A full-cell dcr growth tracing, symmetric cell preparation and electrolyte optimization method

Abstract

The application discloses a full-cell DCR growth tracing, symmetric cell preparation and electrolyte optimization method, and belongs to the technical field of lithium ion batteries, and comprises the following steps: S1, preparing a symmetric soft package cell system: selecting a single-face positive plate or a single-face negative plate from a non-cycled or cycled cell after formation, assembling a positive or negative symmetric cell, and respectively injecting a basic electrolyte and a to-be-tested electrolyte to form two symmetric cells; S2, DCR growth tracing analysis: performing a constant-current discharge test on the symmetric cell to obtain DCR growth rate data, and determining whether the DCR growth of the full cell is mainly from the positive side or the negative side; and combining electrochemical impedance spectroscopy (EIS) and relaxation time distribution (DRT) methods, further distinguishing whether the DCR growth is from SEI film impedance growth or charge transfer impedance growth. The method eliminates the crosstalk effect between the positive and negative electrodes, realizes accurate diagnosis of the DCR growth source, and provides a key basis for targeted design and efficient development of electrolytes.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for tracing the growth of full-cell DCR, preparing symmetrical cells, and optimizing electrolytes. Background Technology

[0002] Over the past decade or so, lithium-ion batteries have become an indispensable energy storage device in future 3C electronic products, new energy vehicles, large-scale energy storage equipment, and military equipment. The electrolyte acts as a medium for transporting Li-ion electrolyte between the positive and negative electrodes of a lithium-ion battery. + The electrolyte carrier is the most important part for realizing multi-condition application of batteries. However, under extreme conditions, the interfacial reaction between the electrolyte and the positive and negative electrodes often leads to the continuous consumption of electrolyte during cycling, resulting in the destruction of the surface structure of the positive and negative electrodes, changes in the intrinsic physicochemical properties of the electrolyte, and ultimately irreversible capacity loss of lithium-ion batteries until battery failure. Especially under fast charging conditions, its external manifestation is a series of parameter changes such as the increase of battery DCR (Direct Current Resistance).

[0003] The growth of DCR is complex and is mainly related to the side reactions of the positive electrode material, negative electrode material, and electrolyte at the electrode interface.

[0004] The electrolyte is the carrier of lithium ions in the battery system and a crucial channel connecting the positive and negative electrodes. The interfacial reactions between the electrolyte and the positive and negative electrodes induce the evolution of the surface lattice of the electrodes and the generation of surface by-reaction products, directly determining the DCR growth rate at the positive and negative electrode interface. Therefore, to suppress DCR growth, researchers typically use two methods: one is the empirical use of film-forming additives to form a stable SEI / CEI film on the positive / negative electrode surfaces; the other is the use of dehydrating and deacidifying agents to inhibit HF from damaging the positive electrode surface structure and the dissolution of transition metal ions to the negative electrode, thereby reducing the DCR growth of both the positive and negative electrodes. For example, adding TMSPi as a dehydrating and deacidifying agent eliminates HF and H2O in the electrolyte, reduces positive electrode surface corrosion and the dissolution of transition metal ions to the negative electrode, and thus inhibits the catalytic decomposition of the electrolyte by highly active transition metal ions on the negative electrode surface, thereby reducing battery DCR growth.

[0005] However, the mechanisms of action of solvents / additives in electrolytes are complex. Without identifying the source of DCR growth, verifying the effects of various solvent / additive-modified electrolytes requires substantial experimental resources. This means that after various Design of Engineering (DOE) validation designs, solvent / additive control, and process adjustments, finished battery cells must be assembled and subjected to hundreds of cycles of formation, capacity testing, and various operating conditions before the post-cycle DCR growth can be tested. This untargeted testing approach leads to wasted experimental resources and increased labor costs, severely reducing R&D and production implementation efficiency, delaying electrolyte development processes, and hindering rapid cell iteration.

[0006] Therefore, research focusing on DCR source analysis can fundamentally solve the problem of untargeted testing, clearly determine whether DCR growth mainly comes from the positive or negative electrode, and then purposefully improve the compatibility between the electrolyte and the positive / negative electrode, thereby quickly optimizing the electrolyte composition and suppressing battery DCR growth.

[0007] Meanwhile, in a full battery system, free radicals and other byproducts generated by the reaction between the electrolyte and the positive / negative electrodes migrate from the working electrode to the counter electrode, leading to crosstalk between the positive and negative electrodes; for example, RH is generated on the surface of the positive electrode by the electrolyte. + Free radicals may migrate to the surface of the negative electrode and further reduce and decompose to generate H2; at the same time, the DCR and other electrochemical parameters of the positive and negative electrodes are difficult to decouple, interfering with the judgment of the source of DCR growth. Therefore, it is urgent to eliminate the influence of the counter electrode on the working electrode through battery structure design, so as to directly determine the source of DCR growth. Summary of the Invention

[0008] To address the existing problems, this invention provides a method for tracing the growth of DCR in full-cell batteries, preparing symmetric cells, and optimizing electrolytes. The specific solution is as follows:

[0009] A method for tracing the growth of full-cell DCR and preparing symmetric cells includes the following steps:

[0010] S1. Preparation of symmetrical battery system: Select single-sided positive electrode or single-sided negative electrode from cells that have not been circulated after formation or cells that have been circulated, and assemble them into positive electrode symmetrical battery and negative electrode symmetrical battery respectively; and inject basic electrolyte and test electrolyte into the positive electrode symmetrical battery and negative electrode symmetrical battery respectively to form two kinds of symmetrical batteries.

[0011] S2, DCR growth source analysis: The symmetrical battery assembled in step S1 was subjected to constant current discharge test to obtain DCR growth rate data, and it was determined whether the DCR growth of the whole battery mainly came from the positive electrode side or the negative electrode side; and combined with electrochemical impedance spectroscopy (EIS) and relaxation time distribution (DRT) methods, it was further distinguished whether the DCR growth mainly came from the SEI film impedance growth or the charge transfer impedance growth.

[0012] This invention establishes a complete method system for tracing the source of DCR growth in full-cell batteries. By constructing symmetrical cells to eliminate crosstalk effects between the positive and negative electrodes, it achieves accurate determination of the source of DCR growth (positive electrode side or negative electrode side). Furthermore, by combining EIS and DRT technologies, it further distinguishes the type of impedance growth (SEI / CEI film impedance or charge transfer impedance), providing a clear diagnostic basis for electrolyte optimization.

[0013] Preferably, in step S1, the preparation of the symmetrical cell specifically includes:

[0014] S11. Electrode Acquisition: Disassemble the pouch cells that have not been cycled after formation based on the base electrolyte and the electrolyte to be tested, as well as the pouch cells that have been cycled a specified number of times and then fixed to 50% SOC, and obtain their positive and negative electrode sheets respectively.

[0015] S12. Electrode processing: Cut out the middle area from the obtained electrode and wipe off the single-sided active material of the electrode to obtain a single-sided electrode.

[0016] S13. Battery assembly: Two identical single-sided positive electrode plates or two identical single-sided negative electrode plates are used as working electrodes and counter electrodes, and are packaged together with a separator to form a pouch cell. The corresponding electrolyte is injected to obtain positive electrode symmetrical pouch cells and negative electrode symmetrical pouch cells, respectively.

[0017] The detailed descriptions of steps S11 to S13 specify the preparation process of the symmetrical battery. By disassembling the actual battery cell to obtain the electrode sheet and performing single-sided processing, the consistency of the electrode state between the symmetrical battery and the full battery is ensured, thereby improving the representativeness and reliability of the test results.

[0018] Preferably, in step S2, a constant current discharge test is performed on the assembled symmetrical battery to obtain the DCR value of each symmetrical battery; by comparing the difference in DCR growth between the positive electrode symmetrical battery and the negative electrode symmetrical battery in the base electrolyte system and the electrolyte system to be tested before and after cycling, it is determined whether the DCR growth of the whole battery mainly comes from the positive electrode side or the negative electrode side; the determination is specifically as follows: if the difference in DCR growth between the positive electrode symmetrical battery and the electrolyte system to be tested is greater than the difference in DCR growth between the negative electrode symmetrical battery, then it is determined that the DCR growth of the whole battery mainly comes from the positive electrode side; otherwise, it is determined that it mainly comes from the negative electrode side.

[0019] The above textual description provides a clear and logical basis for judging the source of DCR growth. By comparing the differences in DCR growth of symmetrical cells in different electrolyte systems, the contribution of the positive and negative electrodes to the DCR growth of the whole cell is quantitatively analyzed, making the judgment process objective and repeatable.

[0020] Preferably, for the side where the DCR growth is mainly determined in step S2, if the change in charge transfer impedance of its symmetrical cell before and after cycling is greater than the change in SEI film impedance, then the DCR growth on that side is determined to be mainly from the charge transfer impedance; otherwise, it is determined to be mainly from the SEI film impedance.

[0021] Based on identifying the source of the growth, the dominant mechanism of impedance growth (charge transfer impedance or SEI / CEI membrane impedance) was further distinguished by the change in EIS parameters, achieving a detailed analysis of the interfacial reaction process and providing a more in-depth basis for targeted regulation of the electrolyte.

[0022] Preferably, the positive electrode material of the symmetrical positive electrode battery includes at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel-cobalt-manganese ternary materials, lithium cobalt oxide, and their composite systems. This method is applicable to a variety of mainstream positive electrode material systems, enhancing the versatility and industrial application value of the invention, and is particularly suitable for DCR analysis of common positive electrode materials in power batteries and energy storage batteries.

[0023] Preferably, the negative electrode material of the negative electrode symmetric battery includes at least one of natural graphite, artificial graphite, hard carbon, silicon, and composite materials thereof. This method is applicable to various mainstream negative electrode material systems, covering commonly used negative electrode types from graphite to silicon-based composite materials, thus improving the applicability of the method in various lithium-ion batteries.

[0024] Preferably, both the base electrolyte and the test electrolyte contain lithium salt and an organic solvent; the organic solvent includes linear carbonates, cyclic carbonates, and their derivatives. The molar ratio of the lithium salt, linear carbonates, and cyclic carbonates and their derivatives is (1~1.5):(3~7):(2~4). This defines the basic composition and ratio range of the electrolyte, ensuring consistency between the symmetric battery test conditions and the actual battery system, and making the analytical results more reflective of the interfacial behavior under real electrochemical conditions.

[0025] Preferably, the lithium salt is at least one selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium difluorooxalateborate. The commonly used lithium salt types listed above cover the current mainstream electrolyte systems, enhancing the applicability and operability of the method in practical electrolyte formulation evaluation.

[0026] Preferably, the linear carbonate is at least one selected from dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; the cyclic carbonate is at least one selected from ethylene carbonate, propylene carbonate, and vinylene carbonate. The specific types of organic solvents described above cover conventional carbonate solvents and their derivatives, making the construction of the electrolyte system closer to actual production formulations and enhancing the engineering guidance significance of the testing.

[0027] This invention also discloses an electrolyte optimization method, which, based on the DCR growth source determined by any of the methods described above, specifically adjusts the electrolyte composition:

[0028] If the growth is determined to originate from the charge transfer impedance on the positive electrode side, the electrolyte solvation structure is adjusted to improve its antioxidant stability.

[0029] If the growth is determined to originate from the charge transfer impedance on the negative electrode side, the solvation structure is adjusted to reduce the lithium-ion desolvation energy barrier.

[0030] If the increase is determined to be due to the impedance of the SEI film on the positive electrode side, then add positive electrode film-forming additives or deacidifying agents.

[0031] If the increase is determined to originate from the SEI film impedance on the negative electrode side, then reduce easily reducible components or add negative electrode film-forming additives.

[0032] Based on the aforementioned diagnostic results of DCR growth sources, targeted electrolyte optimization strategies were provided, forming an integrated "diagnosis-regulation" solution that significantly improves the efficiency and accuracy of electrolyte development and avoids the waste of resources caused by blind experimentation.

[0033] The beneficial effects of this invention are as follows:

[0034] This invention, by constructing symmetrical cells and employing various electrochemical testing methods, can pre-identify the root causes of DCR growth in full-cell batteries. Unlike traditional DCR growth analysis methods, this invention is based on symmetrical cells rather than full or half-cells, thus eliminating the influence of non-working electrodes and obtaining accurate detection results. Furthermore, unlike traditional designs that only focus on macroscopic DCR growth after solvent / additive addition, this invention uses DRT to decouple the EIS results of symmetrical cells and combines them with the macroscopic DCR growth trend. This not only accurately determines whether DCR originates from the positive or negative electrode but also analyzes the specific nodes that dominate DCR growth during lithium-ion migration, thereby clarifying the electrolyte failure mechanism. Based on this precise identification, this invention can finely control solvents and additives to target high DCR growth nodes at the positive / negative electrode interface SEI, the desolvation process on the positive and negative electrode surfaces, the structure of the positive and negative electrode materials, and the electrolyte bulk, thereby effectively curbing DCR growth in the full cell. This invention effectively solves the problem of determining strategies to suppress DCR growth in electrolyte design, significantly reducing the design complexity of electrolytes with low DCR growth, and providing important insights for efficient electrolyte design and modification. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a comparison example of the DCR growth of the positive / negative electrode symmetrical battery in Example 1 before and after cycling;

[0037] Figure 2 This is a comparative example of EIS growth of the positive / negative electrode symmetrical battery in Example 1 before and after cycling; Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] A method for tracing the source of DCR growth in full-cell batteries, preparing symmetrical batteries, and optimizing electrolytes includes the design of a symmetrical battery system fabrication method, the design of a DCR growth analysis method based on the battery system, and an electrolyte optimization strategy proposed based on the aforementioned diagnostic results of DCR growth sources.

[0040] Firstly, the positive and negative electrode materials of the symmetrical battery are derived from the full battery before cycling and after 1000 cycles, and its assembly process includes the following steps:

[0041] (i) Disassemble the battery based on electrolyte 1 that has been formed to meet capacity but has not been cycled to obtain the first positive electrode and the first negative electrode; disassemble the battery based on electrolyte 2 that has been formed to meet capacity but has not been cycled to obtain the second positive electrode and the second negative electrode; disassemble the battery based on electrolyte 1 that has been cycled 1000 times to obtain the third positive electrode and the third negative electrode; disassemble the battery based on electrolyte 2 that has been cycled 1000 times to obtain the fourth positive electrode and the fourth negative electrode; cut the positive / negative electrode obtained from the disassembly to appropriate lengths, wherein the electrode is selected from the middle part of the battery.

[0042] (ii) Fix the disassembled positive / negative electrode sheets onto a smooth, clean plate and secure them with sealing tape. Use NMP to remove the material from one side of the first, second, third, and fourth positive electrode sheets, and use alcohol to remove the material from one side of the first, second, third, and fourth negative electrode sheets. Then, using a 5.4cm × 3.4cm mold, cut the electrode sheets with the removed material from one side into two electrode sheets of the same size, and weld the tabs together.

[0043] (III) Assemble a symmetrical positive / negative single-cell pouch cell with two first positive / negative electrode pieces with welded tabs, using electrolyte 1 without circulation; assemble a symmetrical positive / negative single-cell pouch cell with two second positive / negative electrode pieces, using electrolyte 2 without circulation; assemble a symmetrical positive / negative single-cell pouch cell with two third positive / negative electrode pieces, using electrolyte 1 after circulation; and assemble a symmetrical positive / negative single-cell pouch cell with two fourth positive / negative electrode pieces, using electrolyte 2 after circulation. Finally, inject the corresponding electrolyte 1 and electrolyte 2 to obtain eight symmetrical pouch cells.

[0044] The discharge DCR of eight pouch cells was tested using a DCR program. The values ​​are denoted as R in sequence. 1-DCR R 1+DCR R 2-DCR R 2+DCR R 1-DCR ', R 1+DCR ', R 2-DCR ', R 2+DCR 'Then the negative electrode DCR in electrolyte 1 increases by ΔR.' 1-DCR =R 1-DCR '-R 1-DCR ; The increase in positive electrode DCR ΔR in electrolyte 1 1+DCR =R 1+DCR '-R 1+DCR The DCR impedance growth ΔR in electrolyte 2 2-DCR =R 2-DCR '-R 2-DCR The DCR impedance growth ΔR in electrolyte 2 2+DCR =R 2+DCR '-R 2+DCR If ΔR 2+DCR -ΔR 1+DCR >ΔR 2-DCR -ΔR 1-DCR If the battery DCR increases, the increase mainly comes from the positive electrode side; conversely, if the battery DCR increases, the increase mainly comes from the negative electrode side.

[0045] The DCR growth during battery cycling is influenced by side reactions of the positive and negative electrode materials and the electrolyte on the positive and negative electrode surfaces. Therefore, to suppress crosstalk effects, this invention designs a symmetrical battery scheme, thereby eliminating the influence of the counter electrode on the working electrode. The DCR growth on the positive and negative electrode surfaces is analyzed. Two electrolyte systems with different DCR growth rates are selected for comparison, rather than a direct comparison of one positive and negative electrode, thus eliminating the influence of the intrinsic materials of the positive and negative electrodes and allowing for targeted optimization of the electrolyte design.

[0046] Secondly, after initially identifying the source of battery DCR growth, EIS testing is used to further determine the source of battery DCR growth. The specific technical solution is as follows:

[0047] (iv) The sources of DCR in the full cell of electrolyte 1 after uncirculated (R) and after 1000 cycles (R') are separated into positive / negative electrode surface contact resistance R. 1-0 / R 1-0 ' / R 1+0 / R 1+0 ', the internal resistance R of the SEI on the positive / negative electrode surface 1-SEI / R 1-SEI ' / R 1+SEI / R 1+SEI Electrode desolvation internal resistance R 1-ct / R 1-ct ' / R 1+ct / R 1+ct 'and the intrinsic resistance R of the electrode 1-w / R 1+w The electrolyte in the full cell after no cycling and 1000 cycles was used to determine the source of DCR, which was then separated into positive / negative electrode surface contact resistance R. 2-0 / R 2-0 ' / R 2+0 / R 2+0 ', the internal resistance R of the SEI on the positive / negative electrode surface 2-SEI / R 2-SEI ' / R 2+SEI / R 2+SEI Electrode desolvation internal resistance R 2-ct / R 2-ct ' / R 2+ct / R 2+ct 'and the intrinsic resistance R of the electrode 2-w / R 2+w .

[0048] (v) Before measuring EIS, the battery was pre-cycled and capacitated to 50% SOC with a voltage of 0V. Then, the EIS of eight symmetrical cells was measured using an electrochemical workstation. The voltage perturbation was set to 10 mV, the frequency range was set to 2 MHz~10 mHz, and the number of test points was set to 84.

[0049] (vi) Using EC-lab fitting and DRT analysis of the EIS results of eight symmetrical cells, among which the contact resistance R 1-0 / R 1-0 ' / R 1+0 / R 1+0 ' / R 2-0 / R 2-0 ' / R 2+0 / R 2+0 'The initial value of the curve is >8 kHz (10 -4 The resistance value at point s), the SEI resistor R 1-SEI / R 1-SEI ' / R 1+SEI / R 1+SEI ' / R 2-SEI / R 2-SEI ' / R 2+SEI / R 2+SEI 'The first inflection point is 3 kHz to 100 Hz (10 -2 The resistance at point s) represents the charge transfer impedance R during desolvation. 1-ct / R 1-ct ' / R 1+ct / R 1+ct ' / R 2-ct / R 2-ct ' / R 2+ct / R 2+ct 'For the second inflection point 10~100 mHz (10 1 The resistance value at point s).

[0050] (vii) Based on the results of (vi) and (iii), if R 1-ct '- R 1-ct > R 1-SEI '- R 1-SEI Or R 2-ct '- R 2-ct >R 2-SEI '- R 2-SEI If the negative electrode side DCR growth is mainly due to charge transfer impedance, then the growth is mainly due to SEI film formation impedance. If R 1+ct '- R 1+ct > R 1+SEI '- R 1+SEI Or R 2+ct '- R 2+ct > R 2+SEI '- R 2+SEI If the positive electrode DCR increases, it mainly comes from charge transfer impedance; conversely, if the negative electrode DCR increases, it comes from SEI film formation impedance. During battery cycling, the interfacial electrochemistry and side reactions of the electrolyte at the positive and negative electrode surfaces significantly affect the DCR changes on the electrode surfaces. These effects primarily include the charge transfer impedance (Rc) resulting from the interfacial desolvation process. ct) and from Li + SEI impedance (R) during the SEI penetration process SEI The contact resistance (R0) is small and the difference before and after the cycle is negligible, so it is not included in the analysis.

[0051] The design concept of this invention is to first test the DCR of symmetrical positive and negative electrodes of uncycled and cycle-completed batteries to preliminarily determine whether the DCR growth is dominated by the positive or negative electrode side. Then, further in-depth analysis using EIS testing is conducted to determine whether the DCR growth primarily originates from the desolvation process during battery charging and discharging or from the Li... + Penetrating the SEI. Unlike traditional testing methods, this invention first utilizes a symmetrical battery design to eliminate interference from the Li sheet or positive electrode, achieving DCR growth testing under conditions free from counter-electrode interference. Second, unlike the traditional method of disassembling and assembling coin cells, this invention manufactures pouch cells, which offer superior consistency and more accurate, convincing results compared to coin cells. Furthermore, this invention further detects the EIS of the positive and negative electrodes to determine which step in the electrochemical process the battery's DCR growth originates from, facilitating the targeted design of appropriate electrolytes.

[0052] Once the source of DCR growth in the battery is determined, the electrolyte can be designed accordingly. If the DCR growth is determined to originate from the positive electrode side and is mainly due to interfacial charge transfer resistance, it indicates that there are many electrochemical side reactions in the electrolyte on the positive electrode surface. This can be addressed by controlling the solvation structure of the electrolyte and suppressing PF6 in the solvation structure. - - The interaction of solvent clusters enhances the antioxidant stability of the solvated structure, thereby improving DCR growth; if the DCR growth of the battery is determined to originate from the negative electrode side and mainly from the interfacial charge transfer impedance, it indicates that the Li in the electrolyte... + Desolvation is difficult on the negative electrode surface, leading to Li... + The electrochemical side reactions involved can be mitigated by adjusting the solvation structure of the electrolyte and appropriately weakening the Li in the solvation structure. + - Interactions of solvent clusters reduce Li + Desolvation energy barrier, accelerating Li +Desolvation processes can be used to improve DCR growth. If the DCR growth originates from the positive electrode side and is mainly due to SEI impedance, it indicates that the additives / solvents in the electrolyte have poor antioxidant stability and react too violently on the positive electrode surface, leading to continuous growth of CEI on the positive electrode surface. This can be improved by reducing the content of easily oxidized additives and using positive electrode film-forming additives and HF / H2O inhibitors to suppress CEI growth on the positive electrode surface. If the DCR growth originates from the negative electrode side and is mainly due to SEI impedance, it indicates that there are too many easily reducible additives / solvents in the electrolyte, causing the electrolyte to be continuously consumed during charging and discharging, forming a thick SEI layer. This can be improved by reducing the content of easily reducible additives or controlling the interfacial solvation structure to suppress the presence of easily reducible solvents in the first solvation shell, thereby weakening electrolyte decomposition and SEI growth on the negative electrode surface.

[0053] Example 1:

[0054] like Figure 1-2 In this embodiment, the battery system used is a graphite (Gr)||lithium manganese iron phosphate (LMFP) system. Electrolyte system 1 is a mixture of LiPF6, EC, EMC, DMC, EA, and VC in a ratio of 1:3.3:5.16:3.5:0.1, and electrolyte system 2 is a mixture of LiPF6, EC, EMC, DMC, EA, and VC in a ratio of 1:3.3:5.16:3.5:0.3 with an injection coefficient of 3.4 g / Ah. The initial capacity of the stacked soft-pack battery is 2.4 Ah.

[0055] Symmetrical pouch cell preparation: Four pouch cells were activated at a constant temperature of 25°C, with two charge-discharge cycles at 1 / 10 C and two charge-discharge cycles at 1 / 3 C between 2.5 V and 4.25 V. The resulting cells were uncycled. Subsequently, all four pouch cells were cycled at 1 C for 500 cycles, resulting in cycled cells. The two uncycled cells and the two cycled cells were moved to a drying room and disassembled, and two positive and two negative electrode plates were removed from each. Then, one side of the positive electrode plate was wiped clean with N-methylpyrrolidone (NMP), and one side of the negative electrode plate was wiped clean with anhydrous ethanol. Afterward, the two types of electrode plates were cut into appropriate sizes using a mold, sealed with a separator and aluminum-plastic film, and then injected with electrolyte and heat-pressed for sealing, assembling eight symmetrical single-cell pouch cells.

[0056] Symmetrical pouch cell DCR test: Eight symmetrical pouch cells were left to stand at a constant temperature of 25°C for 3 hours, and then subjected to DCR testing using a Xinwei testing instrument. The specific steps were as follows: 0.1C constant current charging for 1 hour, then stopping charging and letting stand for 9-11 minutes; 0.1C constant current discharging for 1 hour, then stopping discharging and letting stand for 9-11 minutes; pre-cycle charge and discharge 3 times to activate the electrodes and ensure the voltage is around 0 V; let stand for 9-11 minutes; 2C constant current charging for 30 seconds; let stand for 9-11 minutes. The DCR of the eight symmetrical cells was calculated as follows: R... DCR = (2C constant current charging end voltage - resting end voltage) / 2C constant current charging current. The results are shown in Table 1, and it can be found that ΔR 1+DCR -ΔR 2+DCR <ΔR 1-DCR -ΔR 2-DCR This indicates that the DCR growth of electrolyte 1 system mainly comes from the negative electrode side.

[0057] Symmetrical soft-pack EIS test: After the DCR test, the battery was placed on an electrochemical workstation for EIS testing. The specific steps were as follows: 0.1C constant current charging for 1 hour, then stopping charging and letting it stand for 9-11 minutes; 0.1C constant current discharging for 2 hours until the voltage reached 0 V, then stopping discharging and letting it stand for 9-11 minutes; the voltage perturbation was set to 10mV, the frequency range was set to 2 MHz~10 mHz, and the number of test points was set to 84 for EIS testing; the test results are shown in Table 2. It can be found that R... 1-ct '- R 1-ct > R 1-SEI '- R 1-SEI Or R 2-ct '- R 2-ct > R 2-SEI '- R 2-SEI This indicates that the increase in DCR on the negative electrode side mainly comes from the increase in charge transfer impedance.

[0058] Table 1 Comparison of DCR growth of symmetrical pouch cells before and after cycling.

[0059]

[0060] The above test results show that ΔR 1+DCR -ΔR 2+DCR =-0.136 Ω <ΔR 1-DCR -ΔR 2-DCR =0.265 Ω, indicating that the high DCR growth problem in electrolyte 1 system mainly comes from the negative electrode side.

[0061] Table 2 Comparison of EIS growth of symmetrical pouch cells before and after cycling.

[0062]

[0063] As can be seen from the table above, R 1-ct '- R 1-ct > R 1-SEI '- R 1-SEI This indicates that the increase in DCR on the negative electrode side of electrolyte 1 system mainly comes from the increase in charge transfer impedance.

[0064] Given that the main source of DCR growth is determined to be the charge transfer resistance on the negative electrode surface, which is directly related to the desolvation process on the negative electrode surface and the SEI deposition and dissolution side reaction on the negative electrode surface, a weak solvating solvent can be used to reduce the desolvation barrier of the electrolyte, while negative electrode film-forming additives can be used to inhibit electrolyte decomposition on the negative electrode surface and surface SEI dissolution.

[0065] The prior description of this disclosure is provided to enable any person skilled in the art to make or use this disclosure. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles defined herein may be applied to other variations without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not intended to be limited to the examples and designs described herein, but should be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0066] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for tracing the growth of full-cell DCR and preparing symmetrical cells, characterized in that, Includes the following steps: S1. Preparation of symmetrical battery system: Select single-sided positive electrode or single-sided negative electrode from cells that have not been circulated after formation or cells that have been circulated and then fixed to 50% SOC, and assemble them into positive electrode symmetrical battery and negative electrode symmetrical battery respectively; and inject basic electrolyte and test electrolyte into the positive electrode symmetrical battery and negative electrode symmetrical battery respectively to form two kinds of symmetrical batteries. S2, DCR growth source analysis: The symmetrical battery assembled in step S1 is subjected to constant current discharge test to obtain DCR growth rate data, and it is determined whether the DCR growth of the whole battery mainly comes from the positive electrode side or the negative electrode side; and combined with electrochemical impedance spectroscopy (EIS) and relaxation time distribution (DRT) methods, it is further distinguished whether the DCR growth mainly comes from the growth of SEI / CEI film impedance or the growth of charge transfer impedance.

2. The method according to claim 1, characterized in that, In step S1, the fabrication of the symmetrical cell specifically includes: S11. Electrode Acquisition: Disassemble the pouch cells that have not been cycled after formation based on the base electrolyte and the electrolyte to be tested, as well as the pouch cells that have been cycled a specified number of times, and obtain their positive and negative electrode plates respectively. S12. Electrode processing: Cut out the middle area from the obtained electrode and wipe off the single-sided active material of the electrode to obtain a single-sided electrode. S13. Battery assembly: Two identical single-sided positive electrode plates or two identical single-sided negative electrode plates are used as working electrodes and counter electrodes, and are packaged together with a separator to form a pouch cell. The corresponding electrolyte is injected to obtain positive electrode symmetrical pouch cells and negative electrode symmetrical pouch cells, respectively.

3. The method according to claim 1, characterized in that: In step S2, a constant current discharge test is performed on the assembled symmetrical cells to obtain the DCR value of each symmetrical cell. By comparing the difference in DCR growth between the positive electrode symmetrical cell and the negative electrode symmetrical cell in the base electrolyte system and the electrolyte system under test before and after cycling, it is determined whether the DCR growth of the whole cell mainly comes from the positive electrode side or the negative electrode side. Specifically, if the difference in DCR growth between the positive electrode symmetrical cell and the electrolyte system under test is greater than that between the negative electrode symmetrical cell and the electrolyte system under test, then it is determined that the DCR growth of the whole cell mainly comes from the positive electrode side; otherwise, it is determined that it mainly comes from the negative electrode side.

4. The method according to claim 3, characterized in that: After determining the main source of DCR growth in step S2, if the charge transfer impedance R of its symmetrical cell... ct The change before and after the cycle is greater than the change in SEI / CEI membrane impedance R. SEI If so, it is determined that the increase in DCR on this side mainly comes from charge transfer impedance; Conversely, it is determined that the impedance mainly originates from the SEI / CEI film.

5. The method according to any one of claims 1-4, characterized in that: The positive electrode material of the positive electrode symmetrical battery includes at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel-cobalt-manganese ternary materials, lithium cobalt oxide, and their compound systems.

6. The method according to any one of claims 1-4, characterized in that: The negative electrode material of the negative electrode symmetrical battery includes at least one of natural graphite, artificial graphite, hard carbon, silicon, and their composite systems.

7. The method according to claim 1, characterized in that: Both the base electrolyte and the electrolyte to be tested contain lithium salt and organic solvent; the organic solvent includes linear carbonate, cyclic carbonate and its derivatives, wherein the molar ratio of the lithium salt, linear carbonate and cyclic carbonate and its derivative organic solvent is (1~1.5):(3~7):(2~4).

8. The method according to claim 7, characterized in that: The lithium salt is at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium difluorooxalateborate.

9. The method according to claim 7, characterized in that: The linear carbonate is at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; the cyclic carbonate is at least one of ethylene carbonate, propylene carbonate, and vinylene carbonate.

10. A method for optimizing an electrolyte, characterized in that, Based on the DCR growth source determined by the method described in any one of claims 1-9, the electrolyte composition is adjusted accordingly: If the growth is determined to originate from the charge transfer impedance on the positive electrode side, the electrolyte solvation structure is adjusted to improve its antioxidant stability. If the growth is determined to originate from the charge transfer impedance on the negative electrode side, the solvation structure is adjusted to reduce the lithium-ion desolvation energy barrier. If the increase is determined to be due to the impedance of the SEI film on the positive electrode side, then add positive electrode film-forming additives or dehydrating and deacidifying agents. If the increase is determined to originate from the SEI film impedance on the negative electrode side, then reduce easily reducible components or add negative electrode film-forming additives.

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