JP7032221B2 - 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 vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). It is suitably used for driving power supplies and the like.

A non-aqueous electrolytic solution secondary battery generally has a configuration including an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolytic solution. A technique for providing a heat-resistant layer (HRL) containing inorganic particles and a binder in a separator is known in order to improve thermal stability (see, for example, Patent Document 1). Patent Document 1 describes that a binder having a softening temperature of 175 ° C. or higher is used in combination with poly-N-vinylacetamide as a binder having a softening temperature of 175 ° C. or higher. Patent Document 1 has an experimental example in which polyvinylpyrrolidone is used as a binder having a softening temperature of 175 ° C. or higher. Further, Patent Document 1 describes that a non-aqueous electrolytic solution contains lithium bis (oxalate) borate, which is an oxalate complex compound, as an additive.

Japanese Unexamined Patent Publication No. 2014-209414

With the widespread use of non-aqueous electrolyte secondary batteries, further improvement in characteristics (for example, higher capacity) is required for non-aqueous electrolyte secondary batteries. One of the measures to increase the capacity is to thin the heat-resistant layer of the separator. However, in general, the smaller the thickness of the heat-resistant layer, the more easily the heat-resistant layer shrinks due to heat, the higher the temperature tends to rise during overcharging, and the lower the thermal stability tends to be. Although the thermal stability at the time of overcharging is improved by adding the oxalate complex compound to the non-aqueous electrolytic solution, when the thickness of the heat-resistant layer of the separator is small, the effect of improving the thermal stability cannot be sufficiently obtained. There is. Therefore, in the prior art, it is difficult to thin the heat-resistant layer of the separator while ensuring thermal stability.

In view of such circumstances, it is an object of the present invention to provide a non-aqueous electrolyte secondary battery having excellent thermal stability despite the small thickness of the heat resistant layer of the separator.

The non-aqueous electrolytic solution secondary battery disclosed herein includes an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolytic solution. The non-aqueous electrolytic solution contains an oxalate complex compound as an additive. The separator includes a heat-resistant layer containing inorganic particles and a binder, and a base material layer. The binder contains poly-N-vinylacetamide and polyvinylpyrrolidone. The thickness of the heat-resistant layer is 2.7 μm or more and 5.1 μm or less.
According to such a configuration, it is possible to provide a non-aqueous electrolytic solution secondary battery having excellent thermal stability despite the small thickness of the heat-resistant layer of the separator.

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 schematic diagram which shows a part of the laminated structure of the wound electrode body of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a graph which plotted the maximum temperature ratio at the time of supercharging with respect to the thickness of a heat-resistant layer about Examples 1 to 3, Comparative Example 1 and Reference Example 2-4.

Hereinafter, embodiments according to the present invention will be described. 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 which 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 the common general technical knowledge in the art.

In the present specification, the "secondary battery" generally refers to a power storage device capable of being repeatedly charged and discharged, and is a term including a so-called storage 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-angle type lithium ion secondary battery as an example, but the present invention is not intended to be limited to those described in such embodiments.

The lithium ion secondary battery 100 shown in FIG. 1 is a hermetically sealed battery constructed by housing a flat wound electrode body 20 and a non-aqueous electrolytic solution 80 in a flat square battery case (that is, an outer container) 30. It is a type battery. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. ing. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolytic solution 80. The positive electrode terminal 42 is electrically connected to the positive electrode current collector plate 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 lightweight metal material having good thermal conductivity such as aluminum is used.

As shown in FIGS. 1 and 2, in the wound electrode body 20, the positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of the long 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 sheets. It has a form of being overlapped with each other via a separator sheet 70 and wound in the longitudinal direction. The positive electrode active material layer non-formed portion 52a (that is, the positive electrode activity) formed so as to protrude outward from both ends of the winding electrode body 20 in the winding axis direction (that is, the sheet width direction orthogonal to the longitudinal direction). A portion where the positive electrode current collector 52 is exposed without forming the material layer 54) and a portion 62a where the negative electrode active material layer is not formed (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 to each of the above.

As the positive electrode sheet 50 and the negative electrode sheet 60, the same ones used in the conventional lithium ion secondary battery can be used without particular limitation. A typical aspect is shown below.

Examples of the positive electrode current collector 52 constituting the positive electrode sheet 50 include aluminum foil and the like. 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 , etc.), lithium transition metal phosphate compounds (eg, LiFePO ₄ , etc.) and the like. The positive electrode active material layer 54 may contain 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) or other carbon material (eg, graphite or the like) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.

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 negative electrode active material layer 64 may contain 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.

As shown in FIG. 3, the separator 70 includes a heat-resistant layer (HRL) 72 and a base material layer 74. The heat-resistant layer 72 is formed on the main surface of the base material layer 74. The heat-resistant layer 72 may be provided on only one side of the base material layer 74, or may be provided on both sides.
In the illustrated example, the heat-resistant layer 72 faces the positive electrode 50. That is, the heat-resistant layer 72 is provided on the surface of the base material layer 74 facing the positive electrode 50. However, the heat-resistant layer 72 may face the negative electrode 60. It is preferable that the heat-resistant layer 72 faces the positive electrode 50 because the thermal stability is higher.

The heat-resistant layer 72 contains inorganic particles and a binder.
Examples of the material of the inorganic particles include inorganic oxides such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ), and titania (TIO ₂ ), nitrides such as aluminum nitride and silicon nitride, and water. Examples thereof include metal hydroxides such as calcium oxide, 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. These have a high melting point and excellent heat resistance. It also has a relatively high Mohs hardness and is excellent in mechanical strength and durability. Furthermore, since it is relatively inexpensive, the raw material cost can be suppressed.

The heat-resistant layer 72 contains poly-N-vinylacetamide (PNVA) and polyvinylpyrrolidone (PVP) as a binder. By using these two types of polymers as the binder, the binding force and thermal stability of the heat-resistant layer 72 can be enhanced. It is considered that PVP mainly contributes to the improvement of binding force, and the acetamide group of PNVA mainly contributes to the improvement of thermal stability.
The total amount of the binder contained in the heat-resistant layer 72 is not particularly limited, but is preferably 1 part by mass or more and 7 parts by mass or less, and more preferably 1.8 parts by mass or more and 4.5 parts by mass with respect to 100 parts by mass of the inorganic particles. Parts or less, more preferably 2.4 parts by mass or more and 3.6 parts by mass or less.
The amount of PNVA contained in the heat-resistant layer 72 is not particularly limited, but is preferably 0.1 part by mass or more and 2 parts by mass or less, and more preferably 0.2 parts by mass or more with respect to 100 parts by mass of the inorganic particles. It is 5 parts by mass or less, more preferably 0.27 parts by mass or more and 1 part by mass or less.
The heat-resistant layer 72 may contain components other than the inorganic particles and the binder. An example is a thickener. Examples of the thickener for the heat-resistant layer 72 include carboxymethyl cellulose (CMC) and methyl cellulose (MC).
The thickness of the heat-resistant layer 72 is 2.7 μm or more and 5.1 μm or less. If the thickness of the heat-resistant layer 72 is out of this range, the thermal stability is lowered.

The base material layer 74 is typically a porous resin sheet layer. Examples of the resin constituting the porous resin sheet layer include polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. The base material 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 PP layers are laminated on both sides of the PE layer).
The thickness of the base material layer 74 is usually 10 μm or more, and typically 15 μm or more, for example, 17 μm or more. On the other hand, the thickness of the base material layer 74 is usually 40 μm or less, typically 30 μm or less, for example 25 μm or less.

The non-aqueous electrolytic solution 80 contains an oxalate complex compound as an additive. The oxalate compound acts as a film-forming agent. The oxalate complex compound is preferably a lithium salt of the oxalate complex, and examples thereof include lithium bis (oxalate) borate (LiBOB), lithium difluoro (oxalate) borate (LiFOB), and lithium difluorobis (oxalate) phosphate (LPFO). ), Lithium bis (oxalate) silane and the like.
The concentration of the oxalate complex compound in the non-aqueous electrolytic solution 80 is not particularly limited, but is preferably 0.005 mol / L or more and 0.15 mol / L or less, and more preferably 0.01 mol / L or more and 0.10 mol or less. It is less than / L.
The non-aqueous electrolyte 80 typically contains a non-aqueous solvent and a supporting salt in addition to the above additives.
As the non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones and lactones used in the electrolytic solution of a general lithium ion secondary battery shall be used without particular limitation. Can be done. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), and the like. Examples thereof include monofluoromethyldifluoromethyl carbonate (F-DMC) and trifluorodimethyl carbonate (TFDMC). As such a non-aqueous solvent, one kind may be used alone, or two or more kinds may be used in combination as appropriate.
As the supporting salt, for example, a lithium salt (preferably LiPF ₆ ) such as LiPF ₆ , LiBF ₄ , and LiClO ₄ 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 electrolytic solution 80 may contain, for example, a gas generator such as biphenyl (BP) or cyclohexylbenzene (CHB); a dispersant; a thickener, etc., as long as the effects of the present invention are not significantly impaired. good.

As the results of Examples described later show, the non-aqueous electrolytic solution 80 contains an oxalate complex compound as an additive, and the binder of the heat-resistant layer 72 of the separator 70 contains poly-N-vinylacetamide and polyvinylpyrrolidone. Since the thickness of the heat-resistant layer 72 is 2.7 μm or more and 5.1 μm or less, the lithium ion secondary battery 100 is excellent in thermal stability even though the thickness of the heat-resistant layer 72 of the separator 70 is small.

The lithium ion secondary battery 100 configured as described above can be used for various purposes. Suitable applications include drive power supplies mounted on vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). 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 lithium ion secondary batteries in series and / or in parallel.

As an example, a square lithium ion secondary battery 100 including a 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 laminated electrode body. The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery. Further, the technique disclosed herein can be applied to a non-aqueous electrolyte secondary battery other than a lithium ion secondary battery.

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.

<Manufacturing 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 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 N-methylpyrrolidone (NMP) at a mass ratio of AB: PVdF = 90: 8: 2. This slurry was applied to both sides of a long aluminum foil in a strip shape, dried, and then pressed to prepare a positive electrode sheet.
Natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are mixed in a mass ratio of C: SBR: CMC = 98: 1: 1. To prepare a slurry for forming a negative electrode active material layer by mixing with ion-exchanged water. This slurry was applied to both sides of a long copper foil in a strip shape, dried, and then pressed to prepare a negative electrode sheet.
As a separator base material, a porous film having a three-layer structure (PP / PE / PP) having a thickness of 20 μm in which polypropylene layers were formed on both sides of the polyethylene layer was prepared. Then, alumina powder (average particle size (D50) 0.1 μm) as inorganic particles, a binder and CMC as a thickener were dispersed in water to prepare a heat-resistant layer forming layer composition. The types of binders shown in the table were used in the amounts shown in the table. This heat-resistant layer layer forming composition is applied to one side of the separator base material with a gravure roll and dried to form a heat-resistant layer having the thickness shown in the table on the separator base material to prepare a separator sheet. did.
A flat-shaped wound electrode body was produced by laminating the positive electrode sheet, the negative electrode sheet, and the two separator sheets produced above, winding them, and then pressing them from the side surface direction to squeeze them. At the time of laminating, the heat-resistant layer of the separator sheet was made to face the electrodes shown in the table. Next, the positive electrode terminal and the negative electrode terminal were connected to the wound electrode body and housed in a square battery case having an electrolytic solution injection port.
Subsequently, a non-aqueous electrolyte solution was injected from the electrolyte solution injection port of the battery case, and the injection port was hermetically sealed. The non-aqueous electrolytic solution 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 = 3: 4: 3. LiPF ₆ as a salt was dissolved at a concentration of 1.1 mol / L, and when used, the additive shown in the table was dissolved at a concentration of 0.01 mol / L.
In this way, a lithium ion secondary battery for evaluation was produced.

<-10 ° C input test>
The time (seconds) until the battery voltage reached 4.1 V was measured by adjusting the battery to a state of SOC 60% and charging it with constant power at different power rates in an environment of −10 ° C. Then, a 10-second input was calculated from the slope of the linear approximation straight line of the plot of charging time (seconds) -power (W) at this time. The ratio of the 10-second input of each test example was calculated when the 10-second input of Example 2 was set to 100. The evaluation results are shown in the table.

<Maximum temperature during overcharge>
Each evaluation lithium ion secondary battery was CC-CV charged (4.1V, rate 1C, 0.01C cut) in an environment of 25 ° C. After that, an overcharge test was carried out with the upper limit voltage set to 25 V, and the internal temperature of each evaluation lithium ion secondary battery at that time was monitored to measure the maximum temperature at the time of overcharge. The ratio of the maximum temperature values of each test example was obtained when the maximum temperature value of Example 2 was set to 100. The evaluation results are shown in the table.

The non-aqueous electrolytic solution contains an oxalate complex compound as an additive, the binder of the heat-resistant layer of the separator contains PNVA and PVP, and the thickness of the heat-resistant layer is within the range of 2.7 μm or more and 5.1 μm or less. In Examples 1-11, good results were obtained for both -10 ° C input and thermal stability.
On the other hand, in Comparative Example 1 in which the thickness of the heat-resistant layer was as large as 6.6 μm, the maximum temperature during overcharging was high and the thermal stability was low.
In Comparative Example 2 in which PVP was not used as the binder A, the thermal stability was low.
In the reference example in which the acrylic binder was used as the binder A, the thermal stability tended to decrease as the thickness of the heat-resistant layer became smaller. Therefore, FIG. 4 shows a graph plotting the maximum temperature ratio during overcharging with respect to the thickness of the heat-resistant layer for Examples 1 to 3, Comparative Examples 1 and Reference Examples 2 to 4. From FIG. 4, when only an acrylic binder is used as the binder of the heat-resistant layer, the maximum temperature ratio at the time of overcharging increases as the thickness of the heat-resistant layer decreases, and it can be approximated by a straight line having a negative slope. It is a general behavior that the thermal stability deteriorates as the thickness of the heat-resistant layer decreases. However, when PNVA and PVP are used in combination with the heat-resistant layer binder, the maximum temperature ratio during overcharging is small in a region where the thickness of the heat-resistant layer is small (range of 2.7 μm or more and 5.1 μm or less). I was able to confirm the phenomenon. This can be approximated by a downwardly convex curve.
Further, in Comparative Examples 3 to 9 in which the electrolytic solution did not contain an additive, the thermal stability was low.
The reason why such a result was obtained is presumed as follows.
At the time of overcharging, the LiBOB film decomposition product reacts with PNVA on the surface of the negative electrode to generate heat. If the thickness of the heat-resistant layer is large, the amount of reaction increases and heat generation increases, so that the temperature at the time of overcharging tends to rise, and the thermal stability decreases. On the other hand, if the thickness of the heat-resistant layer is small, the amount of reaction is reduced, which is improved. Therefore, the relationship between the thickness of the heat-resistant layer and the maximum temperature ratio during overcharging can be seen, which can be approximated by a downwardly convex curve. While PNVA has a negative effect of causing an exothermic reaction on the surface of the negative electrode, it has a positive effect of being able to function as a binder even at high temperatures and retain a heat-resistant layer because it has high thermal stability for the portion that is not decomposed. It is considered that this positive effect is larger in the region where the thickness of the heat-resistant layer is small (range of 2.7 μm or more and 5.1 μm or less). However, with PNVA alone, the heat-resistant layer has a small binding force, so that the heat shrinkage rate is large and the thermal stability is lowered. It is conceivable to increase PNVA in order to improve the binding property of the heat-resistant layer, but it is considered that when the content of PNVA increases, the exothermic reaction increases and the thermal stability at the time of overcharging decreases. Further, since PVP is excellent in the binding force of the heat-resistant layer, the inclusion of PVP in the binder can reduce heat shrinkage during heating and improve thermal stability. However, when the binder does not contain PNVA, the thermal stability at high temperature is lowered.

The same tendency as described above was obtained in Examples 4 and 5 and Comparative Example 10 in which the types of additives for the non-aqueous electrolytic solution were changed. Like LiBOB, LPFO can form a decomposition film of oxalate on the negative electrode. Therefore, the decomposition product of oxalate and PNVA can undergo an exothermic reaction on the surface of the negative electrode during overcharging, and the effect of thermal stability can be obtained by the same mechanism as described above. Therefore, it can be seen that the effect of the present invention can be obtained if the oxalate complex compound is used as an additive for the non-aqueous electrolytic solution.

In Example 6 in which the heat-resistant layer of the separator was opposed to the positive electrode, the thermal stability was higher. It is considered that this is because the exothermic reaction occurs on the negative electrode as described above, and the reaction with the LiBOB film decomposition product generated on the negative electrode can be reduced by directing the heat-resistant layer toward the positive electrode side.

In Examples 7 to 11, the amount of the binder was examined, but the effect of improving the thermal stability tended to be small when the amount of the binder was increased or decreased. It is considered that this is because the exothermic reaction increases when the amount of binder is large, and the binding force of the heat-resistant layer decreases when the amount of binder is small, and the effect of suppressing heat shrinkage decreases.

From the above results, it can be seen that the non-aqueous electrolytic solution secondary battery according to the present embodiment has excellent thermal stability despite the small thickness of the heat-resistant layer of the separator.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The techniques described in the scope of the claims include various modifications and modifications of the specific examples exemplified 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-formed 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)
72 Heat-resistant layer 74 Base material layer 80 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims (4)

A non-aqueous electrolytic solution secondary battery including an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolytic solution.
The non-aqueous electrolytic solution contains an oxalate complex compound as an additive and contains.
The separator includes a heat-resistant layer containing inorganic particles and a binder, and a base material layer.
The binder contains poly-N-vinylacetamide and polyvinylpyrrolidone.
The thickness of the heat-resistant layer is 2.7 μm or more and 5.1 μm or less.
Non-aqueous electrolyte secondary battery. The non-aqueous electrolytic solution secondary battery according to claim 1, wherein the total amount of the binder contained in the heat-resistant layer is 2.4 parts by mass or more and 3.6 parts by mass or less with respect to 100 parts by mass of the inorganic particles. The secondary non-aqueous electrolytic solution according to claim 2, wherein the amount of poly-N-vinylacetamide contained in the heat-resistant layer is 0.27 parts by mass or more and 1 part by mass or less with respect to 100 parts by mass of the inorganic particles. battery. The non-aqueous electrolytic solution secondary battery according to any one of claims 1 to 3, wherein the heat-resistant layer faces the positive electrode.

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