Method for determining additive amount of LiFSI in electrolyte

Abstract

The invention belongs to the field of lithium ion batteries, and particularly relates to a method for determining the addition amount of LiFSI in an electrolyte. The method comprises the following steps: providing a series of electrolytes with different LiFSI contents, and respectively assembling the electrolytes into batteries; testing the electrolyte through cyclic voltammetry to obtain corrosion current data; performing a DSC test on the negative plate powder of the battery to obtain DSC test data (including at least one of heat release temperature and heat release amount); performing a thermal runaway test on the battery to obtain thermal runaway test data (including at least one of thermal runaway time and maximum thermal runaway temperature); determining the addition amount of LiFSI in the electrolyte according to the corrosion current data, the DSC test data and the thermal runaway test data; and comprehensively evaluating the data, and determining the additive amount of LiFSI in the electrolyte. The method has multiple dimensions, comprehensiveness and high accuracy.

Description

Technical Field

[0001] This application belongs to the field of lithium-ion batteries, specifically relating to a method for determining the amount of LiFSI added to an electrolyte. Background Technology

[0002] LiFSI, as a novel lithium salt, possesses superior chemical stability and ionic conductivity compared to traditional LiPF6, significantly improving battery low-temperature performance, fast-charging response, and cycle life. Furthermore, within a reasonable range, higher addition amounts result in even better electrical performance, indicating broad application prospects. However, LiFSI faces key technical bottlenecks: its reaction with lithium-intercalated graphite generates higher heat, leading to decreased battery thermal stability and increased risk of thermal runaway. High concentrations also accelerate corrosion of the aluminum current collector in the cathode, damaging the electrode structure. Current technologies lack a systematic method for evaluating LiFSI addition amounts, failing to balance its performance-enhancing effects with potential safety and stability risks, thus limiting large-scale application. Therefore, a multi-dimensional evaluation system for accurately identifying LiFSI addition amounts is urgently needed. Summary of the Invention

[0003] This application aims to at least partially address one of the technical problems in related technologies. To this end, this application proposes a method for determining the amount of LiFSI added to an electrolyte in a multi-dimensional, comprehensive, or highly precise manner.

[0004] A first aspect of this application provides a method for determining the amount of LiFSI added to an electrolyte, comprising: A series of electrolytes with different LiFSI contents are provided and assembled into batteries respectively; The electrolyte was tested using cyclic voltammetry to obtain corrosion current data; The negative electrode powder of the battery is subjected to DSC test to obtain DSC test data, which includes at least one of the exothermic temperature and the exothermic amount. The battery is subjected to a thermal runaway test to obtain thermal runaway test data, which includes at least one of thermal runaway time and maximum thermal runaway temperature. The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, and the thermal runaway test data.

[0005] According to an embodiment of this application, the above method specifically includes: preparing electrolytes with LiFSI contents of 1, 2, ..., n, respectively, according to the LiFSI content from low to high, and assembling them into batteries to obtain batteries 1, 2, ..., n. Using aluminum as the working electrode, lithium metal as the reference electrode, and lithium metal as the counter electrode, a three-electrode system is constructed. Cyclic voltammetry is performed on the first electrolyte, the second electrolyte, ..., the nth electrolyte to detect whether a corrosion current peak appears and obtain corrosion current data. After the first battery, the second battery, ..., the nth battery are fully charged at 0.5C, the negative electrode sheet is removed, the negative electrode powder is scraped off and DSC test is performed to obtain DSC test data. The DSC test data includes at least one of the exothermic temperature and the exothermic heat. The first battery, the second battery, ..., the nth battery are fully charged at 0.5C constant current and constant voltage under 25°C conditions, and then thermal runaway test is performed to obtain thermal runaway test data. The thermal runaway test data includes at least one of thermal runaway time and thermal runaway maximum temperature. The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, and the thermal runaway test data.

[0006] According to an embodiment of this application, the solvent in the electrolyte is a mixed solvent of EC and EMC with a mass ratio of 3:7.

[0007] According to embodiments of this application, the cyclic voltammetry test satisfies at least one of the following conditions: The voltage range of the cyclic voltammetry test is 3~5V; The cyclic voltammetry test scan rate is 1 mV / s to 10 mV / s.

[0008] According to embodiments of this application, the DSC test satisfies at least one of the following conditions: The temperature range for the DSC test is 25~400℃; The temperature rise rate of the DSC test is 5℃ / min to 10℃ / min.

[0009] According to an embodiment of this application, the thermal runaway test includes heating the battery to 200°C at a rate of 5°C / min, maintaining it for 30 minutes, triggering the thermal runaway time and the highest thermal runaway temperature.

[0010] According to embodiments of this application, the amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, and the thermal runaway test data, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, then the second upper limit content of LiFSI is determined to be equal to the content of the b-1-th battery. If the DSC test data of cell c satisfies the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, and the DSC test data of cell c-1 does not satisfy the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, then the third upper limit content of LiFSI is determined to be equal to the content of cell c-1. The minimum value among the first upper limit content, the second upper limit content, and the third upper limit content is taken as the upper limit content of LiFSI in the electrolyte.

[0011] According to embodiments of this application, the above method further includes: After storing the first battery, the second battery, ..., the nth battery at 60°C for 3 months, the positive electrode sheet was removed, the positive electrode active material was peeled off, and the positive electrode current collector was obtained. The positive electrode current collector was observed by SEM to obtain the corrosion data of the positive electrode current collector. The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, and the corrosion data of the positive electrode current collector.

[0012] According to embodiments of this application, the amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, and the positive electrode current collector corrosion data, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, then the second upper limit content of LiFSI is determined to be equal to the content of the b-1-th battery. If the DSC test data of cell c satisfies the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, and the DSC test data of cell c-1 does not satisfy the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, then the third upper limit content of LiFSI is determined to be equal to the content of cell c-1. If the m-th cell satisfies the condition that pores appear on the surface of the positive electrode current collector, and the (m-1)-th cell does not have pores on the surface of the positive electrode current collector, then the fourth upper limit content of LiFSI is determined to be equal to the (m-1)-th content. The minimum value among the first upper limit content, the second upper limit content, the third upper limit content, and the fourth upper limit content is taken as the upper limit content of LiFSI in the electrolyte.

[0013] According to embodiments of this application, the above method further includes: After storing the first battery, the second battery, ..., the nth battery at 60°C for 3 months, the negative electrode was removed, and powder was scraped off the surface of the electrode for ICP testing to obtain aluminum content data. The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, the corrosion data of the positive electrode current collector, and the aluminum content data.

[0014] According to embodiments of this application, the amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, the positive electrode current collector corrosion data, and the aluminum content data, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, then the second upper limit content of LiFSI is determined to be equal to the content of the b-1-th battery. If the DSC test data of cell c satisfies the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, and the DSC test data of cell c-1 does not satisfy the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, then the third upper limit content of LiFSI is determined to be equal to the content of cell c-1. If the m-th cell satisfies the condition that pores appear on the surface of the positive electrode current collector, and the (m-1)-th cell does not have pores on the surface of the positive electrode current collector, then the fourth upper limit content of LiFSI is determined to be equal to the (m-1)-th content. If the nth cell satisfies that the aluminum content of the negative electrode is >10ppm and the (n-1)th cell does not satisfy that the aluminum content of the negative electrode is >10ppm, then the fifth upper limit content of LiFSI is determined to be equal to the (n-1)th content. The minimum value among the first upper limit content, the second upper limit content, the third upper limit content, the fourth upper limit content, and the fifth upper limit content is taken as the upper limit content of LiFSI in the electrolyte. Detailed Implementation

[0015] The embodiments of this application are described in detail below and are intended to explain this application, but should not be construed as limiting this application.

[0016] Currently, lithium hexafluorophosphate (LiPF6) is widely used as the lithium salt in commercial electrolytes. While it possesses good ion dissociation capabilities, it suffers from inherent defects such as insufficient thermal stability, poor low-temperature conductivity, and susceptibility to hydrolysis leading to HF corrosion of the electrodes. These drawbacks make it difficult to meet the stringent requirements of fast charging for power batteries, low-temperature operation, and long-cycle operation for energy storage batteries. Lithium bisfluorosulfonyl imide (LiFSI), as a novel high-performance lithium salt, exhibits unique advantages due to its distinctive molecular structure: Firstly, LiFSI possesses higher thermal and chemical stability, is less prone to hydrolysis producing corrosive substances, and helps improve battery storage stability and cycle life. Secondly, its lower dissociation energy significantly improves the ionic conductivity of the electrolyte, especially maintaining excellent ion transport efficiency at low temperatures, thus effectively improving the low-temperature discharge performance and high-rate fast charging capability of lithium-ion batteries. Furthermore, research shows that within a certain range, the higher the LiFSI content in the electrolyte, the more significant the improvement in these electrical performance characteristics, making it a promising candidate for high-performance lithium-ion batteries.

[0017] However, the large-scale application of LiFSI still faces two major technical bottlenecks that urgently need to be overcome: First, the exothermic effect of the interfacial reaction between LiFSI and graphite electrodes during charge and discharge is significantly higher than that of LiPF6. Excessive addition can increase the risk of thermal runaway in the battery system, seriously affecting the battery's safety performance. Second, under high-voltage conditions, LiFSI is prone to corrosion reaction with the battery current collector (aluminum foil), forming an unstable passivation film. This not only reduces the stability of the electrolyte but may also cause current collector damage, a surge in battery internal resistance, and ultimately lead to battery performance degradation or even failure. More importantly, the industry has not yet established a scientific and systematic method to evaluate the appropriate amount of LiFSI added to the electrolyte. Existing studies mostly focus on the effect of LiFSI on improving electrical performance but lack a synergistic consideration of its safety and corrosiveness. It is impossible to accurately define the balance range between "electrical performance optimization" and "safety performance assurance," making it difficult to quantitatively control the amount of LiFSI added, which greatly limits its large-scale application in power batteries and energy storage batteries.

[0018] Based on the aforementioned technological status, in order to address the technological gap in the lack of a method for evaluating the appropriate amount of LiFSI in existing technologies, and to simultaneously consider both the improvement of the electrical performance and the assurance of the safety performance of lithium-ion batteries, this application proposes a method for determining the appropriate amount of LiFSI added to the electrolyte. Through the synergistic screening of multi-dimensional performance indicators (electrolyte-electrode-battery), the range of LiFSI addition amount is accurately determined, which not only fully leverages its role in improving the electrical performance of the battery, but also effectively reduces or avoids the safety and corrosion problems it brings, providing a scientific basis and technical support for the formulation optimization of high-performance lithium-ion battery electrolytes.

[0019] In view of this, this application proposes a method for determining the amount of LiFSI added in an electrolyte, comprising: S10: Provides a series of electrolytes with different LiFSI contents, and assembles them into batteries.

[0020] As a specific example, electrolytes with LiFSI contents of 1, 2, ..., n are prepared according to the LiFSI content from low to high, and then assembled into batteries to obtain battery 1, battery 2, ..., battery n.

[0021] In this step, electrolytes with different LiFSI contents, along with positive electrode plates, negative electrode plates, and separators, are assembled into a battery.

[0022] According to embodiments of this application, the lithium salt concentration in the electrolyte ranges from 0.5M to 1M, specifically 0.5M, 0.6M, 0.7M, 0.8M, 0.9M, 1M, or any two of these ranges.

[0023] According to the embodiments of this application, the difference in lithium salt concentration between any two adjacent electrolyte groups is 0.1-0.2, specifically such as 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, or any two of them. This allows for precise positioning of the critical matching range between LiFSI content and battery electrical performance and safety (thermal stability, corrosivity), avoiding or reducing the possibility of missing the optimal addition amount or performance mutation points due to excessively large gradients.

[0024] It is understood that there are no restrictions on the types of other lithium salts in the electrolyte; the specific selection can be made based on actual conditions. As a specific example, this application uses a mixed lithium salt of LiFSI and LiPF6, with a concentration of 1M in the electrolyte.

[0025] According to an embodiment of this application, the solvent in the electrolyte is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 3:7.

[0026] It is understood that a battery can include a positive electrode, a negative electrode, and a separator. The positive electrode, negative electrode, and separator are all immersed in an electrolyte. During the charging and discharging process of the battery, active ions are inserted and extracted back and forth between the positive and negative electrodes. The electrolyte plays the role of conducting ions between the positive and negative electrodes. The separator is placed between the positive and negative electrodes, mainly to prevent short circuits between the positive and negative electrodes, while allowing active ions to pass through.

[0027] According to an embodiment of this application, the positive electrode sheet includes a positive current collector and a positive active material layer located at least on one side of the positive electrode sheet, the positive active material layer including a positive active material.

[0028] According to embodiments of this application, the positive electrode current collector can be a metal current collector or a composite current collector. Metal current collectors include at least one of aluminum foil current collectors and carbon-coated aluminum foil current collectors; composite current collectors may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. Composite current collectors can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). As a specific example, this application uses carbon-coated aluminum foil as the positive electrode current collector.

[0029] According to embodiments of this application, the positive electrode active material of the battery may include at least one of the following: layered structure positive electrode active material (e.g., nickel-cobalt-manganese ternary positive electrode material, nickel-cobalt-aluminum ternary positive electrode material, lithium nickel oxide / sodium, lithium cobalt oxide / sodium, lithium manganese oxide / sodium, lithium-rich / sodium layered and rock salt phase layered materials), olivine-type phosphate active material (e.g., lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, etc.), spinel structure positive electrode active material (e.g., spinel lithium manganese oxide, spinel lithium nickel manganese oxide, lithium-rich spinel lithium manganese oxide and lithium nickel manganese oxide, etc.). As a specific example, this application uses lithium iron phosphate positive electrode material.

[0030] According to embodiments of this application, the binder in the positive electrode active material layer may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. As a specific example, this application uses polyvinylidene fluoride.

[0031] According to embodiments of this application, the conductive agent in the positive electrode active material layer may include at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. As a specific example, this application uses carbon black.

[0032] According to an embodiment of this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located at least on one side of the negative electrode sheet, wherein the negative electrode active material layer includes a positive electrode active material.

[0033] According to an embodiment of this application, the negative current collector includes copper foil.

[0034] According to embodiments of this application, the negative electrode active material layer may include a negative electrode active material, a conductive agent, and a binder.

[0035] According to embodiments of this application, the negative electrode active material may include carbon-based materials (such as artificial graphite), silicon-based materials, tin-based materials, etc. As a specific example, this application uses artificial graphite.

[0036] According to embodiments of this application, the binder in the negative electrode material layer includes at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). As a specific example, this application uses styrene-butadiene rubber.

[0037] According to embodiments of this application, the conductive agent in the negative electrode material layer includes at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. As a specific example, this application uses carbon black.

[0038] According to embodiments of this application, the diaphragm includes polyethylene diaphragms, polypropylene diaphragms, polyethylene / polypropylene composite diaphragms, etc. As a specific example, this application uses a polyethylene diaphragm.

[0039] S20: The electrolyte is tested by cyclic voltammetry to obtain corrosion current data.

[0040] As a specific example, a three-electrode system is constructed using aluminum as the working electrode, lithium metal as the reference electrode, and lithium metal as the counter electrode. Cyclic voltammetry is then performed on the first electrolyte, the second electrolyte, ..., the nth electrolyte to detect whether a corrosion current peak appears and obtain corrosion current data.

[0041] The working principle of the three-electrode system cyclic voltammetry (CV test) is as follows: aluminum metal (simulating the positive electrode current collector of the battery) is used as the working electrode, and lithium metal sheet is used as the reference electrode (providing a stable potential reference) and the counter electrode (forming a current loop). By applying a continuously scanning voltage signal to the system, the redox reaction occurs on the surface of the working electrode. During the test, the current-voltage change curve (cyclic voltammetry curve) is recorded in real time. If a corrosion reaction occurs on the surface of the aluminum electrode (such as the dissolution of aluminum or oxidation caused by the damage of the passivation film), a characteristic corrosion current peak will appear in the corresponding potential range, thereby obtaining the corrosion current data of each electrolyte (electrolyte 1 to electrolyte n). Observe whether there is a significant corrosion current peak in the cyclic voltammetry curve: if there is no corrosion current peak or the peak intensity is extremely low, it indicates that the electrolyte has good compatibility with metallic aluminum and can inhibit the corrosion of aluminum electrode; if a significant corrosion current peak appears (the larger the peak current and the sharper the peak shape), it indicates that the electrolyte easily induces corrosion of aluminum electrode, and the peak potential can reflect the critical voltage for the corrosion reaction to occur; by comparing whether different electrolytes have corrosion current peaks, the influence of the amount of LiFSI added in each electrolyte on the corrosivity of aluminum current collector can be directly evaluated, providing a key basis for screening the amount of LiFSI added that can take into account both electrical performance and current collector corrosion inhibition effect.

[0042] According to embodiments of this application, the voltage range for the cyclic voltammetry test is 3V to 5V, specifically 3V, 3.5V, 4V, 4.5V, 5V, or any combination thereof. Within this range, the electrochemical behavior of the cathode material can be accurately captured, electrolyte stability limits can be avoided, and current collector interference can be eliminated.

[0043] According to embodiments of this application, the cyclic voltammetry test scan rate is 1 mV / s-10 mV / s, specifically 1 mV / s, 2 mV / s, 3 mV / s, 4 mV / s, 5 mV / s, 6 mV / s, 7 mV / s, 8 mV / s, 9 mV / s, and 10 mV / s. Within this range, accurate identification and peak shape integrity of the corrosion current peak can be ensured, balancing test resolution and efficiency.

[0044] S30: Perform DSC testing on the negative electrode powder of the battery to obtain DSC test data, wherein the DSC test data includes at least one of the exothermic temperature and the exothermic amount.

[0045] As a specific example, after the first battery, the second battery, ..., the nth battery are fully charged at 0.5C, the negative electrode sheet is removed, the negative electrode powder is scraped off and subjected to DSC test to obtain DSC test data. The DSC test data includes at least one of the exothermic temperature and the exothermic heat.

[0046] The working principle of DSC testing is as follows: the first to nth batteries are fully charged to 3.65V at a rate of 0.5C, so that lithium ions are fully intercalated into the negative electrode material to form a lithium-intercalated negative electrode. The negative electrode sheet is removed and the negative electrode powder is scraped off as a DSC test sample. During the programmed temperature rise, the heat difference between the sample and the reference material is measured by differential scanning calorimetry (DSC). The temperature change and heat release during the thermal reaction of the sample are recorded, and the exothermic temperature (the onset / peak temperature of the thermal reaction) and the heat release (the total heat released during the reaction) data are obtained. The analysis then examines the relationship between the exothermic temperature and the amount of heat released: a higher exothermic temperature indicates better thermal stability of the negative electrode, while a smaller heat release indicates a lower degree of thermal reaction and a lower risk of thermal runaway. By comparing the DSC data of different batteries (corresponding to electrolytes with different LiFSI addition amounts), the impact of LiFSI addition amount on the thermal stability of the negative electrode can be directly assessed. If the negative electrode of a battery corresponding to a certain electrolyte exhibits a higher exothermic temperature and a smaller heat release, it indicates that the electrolyte has better compatibility with the negative electrode and can suppress the thermal decomposition reaction of the negative electrode. This provides key data support for screening the appropriate amount of LiFSI and ensuring the thermal safety performance of the battery.

[0047] According to an embodiment of this application, the temperature range for the DSC test is 25℃ to 400℃. The initial temperature of 25℃ corresponds to a normal ambient temperature, which avoids baseline shifts caused by ambient temperature differences and ensures the accuracy of the initial thermal signal. The termination temperature of 400℃ covers the critical thermal reaction range of the negative electrode, and 400℃ does not exceed the safe upper limit of commonly used DSC tests, thus ensuring test safety while comprehensively acquiring exothermic temperature and heat release data related to the thermal stability of the negative electrode.

[0048] According to embodiments of this application, the temperature rise rate of the DSC test is 5℃ / min-10℃ / min, specifically 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, and 10℃ / min. Within this range, test efficiency and data resolution can be balanced, ensuring clear separation of each thermal reaction peak.

[0049] S40: Perform a thermal runaway test on the battery and obtain thermal runaway test data, wherein the thermal runaway test data includes at least one of thermal runaway time and maximum thermal runaway temperature.

[0050] As a specific example, the first battery, the second battery, ..., the nth battery are fully charged at 0.5C constant current and constant voltage under 25°C conditions, and then thermal runaway test is performed to obtain thermal runaway test data. The thermal runaway test data includes at least one of thermal runaway time and thermal runaway maximum temperature.

[0051] The working principle of thermal runaway testing is as follows: the first to nth batteries are fully charged to 3.65V at 0.5C constant current and constant voltage in a room temperature environment of 25℃ to ensure that the batteries reach the rated charging state. Then, the thermal runaway triggering scenario (such as programmed temperature rise, external heating) is simulated by thermal runaway testing equipment (such as accelerated calorimeter ARC). The battery temperature change curve over time is monitored in real time, and the time from the start of the test to the occurrence of thermal runaway (thermal runaway time) and the highest temperature reached during the thermal runaway process (maximum thermal runaway temperature) are recorded. Data analysis of thermal runaway duration and peak thermal runaway temperature reveals the following: a longer thermal runaway duration indicates better battery thermal stability and more time for the safety protection system to respond; a lower peak thermal runaway temperature indicates a weaker thermal runaway reaction and lower safety risk. By comparing thermal runaway test data from different batteries (corresponding to electrolytes with different LiFSI addition amounts), the impact of LiFSI addition amount on the overall thermal safety performance of the battery can be directly assessed. If a battery corresponding to a certain electrolyte exhibits a longer thermal runaway duration and a lower peak thermal runaway temperature, it indicates that the electrolyte helps improve battery thermal stability, delays the occurrence of thermal runaway, and reduces the severity of thermal runaway hazards. This provides a key basis for selecting a reasonable LiFSI addition amount that balances electrical performance and thermal safety.

[0052] According to an embodiment of this application, the thermal runaway test includes heating the battery to 200°C at a rate of 5°C / min, maintaining it for 30 minutes, triggering the thermal runaway time and the highest thermal runaway temperature.

[0053] S50: Determine the amount of LiFSI added to the electrolyte based on the corrosion current data, the DSC test data, and the thermal runaway test data.

[0054] This step involves comprehensively analyzing corrosion current data obtained from cyclic voltammetry, DSC test data of the negative electrode, and battery thermal runaway test data to collaboratively determine the amount of LiFSI added to the electrolyte, thereby balancing the battery's corrosion inhibition effect, negative electrode thermal stability, and overall battery thermal safety performance.

[0055] According to embodiments of this application, the amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, and the thermal runaway test data, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, then the second upper limit content of LiFSI is determined to be equal to the content of the b-1-th battery. If the DSC test data of cell c satisfies the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, and the DSC test data of cell c-1 does not satisfy the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, then the third upper limit content of LiFSI is determined to be equal to the content of cell c-1. The minimum value among the first upper limit content, the second upper limit content, and the third upper limit content is taken as the upper limit content of LiFSI in the electrolyte.

[0056] By comparing the critical changes in corrosion suppression, thermal stability, and thermal safety performance between adjacent groups using a series of electrolytes, electrodes, and batteries with different LiFSI contents, the safety upper limits for each dimension were screened. Specifically: the first upper limit of LiFSI content is equal to the LiFSI content of electrolyte group a-1, meaning the maximum safe content for "not initiating aluminum corrosion" cannot exceed the content of electrolyte group a-1, to avoid corrosion of the current collector and failure due to excessive content; the negative electrode of battery b, which meets the above exothermic temperature and heat release conditions, has poor thermal stability and is prone to violent thermal reactions at lower temperatures, while battery b-1 does not meet the above conditions, indicating good thermal stability and mild thermal reaction at this content. Therefore, the second upper limit of LiFSI content is equal to the LiFSI content of the electrolyte corresponding to battery b-1, meaning the maximum safe content for "ensuring negative electrode thermal stability" cannot exceed the content of group b-1, to avoid aggravating the negative electrode thermal decomposition reaction due to excessive content. Similarly, the third upper limit for LiFSI content is equal to the LiFSI content of the electrolyte corresponding to battery c-1. This means the maximum safe content for "ensuring battery thermal safety" cannot exceed the content of group c-1, avoiding a surge in thermal runaway risk due to excessive content. The minimum value is ultimately taken as the overall upper limit, ensuring the electrolyte balances multi-dimensional performance. This minimum value simultaneously satisfies the maximum restrictions of "not corroding the negative electrode current collector, ensuring negative electrode thermal stability, and controlling the risk of thermal runaway." It ensures that when the LiFSI content in the electrolyte is within this upper limit, all key battery performance characteristics meet preset standards, fully leveraging the performance-enhancing effect of LiFSI while reducing or avoiding safety and stability issues it may cause.

[0057] According to embodiments of this application, the above method further includes: After storing the first, second, ..., nth batteries at 60°C for 3 months, the positive electrode sheet was removed, the positive electrode active material was peeled off, and the positive electrode current collector was obtained. The positive electrode current collector was observed by SEM to obtain the corrosion data of the positive electrode current collector.

[0058] The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, and the corrosion data of the positive electrode current collector.

[0059] The principle of this test is as follows: Batteries 1 through n are stored at 60°C for 3 months to simulate the harsh conditions of long-term battery use or high-temperature storage, accelerating the interfacial reaction between the electrolyte and the positive electrode current collector (aluminum foil). The positive electrode sheet is then removed and the surface active material is peeled off, exposing the complete positive electrode current collector substrate. The microstructure of the current collector surface is observed using a scanning electron microscope (SEM) to obtain corrosion-related characterization data. The presence of pores on the current collector surface is observed—a smooth surface without obvious corrosion indicates good compatibility between the electrolyte and the current collector; obvious pores indicate that the LiFSI addition may be too high, causing current collector corrosion. By comparing the SEM observation results of different batteries, the impact of LiFSI addition on the long-term storage stability of the positive electrode current collector can be directly assessed. Furthermore, a multi-dimensional synergistic comprehensive evaluation can be performed by combining corrosion current data, DSC test data, and thermal runaway test data, effectively avoiding the limitations of a single test dimension, thereby further accurately determining the amount of LiFSI added in the electrolyte.

[0060] According to embodiments of this application, the amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, and the positive electrode current collector corrosion data, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, then the second upper limit content of LiFSI is determined to be equal to the content of the b-1-th battery. If the DSC test data of cell c satisfies the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, and the DSC test data of cell c-1 does not satisfy the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, then the third upper limit content of LiFSI is determined to be equal to the content of cell c-1. If the m-th cell satisfies the condition that pores appear on the surface of the positive electrode current collector, and the (m-1)-th cell does not have pores on the surface of the positive electrode current collector, then the fourth upper limit content of LiFSI is determined to be equal to the (m-1)-th content. The minimum value among the first upper limit content, the second upper limit content, the third upper limit content, and the fourth upper limit content is taken as the upper limit content of LiFSI in the electrolyte.

[0061] It is understandable that even if the first three dimensions (corrosion current, negative electrode thermal stability, and overall thermal safety) meet the standards, the appearance of corrosion pits in the positive electrode current collector during long-term storage can still lead to a shortened battery life and increased safety risks. Therefore, cross-validation by incorporating corrosion data from the surface of the positive electrode current collector can further narrow down the safe range for adding LiFSI, ensuring that the final determined upper limit content can both leverage the electrical performance-enhancing effect of LiFSI and take into account the short-term safety performance and long-term storage reliability of the battery, resulting in a more comprehensive assessment and more accurate judgment.

[0062] According to an embodiment of this application, the above method further includes: storing the first battery, the second battery, ..., the nth battery at 60°C for 3 months, then taking out the negative electrode, scraping off the powder from the surface of the electrode for ICP testing, and obtaining aluminum content data; The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, the corrosion data of the positive electrode current collector, and the aluminum content data.

[0063] The working principle of this aluminum content test is as follows: the first to nth batteries are stored at a high temperature of 60℃ for 3 months to simulate long-term use conditions, accelerating the corrosion reaction of the electrolyte and the positive electrode current collector and the migration process of aluminum elements. Then, the negative electrode sheet is taken out and the surface powder is scraped off. The aluminum content in the powder is quantitatively analyzed by inductively coupled plasma atomic emission spectrometry (ICP) to obtain aluminum content data. The measured aluminum content is compared with the preset value. The lower the aluminum content, the less corrosion of the positive electrode current collector, the less aluminum migration to the negative electrode, and the better the long-term compatibility between the electrolyte and the positive electrode current collector. If the aluminum content exceeds the preset value, it indicates that the LiFSI addition is too high, which leads to aggravated corrosion of the positive electrode current collector and aluminum ions migration to the negative electrode for deposition, which may cause problems such as negative electrode performance degradation and increased battery internal resistance. By conducting a multi-dimensional collaborative evaluation of the aluminum content data along with corrosion current data, DSC test data, thermal runaway test data, and cathode current collector corrosion data, the precise screening of LiFSI addition amount can be achieved. This ensures both the short-term electrochemical performance and safety performance of the battery, and also ensures the long-term reliability of the battery by quantifying the aluminum migration risk. This provides a quantitative chemical characterization basis for determining the reasonable addition amount of LiFSI that takes into account multiple performance dimensions.

[0064] According to embodiments of this application, the amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, the thermal runaway test data, the positive electrode current collector corrosion data, and the aluminum content data, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the condition that the heat release temperature is ≤215℃ and the absolute value of the heat release is >1mW / mg, then the second upper limit content of LiFSI is determined to be equal to the content of the b-1-th battery. If the DSC test data of cell c satisfies the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, and the DSC test data of cell c-1 does not satisfy the condition that the thermal runaway time is <40 min and the maximum thermal runaway temperature is >320℃, then the third upper limit content of LiFSI is determined to be equal to the content of cell c-1. If the m-th cell satisfies the condition that pores appear on the surface of the positive electrode current collector, and the (m-1)-th cell does not have pores on the surface of the positive electrode current collector, then the fourth upper limit content of LiFSI is determined to be equal to the (m-1)-th content. If the nth cell satisfies that the aluminum content of the negative electrode is >10ppm and the (n-1)th cell does not satisfy that the aluminum content of the negative electrode is >10ppm, then the fifth upper limit content of LiFSI is determined to be equal to the (n-1)th content. The minimum value among the first upper limit content, the second upper limit content, the third upper limit content, the fourth upper limit content, and the fifth upper limit content is taken as the upper limit content of LiFSI in the electrolyte.

[0065] The aforementioned four evaluation criteria will not be elaborated upon one by one. Including aluminum content in the quantitative evaluation criteria can provide accurate quantitative evidence for the corrosion of the cathode current collector and the migration of aluminum ions, forming a complementary verification with the previous four evaluation criteria: it can quantify the total amount of corrosion product migration through ICP testing, avoiding the subjective bias of SEM observation and the problem of missing latent corrosion, and can also be directly related to the risk of long-term performance degradation of the battery, further narrowing the safe addition range of LiFSI, strengthening the objectivity, repeatability and rigor of multi-dimensional evaluation, and providing more comprehensive quantitative support for the accurate determination of LiFSI addition amount.

[0066] In this application, the amount of LiFSI added ranges from 0 to the aforementioned upper limit of LiFSI content. Given that a higher amount of LiFSI contributes to improving the overall electrical performance of the battery, in practical applications, the upper limit of LiFSI content or a content close to the upper limit can be preferentially selected to fully leverage its role in optimizing battery electrical performance, provided that it does not cause corrosion of the aluminum current collector, reduce the thermal stability of the negative electrode, or affect the thermal safety of the battery.

[0067] The embodiments of this application are described in detail below.

[0068] Example 1 Electrolyte preparation: EC and EMC were mixed at a mass ratio of 3:7, then LiFSI and LiPF6 were added and mixed evenly to obtain the electrolyte. The total concentration of LiFSI and LiPF6 was 1M, and the amount of LiFSI added was 0M, 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, 0.8M, 0.9M, and 1M, respectively. Positive electrode preparation: Lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride were mixed evenly at a mass ratio of 97.5:1.5:1.0. N-methylpyrrolidone was added, with a solid content of 60%, to obtain the positive electrode active slurry. The positive electrode active slurry was coated on both sides of carbon-coated aluminum foil and dried at 110℃ for 10 min to obtain the positive electrode sheet.

[0069] Preparation of negative electrode sheet: The negative electrode active materials artificial graphite, conductive carbon black and styrene-butadiene rubber are uniformly mixed in a mass ratio of 96.5:1.0:2.5, deionized water is added, and the solid content is 50% to obtain a negative electrode active slurry. The negative electrode active slurry is coated on both sides of copper foil and dried at 100°C for 10 min to obtain a negative electrode sheet. Separator: The separator is a single-sided ceramic separator with a 7μm base membrane of PE and an inorganic layer of 2μm alumina.

[0070] Battery preparation: The above-mentioned positive electrode, negative electrode and separator are stacked, assembled, injected with liquid, formed and capacity tested to obtain a lithium-ion battery.

[0071] The five electrolytes and the batteries made from them were subjected to CV tests, DSC tests, thermal runaway tests, positive electrode current collector corrosion tests, and aluminum content tests, respectively. Based on the test results, a comprehensive analysis was conducted to obtain the optimal amount of LiFSI to be added. The specific test data are shown in Table 1.

[0072] Table 1

[0073] Conclusion: Based on the comprehensive analysis of the data in the table, the reasonable addition amount of LiFSI is 0M-0.5M. Given that a higher LiFSI addition amount generally improves the overall performance of the battery, the addition amount of LiFSI can be selected within the range of 0.4M-0.5M, or 0.5M can be chosen directly.

[0074] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0075] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0076] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method of determining the amount of LiFSI to add to an electrolyte, characterized in that, include: A series of electrolytes with different LiFSI contents are provided and assembled into batteries respectively; The electrolyte was tested using cyclic voltammetry to obtain corrosion current data. The negative electrode powder of the battery is subjected to DSC test to obtain DSC test data, which includes at least one of the exothermic temperature and the exothermic amount. The battery is subjected to a thermal runaway test to obtain thermal runaway test data, which includes at least one of thermal runaway time and maximum thermal runaway temperature. The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, and the thermal runaway test data.

2. The method of claim 1, wherein, include: Electrolytes with LiFSI contents ranging from low to high were prepared as follows: electrolyte 1, electrolyte 2, ..., electrolyte n with LiFSI contents of 1, 2, ..., and electrolyte n respectively. These electrolytes were then assembled into batteries to obtain batteries 1, 2, ..., and n. Using aluminum as the working electrode, lithium metal as the reference electrode, and lithium metal as the counter electrode, a three-electrode system is constructed. Cyclic voltammetry is performed on the first electrolyte, the second electrolyte, ..., the nth electrolyte to detect whether a corrosion current peak appears and obtain corrosion current data. After the first battery, the second battery, ..., the nth battery are fully charged at 0.5C, the negative electrode sheet is removed, the negative electrode powder is scraped off and DSC test is performed to obtain DSC test data. The DSC test data includes at least one of the exothermic temperature and the exothermic heat. The first battery, the second battery, ..., the nth battery are fully charged at 0.5C constant current and constant voltage under 25°C conditions, and then thermal runaway test is performed to obtain thermal runaway test data. The thermal runaway test data includes at least one of thermal runaway time and thermal runaway maximum temperature. The amount of LiFSI added to the electrolyte is determined based on the corrosion current data, the DSC test data, and the thermal runaway test data.

3. The method of claim 1, wherein, The solvent in the electrolyte is a mixture of EC and EMC in a mass ratio of 3:

7.

4. The method of claim 1, wherein, The cyclic voltammetry test satisfies at least one of the following conditions: The voltage range of the cyclic voltammetry test is 3~5V; The cyclic voltammetry test scan rate is 1 mV / s to 10 mV / s.

5. The method of claim 1, wherein, The DSC test satisfies at least one of the following conditions: The temperature range for the DSC test is 25℃~400℃; The temperature rise rate of the DSC test is 5℃ / min to 10℃ / min.

6. The method of claim 1, wherein, The thermal runaway test includes heating the battery to 200°C at a rate of 5°C / min and maintaining it for 30 minutes, triggering the thermal runaway time and the highest thermal runaway temperature.

7. The method of claim 1, wherein, Based on the corrosion current data, the DSC test data, and the thermal runaway test data, the amount of LiFSI added to the electrolyte is determined, including: If the a-th electrolyte exhibits a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not exhibit a corrosion current peak in the cyclic voltammetry test, the first upper limit content of LiFSI is determined to be equal to the a-1-th content; If the DSC test data of the bth battery meets the exothermic temperature ≤ 215℃ and the absolute value of the exothermic quantity > 1mW / mg, and the DSC test data of the b-1th battery does not meet the exothermic temperature ≤ 215℃ and the absolute value of the exothermic quantity > 1mW / mg, it is determined that the second upper limit content of LiFSI is equal to the b-1th content; If the DSC test data of the cth battery meets the thermal runaway time < 40min and the thermal runaway maximum temperature > 320℃, and the DSC test data of the c-1th battery does not meet the thermal runaway time < 40min and the thermal runaway maximum temperature > 320℃, it is determined that the third upper limit content of LiFSI is equal to the c-1th content; Taking the minimum value of the first upper limit content, the second upper limit content and the third upper limit content as the upper limit content of LiFSI in the electrolyte.

8. The method of claim 2, wherein, Also comprising: After storing the 1st battery, the 2nd battery,..., the nth battery at 60℃ for 3 months, taking out the positive electrode sheet, stripping the positive electrode active material, obtaining the positive electrode current collector, and observing the positive electrode current collector by SEM to obtain positive electrode current collector corrosion data; According to the corrosion current data, the DSC test data, the thermal runaway test data and the positive electrode current collector corrosion data, the LiFSI addition amount in the electrolyte is determined.

9. The method of claim 8, wherein, According to the corrosion current data, the DSC test data, the thermal runaway test data and the positive electrode current collector corrosion data, the LiFSI addition amount in the electrolyte is determined, comprising: If the ath electrolyte appears a corrosion current peak in the cyclic voltammetry test, and the a-1th electrolyte does not appear a corrosion current peak in the cyclic voltammetry test, it is determined that the first upper limit content of LiFSI is equal to the a-1th content; If the DSC test data of the bth battery meets the exothermic temperature ≤ 215℃ and the absolute value of the exothermic quantity > 1mW / mg, and the DSC test data of the b-1th battery does not meet the exothermic temperature ≤ 215℃ and the absolute value of the exothermic quantity > 1mW / mg, it is determined that the second upper limit content of LiFSI is equal to the b-1th content; If the DSC test data of the cth battery meets the thermal runaway time < 40min and the thermal runaway maximum temperature > 320℃, and the DSC test data of the c-1th battery does not meet the thermal runaway time < 40min and the thermal runaway maximum temperature > 320℃, it is determined that the third upper limit content of LiFSI is equal to the c-1th content; If the mth battery meets the positive electrode current collector surface appearing holes, and the positive electrode current collector surface of the m-1th battery does not appear holes, it is determined that the fourth upper limit content of LiFSI is equal to the m-1th content; Taking the minimum value of the first upper limit content, the second upper limit content, the third upper limit content and the fourth upper limit content as the upper limit content of LiFSI in the electrolyte.

10. The method of claim 8, wherein, Also comprising: After storing the first battery, the second battery,..., the n-th battery at 60℃ for 3 months, the negative electrode sheet is taken out, the powder on the surface of the electrode sheet is scraped and ICP test is performed to obtain aluminum content data; According to the corrosion current data, the DSC test data, the thermal runaway test data, the positive current collector corrosion data and the aluminum content data, the LiFSI addition amount in the electrolyte is determined.

11. The method of claim 10, wherein, According to the corrosion current data, the DSC test data, the thermal runaway test data, the positive current collector corrosion data and the aluminum content data, the LiFSI addition amount in the electrolyte is determined, comprising: If the a-th electrolyte appears a corrosion current peak in the cyclic voltammetry test, and the a-1-th electrolyte does not appear a corrosion current peak in the cyclic voltammetry test, it is determined that the first upper limit content of LiFSI is equal to the a-1-th content; If the DSC test data of the b-th battery satisfies the heat release temperature ≤ 215℃ and the absolute value of the heat release amount > 1mW / mg, and the DSC test data of the b-1-th battery does not satisfy the heat release temperature ≤ 215℃ and the absolute value of the heat release amount > 1mW / mg, it is determined that the second upper limit content of LiFSI is equal to the b-1-th content; If the DSC test data of the c-th battery satisfies the thermal runaway time < 40min and the thermal runaway maximum temperature > 320℃, and the DSC test data of the c-1-th battery does not satisfy the thermal runaway time < 40min and the thermal runaway maximum temperature > 320℃, it is determined that the third upper limit content of LiFSI is equal to the c-1-th content; If the m-th battery satisfies the appearance of holes on the surface of the positive current collector, and the m-1-th battery does not satisfy the appearance of holes on the surface of the positive current collector, it is determined that the fourth upper limit content of LiFSI is equal to the m-1-th content; If the n-th battery satisfies the aluminum content of the negative electrode sheet > 10ppm and the n-1-th battery does not satisfy the aluminum content of the negative electrode sheet > 10ppm, it is determined that the fifth upper limit content of LiFSI is equal to the n-1-th content; Taking the minimum value of the first upper limit content, the second upper limit content, the third upper limit content, the fourth upper limit content and the fifth upper limit content as the upper limit content of LiFSI in the electrolyte.

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