Device for manufacturing negative electrode for secondary batteries

Abstract

The present invention relates to an apparatus for manufacturing a negative electrode for a secondary battery and a method for manufacturing a negative electrode using the same. The apparatus for manufacturing a negative electrode includes an upper block, a middle block, and a lower block, a double slot die including a first slot and a second slot for discharging a negative electrode slurry through the gaps between the upper block and the middle block and between the middle block and the lower block, and a coating roll, and the upper lip, the middle lip, and the lower lip located at the tip of each block apply a magnetic field of the same polarity, and the coating roll applies a magnetic field of the opposite polarity to each lip. As a result, the carbon-based negative electrode active material in the discharged negative electrode slurry can be applied in a state in which the crystal plane is oriented nearly perpendicular to the surface of the negative electrode current collector. Therefore, the manufactured negative electrode has an advantage that not only the carbon-based negative electrode active material has an excellent degree of orientation, but also that it is easy to apply to a mass production process.

Description

[Technical Field]

[0001] The present invention relates to an apparatus for manufacturing a negative electrode for use in a secondary battery and a method for manufacturing a negative electrode for a secondary battery using the same.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0175563, dated December 15, 2022, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference. [Background technology]

[0003] BACKGROUND ART In recent years, secondary batteries have been widely used not only in small devices such as portable electronic devices but also in medium to large devices such as battery packs for hybrid cars and electric cars or power storage devices.

[0004] Such a secondary battery is a chargeable and dischargeable power generating element having a laminated structure of a positive electrode / separator / negative electrode. Generally, the positive electrode contains a lithium metal oxide as a positive electrode active material, and the negative electrode contains a carbon-based negative electrode active material such as graphite. During charging of the secondary battery, lithium ions released from the positive electrode are absorbed into the carbon-based negative electrode active material of the negative electrode. During discharge, the lithium ions contained in the carbon-based negative electrode active material are absorbed into the lithium metal oxide of the positive electrode, allowing for repeated charging and discharging.

[0005] Examples of the negative electrode active material used in the negative electrode include graphite materials such as natural graphite and artificial graphite. Such graphite has a layered structure, in which carbon atoms form a network structure and many planar layers are stacked. During charging, lithium ions penetrate through the edge surfaces (surfaces where layers overlap) of such graphite layers and diffuse between the layers. During discharging, lithium ions can be desorbed and released from the edge surfaces of the layers. Furthermore, since the electrical resistivity of graphite in the plane direction of the layers is lower than that in the stacking direction of the layers, a detour electron conduction path is formed along the plane direction of the layers.

[0006] In response to this issue, a technique for magnetically orienting graphite contained in a negative electrode has been proposed to improve the charging performance of conventional graphite-based lithium secondary batteries. Specifically, the (002) crystal plane of the graphite is oriented in a magnetic field during the formation of the negative electrode, and then fixed in place. In this case, the edge plane of the graphite faces the positive electrode active layer, facilitating the insertion and desorption of lithium ions. At the same time, the electron conduction path is shortened, improving the electronic conductivity of the negative electrode and thereby improving the charging performance of the battery.

[0007] This graphite orientation can be induced by applying a magnetic field to an undried negative electrode slurry containing graphite coated on a negative electrode current collector. However, considering the production speed during actual mass production of negative electrodes, it is difficult to ensure a sufficient magnetic field application time to induce graphite alignment in the negative electrode slurry. Furthermore, if the strength of the magnetic field applied to the negative electrode slurry coated on the electrode sheet is increased to solve this problem, the graphite is pulled by the magnetic field at the end of the magnetic field, which disrupts the orientation of the graphite aligned perpendicular to the metal sheet, resulting in a limited degree of graphite orientation in the final negative electrode active layer.

[0008] Therefore, there is a need for a negative electrode manufacturing technology that can realize a high degree of graphite orientation and is applicable to mass production of negative electrodes for secondary batteries. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 10-2018-0048131 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a negative electrode manufacturing technology that can realize a high degree of crystalline orientation of a carbon-based negative electrode active material such as graphite contained in a negative electrode active layer and that can be applied to mass production of negative electrodes for secondary batteries. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the present invention provides, in one embodiment, An apparatus for manufacturing an electrode sheet coated with a negative electrode slurry containing a carbon-based negative electrode active material, a double slot die including an upper block, an intermediate block, and a lower block, a first slot provided in the gap between the upper block and the intermediate block and for discharging a first negative electrode slurry, and a second slot provided in the gap between the intermediate block and the lower block and for discharging a second negative electrode slurry; a coating roll that faces the first slot and the second slot of the double slot die and transports the electrode sheet coated with the negative electrode slurry discharged from each slot, the upper block, the intermediate block and the lower block each have an upper lip, an intermediate lip and a lower lip forming a discharge port at a tip thereof, and the upper lip, the intermediate lip and the lower lip exhibit magnetism of the same polarity; The coating roll provides an apparatus for manufacturing a negative electrode for a secondary battery, which exhibits a magnetic polarity opposite to that of the upper lip.

[0012] In this case, the upper lip and the lower lip may have a structure including at least one of a permanent magnet and an electromagnet, thereby applying a magnetic field having a strength of 500 G to 3,000 G when discharging the negative electrode slurry.

[0013] Also, the coating roll may satisfy the following formula 1:

[0014] [Formula 1] G lip / G roll ≦1

[0015] In Equation 1, G lip represents the strength of the magnetic field (unit: G) applied to the upper lip, middle lip, and lower lip, G roll represents the strength of the magnetic field (unit: G) applied to the coating roll.

[0016] The negative electrode manufacturing apparatus may further include a drying unit that dries the negative electrode slurry applied to the electrode sheet.

[0017] The drying unit may be disposed at a position where the negative electrode slurry discharged from the slot die can reach within 20 seconds from the time when the negative electrode slurry is applied to the electrode sheet.

[0018] In one embodiment, the present invention provides: The method includes applying the negative electrode slurry to which the magnetic field is applied onto an electrode sheet using the above-described negative electrode manufacturing apparatus according to the present invention, The present invention provides a method for producing a negative electrode for a secondary battery, wherein the negative electrode slurry contains a carbon-based negative electrode active material.

[0019] In this case, the average thickness of the negative electrode slurry applied to the electrode sheet may be greater than the distance between the slot of a slot die provided in the negative electrode manufacturing apparatus and the coating roll, and the average thickness of the negative electrode slurry applied to the electrode sheet may be 100 μm or more.

[0020] The step of applying the negative electrode slurry to the electrode sheet may be carried out at a speed of 5 to 100 m / min.

[0021] The method may further include, after the step of applying the negative electrode slurry to the electrode sheet, drying the applied negative electrode slurry to form a negative electrode active layer.

[0022] Here, the negative electrode active layer may have an alignment index (OI) of the carbon-based negative electrode active material relative to the surface of the electrode sheet, represented by the following formula 2, of 0.9 or less:

[0023] [Formula 2] OI=I 004 / I 110

[0024] In Equation 2, I 004 represents the area of ​​the peak representing the (004) crystal plane when measuring the negative electrode active layer by X-ray diffraction spectroscopy (XRD), I 110 represents the area of ​​the peak indicating the (110) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD). [Effects of the Invention]

[0025] The apparatus for manufacturing a negative electrode for a secondary battery according to the present invention can apply a carbon-based negative electrode active material in a discharged negative electrode slurry in a state where the carbon-based negative electrode active material is oriented nearly perpendicular to the surface of the negative electrode current collector, thereby providing the advantages of a manufactured negative electrode having an excellent degree of orientation of the carbon-based negative electrode active material and being easily applicable to mass production processes. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a cross-sectional view schematically showing an apparatus for manufacturing a negative electrode for a secondary battery according to the present invention. [Figure 2] 1 is a perspective view showing the structure of a double die coater provided in an apparatus for manufacturing a negative electrode for a secondary battery according to the present invention. [Figure 3] 3 is a conceptual diagram illustrating the principle of applying a carbon-based negative electrode active material of a negative electrode slurry to an electrode sheet in a magnetically aligned state using a dual die coater according to the present invention. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0027] Because the present invention is susceptible to various modifications and can have various embodiments, specific embodiments are described in detail in the detailed description.

[0028] However, this is not intended to limit the invention to any particular embodiment, but rather to be understood as including all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

[0029] In the present invention, terms such as "comprise" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts or combinations thereof described in the specification, and may be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0030] Furthermore, in the present invention, when a layer, film, region, plate, or other portion is described as being "on" another portion, this includes not only the case where it is "directly on" the other portion, but also the case where there is another portion therebetween. Conversely, when a layer, film, region, plate, or other portion is described as being "under" the other portion, this includes not only the case where it is "directly under" the other portion, but also the case where there is another portion therebetween. Furthermore, in this application, being "located on" can include not only the case where it is located at the top, but also the case where it is located at the bottom.

[0031] Furthermore, in the present invention, "comprising as a main component" can mean that the defined component is contained in an amount of 50 wt% or more (or 50 vol% or more), 60 wt% or more (or 60 vol% or more), 70 wt% or more (or 70 vol% or more), 80 wt% or more (or 80 vol% or more), 90 wt% or more (or 90 vol% or more), or 95 wt% or more (or 95 vol% or more) based on the total weight (or total volume). For example, "comprising graphite as a main component as a negative electrode active material" can mean that graphite is contained in an amount of 50 wt% or more, 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, or 95 wt% or more based on the total weight of the negative electrode active material. In some cases, it can mean that the entire negative electrode active material is composed of graphite, with graphite accounting for 100 wt%.

[0032] In addition, in the present invention, the "lip portion" refers to a region including the upper lip of the upper block, the middle lip of the middle block, and the lower lip of the lower block, and may include the space formed between the lips, i.e., the region where each lip is disposed by the first slot and the second slot. Similarly, in the present invention, the "main body portion" refers to a region including the upper body of the upper block, the middle body of the middle block, and the lower body of the lower block, and may include the space formed between the bodies, i.e., the region where each body is disposed by the first slot and the second slot.

[0033] The present invention will now be described in more detail.

[0034] <Secondary battery negative electrode manufacturing equipment>

[0035] In one embodiment, the present invention comprises: An apparatus for manufacturing an electrode sheet coated with a negative electrode slurry containing a carbon-based negative electrode active material, a double slot die including an upper block, an intermediate block, and a lower block, a first slot provided in the gap between the upper block and the intermediate block and for discharging a first negative electrode slurry, and a second slot provided in the gap between the intermediate block and the lower block and for discharging a second negative electrode slurry; a coating roll that faces the first slot and the second slot of the double slot die and transports the electrode sheet coated with the negative electrode slurry discharged from each slot, the upper block, the intermediate block and the lower block each have an upper lip, an intermediate lip and a lower lip forming a discharge port at a tip thereof, and the upper lip, the intermediate lip and the lower lip exhibit magnetism of the same polarity; The coating roll exhibits a magnetic polarity opposite to that of the upper lip, thereby providing an apparatus for manufacturing a negative electrode for a secondary battery.

[0036] The secondary battery negative electrode manufacturing apparatus according to the present invention is an apparatus that is applied when manufacturing a negative electrode to be used in a secondary battery, and has a configuration in which a negative electrode slurry containing a carbon-based negative electrode active material is applied onto a negative electrode current collector to manufacture an electrode sheet.

[0037] Specifically, Fig. 1 is a cross-sectional view showing the structure of an apparatus for manufacturing a secondary battery anode according to the present invention. Referring to Fig. 1, the apparatus for manufacturing anode (10) comprises a double slot die (100) and a coating roll (200). The coating roll (200) rotates to move an electrode sheet (300) and apply anode slurry to the surface of the electrode sheet (300).

[0038] The double slot die 100 includes a first slot 151 and a second slot 152, and can coat two types of negative electrode slurries, which may be the same or different, onto the negative electrode current collector in a double manner. As shown in FIG. 2 , the first slot 151 can be formed between the upper die 110 and the middle die 120, and the second slot 152 can be formed between the middle die 120 and the lower die 130, respectively.

[0039] For example, the double slot die 100 may have first spacers 141 and second spacers 142 sequentially interposed between the upper die 110, the middle die 120, and the lower die 130, respectively, to form gaps therebetween, thereby forming passages, i.e., slots 151 and 152, through which the anode slurry can flow. In this case, the vertical height of the slots may be determined by the thickness (Y-axis direction) of the first spacer 141 and / or second spacer 142, which form the gaps between the dies.

[0040] The thickness of the first spacer 141 and / or the second spacer 142 can be controlled depending on the application and capacity of the secondary battery. Specifically, the thickness of each spacer 141 and 142 can be independently 50 μm to 2,000 μm, more specifically, 50 μm to 1,500 μm, 500 μm to 1,200 μm, 800 μm to 1,200 μm, 80 μm to 200 μm, 200 μm to 500 μm, or 400 μm to 700 μm.

[0041] In addition, the first spacer 141 and / or the second spacer 142 may be cut open in one region to form an opening, and may be interposed in the remaining edge regions of the opposing surfaces of the upper die 110, the intermediate die 120, and the lower die 130, excluding one side. As a result, discharge ports through which the negative electrode slurry can be discharged to the outside, i.e., the first slot 151 and the second slot 152, are formed only between the leading ends of the upper die 110, the intermediate die 120, and the lower die 130. Here, the leading ends of the upper die 110, the intermediate die 120, and the lower die 130 include an upper lip 111, a middle lip 121, and a lower lip 131, and the slots 151 and 152 through which the negative electrode slurry is discharged can be said to be locations formed by separating the lips 111, 121, and 131.

[0042] In addition, the first spacer 141 and the second spacer 142 also function as gaskets to prevent the negative electrode slurry from leaking from the gaps between the dies 110, 120, and 130, except for the areas where the first slot 151 and the second slot 152 are formed, so they are preferably made of a material with sealing properties.

[0043] Meanwhile, the upper die 110, the middle die 120, and the lower die 130 may include an upper lip 111, a middle lip 121, and a lower lip 131 located at each end, as well as an upper body 112, a middle body 122, and a lower body 132 extending from each lip and closely contacting a first spacer 141 and / or a second spacer 142 to form a passage through which the anode slurry can fluidly move.

[0044] Additionally, the upper lip 111, the middle lip 121, and the lower lip 131 exhibit magnetism. Specifically, the upper lip 111, the middle lip 121, and the lower lip 131 may exhibit magnetism having the same polarity. As a result, a magnetic field may be formed in the first slot 151 and the second slot 152 formed by the lips 111, 121, and 131 in the same direction as the direction in which the negative electrode slurry is discharged.

[0045] As shown in FIG. 3, the path along which the negative electrode slurry is discharged and applied can be divided into a section A where the carbon-based negative electrode active material CM (e.g., graphite) is not oriented and a section B where it is oriented. The carbon-based negative electrode active material CM of the negative electrode slurry is not affected by magnetism in the non-oriented section A, which includes the space between the main bodies 112, 122, and 132 of each die (i.e., section I: main body), and thus moves with high freedom of movement. However, the orientation section B includes the space between the upper lip 111, middle lip 121, and lower lip 131 (i.e., section II: lip), which exhibit the same polarity of magnetism. In the lip section, a repulsive force acts, forming a magnetic field direction MD in the same direction as the direction along which the negative electrode slurry is discharged. Therefore, before the negative electrode slurry is applied to the negative electrode current collector, the carbon-based negative electrode active material CM, i.e., graphite, contained in the negative electrode slurry can be oriented so that its (002) crystal plane is approximately perpendicular to the negative electrode current collector. The oriented graphite is applied to the negative electrode current collector while maintaining its oriented state (i.e., corresponds to the III: coating region), and therefore can have a high degree of orientation without the application of a separate magnetic field after application of the negative electrode slurry.

[0046] The upper lip 111, the middle lip 121, and the lower lip 131 may include electromagnets and / or permanent magnets to exhibit magnetism. The electromagnets may include both DC and AC electromagnets. The permanent magnets may include both ferromagnetic and soft magnetic magnets, such as NdFeB magnets, SmCo magnets, ferrite magnets, alnico magnets, FeCrCo magnets, and bond magnets (Nd-Fe-B, Sm-Fe-N, Sm-Co, and ferrite magnets).

[0047] Furthermore, since the negative electrode manufacturing apparatus applies a magnetic field before the negative electrode slurry is applied to the electrode sheet, a magnetic field with a weaker intensity can be applied compared to when the magnetic field is applied after the negative electrode slurry is applied to the electrode sheet. In this case, the magnetic field intensity can be within a predetermined range. In other words, a magnetic field that satisfies a predetermined intensity range can be applied to the upper lip 111, the middle lip 121, and the lower lip 131 when the negative electrode slurry is discharged. Specifically, the same magnetic field can be applied to the upper lip 111, the middle lip 121, and the lower lip 131. The strength of the applied magnetic field may be 500G to 3,000G (Gauss), more specifically 500G to 2,500G, 500G to 2,000G, 500G to 1,500G, 1,000G to 2,000G, 2,000G to 2,500G, 500G to 1,500G, or 500G to 900G.

[0048] By adjusting the strength of the magnetic field applied to the upper lip 111, the middle lip 121, and the lower lip 131 within the above range, the present invention can achieve uniform crystalline orientation of the carbon-based negative electrode active material before the negative electrode slurry is discharged through the first slot 151 and the second slot 152. Furthermore, the present invention can maintain the orientation of the uniformly crystalline oriented carbon-based negative electrode active material after discharge so that it can be applied to the negative electrode current collector without any change in orientation. Furthermore, the present invention can prevent the strength of the magnetic field applied to the upper lip 111, the middle lip 121, and the lower lip 131 from exceeding the above range, which would narrow the separation distance between the lips and make it difficult to discharge the negative electrode slurry.

[0049] The first slot 151 and the second slot 152 may be positioned so that the negative electrode slurry is discharged perpendicular to the surface of the negative electrode current collector. To this end, the double slot die 100 may be positioned so that it is perpendicular to the surface of the electrode sheet 300. Preferably, as shown in FIG. 1 , the negative electrode manufacturing apparatus of the present invention has a rotatable coating roll 200 disposed opposite the first slot 151 and the second slot 152 of the double slot die 100, and the negative electrode slurry may be applied to the surface of the electrode sheet 300 (or the negative electrode current collector) as it moves due to the rotation of the coating roll 200.

[0050] Furthermore, the coating roll 200 may exhibit magnetism on its surface so that the carbon-based negative electrode active material of the negative electrode slurry applied to the electrode sheet 300 can maintain an oriented state inside the double slot die 100. In this case, the magnetism may have a polarity different from that of the lips 111, 121, and 131 of the double slot die 100. By imparting magnetism having a polarity different from that of the lips 111, 121, and 131 of the double slot die 100 to the surface of the coating roll 200, the present invention can prevent the crystal orientation of the carbon-based negative electrode active material applied to the surface of the electrode sheet 300 from being reduced. Such magnetism may be implemented by incorporating at least one of a permanent magnet and an electromagnet on the surface of the coating roll 200.

[0051] In addition, the magnetic field applied to the coating roll 200 may be stronger than the magnetic field strength applied at the lip portions 111, 121, and 131 of the double slot die 100 so that the crystalline orientation of the carbon-based negative electrode active material in the negative electrode slurry can be coated onto the electrode sheet 300 (or the negative electrode current collector) without changing. As an example, the coating roll 200 may satisfy the following formula 1:

[0052] [Formula 1] G lip / G roll ≦1

[0053] In Equation 1, G lip represents the strength of the magnetic field (unit: G) applied to the upper lip, middle lip, and lower lip, G roll represents the strength of the magnetic field (unit: G) applied to the coating roll.

[0054] If a strong magnetic field is applied to the lips of the double slot die 100, the repulsive force between the upper lip 111, the middle lip 121, and the lower lip 131 will be strong, resulting in uneven slot heights. Therefore, the magnetic field applied to the lips of the double slot die 100 is controlled to satisfy a predetermined range, as described above. However, in this case, the crystal orientation of the carbon-based negative electrode active material, which is oriented depending on the fluid momentum and / or direction of the negative electrode slurry when it is discharged, may change. Therefore, the negative electrode manufacturing apparatus of the present invention is configured to apply a magnetic field with a polarity opposite to that applied to the lips and with a strength greater than that of the magnetic field applied to the lips to the coating roll 200 disposed on the other side of the electrode sheet 300 (or negative electrode current collector) to which the negative electrode slurry is applied. This allows the carbon-based negative electrode active material to be applied with its crystal plane nearly perpendicular to the surface of the electrode sheet 300 (or negative electrode current collector). In this case, the coating roll 200 adjusts the above formula 1 to be 1 or less (i.e., G lip / G rol ≦1), specifically 0.1 to 1 (i.e., 0.1≦G lip / G roll ≦1), 0.2~0.9 (i.e., 0.2≦G lip / G roll ≦0.9), 0.4~0.9 (i.e., 0.4≦G lip / G roll ≦0.9), 0.3~0.6 (i.e., 0.3≦G lip / G roll ≦0.6), 0.5~0.9 (i.e., 0.5≦G lip / G roll ≦0.9), 0.51~0.99 (i.e., 0.51≦G lip / G roll ≦0.99), 0.70~0.99 (i.e., 0.70≦G lip / G roll ≦0.99), 0.90~0.99 (i.e., 0.90≦G lip / G roll ≦0.99), 0.6~0.8 (i.e., 0.6≦G lip / G roll ≦0.8), or 0.45 to 0.75 (i.e., 0.45≦G lip / Groll ≦75).

[0055] By providing a coating roll 200 that satisfies Equation 1 within the above-mentioned range, the present invention can prevent the crystal plane of the carbon-based negative electrode active material from being oriented at a low angle relative to the surface of the electrode sheet 300 (negative electrode current collector) due to a slight magnetic field applied by the coating roll 200, while also preventing a decrease in productivity during negative electrode production due to an excessive magnetic field applied.

[0056] Furthermore, the negative electrode manufacturing apparatus 10 according to the present invention may further include a drying unit (not shown) that dries the negative electrode slurry applied to the electrode sheet 300. The drying unit may be disposed at a position where it can reach a drying time within 20 seconds, specifically 0.01 to 20 seconds, 0.01 to 15 seconds, 0.01 to 10 seconds, or 0.01 to 5 seconds, from the time when the negative electrode slurry discharged from the slot die 100 is applied to the electrode sheet.

[0057] The drying unit functions to remove the solvent contained in the negative electrode slurry to form a negative electrode active layer and also to fix the oriented carbon-based negative electrode active material within the negative electrode slurry. To this end, the drying unit may be disposed at a position where the negative electrode slurry can be dried before the crystalline orientation of the carbon-based negative electrode active material coated on the surface of the electrode sheet 300 is reduced.

[0058] The drying unit is also formed to include a wall (not shown) that blocks off the periphery except for an entrance / exit through which the electrode sheet 300 coated with the negative electrode slurry is introduced and removed, and a dryer (not shown) for drying the electrode sheet on the wall on the side through which the electrode sheet 300 coated with the negative electrode slurry is removed.

[0059] When the electrode sheet 300 coated with the negative electrode slurry enters the drying unit through an inlet, energy such as light, heat, or the like supplied from the opposite wall is transferred to the electrode sheet 300. Therefore, the wall is preferably made of a heat insulating material to prevent internal energy from being transferred to the outside and causing heat loss.

[0060] The dryer can apply energy such as light, wavelength, or heat and can be any dryer commonly used in the art, without any particular limitation. For example, the dryer can be an ultraviolet dryer, a near-infrared dryer, a far-infrared dryer, a hot air dryer, a vacuum oven, or the like, which can be used alone or in combination.

[0061] The negative electrode manufacturing apparatus according to the present invention, having the above-described configuration, induces crystalline orientation of the carbon-based negative electrode active material before the negative electrode slurry is dispensed onto the negative electrode sheet, and the carbon-based negative electrode active material can be applied in a state oriented nearly perpendicular to the surface of the negative electrode current collector, thereby resulting in an excellent degree of orientation of the carbon-based negative electrode active material in the manufactured negative electrode. Furthermore, since a separate orientation process of the carbon-based negative electrode active material is not required after the application of the negative electrode slurry, there are advantages in that additional facilities for negative electrode manufacturing are not required and that it is easily applicable to mass production processes.

[0062] <Method of manufacturing negative electrode for secondary battery>

[0063] In one embodiment, the present invention provides: The method includes applying a negative electrode slurry to an electrode sheet while applying a magnetic field thereto using the negative electrode manufacturing apparatus according to the present invention; The negative electrode slurry contains a carbon-based negative electrode active material, and the negative electrode slurry contains a carbon-based negative electrode active material.

[0064] The method for manufacturing a negative electrode for a secondary battery according to the present invention refers to a method for manufacturing a negative electrode using the above-described negative electrode manufacturing apparatus according to the present invention. The method for manufacturing a negative electrode includes applying a negative electrode slurry to an electrode sheet while applying a magnetic field. That is, the method is characterized in that the application of a magnetic field to a negative electrode slurry containing a carbon-based negative electrode active material and the application of the negative electrode slurry to an electrode sheet (i.e., a negative electrode current collector) are performed simultaneously. As a result, the method does not require separate crystal orientation of the carbon-based negative electrode active material after application of the negative electrode slurry, and therefore has the advantages of being economical due to its simple process and easy application to mass production due to its excellent processability.

[0065] Here, the negative electrode slurry may contain a carbon-based negative electrode active material that is magnetically oriented by the negative electrode manufacturing apparatus according to the present invention as described above, and the magnetic orientation may be performed by forming a magnetic field perpendicular to the surface of the electrode sheet before the negative electrode slurry is discharged onto the electrode sheet.

[0066] Furthermore, the manufacturing method may control the distance between the coating roll and the slot of a slot die, i.e., the lip portion including an upper lip, a middle lip, and a lower lip, of a slot die provided in an anode manufacturing apparatus, depending on the average thickness of the anode slurry to be applied to the electrode sheet so that the crystalline orientation of the carbon-based anode active material is maintained. Even if the carbon-based anode active material is oriented before discharging so that a specific crystal plane is at a predetermined angle relative to the surface of the electrode sheet, the angle or degree of orientation may change due to fluid motion corresponding to the angle at which the anode slurry is applied to the surface of the electrode sheet during discharging. Therefore, in the present invention, in order to minimize the amount of fluid momentum corresponding to the angle at which the anode slurry is applied during application of the anode slurry, the average thickness of the anode slurry applied to the electrode sheet may be adjusted to be greater than the distance between the lip portion of a slot die provided in an anode manufacturing apparatus and the coating roll. For example, the average thickness of the negative electrode slurry applied to the electrode sheet can be adjusted to 1.01 to 2.00 times, 1.01 to 1.80 times, 1.01 to 1.50 times, 1.1 to 1.5 times, or 1.2 to 1.9 times the distance between the lip of a slot die and a coating roll provided in the negative electrode manufacturing apparatus.

[0067] Furthermore, the step of applying the negative electrode slurry to the electrode sheet while the magnetic field is applied may be performed on the electrode sheet being transported at a predetermined speed. The transport speed may be the same as the speed at which the negative electrode slurry is applied to the electrode sheet. For example, the step of applying the negative electrode slurry to the electrode sheet may be performed at a speed of 5 to 100 m / min, specifically 5 to 35 m / min, 10 to 35 m / min, 20 to 35 m / min, 10 to 30 m / min, 5 to 20 m / min, 5 to 25 m / min, or 15 to 25 m / min.

[0068] By performing the step of applying the negative electrode slurry within the above speed range, the present invention can prevent the negative electrode slurry from being unevenly applied on the surface of the electrode sheet due to an extremely slow application speed, while preventing a decrease in process efficiency and productivity. Furthermore, the present invention can prevent a decrease in the degree of orientation due to the disruption of the crystalline alignment of the carbon-based negative electrode active material present in the negative electrode slurry due to an excessively fast application speed.

[0069] In addition, the method for manufacturing a negative electrode for a secondary battery according to the present invention may further include, after the step of applying the negative electrode slurry to the electrode sheet while the magnetic field is applied, a step of drying the applied negative electrode slurry to form a negative electrode active layer.

[0070] The step of forming the negative electrode active layer refers to a process of fixing the carbon-based negative electrode active material contained in the negative electrode slurry onto the electrode sheet by drying the negative electrode slurry. This step may be applied without any particular limitation as long as it can fix the carbon-based negative electrode active material of the negative electrode slurry applied to the electrode sheet while minimizing loss of its orientation relative to the surface of the electrode sheet.

[0071] Specifically, the drying may be performed by applying energy such as light, wavelength, or heat, and may be performed using an ultraviolet dryer, a near-infrared dryer, a far-infrared dryer, a hot air dryer, a vacuum oven, or the like, either alone or in combination.

[0072] Meanwhile, the carbon-based negative electrode active material contained in the negative electrode slurry may include a carbon-based negative electrode active material commonly used in lithium secondary batteries. Specifically, the carbon-based negative electrode active material may refer to a material primarily composed of carbon atoms. Such a carbon-based negative electrode active material may include graphite. The graphite may include at least one of natural graphite and artificial graphite, and preferably includes natural graphite or a mixture of natural graphite and artificial graphite.

[0073] The carbon-based negative electrode active material is preferably a spherical graphite granule formed by the aggregation of multiple flake graphite particles. Examples of flake graphite include natural graphite, artificial graphite, mesophase calcined carbon (bulk mesophase) made from tar or pitch, and graphitized cokes (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.). In particular, graphite granules assembled using multiple highly crystalline natural graphite particles are preferred. Furthermore, one graphite granule may be formed by the aggregation of 2 to 100, preferably 3 to 20, flake graphite particles.

[0074] Such a carbon-based negative electrode active material, specifically graphite, may have a spherical particle shape. The sphericity of the graphite particles may be 0.75 or greater, e.g., 0.75 to 1.0, 0.75 to 0.95, 0.8 to 0.95, or 0.90 to 0.99. Here, "sphericity" refers to the ratio of the shortest diameter (minor axis) to the longest diameter (major axis) among any diameters passing through the center of a particle. A sphericity of 1 indicates that the particle shape is spherical. The sphericity may be measured using a particle shape analyzer. The present invention achieves a nearly spherical shape of the carbon-based negative electrode active material, thereby enhancing the electrical conductivity of the negative electrode active layer and improving the capacity of secondary batteries. Furthermore, the present invention advantageously increases the specific surface area of ​​the negative electrode active material, thereby improving the adhesion between the negative electrode active layer and the current collector.

[0075] The carbon-based negative electrode active material has an average particle size (D 50 ), specifically, an average particle size (D 50 ) can be shown.

[0076] The average particle size of spherical natural graphite is preferably as small as possible to maximize the degree of disorder in the direction of expansion of each particle, thereby preventing particle expansion during charging with lithium ions. However, if the particle size of natural graphite is less than 5.0 μm, the number of particles per unit volume increases, requiring a large amount of binder, which can result in low sphericity and sphericity yield. On the other hand, if the maximum particle size exceeds 10 μm, excessive expansion can occur, resulting in a decrease in inter-particle adhesion and adhesion between the particles and the current collector due to repeated charge and discharge, which can significantly reduce cycle performance.

[0077] In addition, the negative electrode slurry may further include a conductive material, a binder, a thickener, etc., in addition to the carbon-based negative electrode active material, and these may be those commonly used in the art.

[0078] The conductive material may include, but is not limited to, one or more of carbon black, acetylene black, ketjen black, carbon nanotubes, carbon fibers, and the like.

[0079] For example, the negative electrode active layer may contain carbon black, carbon nanotubes, carbon fibers, etc., alone or in combination, as a conductive material.

[0080] The content of the conductive material may be 0.1 to 10 parts by weight, specifically 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, 2 to 6 parts by weight, or 0.5 to 2 parts by weight, based on 100 parts by weight of the total negative electrode active layer. By controlling the content of the conductive material within the above range, the present invention can prevent a decrease in charge capacity due to an increase in the resistance of the negative electrode caused by a low content of conductive material. Furthermore, the present invention can prevent problems such as a decrease in charge capacity due to a decrease in the content of the negative electrode active material caused by an excessive amount of conductive material, or a decrease in fast charge characteristics due to an increase in the loading amount of the negative electrode active layer.

[0081] The binder is a component that aids in bonding between the negative electrode active material and the conductive material, etc., and bonding to the negative electrode current collector. It may be suitably used within a range that does not degrade the electrical properties of the electrode. Specifically, the binder may include any one or more of vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, and fluororubber.

[0082] The content of the binder may be 0.1 to 10 parts by weight, specifically 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, or 2 to 6 parts by weight, based on 100 parts by weight of the total negative electrode slurry. By controlling the content of the binder contained in the negative electrode slurry within the above range, the present invention can prevent a decrease in the adhesive strength of the negative electrode active layer due to a low content of binder or a decrease in the electrical properties of the electrode due to an excessive amount of binder.

[0083] The method for manufacturing a secondary battery negative electrode sheet according to the present invention has the above-described configuration, and thus has the advantage of being able to uniformly align the carbon-based negative electrode active material of the negative electrode slurry applied to the negative electrode current collector with a high degree of orientation.

[0084] <Secondary battery negative electrode>

[0085] Furthermore, in one embodiment, the present invention provides a secondary battery negative electrode manufactured by the above-described manufacturing method.

[0086] The negative electrode for a lithium secondary battery according to the present invention includes a negative electrode active layer, mainly composed of a carbon-based negative electrode active material, on at least one surface of a negative electrode current collector. The negative electrode active layer is a layer that realizes the electrical activity of the negative electrode. The negative electrode active layer is fabricated by coating both surfaces of the electrode current collector with an electrode slurry containing a carbon-based negative electrode active material that realizes an electrochemical redox reaction during charge and discharge of the secondary battery, followed by drying and rolling. The negative electrode active layer is formed by coating the negative electrode slurry to an electrode sheet under a magnetic field using the above-described negative electrode manufacturing apparatus, and then drying the coated electrode sheet. The carbon-based negative electrode active material, which is the main component, exhibits high crystalline orientation relative to the surface of the electrode sheet, resulting in high energy density and charge / discharge performance.

[0087] Specifically, in the negative electrode according to the present invention, the carbon-based negative electrode active material contained in the negative electrode active layer may be aligned at a predetermined angle relative to the electrode sheet (or negative electrode current collector). This alignment of the carbon-based negative electrode active material may reduce the degree of disorder in the negative electrode active layer, thereby reducing electrode resistance and providing a path for lithium ion migration. The crystalline orientation of the carbon-based negative electrode active material (e.g., graphite) may be determined by crystal plane analysis of the carbon-based negative electrode active material contained in the negative electrode active layer.

[0088] For example, in the negative electrode active layer, the carbon-based negative electrode active material may be aligned in a certain direction relative to the negative electrode current collector, and when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD), the average degree of alignment (OI) of the carbon-based negative electrode active material, represented by the following Equation 2, may be 0.9 or less:

[0089] [Formula 2] OI=I 004 / I 110

[0090] In Equation 2, I 004 represents the area of ​​the peak representing the (004) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD), I 110represents the area of ​​the peak indicating the (110) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD).

[0091] Equation 2 above can be an index showing the degree to which the crystalline structure of the spherical carbon-based negative electrode active material is aligned in a certain direction, specifically, relative to the surface of the negative electrode current collector, during X-ray diffraction measurement. More specifically, the negative electrode active layer exhibits peaks of 2θ=26.5±0.2°, 42.4±0.2°, 43.4±0.2°, 44.6±0.2°, 54.7±0.2°, and 77.5±0.2°, which are the peaks for the carbon-based negative electrode active material, graphite, during X-ray diffraction measurement, and these peaks represent the (002), (100), (101)R, (101)H, (004), and (110) planes. In addition, the peak appearing at 2θ=43.4±0.2° may be due to the overlap of the peaks corresponding to the (101)R plane of the carbon-based negative electrode active material and the (111) plane of the current collector, for example, copper (Cu).

[0092] The degree of alignment (OI) of the carbon-based negative electrode active material can be measured by the area ratio of the peak at 2θ = 77.5 ± 0.2°, which indicates the (110) plane, to the peak at 2θ = 54.7 ± 0.2°, which indicates the (004) plane, specifically, the area ratio obtained by integrating the intensities of these peaks. Here, the peak at 2θ = 54.7 ± 0.2° is a peak indicating the (004) plane, which is one of the crystal planes of graphite that is tilted relative to the negative electrode current collector. Therefore, the closer the degree of alignment (OI) value is to 0, the closer the tilt relative to the surface of the negative electrode current collector is to 90°, and the larger the value is, the closer the tilt relative to the surface of the negative electrode current collector is to 0° or 180°. In view of this, in the negative electrode active layer according to the present invention, the carbon-based negative electrode active material is aligned nearly perpendicular to the negative electrode current collector, for example, at an angle of 60° or more, 70° or more, 70 to 90°, 80 to 90°, 65 to 85°, or 70 to 85° with respect to the negative electrode current collector. Therefore, the degree of alignment (OI) of the carbon-based negative electrode active material may be lower than when the carbon-based negative electrode active material is not magnetically aligned.

[0093] In the negative electrode according to the present invention, the degree of alignment (OI) according to Equation 2 when measured by X-ray diffraction on the surface of the negative electrode active layer may be 0.9 or less, specifically 0.8 or less, 0.7 or less, 0.6 or less, 0.1 to 0.9, 0.2 to 0.8, 0.3 to 0.7, 0.2 to 0.6, or 0.4 to 0.6.

[0094] Meanwhile, the average thickness of the negative electrode active layer may be 50 μm to 300 μm, specifically 50 μm to 250 μm, 100 μm to 250 μm, or 100 μm to 200 μm. By adjusting the average thickness of the negative electrode active layer within the above range, the present invention can not only increase the energy density of the electrode but also uniformly control the degree of disorder of the carbon-based negative electrode active material contained in the negative electrode active layer.

[0095] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, the negative electrode current collector may include copper, stainless steel, nickel, titanium, calcined carbon, etc. In the case of copper or stainless steel, it may also include those that have been surface-treated with carbon, nickel, titanium, silver, etc. In addition, the average thickness of the negative electrode current collector may be preferably 1 to 500 μm, taking into account the conductivity and total thickness of the negative electrode to be manufactured.

[0096] The present invention will be described in more detail below with reference to examples and experimental examples.

[0097] However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0098] Examples 1 to 5 and Comparative Examples 1 to 3. Production of secondary battery negative electrode sheets

[0099] First, natural graphite and artificial graphite were prepared as carbon-based active materials (average particle size: 4±1 μm, sphericity: 0.94±0.2), and a first negative electrode slurry and a second negative electrode slurry were produced using the prepared carbon-based active materials.

[0100] Specifically, the first negative electrode slurry was prepared by mixing natural graphite and artificial graphite in a weight ratio of 2:8 as the negative electrode active material, carbon black as the conductive material, and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as binders. 95 parts by weight of the mixed graphite, 1 part by weight of carbon black, 1.5 parts by weight of carboxymethyl cellulose (CMC), and 2.5 parts by weight of styrene-butadiene rubber (SBR) were mixed with water to a solids content of 50% to prepare the first negative electrode slurry.

[0101] The second negative electrode slurry was prepared using the same method as the first negative electrode slurry, but only using artificial graphite as the carbon-based active material. The viscosity of the first and second negative electrode slurries was adjusted to 6300±200 cps at 25°C.

[0102] Separately, a negative electrode manufacturing apparatus was prepared, which included a double slot die having a first slot and a second slot into which a negative electrode slurry can be discharged at the tip end, in which an upper block, a first spacer, a middle block, a second spacer, and a lower block are sequentially stacked, and a coating roll disposed to face the slot of the slot die. In this case, the negative electrode manufacturing apparatus was configured to determine i) the type of magnetic polarity imparted to the upper lip of the upper block, the middle lip of the middle block, and the lower lip of the lower block, ii) the type of magnetic polarity imparted to the coating roll, iii) the magnetic field strength applied to the lip portion including the upper lip, the middle lip, and the lower lip, and iv) the magnetic field strength (G lip ) and the magnetic field strength (G roll ) was adjusted as shown in Table 1 below.

[0103] The negative electrode slurry was then loaded into the negative electrode manufacturing equipment and applied to the surface of a copper sheet (thickness: 8 μm) being transported at a speed of 10 m / min. Hot air was then continuously blown onto the applied negative electrode slurry to dry it, forming a negative electrode active layer.

[0104] [Table 1]

[0105] Experimental example

[0106] The orientation exhibited by the carbon-based negative electrode active material of the negative electrode for secondary battery manufactured using the negative electrode manufacturing apparatus according to the present invention was evaluated.

[0107] Specifically, X-ray diffraction spectroscopy (XRD) was performed on three arbitrary points of the negative electrode active layer of each of the negative electrodes produced in Examples 1 to 5 and Comparative Examples 1 to 3. The measurement conditions for X-ray diffraction (XRD) were as follows:

[0108] - Target: Cu (Kα-ray) graphite monochromator - Slit: Divergence slit = 1 degree, receiving slit = 0.1 mm, scattering slit = 1 degree - Measurement area: (110) plane: 76.5 degrees < 2θ < 78.5 degrees / (004) plane: 53.5 degrees < 2θ < 56.0 degrees

[0109] The degree of alignment (OI) of each carbon-based negative electrode active material was calculated from the spectrum measured under the above conditions, and the average degree of alignment (OI) was calculated from the calculated degrees of alignment (OI) of the carbon-based negative electrode active material at three points. The results are shown in Table 2.

[0110] [Formula 2] OI=I 004 / I 110

[0111] In Equation 2, I 004 represents the area of ​​the peak representing the (004) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD), I 110 represents the area of ​​the peak indicating the (110) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD).

[0112] [Table 2]

[0113] As shown in Table 2 above, it can be seen that the negative electrode manufactured using the negative electrode manufacturing apparatus according to the present invention has a high degree of orientation of the carbon-based negative electrode active material contained in the negative electrode active layer.

[0114] This means that when the tip portions of the upper block, middle block, and lower block that make up the double slot die, i.e., each lip portion, are given magnetism of the same polarity when discharging the negative electrode slurry, and at the same time, the coating roll is given magnetism of the opposite polarity, the crystal plane of the carbon-based negative electrode active material is oriented nearly perpendicular to the electrode sheet before the negative electrode slurry is discharged onto the electrode sheet (or negative electrode current collector), and the carbon-based negative electrode active material is applied to the electrode sheet and maintained in this induced orientation, thereby forming a negative electrode active layer.

[0115] From these results, the anode manufacturing apparatus according to the present invention can apply the carbon-based anode active material in the discharged anode slurry in a state where the carbon-based anode active material is oriented at a predetermined angle relative to the surface of the anode current collector, which not only provides an excellent degree of orientation of the carbon-based anode active material in the manufactured anode, but also has the advantage of being easily applicable to mass production processes.

[0116] Although the present invention has been described above with reference to preferred embodiments, it will be understood that a person skilled in the art or having ordinary knowledge in the art can make various modifications and changes to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims below.

[0117] Therefore, the technical scope of the present invention is not limited to the content described in the Summary of the Invention of the specification, but can be defined by the claims. [Explanation of symbols]

[0118] 10: Apparatus for manufacturing a negative electrode for a secondary battery according to the present invention 100: Double slot die 110: Upper die 111: Upper lip 112: Upper body 120: Intermediate die 121: Middle lip 122: Intermediate body 130: Lower die 131: Lower lip 132: Lower body 141: First spacer 142: Second spacer 151: 1st slot 152: Second slot 200: Coating roll 210: Coated roll magnet 300: Electrode sheet A: Unoriented section B: Orientation section I: Main body II: Lip III: Coating area MD: Magnetic field direction CM: Carbon-based negative electrode active material

Claims

1. An apparatus for manufacturing a secondary battery anode, which manufactures an electrode sheet coated with anode slurry containing a carbon-based anode active material, a double slot die including an upper block, an intermediate block, and a lower block, a first slot provided with a gap between the upper block and the intermediate block and for discharging a first negative electrode slurry, and a second slot provided with a gap between the intermediate block and the lower block and for discharging a second negative electrode slurry; a coating roll that faces the first slot and the second slot of the double slot die and transports an electrode sheet coated with the negative electrode slurry discharged from each slot, the upper block, the intermediate block, and the lower block each have an upper lip, an intermediate lip, and a lower lip that form a discharge port at a tip end thereof, and the upper lip, the intermediate lip, and the lower lip exhibit magnetism of the same polarity; The coating roll exhibits a magnetic polarity opposite to that of the upper lip.

2. 2. The apparatus for manufacturing a negative electrode for a secondary battery according to claim 1, wherein the upper lip, the middle lip, and the lower lip apply a magnetic field having a strength of 500 G to 3,000 G when the negative electrode slurry is discharged.

3. The coating roll satisfies the following formula 1: [Formula 1] G ｌｉｐ / G ｒｏｌｌ ≦1 In Formula 1, G ｌｉｐ represents the strength of the magnetic field (unit: G) applied to the upper lip, the middle lip, and the lower lip; G ｒｏｌｌ 2. The apparatus for manufacturing a negative electrode for a secondary battery according to claim 1, wherein σ represents the strength (unit: G) of the magnetic field applied to the coating roll.

4. The apparatus for manufacturing a negative electrode for a secondary battery according to claim 1 , wherein the upper lip, the middle lip, and the lower lip have a structure including at least one of a permanent magnet and an electromagnet.

5. The apparatus for manufacturing a negative electrode for a secondary battery further includes a drying unit that dries the negative electrode slurry applied to the electrode sheet, 2. The apparatus for manufacturing a negative electrode for a secondary battery according to claim 1, wherein the drying unit is disposed at a position where the drying unit reaches within 20 seconds from the time when the negative electrode slurry discharged from the double slot die is applied to the electrode sheet.

6. The method includes applying the negative electrode slurry to an electrode sheet while applying a magnetic field thereto using the apparatus for manufacturing a negative electrode for a secondary battery according to claim 1, The negative electrode slurry contains a carbon-based negative electrode active material.

7. 7. The method of claim 6, wherein an average thickness of the negative electrode slurry applied to the electrode sheet is greater than a distance between a slot of a double slot die and a coating roll provided in the apparatus for manufacturing a negative electrode for a secondary battery.

8. The method for manufacturing a negative electrode for a secondary battery according to claim 6 , wherein the average thickness of the negative electrode slurry applied to the electrode sheet is 100 μm or more.

9. 7. The method for manufacturing a negative electrode for a secondary battery according to claim 6, wherein the step of applying the negative electrode slurry to the electrode sheet is performed at a speed of 5 to 100 m / min.

10. 7. The method of claim 6, further comprising the step of drying the coated negative electrode slurry to form a negative electrode active layer after the step of coating the negative electrode slurry on the electrode sheet.

11. In the negative electrode active layer, the degree of alignment (O.I) of the carbon-based negative electrode active material with respect to the surface of the electrode sheet, represented by the following formula 2, is 0.9 or less: [Formula 2] O.I=I ００４ / I １１０ In Equation 2, I ００４ represents the area of ​​the peak representing the (004) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD), I １１０ The method for producing a negative electrode for a secondary battery according to claim 10 , wherein represents the area of ​​a peak indicating a (110) crystal plane when the negative electrode active layer is measured by X-ray diffraction spectroscopy (XRD).

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