A battery cell, a symmetrical battery and their applications

Abstract

This application relates to the technical field of lithium-ion batteries, and more particularly, to an electrode core, a symmetric battery, and their applications. The electrode core has opposite first and second ends; the electrode core includes a separator, two tabs, and two electrode foils with the same polarity; the electrode foil includes a current collector and an electrode coating; along the length direction of the current collector, one surface of the current collector includes a coating area and a blank foil area; the electrode coating is disposed in the coating area; the separator is disposed between the two electrode foils and the two surfaces of the separator are respectively in contact with the electrode coatings of the two electrode foils, and the blank foil areas of the two electrode foils are respectively located at the first end and the second end of the electrode core; the two tabs are respectively welded to the blank foil areas of the two electrode foils, and the area of the welding area on each electrode foil is at least 80% of the area of the blank foil area. Using the electrode core of this application to prepare a symmetric battery has a high preparation consistency, can accurately measure the ohmic impedance of the electrode foil and the ionic transport impedance in the electrode coating, and is beneficial to the evaluation of the performance of the electrode foil of the lithium-ion battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium - ion batteries, and more particularly, to a battery cell, a symmetric battery and their applications. Background Art

[0002] The components of a lithium - ion battery include a positive electrode plate, a negative electrode plate, a separator, an electrolyte and a battery case. The positive electrode plate and the negative electrode plate both include an electrode active material, a binder and a conductive agent. During the development of lithium - ion batteries, to meet different application requirements, it is necessary to improve the positive and negative active materials, conductive agents, binders, electrolytes or separators in the lithium - ion battery, and after the lithium - ion battery is manufactured, a large number of tests and analyses are carried out to verify the performance of the manufactured lithium - ion battery after improvement, and finally production applications are carried out. Due to the many factors affecting the performance of lithium - ion batteries, the development and testing processes of lithium - ion batteries are extremely costly.

[0003] Currently, usually two electrode plates with the same polarity are used as the positive and negative electrodes of a lithium - ion battery, and electrode tabs are welded to the two electrode plates to assemble a symmetric battery, which is then used to evaluate the performance of the improved lithium - ion battery. However, the existing symmetric batteries have poor preparation consistency and a large internal resistance in electrode tab welding, and it is impossible to accurately know the true internal resistance of the electrode plate and the transmission status of lithium ions inside the electrode coating, and it is impossible to provide accurate feedback and effective reference for electrode design, especially the optimization of the electrode coating structure and the preparation process of the electrode plate. Summary of the Invention

[0004] The purpose of the present application is to provide a battery cell, a symmetric battery and their applications, aiming to solve the technical problem that the existing symmetric batteries have poor preparation consistency and cannot accurately test and evaluate the performance of the electrode plates of lithium - ion batteries.

[0005] In a first aspect, the present application provides a battery cell. The battery cell has opposite first and second ends; the battery cell includes a separator, two electrode tabs and two electrode plates with the same polarity.

[0006] The electrode plate includes a current collector and an electrode coating; along the length direction of the current collector, one surface of the current collector includes a coating area and a bare foil area; the electrode coating is provided on the coating area.

[0007] The separator is disposed between the two electrode plates, and the two surfaces of the separator are respectively in contact with the electrode coatings of the two electrode plates. The bare foil areas of the two electrode plates are respectively located at the first end and the second end of the battery cell.

[0008] The two electrode tabs are respectively welded to the bare foil areas of the two electrode plates. The area of the region on the electrode tab welded to the electrode plate is defined as the welding area, and the area of the welding area welded to each electrode plate is at least 80% of the area of the bare foil area.

[0009] Polar plates with the same polarity are used as the positive and negative electrodes of the battery cell, and the empty foil areas of the two polar plates are respectively arranged at the first end and the second end of the battery cell to form the battery cell matrix of the symmetric battery, which is beneficial to testing the impedance information of the electrode polar plate itself. The tab is welded to the empty foil area of the polar plate, and the area of the welding area welded on the polar plate is at least 80% of the area of the empty foil area, which can effectively reduce the internal resistance of the solder joint caused by welding the tab on the polar plate and improve the welding consistency, so that the subsequent symmetric battery can more accurately measure the ohmic impedance of the lithium-ion battery electrode polar plate and the ion transfer impedance in the electrode coating, which is beneficial to accurately evaluating the performance of the lithium-ion battery electrode polar plate and providing feedback and reference for electrode design, especially the optimization of the electrode coating structure and the preparation process of the polar plate.

[0010] In a possible implementation manner, the area of the welding area welded on each polar plate is at least 90% of the area of the empty foil area.

[0011] The above setting method can further reduce the internal resistance of the solder joint caused by welding the tab on the polar plate.

[0012] In a possible implementation manner, the areas of the empty foil areas of the two polar plates are equal.

[0013] The above setting method can further improve the welding consistency of the two tabs while effectively reducing the internal resistance of the solder joint, so that the subsequent symmetric battery can more accurately measure the ohmic impedance of the lithium-ion battery electrode polar plate and the ion transfer impedance in the electrode coating.

[0014] In a possible implementation manner, the battery cell further includes two insulating clamping plates. The two polar plates are arranged between the two insulating clamping plates, and the insulating clamping plates are in contact with the surface of the current collector facing away from the electrode coating.

[0015] The setting of the insulating clamping plate not only enables the polar plate and the separator to be closely attached to each other, effectively avoiding the sliding misalignment between the polar plate and the separator and affecting the test accuracy; but also can improve the structural strength of the battery cell, effectively avoiding the situation of easy bending and damage caused by the thin battery cell, which is beneficial to improving the operability and practicability of the battery cell and improving the consistency of the preparation of the symmetric battery.

[0016] Optionally, the material of the insulating clamping plate is selected from at least one of polyethylene and polypropylene.

[0017] In a possible implementation manner, the extending end of the separator extends out between the two polar plates and extends and sequentially winds and wraps the surfaces of the two insulating clamping plates facing away from the polar plates.

[0018] The above setting method enables the separator and the insulating clamping plate to cooperate with each other to effectively fix the two polar plates, so that the polar plate and the separator are not easily slid and misaligned.

[0019] In a possible implementation, along the width direction of the current collector, the length of the empty foil area is different from the length of the coating area.

[0020] The above setting method can form a chamfer structure between the empty foil area and the coating area of the electrode sheet. During the process of preparing the battery cell, it is convenient for the coating areas of the two electrode sheets to be more accurately aligned, which is beneficial to avoiding the misalignment of the electrode coatings of the two electrode sheets and affecting the test accuracy.

[0021] In a possible implementation, along the width direction of the current collector, the length of the empty foil area is less than the length of the coating area.

[0022] Optionally, chamfer structures are respectively formed between the two opposite ends in the length direction of the empty foil area and the coating area.

[0023] Optionally, the chamfer structure is a right-angled chamfer structure.

[0024] In a possible implementation, two tab ears are symmetrically arranged at the first end and the second end of the battery cell.

[0025] The above setting method can further improve the welding consistency of the two tab ears while effectively reducing the internal resistance of the solder joints.

[0026] In a second aspect, the present application provides a symmetric battery, including a housing, an electrolyte, and the battery cell provided in the first aspect above; the electrolyte and the battery cell are both located inside the housing, and the free ends of the two tab ears are located outside the housing.

[0027] The symmetric battery provided by the present application can be used to more accurately measure the ohmic impedance of the electrode sheet of the lithium-ion battery and the ion transport impedance in the electrode coating due to its lower internal resistance of the solder joints and the ability to effectively improve the preparation consistency of the symmetric battery, which is beneficial to accurately evaluating the performance of the electrode sheet of the lithium-ion battery and providing feedback and reference basis for electrode design, especially the optimization of the electrode coating structure and the preparation process of the electrode sheet.

[0028] In a third aspect, the present application provides an application of the symmetric battery provided in the second aspect above in evaluating the performance of the electrode sheet of the lithium-ion battery.

[0029] Optionally, the performance of the electrode sheet of the lithium-ion battery includes at least one of ohmic impedance and ion transport impedance. Description of the Drawings

[0030] To more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the accompanying drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present application and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0031] Figure 1 Shows a schematic structural diagram of the battery cell provided by the present application.

[0032] Figure 2 Shows Figure 1 The cross-sectional view along the A-A direction in

[0033] Figure 3 Shows a schematic structural diagram of the electrode provided in the battery cell provided by the present application.

[0034] Figure 4 Shows a schematic structural diagram of the electrode, separator, and fixing element in the battery cell provided by the present application.

[0035] Figure 5 Shows the Nyquist curve diagram of the symmetric battery provided by the embodiment of the present application.

[0036] Figure 6 Shows the Nyquist curve diagram of the symmetric battery provided by the comparative example of the present application.

[0037] Figure 7 Shows the Nyquist curve diagram of the symmetric battery provided by the embodiment of the present application under different electrode coating thicknesses.

[0038] Figure 8 Shows the Nyquist curve diagram of the symmetric battery provided by the embodiment of the present application under different electrode coating compaction densities.

[0039] Figure 9 Shows the Nyquist curve diagram of the symmetric battery provided by the embodiment of the present application under different particle sizes of the electrode active material.

[0040] Figure 10 Shows the comparison diagram of the capacity retention rate of the symmetric battery provided by the embodiment of the present application under different particle sizes of the electrode active material at different rates.

[0041] Icon: 100 - battery cell; 101 - first end; 102 - second end; 110 - electrode; 111 - current collector; 1111 - coating area; 1112 - blank foil area; 120 - separator; 130 - tab; 131 - welding area; 132 - encapsulation area; 133 - exposed area; 140 - insulating clamping plate; 150 - fixing element. Detailed Implementation Modes

[0042] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. For those not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are applied. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0043] During the electrode reaction process of a lithium-ion battery, the process of lithium ions embedding into the electrode mainly includes the following steps: the transport of lithium ions in the pores of the electrode coating, the migration of lithium ions in the interface film, charge transfer, and the solid-state diffusion of lithium ions in the electrode active material. Among them, the transport ability of lithium ions in the pores of the electrode coating has an important impact on the battery capacity and rate performance. The pore structure of the electrode coating includes many influencing parameters such as porosity, pore size and distribution, connectivity, and tortuosity. Moreover, the pore structure in the electrode coating is not evenly distributed, and the pore structure and morphology are relatively complex, making it difficult to effectively evaluate the transport ability of lithium ions in the electrode coating.

[0044] In the prior art, to efficiently and conveniently identify the quality of the electrode sheets of a lithium-ion battery, usually two electrode sheets with the same polarity are used as the positive electrode and the negative electrode to assemble a symmetric battery, and the electrochemical impedance spectroscopy (EIS) information of the symmetric battery is used to evaluate the performance of the electrode sheets of the lithium-ion battery.

[0045] However, the inventor found that due to the existence of only two electrode sheets, the traditional symmetric battery structure has low strength and poor preparation consistency. For example, for a symmetric battery with the same-side electrode tabs, there is a small welding area of the electrode tabs, resulting in a large internal resistance of the solder joints and poor welding consistency. As a result, the ohmic resistance of the electrode sheet measured during the EIS test is affected by the internal resistance of the solder joints, making it difficult to accurately measure the ohmic resistance of the electrode sheet itself and even more impossible to effectively evaluate the transport ability of lithium ions in the electrode coating.

[0046] If the empty foil area of the same-sex electrode sheet is directly used as the electrode tab to prepare a symmetric battery, this method does not require welding and can effectively avoid the situation of increasing the internal resistance of the solder joints caused by welding the electrode tabs on the empty foil area. However, in the method of directly using the empty foil area as the electrode tab, the fragile foil is extremely easy to break, which makes the symmetric battery extremely easy to damage, not easy to perform EIS tests, and has low safety.

[0047] Therefore, to address the above problems, the inventor provides a battery cell Figure 1 shows a schematic structural diagram of the battery cell 100 provided by this application Figure 2 shows Figure 1 a cross-sectional view taken along the A-A direction in Figure 1 and Figure 2, the battery cell 100 includes two electrode plates 110 with the same polarity, a separator 120, and two tab ears 130. The separator 120 is partially disposed between the two electrode plates 110, and the two surfaces of the separator 120 located between the two electrode plates 110 are respectively in contact with the two electrode plates 110.

[0048] It is defined that the battery cell 100 has opposite first end 101 and second end 102. Figure 1 In it, the left end of the battery cell 100 is the first end 101, and the right end of the battery cell 100 is the second end 102. The two tab ears 130 are respectively located at the first end 101 and the second end 102 of the battery cell 100.

[0049] Figure 3 The schematic structural diagram of the electrode plate 110 provided in the battery cell 100 provided by the present application is shown. Please refer to Figure 3 , the electrode plate 110 includes a current collector 111 and an electrode coating (not shown in the figure) provided on the surface of the current collector 111.

[0050] Define Figure 3 In it, the direction from left to right is the length direction of the current collector 111, and it is defined that Figure 3 In it, the direction from top to bottom is the width direction of the current collector 111.

[0051] Along the length direction of the current collector 111, one surface of the current collector 111 includes a coating area 1111 and a bare foil area 1112, and the electrode coating is provided in the coating area 1111.

[0052] Figure 4 The schematic structural diagram of the electrode plate 110, the separator 120, and the fixing element 150 in the battery cell 100 provided by the present application is shown. Please refer to Figure 1-4 , the coating areas 1111 of the two electrode plates 110 in the battery cell 100 are disposed opposite to each other, and the two surfaces of the separator 120 located between the two electrode plates 110 are respectively in contact with the electrode coatings of the two electrode plates 110. The bare foil areas 1112 of the two electrode plates 110 are respectively located at the first end 101 and the second end 102 of the battery cell 100.

[0053] Using electrode plates 110 with the same polarity as the positive electrode and the negative electrode of the battery cell 100, and respectively disposing the bare foil areas 1112 of the two electrode plates 110 at the first end 101 and the second end 102 of the battery cell 100 to form the battery cell 100 matrix of the symmetric battery is beneficial to testing the impedance information of the electrode plate 110 itself.

[0054] The two tab ears 130 are respectively welded to the bare foil areas 1112 of the two electrode plates 110. Please refer to Figure 1, the tab 130 includes a welding area 131, a rubber-coated area 132, and an exposed area 133. The welding area 131 is the area on the tab 130 that is welded to the empty foil area 1112. The rubber-coated area 132 and the exposed area 133 are sequentially arranged on the side of the welding area 131 away from the coating area 1111. The rubber-coated area 132 is used to contact the outer shell and perform molten sealing during the subsequent preparation of the symmetric battery. The exposed area 133 is used to lead out the electrode and connect it to an external test device.

[0055] To effectively reduce the internal resistance of the solder joint caused by welding the tab 130 to the electrode sheet 110, in this application, the area of the welding area 131 welded to each electrode sheet 110 is at least 80% of the area of the empty foil area 1112. The above setting method can effectively reduce the internal resistance of the solder joint caused by welding the tab 130 to the electrode sheet 110, so that the subsequent symmetric battery can more accurately measure the ohmic impedance of the lithium-ion battery electrode sheet 110 and the ion transport impedance in the electrode coating, which is beneficial to accurately evaluating the performance of the lithium-ion battery electrode sheet 110, and then can effectively obtain the lithium-ion transport ability inside the electrode sheet 110, providing feedback and reference basis for electrode design, especially the optimization of the electrode coating structure and the preparation process of the electrode sheet 110; at the same time, since the tab 130 is separately set and the empty foil area 1112 is not directly used as the electrode lead-out element, the structural strength of the entire battery cell 100 is ensured.

[0056] Exemplarily, the area of the welding area 131 welded to each electrode sheet 110 can be 80%, 85%, 90%, or 95% of the area of the empty foil area 1112, etc.

[0057] Furthermore, the area of the welding area 131 welded to each electrode sheet 110 is at least 90% of the area of the empty foil area 1112, which can further reduce the internal resistance of the solder joint caused by welding the tab 130 to the electrode sheet 110.

[0058] In the embodiment of this application, the area of the welding area 131 welded to each electrode sheet 110 is 90% of the area of the empty foil area 1112, and the areas of the empty foil areas 1112 of the two electrode sheets 110 are also equal, which can further improve the welding consistency of the two tabs 130 while effectively reducing the internal resistance of the solder joint, so that the subsequent symmetric battery can more accurately measure the ohmic impedance of the lithium-ion battery electrode sheet 110 and the ion transport impedance in the electrode coating.

[0059] Furthermore, as Figure 4As shown, the empty foil area 1112 is symmetrically arranged at the first end 101 and the second end 102 of the battery cell 100, so that the two pole ears 130 welded to the empty foil area 1112 are also symmetrically arranged at the first end 101 and the second end 102 of the battery cell 100, which can further improve the welding consistency of the two pole ears 130 while effectively reducing the internal resistance of the solder joint.

[0060] In the embodiment of the present application, the sizes of the two electrode sheets 110 are equal, and the sizes of the coated area 1111 and the empty foil area 1112 on the two electrode sheets 110 are also equal. Using two electrode sheets 110 of the same sex with the same size as the positive and negative electrodes of the battery cell 100 can further reduce the test error caused by the inconsistent sizes of the positive and negative electrode sheets 110.

[0061] Furthermore, along the thickness direction of the diaphragm 120 located between the two pole pieces 110 , the coating areas 1111 on the two pole pieces 110 are aligned, which can further improve the test accuracy.

[0062] In order to facilitate more accurate alignment of the coating areas 1111 of the two electrode pieces 110, it is helpful to avoid the electrode coatings of the two electrode pieces 110 being misaligned with each other and affecting the test accuracy. Figure 3 , along the width direction of the current collector 111, the length of the empty foil area 1112 is different from the length of the coated area 1111. The above arrangement can form a chamfered structure between the empty foil area 1112 and the coated area 1111 of the pole piece 110, thereby facilitating more accurate alignment of the coated areas 1111 of the two pole pieces 110 during the preparation of the battery cell 100.

[0063] In the embodiments of the present application, Figure 3 As shown, along the width direction of the current collector 111, the length of the empty foil area 1112 is less than the length of the coated area 1111; and along the width direction of the current collector 111, the opposite ends of the length direction of the empty foil area 1112 respectively form chamfer structures with the coated area 1111. The opposite ends of the length direction of the empty foil area 1112 have chamfer structures, which can more conveniently and accurately align the coated areas 1111 of the two pole pieces 110.

[0064] Furthermore, the chamfer structure is a right-angled chamfer structure, which can make it easier to align the coating areas 1111 of the two pole pieces 110 .

[0065] It should be noted that, in other embodiments, the length of the empty foil area 1112 along the width direction of the current collector 111 may be greater than the length of the coated area 1111; or, a chamfer structure may be provided only at one end of the length direction of the empty foil area 1112; as long as a chamfer structure can be formed between the empty foil area 1112 and the coated area 1111 of the pole piece 110.

[0066] In the embodiment of the present application, the length of the coating area 1111 is 45 - 55 mm, and both opposite ends in the length direction of the empty foil area 1112 are 1 mm shorter than the coating area 1111.

[0067] As described above, since the two surfaces of the separator 120 located between the two electrode plates 110 are respectively in contact with the electrode coatings of the two electrode plates 110. In the embodiment of the present application, the separator 120 completely covers the electrode coatings of the two electrode plates 110, and the empty foil areas 1112 of the two electrode plates 110 are respectively located at the opposite ends of the separator 120 along the direction from the first end 101 to the second end 102.

[0068] Please refer to again Figure 1 and Figure 2 The battery cell 100 further includes two insulating clamping plates 140. The two electrode plates 110 are arranged between the two insulating clamping plates 140, and the insulating clamping plates 140 are in contact with the surface of the current collector 111 facing away from the electrode coating. The arrangement of the insulating clamping plates 140 not only enables the electrode plate 110 and the separator 120 to be closely attached to each other, effectively avoiding the sliding misalignment between the electrode plate 110 and the separator 120 and affecting the test accuracy; but also can improve the structural strength of the battery cell 100, effectively avoiding the situation of being easily bent and damaged due to the thinness of the battery cell 100, which is beneficial to improving the operability and practicality of the battery cell 100.

[0069] In the embodiment of the present application, the size of the insulating clamping plate 140 is the same as the size of the coating area 1111.

[0070] Exemplarily, the material of the insulating clamping plate 140 is selected from at least one of polyethylene and polypropylene, and the thickness of the insulating clamping plate 140 is 1 - 2 mm.

[0071] Furthermore, due to the setting of the insulating clamping plate 140 in the present application, the extending end of the separator 120 extends out between the two electrode plates 110, and extends and sequentially winds around the surfaces of the two insulating clamping plates 140 facing away from the electrode plates 110. The above setting method enables the separator 120 and the insulating clamping plate 140 to cooperate with each other to effectively fix the two electrode plates 110, making it difficult for the electrode plates 110 and the separator 120 to slide and misalign, and improving the test accuracy.

[0072] Still further, a fixing element 150 is used to fix the extending end of the separator 120. In the embodiment of the present application, the fixing element 150 is a tape. It should be noted that the present application does not specifically limit the type of the fixing element 150.

[0073] The battery cell 100 provided by the present application has at least the following advantages:

[0074] Use pole pieces with the same polarity as the positive and negative electrodes of the battery cell 100, and respectively arrange the empty foil areas 1112 of the two pole pieces 110 at the first end 101 and the second end 102 of the battery cell 100 to form the battery cell matrix of the symmetric battery, which is beneficial to testing the impedance information of the pole piece 110 itself. The tab 130 is welded to the empty foil area 1112 of the pole piece 110, and the area of the welding area 131 welded on the pole piece 110 is at least 80% of the area of the empty foil area 1112, which can effectively reduce the internal resistance of the solder joint caused by welding the tab 130 to the pole piece 110, so that the manufactured symmetric battery can more accurately measure the ohmic impedance of the lithium-ion battery pole piece 110 and the ion transfer impedance in the electrode coating, which is beneficial to accurately evaluating the performance of the electrode pole piece 110 of the lithium-ion battery, and further can effectively obtain the lithium-ion transfer ability inside the pole piece 110, providing feedback and reference basis for electrode design, especially the optimization of the electrode coating structure and the preparation process of the pole piece 110.

[0075] The present application also provides a symmetric battery, including a housing, an electrolyte, and the battery cell provided above. The electrolyte and the battery cell are both located inside the housing, and the free ends of the two tabs (i.e., the exposed areas of the tabs) are located outside the housing.

[0076] In the embodiment of the present application, the symmetric battery is a soft-pack battery, and the material of the housing is aluminum-plastic film. It should be noted that the present application does not limit the material of the housing.

[0077] The symmetric battery provided by the present application has at least the following advantages:

[0078] The symmetric battery provided by the present application can be used to more accurately measure the ohmic impedance of the lithium-ion battery electrode pole piece and the ion transfer impedance in the electrode coating due to its low internal resistance of the solder joint and the ability to effectively improve the preparation consistency of the symmetric battery, obtain the lithium-ion transfer condition inside the electrode pole piece, and further effectively evaluate the performance of the electrode pole piece of the lithium-ion battery, providing feedback and reference basis for electrode design, especially the optimization of the electrode coating structure and the preparation process of the pole piece.

[0079] The present application also provides an application of the symmetric battery provided above in evaluating the performance of the electrode pole piece of the lithium-ion battery.

[0080] Optionally, the performance of the lithium-ion battery electrode pole piece includes at least one of ohmic impedance and ion transfer impedance.

[0081] Embodiment

[0082] This embodiment provides a symmetric battery, which is prepared by the following method:

[0083] Step (1):

[0084] Select an aluminum foil as the current collector. One surface of the aluminum foil consists of a coating area and a blank foil area. An electrode coating is applied to the coating area to obtain a pole piece.

[0085] Among them, the length and width of the coating area are both 50 mm. The length of the blank foil area is 48 mm, the width is 10 mm, and both ends in the length direction of the blank foil area are 1 mm shorter than the coating area. The electrode coating is composed of lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black and polyvinylidene fluoride, and the surface density of the electrode coating is 18 mg / cm 2 , and the tap density of the electrode coating is 3.4 g / cm 3 .

[0086] Step (2):

[0087] Place the electrode coatings of the two pole pieces prepared in step (1) opposite and aligned, and make the blank foil areas of the two pole pieces symmetrically distributed on the opposite sides of the electrode coating. Place the separator between the two pole pieces so that both sides of the separator cover the electrode coatings of the two pole pieces respectively. Press two polyethylene plates against the sides of the two pole pieces away from the separator and align them with the electrode coating. Extend the ends of the separator out between the two pole pieces and successively wind and wrap the surfaces of the two polyethylene plates facing away from the pole pieces, and fix the extended ends of the separator with tape to obtain a bare battery cell. Hot press the bare battery cell, and weld two aluminum sheets (as pole ears) on the blank foil areas of the two pole pieces respectively to obtain a battery cell.

[0088] Among them, the length of the separator is 180 mm and the width is 51 mm. The length and width of the two polyethylene plates are both 51 mm, and the thickness is 2 mm. The length of the two aluminum sheets is 48 mm and the width is 25 mm; and the area of the aluminum sheets welded on the blank foil areas of the two pole pieces is 48 mm * 9 mm.

[0089] Step (3):

[0090] Place the battery cell prepared in step (2) into the shell, place the free ends of the two pole ears outside the shell, and inject the electrolyte to obtain a symmetric battery.

[0091] Among them, the material of the shell is aluminum plastic film. The electrolyte is a lithium hexafluorophosphate solution with a mass fraction of 12%, and the solvent of the electrolyte consists of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate with a mass ratio of 1:1:1.5.

[0092] Comparative Example

[0093] The difference between this comparative example and the example is that: the width of the two aluminum sheets is 10 mm and the length is 25 mm, and the area of the aluminum sheets welded on the blank foil areas of the two pole pieces is 10 mm * 9 mm.

[0094] Experimental Example 1

[0095] The symmetrical batteries provided in the embodiment and the comparative example were subjected to EIS tests respectively. The EIS results of the embodiment are shown in FIG. Figure 5 As shown, the EIS results of the comparative example are as follows Figure 6 As shown. Among them, Figure 5 P-1(1), P-1(2) and P-1(3) respectively represent three symmetrical batteries prepared in parallel according to the embodiment. Figure 6 S-1(1), S-1(2) and S-1(3) represent three symmetrical batteries prepared in parallel according to the comparative example. EIS test method: A symmetrical battery is prepared by using fresh homologous electrodes according to the schemes of the embodiment and the comparative example, and is placed at 25°C for 12 hours. Then, an EIS test is performed using an electrochemical workstation, and the frequency of the disturbing alternating current is 1Hz-100kHz and the amplitude is 5mV.

[0096] from Figure 5 It can be seen that the Nyquist curves of the three symmetrical batteries prepared in parallel according to the embodiment have good overlap, indicating that the design of the symmetrical battery provided in the embodiment is reliable and the preparation consistency is high, and it can be used to evaluate the performance of the electrode sheet of the lithium-ion battery. The Nyquist curves of the three symmetrical batteries prepared in parallel according to the embodiment have a starting point value of about 0.15 on the horizontal axis, indicating that the ohmic internal resistance R of the three symmetrical batteries prepared in parallel according to the embodiment is s About 0.15Ω.

[0097] Further, Figure 5 The Nyquist curves of the three symmetrical batteries prepared in parallel are all composed of a 45° oblique line in the high-frequency region and an approximately vertical straight line in the low-frequency region, and the Nyquist curves do not show a semicircular arc shape, indicating that the symmetrical battery provided in the embodiment has only non-Faraday process during the EIS test, and no charge transfer occurs at the electrode-solution interface (such as the embedding of lithium ions into the electrode active material), but only double-layer capacitance behavior. Therefore, the 45° oblique line portion of the Nyquist curve of the symmetrical battery of the embodiment of the present application in the high-frequency region reflects the transmission process of lithium ions in the pores of the electrode coating. Under the same test parameters, the longer the length of the 45° oblique line in the high-frequency region, the more difficult it is for lithium ions to be transmitted in the pores of the electrode coating, that is, the ion transfer impedance R of the electrode coating. ion The bigger.

[0098] from Figure 6 It can be seen that the Nyquist curves of the three symmetrical batteries prepared in parallel in the comparative example do not overlap highly, indicating that the consistency between the parallel prepared samples is poor. By comparing with the examples, it is found that the three groups of R sGenerally greater than 0.15Ω, R s The differences in size should mainly stem from the differences in the internal resistance of tab welding. A smaller welding area is likely to lead to an increase in the internal resistance of welding and a deterioration in the consistency of the welding effect. Additionally, in the Nyquist curves of the three groups of parallel samples, there are also differences in the length of the 45° diagonal line, indicating that the consistency of symmetric cell preparation will affect the accurate evaluation of the transport performance of lithium ions in the electrode coating by the symmetric cell.

[0099] Experimental Example 2

[0100] Symmetric cells P-1, P-2, and P-3 were prepared respectively with reference to the method for preparing symmetric cells provided in the Examples. The differences between symmetric cells P-1, P-2, and P-3 are as follows: the areal density of the electrode coating is different; the areal densities of the electrode coatings of P-1, P-2, and P-3 are 18mg / cm 2 , 20mg / cm 2 and 22mg / cm 2 respectively, that is, the thicknesses of the electrode coatings of P-1, P-2, and P-3 increase in sequence. Using the same testing method as in Experimental Example 1, EIS tests were conducted on P-1, P-2, and P-3 respectively, and the test results are as Figure 7 shown.

[0101] From Figure 7 it can be seen that the starting point values of the Nyquist curves of P-1, P-2, and P-3 on the horizontal axis are basically the same, indicating that changing the thickness of the electrode coating on the counter electrode has little effect on the value of the ohmic internal resistance R s .

[0102] Furthermore, by respectively drawing auxiliary lines perpendicular to the horizontal axis at the inflection points of the Nyquist curves of P-1, P-2, and P-3, it can be found that as the thickness of the electrode coating increases, the length of the 45° diagonal line in the high-frequency region of the corresponding Nyquist curve extends, indicating that under the same conditions, the thicker the electrode coating, the greater the transport impedance R ion of lithium ions. In fact, under the same conditions, the thicker the electrode coating, the longer the transport path of lithium ions in the electrode coating, and thus it should have a greater R ion . Therefore, it is proved that the method of analyzing and judging the size of R ion by the length of the 45° diagonal line in the high-frequency region of the Nyquist curve of the symmetric cell provided in the embodiments of the present application for evaluating the transport ability of lithium ions in the pores of the electrode coating is accurate and effective.

[0103] Experimental Example 3

[0104] The symmetrical battery P-4 was prepared with reference to the method for preparing a symmetrical battery provided in the embodiment. The difference between the symmetrical battery P-4 and P-1 is that the compaction density of the electrode coating is different; the compaction density of the electrode coating of P-4 is 3.6 mg / cm 3 , which is greater than 3.4 mg / cm of P-1 3 . Using the same test method as in Experimental Example 1, EIS testing was performed on P-4, and the test results are as shown in Figure 8 . P-4 has a smaller Rs than P-1, which is based on the fact that the increase in compaction density effectively improves the contact internal resistance between the electrode coating and the electrode foil. However, the length of the 45° diagonal line in the Nyquist curve of P-4 is greater than that in P-1, indicating that the transport of lithium ions in the electrode coating of P-4 becomes more difficult. This is because the increase in compaction density will reduce the porosity in the electrode coating, resulting in a longer transport path for lithium ions in the electrode coating. This test result shows that the symmetrical battery of the present application has high sensitivity and reliability.

[0105] Experimental Example 4

[0106] The symmetrical batteries P-1 and P-5 were prepared with reference to the method for preparing a symmetrical battery provided in the embodiment. The difference between the symmetrical batteries P-1 and P-5 is that the active materials in the electrode coating are different; the active materials in the electrode coatings of P-1 and P-4 are both LiNi 0.8 Co 0.1 Mn 0.1 O2 materials, but the active material in P-5 is a secondary spherical structure with a particle size D 50 = 5 μm, which is smaller than the secondary spherical structure D in P-1 50 = 12 μm. Using the same test method as in Experimental Example 1, EIS testing was performed on P-5, and the test results are as shown in Figure 9 .

[0107] It can be seen from Figure 9 that there are significant differences in the Nyquist curves between P-5 and P-1. P-5 has a smaller ohmic internal resistance R s = 0.11 Ω, which is smaller than 0.15 Ω of P-1, indicating that the conductivity of the active material in P-5 is better than that of the active material in P-1.

[0108] In addition, by making an auxiliary line perpendicular to the horizontal axis at the inflection point of the Nyquist curves of P-5 and P-1, it can be seen that the length of the 45° diagonal line in the high-frequency region of P-5 is shorter than that of P-1, that is, it shows that P-5 has a smaller R ion , indicating that the electrode coating in P-5 has better lithium ion transport ability than P-1.

[0109] To further corroborate the accuracy of the symmetric battery in this application, the electrode sheets of P-1 and P-5 were fabricated into coin-type half-cells for rate performance verification, and the test results are as Figure 10 shown. At low rates, the capacity retention rates of P-5 and P-1 are comparable. However, at high rates, P-5 exhibits a more excellent capacity retention rate than P-1, indicating that the P-5 electrode coating has a more excellent lithium-ion transport ability. This proves that the symmetric battery provided in the embodiments of this application can be used to screen out electrode active materials with more excellent effects and can be used to evaluate the lithium-ion transport ability of different electrode coatings.

[0110] In summary, due to the low solder joint internal resistance, the symmetric battery provided in this application can be used to more accurately measure the Ohmic impedance of the lithium-ion battery electrode sheet and the ion transport impedance in the electrode coating, which is beneficial to accurately evaluate the performance of the lithium-ion battery electrode sheet. Furthermore, it can effectively obtain the lithium-ion transport condition inside the electrode sheet, providing feedback and reference for electrode design, especially the optimization of the electrode coating structure and the preparation process of the electrode sheet.

[0111] The above are only the preferred embodiments of this application and are not intended to limit this application. For those skilled in the art, this application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included within the protection scope of this application.

Claims

1. A battery cell, characterized in that, The battery cell has opposite first and second ends; the battery cell includes a separator, two tabs, and two electrode foils with the same polarity. The electrode foil includes a current collector and an electrode coating; along the length direction of the current collector, one surface of the current collector includes a coating area and a bare foil area; the electrode coating is disposed on the coating area. The separator is disposed between the two electrode foils, and two surfaces of the separator are respectively in contact with the electrode coatings of the two electrode foils; the bare foil areas of the two electrode foils are respectively located at the first end and the second end of the battery cell. Along the width direction of the tab, the two tabs are respectively welded to the bare foil areas of the two electrode foils; a region on the tab welded to the electrode foil is defined as a welding region, and the area of the welding region welded to each electrode foil is at least 80% of the area of the bare foil area; wherein, the length of the tab is equal to the length of the bare foil area. The battery cell further includes a fixing element and two insulating clamping plates; the two electrode foils are disposed between the two insulating clamping plates, and the insulating clamping plates are in contact with a surface of the current collector facing away from the electrode coating; an extending end of the separator extends out between the two electrode foils, and extends and sequentially winds around surfaces of the two insulating clamping plates facing away from the electrode foils, and the fixing element fixes the extending end of the separator.

2. The battery cell according to claim 1, wherein The area of the welding region welded to each electrode foil is at least 90% of the area of the bare foil area.

3. The battery cell according to claim 2, wherein The areas of the bare foil areas of the two electrode foils are equal.

4. The battery cell according to claim 1, wherein, The material of the insulating clamping plate is selected from at least one of polyethylene and polypropylene.

5. The battery cell according to claim 1, characterized in that, Along the width direction of the current collector, the length of the bare foil area is different from the length of the coating area.

6. The battery cell according to claim 5, wherein Along the width direction of the current collector, the length of the bare foil area is less than the length of the coating area.

7. The cell according to claim 6, wherein Chamfer structures are respectively formed between opposite ends in the length direction of the bare foil area and the coating area.

8. The battery cell according to claim 7, wherein, The chamfer structure is a right-angled chamfer structure.

9. The battery cell according to any one of claims 1-8, characterized in that, The two tabs are symmetrically disposed at the first end and the second end of the battery cell.

10. A symmetric battery, characterized in that, It includes a housing, an electrolyte, and the battery cell according to any one of claims 1-9. The electrolyte and the battery cell are both located in the housing, and free ends of the two tabs are located outside the housing.

11. Application of the symmetric battery according to claim 10 in evaluating the performance of an electrode foil of a lithium-ion battery.

12. The application according to claim 11, wherein The performance of the electrode foil of the lithium-ion battery includes at least one of ohmic impedance and ion transfer impedance.

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