JP7689045B2 - Method for manufacturing non-aqueous secondary battery - Google Patents

Description

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

Non-aqueous secondary batteries such as lithium ion secondary batteries have a so-called SEI (Solid Electrolyte Interface) film on the surface of the negative electrode composite. This film exists between the negative electrode composite and the non-aqueous electrolyte, and facilitates the absorption and release of lithium ions into and from the negative electrode, while also suppressing further decomposition of the non-aqueous electrolyte, making it essential for maintaining good battery characteristics. In addition, this film is formed by decomposition products of the non-aqueous electrolyte and additives, and the decomposition products take in lithium ions during the formation process. For this reason, if the film becomes too thick, it can lead to a decrease in battery capacity.

To solve this problem, it has already been proposed to add a film-forming material containing lithium to the non-aqueous electrolyte in advance. The film-forming material is a compound containing lithium element such as a lithium salt, for example, lithium bis( _oxalato )borate (LiBOB, LiB( _C2O4 ) ₂ ). This method can promote the formation of the film while ensuring a sufficient amount of lithium ions that migrate between the positive and negative electrodes and directly contribute to the battery reaction.

On the other hand, bis(oxalato)borate ions (BOB, B(C ₂ O ₄ ) ₂ ⁻ ), which are anions ionized from LiBOB, also react with sodium ions contained as impurities in the negative electrode mixture layer, and become part of the coating on the negative electrode sheet. The coating derived from sodium ions tends to be formed more in the center of the negative electrode sheet of the electrode body formed by winding the positive electrode sheet, negative electrode sheet, and separator. This is because sodium ions tend to gather in the center of the negative electrode sheet, where the sodium ions and bis(oxalato)borate ions react actively.

The center of the negative electrode sheet, where the amount of coating is excessively large, has a higher resistance than the surrounding area, making lithium more likely to precipitate. Because the sodium element contained in the negative electrode mixture layer can cause lithium to precipitate, it has been proposed to reduce the content of sodium in the negative electrode by, for example, washing the negative electrode with a nonaqueous electrolyte (see Patent Document 1).

JP 2014-26932 A

A negative electrode whose sodium content has been reduced by cleaning or other methods has a smaller resistance difference within the negative electrode sheet than a negative electrode whose sodium content has not been reduced, and this makes it possible to suppress lithium deposition on the negative electrode sheet. However, since the resistance difference within the negative electrode sheet also affects deterioration of the positive electrode, it is desirable to consider these factors in combination, and there is room for further improvement.

The method for manufacturing a nonaqueous secondary battery that solves the above problem is a method for manufacturing a nonaqueous secondary battery having a positive electrode sheet with a positive electrode composite layer, a negative electrode sheet with a negative electrode composite layer, and a nonaqueous electrolyte containing a film-forming material containing lithium element, and includes a positive electrode resistance measurement process for measuring a positive electrode resistance difference, which is the difference between the minimum and maximum resistance values of the positive electrode sheet, an adjustment value specification process for specifying, as an adjustment value, a sodium content corresponding to the same negative electrode resistance difference as the measured positive electrode resistance difference, using information indicating the relationship between the negative electrode resistance difference, which is the difference between the minimum and maximum resistance values of the negative electrode sheet, and the sodium content of the negative electrode composite layer, and a design process for designing the negative electrode composite layer so that the sodium content of the negative electrode composite layer becomes the adjustment value specified in the adjustment value specification process.

The resistance of the center of both the positive and negative electrode sheets tends to be high. When sodium ions, which are one of the factors that cause the resistance of the negative electrode to vary, are removed, the amount of coating produced by the reaction between the sodium ions and the coating material does not vary, and the resistance of the negative electrode sheet is uniformed. When the resistance of the negative electrode sheet is uniformed, the amount of lithium ions released from the negative electrode sheet during discharge is also uniformed, but the absorption of lithium ions in the high resistance part in the center of the positive electrode sheet cannot keep up, and the center of the positive electrode sheet may deteriorate due to oxidation reactions, etc. According to the above method, the sodium content of the negative electrode mixture layer is adjusted so that the resistance difference of the positive electrode and the resistance difference of the negative electrode are equal. Since the high resistance parts of the positive electrode sheet and the negative electrode sheet are located at almost the same position, the amount of lithium ions released from the high resistance part of the negative electrode sheet during discharge and the amount of lithium ions absorbed in the high resistance part of the positive electrode sheet can be balanced with no excess or deficiency. Therefore, the deterioration reaction in the positive electrode sheet can also be suppressed.

In the above-described method for manufacturing a nonaqueous secondary battery, it is preferable that the positive electrode mixture layer contains a lithium composite metal oxide as a positive electrode active material, and that in the designing step, the sodium content is set to the adjusted value by replacing sodium element contained in the negative electrode mixture layer with lithium element.
According to the above method, the deterioration reaction in the positive electrode sheet can be suppressed without significantly changing the properties of the material used in the negative electrode mixture layer.

In the manufacturing method of the nonaqueous secondary battery, the design step preferably adjusts the sodium content to the adjusted value by adding lithium carboxymethyl cellulose salt to the negative electrode mixture paste that forms the negative electrode mixture layer.

According to the above method, sodium carboxymethylcellulose can be added to the negative electrode composite paste without being added or in a reduced amount. This makes it possible to suppress the precipitation of lithium on the negative electrode sheet while maintaining the viscosity required for the negative electrode composite paste in manufacturing.

In the manufacturing method of the nonaqueous secondary battery, the design step preferably adjusts the sodium content to the adjusted value by adding a styrene-butadiene copolymer containing lithium to the negative electrode mixture paste that forms the negative electrode mixture layer.

According to the above method, it is possible to add no or a small amount of styrene-butadiene copolymer containing sodium to the negative electrode composite paste. Therefore, in manufacturing, it is possible to suppress the precipitation of lithium in the negative electrode sheet while maintaining good adhesion of the negative electrode composite layer.

According to the present invention, by controlling the resistance difference within the negative electrode of a nonaqueous secondary battery, it is possible to suppress lithium deposition on the negative electrode sheet and at the same time suppress the deterioration reaction on the positive electrode sheet.

FIG. 1 is a schematic diagram showing an electrode assembly of a lithium secondary battery as a nonaqueous secondary battery in one embodiment of the nonaqueous secondary battery. FIG. 2 is a diagram showing a schematic structure of a positive electrode sheet and a negative electrode sheet in the embodiment. FIG. 2 is a diagram showing a schematic structure of a positive electrode sheet and a negative electrode sheet in the embodiment. FIG. 4 is a diagram showing the relationship between a charged state and resistance in the embodiment. FIG. 4 is a diagram showing the amount of lithium ions moving between the positive and negative electrodes in the embodiment. FIG. 4 is a diagram showing the amount of lithium ions moving between the positive and negative electrodes in the embodiment. FIG. 13 is a graph showing a Na amount-resistance difference line in the embodiment. FIG. 4 is a schematic diagram of a measurement device for measuring resistance in the embodiment. FIG. 4 is a diagram showing the resistance distribution of a positive electrode in the embodiment. FIG. 4 is a diagram showing the resistance distribution of the negative electrode after adjusting the sodium content in the embodiment.

One embodiment of the present invention will be described below. In this embodiment, the nonaqueous secondary battery will be described as a lithium ion secondary battery. The lithium ion secondary battery 10 is a battery that charges and discharges by moving lithium ions between a positive electrode and a negative electrode. The lithium ion secondary battery 10 is used, for example, as a power source for driving electric vehicles (EVs) and hybrid vehicles (HVs).

The configuration of the lithium ion secondary battery 10 will be described with reference to FIG. 1. The lithium ion secondary battery 10 includes an electrode body 11 in a case not shown. The electrode body 11 is a wound body in which a plurality of sheets are wound. The electrode body 11 is formed by stacking 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 a long shape and includes a positive electrode current collector 18 and a positive electrode composite layer 19 provided on both sides of the positive electrode current collector 18. The positive electrode composite layer 19 is a layer formed by a process of applying and drying a positive electrode composite paste. The negative electrode sheet 16 has a long shape and includes a sheet-shaped negative electrode current collector 20 and a negative electrode composite layer 21 provided on both sides of the negative electrode current collector 20. The negative electrode composite layer 21 is a layer formed by a process of applying and drying a negative electrode composite paste. The stack before winding is stacked in the order of positive electrode sheet 15, separator 17, negative electrode sheet 16, and separator 17 so that the longitudinal directions of the positive electrode sheet 15 and negative electrode sheet 16 are aligned. The stack is wound so that the negative electrode sheet 16 is on the innermost side. The longitudinal direction of the positive electrode sheet 15 and negative electrode sheet 16 is the "longitudinal direction Y", and the direction perpendicular to this is the "width direction X".

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 periphery. At one end of the positive electrode sheet 15 in the width direction X, an uncoated portion 15A is provided where the positive electrode composite layer 19 is not formed and the positive electrode current collector 18 is exposed. The length of the uncoated portion 15A in the width direction X is, for example, 12 mm or less.

At one end of the negative electrode sheet 16 in the width direction X, there is an uncoated portion 16A where the negative electrode composite layer 21 is not formed and the negative electrode current collector 20 is exposed. The length of the uncoated portion 16A in the width direction X is, for example, 12 mm or less. The negative electrode sheet 16 also has an unopposed portion 25, which is a negative electrode composite layer that does not face the positive electrode composite layer 19 via the separator. The length of the unopposed portion 25 in the width direction X is, for example, 3.5 mm.

Next, the positive electrode will be described. A metal foil such as an aluminum foil is used for the positive electrode current collector 18. The positive electrode mixture layer 19 includes a positive electrode active material, a conductive material, and a binder. The positive electrode active material can be one or more of various materials known to be usable as a positive electrode active material for a lithium ion secondary battery. Suitable examples include lithium composite metal oxides such as layered and spinel-based oxides (e.g., LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , and LiFePO ₄ ). Examples of conductive materials include carbon black such as acetylene black and ketjen black, and other powdered carbon materials (e.g., graphite). Examples of the binder include polyvinylidene fluoride (PVDF), styrene butadiene copolymer (SBR), and polytetrafluoroethylene (PTFE).

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). It may also be 70% by mass or more and 95% by mass or less. When a conductive material is used, 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 a binder is used, 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.

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 composite layer 21 includes a negative electrode active material, a conductive material, a binder, and the like. The negative electrode active material can be one or more of various materials known to be usable as a negative electrode active material for lithium ion secondary batteries. For example, carbon materials such as graphite, hard carbon, soft carbon, and carbon nanotubes can be mentioned. Among them, graphite-based materials such as natural graphite and artificial graphite (particularly natural graphite) can be preferably used because they have excellent conductivity and can obtain high energy density. The same binder as that of the positive electrode can be used. In addition, a thickener, a dispersant, and the like can be appropriately used. For example, carboxymethyl cellulose (CMC) and methyl cellulose (MC) can be used as the thickener.

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 a binder is used, 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 a thickener is used, the proportion of the thickener 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.

The separator 17 has a porous layer formed from a resin. The porous layer has a single-layer structure made of, for example, porous polyethylene, porous polyolefin, and porous polyvinyl chloride, or a laminated structure made of multiple materials. The porous layer can also contain a filler for the purpose of improving strength. An adhesive layer made of an adhesive may be interposed between the separator 17 and the negative electrode sheet 16.

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 of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be used. In addition, as the supporting salt, one or more of 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.

The electrolyte also contains a film-forming material that forms an SIE film on the negative electrode composite layer 21. For example, lithium bis(oxalato)borate (LiBOB) can be used as the film-forming material. To form a film of an appropriate thickness, the concentration of LiBOB in the electrolyte is, for example, 0.05 mol/kg or less.

Before shipping, the lithium-ion secondary battery 10 undergoes a conditioning process in which a coating derived from LiBOB is formed on the surface of the negative electrode composite layer 21. The conditioning process can be performed by repeatedly charging and discharging the lithium-ion secondary battery 10 at a predetermined rate.

The configuration of the positive electrode sheet 15 and the negative electrode sheet 16 and the resistance distribution of the positive electrode will be described with reference to Figures 2 and 3. Figures 2 and 3 are schematic diagrams showing the positive electrode sheet 15 and the negative electrode sheet 16 when the laminate in Figure 1 is cut at the 2-2 position along the width direction X. For ease of explanation, the separator 17 is omitted in Figures 2 and 3, and the positive electrode sheet 15 and the negative electrode sheet 16 are shown in an unwound state.

The lithium ion secondary battery 10 is a positive electrode regulated battery in which the battery capacity is regulated by the positive electrode capacity. The length in the width direction X of the negative electrode mixture layer 21 is longer than the length in the width direction X of the positive electrode mixture layer 19. As a result, the entire positive electrode mixture layer 19 faces the negative electrode mixture layer 21, while the negative electrode mixture layer 21 has an opposing portion 24 that faces the positive electrode mixture layer 19 via the separator 17, and a non-opposing portion 25 that does not face the positive electrode mixture layer 19 via the separator 17.

Figure 2 shows the movement of lithium ions 50 during charging. During charging, lithium ions 50 move from the positive electrode to the negative electrode. At this time, lithium ions 50 released from the end of the positive electrode mixture layer 19 move not only to the facing portion 24 of the negative electrode mixture layer 21, but also to the non-facing portion 25.

Figure 3 shows the movement of lithium ions 50 during discharge. The lithium ions 50 stored in the non-facing portion 25 of the negative electrode composite layer 21 do not move because there is no opposing positive electrode composite layer 19. Therefore, the amount of lithium ions stored at the end of the facing portion 24 of the positive electrode composite layer 19 is relatively small compared to the other parts of the facing portion 24. Therefore, the SOC (State of Charge) of the positive electrode after discharge is low in the center of the positive electrode composite layer 19 and high at the end. This causes SOC variation within the positive electrode composite layer 19.

Figure 4 is a graph showing the relationship between SOC and resistance during positive electrode discharge. The horizontal axis shows the positive electrode SOC, and the vertical axis shows resistance. As shown in this graph, the resistance decreases as the SOC increases. In other words, the resistance distribution of the positive electrode composite layer 19 during discharge is such that the ends with high SOC have low resistance, and the center with low SOC has high resistance.

5 and 6, the deterioration reaction proceeds on the positive electrode sheet when the electrode body 11 does not contain sodium element. In general, additives such as carboxymethyl cellulose and styrene butadiene copolymer contain sodium element. For example, carboxymethyl cellulose sodium is added to the positive electrode mixture paste and the negative electrode mixture paste as carboxymethyl cellulose having a function of a thickener. Styrene butadiene copolymer having a function of a binder contains sodium element. When sodium element is removed in the manufacturing process by washing with a nonaqueous electrolyte, the variation in the amount of coating caused by the reaction of bisoxalate borate ions derived from LiBOB and sodium ions is reduced. Therefore, the variation in resistance in the width direction X of the negative electrode mixture layer 21 is reduced. When the variation in resistance in the width direction X in the negative electrode sheet 16 is reduced, the amount of lithium ions 50 released during discharge is also uniform in the width direction X.

However, as shown in FIG. 5, due to the presence of unopposed portions 25 in the negative electrode composite layer 21, the resistance of the central portion 27 in the width direction X of the positive electrode sheet 15 is greater than that of the end portions 28. In other words, while the variation in resistance of the negative electrode sheet 16 is reduced by reducing the sodium content, the variation in resistance of the positive electrode sheet 15 remains large. As a result, the absorption of lithium ions 50 cannot keep up in the central portion of the positive electrode sheet 15 during discharge, and an oxidation reaction occurs in the central portion 27, causing deterioration to proceed.

To suppress the deterioration reaction in the positive electrode sheet 15, the inventors of the present application focused on matching the resistance distribution of the positive electrode sheet 15 with the resistance distribution of the negative electrode sheet 16, and discovered that the sodium content of the negative electrode composite layer 21 can be adjusted to the extent that these resistance distributions are matched.

A coating produced by a reaction between sodium ions and bis(oxalatoborate) ions (B(C ₂ O ₄ ) ₂ ⁻ ) is likely to be formed in central portion 27 of negative electrode mixture layer 21. For this reason, by adjusting the sodium content in negative electrode mixture layer 21, a coating is intentionally formed in central portion 27 of negative electrode sheet 16, thereby slightly increasing the resistance of central portion 27 and matching the tendency of variation in resistance in the width direction of positive electrode sheet 15.

Figure 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 center portion 27 of the negative electrode sheet 16 is slightly increased. When the amount of lithium ions 50 released from the center portion 27 of the negative electrode sheet 16 during discharge decreases, the absorption of lithium ions 50 in the center portion 27 of the positive electrode sheet 15 does not overtake. As a result, the deterioration reaction in the center portion 27 of the positive electrode sheet 15 is suppressed. In other words, by matching the sodium content to the resistance distribution of the positive electrode sheet 15, the progress of the deterioration reaction in the positive electrode sheet is suppressed compared to the case where the sodium element is completely removed from the negative electrode mixture layer 21.

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

A step of identifying the relationship between the resistance variation of the negative electrode and the sodium content in the negative electrode (step S1)
Negative electrode resistance measurement step for the resistance distribution of the negative electrode sheet 16 (step S2)
A step of determining an optimal sodium content based on the resistance variation of the negative electrode and the graph (step S3)
A design process (step S4) of designing the negative electrode mixture layer 21 so that the sodium content of the negative electrode mixture layer 21 becomes the adjustment value specified in the adjustment value specifying process.
First, the specification step (step S1) will be described with reference to FIGS.

Figure 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 layer 21, using a Na content-resistance difference line 55. The horizontal axis shows the sodium content (ppm) of the negative electrode composite layer 21, and the vertical axis shows the difference (Ω) between the minimum and maximum resistance values of the negative electrode sheet 16. The sodium content is in parts per million per unit weight of the negative electrode composite. The resistance difference increases as the sodium content increases. The Na content-resistance difference line 55 can be obtained by measuring the resistance variation of multiple negative electrode sheets 16 with different sodium contents contained in the negative electrode composite layer 21.

8 is a schematic diagram of a measuring device 60 for measuring the resistance of the negative electrode sheet 16. The measuring device 60 has a probe 61, a stage 62 on which the measurement target is placed, 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 the 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 when measuring the AC impedance. For example, a carbon material, various metal materials, etc. can be used for the counter electrode 66.

The impedance measuring unit 63 inputs an AC current or AC voltage between the probe 61 and the negative electrode sheet 16 to be measured, and measures the impedance by the AC impedance method. The AC impedance method inputs an AC voltage or AC current signal to the measurement object while changing the frequency, and measures the response current or response voltage at that time. The transfer function (impedance) of the electrode reaction is then obtained by comparing the sine wave input to the measurement object with the response signal. The impedance measuring unit 63 also outputs the impedance measurement results to the control unit 64. The control unit 64 stores a program for measuring and statistically compiling AC impedance.

When measuring the resistance of the negative electrode sheet 16, the negative electrode sheet 16 is first placed on the stage 62. The sodium content of the negative electrode sheet 16 is determined in advance. The negative electrode sheet 16 is obtained by disassembling a lithium ion secondary battery 10 that has been inspected to be a good product and is ready for shipment. The negative electrode sheet 16 contains a non-aqueous electrolyte.

Next, the control unit 64 moves the probe 61 or the stage 62 relatively 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 AC current or an AC 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 measuring the AC impedance. Then, of the components obtained from the Nyquist plot, the DC resistance is taken as the resistance of the negative electrode sheet.

The control unit 64 then moves the probe 61 or stage 62 relative to the target to measure the resistance at the next measurement point in the same manner. When the resistance measurements are completed for the multiple measurement points set in the width direction X, the measurement is terminated. This allows the resistance distribution of the negative electrode sheet 16 in the width direction X to be measured.

The control unit 64 determines the minimum and maximum resistance values measured at multiple measurement points. The control unit 64 then stores the sodium content of the negative electrode sheet 16 after the measurement and the minimum and maximum resistance values in the memory unit. This series of steps is repeated while changing the sodium content of the negative electrode sheet 16 to obtain the graph shown in Figure 7.

Next, the negative electrode resistance measurement process (step S2) will be described. In the negative electrode resistance measurement process, the resistance distribution in the width direction X of the negative electrode sheet 16 is measured using a method using a measuring device 60, similar to step S1. The minimum and maximum values of the measured resistance values are then obtained. The negative electrode sheet 16 measured here is the one provided in the lithium ion secondary battery 10 being designed.

Next, the sodium content determination step (step S3) will be described. The difference between the minimum and maximum resistance values of the negative electrode measured in step S2 is set as the negative electrode resistance difference ΔRn. With reference 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 determined. This sodium content Cn is then set as the adjustment value for the sodium content of the negative electrode composite layer 21. This process may be performed by the control unit 64.

The design process (step S4) will be described. Once the sodium content Cn, which is the adjustment value, is determined, the sodium content of the negative electrode composite 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 layer 21 is specified. The sodium content can be measured, for example, using a high-frequency inductively coupled plasma emission spectrometer. The sodium content of the negative electrode composite layer 21 may be specified from the amount of sodium contained in each material that constitutes the negative electrode composite layer 21.

If the specified sodium content is greater than the sodium content Cn, which is the adjustment value, 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 adjustment value. For example, if sodium carboxymethylcellulose is added as a thickener to the negative electrode mixture layer 21, at least a part of the thickener may be lithium carboxymethylcellulose. Alternatively or in addition to this, if a styrene-butadiene copolymer containing mainly sodium elements as impurities is added to the negative electrode mixture layer 21 as a binder, at least a part of the binder may be a styrene-butadiene copolymer containing mainly lithium elements as impurities. Alternatively, the sodium content of materials other than carboxymethylcellulose and styrene-butadiene copolymer may be reduced. Also, if the specified sodium content is less than the sodium content Cn, which is the adjustment value, the content of the additive containing sodium salt may be increased.

For example, the amount of sodium per unit weight of sodium carboxymethylcellulose (CMC-Na) is 1.0×10 ⁵ ppm in parts per million. Lithium carboxymethylcellulose (CMC-Li) contains a small amount of sodium, but the amount of sodium per unit weight is 2.0×10 ³ ppm. Therefore, by using lithium carboxymethylcellulose instead of sodium carboxymethylcellulose, the sodium content of the thickener can be greatly reduced. In addition, 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 styrene butadiene copolymer containing lithium element instead of styrene butadiene copolymer containing sodium element, the sodium content of the binder can be greatly reduced.

By adjusting the sodium content of negative electrode mixture layer 21 in this manner, the resistance difference ΔRn of the negative electrode can be made to match the resistance difference of the positive electrode.
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 mixture layer 21. The positive electrode sheet 15 and the negative electrode sheet 16 were obtained from a lithium ion secondary battery 10 in a ready-to-ship state after conditioning and inspection. As shown in a resistance distribution curve 70 in FIG. 9, the resistance difference ΔRp of the positive electrode sheet 15 is highest at the center of the positive electrode sheet 15 in the width direction X, and is "6 Ω" in this example.

Figure 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 the sodium content is adjusted to the optimal value. The resistance difference ΔRn in 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 in the center equal between the positive and negative electrodes in this way, it is possible to achieve a balanced state between the amount of lithium ions 50 released from the center of the negative electrode sheet 16 during discharge and the amount of lithium ions absorbed in the center of the positive electrode sheet 15. This makes it possible to suppress the progression of the deterioration reaction in the positive electrode sheet 15.

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 mixture 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 equivalent. Since the high resistance parts of the positive electrode sheet 15 and the negative electrode sheet 16 are located at approximately the same position, the amount of lithium ions released from the high resistance part of the negative electrode sheet 16 during discharge and the amount of lithium ions absorbed in the high resistance part of the positive electrode sheet can be kept in a balanced state in which excess and deficiency are suppressed. Therefore, even if the resistance of the negative electrode sheet 16 is reduced overall by reducing the sodium content of the negative electrode mixture layer 21, the progress of the deterioration reaction in the positive electrode sheet 15 can be suppressed.

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

(3) When the sodium content is adjusted by adding lithium carboxymethylcellulose salt to the negative electrode composite paste that forms the negative electrode composite layer 21, sodium carboxymethylcellulose may not be added or may be added in a small amount. This makes it possible to suppress the deposition of lithium on the negative electrode sheet while maintaining the viscosity required for the negative electrode composite paste in manufacturing.

(4) When a styrene-butadiene copolymer containing lithium elements is added to the negative electrode composite paste that forms the negative electrode composite layer 21, it is possible to add no styrene-butadiene copolymer containing sodium elements or to add a small amount of styrene-butadiene copolymer. Therefore, in manufacturing, it is possible to suppress the deposition of lithium on the negative electrode sheet while maintaining good adhesion of the negative electrode composite layer 21.

Other Embodiments
The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other to the extent that no technical contradiction occurs.

The electrode body 11 is not limited to an electrode structure in which the positive electrode sheet 15 and the negative electrode sheet 16 are wound with the separator 17 interposed therebetween, and may be modified as appropriate depending on the shape and intended use of the lithium-ion secondary battery 10. For example, the electrode body 11 may have an electrode structure in which the positive electrode sheet 15 and the negative electrode sheet 16 are stacked with the separator 17 interposed therebetween, without being wound.

- The lithium ion secondary battery 10 may be used for purposes other than as a driving source for electric vehicles and hybrid vehicles. For example, the lithium ion secondary battery 10 may be installed in vehicles such as gasoline vehicles and diesel vehicles. The lithium ion secondary battery 10 may also be used as a power source for moving objects such as trains, ships, and aircraft, and for electrical products such as robots and information processing devices.

- In the above embodiment, a non-aqueous secondary battery having a non-aqueous electrolyte solution was described as being embodied in the lithium ion secondary battery 10, but as long as a coating material is used and the amount of coating can vary, the present invention can also be applied to batteries having a non-aqueous electrolyte solution other than lithium ion secondary batteries.

Reference Signs List 1... Lithium ion secondary battery 15... Positive electrode sheet 16... Negative electrode sheet 19... Positive electrode mixture layer 21... Negative electrode mixture layer

Claims (4)

A method for producing a non-aqueous secondary battery having a positive electrode sheet having a positive electrode mixture layer, a negative electrode sheet having a negative electrode mixture layer, and a non-aqueous electrolyte solution containing a coating material containing lithium element, comprising:
a positive electrode resistance measuring step of measuring a positive electrode resistance difference which 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, using information indicating a relationship between a negative electrode resistance difference, which 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 mixture layer;
a designing step of designing the negative electrode mixture layer so that the sodium content of the negative electrode mixture layer becomes the adjustment value specified in the adjustment value specifying step. The positive electrode mixture layer contains a lithium composite metal oxide as a positive electrode active material,
The method for producing a nonaqueous secondary battery according to claim 1 , wherein the designing step includes replacing sodium element contained in the negative electrode mixture layer with lithium element to set the sodium content to the adjusted value. The method for producing a nonaqueous secondary battery according to claim 1 , wherein the designing step sets the sodium content to the adjusted value by adding a lithium carboxymethyl cellulose salt to a negative electrode mixture paste that forms the negative electrode mixture layer. The method for producing a nonaqueous secondary battery according to any one of claims 1 to 3, wherein the designing step sets the sodium content to the adjusted value by adding a styrene-butadiene copolymer containing lithium element to a negative electrode composite paste that forms the negative electrode composite layer.

Images