Method for manufacturing a non-aqueous secondary battery

Abstract

To control the resistance distribution in the anode of a non-aqueous secondary battery so as to suppress the deposition of metal.SOLUTION: Provided is a method for manufacturing a non-aqueous secondary battery that comprises a cathode sheet 15 that includes a cathode mixture layer, an anode sheet 16 that includes an anode mixture layer, and a non-aqueous solution that contains a film formation material containing a lithium element. The method includes: a cathode resistance measurement step for measuring a cathode resistance difference which is the difference between the maximum and minimum resistance values of the cathode sheet 15; an adjustment value identification step for identifying a sodium content corresponding to the same anode resistance difference as the measured cathode resistance value, as an adjustment value, using information that indicates a relationship between an anode resistance difference which is the difference between the maximum and minimum resistance values of the anode sheet 16 and the sodium content of the anode mixture layer; and a design step for designing the anode mixture layer so that the sodium content of the anode mixture layer equals the adjustment value identified in the adjustment value identification step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a non-aqueous secondary battery.

Background Art

[0002] Non-aqueous secondary batteries such as lithium-ion secondary batteries have a so-called SEI film (Solid Electrolyte Interface) on the surface of the negative electrode composite material. This film exists between the negative electrode composite material and the non-aqueous electrolyte, has a function of smoothing the occlusion and release of lithium ions to the negative electrode and suppressing further decomposition of the non-aqueous electrolyte, and is essential for maintaining good battery characteristics. Further, this film is formed by decomposition products of the non-aqueous electrolyte and additives, and the decomposition products take in lithium ions during the generation process. Therefore, when the film becomes thick, it may cause a decrease in battery capacity.

[0003] In response to this problem, it has already been proposed to previously add a film-forming material containing lithium to the non-aqueous electrolyte. The film-forming material is a compound having a lithium element such as a lithium salt, for example, lithium bisoxalate borate (LiBOB, LiB(C 2 O 4 ) 2 )). According to this method, it is possible to promote the formation of the film while sufficiently securing lithium ions that move between the positive electrode and the negative electrode and directly contribute to the battery reaction.

[0004] On the other hand, bisoxalate borate ion (BOB, B(C 2 O 4 ) 2 -) also reacts with sodium ions contained as impurities in the negative electrode composite layer and becomes part of the coating on the negative electrode sheet. The coating derived from sodium ions tends to be formed more in the central part of the negative electrode sheet of the electrode body obtained by winding the positive electrode sheet, the negative electrode sheet, and the separator. The reason is that sodium ions tend to gather in the central part of the negative electrode sheet, and sodium ions and bisoxalate borate ions react actively in the central part.

[0005] The central part of the negative electrode sheet with an excessively large coating amount has a higher resistance than the surrounding area, and lithium is likely to precipitate. Since the sodium element contained in the negative electrode composite layer may cause lithium precipitation in this way, it has been proposed to reduce the content of the sodium component in the negative electrode by washing the negative electrode with a non-aqueous electrolyte or the like (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] The negative electrode with a reduced sodium content by washing or the like has a smaller resistance difference in the negative electrode sheet than the negative electrode with an unchanged sodium content, so the precipitation of lithium on the negative electrode sheet can be suppressed. However, since the resistance difference in the negative electrode sheet also affects the deterioration of the positive electrode, it is desirable to consider these factors comprehensively, and there is room for further improvement.

Means for Solving the Problems

[0008] A method for manufacturing a non-aqueous secondary battery for solving the above problems is a method for manufacturing a non-aqueous secondary battery having a positive electrode sheet provided with a positive electrode composite material layer, a negative electrode sheet having a negative electrode composite material layer, and a non-aqueous electrolyte containing a film-forming material containing a lithium element, the method including: a positive electrode resistance measurement step of measuring a positive electrode resistance difference that is a difference between a minimum value and a maximum value of the resistance of the positive electrode sheet; an adjustment value specifying step of specifying, using information indicating a relationship between a negative electrode resistance difference that is a difference between a minimum value and a maximum value of the resistance of the negative electrode sheet and a sodium content of the negative electrode composite material layer, a sodium content corresponding to the same negative electrode resistance difference as the measured positive electrode resistance difference as an adjustment value; and a design step of designing the negative electrode composite material layer such that the sodium content of the negative electrode composite material layer becomes the adjustment value specified in the adjustment value specifying step.

[0009] The resistance at the central portion of both the positive electrode sheet and the negative electrode sheet tends to be high. When sodium ions, which are one of the factors causing variations in the resistance of the negative electrode, are removed, there is no variation in the amount of the film generated by the reaction between the sodium ions and the film-forming material, and the resistance of the negative electrode sheet is made uniform. When the resistance of the negative electrode sheet is made uniform, the amount of lithium ions released from the negative electrode sheet during discharge is also made uniform. However, the occlusion of lithium ions cannot catch up with the high-resistance portion at the center of the positive electrode sheet, and the central portion of the positive electrode sheet may deteriorate due to an oxidation reaction or the like. According to the above method, the sodium content of the negative electrode composite material layer is adjusted so that the resistance difference of the positive electrode and the resistance difference of the negative electrode become equal. Since the high-resistance portions of the positive electrode sheet and the negative electrode sheet are substantially at the same position, the amount of lithium ions released from the high-resistance portion of the negative electrode sheet and the amount of lithium ions occluded in the high-resistance portion of the positive electrode sheet during discharge can be made to be in a balanced state with less excess or deficiency. Therefore, the deterioration reaction in the positive electrode sheet can also be suppressed.

[0010] Regarding the above method for manufacturing a non-aqueous secondary battery, it is preferable that the positive electrode composite material layer uses a lithium composite metal oxide as a positive electrode active material, and in the design step, the sodium element contained in the negative electrode composite material layer is replaced with a lithium element so that the sodium content becomes the adjustment value. According to the above method, it is possible to suppress the deterioration reaction in the positive electrode sheet without significantly changing the properties of the materials used in the negative electrode composite layer.

[0011] Regarding the method for manufacturing the non-aqueous secondary battery, in the design step, it is preferable to adjust the sodium content to the adjusted value by adding lithium carboxymethyl cellulose to the negative electrode composite paste for forming the negative electrode composite layer.

[0012] According to the above method, it is possible not to add sodium carboxymethyl cellulose to the negative electrode composite paste or to reduce the addition amount. Therefore, it is possible to suppress the precipitation of lithium in the negative electrode sheet while maintaining the viscosity required for the negative electrode composite paste in manufacturing.

[0013] Regarding the method for manufacturing the non-aqueous secondary battery, in the design step, it is preferable to adjust the sodium content to the adjusted value by adding a styrene-butadiene copolymer containing a lithium element to the negative electrode composite paste for forming the negative electrode composite layer.

[0014] According to the above method, it is possible not to add a styrene-butadiene copolymer containing a sodium element to the negative electrode composite paste or to reduce the addition amount. Therefore, it is possible to suppress the precipitation of lithium in the negative electrode sheet while maintaining good binding properties of the negative electrode composite layer in manufacturing.

Effects of the Invention

[0015] According to the present invention, by controlling the resistance difference in the negative electrode of the non-aqueous secondary battery, it is possible to suppress the precipitation of lithium on the negative electrode sheet and simultaneously suppress the deterioration reaction in the positive electrode sheet.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

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Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0017] Hereinafter, an embodiment of the present invention will be described. In this embodiment, a non-aqueous secondary battery will be described as a lithium-ion secondary battery. The lithium-ion secondary battery 10 is a battery that performs charge and discharge by moving lithium ions between a positive electrode and a negative electrode. Note that this lithium-ion secondary battery 10 is used, for example, as a driving power source for an electric vehicle (EV) or a hybrid vehicle (HV).

[0018] Referring to FIG. 1, the configuration of the lithium-ion secondary battery 10 will be described. The lithium-ion secondary battery 10 includes an electrode body 11 in a case (not shown). The electrode body 11 is a wound body formed by winding a plurality of sheets. The electrode body 11 is formed by laminating a positive electrode sheet 15 and a negative electrode sheet 16 with a separator 17 interposed therebetween and winding the laminate. The positive electrode sheet 15 has an elongated shape and includes a positive electrode current collector 18 and a positive electrode mixture layer 19 provided on both sides of the positive electrode current collector 18. The positive electrode mixture layer 19 is a layer formed by a process of coating and drying a positive electrode mixture paste. The negative electrode sheet 16 has an elongated shape and includes a sheet-like negative electrode current collector 20 and a negative electrode mixture layer 21 provided on both sides of the negative electrode current collector 20. The negative electrode mixture layer 21 is a layer formed by a process of coating and drying a negative electrode mixture paste. Before winding, the laminate is laminated in the order of the positive electrode sheet 15, the separator 17, the negative electrode sheet 16, and the separator 17 so that the longitudinal directions of the positive electrode sheet 15 and the negative electrode sheet 16 coincide. The laminate is wound so that the negative electrode sheet 16 is on the innermost side. The longitudinal direction of the positive electrode sheet 15 and the negative electrode sheet 16 is defined as the "longitudinal direction Y", and the direction orthogonal thereto is defined as the "width direction X".

[0019] The electrode body 11 is formed into a flat shape by winding the positive electrode sheet 15 and the negative electrode sheet 16 along the longitudinal direction Y and pressing the wound laminate from its peripheral surface. At one end of the positive electrode sheet 15 in the width direction X, an uncoated portion 15A is provided where the positive electrode current collector 18 is exposed without the positive electrode mixture layer 19 being formed. The length of the uncoated portion 15A in the width direction X is, for example, 12 mm or less.

[0020] At one end of the negative electrode sheet 16 in the width direction X, an uncoated portion 16A is provided where the negative electrode current collector 20 is exposed without the negative electrode mixture layer 21 being formed. The length of the uncoated portion 16A in the width direction X is, for example, 12 mm or less. Further, the negative electrode sheet 16 is provided with a non-opposing portion 25 which is a negative electrode mixture layer not opposing the positive electrode mixture layer 19 via a separator. The length of the non-opposing portion 25 in the width direction X is, for example, 3.5 mm.

[0021] Next, the positive electrode will be described. As the positive electrode current collector 18, a metal foil such as aluminum foil is used. The positive electrode mixture layer 19 contains a positive electrode active material, a conductive material, a binder, and the like. As the positive electrode active material, one or more of various materials known to be usable as the positive electrode active material of a lithium-ion secondary battery can be used. As a preferred example, lithium composite metal oxides such as layered systems and spinel systems (for example, LiNiO 2 、LiCoO 2 、LiFeO 2 、LiMn 2 O 4 、LiNi 0.5 Mn 1.5 O 4 ,LiCrMnO 4 、LiFePO 4 ) are mentioned. Examples of the conductive material include carbon blacks such as acetylene black and ketjen black, and other powdery carbon materials (such as graphite). Examples of the binder include polyvinylidene fluoride (PVDF), styrene-butadiene copolymer (SBR), polytetrafluoroethylene (PTFE), and the like.

[0022] The proportion of the positive electrode active material in the entire positive electrode mixture is preferably 60% by mass or more (typically 60% by mass or more and 99% by mass or less). Or it may be 70% by mass or more and 95% by mass or less. When using a conductive material, the proportion of the conductive material in the entire positive electrode mixture is preferably 2% by mass or more and 20% by mass or less, and may be 3% by mass or more and 10% by mass or less. When using a binder, the proportion of the binder in the entire positive electrode mixture is preferably 0.5% by mass or more and 10% by mass or less, and may be 1% by mass or more and 5% by mass or less.

[0023] Next, the material of the negative electrode will be described. The negative electrode current collector 20 is formed of a metal foil such as copper or nickel. The negative electrode mixture layer 21 contains a negative electrode active material, a conductive material, a binder, and the like. As the negative electrode active material, one or more of various materials known to be usable as a negative electrode active material of a lithium-ion secondary battery can be used. For example, carbon materials such as graphite, non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), and carbon nanotubes can be mentioned. Among them, graphite-based materials (particularly natural graphite) such as natural graphite and artificial graphite can be preferably used because of their excellent conductivity and high energy density. As the binder, the same ones as those for the positive electrode can be used. In addition, a thickening agent, a dispersant, and the like can be appropriately used. For example, carboxymethyl cellulose (CMC) or methyl cellulose (MC) can be used as the thickening agent.

[0024] The proportion of the negative electrode active material in the entire negative electrode mixture layer is preferably 50% by mass or more, and may be 90% by mass or more and 99% by mass or less. When using a binder, the proportion of the binder in the entire negative electrode mixture layer 21 is preferably 0.5% by mass or more and 10% by mass or less, and may be 0.5% by mass or more and 5% by mass or less. When using a thickening agent, the proportion of the thickening agent in the entire negative electrode mixture layer 21 is preferably 0.5% by mass or more and 10% by mass, and may be 0.5% by mass or more and 5% by mass or less.

[0025] The separator 17 has a porous layer formed of a resin. The porous layer is, for example, a single-layer structure composed of porous polyethylene, porous polyolefin, porous polyvinyl chloride, or the like, or a laminated structure composed of a plurality of materials. Further, a filler can be contained in the porous layer for the purpose of improving strength and the like. An adhesive layer made of an adhesive may be interposed between the separator 17 and the negative electrode sheet 16.

[0026] The non-aqueous electrolyte is a composition in which a supporting salt is contained in a non-aqueous solvent. Here, as the non-aqueous solvent, one or more materials such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be used. Also, as the supporting salt, one or more lithium compounds (lithium salts) selected from LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN(CF 3 SO 2 ) 2 , LiC(CF 3 SO 2 ) 3 , LiI, etc. can be used.

[0027] Also, the electrolyte contains a film-forming material that forms an SIE film on the negative electrode composite layer 21. As the film-forming material, for example, lithium bisoxalate borate (LiBOB) can be used. In forming a film with an appropriate thickness, the concentration of LiBOB in the electrolyte is, for example, 0.05 mol / kg or less.

[0028] For the lithium ion secondary battery 10, before shipment, a conditioning treatment for forming a film derived from LiBOB on the surface of the negative electrode composite layer 21 is performed. The conditioning treatment can be carried out by repeating the charging and discharging of the lithium ion secondary battery 10 at a predetermined rate.

[0029] With reference to FIGS. 2 and 3, the configurations of the positive electrode sheet 15 and the negative electrode sheet 16 and the resistance distribution of the positive electrode will be described. FIGS. 2 and 3 schematically show the positive electrode sheet 15 and the negative electrode sheet 16 when the laminate is cut at the 2-2 position along the width direction X in FIG. 1. Further, in FIGS. 2 and 3, for convenience of explanation, the separator 17 is omitted and the positive electrode sheet 15 and the negative electrode sheet 16 are shown in an unwound state.

[0030] The lithium ion secondary battery 10 is a positive electrode regulation in which the battery capacity is regulated by the positive electrode capacity. The length of the negative electrode composite layer 21 in the width direction X is longer than the length of the positive electrode composite layer 19 in the width direction X. As a result, all of the positive electrode composite layer 19 faces the negative electrode composite layer 21, while the negative electrode composite layer 21 is provided with an opposed portion 24 that faces the positive electrode composite layer 19 with the separator 17 interposed therebetween and an unopposed portion 25 that does not face the positive electrode composite layer 19 with the separator 17 interposed therebetween.

[0031] FIG. 2 shows the movement of lithium ions 50 during charging. During charging, the lithium ions 50 move from the positive electrode to the negative electrode. At this time, the lithium ions 50 released from the end of the positive electrode composite layer 19 move not only to the opposed portion 24 of the negative electrode composite layer 21 but also to the unopposed portion 25.

[0032] FIG. 3 shows the movement of lithium ions 50 during discharging. The lithium ions 50 occluded in the unopposed portion 25 of the negative electrode composite layer 21 do not move because there is no opposed positive electrode composite layer 19. For this reason, the amount of lithium ions occluded at the end of the opposed portion 24 of the positive electrode composite layer 19 is relatively smaller than that of the portion other than the end in the opposed portion 24. For this reason, the SOC (State Of Charge) of the positive electrode after discharging becomes low at the central portion of the positive electrode composite layer 19 and high at the end portion. As a result, variations in SOC occur within the positive electrode composite layer 19.

[0033] FIG. 4 is a graph showing the relationship between the SOC and the resistance of the positive electrode during discharge. The horizontal axis represents the SOC of the positive electrode, and the vertical axis represents the resistance. As shown in this graph, the resistance decreases as the SOC increases. That is, in the resistance distribution of the positive electrode composite layer 19 during discharge, the resistance is low at the high-SOC end and high at the low-SOC central part.

[0034] Next, with reference to FIGS. 5 and 6, the progress of the deterioration reaction on the positive electrode sheet when the electrode body 11 does not contain sodium element will be described. Generally, additives such as carboxymethyl cellulose and styrene-butadiene copolymer contain sodium element. For example, as carboxymethyl cellulose having a thickening function, sodium carboxymethyl cellulose is added to the positive electrode composite paste and the negative electrode composite paste. The styrene-butadiene copolymer having a binding function contains sodium element. When the sodium element is removed by washing with a non-aqueous electrolyte in the manufacturing process or the like, the variation in the amount of the film formed by the reaction of bisoxalate borate ions and sodium ions derived from LiBOB becomes small. Therefore, the variation in the resistance in the width direction X of the negative electrode composite layer 21 becomes small. When the variation in the resistance in the width direction X in the negative electrode sheet 16 becomes small, the amount of lithium ions 50 released during discharge is also made uniform in the width direction X.

[0035] However, as shown in FIG. 5, due to the presence of the unopposed portion 25 in the negative electrode composite layer 21, the central portion 27 in the width direction X of the positive electrode sheet 15 has a greater resistance than the end portion 28. That is, while reducing the variation in the resistance of the negative electrode sheet 16 by reducing the sodium component, the variation in the resistance of the positive electrode sheet 15 remains large. For this reason, the occlusion of lithium ions 50 cannot catch up at the central portion of the positive electrode sheet 15 during discharge, and an oxidation reaction occurs at the central portion 27, and the deterioration progresses.

[0036] In order to suppress the degradation reaction in the positive electrode sheet 15, the inventor of the present application focused on matching the resistance distribution of the negative electrode sheet 16 with the resistance distribution of the positive electrode sheet 15, and found that the sodium content of the negative electrode composite layer 21 was adjusted to such an extent that their resistance distributions were matched.

[0037] The film formed by the reaction of sodium ions with bis(oxalato)borate ions (B(C 2 O 4 ) 2 - ) is likely to be formed in the central portion 27 of the negative electrode composite layer 21. Therefore, by adjusting the sodium content contained in the negative electrode composite layer 21, a film is intentionally generated in the central portion 27 of the negative electrode sheet 16 to slightly increase the resistance of the central portion 27, so as to match the tendency of the resistance variation in the width direction of the positive electrode sheet 15.

[0038] FIG. 6 is a schematic diagram of the resistance distribution of the negative electrode sheet and the movement of lithium ions 50 between the positive and negative electrodes when the resistance of the central portion 27 of the negative electrode sheet 16 is slightly increased. When the amount of lithium ions 50 released from the central portion 27 of the negative electrode sheet 16 during discharge decreases, the occlusion of lithium ions 50 in the central portion 27 of the positive electrode sheet 15 will not be unable to catch up. As a result, the degradation reaction in the central portion 27 of the positive electrode sheet 15 is suppressed. That is, by matching the sodium content with the resistance distribution of the positive electrode sheet 15, the progress of the degradation reaction in the positive electrode sheet is suppressed as compared with the case where the sodium element in the negative electrode composite layer 21 is completely removed.

[0039] With reference to FIGS. 7 to 10, a method of matching the resistance distribution between the positive and negative electrodes by adjusting the content of the sodium component will be described. This method is a method performed at the design stage of the lithium-ion secondary battery 10 and has the following steps.

[0040] · Specific step (step S1) of specifying the relationship between the resistance variation of the negative electrode and the sodium content in the negative electrode · Negative electrode resistance measurement step (step S2) of the resistance distribution of the negative electrode sheet 16 ·Adjustment value specifying step (step S3) for obtaining an optimal sodium content from the resistance variation of the negative electrode and the above graph ·Design step (step S4) for designing the negative electrode composite material layer 21 such that the sodium content of the negative electrode composite material layer 21 becomes the adjustment value specified in the adjustment value specifying step First, with reference to FIGS. 7 and 8, the specifying step (step S1) will be described.

[0041] FIG. 7 is a graph showing the relationship between the resistance variation in the width direction X of the negative electrode sheet 16 and the sodium content in the negative electrode composite material layer 21, indicated by the Na amount-resistance difference line 55. The horizontal axis represents the sodium content (ppm) of the negative electrode composite material layer 21, and the vertical axis represents the difference (Ω) between the minimum value and the maximum value of the resistance of the negative electrode sheet 16. The sodium content is the parts per million per unit weight of the negative electrode composite material. As the sodium content increases, the resistance difference increases. The Na amount-resistance difference line 55 can be obtained by measuring the resistance variations of a plurality of negative electrode sheets 16 with the sodium content contained in the negative electrode composite material layer 21 changed.

[0042] FIG. 8 is a schematic diagram of a measuring device 60 for measuring the resistance of the negative electrode sheet 16. The measuring device 60 includes a probe 61, a stage 62 for placing the measurement object, an impedance measuring unit 63, and a control unit 64. The probe 61 is an electrode with a known potential and is a reference electrode for measuring the potential of the negative electrode sheet 16, which is a working electrode placed on the stage 62. The probe 61 has a non-aqueous electrolyte 65 and a counter electrode 66. The non-aqueous electrolyte 65 is not particularly limited, but for example, the same non-aqueous electrolyte used in the lithium-ion secondary battery 10 can be used. The counter electrode 66 is not particularly limited as long as it is stable in the electrolyte used under the conditions input during the measurement of the alternating current impedance. For the counter electrode 66, for example, a carbon material, various metal materials, etc. can be used.

[0043] The impedance measurement unit 63 inputs an alternating current or an alternating voltage between the probe 61 and the negative electrode sheet 16 which is the measurement object, and measures the impedance by the alternating current impedance method. The alternating current impedance method inputs an alternating voltage or an alternating current signal to the measurement object while changing the frequency, and measures the response current or the response voltage at that time. Then, by comparing the sine wave input to the measurement object with the response signal, the transfer function (impedance) of the electrode reaction is obtained. Further, the impedance measurement unit 63 outputs the measurement result of the impedance to the control unit 64. A program for measuring and statistically analyzing the alternating current impedance is stored in the control unit 64.

[0044] When measuring the resistance of the negative electrode sheet 16, first place the negative electrode sheet 16 on the stage 62. The sodium content of the negative electrode sheet 16 is obtained in advance. The negative electrode sheet 16 is obtained by disassembling the lithium-ion secondary battery 10 which has passed the inspection and is in a state where it can be shipped. The negative electrode sheet 16 contains a non-aqueous electrolyte.

[0045] Next, the control unit 64 relatively moves the probe 61 or the stage 62 in the vertical direction to bring the probe 61 into contact with the negative electrode composite layer 21 of the negative electrode sheet 16. Then, an alternating current or an alternating voltage is input between the probe 61 and the negative electrode sheet 16 to measure the impedance. The control unit 64 obtains a Nyquist plot (Cole-Cole plot) based on the data obtained by the measurement of the alternating current impedance. Then, among the components obtained from the Nyquist plot, the direct current resistance is taken as the resistance of the negative electrode sheet.

[0046] Furthermore, the control unit 64 relatively moves the probe 61 or the stage 62 and similarly measures the resistance for the next measurement point. When the measurement of the resistance for a plurality of measurement points set in the width direction X is completed, the measurement is terminated. Thereby, the resistance distribution in the width direction X of the negative electrode sheet 16 can be measured.

[0047] The control unit 64 obtains the minimum value and the maximum value among the resistance values measured at a plurality of measurement points. Then, the sodium content of the negative electrode sheet 16 after the measurement is completed, and the minimum value and the maximum value of the resistance are stored in the storage unit. This series of steps is performed by changing the sodium content of the negative electrode sheet 16 to obtain the graph of FIG. 7.

[0048] Next, the negative electrode resistance measurement step (step S2) will be described. In the negative electrode resistance measurement step, the resistance distribution in the width direction X of the negative electrode sheet 16 is measured by the same method using the measuring device 60 as in step S1. Then, the minimum value and the maximum value of the measured resistance values are obtained. The negative electrode sheet 16 to be measured here is the one provided in the lithium ion secondary battery 10 to be designed.

[0049] Next, the sodium content specifying step (step S3) will be described. The difference between the minimum value and the maximum value of the resistance value of the negative electrode measured in step S2 is defined as the negative electrode resistance difference ΔRn. Referring to the Na amount-resistance difference line 55 shown in FIG. 7, the sodium content Cn when the positive electrode resistance difference is the same as the negative electrode resistance difference ΔRn is obtained. Then, this sodium content Cn is used as the adjustment value of the sodium content of the negative electrode composite material layer 21. Note that this process may be performed by the control unit 64.

[0050] The design step (step S4) will be described. When the sodium content Cn, which is the adjustment value, is obtained, the sodium content of the negative electrode composite material layer 21 to be designed is adjusted to match the sodium content Cn, which is the adjustment value. Specifically, the sodium content of the negative electrode composite material layer 21 is specified. The measurement of the sodium content can be performed using, for example, a high-frequency inductively coupled plasma optical emission spectrometer. Note that the sodium content of the negative electrode composite material layer 21 may be specified from the amount of sodium contained in each material constituting the negative electrode composite material layer 21.

[0051] When the specified sodium content is greater than the sodium content Cn which is the adjusted value, a lithium salt is used instead of the sodium salt contained in the additive so that the specified sodium content becomes the sodium content Cn which is the adjusted value. For example, when sodium carboxymethyl cellulose is added as a thickening agent to the negative electrode composite material layer 21, at least a part of the thickening agent may be lithium carboxymethyl cellulose. Instead of or in addition to this, when a styrene-butadiene copolymer containing mainly sodium element as an impurity is added as a binder to the negative electrode composite material layer 21, at least a part of the binder may be a styrene-butadiene copolymer containing mainly lithium element as an impurity. Or the sodium content of materials other than carboxymethyl cellulose and styrene-butadiene copolymer may be decreased. Also, when the specified sodium content is less than the sodium content Cn which is the adjusted value, the content of the additive containing a sodium salt may be increased.

[0052] For example, the amount of sodium per unit weight of sodium carboxymethyl cellulose (CMC-Na) is 1.0×10 5 ppm in parts per million. Lithium carboxymethyl cellulose (CMC-Li) contains a little sodium, but the amount of sodium per unit weight is 2.0×10 3 ppm. Therefore, by using lithium carboxymethyl cellulose instead of sodium carboxymethyl cellulose, the sodium content of the thickening agent can be greatly decreased. Also, the amount of sodium per unit weight of styrene-butadiene copolymer containing sodium element (SBR-Na) is 5.0×10 3 ppm, and the amount of sodium per unit weight of styrene-butadiene copolymer containing lithium element (SBR-Li) is 2.0×10 3 ppm. Therefore, by using a styrene-butadiene copolymer containing lithium element instead of a styrene-butadiene copolymer containing sodium element, the sodium content of the binder can be greatly decreased.

[0053] By adjusting the sodium content of the negative electrode composite layer 21 in this way, the resistance difference ΔRn of the negative electrode can be made to match the resistance difference of the positive electrode. Figs. 9 and 10 show the resistance distribution of the positive electrode sheet 15 and the resistance distribution of the negative electrode sheet 16 after adjusting the sodium content of the negative electrode composite layer 21. The positive electrode sheet 15 and the negative electrode sheet 16 were obtained from the lithium-ion secondary battery 10 in a state ready for shipment after undergoing conditioning treatment and inspection. As shown by the resistance distribution curve 70 in Fig. 9, the resistance difference ΔRp of the positive electrode sheet 15 is highest at the center in the width direction X of the positive electrode sheet 15, and is "6 Ω" in this example.

[0054] Fig. 10 shows the resistance distribution of the negative electrode sheet 16. The resistance distribution curve 71 shown by the solid line is the resistance distribution after adjusting the sodium content to the optimum value. The resistance difference ΔRn at the center is the same value as the resistance difference of the positive electrode (for example, "6 Ω"). By making the resistance differences ΔRp and ΔRn at the center equal between the positive and negative electrodes in this way, it is possible to make the amount of lithium ions 50 released from the center of the negative electrode sheet 16 during discharge match the amount of lithium ions occluded in the center of the positive electrode sheet 15. For this reason, the progress of the degradation reaction in the positive electrode sheet 15 can be suppressed.

[0055] As described above, according to the above embodiment, the following effects can be obtained. (1) In the above embodiment, the sodium content of the negative electrode composite layer 21 is adjusted so that the resistance difference of the positive electrode sheet 15 and the resistance difference of the negative electrode sheet 16 are equal. Since the high-resistance portions of the positive electrode sheet 15 and the negative electrode sheet 16 are at almost the same position, the amount of lithium ions released from the high-resistance portion of the negative electrode sheet 16 during discharge and the amount of lithium ions occluded in the high-resistance portion of the positive electrode sheet can be made to be in a balanced state with little excess or deficiency. For this reason, even when the resistance of the negative electrode sheet 16 is overall decreased by reducing the sodium content of the negative electrode composite layer 21, the progress of the degradation reaction in the positive electrode sheet 15 can be suppressed.

[0056] (2) In the above embodiment, in the design process (step S4), the sodium element contained in the negative electrode composite material layer 21 is replaced with a lithium element, and the sodium content of the negative electrode composite material layer 21 is set to the adjustment value obtained in the adjustment value specifying step. Therefore, the deterioration reaction in the positive electrode sheet 15 can be suppressed without significantly changing the characteristics of the material used for the negative electrode composite material layer 21.

[0057] (3) When the sodium content is set to the adjustment value by adding lithium carboxymethyl cellulose to the negative electrode composite material paste for forming the negative electrode composite material layer 21, sodium carboxymethyl cellulose may not be added or the addition amount may be reduced. Therefore, during manufacturing, precipitation of lithium on the negative electrode sheet can be suppressed while maintaining the viscosity required for the negative electrode composite material paste.

[0058] (4) When adding a styrene-butadiene copolymer containing a lithium element to the negative electrode composite material paste for forming the negative electrode composite material layer 21, a styrene-butadiene copolymer containing a sodium element may not be added or the addition amount may be reduced. Therefore, during manufacturing, precipitation of lithium on the negative electrode sheet can be suppressed while maintaining good binding properties of the negative electrode composite material layer 21.

[0059] (Other Embodiments) The above embodiment can be implemented with the following modifications. The above embodiment and the following modification examples can be implemented in combination with each other as long as they do not technically conflict.

[0060] · The electrode body 11 is not limited to the electrode structure in which the positive electrode sheet 15 and the negative electrode sheet 16 are wound via the separator 17, and may be appropriately changed according to the shape and purpose of use of the lithium-ion secondary battery 10. For example, the positive electrode sheet 15 and the negative electrode sheet 16 may have a non-wound electrode structure in which they are laminated via the separator 17.

[0061] · The lithium-ion secondary battery 10 may be used for applications other than as a drive source for electric vehicles and drive sources for hybrid vehicles. For example, the lithium-ion secondary battery 10 may be mounted on vehicles such as gasoline vehicles and diesel vehicles. Further, the lithium-ion secondary battery 10 may be used as a power source for moving bodies such as railways, ships, and aircraft, robots, and electrical products such as information processing devices.

[0062] · In the above embodiment, a non-aqueous secondary battery having a non-aqueous electrolyte was embodied and described as the lithium-ion secondary battery 10. However, as long as a film-forming material is used and variations in the film amount can occur, it is also possible to apply it to a battery having a non-aqueous electrolyte other than a lithium-ion secondary battery.

Explanation of Reference Numerals

[0063] 1... Lithium-ion secondary battery 15... Positive electrode sheet 16... Negative electrode sheet 19... Positive electrode composite layer 21... Negative electrode composite layer

Claims

1. A method for manufacturing a non-aqueous secondary battery having a positive electrode sheet including a positive electrode composite material layer, a negative electrode sheet having a negative electrode composite material layer, and a non-aqueous electrolyte containing a film-forming material containing a lithium element, a positive electrode resistance measurement step of measuring a positive electrode resistance difference that is a difference between a minimum value and a maximum value of the resistance of the positive electrode sheet; an adjustment value specifying step of specifying, as an adjustment value, a sodium content corresponding to the same negative electrode resistance difference as the measured positive electrode resistance difference by using information indicating a relationship between a negative electrode resistance difference that is a difference between a minimum value and a maximum value of the resistance of the negative electrode sheet and the sodium content of the negative electrode composite material layer; a design step of designing the negative electrode composite material layer such that the sodium content of the negative electrode composite material layer becomes the adjustment value specified in the adjustment value specifying step. A method for manufacturing a non-aqueous secondary battery.

2. The positive electrode composite material layer uses a lithium composite metal oxide as a positive electrode active material, The design step replaces a sodium element contained in the negative electrode composite material layer with a lithium element to make the sodium content the adjustment value. The method for manufacturing a non-aqueous secondary battery according to Claim 1.

3. The design step makes the sodium content the adjustment value by adding lithium carboxymethyl cellulose to the negative electrode composite material paste forming the negative electrode composite material layer. The method for manufacturing a non-aqueous secondary battery according to Claim 1 or 2.

4. The design step makes the sodium content the adjustment value by adding a styrene-butadiene copolymer containing a lithium element to the negative electrode composite material paste forming the negative electrode composite material layer. The method for manufacturing a non-aqueous secondary battery according to any one of Claims 1 to 3.

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