JP2017103163A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery arranged by use of a negative electrode active material high in potential and low in electric conductivity, which has a high capacity per unit weight of a negative electrode active material layer (specific capacity of a negative electrode).SOLUTION: A nonaqueous electrolyte secondary battery herein disclosed comprises: an electrode body including a positive electrode and a negative electrode having a negative electrode active material layer; and a nonaqueous electrolyte solution. The negative electrode active material layer includes: a negative electrode active material which enables the charge and discharge with a potential of 0.5 V or higher and has an electric conductivity of 10S/cm or less; and a conductive material essentially consisting of LiTiO. The conductive material is included by 1-40 wt% to a total weight of the negative electrode active material layer.SELECTED DRAWING: Figure 3

Description

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

  Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries (lithium secondary batteries) are lighter and have higher energy density than existing batteries. It is used as a driving power source. In particular, lithium-ion secondary batteries that are lightweight and provide high energy density will become increasingly popular as high-output power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). It is expected to do.

  Graphite is widely used as a negative electrode active material for lithium ion secondary batteries. However, graphite has a potential for inserting and extracting lithium of about 0.1 V, which is close to that of metallic lithium. Therefore, when graphite is used as the negative electrode active material, attention must be paid to the deposition of metallic lithium on the negative electrode active material, particularly at a low temperature.

On the other hand, it is effective to use a negative electrode active material having a higher potential in order to suppress the deposition of metallic lithium. However, among high potential negative electrode active materials, negative electrode active materials such as TiO ₂ and Li ₃ VO ₄ have low electrical conductivity. Therefore, these negative electrode active materials are generally used in combination with carbon black-based conductive materials such as acetylene black and ketjen black (see, for example, Patent Document 1).

JP 2014-063644 A

  However, as a result of intensive studies by the present inventors, when a negative electrode active material having a low electric conductivity as described above is used in combination with a carbon black-based conductive material such as acetylene black, the unit weight of the negative electrode active material layer It has been found that there is a problem that the capacity (specific capacity of the negative electrode) is low.

  Accordingly, an object of the present invention is a non-aqueous electrolyte secondary battery using a negative electrode active material having a low electric conductivity while having a high potential, and has a high capacity per unit weight (negative electrode specific capacity) of the negative electrode active material layer. An object is to provide a non-aqueous electrolyte secondary battery.

As a result of intensive studies, the present inventors have found that the reason why the specific capacity of the negative electrode is reduced when a negative electrode active material having a high electric potential but low electrical conductivity is used in combination with a carbon black conductive material such as acetylene black is I found out that the capacity is small. As a result of further investigation, when Li ₄ Ti ₅ O ₁₂ is added to a negative electrode active material having a high potential but low electrical conductivity, Li is inserted into Li ₄ Ti ₅ O ₁₂ when using a non-aqueous electrolyte secondary battery. It showed high electrical conductivity, and it was found that Li ₄ Ti ₅ O ₁₂ functions as a conductive material. Li ₄ Ti ₅ O ₁₂ has a higher capacity than a carbon black conductive material such as acetylene black. Accordingly, the present inventor has added a predetermined amount of Li ₄ Ti ₅ O ₁₂ as a conductive material to a negative electrode active material having a low electric conductivity while having a high potential, whereby a non-aqueous electrolyte secondary battery having a high negative electrode specific capacity is obtained. It was found that it can be obtained.

That is, the non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode and an electrode body including a negative electrode having a negative electrode active material layer, and a non-aqueous electrolyte. The negative electrode active material layer can be charged / discharged at a potential of 0.5 V or more, and is essentially composed of a negative electrode active material having an electric conductivity of 10 ⁻¹ S / cm or less and Li ₄ Ti ₅ O _12. A conductive material. The conductive material is included in an amount of 1 to 40% by weight with respect to the total weight of the negative electrode active material layer.
According to such a configuration, a non-aqueous electrolyte secondary battery using a negative electrode active material having a low electric conductivity while having a high potential is provided. The non-aqueous electrolyte secondary battery has a high negative electrode specific capacity. Can do.

It is sectional drawing which shows typically the internal structure of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the winding electrode body of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a graph which shows the evaluation result of the specific capacity of the negative electrode of each lithium ion ion secondary battery produced as a test example.

  Embodiments according to the present invention will be described below with reference to the drawings. Note that matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, a general configuration and manufacturing process of a non-aqueous electrolyte secondary battery that does not characterize the present invention) ) Can be understood as a design matter of those 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 this specification and common technical knowledge in the field. Moreover, in the following drawings, the same code | symbol is attached | subjected and demonstrated to the member and site | part which show | plays the same effect | action. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

In the present specification, the “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term including 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 by taking a flat rectangular lithium ion secondary battery as an example, but the present invention is not intended to be limited to those described in the embodiment.

  The lithium ion secondary battery 100 shown in FIG. 1 has a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) accommodated in a flat rectangular battery case (that is, an exterior container) 30. This is a sealed lithium ion secondary battery 100 to be constructed. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 set so as to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Yes. In addition, the battery case 30 is provided with an inlet (not shown) for injecting a non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive current collector 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector plate 44a. As the material of the battery case 30, for example, a light metal material having good thermal conductivity such as aluminum is used.

  As shown in FIGS. 1 and 2, the wound electrode body 20 has a positive electrode active material layer 54 formed along the longitudinal direction on one side or both sides (here, both sides) of an elongated positive electrode current collector 52. The positive electrode sheet 50 and the negative electrode sheet 60 in which the negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of the long negative electrode current collector 62 are two long shapes. The separator sheet 70 is overlapped and wound in the longitudinal direction. The positive electrode active material layer non-forming portion 52a (that is, the positive electrode) formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20 (referred to as the sheet width direction orthogonal to the longitudinal direction). The portion where the active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer non-formed portion 62a (that is, the portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed). ) Are joined with a positive current collector 42a and a negative current collector 44a, respectively.

Examples of the positive electrode current collector 52 constituting the positive electrode sheet 50 include aluminum foil. Examples of the positive electrode active material included in the positive electrode active material layer 54 include lithium transition metal oxides (eg, 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 etc.), lithium transition metal phosphate compounds (eg, LiFePO ₄ etc.) and the like. The positive electrode active material layer 54 can include components other than the active material, such as a conductive material and a binder. As the conductive material, for example, carbon black such as acetylene black (AB) and other (eg, graphite) carbon materials can be suitably used. As the binder, for example, polyvinylidene fluoride (PVDF) can be used.

Examples of the negative electrode current collector 62 constituting the negative electrode sheet 60 include aluminum foil and copper foil.
The negative electrode active material layer 64 includes a negative electrode active material and a conductive material. The negative electrode active material used here is a negative electrode active material having a high potential but low electrical conductivity. That is, it is a negative electrode active material that can be charged and discharged at a potential of 0.5 V or more and has an electric conductivity of 10 ⁻¹ S / cm or less. In this embodiment, since the non-aqueous electrolyte secondary battery is the lithium ion secondary battery 100, the negative electrode active material has a potential of 0.5 V or more with respect to Li / Li ⁺ . By using the negative electrode active material having such a high potential for the lithium ion secondary battery 100, the deposition of metallic lithium on the negative electrode active material, particularly at a low temperature, is suppressed. Examples of the negative electrode active material that can be charged and discharged at a potential of 0.5 V or more and have an electric conductivity of 10 ⁻¹ S / cm or less include TiO ₂ and Li ₃ VO ₄ .
The conductive material used here is a conductive material consisting essentially of Li ₄ Ti ₅ O ₁₂ . Here, “the conductive material consists essentially of Li ₄ Ti ₅ O ₁₂ ” means that a part of Li ₄ Ti ₅ O ₁₂ may take the form of Li ₇ Ti ₅ O _12. Does not contain other conductive material components other than Li ₄ Ti ₅ O ₁₂ .

When a negative electrode active material having a low electric conductivity with high potential is used in combination with a carbon black conductive material such as acetylene black as in the past, the specific capacity of the negative electrode is low because the capacity of the carbon black conductive material is small. Becomes lower. In contrast, according to the study of the present inventors, the addition of Li ₄ Ti ₅ O ₁₂ is the negative electrode active material having low electrical conductivity while high potential, Li ₄ Ti during a non-aqueous electrolyte secondary battery using ₅ If O ₁₂ to Li is inserted _Li 7 _Ti 5 _{O 12,} and this _Li 7 _Ti 5 _{O 12} indicates high electrical conductivity, thereby _Li 4 _Ti 5 _{O 12} is a non-aqueous electrolyte secondary battery It has been found that it functions as a negative electrode conductive material. Li ₄ Ti ₅ O ₁₂ has a higher capacity (about 170 mAh / g) than a carbon black conductive material such as acetylene black. Therefore, by adding Li ₄ Ti ₅ O ₁₂ as a conductive material to a negative electrode active material having a low electric conductivity while having a high potential, a nonaqueous electrolyte secondary battery having a high negative electrode specific capacity can be obtained.

_Here, content relative to the total weight of the negative electrode active material layer 64 of conductive material consists essentially of Li ₄ Ti _{5 O 12} is 1 to 40 wt%. When the content is less than 1% by weight, the effect of improving the conductivity by Li ₄ Ti ₅ O ₁₂ is insufficient, the resistance of the negative electrode active material layer is increased, and as a result, the specific capacity of the negative electrode is decreased. On the other hand, when the content is larger than 40% by weight, the content ratio of the negative electrode active material becomes too small, and the specific capacity of the negative electrode becomes small. Since the specific capacity of the negative electrode is particularly high, the content of the conductive material consisting essentially of Li ₄ Ti ₅ O _{12 with} respect to the total weight of the negative electrode active material layer 64 is preferably 1 to 10% by weight, and preferably 1 to 5% by weight. Is more preferable.

  The negative electrode active material layer 64 can include components other than the negative electrode active material and the conductive material, such as a binder and a thickener. As the binder, for example, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), or the like can be used. As the thickener, for example, carboxymethyl cellulose (CMC) can be used.

  Examples of the separator 70 include a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. 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). A heat resistant layer (HRL) may be provided on the surface of the separator 70.

The non-aqueous electrolyte can be the same as that of a conventional lithium ion secondary battery. Typically, a non-aqueous electrolyte containing a supporting salt in an organic solvent (non-aqueous solvent) can be used. As the non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, lactones and the like used in electrolytes of general lithium ion secondary batteries are used without particular limitation. Can do. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), Examples thereof include monofluoromethyl difluoromethyl carbonate (F-DMC) and trifluorodimethyl carbonate (TFDMC). Such a non-aqueous solvent can be used individually by 1 type or in combination of 2 or more types as appropriate. As the supporting salt, for example, a lithium salt such as LiPF ₆ , LiBF ₄ , LiClO ₄ (preferably LiPF ₆ ) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

  In addition, the non-aqueous electrolyte is a gas generating agent such as biphenyl (BP) or cyclohexylbenzene (CHB); an oxalato complex compound containing a boron atom and / or a phosphorus atom, as long as the effects of the present invention are not significantly impaired. And film forming agents such as vinylene carbonate (VC); dispersants; various additives such as thickeners.

  The lithium ion secondary battery 100 configured as described above can be used for various applications. Suitable applications include driving power sources mounted on vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). The lithium ion secondary battery 100 can also be used in the form of a battery pack typically formed by connecting a plurality of lithium ion secondary batteries 100 in series and / or in parallel.

  As an example, the rectangular lithium ion secondary battery 100 including the flat wound electrode body 20 has been described. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery including a stacked electrode body. The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery. Moreover, the technique disclosed here is applicable also to nonaqueous electrolyte secondary batteries other than a lithium ion secondary battery.

  EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Production of lithium ion secondary battery>
[Test Example 1]
TiO ₂ as a negative electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed with N-methylpyrrolidone (NMP) in the blending ratio (% by weight) shown in Table 1. A negative electrode slurry was prepared. This negative electrode slurry was applied to both sides of a long aluminum foil (negative electrode current collector) in a band shape, dried and pressed to obtain a negative electrode. The pressing conditions were such that the density of the negative electrode active material layer was 2 g / cm ³ .
Meanwhile, a positive electrode using metallic lithium was prepared.
In addition, a porous sheet having a three-layer structure of PP / PE / PP was prepared as a separator.
The negative electrode and the positive electrode were laminated via a separator to produce an electrode body. Next, a positive electrode terminal and a negative electrode terminal were connected to the electrode body and accommodated in a laminate case together with a non-aqueous electrolyte solution to obtain a laminate type lithium ion ion secondary battery of Test Example 1. In addition, the non-aqueous electrolyte includes a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC: DEC = 3: 7, and LiPF ₆ as a supporting salt at a concentration of 1 mol / L. What was dissolved in was used.

[Test Example 2]
When producing the negative electrode, TiO ₂ as the negative electrode active material and polyvinylidene fluoride (PVDF) as the binder were used in the same manner as in Test Example 1, except that the blending ratio (% by weight) shown in Table 1 was used. A lithium ion ion secondary battery of Test Example 2 was produced.

[Test Examples 3 to 11]
When producing a negative electrode, TiO ₂ as a negative electrode active material, Li ₄ Ti ₅ O ₁₂ as a conductive material, and polyvinylidene fluoride (PVDF) as a binder are used in a blending ratio (% by weight) shown in Table 1. The lithium ion secondary batteries of Test Examples 3 to 11 were produced in the same manner as in Test Example 1 except for the above.

<Evaluation of lithium ion secondary battery>
The lithium ion secondary battery of each test example was placed in a temperature environment of 25 ° C. After a constant current (CC) discharge to 1.0 V at a rate of 1/10 C, it was paused for 10 minutes, and after a constant current (CC) charge to 3.0 V at a rate of 1/10 C, it was paused for 10 minutes. The charge capacity at this time was determined, and the value obtained by dividing the charge capacity by the weight of the negative electrode active material layer was defined as the specific capacity of the negative electrode. The evaluation results are shown in FIG.

In FIG. 3, the evaluation result (specific capacity: 175 mAh / g) of Test Example 1 corresponding to the prior art is indicated by a dotted line as a reference line, and the evaluation results (specific capacity values) of Test Examples 2 to 11 are indicated by rhombus marks. . 3 that the specific capacity of Test Examples 4 to 10 in which the blending ratio of Li ₄ Ti ₅ O ₁₂ is in the range of 1 to 40 wt% is higher than the specific capacity of Test Example 1. This can be said that Li ₄ Ti ₅ O ₁₂ functions as a conductive material having a large capacity. Also, specific _capacity, Li ₄ Ti _{5 O 12} proportion is 110 wt.% Of, more at 1-5 wt%, it can be seen that has particularly high. On the other hand, in Test Examples 2 and 3 in which the blending ratio of Li ₄ Ti ₅ O ₁₂ was 0 wt% and 0.5 wt%, respectively, the specific capacity of Test Example 1 was lower. This is probably because the resistance of the negative electrode active material layer was high (conductivity was low). Also in Test Example 11 in which the blending ratio of Li ₄ Ti ₅ O ₁₂ was 60 wt%, the specific capacity of Test Example 1 was lower. This is considered to be because the content ratio of the negative electrode active material in the negative electrode active material layer was small.

  In the test examples shown above, metallic lithium was used for the positive electrode in order to evaluate the characteristics of the negative electrode, but the positive electrode such as a lithium transition metal oxide or lithium transition metal phosphate compound was used as the positive electrode on the current collector. It is understood that the same effect can be obtained even when a positive electrode sheet including a positive electrode active material containing an active material is used.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

20 wound electrode body 30 battery case 36 safety valve 42 positive electrode terminal 42a positive electrode current collector plate 44 negative electrode terminal 44a negative electrode current collector plate 50 positive electrode sheet (positive electrode)
52 Positive Electrode Current Collector 52a Positive Electrode Active Material Layer Non-Forming Portion 54 Positive Electrode Active Material Layer 60 Negative Electrode Sheet (Negative Electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formed portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 Lithium ion secondary battery

Claims (1)

An electrode body including a positive electrode and a negative electrode having a negative electrode active material layer;
A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte,
The negative electrode active material layer can be charged / discharged at a potential of 0.5 V or more, and is essentially composed of a negative electrode active material having an electric conductivity of 10 ⁻¹ S / cm or less and Li ₄ Ti ₅ O _12. The conductive material is contained in an amount of 1 to 40% by weight with respect to the total weight of the negative electrode active material layer.
Non-aqueous electrolyte secondary battery.

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