Negative electrode plate manufacturing method

Abstract

Conventional methods for manufacturing secondary batteries have the problem of not being able to suppress lot-to-lot variations in the output performance of secondary batteries. [Solution] The present invention provides a method for manufacturing a secondary battery, comprising: a crystallinity measurement step (S2) in which the crystallinity of a carbon material containing hard carbon and graphite is measured for each lot of the carbon material and the crystallinity measurement value is obtained for each lot; a mixing adjustment amount calculation step (S3) in which a mixing adjustment amount is calculated to adjust the ratio of hard carbon contained in the carbon material to a reference ratio corresponding to the reference value based on the difference between the crystallinity measurement value and a reference value of the crystallinity measurement value; a mixing step (S5) in which the ratio of hard carbon in the carbon material is adjusted according to the mixing adjustment amount; and a kneading step (S6) in which the carbon material after the mixing step is kneaded to produce a slurry.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a negative electrode plate used in a secondary battery such as a lithium-ion battery, for example.

Background Art

[0002] Graphite is used as a negative electrode active material in a lithium-ion battery. Due to the influence of temperature unevenness during firing, crystallinity variations may occur between lots in this graphite. This crystallinity variation affects the specific capacity of the lithium-ion battery. Also, this specific capacity is caused by a deviation in the negative electrode potential and affects the output performance of the lithium-ion battery. Therefore, technologies related to lithium-ion batteries using graphite containing hard carbon as a negative electrode active material are described in Patent Documents 1 to 3.

[0003] Patent Document 1 discloses a negative electrode for a lithium-ion secondary battery, which is composed of a laminate of a negative electrode material layer made of a carbon material containing hard carbon and a negative electrode current collector, wherein the limiting curvature radius of the negative electrode in a dry state is 15 mm or less, and the content of hard carbon is 5 to 45% by weight.

[0004] Patent Document 2 discloses an electricity storage element including a positive electrode and a negative electrode, wherein the negative electrode has graphite and non-graphitizable carbon, the D50 particle diameter at which the cumulative volume in the particle size distribution of the graphite becomes 50% is 2 μm or more, the ratio of the mass of non-graphitizable carbon to the total mass of the mass of graphite and the mass of non-graphitizable carbon is 5% to 45% by mass, and the ratio of the D50 particle diameter of the graphite to the D50 particle diameter of the non-graphitizable carbon is 1.02 or less.

[0005] Patent Document 3 discloses an electrode for a non-aqueous electrolyte secondary battery, comprising a current collector and an active material layer, wherein the active material layer has at least two layers: a first layer close to the current collector and a second layer close to the surface of the electrode, both the first and second layers contain graphite as the main active material, the second layer further contains low-crystalline carbon, the graphite content of the first layer is higher than the graphite content of the second layer, and the low-crystalline carbon content in the active material layer is 8% by mass or less of the active material material in the active material layer. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-080659 [Patent Document 2] Japanese Patent Publication No. 2016-225137 [Patent Document 3] Japanese Patent Publication No. 2017-062911 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, Patent Documents 1 to 3 do not address lot-to-lot variations in the crystallinity of hard carbon, which presents a problem in that they cannot suppress lot-to-lot variations in the output performance of secondary batteries.

[0008] This invention has been made in view of the above circumstances, and aims to suppress lot-to-lot variations in the output performance of secondary batteries. [Means for solving the problem]

[0009] One embodiment of the method for manufacturing a negative electrode plate according to the present invention includes: a crystallinity measurement step of measuring the crystallinity of a carbon material containing hard carbon and graphite for each lot of hard carbon and obtaining the crystallinity measurement value of the hard carbon for each lot; a mixing adjustment amount calculation step of calculating a mixing adjustment amount to adjust the ratio of hard carbon contained in the carbon material to a reference ratio corresponding to the reference value based on the difference between the crystallinity measurement value and a reference value of the crystallinity measurement value; a mixing step of adjusting the ratio of hard carbon in the carbon material according to the mixing adjustment amount; a kneading step of kneading the carbon material after the mixing step; and a coating step of coating the carbon material after the kneading step onto a negative electrode foil. [Effects of the Invention]

[0010] According to the negative electrode plate manufacturing method of the present invention, it is possible to manufacture secondary batteries with less lot-to-lot variation in output performance. [Brief explanation of the drawing]

[0011] [Figure 1] This graph illustrates the change in cell voltage caused by the hard carbon mixing ratio in a secondary battery. [Figure 2] This graph illustrates the relationship between the negative electrode potential and the negative electrode's reaction resistance ratio in a secondary battery. [Figure 3] This graph illustrates the relationship between the hard carbon mixing ratio and the negative electrode potential in a secondary battery. [Figure 4] This is a flowchart illustrating the process for manufacturing the negative electrode plate according to Embodiment 1. [Figure 5] This table illustrates an example of variation in crystallinity between batches. [Figure 6] This graph illustrates the relationship between the R value and firing temperature. [Figure 7] This graph illustrates the relationship between firing temperature and negative electrode potential. [Figure 8] This table explains the hard carbon blend adjustment amount for each lot. [Modes for carrying out the invention]

[0012] For clarity of explanation, the following descriptions and drawings have been omitted and simplified as appropriate. Furthermore, each element shown in the drawings as a functional block performing various processes can be composed of a CPU (Central Processing Unit), memory, and other circuits in hardware terms, and implemented in software terms by programs loaded into memory. Therefore, it will be understood by those skilled in the art that these functional blocks can be implemented in various ways using hardware alone, software alone, or a combination thereof, and are not limited to any one of these. In each drawing, the same elements are denoted by the same reference numeral, and redundant explanations have been omitted where necessary.

[0013] Furthermore, the program described above includes, when loaded into a computer, a set of instructions (or software code) for causing the computer to perform one or more of the functions described in the embodiments. The program may be stored in a non-temporary computer-readable medium or a physical storage medium. Examples, but not limited to, include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technologies, CD-ROM, digital versatile disc (DVD), Blu-ray® disc or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage devices. The program may be transmitted over a temporary computer-readable medium or a communication medium. Examples, but not limited to, include temporary computer-readable medium or a communication medium that includes electrically, optically, acoustically, or otherwise propagating signals.

[0014] Embodiment 1 In the method for manufacturing a negative electrode plate described below, a number of arithmetic processes are performed. These arithmetic processes are executed by a program in an arithmetic device such as a computer. By inputting measured values, the mixing adjustment amount of hard carbon is automatically calculated, and the manufacturing of the negative electrode plate is advanced based on this mixing adjustment amount of hard carbon.

[0015] First, the influence of hard carbon in a lithium-ion battery (hereinafter simply referred to as a secondary battery) using a negative electrode in which graphite and hard carbon are mixed will be described. FIG. 1 shows a graph for explaining the change in cell voltage caused by the hard carbon mixing ratio in the secondary battery. The graph shown in FIG. 1 has the charging rate (cell SOC) of the secondary battery on the horizontal axis and the cell voltage, which is the output voltage of the secondary battery, on the vertical axis. Further, the graph in FIG. 1 shows an SOC curve indicating the relationship between the cell SOC and the cell voltage for each content rate of the amount of hard carbon in the negative electrode active material.

[0016] As shown in FIG. 1, in the secondary battery, as the content of hard carbon in the negative electrode active material increases, the amount of decrease in the cell voltage when the charging rate decreases becomes larger. The cell voltage is determined by the difference between the negative electrode potential and the positive electrode potential. And in the verification example shown in FIG. 1, since the positive electrode potential is not changed, it can be considered that the change in the cell voltage is due to the change in the negative electrode potential caused by the hard carbon content.

[0017] Here, the reaction resistance ratio is important for the output performance of the secondary battery. Therefore, FIG. 2 shows a graph for explaining the change in cell voltage caused by the hard carbon mixing ratio in the secondary battery, and the relationship between the negative electrode potential and the reaction resistance ratio will be explained. The graph shown in FIG. 2 has the negative electrode potential on the horizontal axis and the reaction resistance ratio on the vertical axis.

[0018] It can be seen from FIG. 1 that as the cell SOC of the secondary battery decreases, the negative electrode potential increases. And referring to FIG. 2, it can be seen that the reaction resistance ratio increases as the negative electrode potential increases. That is, it can be understood that as the negative electrode potential increases, not only does the cell voltage decrease, but also a decrease in output performance occurs where the reaction resistance ratio increases.

[0019] Figures 1 and 2 show that in secondary batteries, variations in output performance occur due to changes in the negative electrode potential and reaction resistivity caused by the mixing ratio of hard carbon in the negative electrode active material. Therefore, Figure 3 shows a graph illustrating the relationship between the hard carbon mixing ratio and the negative electrode potential in secondary batteries. In Figure 3, the horizontal axis shows the mixing ratio of hard carbon in the negative electrode active material, and the vertical axis shows the negative electrode potential. As shown in Figure 3, it can be seen that the negative electrode potential increases as the mixing ratio of hard carbon in the negative electrode active material increases.

[0020] In the negative electrode plate manufacturing method according to Embodiment 1, variations in output performance between lots are reduced by suppressing variations in crystallinity through adjusting the mixing ratio (or amount) of hard carbon in the negative electrode active material. The negative electrode plate manufacturing method according to Embodiment 1 will be described in detail below. Figure 4 shows a flowchart illustrating the flow of the negative electrode plate manufacturing method according to Embodiment 1. Furthermore, in the following description, the negative electrode plate manufacturing method will be explained in detail with reference to Figures 5 to 8 as appropriate. Note that in Figure 4, hard carbon is denoted as HC.

[0021] As shown in Figure 4, in the negative electrode plate manufacturing method according to Embodiment 1, it is determined at the start of manufacturing the negative electrode plate whether the hard carbon, which is part of the carbon material that will become the negative electrode active material, is from a new lot (step S1). If the hard carbon to be used is from a new lot, the processes in steps S2 and S3 are performed.

[0022] In step S2, the crystallinity of the hard carbon (HC in the figure) is measured. In other words, in the negative electrode plate manufacturing method according to Embodiment 1, step S2 involves a crystallinity measurement process in which the crystallinity of the hard carbon is measured for each lot and the crystallinity measurement value is obtained for each lot.

[0023] In this crystallinity measurement, the R value obtained using X-ray diffraction or Raman spectroscopy for hard carbon is used as the crystallinity measurement. X-ray diffraction or Raman spectroscopy yields a Raman spectrum, which shows peaks in the G band (e.g., I1580) common to graphite, and peaks in the D band (e.g., I1350) resulting from disorder and defects in the graphite structure. The R value is calculated as the ratio of the G band to the D band (I1350 / I1580).

[0024] Here, we will explain an example of lot-to-lot crystallinity variation obtained in the crystallinity measurement process. Figure 5 shows a table illustrating an example of lot-to-lot crystallinity variation. In the example shown in Figure 5, the R value of the carbon material was obtained for each of the five lots, the average value of the five lots was calculated, the average value was set to 100%, and the deviation of the crystallinity variation from the average value was shown as a ratio. As shown in Figure 5, hard carbon exhibits variation in crystallinity from lot to lot.

[0025] Next, in the negative electrode plate manufacturing method according to Embodiment 1, a mixing adjustment amount calculation step is performed as step S3. In the mixing adjustment amount calculation step, a mixing adjustment amount is calculated to adjust the ratio of hard carbon contained in the carbon material to a reference ratio corresponding to the reference value, based on the difference between the crystallinity measurement value (e.g., R value) and the reference value of the crystallinity measurement value (average value of R value in Figure 5).

[0026] Specifically, the variation in crystallinity (variation in R-values) shown in Figure 5 is caused by variations in firing temperature during the firing process performed on carbon materials. In the following explanation, the average value of the R-values ​​in Figure 5 will be used as the reference value for crystallinity measurement. In other words, the calculations described below will use the crystallinity variation value as input. Note that the reference value for crystallinity measurement may be a value predetermined during the design phase of the secondary battery, for example.

[0027] Therefore, Figure 6 shows a graph illustrating the relationship between the R value and the firing temperature. In the graph shown in Figure 6, the horizontal axis shows the ratio of the R value, and the vertical axis shows the firing temperature in the firing process performed on the carbon material. In Figure 6, the firing temperature is shown on the vertical axis as the temperature difference from the reference firing temperature Ref. As shown in Figure 6, it can be seen that there is a relationship between the R value and the firing temperature that can be expressed by a linear function. Then, by applying the amount of crystallinity variation shown in Figure 5 to the linear function derived from Figure 6, it is possible to calculate the firing temperature unevenness amount, which indicates the magnitude of the unevenness in firing temperature corresponding to the amount of deviation in the R value. For example, in the example shown in Figure 6, if there is a deviation of 4.4% in the R value (for example, lot number 4 in Figure 4), a value of approximately 38.9°C is calculated as the firing temperature unevenness amount.

[0028] Furthermore, the firing temperature is correlated with the negative electrode potential of the negative electrode plate. Therefore, Figure 7 shows a graph illustrating the relationship between firing temperature and negative electrode potential. In the graph shown in Figure 7, the horizontal axis shows the firing temperature (temperature difference relative to the reference firing temperature Ref), and the vertical axis shows the negative electrode potential. As shown in Figure 7, there is a relationship between the negative electrode potential and the firing temperature that can be expressed as a linear function, where the negative electrode potential increases as the firing temperature decreases. Therefore, in the mixing amount adjustment calculation process, the linear function shown in Figure 7 is used as the second equation, and by applying the firing temperature unevenness amount calculated from the first equation to this second equation, the negative electrode potential deviation amount, which is the deviation amount of the negative electrode potential corresponding to the firing temperature unevenness amount, can be calculated.

[0029] Here, there is a relationship between the negative electrode potential and the hard carbon ratio in the carbon material, as shown in Figure 3. As shown in Figure 3, there is a relationship between the negative electrode potential and the hard carbon ratio in the carbon material that can be expressed by a linear function. Therefore, in the mixing adjustment amount calculation process, the relationship shown in Figure 3 is used as the third equation, and the amount of negative electrode potential shift is applied to the third equation to calculate the mixing adjustment amount, which is the adjustment amount for the hard carbon ratio in the carbon material.

[0030] In other words, the mixing adjustment amount calculation process calculates the mixing adjustment amount by performing three calculation processes: a first calculation process, a second calculation process, and a third calculation process. In the first calculation process, the firing temperature unevenness amount, which indicates the difference in firing temperature corresponding to the difference between the crystallinity measurement value and the reference value (for example, the crystallinity variation in Figure 5), is calculated in the first equation, which represents the relationship between the crystallinity measurement value (for example, the R value) and the firing temperature of the carbon material. In the second calculation process, the firing temperature unevenness amount is applied to the second equation, which represents the relationship between the firing temperature and the negative electrode potential of the negative electrode plate, to calculate the negative electrode potential deviation amount, which indicates the deviation in the negative electrode potential corresponding to the firing temperature unevenness amount. In the third calculation process, the negative electrode potential deviation amount is applied to the third equation, which represents the relationship between the negative electrode potential and the hard carbon ratio in the carbon material, to calculate the mixing adjustment amount, which is the adjustment amount for the hard carbon ratio in the carbon material.

[0031] Here, we will explain an example of the mixing adjustment amount calculated for the five lots shown in Figure 5. Figure 8 shows a table illustrating the hard carbon mixing adjustment amount for each lot. The table in Figure 8 shows the results of performing the crystallinity measurement process in step S2 and the mixing adjustment amount calculation process in step S3 for the five lots shown in Figure 5. Taking the first lot shown in Figure 8 as an example, applying the crystallinity variation of 1.6% to the first equation derived from the graph in Figure 6 yields a firing temperature unevenness of 14.6°C. Then, using this firing temperature unevenness as the second equation derived from the graph in Figure 7, a negative electrode potential shift of -0.009V is calculated. Furthermore, applying this negative electrode potential shift to the third equation derived from the graph in Figure 3 yields a mixing adjustment amount of 2.2% increase in the hard carbon ratio.

[0032] Furthermore, referring to Figure 2, for hard carbon lots for which the mixing adjustment amount has already been calculated, the calculated mixing adjustment amount is read from memory or the like (step S4), and the processes from step S5 onward are carried out.

[0033] As shown in Figure 2, in the negative electrode plate manufacturing method according to Embodiment 1, after step S3 or step S4 is completed, the negative electrode plate is manufactured by carrying out a mixing step (step S5), a kneading step (step S6), a coating step, a pressing step, and a slitting step (step S7).

[0034] In the mixing step (step S5), the hard carbon ratio in the carbon material is adjusted according to the mixing adjustment amount obtained in step S3 or step S4. In the kneading step (step S6), the carbon material after the mixing step is kneaded to produce a slurry. In the coating step, the slurry after the kneading step is coated onto the negative electrode foil. In the pressing step, the slurry on the negative electrode foil after the coating step is pressed. In the slitting step, the negative electrode foil after the pressing step is cut into predetermined lengths to form negative electrode plates.

[0035] As described above, in the negative electrode plate manufacturing method according to Embodiment 1, a slurry is generated and coated onto the negative electrode plate after eliminating the deviation in negative electrode potential caused by variations in the crystallinity of hard carbon that occur from lot to lot due to uneven firing temperature of the material, based on the mixing adjustment amount calculated in the crystallinity measurement step and the mixing adjustment amount calculation step. As a result, the negative electrode plate manufactured using the negative electrode plate manufacturing method according to Embodiment 1 has a uniform ratio of hard carbon contained in the active material from lot to lot, thus suppressing variations in output characteristics. In this way, by suppressing variations in output characteristics between lots, it is possible not only to improve the productivity of secondary batteries but also to reduce the margin set when using secondary batteries, thereby improving the actual battery performance.

[0036] It should be noted that the present invention is not limited to the embodiments described above, and can be modified as appropriate without departing from the spirit of the invention.

Claims

1. A crystallinity measurement step involves measuring the crystallinity of a carbon material containing hard carbon and graphite for each lot of the hard carbon, and obtaining the crystallinity measurement value of the hard carbon for each lot. A mixing adjustment amount calculation step, which calculates a mixing adjustment amount to adjust the ratio of hard carbon contained in the carbon material to a reference ratio corresponding to the reference value, based on the difference between the crystallinity measurement value and the reference value of the crystallinity measurement value, A mixing step to adjust the hard carbon ratio in the carbon material according to the aforementioned mixing adjustment amount, A kneading step to knead the carbon material after the mixing step to produce a slurry, A coating step in which the slurry after the kneading step is coated onto the negative electrode foil, A method for manufacturing a negative electrode plate containing [a specific component].

2. The method for manufacturing a negative electrode plate according to claim 1, wherein the crystallinity measurement step is performed by obtaining the crystallinity measurement value using X-ray diffraction or Raman spectroscopy.

3. The method for manufacturing a negative electrode plate according to claim 2, wherein the R value calculated based on the intensity ratio of the Raman band in the X-ray diffraction method or Raman spectroscopy method is used as the crystallinity measurement value.

4. In the above mixing adjustment amount calculation step, A first calculation process is used to calculate the amount of firing temperature unevenness, which indicates the difference in firing temperature corresponding to the difference between the crystallinity measurement value and the reference value, in the first equation representing the relationship between the crystallinity measurement value and the firing temperature of the hard carbon. A second calculation process involves applying the firing temperature variation amount to a second equation representing the relationship between the firing temperature and the negative electrode potential of the negative electrode plate, and calculating a negative electrode potential deviation amount that indicates the deviation amount of the negative electrode potential corresponding to the firing temperature variation amount. A third calculation process involves applying the negative electrode potential deviation amount to a third equation representing the relationship between the negative electrode potential and the hard carbon ratio in the carbon material to calculate the mixing adjustment amount, which is an adjustment of the hard carbon ratio in the carbon material. A method for manufacturing a negative electrode plate according to claim 1, wherein the method is carried out as described in claim 1.

5. The method for manufacturing a negative electrode plate according to claim 1, wherein the reference value is the average value of the crystallinity measurements of a plurality of lots.

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