as the conventional one, and is not particularly limited. The non-aqueous electrolyte may be liquid or polymer-like (gel-like) within the operating temperature range of the non-aqueous lithium ion secondary battery, for example, within the temperature range of -20 to + 60 ° C. The non-aqueous electrolyte may be a non-aqueous electrolyte solution. The non-aqueous electrolyte typically has a composition in which a non-aqueous solvent contains a supporting salt. Examples of the non-aqueous solvent include carbonates such as ethylene carbonate, diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate, ethers and esters. Examples of the supporting salt include lithium salts such as LiPF ₆ and LiBF ₄ .

The non-aqueous lithium ion secondary battery disclosed herein can be used for various purposes, but is preferably used in applications where high-rate charging / discharging of, for example, 2C or more, 5C or more, and further 10C or more, for example, 10 to 30C is repeated. Can be done. Note that "1C" means a current value capable of charging the rated capacity (Ah) of the battery predicted from the theoretical capacity in one hour. Suitable applications include drive power supplies mounted on vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). The non-aqueous lithium ion secondary battery can be used in the form of an assembled battery in which a plurality of batteries are connected in series and / or in parallel.

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

[Comparative Example 1]
<Preparation of Positive Electrode> First, in Comparative Example 1, LiNiComnO ₂ (NCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were used as NCM: AB. A positive electrode slurry was prepared by mixing at a mass ratio of PVdF = 92: 5: 3. Next, the prepared positive electrode slurry was applied to the surface of a band-shaped aluminum foil (positive electrode current collector, average thickness 15 μm), dried, and pressed to a predetermined thickness to prepare a band-shaped positive electrode.

<Preparation of Negative Electrode> First, graphite (C, average particle size 10 μm) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are used as C: SBR. A negative electrode slurry was prepared by mixing at a mass ratio of: CMC = 99: 0.5: 0.5. Next, the above-prepared negative electrode slurry was applied to the surface of a band-shaped copper foil (negative electrode current collector, average thickness 10 μm), dried, and pressed to a predetermined thickness to prepare a band-shaped negative electrode. FIG. 2 is a cross-sectional view schematically showing the negative electrode 20 of Comparative Example 1. The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24. The negative electrode active material layer 24 contains the negative electrode active material 26, but does not contain a gas adsorbent.

<Construction of Non-Aqueous Lithium Ion Secondary Battery> First, the band-shaped positive electrode and the band-shaped negative electrode produced above were wound via a band-shaped separator to prepare a flat-shaped wound electrode body. The separator is a resin porous film (average thickness 24 μm) having a three-layer structure of PP / PE / PP in which PP layers are laminated on both sides of the PE layer, and ceramic particles provided on one side thereof. A heat-resistant layer (average thickness 4 μm) including the heat-resistant layer was used. Further, the separator was arranged so that the heat-resistant layer faces the positive electrode. Next, the wound electrode body was housed in a square outer body having a liquid injection hole, and the opening of the outer body was sealed. 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 = 3: 3: 4. Was dissolved at a concentration of 1.0 mol / L to prepare a non-aqueous electrolytic solution. Next, a non-aqueous electrolytic solution was injected from the liquid injection hole, and a sealing screw was tightened in the liquid injection hole to hermetically seal the exterior body. Then, after leaving it for a predetermined time so that the wound electrode body was impregnated with the non-aqueous electrolytic solution, initial charging and aging treatment at 60 ° C. were performed.
In this way, the non-aqueous lithium ion secondary battery of Comparative Example 1 was constructed.

[Comparative Example 2]
When the negative electrode was manufactured, activated carbon (AC, BET specific surface area: 780 m ² / g, peak pore diameter: 80.7 nm) was used as the gas-adsorbed carbon material, and C: AC: SBR: CMC = 89: 10: 0. A negative electrode slurry was prepared by mixing at a mass ratio of 5: 0.5. Further, the negative electrode slurry was applied so that the amount of the negative electrode active material was the same as that in Comparative Example 1. FIG. 3 is a cross-sectional view schematically showing the negative electrode 30 of Comparative Example 2. The negative electrode 30 includes a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 contains a negative electrode active material 36 and a gas-adsorbed carbon material 38 made of activated carbon. Except for this, the non-aqueous lithium ion secondary battery of Comparative Example 2 was constructed in the same manner as in Comparative Example 1.

[Examples 1 to 5, Comparative Example 3]
In Examples 1 to 5 and Comparative Example 3, a gas adsorbent was first produced.
In Example 1, a gas adsorbent (Example 1) was prepared by coating the surface of acetylene black as a base material with aluminum oxide as a child material by a sputtering method.
In Examples 2 to 4 and Comparative Example 3, the surface of activated carbon as a base material is coated with aluminum oxide (Al ₂ O ₃ ) as a child material by a sputtering method, and a gas adsorbent (Examples 2 to 4) is coated. , Comparative Example 3) was prepared. The pore structure of activated carbon as a base material is different in each test example.
In Example 5, a gas adsorbent (Example 5) was prepared by coating the surface of activated carbon as a base material with barium titanate (BaTIO ₃ ) as a child material by a sputtering method.
In all the test examples, a sputtering apparatus (model: E-200S) manufactured by Canon Anerva Co., Ltd. was used for sputtering. As the sputtering target, a 99% pure one manufactured by High Purity Chemical Laboratory Co., Ltd. was used.

Next, nitrogen gas adsorption measurement of the gas adsorbent (Examples 1 to 5 and Comparative Example 3) produced above was performed. Then, the BET specific surface area was determined from the obtained nitrogen adsorption isotherm by the BET multipoint method. Further, a pore distribution curve was obtained from the obtained nitrogen adsorption isotherm by the BJH method. FIG. 4 shows, as an example, the pore distribution curves of the gas adsorbent according to Example 3 and Comparative Example 3. The pore volume distribution curve shown in FIG. 4 is a differential pore volume obtained by plotting a value (dV / dD) obtained by differentiating the pore volume (V) by the pore diameter (D) with respect to the pore diameter (D) nm. It is a distribution. Then, the pore diameter showing the maximum peak on the pore distribution curve was determined as the peak pore diameter. The results are shown in Table 1.

Then, the non-aqueous lithium ion secondary batteries of Examples 1 to 5 and Comparative Example 3 were constructed in the same manner as in Comparative Example 2 except that a gas adsorbent was used instead of activated carbon when producing the negative electrode.

<Measurement of Gas Amount> First, the non-aqueous lithium ion secondary battery constructed above was adjusted to a state of charging with an SOC of 80%, and then left to stand in a temperature environment of 60 ° C. for 30 days. Then, the gas increase rate was calculated from the comparison of the amount of gas before and after the test. The results are shown in Table 1. In Table 1, the results are shown by relative values based on the gas increase rate of Comparative Example 1 (100%). As for the amount of gas, the smaller the value, the less gas accumulates in the battery, indicating that the battery swelling is suppressed.

<Evaluation of Li Precipitation Resistance> First, the non-aqueous lithium ion secondary battery constructed above is adjusted to a state of charge with an SOC of 56%, and then the charge / discharge pattern (charge / discharge) shown in FIG. 5 is observed in a temperature environment of −10 ° C. High-rate constant current charging / discharging was repeated for 1000 cycles at a current value (rate of 20C). Next, the discharge capacity before and after the test was measured using the charge / discharge pattern (charge / discharge current value: 0.5 C rate) shown in FIG. Then, the capacity retention rate was calculated by dividing the discharge capacity after the cycle by the discharge capacity before the cycle and multiplying by 100. The obtained results are shown in Table 1 as Li precipitation resistance. In Table 1, the results are shown by relative values based on the capacity retention rate of Comparative Example 1 (100%). As for the Li precipitation resistance (capacity retention rate), the larger the value, the higher the Li precipitation resistance and the smaller the capacity deterioration.

As shown in the column of "gas amount" in Table 1, in Comparative Example 2 in which the activated carbon itself is contained in the negative electrode as a gas adsorbent, the amount of gas in the battery is increased as compared with Comparative Example 1 in which the gas adsorbent is not included. It was. It is considered that the reason for this is that since the activated carbon has conductivity, the non-aqueous electrolytic solution is reduced and decomposed at the negative electrode, and the presence of the activated carbon promotes gas generation. On the other hand, in Examples 1 to 5 and Comparative Example 3, the amount of gas in the battery can be suppressed to be lower than that in Comparative Examples 1 and 2 by using the gas adsorbent whose surface of the carbon material is coated with insulation. Was there. It is considered that the reason for this is that the non-aqueous electrolyte is less likely to be decomposed on the surface of the gas adsorbent, and the generation of gas is suppressed. Further, it is considered that the gas adsorbent exerts its effect, for example, the gas generated at the negative electrode is adsorbed by the gas adsorbent, and as a result, the gas outflow from the negative electrode is suppressed.

As shown in the column of "Li precipitation resistance" in Table 1, in Comparative Example 2, the Li precipitation resistance was significantly deteriorated as compared with Comparative Example 1 which did not contain the gas adsorbent. The reason for this is that the gas generated at the negative electrode exists as bubbles at the interface between the separator and the negative electrode, and as a result of unevenness in the distance between the positive and negative electrodes, there are unreacted portions in the negative electrode that do not contribute to charging and discharging. It is possible that it has occurred. That is, it is considered that the resistance increased as a result of the substantial reaction region of the negative electrode decreasing. On the other hand, in Examples 1 to 5 in which the negative electrode contained a gas adsorbent having a peak pore diameter of 1 nm or more and 50 nm or less, the Li precipitation resistance was significantly improved as compared with Comparative Examples 1 and 2. The reason for this is considered to be that the reaction region of the negative electrode could be stably maintained as a result of suppressing the outflow of gas from the negative electrode by the action of the gas adsorbent.

From the above, by including a gas adsorbent in which the surface of the carbon material is insulatingly coated and the peak pore diameter is 1 nm or more and 50 nm or less in the negative electrode, gas generation in the battery can be suppressed, which is excellent. It was found that Li precipitation resistance can be exhibited. Such a result shows the technical significance of the present invention.

Further, as shown in the column of "Li precipitation resistance" in Table 1, in Example 5 in which barium titanate was used for the insulating coating of the gas adsorbent, the Li precipitation resistance could be improved at a higher level. The reason for this is not clear, but as a result of the dielectric polarization of barium titanate, for example, desolvation of charge carriers (lithium ions) is promoted at the interface between the negative electrode active material and the gas adsorbent, and the interface resistance is reduced. It is considered that the reaction on the surface of the negative electrode active material was promoted.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the specific examples illustrated above.

10 Negative electrode 12 Negative electrode current collector 14 Negative electrode active material layer 16 Negative electrode active material 18 Gas adsorbent 18A Carbon material 18B Insulation coat layer

Claims (1)

A non-aqueous lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte.
The negative electrode includes a negative electrode active material and a gas adsorbent.
The gas adsorbent is
It is provided with a carbon material and an insulating coat layer arranged on the surface of the carbon material, and
In the pore distribution curve obtained by the BJH method from the nitrogen adsorption isotherm, the peak pore diameter is in the range of 1 nm or more and 50 nm or less.
Non-aqueous lithium-ion secondary battery.

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