JP6778396B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

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

In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been used for portable power sources such as personal computers and mobile terminals, and for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). It is suitably used for power supplies and the like.

With the widespread use of non-aqueous electrolyte secondary batteries, further improvement in performance is desired for non-aqueous electrolyte secondary batteries. Therefore, non-aqueous electrolyte secondary batteries have been developed from various viewpoints. Among them, as an example of development regarding a non-aqueous electrolyte, Patent Document 1 describes a carbonate solvent and an ether solvent as non-aqueous solvents, a supporting salt, and lithium bis (oxalate) borate (Li [B (C ₂ O)) as an additive. ₄ ) It is disclosed that a non-aqueous electrolyte secondary battery is produced using a non-aqueous electrolyte containing ₂ ]). Patent Document 1 states that by using such a non-aqueous electrolyte, precipitation of Na [B (C ₂ O ₄ ) ₂ ] on the electrode body due to contamination of a sodium component which is an unavoidable impurity can be prevented. Is described. According to Patent Document 1, it is possible to suppress the variation in the amount of the coating film formed on the surface of the negative electrode active material due to the decomposition of [B (C ₂ O ₄ ) ₂ ] of lithium bis (oxalate) borate, and as a result. It is described that an increase in internal resistance can be suppressed when charging and discharging are repeated, and that precipitation of a substance derived from a charge carrier (for example, a metal such as metallic lithium) can be suppressed.

Japanese Unexamined Patent Publication No. 2014-053193

However, as a result of diligent studies by the present inventor, it has been found that the technique described in Patent Document 1 has room for improvement in suppressing the precipitation of substances derived from charge carriers. It was also found that there is room for improvement in the voltage drop due to self-discharge.
Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery in which precipitation of a substance derived from a charge carrier is suppressed and a voltage drop due to self-discharge is suppressed.

The non-aqueous electrolyte secondary battery disclosed herein includes an electrode body including a positive electrode and a negative electrode, and a non-aqueous electrolyte. The non-aqueous electrolyte contains a carbonate-based solvent, an ether-based solvent, a supporting salt, and lithium bis (oxalate) borate (LiBOB). The supporting salt is lithium bis (fluorosulfonyl) imide (LiFSI). The concentration of lithium bis (fluorosulfonyl) imide in the non-aqueous electrolyte is 0.8 mol / L or more and 3 mol / L or less. The weight ratio of lithium bis (oxalate) borate to the non-aqueous electrolyte is 0.2% by mass or more and 0.4% by mass or less. The mass ratio of the ether solvent to lithium bis (oxalate) oxalate (ether solvent / lithium bis (oxalate) oxalate) is 25 or more.

According to such a configuration, when the non-aqueous electrolyte contains a highly polar ether solvent, the solubility of Na [B (C ₂ O ₄ ) ₂ ] in the non-aqueous electrolyte is improved, and Na [B] is improved. Precipitation of (C ₂ O ₄ ) ₂ ] on the electrode body can be suppressed. Therefore, the coating film derived from LiBOB formed on the electrode body can be made uniform, and as a result, the precipitation resistance of the substance derived from the charge carrier (metallic lithium in the present embodiment) can be improved. On the other hand, when the non-aqueous solvent contains an ether solvent, the polarity of the non-aqueous solvent becomes high, so that the decomposition of the supporting salt easily proceeds. However, by using LiFSI having a high binding strength as the supporting salt, it is possible to make it difficult for the decomposition of the supporting salt to proceed even when the non-aqueous solvent contains an ether solvent. Therefore, this makes it possible to suppress the voltage drop due to self-discharge. The concentration of LiFSI in the non-aqueous electrolyte is in a specific range, the mass ratio of LiBOB to the non-aqueous electrolyte is in a specific range, and the mass ratio of the ether solvent to LiBOB is in a specific range. It is possible to both suppress the precipitation of the derived substance and suppress the voltage drop due to self-discharge.

It is sectional drawing which shows typically the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the electrode body used for a lithium ion secondary battery.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. 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, general configurations and manufacturing processes of non-aqueous electrolyte secondary batteries that do not characterize the present invention). Can be grasped as a design matter 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 following drawings, members / parts having the same action are described with the same reference numerals. In addition, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

In the present specification, the "secondary battery" generally refers to a power storage device that can be charged and discharged repeatedly, and is a term that includes a so-called storage battery such as a lithium ion secondary battery and a power storage element such as an electric double layer capacitor. Hereinafter, the present invention will be described in detail using a lithium ion secondary battery as an example.
It should be noted that the present invention is not intended to be limited to those described in such an embodiment, and the technical idea of the present invention is another non-aqueous electrolyte secondary battery provided with another charge carrier (for example, sodium ion). It also applies to (for example, sodium ion secondary batteries).

FIG. 1 is a cross-sectional view schematically showing a lithium ion secondary battery 10 according to an embodiment. FIG. 2 is a schematic view showing the configuration of the electrode body 40 used in the lithium ion secondary battery 10. As shown in FIG. 1, the lithium ion secondary battery 10 includes a battery case 20 and an electrode body 40 (in FIG. 1, a wound electrode body).

The battery case 20 includes a case body 21 and a sealing plate 22. The case body 21 has a box shape having a rectangular opening at one end. Here, the case main body 21 has a bottomed rectangular parallelepiped shape in which one surface corresponding to the upper surface of the lithium ion secondary battery 10 in a normal use state is open. The sealing plate 22 is a member that closes the opening of the case body 21. The sealing plate 22 is composed of a substantially rectangular plate. The battery case 20 having a substantially hexahedral shape is formed by welding the sealing plate 22 to the peripheral edge of the opening of the case body 21. The battery case 20 is mainly composed of, for example, a lightweight metal material having good thermal conductivity. Examples of such a metal material include aluminum, stainless steel, nickel-plated steel and the like. Although a rectangular case is illustrated here, the battery case 20 is not limited to such a shape. For example, the battery case 20 may be a cylindrical case. Further, the battery case 20 may have a bag-like shape that wraps the electrode body, or may be a so-called laminated type battery case 20.

In the example shown in FIG. 1, a positive electrode terminal 23 (external terminal) and a negative electrode terminal 24 (external terminal) for external connection are attached to the sealing plate 22. A safety valve 30 and a liquid injection port 32 are formed on the sealing plate 22. The safety valve 30 is configured to release the internal pressure when the internal pressure of the battery case 20 rises above a predetermined level (for example, the set valve opening pressure is about 0.3 MPa to 1.0 MPa). Further, FIG. 1 shows a state in which the liquid injection port 32 is sealed by the sealing material 33 after the non-aqueous electrolyte 80 is injected.

As shown in FIG. 2, the electrode body 40 includes a band-shaped positive electrode (positive electrode sheet 50), a band-shaped negative electrode (negative electrode sheet 60), and a band-shaped separator (separators 72 and 74).

The positive electrode sheet 50 includes a strip-shaped positive electrode current collecting foil 51 and a positive electrode active material layer 53. For the positive electrode current collecting foil 51, for example, an aluminum foil or the like can be used. An exposed portion 52 is provided along the edge portion on one side of the positive electrode current collecting foil 51 in the width direction. In the illustrated example, the positive electrode active material layer 53 is formed on both sides of the positive electrode current collecting foil 51, except for the exposed portion 52 provided on the positive electrode current collecting foil 51. However, the positive electrode active material layer 53 may be formed on only one side of the positive electrode current collecting foil 51.

The positive electrode active material layer 53 typically contains a positive electrode active material, a conductive material, a binder, and the like. Examples of the positive electrode active material include lithium composite metal oxides such as a layered structure and a spinel structure (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O). ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiFePO _4, etc.) can be adopted. As the conductive material, a carbon material such as carbon black (for example, acetylene black or Ketjen black) can be adopted. As the binder, various polymer materials such as polyvinylidene fluoride (PVdF) and polyethylene oxide (PEO) can be adopted.

The negative electrode sheet 60 includes a strip-shaped negative electrode current collecting foil 61 and a negative electrode active material layer 63. For the negative electrode current collecting foil 61, for example, a copper foil or the like can be used. An exposed portion 62 is provided along the edge portion on one side of the negative electrode current collecting foil 61 in the width direction. In the illustrated example, the negative electrode active material layer 63 is formed on both sides of the negative electrode current collector foil 61, except for the exposed portion 62 provided on the negative electrode current collector foil 61. However, the negative electrode active material layer 63 may be formed on only one side of the negative electrode current collecting foil 61.

The negative electrode active material layer 63 typically contains a negative electrode active material, a binder, a thickener, and the like. As the negative electrode active material, for example, carbon materials such as graphite (natural graphite, artificial graphite) and low crystalline carbon (hard carbon, soft carbon) can be used, and graphite can be preferably adopted. As the binder, various polymer materials such as styrene-butadiene rubber (SBR) can be adopted. As the thickener, various polymer materials such as carboxymethyl cellulose (CMC) can be adopted.

As the separators 72 and 74, for example, a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide or the like can be used, and a polyolefin resin (eg, PE) is preferable. , PP), and a porous sheet is used. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separators 72 and 74 may be provided with a heat resistant layer (HRL).

In the electrode body 40, as shown in FIG. 2, the positive electrode sheet 50, the negative electrode sheet 60, and the separators 72 and 74 are wound in this order, and the wound electrode body (wound electrode body). Reference numeral 40 denotes a shape that is flatly bent along a plane including the winding axis WL. The width b1 of the negative electrode active material layer 63 is slightly wider than the width a1 of the positive electrode active material layer 53. Further, the widths c1 and c2 of the separators 72 and 74 are slightly wider than the width b1 of the negative electrode active material layer 63 (c1, c2>b1> a1). In the example shown in FIG. 2, the exposed portion 52 of the positive electrode current collecting foil 51 and the exposed portion 62 of the negative electrode current collecting foil 61 are spirally exposed on both sides of the separators 72 and 74, respectively. In the present embodiment, as shown in FIG. 1, in the electrode body 40, the intermediate portions of the positive and negative exposed portions 52 and 62 protruding from the separators 72 and 74 are gathered together, and the positive and negative electrodes are arranged inside the battery case 20. It is welded to the tips 23a and 24a of the internal terminals 23 and 24.
In the present embodiment, the electrode body 40 is configured as a wound electrode body, but it may be configured as a laminated electrode body.

Normally, as an unavoidable impurity, a sodium component is contained in the lithium ion secondary battery 10, particularly at least one of the positive electrode sheet 50 and the negative electrode sheet 60. The sodium (Na) component may exist as sodium alone (typically in an ionic state) or as a compound containing Na as a constituent element.

In the form shown in FIG. 1, a flat winding electrode body 40 is housed in the battery case 20 along one plane including the winding shaft WL. The battery case 20 further contains a non-aqueous electrolyte 80. The non-aqueous electrolyte 80 penetrates into the electrode body 40 from both sides in the axial direction of the winding shaft WL (see FIG. 2). Note that FIG. 1 does not strictly show the amount of the non-aqueous electrolyte 80 injected into the battery case 20.

The non-aqueous electrolyte 80 contains a carbonate-based solvent and an ether-based solvent as non-aqueous solvents.
As the carbonate solvent, the same solvent as that used for the non-aqueous electrolyte of a general lithium ion secondary battery or the same solvent can be used, and examples thereof include ethylene carbonate (EC) and propylene carbonate (PC). ), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluoro Examples thereof include dimethyl carbonate (TFDMC). The carbonate solvent can be used alone or in combination of two or more.
As the ether solvent, the same or similar one as that used for the non-aqueous electrolyte of a general lithium ion secondary battery can be used, and examples thereof include dimethoxyethane (DME), 1,2-. Examples thereof include chain ethers such as dimethoxypropane (DMP) and cyclic ethers such as tetrahydrofuran and dioxane. The ether solvent can be used alone or in combination of two or more.
When the non-aqueous electrolyte 80 contains an ether solvent which is a highly polar non-aqueous solvent, the solubility of Na [B (C ₂ O ₄ ) ₂ ] in the non-aqueous solvent is enhanced, and Na [B (C) ₂ ] is enhanced. ₂ O ₄ ) ₂ ] precipitation can be suppressed. Therefore, the coating film derived from LiBOB formed on the electrode body can be made uniform, and as a result, the precipitation resistance of the substance derived from the charge carrier (metallic lithium in the present embodiment) can be improved.
The proportion of the ether solvent in the non-aqueous solvent is preferably 5% by mass or more, more preferably 10% by mass or more, because the precipitation resistance of the substance derived from the charge carrier (metallic lithium in this embodiment) is particularly high. , 20% by mass or more is more preferable, and 40% by mass or more is most preferable. On the other hand, since the ether solvent has a low flash point, the proportion of the ether solvent in the non-aqueous solvent is preferably 70% by mass or less from the viewpoint of safety.
The non-aqueous solvent may contain a solvent other than the carbonate-based solvent and the ether-based solvent as long as the effects of the present invention are not impaired. Examples thereof include aprotic solvents such as ester solvents, nitrile solvents, sulfone solvents, and lactone solvents.

The non-aqueous electrolyte 80 contains a supporting salt, and lithium bis (fluorosulfonyl) imide (LiFSI) is used as the supporting salt. When the non-aqueous solvent contains an ether solvent, the polarity of the non-aqueous solvent becomes high, so that the decomposition of the supporting salt easily proceeds. However, by using LiFSI having a high binding strength as the supporting salt, it is possible to make it difficult for the decomposition of the supporting salt to proceed even when the non-aqueous solvent contains an ether solvent. Therefore, this makes it possible to suppress the voltage drop due to self-discharge.
The concentration of LiFSI in the non-aqueous electrolyte 80 is 0.8 mol / L or more and 3 mol / L or less. If the concentration of LiFSI is less than 0.8 mol / L, the supporting salt becomes insufficient, the ionic conductivity becomes too small, and the characteristic characteristics of the battery deteriorate. On the other hand, when the concentration of LiFSI exceeds 3 mol / L, the amount of decomposition of the supporting salt increases, and the amount of voltage drop due to self-discharge increases.

The non-aqueous electrolyte 80 contains lithium bis (oxalate) borate (LiBOB) as a film-forming agent.
The weight ratio of LiBOB to the non-aqueous electrolyte 80 is 0.2% by mass or more and 0.4% by mass or less. If the weight ratio of LiBOB is less than 0.2% by mass, a sufficient amount of film cannot be formed. On the other hand, when the weight ratio of LiBOB exceeds 0.4% by mass, Na [B (C ₂ O ₄ ) ₂ ] is likely to be precipitated, and the LiBOB-derived film formed on the electrode body becomes non-uniform. As a result, the precipitation resistance of the substance derived from the charge carrier (metallic lithium in this embodiment) is lowered.

Further, in the non-aqueous electrolyte 80, the mass ratio of the ether solvent to LiBOB (ether solvent / LiBOB) is 25 or more. If the mass ratio is less than 25, the effect of suppressing the precipitation of Na [B (C ₂ O ₄ ) ₂ ] by the ether solvent cannot be sufficiently obtained, and the substance derived from the charge carrier (in this embodiment, metallic lithium). ), The effect of improving the precipitation resistance cannot be sufficiently obtained.

As described above, the concentration of LiFSI in the non-aqueous electrolyte is in a specific range, the mass ratio of LiBOB to the non-aqueous electrolyte is in a specific range, and the mass ratio of the ether solvent to LiBOB is in a specific range. It is possible to both suppress the precipitation of the substance derived from the above and suppress the voltage drop due to self-discharge.

In the non-aqueous electrolyte 80, components other than the above-mentioned non-aqueous solvent and supporting salt, for example, a gas generating agent such as biphenyl (BP) and cyclohexylbenzene (CHB), as long as the effects of the present invention are not significantly impaired. It may contain various additives such as dispersant; thickener; etc.

The lithium ion secondary battery 10 configured as described above can be used for various purposes, and suitable applications include electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV), and the like. The drive power supply mounted on the vehicle of. The lithium ion secondary battery 10 can also be used in the form of an assembled battery, which typically consists of a plurality of batteries connected in series and / or in parallel.

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

<Making batteries for evaluation>
[Battery No. 1]
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material powder, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are used as LNCM: A slurry for forming a positive electrode active material layer was prepared by mixing with NMP at a mass ratio of AB: PVdF = 94: 3: 3. This slurry was applied to both sides of a long aluminum foil having a thickness of 15 μm in a strip shape, dried, and then pressed to prepare a positive electrode.
Further, spheroidized graphite (C) 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: CMC = 98: 1: 1. A slurry for forming a negative electrode active material layer was prepared by mixing with ion-exchanged water at the mass ratio of. This slurry was applied to both sides of a long copper foil having a thickness of 10 μm in a strip shape, dried, and then pressed to prepare a negative electrode.
Further, two separator sheets (a porous polyolefin sheet having a PP / PE / PP three-layer structure having a thickness of 20 μm) were prepared.
The prepared positive electrode, the negative electrode, and the two prepared separator sheets were superposed and wound to prepare a wound electrode body. At this time, a separator is interposed between the positive electrode and the negative electrode.
The wound electrode body produced was housed in a square battery case having a liquid injection port.
Subsequently, a non-aqueous electrolyte is injected from the liquid injection port of the battery case, and the liquid injection port is hermetically sealed to form No. A lithium ion secondary battery (battery capacity: 5Ah) of 1 was produced.
The non-aqueous electrolytes include ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and dimethoxyethane (DME), and EC: DMC: EMC: DME = 35: 33: 27: 5. Lithium bis (oxalat) borate (LiBOB) as an additive is added to the non-aqueous solvent contained in the mass ratio of the above so that the weight ratio to the non-aqueous electrolyte is 0.2% by mass, and lithium bis as a supporting salt. The one to which (fluorosulfonyl) imide (LiFSI) was added so that the concentration in the non-aqueous electrolyte was 1.0 mol / L was used.

[Battery No. 2-No. 11]
Battery No. 1 except that the composition of the non-aqueous electrolyte was changed as shown in Table 1. In the same manner as in No. 1. 2-No. Eleven lithium-ion secondary batteries were produced. In addition, No. 9 and No. In No. 10, LiPF ₆ was used as the supporting salt instead of LiFSI.

<Battery evaluation>
[Limited precipitation resistance]
The above-mentioned No. 1-No. After the lithium ion secondary battery of No. 11 was initially charged, it was placed in an environment of 25 ° C. and adjusted to a charged state of SOC 50%. Then, constant current (CC) charging was performed for 500 seconds at a predetermined current value. After that, each lithium ion secondary battery was disassembled, and the presence or absence of precipitation of metallic lithium on the negative electrode was confirmed. Among the current values in which the precipitation of metallic lithium was not confirmed on the negative electrode, the maximum current value (C) was determined. The results are shown in Table 1.

[Voltage drop]
The above-mentioned No. 1-No. The 11 lithium ion secondary batteries were initially charged. It was placed in an environment of 25 ° C. and discharged with a constant current (CC) until it reached 3.0 V. After that, it was left to stand for 24 hours to self-discharge, and the amount of voltage drop at this time was measured. The results are shown in Table 1.

From Table 1, No. 1 corresponding to the non-aqueous electrolyte secondary battery according to this embodiment. 1-No. It can be seen that the lithium ion secondary battery of No. 7 has a high limit precipitation resistance (current value of 35 C or more) and a small amount of voltage drop during self-discharge (voltage drop of 0.050 V or less). In addition, No. 1-No. From the comparison of the lithium ion secondary batteries of No. 4, it can be seen that as the amount of the ether solvent added is increased, the critical precipitation resistance becomes higher.
On the other hand, No. No. 1 lithium ion secondary battery and No. Compared with the lithium ion secondary battery of No. ₈ , when a conventional general non-aqueous electrolyte in which the non-aqueous solvent does not contain an ether solvent and the supporting salt is LiPF ₆ is used, the limit precipitation resistance is high. It turns out to be low.
No. No. 4 lithium ion secondary battery and No. Compared with the lithium ion secondary battery of No. ₉ , when the non-aqueous electrolyte in which the supporting salt is LiPF ₆ contains an ether solvent, the limit precipitation resistance is high, but the amount of decomposition of the supporting salt is large, so that it is self-discharged. It can be seen that the amount of voltage drop is very large.
No. No. 4 lithium ion secondary battery and No. From the comparison with the lithium ion secondary battery of 10, it can be seen that if the amount of LiBOB added is too large, Na [B (C ₂ O ₄ ) ₂ ] is precipitated, and as a result, the limit precipitation resistance is lowered.
No. 4 to No. No. 7 lithium ion secondary battery and No. From the comparison with the lithium ion secondary battery of No. 11, it can be seen that if the concentration of LiFSI is too high, the amount of decomposition of the supporting salt is large, and therefore the amount of voltage drop due to self-discharge becomes large.

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 Lithium ion secondary battery 20 Battery case 21 Case body 22 Seal plate 23 Positive electrode terminal 24 Negative electrode terminal 30 Safety valve 32 Liquid injection port 33 Encapsulant 40 Winding electrode body 50 Positive electrode sheet 51 Positive electrode current collecting foil 52 Exposed part 53 Positive electrode activity Material layer 60 Negative electrode sheet 61 Negative electrode current collecting foil 62 Exposed part 63 Negative electrode active material layer 72, 74 Separator 80 Non-aqueous electrolyte WL Winding shaft

Claims (1)

A non-aqueous electrolyte secondary battery comprising an electrode body including a positive electrode and a negative electrode and a non-aqueous electrolyte.
The non-aqueous electrolyte contains a carbonate-based solvent, an ether-based solvent, a supporting salt, and lithium bis (oxalate) borate.
The supporting salt is lithium bis (fluorosulfonyl) imide.
The concentration of lithium bis (fluorosulfonyl) imide in the non-aqueous electrolyte is 0.8 mol / L or more and 3 mol / L or less.
The weight ratio of lithium bis (oxalate) borate to the non-aqueous electrolyte is 0.2% by mass or more and 0.4% by mass or less.
The mass ratio of the ether solvent to lithium bis (oxalate) oxalate (ether solvent / lithium bis (oxalate) oxalate) is 25 or more.
Non-aqueous electrolyte secondary battery.

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