JP7542571B2 - Nonaqueous electrolyte secondary battery and method of manufacturing same - Google Patents

Description

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

In non-aqueous electrolyte secondary batteries, it is known to vary the density or porosity of the negative electrode active material layer in each region (e.g., Patent Documents 1 to 3). It is also known to provide a non-facing region in the negative electrode active material layer of a non-aqueous electrolyte secondary battery that does not face the positive electrode active material layer (e.g., Patent Document 2).

JP 2005-209411 A JP 2014-207201 A JP 2007-220450 A

It has been found that when a non-aqueous electrolyte secondary battery having a negative electrode active material layer with a non-facing region is stored in a high SOC (State Of Charge) state, lithium ions diffuse from the region facing the positive electrode active material layer to the non-facing region in the negative electrode active material layer, causing capacity degradation of the non-aqueous electrolyte secondary battery.

The objective of this disclosure is to provide a non-aqueous electrolyte secondary battery that can suppress capacity degradation when stored at a high SOC.

[1] A nonaqueous electrolyte secondary battery in which a negative electrode active material layer and a positive electrode active material layer face each other via an electrolyte and a separator,
the negative electrode active material layer has a first region and a second region having different densities,
the first region includes an opposing region opposing the positive electrode active material layer, and the opposing region occupies the largest proportion of the area in a plan view;
the second region includes a non-facing region that does not face the positive electrode active material layer, and the non-facing region occupies the largest proportion of the area in a plan view;
a density d1 [g/cm ³ ] of the first region and a density d2 [g/cm ³ ] of the second region satisfy the relationship of the following formula (1):
d2/d1≧1.22 (1)
[2] The first region and the second region contain a negative electrode active material,
The nonaqueous electrolyte secondary battery according to [1], wherein the negative electrode active material contains a Si-based compound.
[3] The nonaqueous electrolyte secondary battery according to [1] or [2], wherein an area Sn of the negative electrode active material layer in a planar view and an area S1 of the first region in a planar view satisfy the relationship of the following formula (2):
1.02≦Sn/S1≦1.20 (2)
[4] The nonaqueous electrolyte secondary battery according to any one of [1] to [3], wherein the first region includes the non-facing region.
[5] The nonaqueous electrolyte secondary battery according to any one of [1] to [4], wherein the second region includes an end portion of the negative electrode active material layer.

This disclosure provides a nonaqueous electrolyte secondary battery that can suppress capacity degradation when stored at a high SOC.

1 is a schematic diagram showing a configuration of an electrode body included in a nonaqueous electrolyte secondary battery according to one embodiment of the present invention. 2 is a cross-sectional view showing a schematic cross section of line II-II' in FIG. 1.

(Nonaqueous electrolyte secondary battery)
Fig. 1 is a schematic diagram showing the configuration of an electrode assembly included in a nonaqueous electrolyte secondary battery according to one embodiment of the present invention. In the non-aqueous electrolyte secondary battery of this embodiment (hereinafter sometimes referred to as the "battery"), a negative electrode active material layer 15 and a positive electrode active material layer 25 are disposed between an electrolyte (not shown) and a separator. 30.

The negative electrode active material layer 15, the positive electrode active material layer 25, and the separator 30 constitute the electrode body 1 of the battery. The electrode body may be of a wound type or a laminated type. In the electrode body 1, as shown in Figures 1 and 2, the negative electrode body 10 and the positive electrode body 20 are laminated with the separator 30 interposed therebetween. The negative electrode body 10 has a negative electrode current collector 16 and a negative electrode active material layer 15 formed on the negative electrode current collector 16. The positive electrode body 20 has a positive electrode current collector 26 and a positive electrode active material layer 25 formed on the positive electrode current collector 26.

The negative electrode collector 16 is a metal foil made of, for example, a copper material such as copper or a copper alloy. The positive electrode collector 26 is a metal foil made of, for example, an aluminum material such as aluminum or an aluminum alloy.

The battery may include an exterior body that contains the above-mentioned electrode body and electrolyte.

(Negative Electrode Active Material Layer)
In the present battery, the negative electrode active material layer 15 has a first region 11 and a second region 12 having different densities. In the negative electrode active material layer 15, the density d1 [g/cm ³ ] of the first region 11 and the density d2 [g/cm ³ ] of the second region 12 satisfy the relationship of the following formula (1).
d2/d1≧1.22 (1)

In the negative electrode active material layer 15 shown in Figures 1 and 2, the first region 11 is a region that is composed only of a facing region that faces the positive electrode active material layer 25 through the separator 30 and the electrolyte, and the second region 12 is a region that is composed only of a non-facing region that does not face the positive electrode active material layer 25 through the separator 30 and the electrolyte.

The first region 11 may be composed of only the facing region, but may also include a non-facing region as long as the facing region occupies the largest proportion of the area of the first region 11 in a planar view. In the area of the positive electrode active material layer 25 in a planar view, the proportion of the area of the facing region to the area of the first region 11 may be 90% or more, 95% or more, or even 100%. It is preferable that the first region 11 includes a non-facing region.

The second region 12 may be composed of only the non-facing region, but may also include the facing region as long as the proportion of the non-facing region in the area of the second region 12 in plan view is the largest. If the proportion of the facing region and the non-facing region in the first region 11 and the second region 12 is within the above range, the boundary position between the first region 11 and the second region 12 in the negative electrode active material layer 15 may be in the facing region or in the non-facing region. In the area of the positive electrode active material layer 25 in plan view, the proportion of the area of the non-facing region to the area of the second region 12 may be 90% or more, 95% or more, or even 100%. When two or more second regions 12 are included as described later, the above proportion in the second region 12 is the proportion in one second region 12.

In a non-aqueous electrolyte secondary battery having a negative electrode active material layer with a facing region and a non-facing region, when the battery is charged, charge carriers (lithium ions) are more likely to be absorbed in the facing region than in the non-facing region of the negative electrode active material layer. Therefore, as the battery is charged, the potential difference between the facing region and the non-facing region of the negative electrode active material layer increases, and the charge carriers are more likely to diffuse from the facing region to the non-facing region through the electrolyte present in the pores (gaps) of the negative electrode active material layer. Charge carriers that have diffused to the non-facing region tend not to return to the positive electrode active material layer even when the battery is discharged, so the diffusion of charge carriers from the facing region to the non-facing region can cause the capacity of the battery to deteriorate.

The negative electrode active material layer 15 of this battery has a facing region and a non-facing region, but since the first region 11 and the second region 12 satisfy the relationship of the above formula (1), it is considered that the volume of the pores in the second region 12 is smaller than the volume of the pores in the first region 11. As a result, even if the charging of this battery progresses and the potential difference between the facing region and the non-facing region increases, the diffusion of charge carriers from the first region 11 to the second region 12 can be suppressed. This makes it possible to suppress the occlusion of charge carriers in the second region 12, thereby suppressing the capacity deterioration when this battery is stored in a high SOC state.

In the above formula (1), d2/d1 is preferably 1.25 or more, may be 1.30 or more, may be 1.40 or more, may be 1.45 or more, and is usually 2.0 or less. The density d1 may be, for example, 0.7 g/cm ³ or more and 1.8 g/cm ³ or less, or 1.0 g/cm ³ or more and 1.5 g/cm ³ or less. The density d2 may be, for example, 0.8 g/cm ³ or more and 2.2 g/cm ³ or less, or 1.2 g/cm ³ or more and 1.8 g/cm ³ or less. By having d2/d1 in the above range, the capacity deterioration of the present battery when stored in a high SOC state can be well suppressed. The densities d1 and d2 in the above formula (1) can be determined by measuring the weight and thickness per unit area of the negative electrode active material layer in each region.

As shown in FIG. 1 and FIG. 2, the first region 11 of the negative electrode active material layer 15 is preferably provided in the center of the negative electrode active material layer 15 in a plan view, and preferably does not include the end of the negative electrode active material layer 15. The second region 12 is preferably provided so as to include the end of the negative electrode active material layer 15 in a plan view. The end of the negative electrode active material layer 15 included in the second region 12 may be one, or may be two as shown in FIG. 1 and FIG. 2, or may be three or more, and may include all the ends of the negative electrode active material layer 15. When there are two or more second regions 12 in the negative electrode active material layer 15, it is sufficient that any one of the second regions 12 satisfies the relationship of the above formula (1), but it is preferable that all the second regions 12 satisfy the relationship of the above formula (1). It is preferable that the first region 11 and the second region 12 that satisfy the relationship of the above formula (1) are adjacent to each other in a plan view.

The thickness of the first region 11 and the thickness of the second region 12 may be the same or different. The thickness of the first region 11 may be greater than the thickness of the second region 12.

The method of forming the first region 11 and the second region 12 in the negative electrode active material layer 15 is not particularly limited. For example, a method of forming a negative electrode mixture layer with a constant weight per unit area by applying a negative electrode mixture slurry for forming the negative electrode active material layer 15 to the negative electrode current collector 16 and drying it, and then forming regions with different densities in the negative electrode active material layer by changing the amount of compression of the negative electrode mixture layer; a method of forming a negative electrode mixture layer with a different weight per unit area by changing the amount of application of the negative electrode mixture slurry, compressing the negative electrode mixture layer at the same compression rate, and forming regions with different densities in the negative electrode active material layer, and the like can be mentioned.

When the area S1 of the first region 11 in a plan view is taken as Sn, it is preferable that the area S1 of the first region 11 in a plan view satisfies the relationship of the following formula (2).
1.02≦Sn/S1≦1.20 (2)

In the above formula (2), Sn/S1 may be 1.03 or more and 1.15 or less, 1.05 or more and 1.13 or less, or 1.10 or more and 1.14 or less. The ratio Sn/S2 of the area S2 of the second region 12 in a planar view to the area Sn of the negative electrode active material layer 15 in a planar view may be, for example, 5 or more and 50 or less, 7 or more and 35 or less, or 7 or more and 20 or less.

By setting Sn/S1 within the above range, the capacity degradation of the battery when stored at a high SOC can be more effectively suppressed. By setting Sn/S2 within the above range, the capacity degradation of the battery when stored at a high SOC can be more effectively suppressed. The areas Sn, S1, and S2 are measured as the areas of the negative electrode active material layer 15 in a plan view. When there are two or more second regions 12 in the negative electrode active material layer 15, the area S2 of the second regions is calculated as the sum of the areas of the second regions 12 that satisfy the relationship of the above formula (1).

The first region and the second region include a negative electrode active material. The negative electrode active material may be a known material that can be used as a negative electrode active material for a non-aqueous electrolyte secondary battery. Examples of the negative electrode active material include carbon-based active materials containing carbon (C), such as graphite, hard carbon, soft carbon, and amorphous coated graphite; and metal-based active materials containing metal elements such as metal simple substances or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). The negative electrode active material preferably contains a metal-based active material in addition to the carbon-based active material, and more preferably contains a Si-based active material as the metal-based active material. The negative electrode active material preferably contains graphite and a Si-based active material. Examples of the Si-based active material include silicon simple substance, SiOx, LixSiyOz, and the like. In a negative electrode active material layer that contains a Si-based active material, the potential difference between the facing region and the non-facing region is likely to increase, and charge carrier diffusion is likely to occur. However, by using a negative electrode active material layer 15 that satisfies the relationship of the above formula (1), the diffusion of charge carriers can be effectively suppressed.

The types of negative electrode active materials contained in the first region 11 and the second region 12 may be different from each other, but are preferably the same. When the first region 11 and the second region 12 each contain two or more negative electrode active materials, it is preferable that the types of negative electrode active materials contained in these regions are the same from each other, and that the content ratios (weight ratios) of the two or more negative electrode active materials are also the same.

The negative electrode active material layer 15 may contain additives other than the negative electrode active material, such as binders, thickeners, dispersants, and fibrous carbon. These additives may be any known material that can be used as an additive for non-aqueous electrolyte secondary batteries. Examples of binders include styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), and polytetrafluoroethylene (PTFE). Examples of thickeners include carboxymethyl cellulose (CMC) and methyl cellulose (MC). Examples of fibrous carbon include carbon nanotubes (CNT).

The negative electrode active material layer 15 can be formed by applying a negative electrode mixture slurry, which is prepared by mixing a negative electrode mixture containing a negative electrode active material and an additive with a solvent, to a negative electrode current collector, drying the slurry, and compressing the slurry.

(Positive Electrode Active Material Layer)
The positive electrode active material layer 25 includes a positive electrode active material. The positive electrode active material may be any known material that can be used as a positive electrode active material for non-aqueous electrolyte secondary batteries. Examples of the positive electrode active material include layered or spinel lithium transition metal oxides (e.g., _LiNiCoMnO2 , _LiNiO2 , _{LiCoO2 , LiFeO2 , LiMn2O4 , LiNi0.5Mn1.5O4 ,} LiCrMnO4, _LiFePO4 , _{LiNi1 /} 3Co1/ _3Mn1 / _3O2 ).

The positive electrode active material layer 25 may contain additives other than the positive electrode active material, such as binders and conductive materials. These additives may be any known material that can be used as an additive for non-aqueous electrolyte secondary batteries. Examples of binders include styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), and polytetrafluoroethylene (PTFE). Examples of conductive materials include carbon materials such as carbon black (e.g., acetylene black, ketjen black), coke, activated carbon, graphite, carbon fiber, and carbon nanotubes. Examples of binders include polyvinylidene fluoride (PVdF) and polyethylene oxide (PEO).

The positive electrode active material layer 25 can be formed by applying a positive electrode mixture slurry, which is prepared by mixing a positive electrode mixture containing a positive electrode active material and an additive with a solvent, to a positive electrode current collector, drying the slurry, and compressing the slurry.

(Separator)
The separator may be a known material that can be used as an electrolyte for a non-aqueous electrolyte secondary battery. Examples of the separator include a porous sheet (film, nonwoven fabric) made of a resin such as polyethylene, polypropylene, polyester, cellulose, or polyamide. The porous sheet may have a single layer structure or a multilayer structure of two or more layers.

(Electrolytes)
The electrolyte may be a known material that can be used as an electrolyte for a non-aqueous electrolyte secondary battery. The electrolyte may be a non-aqueous electrolyte solution, for example, a non-aqueous solvent such as an organic solvent containing a supporting salt. The non-aqueous solvent may be one or more selected from the group consisting of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The supporting salt may be, for example, a lithium salt (LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , etc.).

The present disclosure will be described more specifically below with reference to examples and comparative examples.
Example 1
(Preparation of positive electrode plate (positive electrode body))
A positive electrode mixture was prepared by mixing 1 part by weight of acetylene black (AB) as a conductive material and 1 part by weight of polyvinylidene fluoride (pVdF) as a binder with 100 parts by weight of LiNiCoMnO ₂ as a positive electrode active material. The positive electrode mixture and N-methyl-2-pyrrolidone (NMP) were mixed to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum foil as a positive electrode current collector and dried to form a positive electrode mixture layer. Next, the positive electrode mixture layer was compressed to form a positive electrode active material layer on the aluminum foil, which was then cut out to form a positive electrode plate.

(Preparation of negative electrode plate (negative electrode body))
A negative electrode mixture was prepared by mixing 1 part by weight of styrene butadiene rubber (SBR) as a binder and 1 part by weight of carboxymethyl cellulose (CMC) as a thickener with 100 parts by weight of graphite as a negative electrode active material. The negative electrode mixture and water were mixed to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to a copper foil as a negative electrode current collector and dried to form a negative electrode mixture layer. The weight of the negative electrode mixture layer was made constant throughout. Next, the entire surface of the negative electrode mixture layer was compressed using a roller, and then a region that would become a non-opposing region that does not face the positive electrode active material layer in a battery described later was compressed using a flat plate press. As a result, a negative electrode active material layer having a second region compressed by a flat plate press and a first region not compressed by a flat plate press was formed on the copper foil, and this was cut out to form a negative electrode plate. As shown in Figures 1 and 2, the negative electrode active material layer of the negative electrode plate had two second regions each including an end portion, and a first region adjacent to the two second regions and disposed in the center. In a plan view, the ratio of the area Sn of the negative electrode active material layer of the negative electrode plate to the area S1 of the first region was 1.1. The density d1 of the first region and the density d2 of the second region were determined by the procedure described below. The results are shown in Table 1.

(Battery Construction)
A three-layer separator in which polypropylene/polyethylene/polypropylene were laminated in this order was prepared. The negative electrode plate and the positive electrode plate were laminated through the separator so that the positive electrode active material layer faced the first region of the negative electrode active material layer to prepare an electrode body. In the electrode body, the negative electrode plate and the positive electrode plate were laminated so that the aluminum foil of the positive electrode current collector was exposed at one end and the copper foil of the negative electrode current collector was exposed at the other end. The aluminum foil exposed at one end of the electrode body was welded to an aluminum plate for external current collection, and the copper foil exposed at the other end was welded to a copper plate for external current collection, which was then inserted into an exterior body of an aluminum laminate film and welded. Then, an electrolyte was injected into the exterior body and the opening of the exterior body was sealed to prepare a nonaqueous electrolyte secondary battery. The electrolyte used was an electrolyte solution prepared by dissolving _LiPF6 at a concentration of 1 mol/L in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of EC:EMC = 1:3. In the electrode body of the produced battery, the first regions were all opposing regions, the second regions were all non-opposing regions, and the ends included in the second region of the negative electrode active material layer were ends of the negative electrode active material layer located at positions corresponding to the one end and the other end of the electrode body.

[Comparative Examples 1 and 2, Examples 2 and 3]
A negative electrode plate was produced in the same manner as in Example 1, except that compression by flat pressing was performed so that the density d2 of the second region became the density shown in Table 1, and a battery was produced using this negative electrode plate.

[Examples 4 to 6]
A negative electrode plate was prepared in the same manner as in Example 1, except that the ratio of the area Sn of the negative electrode active material layer to the area S1 of the first region in a plan view was set to the ratio shown in Table 1, and a battery was prepared using this negative electrode plate.

[Comparative Examples 3 and 4, Examples 7 to 9]
A negative electrode plate was prepared in the same manner as in Comparative Examples 1 and 2 and Examples 1 to 3, except that graphite and SiO were used as the negative electrode active material, and a battery was prepared using this negative electrode plate.

[Measurement of Density of Negative Electrode Active Material Layer]
The first and second regions were cut out from the negative electrode plate prepared above, and the weight per unit area of the negative electrode active material layer in the first and second regions and the thickness of the negative electrode active material layer in the first and second regions were measured, excluding the weight and thickness of the negative electrode current collector, to calculate the density d1 of the first region and the density d2 of the second region. The results are shown in Table 1.

[Ratio of Area Sn of Negative Electrode Active Material Layer to Area S1 of First Region]
In the negative electrode plate produced above, the area Sn of the negative electrode active material layer in a plan view and the area S1 of the first region were measured, and the ratio Sn/S1 was calculated.

[Evaluation of Capacity Retention Rate]
In an environment at a temperature of 25° C., the battery prepared above was charged at a current value of C/3 to 4.2 Vcccv, and then discharged to 2.5 V at a current value of C/3, and the discharge capacity at this time was taken as the initial capacity.

The battery prepared above was charged at 4.2 Vcccv at a current value of C/3 in an environment at a temperature of 25° C., and then stored in this charged state for 20 days in an environment at a temperature of 60° C. Next, after leaving it in an environment at a temperature of 25° C. for 4 hours, it was discharged to 2.5 V at a current value of C/3, and the discharge capacity at this time was taken as the capacity after storage. The capacity retention rate was calculated according to the following formula. The results are shown in Table 1.
Capacity retention rate [%] = (capacity after storage/initial capacity) x 100

Compared to Comparative Example 1, Comparative Example 2 and Examples 1 to 3 have improved capacity retention rates, and it is estimated that the capacity retention rate is saturated when the ratio d2/d1 is 1.25 or more. Compared to Comparative Example 3, Comparative Example 4 and Examples 7 to 9 have improved capacity retention rates, and it is estimated that the capacity retention rate is saturated when the ratio d2/d1 is 1.25 or more. From these results, it is believed that by achieving the relationship of the above formula (1), the diffusion of lithium ions from the first region to the second region is sufficiently suppressed when the battery is stored in a high SOC state, and the capacity deterioration of the battery can be sufficiently suppressed.

In Comparative Example 3, in which graphite and SiOx were used as the negative electrode active material, the capacity retention rate was lower than in Comparative Example 1, in which graphite was used as the negative electrode active material. In Comparative Example 3, the potential difference between the first and second regions was increased when the battery was stored in a high SOC state, and this is thought to have led to a lower capacity retention rate compared to Comparative Example 1. For this reason, when the first and second regions contain Si-based active materials, it can be said that it is preferable to have the relationship of the above formula (1).

The results of Examples 1 and 4 to 6 show that the smaller the area ratio Sn/S1, the better the capacity retention rate, and that the larger the area ratio Sn/S1, the lower the capacity retention rate tends to be.

The embodiments and examples disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

1 Electrode body, 10 Negative electrode body, 11 First region, 12 Second region, 15 Negative electrode active material layer, 16 Negative electrode current collector, 20 Positive electrode body, 25 Positive electrode active material layer, 26 Positive electrode current collector, 30 Separator.

Claims (6)

A non-aqueous electrolyte secondary battery in which a negative electrode active material layer and a positive electrode active material layer face each other via an electrolyte and a separator,
the negative electrode active material layer has a first region and a second region having the same composition and different densities;
the first region and the second region contain a negative electrode active material,
The negative electrode active material includes a Si-based compound,
the first region includes an opposing region opposing the positive electrode active material layer, and the opposing region occupies the largest proportion of the area in a plan view;
The second region includes a non-facing region that does not face the positive electrode active material layer and the facing region , and the non-facing region occupies the largest proportion of an area in a plan view (excluding a case where the second region is disposed so as to surround the entire periphery of the first region in a plan view of the negative electrode active material layer).
The thickness of the first region is greater than or equal to the thickness of the second region;
a density d1 [g/cm ³ ] of the first region and a density d2 [g/cm ³ ] of the second region satisfy the relationship of the following formula (1):
d2/d1≧1.22 (1) The nonaqueous electrolyte secondary battery according to claim 1 , wherein the negative electrode active material further comprises a carbon-based active material . 2 . The nonaqueous electrolyte secondary battery according to claim 1 , wherein an area Sn of the negative electrode active material layer in a plan view and an area S1 of the first region in a plan view satisfy the relationship of the following formula (2):
1.02≦Sn/S1≦1.20 (2) The nonaqueous electrolyte secondary battery according to claim 1 , wherein the second region includes an end portion of the negative electrode active material layer. the second region is two regions each including an end portion of the negative electrode active material layer that faces each other in a plan view of the negative electrode active material layer,
The nonaqueous electrolyte secondary battery according to claim 4 , wherein each of the two regions of the second region is adjacent to the first region. A method for producing a nonaqueous electrolyte secondary battery according to any one of claims 1 to 5, comprising the steps of:
forming a negative electrode mixture layer by applying and drying a negative electrode mixture slurry for forming the negative electrode active material layer; and compressing the negative electrode mixture layer;
A method for producing a nonaqueous electrolyte secondary battery, comprising forming the first region and the second region by the following [i] or [ii]:
[i] The negative electrode mixture layer is formed so that its basis weight is constant throughout, and the size to which the negative electrode mixture layer is compressed is changed.
[ii] The negative electrode mixture layer is formed so that the coating amount of the negative electrode mixture slurry is changed to have different coating weights in different regions, and the negative electrode mixture layer is compressed at the same compression ratio.