Non-aqueous electrolyte secondary batteries

Abstract

To suppress variations of a shutdown timing of a separator.SOLUTION: A non-aqueous electrolyte solution secondary battery comprises an electrode body in which a positive electrode and a negative electrode are laminated via a separator and are wound, in which a thickness ThIN of an inner side separator 120b of the electrode body becomes thicker than a thickness ThOUT of an outer side separator 120a, and a heat capacity [J / K] per a unit area [cm2] of the separator in an inner side of the electrode body is larger than that of the outer side separator. Also, both of the inner side separator 120b and the outer side separator 120a are constructed so that a first layer made of a first resin having a low conventional fusion point is nipped with a pair of second layers made of a second resin having a high conventional fusion point. A ratio of the second layer against the first layer is larger than the inner side separator 120b of the electrode body.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a nonaqueous electrolyte secondary battery, and more particularly to a nonaqueous electrolyte secondary battery with little variation in separator shutdown timing. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been used as power sources for electric vehicles and other devices, with many cell batteries connected in series and parallel to supply high voltage and high current. To accommodate these multiple cell batteries in a compact space, stacked batteries with many stacked electrode plates are being used. Furthermore, cell batteries with thin stacked electrode bodies are increasingly being used to improve cooling efficiency and ensure close contact between the electrode plates.

[0003] In such flat electrode assemblies, positive and negative electrode plates are stacked with a separator interposed between them. The separator is made of a porous resin material to prevent short circuits between the positive and negative electrode plates and to allow electrolyte alternation. In such nonaqueous electrolyte secondary batteries, the temperature of the electrode assembly may rise due to overcharging or other reasons. In such cases, the separator dissolves when the battery reaches a set temperature, interrupting the ionic alternation and stopping the reaction between the electrodes.

[0004] For example, Patent Document 1 describes a battery that uses a polyethylene (PE) separator with a low melting temperature in the battery's outer layer and a polypropylene (PP) separator with a high melting temperature inside the battery. When heat is generated due to overcharging or other reasons, a shutdown phenomenon occurs in the outer separator. Overcharging then stops and discharge begins in the outer power-generating element, and then charging also stops and discharge begins in the inner power-generating element. Because cooling is efficient on the outer side of the battery, stopping heat generation in this area early and suppressing temperature rise can also improve heat dissipation within the battery. Furthermore, because the entire battery discharges over a long period of time, the rate of temperature rise during discharge can be reduced. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-331922 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in Patent Document 1, when heat is generated due to overcharging or the like, the shutdown phenomenon occurs first in the outer separator. In this case, there is a problem that current concentrates in the electrode body on the inner side of the battery, which may actually cause overcharging to progress and overheat.

[0007] Furthermore, if the separators of the electrode bodies are of the same structure as in the past, the separator on the inside will shut down first, but in this case, current will concentrate on the electrode body on the outside of the battery, which again poses the problem of overcharging progressing and overheating.

[0008] The problem to be solved by the non-aqueous electrolyte secondary battery of the present invention is to suppress variations in the shutdown timing of the separator. [Means for solving the problem]

[0009] The nonaqueous electrolyte secondary battery of the present invention is a nonaqueous electrolyte secondary battery having an electrode assembly in which a positive electrode plate and a negative electrode plate are stacked with a separator interposed therebetween, and the thickness Th of the separator inside the electrode assembly IN the thickness Th of the separator outside the electrode body OUT By making it thicker, the unit area of ​​the separator inside the electrode [cm 2 It is characterized by its large heat capacity [J / K] per unit mass.

[0010] The thickness Th of the separator inside the electrode body IN and the thickness Th of the separator outside the electrode body OUTIt is desirable to set the difference in heat capacity so as to reduce the temperature difference between the separator inside the electrode assembly and the separator outside the electrode assembly in an overcharged state.

[0011] The number of layers of the electrode body is divided into outer / inner / outer layers in the ratio of 1 / 2 / 1, and the thickness of the separator on the inner side of the electrode body is set to Th IN The thickness of the separator on the outside of the electrode body is Th OUT When the total number of layers is LN and the coefficient is α, the thickness Th of the separator inside the electrode body is IN , thickness Th IN = thickness Th OUT × (1 + coefficient α × total number of layers LN).

[0012] The separator inside the electrode body and the separator outside the electrode body are both configured so that a first layer made of a first resin with a relatively low melting point is sandwiched between a pair of second layers made of a second resin with a relatively high melting point, and the ratio of the second layer to the first layer can be greater in the separator inside the electrode body.

[0013] The first resin may be polyethylene (PE), and the second resin may be polypropylene (PP). The ratio of the second layer to the first layer can also be set so that the shutdown timing of the separator inside the electrode body and the shutdown timing of the separator outside the electrode body are close to each other depending on the temperature difference between the separator inside the electrode body and the separator outside the electrode body. [Effects of the Invention]

[0014] The nonaqueous electrolyte secondary battery of the present invention can suppress variations in the shutdown timing of the separator. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a perspective view of a lithium ion secondary battery. [Figure 2]FIG. 2 is a perspective view showing the configuration of a laminate of electrode bodies of a lithium ion secondary battery. [Figure 3] FIG. 2 is an exploded perspective view of a laminate of an electrode body. [Figure 4] FIG. 2 is a schematic diagram showing the positions of the inner and outer separators of the electrode assembly. [Figure 5] FIG. 2 is a schematic diagram showing the configuration of the inside and outside of the electrode body. [Figure 6] FIG. 2 is a schematic diagram showing the configuration of a separator on the outside of an electrode assembly. [Figure 7] FIG. 2 is a schematic diagram showing the configuration of a separator inside an electrode assembly. [Figure 8] 1 is a graph showing the voltage and temperature of the separator of the present embodiment. [Figure 9] 1 is a graph showing voltage and temperature for a prior art separator. DETAILED DESCRIPTION OF THE INVENTION

[0016] (Outline of this embodiment) <Principle of this embodiment> As described in the prior art section, in Patent Document 1, when heat is generated due to overcharging or the like, the shutdown phenomenon occurs first in the outer separator. In this case, there is a problem that current concentrates in the electrode body on the inner side of the battery, which may actually cause overcharging to progress and overheat.

[0017] Therefore, the present inventors have focused on the fact that such a problem can be solved by suppressing the variation in the shutdown timing of the separator. There are two ways to achieve this goal. One is to reduce the temperature difference ΔTmp [°C] between the inside and outside of the electrode assembly 10 (see FIG. 4). Specifically, the thickness Th of the inner separator 120b on the inside IN shown in FIG. IN [μm] is the thickness of the outer separator on the outside (OUT) side. OUT [μm]. This reduces the unit area UA [cm 2This can be achieved by increasing the heat capacity HC [J / K] per unit area and reducing the internal and external temperature difference ΔTmp [°C].

[0018] The second is to allow the difference in temperature between the inside and outside ΔTmp [°C] and the shutdown temperature ST of the inner separator 120b. IN [°C] is the shutdown temperature ST of the outer separator 120a. OUT [°C]. This allows the shutdown timing to be closer.

[0019] Furthermore, these two methods can be combined. The present invention will be described in detail below with reference to FIGS. 1 to 9, taking one embodiment of a lithium ion secondary battery 1 as an example.

[0020] (Specific configuration of this embodiment) <Basic structure of lithium-ion secondary battery 1> First, the configuration of a lithium ion secondary battery 1, which is the premise of this embodiment, will be briefly described.

[0021] FIG. 1 is a perspective view of a lithium-ion secondary battery 1 according to this embodiment. As shown in FIG. 1, the lithium-ion secondary battery 1 is configured as a cell battery. It includes a rectangular parallelepiped battery case 11 with an opening on the upper side. The battery case 11 includes a lid 12 that seals the battery case 11. An electrode assembly 10 is housed inside the battery case 11. A nonaqueous electrolyte 17 is injected into the battery case 11 through an inlet 18. The battery case 11 and the lid 12 are made of a metal such as an aluminum alloy. The lithium-ion secondary battery 1 forms a sealed battery container by attaching the lid 12 to the battery case 11. The lithium-ion secondary battery 1 also includes a negative electrode external terminal 14 and a positive electrode external terminal 16, which are used for charging and discharging power, on the lid 12.

[0022] <Electrode body 10> FIG. 2 is a perspective view showing the configuration of the laminate of the electrode body 10 of the lithium-ion secondary battery 1. FIG. 3 is an exploded perspective view of the laminate of the electrode body 10. The electrode body 10 shown in FIG. 2 is configured by laminating a negative electrode plate 100, a positive electrode plate 110, and a separator 120, as shown in FIG. 3. The negative electrode plate 100 has a negative electrode composite layer 102 on both sides of a negative electrode substrate 101. The positive electrode plate 110 has a positive electrode composite layer 112 on both sides of a positive electrode substrate 111. The negative electrode plate 100 and the positive electrode plate 110 are stacked with the separator 120 interposed therebetween to form the electrode body 10.

[0023] Negative electrode connection part 103 formed so as to protrude upward from one end of the upper part of negative electrode plate 100 functions as a current collector that extracts electricity from negative electrode composite layer 102 of negative electrode plate 100. Positive electrode connection part 113 formed so as to protrude upward from the other end of the upper part of positive electrode plate 110 functions as a current collector that extracts electricity from positive electrode composite layer 112 of positive electrode plate 110.

[0024] <Configuration of End Portion of Electrode Body 10> In the perspective view of the stacked electrode body 10 in Fig. 2, the direction parallel to the longitudinal direction of the lithium ion secondary battery 1 is referred to as the "width direction W." The direction perpendicular to the surface of the electrode body 10 is referred to as the "thickness direction T." The direction perpendicular to the width direction W and the thickness direction T is referred to as the "length direction L."

[0025] The electrode assembly 10 is shaped into a flat shape in a flattening press process using a press machine in a thickness direction T perpendicular to the width direction W. In the flattening press process, the electrodes are tightly attached via a separator 120. The negative electrode connection portion 103 is compressed and the negative electrode current collector 13 is welded to it. The positive electrode connection portion 113 is compressed and the positive electrode current collector 15 is welded to it. Methods for welding the connection portions to the current collectors include ultrasonic welding, resistance welding, and electric welding. Then, a negative electrode external terminal 14 is connected to the negative electrode current collector 13 through the lid 12, and a positive electrode external terminal 16 is connected to the positive electrode current collector 15. Then, as shown in FIG. 1 , the flat electrode assembly 10 is housed in a battery case 11 together with the lid 12.

[0026] <Separator 120> The separator 120 shown in FIG. 3 is an insulator between the negative electrode plate 100 and the positive electrode plate 110, and is also a sheet for holding the non-aqueous electrolyte 17. When the electrode assembly 10 is immersed in the non-aqueous electrolyte 17, the non-aqueous electrolyte permeates from the edge of the separator 120 toward the center. The positive electrode plate 110 and the negative electrode plate 100 are connected to each other by the non-aqueous electrolyte 17 held in the separator 120. + ) to charge and discharge the battery.

[0027] <Outer separator 120a and inner separator 120b> FIG. 4 is a schematic diagram showing the positions of the outer separator 120a on the outside of the electrode assembly 10 and the inner separator 120b on the inside. In FIG. 4, the left-right direction is the thickness direction T. In FIG. 4, the negative electrode plate 100 and the positive electrode plate 110 are omitted, and only the separator 120 is shown. The number of stacked layers LN of the electrode assembly 10 of this embodiment is 64, but in FIG. 4, the number of stacked layers of the separator 120 is shown in a simplified manner. The separator 120 of this embodiment consists of an inner separator 120b arranged on the inside of the electrode assembly 10 and an outer separator 120a arranged on the outside of the electrode assembly 10, as shown in FIG. 4.

[0028] In this embodiment, the electrode body 10 is divided into four equal parts in the thickness direction T (left-right direction in the figure) of the electrode body 10 shown in Fig. 4, and the portions located on the outside of both ends of the electrode body 10 are referred to as "outside". Also, the portion located on the inside near the center of the electrode body 10 is referred to as "inside".

[0029] In this embodiment, as shown in FIG. 3, the electrode assembly 10 is a laminated body made up of a combination of "separator-negative electrode plate-separator-positive electrode plate," and this combination is referred to as "one layer." That is, in this embodiment, as shown in FIG. 4, the number of layers LN (here, 64 layers) of the electrode assembly 10 is divided into 1 / 2 / 1 for outer / inner / outer. Therefore, there is a laminated body of 16 layers on the outer side, 32 layers on the inner side, and 16 layers on the opposite outer side. Separators 120 are always placed on both ends.

[0030] 6 is a schematic diagram showing the configuration of the outer separator 120a arranged on the outside of the electrode assembly 10. Also, FIG. 7 is a schematic diagram showing the configuration of the inner separator 120b arranged on the inside of the electrode assembly 10. These configurations correspond to those of Example 2 or 3.

[0031] As shown in FIGS. 6 and 7, the outer separator 120a and the inner separator 120b each have a first layer L at the center, which is made of a nonwoven fabric of a first resin (here, polyethylene (PE)) having a relatively low melting point. IN In addition, this first layer L IN A pair of second layers L made of nonwoven fabrics of a second resin (here, polypropylene (PP)) having a relatively high melting point sandwiched between them. OUT It is made up of three layers, sandwiched between the inner first layer L IN The outer second layer L OUT The ratio R of the inner separator 120b is larger than that of the inner separator 120b. Even if the PE layer melts first, the PP layer prevents a short circuit between the positive electrode plate 110 and the negative electrode plate 100. Furthermore, when the PE layer melts, the first layer L made of porous nonwoven fabric melts. IN The voids are sealed and the lithium ions (L + ) movement is blocked. Therefore, the main reaction of the battery stops by shutting down, and the shut down part will not be overcharged any more, and the overheating state can also be suppressed. When PP and PE are combined, the melting point is between the melting points of PP and PE. Therefore, the first layer L IN and the second layer L OUT The melting point can be controlled by changing the ratio R. Therefore, the shutdown temperature ST of the outer separator 120a can be controlled. OUT [°C] and the shutdown temperature ST of the inner separator 120b IN [°C] can be set individually.

[0032] <Negative electrode plate 100> Negative electrode plate 100 is constructed by forming negative electrode composite layers 102 on both sides of negative electrode substrate 101. In this embodiment, negative electrode substrate 101 is made of Cu foil. Negative electrode substrate 101 serves as a base for the aggregate of negative electrode composite layer 102 and also functions as a current collecting member that collects electricity from negative electrode composite layer 102. In negative electrode plate 100, negative electrode composite layer 102 is formed on metallic negative electrode substrate 101. In the first embodiment, the negative electrode active material is a material capable of absorbing and releasing lithium ions, and a powdered carbon material made of graphite or the like is used.

[0033] Negative electrode plate 100 is produced, for example, by kneading a negative electrode active material, a solvent, and a binder, and then applying the kneaded negative electrode mixture to negative electrode substrate 101 and drying it. <Positive electrode plate 110> Positive electrode plate 110 is configured by forming positive electrode composite layers 112 on both sides of positive electrode substrate 111. In this embodiment, positive electrode substrate 111 is configured from Al foil or Al alloy foil. Positive electrode substrate 111 serves as a base as an aggregate for positive electrode composite layer 112 and also functions as a current collecting member that collects electricity from positive electrode composite layer 112.

[0034] The positive electrode plate 110 has a positive electrode composite layer 112 formed on the surface of a positive electrode substrate 111. The positive electrode composite layer 112 contains a positive electrode active material. The positive electrode active material is a material capable of absorbing and releasing lithium, and examples of the positive electrode active material that can be used include lithium cobalt oxide (LiCoO), lithium manganese oxide (LiMnO), and lithium nickel oxide (LiNiO). Alternatively, a material in which LiCoO, LiMnO, and LiNiO are mixed in any ratio may be used.

[0035] Positive electrode mixture layer 112 also contains a conductive material, which may be, for example, carbon black such as acetylene black (AB) or ketjen black, or graphite.

[0036] The positive electrode plate 110 is produced, for example, by kneading a positive electrode active material, a conductive material, a solvent, and a binder, and then applying the kneaded positive electrode mixture to the positive electrode substrate 111 and drying it. <Nonaqueous electrolyte 17> The nonaqueous electrolyte 17 is a composition in which a supporting salt is contained in a nonaqueous solvent. Ethylene carbonate (EC) can be used as the nonaqueous solvent. Alternatively, one or more materials selected from the group consisting of propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be used. Alternatively, one or more lithium compounds (lithium salts) selected from these can be used.

[0037] (Example) At the end of the overcharge period of a conventional lithium-ion secondary battery 1, a temperature difference ΔTmp between the surface of the electrode body 10 and the innermost layer of the electrode body 10 occurs, which is approximately 15 to 25°C. To reduce the difference in shutdown timing between the inside and outside of the electrode body 10 due to such a temperature difference, the following method can be proposed.

[0038] Example 1 In Example 1, the thickness Th of the inner separator 120b of the electrode assembly 10 IN The thickness Th of the outer separator 120a OUT This increases the unit area [cm 2 ] of the inner separator 120b of the electrode assembly 10. 2 In Example 1, the outer separator 120a and the inner separator 120b are made of the same material and have the same melting point. 2 In this case, the thickness Th of the inner separator 120b of the electrode assembly 10 is adjusted. IN and the thickness Th of the outer separator 120a of the electrode assembly 10 OUT is set as follows: That is, the temperature Tmp of the inner separator 120b of the electrode assembly 10 in an overcharged state IN[°C] and the temperature Tmp of the outer separator 120a OUT The difference between the internal and external temperature difference ΔTmp [°C] and the heat capacity HC [J / K] that reduces the temperature difference between the internal and external temperatures is set to be equal to the difference between the internal and external temperatures ΔTmp [°C] and the heat capacity HC [J / K] that reduces the temperature difference between the internal and external temperatures ΔTmp [°C].

[0039] Specifically, in this embodiment, the number of layers LN of the electrode body 10 is divided into 1 / 2 / 1 for outer / inner / outer. In this embodiment, the number of layers LN of the electrode body 10 is 64, so that the outer / inner / outer ratio is 16 layers / 32 layers / 16 layers. The thickness of the inner separator 120b is Th IN The thickness of the outer separator 120a is Th OUT When the total number of layers is LN and the coefficient is α, the thickness Th of the separator inside the electrode body is IN is calculated using the following equation 1.

[0040] Thickness Th IN = thickness Th OUT × (1 + coefficient α × total number of layers LN)…Equation 1 <Shutdown timing adjustment by reducing the temperature difference ΔTmp inside and outside the electrode> As mentioned above, one of the means for shortening the shutdown timing is to reduce the temperature difference ΔTmp between the inside and outside of the electrode body 10. Specifically, this can be achieved by creating a difference in heat capacity HC [J / K] between the inside and outside of the electrode body 10 due to the thickness of the separator, thereby alleviating the temperature difference ΔTmp between the inside and outside. For example, to eliminate the temperature difference ΔTmp = 15°C (for example, 130°C at the innermost periphery and 145°C at the outermost periphery), the heat capacity ratio of the innermost periphery to the outermost periphery should be 145 / 130. To achieve this with the thickness Th [μm] of the separator 120, the thickness Th of the outer separator 120a should be OUT [μm] and the thickness Th of the inner separator 120b IN Thickness ratio Th [μm] IN / Th OUT However, since it is only necessary to reduce the temperature difference between the inside and outside of the building, it is not necessary to set the temperature difference to 1.7 times, but it is sufficient to be able to reduce the temperature difference between the inside and outside of the building, ΔTmp.

[0041] Since the temperature difference between the inside and outside of the electrode body 10 changes depending on the number of layers LN of the electrode body 10, a design method for reducing the temperature difference between the inside and outside of the electrode body 10 based on the number of layers LN of the electrode body 10 is described below. IN = thickness Th OUT × (1 + coefficient α × total number of layers LN)...This is generalized as Equation 1.

[0042] Specifically, when the number of layers LN is divided into outer / inner / outer layers in a ratio of 1 / 2 / 1, the thickness Th of the inner separator 120b is IN [μm] is the thickness Th of the outer separator 120a OUT It is desirable to design it so that [μm] satisfies the following relational expression.

[0043] Thickness Th IN [μm] = Thickness Th OUT [μm] × (1 + 0.0055 × total number of layers LN) For example, when the total number of layers is LN=64, the thickness is Th IN [μm] is the thickness Th OUT It goes without saying that the coefficient α is not limited to this example, and can be optimized by a person skilled in the art depending on the battery to which the invention of this embodiment is applied.

[0044] <Function of Example 1> FIG. 8 is a graph showing time [seconds], voltage [V], and temperature [°C] for the outer separator 120a and inner separator 120b of this embodiment. FIG. 9 is a graph showing time [seconds], voltage [V], and temperature [°C] for the outer separator 120a and inner separator 120b of the prior art. The horizontal axis shows elapsed time [seconds], and the left vertical axis shows the voltage [V] of the battery voltage graph G1. The right vertical axis shows the Tmp of the inner separator 120b. IN Graphs G3 and G3' showing the temperature Tmp of the outer separator 120a [°C] OUT Graph G3 shows the temperature [°C].

[0045] As shown in Figure 9, conventionally, when overcharging occurs, the battery voltage V [V] rises over time. At this time, the electrode assembly 10 itself generates heat, and the temperatures [°C] of the outer separator 120a and inner separator 120b also rise. At this time, the outer separator 120a and inner separator 120b have the same configuration and the same heat capacity HC [J / K]. For this reason, the temperature of the inner electrode assembly 10 is higher than that of the outer electrode assembly 10. As a result, the temperature Tmp of the inner separator 120b rises. IN [°C] is the temperature Tmp of the outer separator 120a. OUT It rises faster than [°C].

[0046] The outer separator 120a and the inner separator 120b have the same configuration, and the shutdown temperature ST OUT [°C] and the shutdown temperature ST of the inner separator 120b IN As a result, the inner separator 120b reaches the shutdown temperature ST earlier than the inner separator 120b. IN [°C]. Then, ions and electrons do not flow in the inner part of the electrode assembly 10 due to the inner separator 120b that was shut down earlier, and the ions and electrons flow intensively in the outer part of the electrode assembly 10. As a result, overcharging progresses further in the outer part of the electrode assembly 10, and the temperature of the outer separator 120a also rises.

[0047] In the lithium ion secondary battery 1 shown in Example 1, the thickness Th of the inner separator 120b IN [μm] is the thickness Th of the outer separator 120a OUT [μm] thick. Therefore, the heat capacity HC [J / K] of the inner separator 120b is larger than the heat capacity HC [J / K] of the outer separator 120a. In other words, when the electrode assembly 10 generates heat, the temperature of the inner separator 120b is less likely to rise than that of the outer separator 120a.

[0048] Tmp of the inner separator 120b shown in FIG. IN 8. When compared with the graph G2 showing the temperature Tmp [°C] of the inner separator 120b of Example 1 shown in FIG. INAs a result, the graph G2' showing the temperature rise of the inner separator 120b in Example 1 is smaller than the Tmp of the outer separator 120a. OUT Therefore, when the electrode assembly 10 is overheated due to overcharging, the Tmp of the inner separator 120b is IN [°C] and the temperature Tmp of the outer separator 120a OUT The temperature [°C] rises at the same rate. Then, the shutdown temperature ST [°C] is reached at approximately the same time, causing shutdown at approximately the same time. This prevents a large current from flowing to either the inside or outside of the electrode body 10, preventing further overcharging.

[0049] Example 2 Fig. 6 is a schematic diagram showing the configuration of the outer separator 120a arranged on the outside of the electrode assembly 10. Fig. 8 is a schematic diagram showing the configuration of the inner separator 120b arranged on the inside of the electrode assembly 10.

[0050] In Example 1, the thickness Th of the outer separator 120a OUT [μm] and the thickness Th of the inner separator 120b IN On the other hand, Example 2 is an invention in which the outer and inner separators 120 have different structures and different shutdown temperatures ST [°C], thereby adjusting the shutdown timing. Note that the thickness Th of the outer separator 120a in Figures 6 and 7, which are referred to for the explanation, OUT [μm] and the thickness Th of the inner separator 120b IN However, in Example 2, the thickness Th of the outer separator 120a is different. OUT [μm] and the thickness Th of the inner separator 120b IN The condition is that [μm] is the same.

[0051] The inner separator 120b and the outer separator 120a of the electrode assembly 10 are both made of a first resin having a relatively low melting point. INA pair of second layers L made of a second resin having a relatively high melting point OUT The first layer L IN The second layer L OUT The ratio R of the inner separator 120b is set to be larger than that of the inner separator 120b.

[0052] In this embodiment, the first resin is polyethylene (PE) and the second resin is polypropylene (PP). A second method for shortening the shutdown timing inside and outside the electrode assembly 10 is to use separators 120 with different shutdown temperatures ST [°C] inside and outside the electrode assembly 10. For example, a single layer of PE has a shutdown temperature of nearly 130°C, a single layer of PP has a shutdown temperature of nearly 150°C, and a combination of these, PP / PE / PP, has a shutdown temperature of nearly 140°C. Therefore, by combining these, it is possible to adjust the shutdown temperature ST [°C] of the inner separator 120b and the outer separator 120a.

[0053] As shown in FIG. 6, the outer separator 120a is made of a pair of second layers L made of PP, each of which is disposed on the outer side. OUT The width is Th PP1 In addition, a pair of second layers L OUT The second layer L is made of PE sandwiched between OUT The width is Th PE1 At this time, the first layer L IN The second layer L OUT The ratio R is R=Th PP1 ×2 / Th PE1 It is calculated as follows.

[0054] FIG. 7 shows that the inner separator 120b is made of a pair of second layers L made of PP, each layer being arranged on the outside. OUT The width is Th PP2 In addition, a pair of second layers L OUT The second layer L is made of PE sandwiched between OUT The width is Th PE2 At this time, the first layer L INThe second layer L OUT The ratio R is R=Th PP2 ×2 / Th PE2 It is calculated as follows.

[0055] Here, the ratio R of the outer separator 120a shown in Fig. 6 is smaller than the ratio R of the inner separator 120b shown in Fig. 7. That is, since the ratio of PP having a high melting point to PE having a low melting point is high, the shutdown temperature ST OUT [°C] is the shutdown temperature ST of the inner separator 120b. IN It is set lower than [°C].

[0056] Both the outer separator 120a shown in FIG. 6 and the inner separator 120b shown in FIG. 7 are made of a combination of PP / PE / PP. In this case, the outer separator 120a shown in FIG. 6 may be made entirely of PE and have a low melting point, and the inner separator 120b shown in FIG. 7 may be made entirely of PP and have a high melting point.

[0057] In this embodiment and in the design, the shutdown temperature ST [°C] of the separator 120 must be adjusted based on the number of layers LN. When the number of layers LN is divided into outer / inner / outer in a ratio of 1 / 2 / 1, the average temperature difference between the inner and outer layers is "total number of layers LN × 0.22 [°C]." Taking this into consideration, the shutdown temperature can be adjusted by using a PE monolayer (around 130°C), a PP / PE / PP monolayer (around 130-150°C, adjustable by changing the PP / PE / PP ratio), or a PP monolayer (150°C).

[0058] Example 3 In Example 1, adjustment is made to synchronize the shutdown timing by focusing on the thickness Th of the separator 120. In Example 2, adjustment is made to synchronize the shutdown timing by focusing on the material of the separator 120. In the above embodiment, Example 3, which is a combination of Example 1 and Example 2, is described.

[0059] Those skilled in the art will recognize that the temperature Tmp of the inner separator 120b of the electrode assembly 10 IN [°C] and the temperature Tmp of the outer separator 120a OUT The thickness Th and material of the separator 120 can be changed according to the temperature difference ΔTmp [°C].

[0060] For example, in Example 1, the thickness ratio Th of the inner separator 120b / the outer separator 120a is IN / Th OUT On the other hand, in Example 3, the thickness ratio Th of the separator 120 was changed by changing the material of the separator. IN / Th OUT Even if the power consumption is 1.3 times higher, the deviation in the shutdown timing can be suppressed.

[0061] (Effects of this embodiment) (1) The lithium ion secondary battery of this embodiment has the advantage of being able to suppress variations in the shutdown timing of the separator 120.

[0062] (2) As in Example 1, the thickness Th of the separator 120 is set to the thickness Th calculated assuming overcharging, and the heat capacity HC [J / K] is adjusted accordingly. This has the effect of making it possible to set the temperature rise during overheating to be approximately the same.

[0063] (3) The number of layers LN of the electrode body 10 is divided into 1 / 2 / 1 in the outer / inner / outer directions, and the thickness of the inner separator 120b is set to Th IN The thickness of the outer separator 120a is Th OUT The total number of layers is LN, and the coefficient is α. In this case, the thickness Th of the inner separator 120b of the electrode assembly 10 is IN , thickness Th IN = thickness Th OUT × (1 + coefficient α × total number of layers LN). Therefore, the thickness Th of the inner separator 120b can be calculated by simple calculation. IN and the thickness Th of the outer separator 120a OUT This has the effect of making it possible to set the following.

[0064] (4) As in Example 2, the inner separator 120b and the outer separator 120a are both made of a first resin having a relatively low melting point. IN A pair of second layers L made of a second resin having a relatively high melting point OUT The first layer L IN The second layer L OUT By using PP, the melting point of the first layer L can be set to an intermediate value between these melting points. IN The second layer L OUT The ratio R of the inner separator 120b to the outer separator 120c is set to be larger. By using different commonly available materials in this way, the desired shutdown temperature ST [°C] can be easily achieved. This has the effect of allowing the shutdown temperature ST [°C] to be changed to match the characteristics of the target battery, thereby making it possible to synchronize the shutdown timing.

[0065] (5) Furthermore, by combining the technical concepts of the first and second embodiments, it is possible to achieve the effect of further matching the shutdown timing to the characteristics of the target battery. (Another example) In this embodiment, the present invention has been described using as an example a laminated electrode body 10 in which rectangular negative electrode plates 100, positive electrode plates 110, and separators 120 are laminated. The present invention is not limited to this, and may be one in which long negative electrode plates 100, positive electrode plates 110, and separators 120 are wound and laminated into a cylindrical shape. Furthermore, the present invention may be applied to a wound electrode body 10 in which this is flattened and the cross section is shaped like a racing track.

[0066] In this embodiment, for the sake of simplicity, the total number of layers LN is 64, with 16 layers on both outer edges being the "outside" and 32 layers on the inside being the "inside", but this is not limiting and the number of layers LN on the "outside" and "inside" may be different.

[0067] Alternatively, a "middle portion" may be provided between the "outside" and "inside" portions to vary the thickness Th of the separator 120 and the ratio R of the second layer to the first layer. Of course, it goes without saying that it can be further subdivided.

[0068] In the embodiment, the separator 120 is exemplified by a three-layer structure of PP / PE / PP or a single-layer structure of PP or PE, but the present invention is not limited to these. For example, the separator 120 may be made of a porous polymer membrane such as a porous polyethylene membrane, a porous polyolefin membrane, or a porous polyvinyl chloride membrane, or a lithium ion or ion-conductive polymer electrolyte membrane. These may also be used alone or in combination.

[0069] The lithium ion secondary battery 1 of this embodiment is an example of a nonaqueous electrolyte secondary battery of the present invention, and is not limited to the shape of the plate-shaped lithium ion secondary battery 1 for vehicle use as shown in the figure. Furthermore, it is not limited to vehicle use, and can also be used as a stationary battery for home or factory use. Furthermore, the battery case is not limited to being made of metal such as aluminum, and may be made of resin or ceramic.

[0070] The numerical ranges shown are examples of preferred embodiments of this invention, and the present invention is not limited to these numerical limits and can be optimized by those skilled in the art. The shutdown timing varies depending on the battery characteristics, the degree of overcharging, and the ambient temperature, but the purpose of this invention is not to perfectly match the shutdown timing. It goes without saying that those skilled in the art can appropriately set conditions to suppress deviations in accordance with the usage of the battery and optimize the numerical range accordingly.

[0071] It goes without saying that the present invention can be implemented by those skilled in the art by adding, deleting, or modifying its configuration within the scope of the claims. [Explanation of symbols]

[0072] G1, G2, G2´, G3… graphs W: Width direction W T...thickness direction L...Length direction L Th [μm]…Separator thickness Th IN [μm]…Inner separator thickness Th OUT [μm]…Outer separator thickness Th IN / Th OUT …Thickness ratio UA [cm 2 ]…Separator unit area HC[J / K]…Heat capacity Tmp IN [°C]…Temperature of inner separator Tmp OUT [°C]…Outer separator temperature ΔTmp[°C]…Difference between inside and outside temperature ST [°C]: Shutdown temperature ST IN [°C]...Shutdown temperature ST OUT [°C]...Shutdown temperature LN: (Total) number of layers of electrode body α...coefficient L IN ...first layer Th PE1 , Th PE2 …(first layer) width L OUT ...Second layer Th PP1 , Th PP2 …(second layer) width R: Ratio of the second layer to the first layer 1...Lithium-ion secondary battery 10...Electrode body 11...Battery case 12...lid body 13...Negative electrode current collector 14...Negative external terminal 15...Positive electrode current collector 16...Positive external terminal 17...Nonaqueous electrolyte 100...Negative electrode plate 101...Negative electrode substrate 102...Negative electrode composite material layer 103...Negative electrode connection part 110...Positive electrode plate 111...Positive electrode substrate 112...Positive electrode mixture layer 113...Positive electrode connection part 120...Separator 120a...Outer separator 120b...Inner separator

Claims

1. A non-aqueous electrolyte secondary battery having an electrode assembly in which a plurality of laminates each having a positive electrode plate, a negative electrode plate, and a separator are stacked, When the separator arranged on the inner side in the stacking direction close to the center of the electrode assembly is defined as an "inner separator," and the portion of the electrode assembly arranged on the outer side of the inner separator in the stacking direction is defined as an "outer separator," The thickness Th of the inner separator of the electrode assembly ＩＮ the thickness Th of the outer separator of the electrode assembly ＯＵＴ By making it thicker, the unit area [cm 2 ] of the inner separator of the electrode assembly ２ ]. A non-aqueous electrolyte secondary battery characterized by having an increased heat capacity [J / K] per unit area.

2. The thickness Th of the inner separator of the electrode assembly ＩＮ and the thickness Th of the outer separator of the electrode body ＯＵＴ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the difference in heat capacity is set to be a difference that alleviates the temperature difference between the inner separator and the outer separator of the electrode body in an overcharged state.

3. The number of layers of the electrode body is divided into outer / inner / outer layers in a ratio of 1 / 2 / 1, The thickness of the inner separator of the electrode body is Th ＩＮ and the thickness of the outer separator of the electrode body is Th ＯＵＴ The total number of layers is LN. Thickness Th IN = Thickness Th OUT × (1 + coefficient α × total number of stacked layers LN)...Equation 1 In this case, a coefficient α for reducing the internal and external temperature difference ΔTmp is calculated according to the configuration of the applied battery, Thickness Th according to total number of layers LN ＩＮ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein:

4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the inner separator of the electrode body and the outer separator of the electrode body are both configured such that a first layer made of a first resin having a relatively low melting point is sandwiched between a pair of second layers made of a second resin having a relatively high melting point, and the ratio of the second layer to the first layer is higher in the separator on the inner side of the electrode body.

5. 5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the first resin is polyethylene (PE) and the second resin is polypropylene (PP).

6. 6. The nonaqueous electrolyte secondary battery according to claim 5, wherein the ratio of the second layer to the first layer is set so that the shutdown timing of the inner separator of the electrode body and the shutdown timing of the outer separator of the electrode body become close to each other depending on the temperature difference between the inner separator of the electrode body and the outer separator of the electrode body.

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