JP2020077478A - Nonaqueous electrolytic solution lithium ion secondary battery - Google Patents

Abstract

To provide a nonaqueous electrolyte lithium ion secondary battery being a nonaqueous electrolyte lithium ion secondary battery provided with a wound electrode body and being excellent in resistance against metal lithium deposition and low-temperature output characteristics during repetitive charging and discharging.SOLUTION: A nonaqueous electrolyte lithium ion secondary battery 100 includes: a wound electrode body 20 in which a positive electrode 50, a negative electrode 60 and separators 70 are overlapped and wound; a nonaqueous electrode; and a battery case 30 for housing the wound electrode body 20 and the nonaqueous electrode. The separators 70 have a heat-resistant layer. The heat-resistant layer faces the positive electrode 50. A spring constant in a thickness direction of the positive electrode 50 is 288.4kN/mm or more and 332.5kN/mm or below.SELECTED DRAWING: Figure 1

Description

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

  2. Description of the Related Art In recent years, non-aqueous electrolyte lithium ion secondary batteries have been used as portable power sources for personal computers, mobile terminals, etc., and as vehicle drive power sources for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV), etc. It is preferably used.

A general non-aqueous electrolyte lithium ion secondary battery includes an electrode body in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween. This electrode body is roughly classified into a wound electrode body and a laminated electrode body.
Patent Document 1 discloses an example of a non-aqueous electrolyte secondary battery including a wound electrode body. In Patent Document 1, by defining the spring constant in the thickness direction of the positive electrode in the range of 13 kN / mm or more and 14.3 kN / mm or less, the amount of the non-aqueous electrolyte discharged from the negative electrode and the amount of the non-aqueous electrolyte discharged from the positive electrode It is described that it is possible to alleviate or eliminate the uneven distribution of the non-aqueous electrolyte inside the electrode body by balancing with the amount of the non-aqueous electrolyte.

JP, 2017-123236, A

  As a result of intensive studies by the present inventors, in the prior art, it was found that metallic lithium is likely to precipitate in the R portion (curved portion) of the wound electrode body when repeatedly charged and discharged. That is, it was found that there is room for improvement in metal lithium deposition resistance in the prior art. Furthermore, they have found that there is room for improvement in low-temperature output characteristics.

  Therefore, an object of the present invention is a non-aqueous electrolyte lithium ion secondary battery comprising a wound electrode body, which is excellent in metal lithium deposition resistance during repeated charging and discharging, and low temperature output characteristics. To provide batteries.

As a result of diligent studies by the present inventors, the heat-resistant layer of the separator is opposed to the positive electrode, and by appropriately managing the spring constant in the thickness direction of the positive electrode, metal lithium precipitation resistance during repeated charging and discharging, and low-temperature output characteristics We have found that both can be raised.
That is, the non-aqueous electrolyte lithium ion secondary battery disclosed herein includes a wound electrode body in which a positive electrode, a negative electrode, and a separator are stacked and wound, a non-aqueous electrolyte solution, and the wound electrode body. And a battery case containing the non-aqueous electrolyte. The separator has a heat resistant layer. The heat-resistant layer faces the positive electrode. The spring constant of the positive electrode in the thickness direction is 288.4 kN / mm or more and 332.5 kN / mm or less.
According to such a configuration, a non-aqueous electrolyte lithium ion secondary battery having a wound electrode body, which is excellent in metal lithium deposition resistance during repeated charging and discharging and excellent in low temperature output characteristics, is provided. A secondary battery can be provided.

FIG. 1 is a cross-sectional view schematically showing an internal structure of a lithium ion secondary battery according to an embodiment of the present invention. It is a schematic diagram which shows the structure of the wound electrode body of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows a part of laminated structure of the wound electrode body of the lithium ion secondary battery which concerns on one Embodiment of this invention.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. Note that matters other than matters particularly referred to in the present specification and necessary for carrying out the present invention (for example, a general configuration of a non-aqueous electrolyte lithium ion secondary battery that does not characterize the present invention and (Manufacturing process) can be grasped 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 the common general technical knowledge in the field. Further, in the following drawings, the same reference numerals are given to the members / sites that have the same effect. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

In the present specification, the term “secondary battery” refers to a general electric storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries such as lithium-ion secondary batteries and electric storage elements such as electric double layer capacitors.
Further, in the present specification, the “lithium ion secondary battery” refers to a secondary battery in which lithium ions are used as charge carriers and charge / discharge is realized by movement of charges due to lithium ions between the positive and negative electrodes.

  Hereinafter, the present invention will be described in detail by taking a flat rectangular non-aqueous electrolyte lithium ion secondary battery as an example, but the present invention is not intended to be limited to those described in the embodiments. ..

  In the lithium ion secondary battery 100 shown in FIG. 1, the flat wound electrode body 20 and the non-aqueous electrolyte (not shown) are housed in a flat rectangular battery case (that is, an outer container) 30. It is a sealed battery constructed. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure of the battery case 30 when the internal pressure rises above a predetermined level. There is. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolytic solution. The positive electrode terminal 42 is electrically connected to the positive electrode collector plate 42a. The negative electrode terminal 44 is electrically connected to the negative electrode collector plate 44a. As a material for the battery case 30, for example, a lightweight and highly heat-conductive metal material such as aluminum is used.

  In the wound electrode body 20, as shown in FIGS. 1 and 2, a positive electrode active material layer 54 is formed on one surface or both surfaces (here, both surfaces) of a long-shaped positive electrode current collector 52 along the longitudinal direction. The positive electrode sheet 50 and the negative electrode sheet 60 in which the negative electrode active material layer 64 is formed on one surface or both surfaces (here, both surfaces) of the elongated negative electrode current collector 62 along the longitudinal direction are two long sheets. It has a form in which it is overlapped with the separator sheet 70 in between and wound in the longitudinal direction. The positive electrode active material layer-free portion 52a formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axis direction (referred to as the sheet width direction orthogonal to the longitudinal direction) (that is, the positive electrode). A portion where the positive electrode current collector 52 is exposed without forming the active material layer 54) and a negative electrode active material layer non-forming portion 62a (that is, a portion where the negative electrode current collector 62 is exposed without forming the negative electrode active material layer 64). ), A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined respectively.

Examples of the positive electrode current collector 52 constituting the positive electrode sheet 50 include aluminum foil and the like.
The positive electrode active material layer 54 contains a positive electrode active material. Examples of the positive electrode active material contained 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 and the} like), lithium transition metal phosphate compounds (eg, LiFePO _{4 and the} like), and the like. The content of the positive electrode active material is preferably 70% by mass or more in the positive electrode active material layer 54 (that is, based on the total mass of the positive electrode active material layer 54), more preferably 75% by mass or more, and further preferably 80% by mass or more. ..

The positive electrode active material layer 54 may 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 carbon materials such as graphite can be preferably used. The content of the conductive material in the positive electrode active material layer 54 is preferably 1% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 12% by mass or less.
As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used. The content of the binder in the positive electrode active material layer 54 is preferably 1% by mass or more and 15% by mass or less, and more preferably 2% by mass or more and 12% by mass or less.

In the present embodiment, the spring constant of the positive electrode sheet 50 in the thickness direction is 288.4 kN / mm or more and 332.5 kN / mm or less. This thickness direction is a direction perpendicular to the main surface of the positive electrode sheet 50.
When the spring constant in the thickness direction of the positive electrode sheet 50 is less than 288.4 kN / mm, the amount of the nonaqueous electrolytic solution held in the wound electrode body 20 is reduced and the diffusion resistance of lithium ions is increased. Low temperature output becomes low. On the other hand, when the spring constant of the positive electrode sheet 50 in the thickness direction exceeds 332.5 kN / mm, it becomes difficult to maintain the shape in the R portion (curved portion) of the wound electrode body 20, and the gap between the electrodes is increased, and the wound electrode is wound. Non-uniformity occurs in the distance between the electrodes of the body (distance between the electrodes). As a result, resistance unevenness occurs due to the difference in the distance between the electrodes, and the current concentrates on the boundary portion of the R portion of the wound electrode body where the distance between the electrodes is small, and metal lithium is easily deposited. That is, the metal lithium deposition resistance deteriorates.
The spring constant of the positive electrode sheet 50 in the thickness direction is preferably 300 kN / mm or more and 320 kN / mm or less.
The spring constant in the thickness direction of the positive electrode sheet 50 can be controlled by a method of changing the particle size distribution of the positive electrode active material, a method of changing the porosity of the positive electrode active material, a combination thereof, or the like.
The spring constant of the positive electrode sheet 50 in the thickness direction can be measured using, for example, an autograph.

Examples of the negative electrode current collector 62 constituting the negative electrode sheet 60 include copper foil and the like. As the negative electrode active material contained in the negative electrode active material layer 64, a carbon material such as graphite, hard carbon, or soft carbon can be used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form in which graphite is coated with an amorphous carbon material.
The negative electrode active material layer 64 may include components other than the active material, such as a binder and a thickener. As the binder, for example, styrene butadiene rubber (SBR) or the like can be used. As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used.
90 mass% or more is preferable and, as for content of the negative electrode active material in a negative electrode active material layer, 95 mass% or more and 99 mass% or less is more preferable. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

  In this embodiment, as shown in FIG. 3, a separator 70 having a heat resistant layer (HRL) 72 is used. In FIG. 3, the separator 70 has a heat resistant layer 72 and a base material layer (here, a porous resin sheet layer) 74.

The heat-resistant layer 72 may include the material usually used for the heat-resistant layer of the separator of the lithium ion secondary battery. Specifically, it contains an inorganic filler, and may contain a binder, a thickener, etc., if necessary.
Examples of the inorganic filler include inorganic oxides such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ), titania (TiO ₂ ), nitrides such as aluminum nitride and silicon nitride, calcium hydroxide. , Metal hydroxides such as magnesium hydroxide and aluminum hydroxide, clay minerals such as mica, talc, boehmite, zeolite, apatite and kaolin, and glass fibers. Of these, alumina, boehmite, and magnesia are preferably used.
Examples of the binder for the heat-resistant layer 72 include fluorine-based polymers such as polytetrafluoroethylene (PTFE), acrylic binders, styrene-butadiene rubber (SBR), and polyolefin-based binders. Examples include carboxymethyl cellulose (CMC) and methyl cellulose (MC).

  Examples of the resin forming the porous resin sheet layer 74 include polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide and the like. The porous resin sheet layer 74 may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both surfaces of a PE layer).

  In this embodiment, as shown in FIG. 3, the heat-resistant layer 72 of the separator 70 faces the positive electrode sheet 50. Although the viscosity of the non-aqueous electrolyte increases at low temperatures, the heat-resistant layer 72 of the separator 70 faces the positive electrode sheet 50, and thus the retention of the non-aqueous electrolyte at the interface between the positive electrode sheet 50 and the separator 70 is improved. Then, the diffusion resistance of lithium ions decreases. As a result, the low temperature output characteristic is improved.

As the non-aqueous electrolyte, the same one as the conventional lithium ion secondary battery can be used, and typically, an organic solvent (non-aqueous solvent) containing a supporting salt can be used. As the non-aqueous solvent, organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, and lactones used in the electrolyte of a general lithium ion secondary battery can be used without particular limitation. You can As specific examples, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), Examples include monofluoromethyldifluoromethyl carbonate (F-DMC) and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents may be used alone or in appropriate combination of two or more. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ (preferably LiPF ₆ ) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

  The non-aqueous electrolyte solution is, for example, a gas generating agent such as biphenyl (BP) or cyclohexylbenzene (CHB); an oxalato complex compound containing a boron atom and / or a phosphorus atom, unless the effect of the present invention is significantly impaired. , Film forming agents such as vinylene carbonate (VC); dispersants; thickeners and other various additives.

The lithium-ion secondary battery 100 configured as described above is excellent in metal lithium deposition resistance during repeated charge / discharge and low-temperature output characteristics.
The lithium-ion secondary battery 100 can be used for various purposes. Suitable applications include a driving power source mounted on a vehicle such as an electric vehicle (EV), a hybrid vehicle (HV), and a plug-in hybrid vehicle (PHV). The lithium ion secondary battery 100 can also be used in the form of an assembled battery, which is typically formed by connecting a plurality of batteries in series and / or in parallel.

  In addition, as an example, the prismatic 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 cylindrical lithium ion secondary battery, a laminated lithium ion secondary battery, or the like.

  Examples of 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 for evaluation>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were added to LNCM: AB : PVdF = 87: 10: 3 was mixed with N-methylpyrrolidone (NMP) in a mass ratio to prepare a paste for forming a positive electrode active material layer. This paste was applied on both sides of an aluminum foil so that one end of the aluminum foil was exposed, dried, and then pressed to produce a positive electrode sheet.
At this time, the spring constant in the thickness direction of the positive electrode was changed as shown in Table 1 by using positive electrode active materials having different particle size distributions. The spring constant in the thickness direction of the positive electrode was measured by the method described later.
Further, natural graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed with C: SBR: CMC = 98: 1: 1. By mixing with ion-exchanged water in a mass ratio, a paste for forming a negative electrode active material layer was prepared. This paste was applied to both surfaces of a copper foil so that one end of the copper foil was exposed, dried, and then pressed to produce a negative electrode sheet.
In addition, two separators were prepared in which a heat resistant layer (HRL) made of an inorganic filler was formed on one surface of a polyolefin porous substrate having a three-layer structure of PP / PE / PP.
The produced positive electrode, the produced negative electrode, and two separators were stacked and wound to produce a wound electrode body. At this time, in the lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2 and Examples 1 to 3, the HRL of the separator was opposed to the positive electrode. In the lithium ion secondary batteries of Comparative Examples 3 to 7, the HRL of the separator was opposed to the negative electrode.
The produced wound electrode body was housed in a battery case mainly made of aluminum. Subsequently, a non-aqueous electrolyte was injected from the opening of the battery case, and the opening was hermetically sealed to produce the lithium ion secondary batteries of Examples and Comparative Examples. The non-aqueous electrolyte is supported by a mixed solvent containing ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of EC: EMC: DMC = 1: 1: 1. LiPF ₆ as a salt was used at a concentration of 1.0 mol / L.

<Measurement of spring constant in positive electrode thickness direction>
Forty positive electrodes having a size of 45 mm × 45 mm were cut out from each of the positive electrode sheets used for the production of each lithium ion secondary battery and laminated to produce a sample. A load was applied to this sample in the stacking direction using an autograph. Note that this stacking direction is also the thickness direction of the positive electrode sheet. The spring constant in the thickness direction of the positive electrode was obtained from the inclination of the displacement with respect to the load at this time. The results are shown in Table 1.

<Evaluation of metal lithium deposition resistance (capacity maintenance rate measurement)>
The capacity retention rate during repeated charge and discharge was evaluated as an index of metal lithium deposition resistance. Specifically, the lithium-ion secondary batteries of Examples and Comparative Examples were CC-charged at a rate of ⅓ C under a 25 ° C. environment until the voltage became 4.1 V, and then the current value was 0. It was charged by CV until it reached 02C. After that, constant current discharge was performed at a rate of 1/3 C until the voltage reached 3 V, and the discharge capacity at this time was taken as the initial capacity.
Next, the lithium ion secondary battery was adjusted to SOC (State of charge) 80% and placed in an environment of −10 ° C. Then, a rectangular wave charge / discharge for 10 seconds was performed 1000 cycles at a rate of 200A. Then, the capacity was measured in the same manner as above, and the capacity retention rate (%) was determined from (battery capacity after 1000 cycles of charge / discharge / initial capacity) × 100. The results are shown in Table 1.

<Low-temperature output characteristic evaluation>
Each of the prepared lithium secondary batteries for evaluation was adjusted to SOC 27% and left standing in an environment of -35 ° C for 6 hours. After that, constant power output was performed under a plurality of conditions, and an output value that reached a predetermined voltage in a predetermined time was calculated. When the output value of Example 3 was set to 100, the ratios of the output values of the other Examples and each Comparative Example were obtained. The results are shown in Table 1.

As can be seen from the results in Table 1, when the HRL faces the positive electrode and the spring constant in the thickness direction of the positive electrode is 288.4 kN / mm or more and 332.5 kN / mm or less, the capacity is maintained during repeated charge / discharge. It can be seen that both the rate and the low temperature output characteristics are high. The high capacity retention rate is due to the high metal lithium deposition resistance during repeated charge and discharge.
Therefore, it can be seen that the non-aqueous electrolyte lithium ion secondary battery disclosed herein has high resistance to metal lithium deposition during repeated charging / discharging and low-temperature output characteristics.

  Specific examples of the present invention have been described above in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

20 winding 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 Part 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)
72 Heat-resistant layer 74 Porous resin sheet layer 100 Lithium ion secondary battery

Claims (1)

A wound electrode body in which a positive electrode, a negative electrode, and a separator are stacked and wound,
Non-aqueous electrolyte,
A battery case containing the wound electrode body and the non-aqueous electrolyte,
A non-aqueous electrolyte lithium ion secondary battery comprising:
The separator has a heat resistant layer,
The heat-resistant layer faces the positive electrode,
The spring constant in the thickness direction of the positive electrode is 288.4 kN / mm or more and 332.5 kN / mm or less,
Non-aqueous electrolyte lithium-ion secondary battery.

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