Nonaqueous electrolyte secondary battery and method for manufacturing the same

Abstract

To provide a nonaqueous electrolyte secondary battery capable of suppressing decrease in peel strength of a negative electrode and appropriately forming a coating film derived from lithium bis oxalate borate.SOLUTION: Sodium concentration in a negative electrode constituting an electrode 20 is higher than 532 ppm and lower than 71100 ppm. A battery cell 10 includes lithium bisoxalate borate or a LiBOB equivalent that is a material formed by reaction of the lithium bisoxalato borate. Concentration of the LiBOB equivalent in nonaqueous electrolytic solution 14 is 0.35 wt% or more and 0.56 wt% or less in terms of concentration of the lithium bisoxalato borate.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a nonaqueous electrolyte secondary battery and a method for manufacturing a nonaqueous electrolyte secondary battery. [Background technology]

[0002] For example, the following Patent Document 1 describes a manufacturing process for a nonaqueous electrolyte secondary battery. This manufacturing process includes a step of preparing a positive electrode and a negative electrode of the nonaqueous electrolyte secondary battery and then removing sodium contained therein. The manufacturing process also includes a step of injecting a nonaqueous electrolyte to which lithium bis(oxalato)borate has been added into a battery case.

[0003] The lithium bis(oxalato)borate forms a coating on the negative electrode, which serves to protect the surface of the negative electrode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-11969 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, if sodium is present in the negative electrode, the formation of the coating is likely to become uneven due to a reaction between lithium bis(oxalato)borate and sodium. [Means for solving the problem]

[0006] Means for solving the above problems and their effects will be described below. 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains a sodium concentration greater than 532 ppm and less than 71,100 ppm, and the battery contains a LiBOB equivalent, the LiBOB equivalent being lithium bis(oxalato)borate present in the non-aqueous electrolyte or a substance produced by the reaction of the lithium bis(oxalato)borate with another substance, and the LiBOB equivalent, when converted into the mass of the lithium bis(oxalato)borate, has a virtual concentration in the non-aqueous electrolyte of 0.35 wt% or more and 0.56 wt% or less.

[0007] If the amount of sodium relative to lithium bis(oxalato)borate is too large, the formation of the coating derived from lithium bis(oxalato)borate on the negative electrode is likely to be highly uneven. On the other hand, if the amount of sodium in the negative electrode is too small, the peel strength of the negative electrode decreases. Also, if the amount of lithium bis(oxalato)borate is too large, the resistance of the negative electrode becomes too large.

[0008] In this respect, according to the above-mentioned configuration, the amount of sodium can be set to an amount that maintains high peel strength, and the relative amount of the amount of sodium and the amount of LiBOB equivalent can be set to an appropriate amount, so that it is possible to appropriately form a coating film derived from lithium bis(oxalato)borate while suppressing the decrease in the peel strength of the negative electrode.

[0009] 2. The nonaqueous electrolyte secondary battery according to the above item 1, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer, and the density of the negative electrode active material layer is 1.14 grams per cubic centimeter or more.

[0010] When the density of the negative electrode is low, the reaction between lithium bis(oxalato)borate and sodium tends to increase the unevenness of the coating film derived from lithium bis(oxalato)borate. In contrast, in the above-mentioned configuration, the density of the negative electrode is ensured, thereby suppressing the unevenness of the coating film derived from lithium bis(oxalato)borate.

[0011] 3. The nonaqueous electrolyte secondary battery according to the above item 1 or 2, wherein the viscosity of the nonaqueous electrolyte is 3.9 [cP] or less. When the viscosity of the non-aqueous electrolyte is high, the reaction between lithium bis(oxalato)borate and sodium tends to increase the unevenness of the coating film derived from lithium bis(oxalato)borate. In contrast, in the above-mentioned configuration, the viscosity of the non-aqueous electrolyte is limited to the small side, thereby suppressing the unevenness of the coating film derived from lithium bis(oxalato)borate.

[0012] 4. The nonaqueous electrolyte secondary battery according to any one of the above 1 to 3, wherein the negative electrode has a sodium concentration of 700 ppm or more. According to the above-mentioned configuration, the peel strength of the negative electrode can be sufficiently increased.

[0013] 5. The nonaqueous electrolyte secondary battery according to any one of 1 to 4 above, wherein the positive electrode and the negative electrode are wound with a separator sandwiched therebetween to form a wound electrode body, and the resistance value of the negative electrode increases as it moves from the end portion to the center portion in the direction parallel to the winding axis in the wound electrode body.

[0014] In the case of a configuration in which the resistance value of the negative electrode is maximum on both sides in the direction parallel to the winding axis, the unevenness of the coating derived from lithium bis(oxalato)borate tends to increase. In contrast, in the above configuration, the resistance value of the negative electrode increases as it moves from the end to the center in the direction parallel to the winding axis. Therefore, compared to a configuration in which the resistance value of the negative electrode is maximum on both sides, the unevenness of the coating derived from lithium bis(oxalato)borate can be suppressed.

[0015] 6. A method for manufacturing a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, the method comprising: a negative electrode obtaining step; a housing step; and an injection step. The negative electrode obtaining step is a step of obtaining the negative electrode having a sodium concentration greater than 532 ppm and less than 71,100 ppm. The housing step is a step of housing an electrode body comprising the positive electrode and the negative electrode in a battery case. The injection step is a step of injecting the nonaqueous electrolyte into the battery case. The nonaqueous electrolyte injected in the injection step contains a LiBOB equivalent, the LiBOB equivalent being lithium bis(oxalato)borate or a substance produced by reacting the lithium bis(oxalato)borate with another substance. The amount of the LiBOB equivalent in the nonaqueous electrolyte injected in the injection step, when converted into the mass of the lithium bis(oxalato)borate, is 0.35 wt% or more and 0.56 wt% or less in the nonaqueous electrolyte.

[0016] In the injection process, the non-aqueous electrolyte flows from the end to the center of the electrode body. In this case, if the relative amount of sodium in the negative electrode relative to the amount of lithium bis(oxalato)borate is excessively large, the amount of lithium bis(oxalato)borate reacting with sodium at the end of the electrode body increases. Therefore, the coating derived from lithium bis(oxalato)borate tends to be localized at the end.

[0017] On the other hand, if the amount of sodium is too small, the peel strength of the negative electrode decreases. Also, if the amount of lithium bis(oxalato)borate is too large, the amount of the coating film derived from lithium bis(oxalato)borate becomes too large. Therefore, the resistance value of the negative electrode increases.

[0018] Therefore, in the above method, the amount of lithium bis(oxalato)borate and the amount of sodium are adjusted to appropriate amounts, thereby achieving a compromise between maintaining a high peel strength of the negative electrode and appropriately forming a coating film derived from lithium bis(oxalato)borate.

[0019] 7. The method for producing a nonaqueous electrolyte secondary battery according to claim 6, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer, and the density of the negative electrode active material layer is 1.14 grams per cubic centimeter or more.

[0020] If the density of the negative electrode active material is too small, the speed at which the non-aqueous electrolyte flows from the end to the center of the negative electrode in the injection process is reduced. Therefore, the amount of lithium bis(oxalato)borate reacting with sodium at the end increases. This increases the unevenness of the coating derived from lithium bis(oxalato)borate. Therefore, in the above method, the density of the negative electrode active material is adjusted so as not to be too small. This can suppress the unevenness of the coating derived from lithium bis(oxalato)borate.

[0021] 8. The method for producing a nonaqueous electrolyte secondary battery according to the above item 6 or 7, wherein the viscosity of the nonaqueous electrolyte is 3.9 [cP] or less. If the viscosity of the non-aqueous electrolyte is excessively high, the speed at which the non-aqueous electrolyte flows from the end to the center of the negative electrode during the injection process is reduced. Therefore, the amount of lithium bis(oxalato)borate reacting with sodium at the end is increased. This increases the unevenness of the coating derived from lithium bis(oxalato)borate. Therefore, in the above method, the viscosity of the non-aqueous electrolyte is adjusted so as not to be excessively high. This can suppress the unevenness of the coating derived from lithium bis(oxalato)borate.

[0022] 9. The method for producing a nonaqueous electrolyte secondary battery according to any one of the above 6 to 8, wherein the sodium concentration contained in the negative electrode is 700 ppm or more. According to the above method, the peel strength of the negative electrode can be sufficiently increased. [Brief description of the drawings]

[0023] [Figure 1] 1 is a perspective view of a lithium-ion secondary battery according to an embodiment of the present invention; [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of an electrode body of the lithium ion secondary battery according to the embodiment. [Diagram 3]4 is a flowchart showing a manufacturing process of the lithium ion secondary battery according to the embodiment. [Figure 4] FIG. 11 is a diagram showing reasons for selecting parameters according to the embodiment. [Diagram 5] 6(a) to 6(d) are diagrams illustrating the reasons for selecting parameters according to the embodiment. [Figure 6] FIG. 4 is a diagram showing the resistance distribution of the negative electrode sheet in the present embodiment and the comparative example. [Figure 7] FIG. 1 is a graph showing the relationship between the concentration of LiBOB added to a non-aqueous electrolyte and the degradation rate. [Figure 8] FIG. 13 is a graph showing the relationship between the sodium concentration in a negative electrode sheet and the peel strength of the negative electrode sheet. [Figure 9] FIG. 11 is a diagram showing reasons for selecting parameters according to the embodiment. [Figure 10] FIG. 11 is a diagram showing reasons for selecting parameters according to the embodiment. [Figure 11] FIG. 11 is a diagram showing reasons for selecting parameters according to the embodiment. [Figure 12] FIG. 11 is a diagram showing reasons for selecting parameters according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, an embodiment will be described with reference to the drawings. <Configuration of lithium-ion secondary battery 10> FIG. 1 is a perspective view showing the outline of the configuration of a lithium-ion secondary battery 10 of this embodiment.

[0025] As shown in FIG. 1, the lithium ion secondary battery 10 is configured as a cell battery. A plurality of lithium ion secondary batteries 10 are connected in series and mounted on a vehicle. The lithium ion secondary battery 10 includes a rectangular parallelepiped battery case 12 having an opening on the upper side. An electrode body 20 is housed inside the battery case 12. A nonaqueous electrolyte 14 is filled into the battery case 12 through an inlet hole in a lid portion 19. The battery case 12 is made of a metal such as an aluminum alloy. The lithium ion secondary battery 10 also includes a positive electrode external terminal 16 and a negative electrode external terminal 18 used for charging and discharging power. The shapes of the positive electrode external terminal 16 and the negative electrode external terminal 18 are not limited to those shown in FIG. 1.

[0026] <Electrode body 20> 2 is a schematic diagram showing the configuration of the wound electrode body 20. The electrode body 20 is formed by winding a large number of negative electrode sheets 30, positive electrode sheets 40, and separators 50 arranged between them in a flat shape. In the negative electrode sheet 30, a negative electrode mixture layer 34 is formed on a negative electrode current collector 32 serving as a base material. The negative electrode mixture layer 34 is not formed on one end side of the negative electrode sheet 30 in a direction W perpendicular to the winding direction L. The region where the negative electrode mixture layer 34 is not formed is a negative electrode connection part 36 where the negative electrode current collector 32 is exposed.

[0027] In the positive electrode sheet 40, a positive electrode mixture layer 44 is formed on a positive electrode current collector 42 serving as a base material. As shown in Fig. 2, a positive electrode connection part 46 is provided on the other end side in the direction W (opposite the negative electrode connection part 36) of the positive electrode current collector 42. The positive electrode connection part 46 is a region of the positive electrode sheet 40 where the positive electrode mixture layer 44 is not formed. In other words, the positive electrode connection part 46 is a region where the metal of the positive electrode current collector 42 is exposed.

[0028] In this embodiment, an insulating protective layer 48 is provided adjacent to an end of the positive electrode mixture layer 44 and facing the negative electrode mixture layer 34. The insulating protective layer 48 is provided so as to cover the exposed positive electrode current collector 42.

[0029] <Manufacturing process> FIG. 3 shows part of the manufacturing process for the lithium ion secondary battery 10. In the series of steps shown in FIG. 3, first, a positive electrode sheet 40 is formed (S10). In this step, a positive electrode current collector 42 is formed using, for example, a metal foil made of aluminum or an alloy mainly composed of aluminum. Next, a positive electrode composite paste is applied to the positive electrode current collector 42. The positive electrode composite paste may include a positive electrode active material, a positive electrode solvent, a positive electrode conductive material, and a positive electrode binder. For the positive electrode active material, a lithium-containing composite metal oxide capable of absorbing and releasing lithium ions, which are charge carriers in the lithium-ion secondary battery 10, is used. Then, the positive electrode composite paste is dried to form a positive electrode composite layer 44 on the positive electrode current collector 42. The positive electrode composite layer 44 is formed on each of two opposing surfaces of the positive electrode current collector 42. The thickness of the positive electrode composite layer 44 may be adjusted by applying a force to the positive electrode composite layer 44 formed on both surfaces of the positive electrode current collector 42.

[0030] Next, the negative electrode sheet 30 is formed (S12). In this step, first, the negative electrode current collector 32 is formed using, for example, a metal foil made of copper or an alloy mainly composed of copper. Next, the negative electrode composite paste is applied to the negative electrode current collector 32. The negative electrode composite paste may include a negative electrode active material, a negative electrode solvent, a negative electrode thickener, and a negative electrode binder. The negative electrode active material is a material capable of absorbing and releasing lithium ions. For example, carbon materials such as graphite, non-graphitizable carbon, easily graphitizable carbon, and carbon nanotubes are used as the negative electrode active material. For example, the negative electrode solvent is water. For example, CMC (carboxymethyl cellulose) can be used as a thickener containing a sodium salt as the negative electrode thickener. For example, the negative electrode binder can be the same as the positive electrode binder. For example, SBR (styrene butadiene copolymer) can be used as a binder containing a sodium salt as the negative electrode binder. Next, the negative electrode mixture paste is dried in a drying device to form a negative electrode mixture layer 34 on the negative electrode current collector 32. The negative electrode mixture layers 34 are formed one on each of the two opposing surfaces of the negative electrode current collector 32. The thickness of the negative electrode mixture layers 34 may be adjusted by pressing the negative electrode mixture layers 34 formed on both surfaces of the negative electrode current collector 32.

[0031] In the step S12, the density of negative electrode mixture layer 34 is set to 1.14 grams per cubic centimeter or more. Next, sodium is removed from the negative electrode sheet 30 (S14). This is achieved by washing the negative electrode sheet 30 formed in the step S12 with a nonaqueous electrolyte. The nonaqueous electrolyte may be a liquid in which a supporting salt is dissolved in an organic solvent. The supporting salt is, for example, a lithium salt. The step S14 may include a step of immersing the negative electrode sheet 30 in the nonaqueous electrolyte for a predetermined time, a step of washing the surface of the negative electrode sheet 30 with, for example, an organic solvent, and a step of drying the negative electrode sheet 30. After the above three steps are performed in order, the three steps may be repeated again. Here, when the negative electrode thickener contains CMC as a thickener, the sodium contained in the thickener tends to be particularly large. In that case, the sodium is removed by the above step through the following reaction.

[0032] CMC-Na+LiOH → CMC-Li+NaOH That is, sodium is removed by reacting CMC-Na with LiOH to replace the sodium in CMC-Na with lithium.

[0033] In step S14, the sodium concentration of the negative electrode sheet 30 is set to be greater than 532 ppm and less than 71100 ppm, more preferably, to be 700 ppm or more.

[0034] Next, the negative electrode sheet 30 and the positive electrode sheet 40 are stacked with the separator 50 interposed therebetween and wound to produce the electrode body 20 (S16). In detail, the negative electrode sheet 30 and the positive electrode sheet 40 are stacked with the separator 50 interposed therebetween, and are wound in the direction L shown in FIG. 2 while being supported around the winding axis.

[0035] Next, the electrode body 20 is housed in the battery case 12 (S18). In the step of S18, the positive electrode connection part 46 is electrically connected to the positive electrode external terminal 16. Also, the negative electrode connection part 36 is electrically connected to the negative electrode external terminal 18. Then, the battery case 12 and the lid part 19 are sealed by laser welding or the like, and the opening of the battery case 12 is closed by the lid part 19. At this stage, the nonaqueous electrolyte 14 has not yet been injected, and the injection hole of the lid part 19 is open.

[0036] Next, the non-aqueous electrolyte 14 is injected into the battery case 12 (S20). That is, the injection step is a step of injecting the non-aqueous electrolyte 14 into the battery case 12 with the electrode assembly 20 housed therein.

[0037] The nonaqueous electrolyte 14 is a composition in which a supporting salt is contained in a nonaqueous solvent. As the nonaqueous solvent, one or more materials selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, etc. can be used. In addition, as the supporting salt, one or more lithium compounds (lithium salts) can be used. Here, the lithium compounds are LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, etc.

[0038] In this embodiment, ethylene carbonate is used as the non-aqueous solvent. Lithium bis(oxalato)borate is added as a lithium salt as an additive to the non-aqueous electrolyte 14. Hereinafter, lithium bis(oxalato)borate is referred to as LiBOB.

[0039] The concentration of LiBOB in the nonaqueous electrolyte 14 injected in step S20 is set to be 0.35 wt % or more and 0.56 wt % or less. Furthermore, the viscosity of the non-aqueous electrolyte 14 injected in step S20 is set to 3.9 [cP] or less. Here, the viscosity is measured by an Ubbelohde viscometer.

[0040] Next, charging and discharging of the lithium ion secondary battery 10 is repeated a predetermined number of times (S22). The purpose of the step S22 is to form a LiBOB-derived SEI (Solid Electrolyte Interphase) coating.

[0041] <Actions and Effects of the Present Embodiment> Fig. 4 shows the resistance value of the negative electrode sheet 30 when the sodium concentration in the negative electrode sheet 30 produced through the process of S14 and the LiBOB concentration in the nonaqueous electrolyte 14 injected in the process of S20 are variously changed. In detail, the right side of Fig. 4 shows a curve showing the distribution of the resistance value of the negative electrode sheet 30 in the direction W of the winding axis shown in Fig. 2. Moreover, each curve on the right side of Fig. 4 corresponds to each of the regions A1 to A4 divided by the sodium concentration and LiBOB concentration on the left side of Fig. 4. Region A3 is the region adopted in this embodiment.

[0042] 4, when region A1 is used, the resistance value of the negative electrode sheet 30 has two maximum values ​​in the direction parallel to the winding axis. This is because the concentration of LiBOB is excessively small.

[0043] That is, the electrode body 20 is formed by winding a laminate of the negative electrode sheet 30, the positive electrode sheet 40, and the separator 50. Therefore, when the nonaqueous electrolyte solution 14 is injected in the step S20, the nonaqueous electrolyte solution 14 flows into the negative electrode sheet 30 from both ends in the direction W parallel to the winding axis.

[0044] Here, as shown in FIG. 5(a), if the concentration of LiBOB is excessively low, most of the LiBOB reacts with sodium at both ends. Therefore, LiBOB hardly penetrates into the center of the negative electrode sheet 30 in the direction W parallel to the winding axis. Therefore, the coating derived from LiBOB in the negative electrode sheet 30 becomes significantly uneven. This leads to a decrease in the life of the negative electrode sheet 30.

[0045] 4, even when region A2 is used, the resistance value of the negative electrode sheet 30 has two maximum values ​​in the direction parallel to the winding axis. This is because the concentration of sodium in the negative electrode sheet 30 is excessively high.

[0046] That is, as shown in FIG. 5(b), when the sodium concentration in the negative electrode sheet 30 is excessively high, most of the LiBOB that flows in from both ends in the direction W parallel to the winding axis reacts with sodium at both ends. Therefore, LiBOB hardly penetrates into the center of the negative electrode sheet 30 in the direction W parallel to the winding axis. Therefore, the coating derived from LiBOB in the negative electrode sheet 30 becomes significantly uneven. This leads to a decrease in the life of the negative electrode sheet 30.

[0047] 4, when region A4 is used, the fluctuation in the resistance value of negative electrode sheet 30 is small. However, in this case, the peel strength of negative electrode mixture layer 34 in negative electrode sheet 30 decreases.

[0048] That is, for example, when removing sodium from CMC-Na as described above, the sodium in CMC-Na is replaced with lithium to produce CMC-Li, where CMC-Li has a smaller molecular weight than CMC-Na.

[0049] Therefore, as shown on the right side of Fig. 5(c), the molecular chains are shorter and less likely to be entangled than in the case shown on the left side of Fig. 5(c), resulting in a decrease in the adhesive strength between the negative electrode active materials. This leads to a decrease in the peel strength.

[0050] 4, when region A5 is used, the resistance value of the negative electrode sheet 30 varies little, but is large. This is because the concentration of LiBOB in the nonaqueous electrolyte 14 is excessively high.

[0051] That is, as shown in Fig. 5(d), when the concentration of LiBOB is excessively high, the coating produced by the reaction of LiBOB with sodium is thickly covered with a coating derived from LiBOB that did not react with sodium. Since the resistance value of this coating is high, the resistance value of the negative electrode sheet 30 becomes excessively high.

[0052] For these reasons, the present embodiment employs the area A3. In Fig. 6, the measurement data of the distribution of the resistance value of the negative electrode sheet 30 in this embodiment is illustrated by a solid line. In Fig. 6, the measurement data of the distribution of the resistance value in the region A2 is illustrated by a dashed line. As shown in Fig. 6, in this embodiment, the resistance value of the negative electrode sheet 30 is less than 28.07 ohms in the entire region.

[0053] FIG. 7 shows the relationship between the LiBOB concentration in the non-aqueous electrolyte 14 and the capacity degradation rate of the lithium-ion secondary battery 10. The capacity degradation rate is quantified by the absolute value of the slope of the normalized capacity of the lithium-ion secondary battery 10 versus the number of days. Here, the normalized capacity is the ratio of the capacity during the storage test to the capacity before storage, which is the initial capacity in the storage test. The number of days is the number of days elapsed since the start of the storage test. The larger the absolute value of the slope of the normalized capacity, the greater the life reduction. As shown in FIG. 7, the higher the LiBOB concentration in the non-aqueous electrolyte 14, the lower the degradation rate. However, there is a tradeoff in that the resistance value of the negative electrode sheet 30 increases as the LiBOB concentration in the non-aqueous electrolyte 14 increases.

[0054] Fig. 8 shows the relationship between the sodium concentration in the negative electrode sheet 30 and the peel strength of the negative electrode sheet 30. As shown in Fig. 8, the lower the sodium concentration, the lower the peel strength. However, there is a tradeoff in that the higher the sodium concentration in the negative electrode sheet 30, the greater the unevenness of the coating derived from LiBOB in the negative electrode sheet 30.

[0055] 9 shows the evaluation results of the characteristics when the sodium concentration in the negative electrode sheet 30 and the LiBOB concentration in the nonaqueous electrolyte 14 are set to one point in each of the regions A1 to A5. In the regions A1 and A2, the coating derived from LiBOB has large unevenness, resulting in poor life characteristics. In the region A4, the peel strength of the negative electrode sheet 30 is low. In the region A5, the resistance value of the negative electrode sheet 30 is excessively high, resulting in poor input / output characteristics.

[0056] The basis for setting the parameters according to this embodiment will be described below with reference to FIGS. Fig. 10 shows the relationship between the concentration of LiBOB added to the non-aqueous electrolyte 14 and the capacity degradation characteristics. The value "1" on the vertical axis shown in Fig. 10 is based on the case where the concentration of LiBOB added to the non-aqueous electrolyte 14 is 0.5 wt%.

[0057] As shown in Fig. 10, when the concentration of LiBOB added to the non-aqueous electrolyte 14 is 0.35 wt% or more, the capacity degradation rate is almost constant. In contrast, when the concentration of LiBOB added to the non-aqueous electrolyte 14 is less than 0.35 wt%, the capacity degradation characteristics deteriorate. Therefore, in this embodiment, the concentration of LiBOB added to the non-aqueous electrolyte 14 is set to 0.35 wt% or more.

[0058] Fig. 11 shows the relationship between the concentration of LiBOB added to the non-aqueous electrolyte 14 and the input / output ratio, which is the ratio of the input and output of the lithium-ion secondary battery 10. Specifically, the relationship shown in Fig. 11 is the relationship when the lithium-ion secondary battery 10 is set to "-10°C." Also, "1" on the vertical axis of Fig. 11 is based on the case where the concentration of LiBOB added to the non-aqueous electrolyte 14 is 0.5 wt%.

[0059] 11, when the concentration of LiBOB added to the nonaqueous electrolyte 14 is 0.56 wt% or less, the input / output ratio of the negative electrode sheet 30 is the same as when the concentration is 0.5 wt%. On the other hand, when the concentration of LiBOB added to the nonaqueous electrolyte 14 exceeds 0.56 wt%, the input / output ratio of the negative electrode sheet 30 deteriorates.

[0060] For these reasons, in this embodiment, the concentration of LiBOB added to the nonaqueous electrolyte solution 14 is set to 0.35 wt % or more and 0.56 wt % or less. FIG. 12 shows the relationship between the sodium concentration in the negative electrode sheet 30 and the peel strength, together with the lower limit of the allowable range of the peel strength. As shown in FIG. 12, the lower limit of the peel strength is set to 1.5 (N / m). In this case, the sodium concentration in the negative electrode sheet 30 is required to be greater than 532 ppm. In order to ensure sufficient peel strength of the negative electrode sheet 30, it is desirable that the sodium concentration in the negative electrode sheet 30 is 700 ppm or more. Incidentally, the upper limit of the sodium concentration in the negative electrode sheet 30 was determined according to the upper limit of the resistance value of the negative electrode sheet 30 shown in FIG. 6 and the amount of the LiBOB-derived coating formed in the center of the negative electrode sheet 30.

[0061] In this manner, in the present embodiment, by adjusting the concentration of LiBOB added to the nonaqueous electrolyte 14 and the sodium concentration of the negative electrode sheet 30, the input / output characteristics, life span, and peel strength of the negative electrode sheet 30 of the lithium ion secondary battery 10 can be made to meet high standards.

[0062] Furthermore, in this embodiment, the density of the negative electrode mixture layer 34 is set to 1.14 grams per cubic centimeter or more. This is because, when the density of the negative electrode mixture layer 34 is low, the speed at which the nonaqueous electrolyte solution 14 permeates the negative electrode sheet 30 in the step S20 becomes slow. When the speed at which the nonaqueous electrolyte solution 14 permeates the negative electrode sheet 30 becomes slow, the amount of LiBOB that reacts with sodium at both ends in the direction W parallel to the winding axis increases, and the amount of LiBOB that reaches the center in the same direction W tends to decrease.

[0063] In this embodiment, the viscosity of the nonaqueous electrolyte 14 is set to 3.9 [cP] or less. This is because if the viscosity is high, the speed at which the nonaqueous electrolyte 14 permeates the negative electrode sheet 30 in the step S20 decreases. If the speed at which the nonaqueous electrolyte 14 permeates the negative electrode sheet 30 decreases, the amount of LiBOB that reacts with sodium at both ends in the direction W parallel to the winding axis increases, and the amount of LiBOB that reaches the center in the same direction W tends to decrease.

[0064] <Correspondence> The correspondence between the matters in the above embodiment and the matters described in the above "Means for solving the problem" column is as follows. In the following, the correspondence is shown for each number of the means for solving the problem described in the "Means for solving the problem" column. [1,3-5] The LiBOB equivalent corresponds to the substance injected in the step S20 or the coating or the like generated by reacting the substance with sodium or the like in the step S22. [2,7] The negative electrode active material layer corresponds to the negative electrode mixture layer 34. [6,8,9] The negative electrode obtaining step corresponds to the steps S12 and S14. The accommodation step corresponds to the step S18. The injection step corresponds to the step S20.

[0065] <Other embodiments> This embodiment can be modified as follows: This embodiment and the following modifications can be combined with each other to the extent that no technical contradiction occurs.

[0066] "About the negative electrode acquisition process" The negative electrode obtaining step does not necessarily include step S14. The sodium concentration in the negative electrode may be set to an amount that satisfies the above condition without including the step of removing Na.

[0067] "About the injection process" The LiBOB equivalent added to the non-aqueous electrolyte in the injection step is not limited to lithium bis(oxalato)borate itself. For example, lithium bis(oxalato)borate may be reacted with some other substance as long as it forms a coating equivalent to lithium bis(oxalato)borate in step S22.

[0068] "About the electrode body" The electrode body is not limited to a flat wound body, and may be, for example, a cylindrically wound body.

[0069] The electrode body 20 may not be a wound body, but may be a laminate in which a positive electrode sheet 40 and a negative electrode sheet 30 are laminated with a separator 50 interposed therebetween, and housed in a battery case 12. Even in this case, for example, if the concentration of sodium is excessively high, lithium bis(oxalato)borate reacts with sodium in large amounts at the ends of the electrode body during the injection process, which tends to result in large unevenness. Therefore, it is effective to satisfy the above conditions.

[0070] "About non-aqueous electrolyte secondary batteries" The non-aqueous electrolyte secondary battery is not limited to a thin plate battery. For example, it may be a cylindrical battery. In addition, it may be a battery for marine use, aircraft use, or even stationary use, not limited to vehicle use. [Explanation of symbols]

[0071] 10...Lithium-ion secondary battery 12…Battery case 14...Nonaqueous electrolyte 16…Positive external terminal 18...Negative external terminal 19...Lid part 20...Electrode body 30...Negative electrode sheet 32...Negative electrode current collector 34...Negative electrode composite material layer 36…Negative electrode connection part 40…Positive electrode sheet 42...Positive electrode current collector 44…Positive electrode mixture layer 46…Positive electrode connection 48…Insulating protective layer 50…Separator

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The sodium concentration contained in the negative electrode is greater than 532 ppm and less than 71100 ppm, Contains LiBOB equivalent, The LiBOB equivalent is lithium bis(oxalato)borate present in the non-aqueous electrolyte solution or a substance produced by reacting the lithium bis(oxalato)borate with another substance; The LiBOB equivalent has a virtual concentration in the non-aqueous electrolyte of 0.35 wt % or more and 0.56 wt % or less when converted into the mass of the lithium bis(oxalato)borate; The negative electrode includes a negative electrode current collector and a negative electrode active material layer, The density of the negative electrode active material layer is 1.14 grams per cubic centimeter or more, The viscosity of the nonaqueous electrolyte in the nonaqueous electrolyte secondary battery is 3.9 cP or less.

2. 2. The nonaqueous electrolyte secondary battery in accordance with claim 1, wherein the concentration of sodium contained in said negative electrode is 700 ppm or more.

3. The positive electrode and the negative electrode are wound with a separator sandwiched therebetween to form a wound electrode body, 2. The nonaqueous electrolyte secondary battery in accordance with claim 1, wherein the resistance value of the negative electrode increases from the end portion to the center portion in the direction parallel to the winding axis of the wound electrode body.

4. A method for manufacturing a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, comprising the steps of: The method includes a negative electrode obtaining step, a housing step, and a filling step, the negative electrode obtaining step is a step of obtaining the negative electrode having a sodium concentration greater than 532 ppm and less than 71,100 ppm; the housing step is a step of housing an electrode assembly including the positive electrode and the negative electrode in a battery case, the injection step is a step of injecting the nonaqueous electrolyte into the battery case, The non-aqueous electrolyte solution injected in the injection step contains a LiBOB equivalent, The LiBOB equivalent is lithium bis(oxalato)borate or a substance produced by reacting the lithium bis(oxalato)borate with another substance; The amount of the LiBOB equivalent in the non-aqueous electrolyte injected in the injection step is such that the concentration in the non-aqueous electrolyte is 0.35 wt % or more and 0.56 wt % or less when converted into the mass of the lithium bis(oxalato)borate; The negative electrode includes a negative electrode current collector and a negative electrode active material layer, The density of the negative electrode active material layer is 1.14 grams per cubic centimeter or more, The method for producing a nonaqueous electrolyte secondary battery includes controlling the viscosity of the nonaqueous electrolyte to 3.9 cP or less.

5. 5. The method for producing a nonaqueous electrolyte secondary battery in accordance with claim 4, wherein the concentration of sodium contained in the negative electrode is 700 ppm or more.

Images