JP2017021989A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery that suppresses reduction in capacitance when repeating charging/discharging at a high temperature.SOLUTION: The nonaqueous electrolyte secondary battery includes: a battery case; an electrode body accommodated in the battery case; and a nonaqueous electrolyte accommodated in the battery case. The nonaqueous electrolyte contains N-methyl-2- pyrolidone greater than 18 μL and less than 1500 μL per battery capacitance of 1 Ah.SELECTED DRAWING: Figure 1

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. Particularly, lithium ion secondary batteries that are lightweight and obtain high energy density are preferably used as high-output power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). Yes.

  A nonaqueous electrolyte secondary battery typically has a configuration in which an electrode body and a nonaqueous electrolyte are accommodated in a battery case (see, for example, Patent Documents 1 and 2). The non-aqueous electrolyte serves as a transfer medium for ions that are charge carriers. Further, the non-aqueous electrolyte plays a role of forming a protective film (Solid Electrolyte Interface: SEI film) on the negative electrode surface by being reduced and decomposed.

  Non-aqueous electrolyte secondary batteries are desired to have as small a decrease in capacity as possible when charging and discharging are repeated. In particular, it is desired that the capacity decrease when charging / discharging is repeated at a high temperature of about 60 ° C. is as small as possible.

JP 2002-252038 A JP 2008-538448 A

  Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery in which a decrease in capacity when charging / discharging is repeated at a high temperature is suppressed.

As a result of intensive studies by the present inventors, one of the causes of the decrease in battery capacity when the nonaqueous electrolyte secondary battery is repeatedly charged and discharged at a high temperature is that lithium ions released from the positive electrode during charging are on the negative electrode surface. It was found that the potential of the positive electrode is increased without the lithium ions immobilized on the SEI film and the like being returned to the positive electrode. As a result of further intensive studies by the present inventors, it has been found that if an appropriate amount of N-methyl-2-pyrrolidone (NMP) is added to the non-aqueous electrolyte, NMP acts to suppress the potential increase of the positive electrode.
That is, the nonaqueous electrolyte secondary battery disclosed herein includes a battery case, an electrode body accommodated in the battery case, and a nonaqueous electrolyte solution accommodated in the battery case. The non-aqueous electrolyte contains NMP more than 18 μL and less than 1500 μL per 1Ah of battery capacity.
According to such a configuration, it is possible to provide a non-aqueous electrolyte secondary battery in which a decrease in capacity when charging / discharging is repeated at a high temperature (for example, about 60 ° C.) is suppressed.

It is sectional drawing which shows typically the lithium ion secondary battery 10 which concerns on one Embodiment of this invention. 2 is a schematic diagram showing a configuration of an electrode body 40 used in the lithium ion secondary battery 10. FIG. 4 is a graph showing measurement results of initial capacity of non-aqueous electrolyte secondary batteries according to Example 1 and Example 2. It is a graph which shows the measurement result of the capacity | capacitance maintenance factor in the high temperature endurance test of the nonaqueous electrolyte secondary battery which concerns on Example 1 and Example 2. FIG. 4 is a graph showing measurement results of a remaining capacity of a positive electrode of a nonaqueous electrolyte secondary battery according to Example 1 and Example 2.

  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 lithium ion secondary battery as an example. It should be noted that the present invention is not intended to be limited to the one described in the embodiment, and the technical idea of the present invention is that other secondary nonaqueous electrolyte solutions having other charge carriers (for example, sodium ions) are used. The present invention is also applied to a battery (for example, a sodium ion secondary battery).

  FIG. 1 is a cross-sectional view schematically showing a lithium ion secondary battery 10 according to an embodiment. FIG. 2 is a schematic diagram 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 main 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 in the normal use state of the lithium ion secondary battery 10 is opened. The sealing plate 22 is a member that closes the opening of the case body 21. The sealing plate 22 is a rectangular plate. The sealing plate 22 is welded to the peripheral edge of the opening of the case body 21 to form a substantially hexahedral battery case 20. As the material of the battery case 20, for example, a battery case 20 mainly composed of a metal material that is lightweight and has good thermal conductivity is preferably used. 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 shape that wraps the electrode body, or may be a so-called laminate 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 to a predetermined level (for example, a set valve opening pressure of about 0.3 MPa to 1.0 MPa) or more. Further, FIG. 1 shows a state in which the liquid injection port 32 is sealed with the sealing material 33 after the nonaqueous electrolytic solution 80 is injected.

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

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

The positive electrode active material layer 53 typically includes a positive electrode active material, a conductive material, a binder (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.). 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 employed.

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

  The negative electrode active material layer 63 typically includes 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 among them, graphite can be preferably used. 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 employed.

  As the separators 72 and 74, for example, a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide can be used. Preferably, a polyolefin resin (eg, PE , PP) 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 order, and the wound electrode body (wound electrode body). 40 has a shape that is flatly pushed and bent along one 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. Furthermore, 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 collector foil 51 and the exposed portion 62 of the negative electrode current collector foil 61 are spirally exposed on both sides of the separators 72 and 74, respectively. In the present embodiment, as shown in FIG. 1, the electrode body 40 is formed by gathering the intermediate portions of the positive and negative exposed portions 52 and 62 protruding from the separators 72 and 74, and arranging the positive and negative electrodes disposed inside the battery case 20. The inner terminals 23 and 24 are welded to the tip portions 23a and 24a.

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

  The non-aqueous electrolyte 80 contains NMP more than 18 μL and less than 1500 μL per battery capacity 1 Ah. According to the study by the present inventors, when the non-aqueous electrolyte secondary battery is repeatedly charged and discharged at a high temperature, one of the causes that the battery capacity is reduced is that the lithium ions released from the positive electrode during charging are the SEI film on the negative electrode surface. It has been found that the potential of the positive electrode rises without the lithium ions fixed to the positive electrode returning to the positive electrode and irreversible capacity is generated. When the surface area of the negative electrode is increased in order to increase the input / output of the battery, the irreversible capacity increases, so that the decrease in capacity becomes more remarkable. On the other hand, as a result of further studies by the present inventors, it was found that if an appropriate amount of NMP is added to the nonaqueous electrolyte, NMP causes an oxidation reaction on the positive electrode, thereby suppressing an increase in potential of the positive electrode. That is, the present inventors have found that NMP functions as a positive electrode potential increase inhibitor, and can suppress a decrease in battery capacity when a nonaqueous electrolyte secondary battery is repeatedly charged and discharged at a high temperature.

  Patent Document 1 describes that NMP in the battery (in the positive and negative electrodes) is controlled to be as small as possible in order to provide a non-aqueous electrolyte secondary battery with less self-discharge. However, in this embodiment, NMP is positively added in order to suppress the potential increase of the positive electrode. In order to fully exhibit the role of NMP as a positive electrode potential increase inhibitor, the amount of NMP contained in the non-aqueous electrolyte is more than 18 μL per 1 Ah of battery capacity.

  Patent Document 2 describes that NMP can be used as the non-aqueous electrolyte itself. The use of NMP as the non-aqueous electrolyte itself means that the non-aqueous electrolyte contains NMP at a high rate. However, if the proportion of NMP in the non-aqueous electrolyte is too high, the battery resistance becomes too high. Therefore, in the present embodiment, the amount of NMP contained in the nonaqueous electrolytic solution is less than 1500 μL per battery capacity 1 Ah.

As long as the non-aqueous electrolyte contains NMP in the above amount, the type thereof is not particularly limited, and the same non-aqueous electrolyte as that of a conventional lithium ion secondary battery can be used. Typically, an organic solvent (non-aqueous solvent) containing a supporting salt can be used. Examples of the non-aqueous solvent include various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones that are used in non-aqueous electrolytes of general lithium ion secondary batteries. Can be used. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like. 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 (for example, 1.0 mol / L).

  In the non-aqueous electrolyte, gas generation of components other than the above-mentioned non-aqueous solvent and supporting salt, for example, biphenyl (BP), cyclohexylbenzene (CHB), etc., is provided as long as the effects of the present invention are not significantly impaired. Agents; film forming agents such as oxalato complex compounds containing boron and / or phosphorus atoms, vinylene carbonate (VC), fluoroethylene carbonate (FEC); dispersants; thickeners; .

The lithium ion secondary battery 10 according to the present embodiment constructs a step of constructing a battery cell in which the electrode body 40 is accommodated in the battery case 20, and a nonaqueous electrolytic solution containing NMP of more than 18 μL and less than 1500 μL per 1Ah of battery capacity. It can manufacture by implementing the process inject | poured into the battery cell. An activation step (for example, an activation step performed at 60 ° C. for 20 hours or more) may be further performed.
Note that as an example of a method for manufacturing an electrode including an active material layer, there is a method in which a paste containing an active material and a solvent is applied onto a current collector foil and then dried. NMP may be used as this solvent. At this time, NMP may remain in the active material layer. In this case, the amount of NMP contained in the nonaqueous electrolytic solution may be appropriately adjusted in consideration of the amount of remaining NMP. (That is, the step of injecting the non-aqueous electrolyte into the constructed battery cell is performed so that the injected non-aqueous electrolyte contains more than 18 μL and less than 1500 μL of NMP per 1 Ah of battery capacity.)
Note that when a large amount of NMP remains in the active material layer, the peel strength of the active material layer may decrease. Therefore, it is preferable to reduce the amount of NMP remaining in the active material layer as much as possible and add NMP to the non-aqueous electrolyte.

  Moreover, it is also possible to construct the lithium ion secondary battery 10 according to the present embodiment by reinjecting a non-aqueous electrolyte solution containing NMP more than 18 μL and less than 1500 μL per 1 Ah of battery capacity into a battery whose capacity has decreased. In this case, the battery capacity can be recovered.

  The lithium ion secondary battery 10 configured as described above can be used for various applications, and preferred applications include an electric vehicle (EV), a hybrid vehicle (HV), a plug-in hybrid vehicle (PHV), and the like. Driving power source mounted on the vehicle. The lithium ion secondary battery 10 can also be used in the form of a battery pack typically formed by connecting a plurality of lithium ion secondary batteries 10 in series and / or in parallel.

  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.

<Example 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 LNCM: AB: PVdF = 92: 5: 3 was mixed with NMP to prepare a positive electrode active material layer forming slurry. The slurry was applied in a strip shape on both sides of a long aluminum foil, dried, and then pressed to prepare a positive electrode.
Further, graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener have a mass of C: SBR: CMC = 98: 1: 1. The slurry for negative electrode active material layer formation was prepared by mixing with ion exchange water in a ratio. The slurry was applied in a strip shape on both sides of a long copper foil, dried, and then pressed to prepare a negative electrode.
In addition, two separator sheets (porous polyolefin sheets) were prepared.

The produced positive electrode and negative electrode and two prepared separator sheets were superposed and wound to produce a wound electrode body. At this time, a separator was interposed between the positive electrode and the negative electrode.
The produced wound electrode body was accommodated in a battery case having a liquid inlet.

Subsequently, a non-aqueous electrolyte was injected from the injection port of the battery case, and the injection port was hermetically sealed to produce a non-aqueous electrolyte secondary battery according to Example 1. The non-aqueous electrolyte is supported by a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC: DMC: EMC = 1: 1: 1. LiPF ₆ as a salt was dissolved at a concentration of 1.0 mol / L, and NMP was further added. The amount of NMP per battery capacity 1 Ah in the non-aqueous electrolyte was 400 μL. The initial capacity value obtained below was used as the battery capacity.

<Example 2>
A non-aqueous electrolyte secondary battery according to Example 2 was fabricated in the same manner as Example 1 except that a non-aqueous electrolyte solution having the same composition as Example 1 was used except that NMP was not included.

<Activation processing>
The prepared non-aqueous electrolyte secondary batteries according to Example 1 and Example 2 were initially charged and then stored in a constant temperature bath at 60 ° C. for 24 hours.

<Initial capacity>
In the temperature environment of 25 ° C., in the voltage range of 3.0 V to 4.1 V, the initial stage of the non-aqueous electrolyte secondary battery according to Example 1 and Example 2 after the activation treatment according to the following procedure 1 to procedure 4 The capacity was measured.
[Procedure 1] After charging to 4.1 V with a constant current of 1 C, pause for 5 minutes.
[Procedure 2] After discharging to 3.0 V with a constant current of 1 C, pause for 5 minutes.
[Procedure 3] After charging to 4.1 V with a constant current of 1 C, charge at a constant voltage until the current value reaches 0.1 C, and then rest for 10 seconds.
[Procedure 4] After discharging to 3.0 V with a constant current of 1 C, constant voltage discharge until the current value reaches 0.1 C, and then rest for 10 seconds.
And the discharge capacity (CCCV discharge capacity) in the procedure 4 was made into the initial capacity. The measurement results of the initial capacity of the nonaqueous electrolyte secondary batteries according to Example 1 and Example 2 are shown in FIG. In FIG. 3, the initial capacity of the non-aqueous electrolyte secondary battery according to Example 1 is shown as a relative value when the initial capacity of the non-aqueous electrolyte secondary battery according to Example 2 is 100%.

<High temperature durability test>
Next, after the non-aqueous electrolyte secondary battery according to Example 1 and Example 2 after the initial capacity measurement was left in a thermostatic chamber set at a temperature of 60 ° C. for 2 hours or more, the following charge / discharge operation (1 ) And (2) were repeated 200 cycles to evaluate high temperature durability.
(1) After charging CC to 4.1 V at a rate of 2C, pause for 10 minutes.
(2) After discharging CC to 3.0 V at a rate of 2 C, pause for 10 minutes.
After the end of the high temperature durability test, from the CC discharge capacity of the first cycle and the CC discharge capacity of the 200th cycle, the following formula:
(CC discharge capacity at the 200th cycle / CC discharge capacity at the first cycle) × 100
The capacity retention rate (%) was calculated by FIG. 4 shows the measurement results of the capacity retention rates of the nonaqueous electrolyte secondary batteries according to Example 1 and Example 2.

  From FIG. 3, the non-aqueous electrolyte secondary battery according to Example 1 in which the non-aqueous electrolyte contains NMP has an initial capacity higher than that of the non-aqueous electrolyte secondary battery in accordance with Example 2 where the non-aqueous electrolyte contains NMP. I understand that it is expensive. Further, as shown in FIG. 4, the nonaqueous electrolyte secondary battery according to Example 1 in which the nonaqueous electrolyte contains NMP is the same as the nonaqueous electrolyte secondary battery according to Example 2 in which the nonaqueous electrolyte contains NMP. The capacity maintenance rate was high. Therefore, it can be seen that the non-aqueous electrolyte secondary battery according to Example 1 in which the non-aqueous electrolyte contains NMP suppresses a decrease in capacity when charging and discharging are repeated at a high temperature.

  Further, the remaining capacity per unit active material weight of the positive electrode (mAh / g) was determined for the non-aqueous electrolyte secondary batteries according to Example 1 and Example 2 subjected to the high temperature durability test. The results are shown in FIG. FIG. 5 shows that the remaining capacity of the positive electrode is low in the nonaqueous electrolyte secondary battery according to Example 1 in which the nonaqueous electrolyte contains NMP. This result means that the potential increase of the positive electrode is suppressed. Therefore, in the non-aqueous electrolyte secondary battery according to Example 1 in which the non-aqueous electrolyte contains NMP, NMP functions as a positive electrode potential increase inhibitor, so that the capacity decreases when charging and discharging are repeated at a high temperature. It can be said that it was suppressed.

  Further, some non-aqueous electrolyte secondary batteries were produced in the same manner as in Example 1 except that the amount of NMP in the non-aqueous electrolyte was changed, and the above high-temperature durability test was conducted. The amount was examined. As a result, good results were obtained when the content α (μL / Ah) of NMP per 1 Ah of battery capacity in the non-aqueous electrolyte was in the range of 18 <α <1500.

  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.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 20 Battery case 21 Case main body 22 Sealing plate 23 Positive electrode terminal 24 Negative electrode terminal 30 Safety valve 32 Injection port 33 Sealing material 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 collector foil 62 Exposed portion 63 Negative electrode active material layers 72 and 74 Separator 80 Nonaqueous electrolyte WL Winding shaft

Claims (1)

A battery case,
An electrode body housed in the battery case;
A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte housed in the battery case,
The non-aqueous electrolyte secondary battery in which the non-aqueous electrolyte contains N-methyl-2-pyrrolidone more than 18 μL and less than 1500 μL per battery capacity 1 Ah.

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