Electrolyte, lithium ion battery and preparation method of lithium ion battery

Abstract

The application belongs to the technical field of lithium ion batteries, and provides an electrolyte, a lithium ion battery and a preparation method of the lithium ion battery.The electrolyte comprises a lithium salt electrolyte, an organic solvent, a sulfur-containing additive, a carbonate additive, a lithium salt additive and a functional additive, wherein the functional additive is selected from any one of the following compounds:Based on the lithium ion battery of the electrolyte, the lithium ion battery is subjected to segmented aging according to the additives.The application solves the problem that the existing electrolyte system and the conventional aging process cannot meet the strict requirements of the wind power energy storage system on the lithium ion battery monomer, such as high consistency and low self-discharge rate.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to an electrolyte, a lithium-ion battery, and a method for preparing a lithium-ion battery. Background Technology

[0002] Wind power has become a core clean energy source for the construction of new power systems, with large-scale expansion of onshore wind power and accelerated deployment of offshore wind power, resulting in a continuous increase in total installed wind power capacity. However, wind power output is inherently intermittent, fluctuating, and random, which not only severely restricts wind power absorption efficiency but also poses a serious challenge to the power balance and stable operation of the power grid. The construction of wind power energy storage systems has become a standard feature of wind power project development, serving as a core supporting technology for smoothing wind power fluctuations, shifting peak and off-peak electricity times, improving grid absorption capacity, and ensuring stable grid connection of wind power projects.

[0003] Wind power energy storage systems typically employ battery packs composed of thousands to tens of thousands of lithium-ion battery cells connected in series and parallel. The overall performance, service life, and operational safety of the system are highly dependent on the consistency of these individual lithium-ion battery cells. During long-term operation, the initial performance differences between lithium-ion battery cells gradually accumulate and amplify under harsh environments of frequent charging and discharging, wide operating condition fluctuations, and high outdoor temperatures. This leads to continuous differentiation in the state of charge (SOC), internal resistance, and capacity state among the cells within the battery pack. This differentiation not only causes a significant loss in the overall usable capacity of the wind power energy storage system, directly reducing its wind power absorption capacity, but also exacerbates overcharging and over-discharging of local cells and accelerates aging, even triggering thermal runaway safety risks, and significantly increasing the total lifecycle operation and maintenance costs of the wind power energy storage system.

[0004] Existing technologies generally adopt the mode of "mixed use of additives + single-temperature constant temperature aging" to improve the consistency of lithium-ion battery cells. However, this approach has two major pain points: First, highly active additives react slowly at room temperature and are easily consumed excessively at high temperatures, resulting in significant differences in film formation process and film quality among different lithium-ion battery cells. Second, the solid electrolyte interphase (SEI) film on the negative electrode surface is unevenly formed, with a loose structure in the early stage and insufficient density in the later stage, resulting in high self-discharge rate of lithium-ion battery cells and large individual differences. Under long-term static conditions in wind power energy storage systems, the capacity decay is severely differentiated.

[0005] In summary, existing electrolyte systems and conventional aging processes are insufficient to meet the stringent requirements of wind power energy storage systems for high consistency and low self-discharge rates in lithium-ion battery cells. Therefore, there is an urgent need to develop an electrolyte and supporting process that can fundamentally improve the consistency of lithium-ion battery cells, reduce self-discharge rates, and suppress long-term performance degradation. Summary of the Invention

[0006] To address the problem that existing electrolyte systems and conventional aging processes cannot meet the stringent requirements of wind power energy storage systems for high consistency and low self-discharge rates of lithium-ion battery cells, this invention provides an electrolyte, a lithium-ion battery, and a method for preparing the lithium-ion battery.

[0007] This invention is achieved through the following technical solution: In a first aspect, the present invention provides an electrolyte comprising: a lithium salt electrolyte, an organic solvent, a sulfur-containing additive, a carbonate additive, a lithium salt additive, and a functional additive, wherein the functional additive is selected from any one of the following compounds: .

[0008] Preferably, the functional additive has a mass percentage of 0.2% to 0.6% in the electrolyte.

[0009] Preferably, the sulfur-containing additive is 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, or vinyl sulfate.

[0010] Preferably, the carbonate additive is vinylene carbonate.

[0011] Preferably, the lithium salt additive is lithium bis(oxalato)borate or lithium difluorophosphate.

[0012] Preferably, the lithium salt electrolyte is lithium hexafluorophosphate.

[0013] Preferably, the organic solvent includes cyclic carbonates and chain carbonates.

[0014] In a second aspect, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the electrolyte is the electrolyte as described above.

[0015] Thirdly, the present invention provides a method for preparing the lithium-ion battery, wherein a positive electrode, a negative electrode, an electrolyte, and a separator are assembled into a battery, and then the battery is obtained through formation, capacity testing, and aging; wherein the aging includes three stages: the first stage, placing the battery in an environment of 50~55℃ and letting it stand for 36~48h; the second stage, cooling the battery to 15~20℃ and letting it stand for 48~72h; and the third stage, heating the battery to 40~45℃ and letting it stand for 48~72h.

[0016] Preferably, after the second stage ends and before the third stage begins, a charge is performed, preferably to 50% SOC.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The electrolyte system of this invention includes a sulfur-containing additive, a carbonate additive, a lithium salt additive, and a functional additive. The sulfur-containing additive exhibits high reactivity at high temperatures and can rapidly form an initial SEI film on the negative electrode surface, suppressing initial self-discharge. The functional additive contains a highly polar cyano group (-CN), which adsorbs onto the negative electrode surface at low temperatures and can guide Li... + The orderly arrangement of the SEI film promotes its crystallization, forming a dense and stable SEI film and reducing defects. Simultaneously, the conjugated framework in the functional additives can fill micro-cracks in the SEI film, further improving its integrity. This dense and stable SEI film not only improves the self-discharge behavior of lithium-ion batteries but also makes the interface structure and internal state between different lithium-ion battery cells more consistent, thereby improving the consistency of lithium-ion battery cells and overcoming the differentiation problem during long-term operation.

[0018] Furthermore, when the amount of the functional additive is between 0.2% and 0.6%, it can effectively reduce the self-discharge rate of lithium-ion batteries and improve the consistency of lithium-ion battery cells. If too many functional additives are added, the high-energy defect sites on the negative electrode surface will be completely covered and unable to adsorb more functional additives. Instead, it will slightly increase the viscosity of the electrolyte and have an adverse effect on the performance of lithium-ion batteries.

[0019] The lithium-ion battery preparation method of this invention divides the aging process into three stages, providing precise action windows for different additives. In the first stage (active film formation period), the aging temperature is 50-55°C. At this temperature, the sulfur atoms (S) in the sulfur-containing additives exhibit a surge in reactivity, preferentially undergoing ring-opening polymerization on the negative electrode surface to form a porous initial SEI film rich in lithium sulfonate (-SO3Li), rapidly blocking electron transport and suppressing initial self-discharge. In the second stage (structural stabilization period), the temperature is lowered to 15-20°C, slowing down the reaction of the sulfur-containing additives. The cyano groups (-CN) of the functional additives are highly polar and adsorb onto the negative electrode surface at low temperatures, guiding the Li... + The orderly arrangement of the electrolyte promotes SEI film crystallization, forming a dense and stable structure and reducing SEI film defects. In the third stage (defect elimination period), the temperature is raised to 40-45℃. The moderately high temperature accelerates the reaction and precipitation of residual impurities in the electrolyte, eliminates interfacial bubbles, and improves the contact tightness between the electrolyte and the negative electrode. At the same time, the conjugate framework of the functional additives fills the tiny cracks in the SEI film, forming a complete protective layer. This invention optimizes the aging process and combines it with specific electrolyte additives to make the formed SEI film complete and stable with reduced interfacial defects. As a result, the self-discharge behavior of lithium-ion batteries is significantly suppressed, and the interfacial structure and internal state between different lithium-ion battery cells become more consistent, thereby significantly improving the self-discharge rate and cell consistency of lithium-ion batteries.

[0020] Furthermore, after the second stage ends and before the third stage begins, the present invention performs a charge to activate the active sites on the negative electrode surface, promote the binding of functional additives with the negative electrode, improve the stability of the SEI film, and thus reduce the self-discharge rate. Attached Figure Description

[0021] 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.

[0022] Figure 1 The DC internal resistance growth rate of the lithium-ion battery cells corresponding to Examples 1, 5, 14, 16, 18 and Comparative Examples 1-2 of the present invention when stored at 60°C for 14 days; Figure 2 The DC internal resistance growth rate of the lithium-ion battery cells corresponding to Examples 1, 5, 14, 16, 18 and Comparative Examples 1-2 of the present invention after being stored at 60°C for 28 days. Detailed Implementation

[0023] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0024] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0025] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0026] The electrolyte provided by this invention comprises: a lithium salt electrolyte, an organic solvent, a sulfur-containing additive, a carbonate additive, a lithium salt additive, and a functional additive, wherein the functional additive is selected from any one of the following compounds: .

[0027] The CAS number of compound I-1 is 149520-59-2, the CAS number of compound I-2 is 149520-70-7, the CAS number of compound I-3 is 2135823-46-8, the CAS number of compound I-4 is 268205-27-2, and the CAS number of compound I-5 is 109879-48-3.

[0028] This invention relates to an electrolyte system comprising a sulfur-containing additive, a carbonate additive, a lithium salt additive, and a functional additive. The carbonate additive generally has a lower decomposition potential than carbonate solvents, preferentially reducing and decomposing on the negative electrode surface to form a dense, flexible SEI film rich in organic carbonates. Simultaneously, it oxidizes on the positive electrode surface to generate a stable cathode electrolyte interphase (CEI) film, effectively inhibiting the continuous decomposition of carbonate solvents, structural damage to electrode materials, and transition metal dissolution, significantly improving the performance of lithium-ion batteries. The lithium salt additive participates in the construction of the SEI and CEI films through its functional anions, introducing inorganic components containing boron, phosphorus, fluorine, etc., making the SEI / CEI films denser, with better ionic conductivity and higher mechanical strength. The sulfur-containing additive exhibits high reactivity at high temperatures, rapidly forming an initial SEI film on the negative electrode surface and suppressing initial self-discharge. The functional additive contains a highly polar cyano group (-CN), which adsorbs onto the negative electrode surface at low temperatures, guiding Li... + The orderly arrangement of the SEI film promotes its crystallization, forming a dense and stable SEI film and reducing defects. Simultaneously, the conjugated framework in the functional additives can fill micro-cracks in the SEI film, further improving its integrity. This dense and stable SEI film not only improves the self-discharge behavior of lithium-ion batteries but also makes the interface structure and internal state between different lithium-ion battery cells more consistent, thereby improving the consistency of lithium-ion battery cells and overcoming the differentiation problem during long-term operation.

[0029] In some embodiments of the present invention, the mass percentage of the functional additive in the electrolyte is 0.2% to 0.6%, for example, it can be 0.2%, 0.4%, 0.6%, and more preferably 0.2%. When the mass percentage of the functional additive in the electrolyte is 0.2%, the improvement effect on the self-discharge rate of the lithium-ion battery and the consistency of the lithium-ion battery cells basically reaches a stable level, and further increasing its addition amount does not significantly improve the improvement effect. Therefore, 0.2% is the optimal addition amount of the functional additive.

[0030] In some embodiments of the present invention, the sulfur-containing additive is 1,3-propene sulfonate lactone (Prop-1-ene-1,3-sultone, PST), 1,3-propane sulfonate lactone (1,3-Propane Sultone, PS), or 1,3,2-dioxathiolane-2,2-dioxide (DTD); the mass percentage of the sulfur-containing additive in the electrolyte is 0.5% to 1%, for example, it can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%.

[0031] In some embodiments of the present invention, the carbonate additive is vinylene carbonate (VC); the mass percentage of the carbonate additive in the electrolyte is 0.3% to 0.5%, for example, it can be 0.3%, 0.4%, or 0.5%.

[0032] In some embodiments of the present invention, the lithium salt additive is lithium bis(oxalato)borate (LiBOB) or lithium difluorophosphate (LiPO2F2); the mass percentage of the lithium salt additive in the electrolyte is 0.5% to 1%, for example, it can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%.

[0033] In some embodiments of the present invention, the lithium salt electrolyte is lithium hexafluorophosphate, and the mass percentage of the lithium salt electrolyte in the electrolyte is 12% to 15%, for example, it can be 12%, 13%, 14%, or 15%.

[0034] In some embodiments of the present invention, the organic solvent includes cyclic carbonates and chain carbonates. The cyclic carbonates include at least two selected from fluoroethylene carbonate (FEC), ethylene carbonate (EC), and propylene carbonate (PC); the total mass of the cyclic carbonates is 15% to 25% of the total mass of the organic solvents, calculated as 100% of the total mass of the organic solvents. The chain carbonates include at least one selected from diethyl carbonate (DEC) and dimethyl carbonate (DMC). The total mass of the chain carbonates is 75% to 85% of the total mass of the organic solvents, calculated as 100% of the total mass of the organic solvents.

[0035] Based on the electrolyte described above, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator.

[0036] The method for preparing a lithium-ion battery according to the present invention involves assembling a positive electrode, a negative electrode, an electrolyte, and a separator, followed by formation, capacity testing, and aging to obtain a lithium-ion battery. The aging process includes three stages: a first stage, placing the battery in an environment of 50-55°C and allowing it to stand for 36-48 hours; a second stage, cooling the battery to 15-20°C and allowing it to stand for 48-72 hours; and a third stage, heating the battery to 40-45°C and allowing it to stand for 48-72 hours.

[0037] This invention designs a multi-stage gradient temperature aging process based on the differences in reactivity of different additives in the electrolyte. First, it promotes rapid film formation of highly active additives; then, it stabilizes the film structure at low temperatures; and finally, it eliminates interface defects at high temperatures, solving the consistency problem caused by uneven additive reactions in existing technologies. Specifically, this invention divides the aging process into three stages, providing precise action windows for different additives. In the first stage (active film formation period), the aging temperature is 50-55℃. At this temperature, the reactivity of sulfur atoms (S) in the sulfur-containing additives surges, preferentially undergoing ring-opening polymerization on the negative electrode surface to form a porous initial SEI film rich in lithium sulfonate (-SO3Li), rapidly blocking electron transport and suppressing initial self-discharge. In the second stage (structural stabilization period), the temperature is lowered to 15-20℃, slowing down the reaction of the sulfur-containing additives. The cyano groups (-CN) of the functional additives are highly polar and adsorb onto the negative electrode surface at low temperatures, guiding the Li... +The orderly arrangement promotes SEI film crystallization, forming a dense and stable structure and reducing SEI film defects; in the third stage (defect elimination period), the temperature is raised to 40~45℃, and the moderate high temperature accelerates the reaction and precipitation of residual impurities in the electrolyte, eliminates interfacial bubbles, and improves the contact tightness between the electrolyte and the electrode; at the same time, the conjugated skeleton in the functional additives fills the tiny cracks in the SEI film, forming a complete protective layer.

[0038] This invention optimizes the aging process and combines it with specific electrolyte additives to make the formed SEI film complete and stable with reduced interface defects. As a result, the self-discharge behavior of lithium-ion batteries is significantly suppressed, and the interface structure and internal state between different lithium-ion battery cells are more consistent. This significantly improves the self-discharge rate and consistency of lithium-ion battery cells.

[0039] In a more preferred embodiment of the present invention, after the second stage ends and before the third stage begins, a charge is performed, for example, to 50% SOC, to activate the active sites on the negative electrode surface and promote the binding of functional additives to the negative electrode.

[0040] The present invention will now be described in detail with reference to specific embodiments.

[0041] Example 1 The electrolyte formulation used in this embodiment, by mass percentage, is as follows: functional additive (compound of formula I-1): 0.2%; vinylene carbonate: 0.3%; 1,3-propanesulfonate lactone: 0.5%; lithium bis(oxalato)borate (LiBOB): 0.5%; lithium hexafluorophosphate (LiPF6): 12%; the balance is organic solvent.

[0042] The organic solvent in the electrolyte is composed of fluoroethylene carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and dimethyl carbonate. Based on the total mass of the organic solvent being 100%, fluoroethylene carbonate accounts for 3% of the mass, ethylene carbonate accounts for 13% of the mass, propylene carbonate accounts for 4% of the mass, diethyl carbonate accounts for 20% of the mass, and dimethyl carbonate accounts for 60% of the mass.

[0043] Preparation of the electrolyte: In a glove box with water and oxygen content both less than 0.1 ppm, functional additives, vinylene carbonate, 1,3-propanesulfonate lactone, and lithium bis(oxalato)borate were added to the organic solvent according to the above mass percentages. Then lithium hexafluorophosphate was added, and the mixture was stirred and mixed at 10°C for 5 hours to obtain the electrolyte of this embodiment.

[0044] Preparation of lithium-ion batteries: A ternary cathode material (NCM811 / NCM523 / NCM622), conductive agent Super P, and binder polyvinylidene fluoride (PVDF) were mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) at a mass ratio of 93:3:4 to form a slurry. This slurry was coated onto aluminum foil, dried, rolled, and die-cut to form the cathode. Graphite, conductive agent acetylene black, binder sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (Styrene-Butadiene) were mixed at a mass ratio of 94:1:2:3. Rubber (SBR) is stirred into a slurry, coated onto copper foil, and then dried, rolled, and die-cut to form a negative electrode. Then, the positive electrode, negative electrode, and polypropylene separator are wound together, placed into a shell, injected with the above-mentioned electrolyte, welded cap, formed, capacity tested, and aged to obtain a lithium-ion battery. This lithium-ion battery is a single lithium-ion battery cell.

[0045] The aging process is as follows: First stage: immediately after separation, place in an environment of 55℃ and let stand for 36 hours; Second stage: cool down to 20℃ and let stand for 48 hours; Third stage: heat up to 45℃ and let stand for 72 hours.

[0046] After the second stage, perform a small current charge of 0.05C (charge to 50% SOC) before proceeding to the third stage.

[0047] Following the above-described method for preparing lithium-ion batteries, three parallel lithium-ion battery cells were prepared and named Cell 1, Cell 2, and Cell 3, respectively.

[0048] Example 2 The only difference between this embodiment and Embodiment 1 is that the amount of functional additive added to the electrolyte is 0.4%, while the other conditions are the same as in Embodiment 1.

[0049] Example 3 The only difference between this embodiment and Embodiment 1 is that the amount of functional additive added to the electrolyte is 0.6%, while the other conditions are the same as in Embodiment 1.

[0050] Example 4 The only difference between this embodiment and Example 1 is that the functional additive in the electrolyte is replaced by compound I-2 instead of compound I-1, while the other conditions are the same as in Example 1.

[0051] Example 5 The only difference between this embodiment and Example 1 is that the functional additive in the electrolyte is replaced by compound I-3 instead of compound I-1, while the other conditions are the same as in Example 1.

[0052] Example 6 The only difference between this embodiment and Example 1 is that the functional additive in the electrolyte is replaced by compound I-4 instead of compound I-1, while the other conditions are the same as in Example 1.

[0053] Example 7 The only difference between this embodiment and Example 1 is that the functional additive in the electrolyte is replaced by compound I-5 instead of compound I-1, while the other conditions are the same as in Example 1.

[0054] Example 8 The only difference between this embodiment and Example 1 is that the amount of sulfur-containing additive 1,3-propanesulfonate lactone added to the electrolyte is 1%, while other conditions are the same as in Example 1.

[0055] Example 9 The only difference between this embodiment and Example 1 is that the sulfur-containing additive in the electrolyte is 1,3-propenesulfonate lactone; all other conditions are the same as in Example 1.

[0056] Example 10 The only difference between this embodiment and Example 1 is that the sulfur-containing additive in the electrolyte is vinyl sulfate; all other conditions are the same as in Example 1.

[0057] Example 11 The only difference between this embodiment and Embodiment 1 is that the lithium salt additive in the electrolyte is lithium difluorophosphate; all other conditions are the same as in Embodiment 1.

[0058] Example 12 The only difference between this embodiment and Example 1 is that the amount of vinylene carbonate added to the electrolyte is 0.5%, while the other conditions are the same as in Example 1.

[0059] Example 13 The only difference between this embodiment and Example 1 is that the amount of lithium hexafluorophosphate added to the electrolyte is 15%, while the other conditions are the same as in Example 1.

[0060] Example 14 The electrolyte formulation used in this embodiment, by mass percentage, is as follows: functional additive (compound of formula I-1): 0.2%; vinylene carbonate: 0.3%; 1,3-propanesulfonyl lactone: 0.5%; lithium bis(oxalato)borate: 0.5%; lithium hexafluorophosphate: 12%; the balance is organic solvent.

[0061] The organic solvent in the electrolyte is composed of fluoroethylene carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and dimethyl carbonate. Assuming the total mass of the organic solvent is 100%, the mass percentage of fluoroethylene carbonate is 3%, the mass percentage of ethylene carbonate is 13%, the mass percentage of propylene carbonate is 4%, the mass percentage of diethyl carbonate is 20%, and the mass percentage of dimethyl carbonate is 60%.

[0062] Preparation of the electrolyte: In a glove box where the water and oxygen contents are both less than 0.1 ppm, functional additives, vinylene carbonate, 1,3-propanesulfonate lactone, and lithium bis(oxalato)borate are added to the organic solvent according to the above mass percentages. Then lithium hexafluorophosphate is added, and the mixture is stirred and mixed at 10°C for 5 hours to obtain the electrolyte of this embodiment.

[0063] Preparation of lithium-ion batteries: Ternary cathode materials (NCM811 / NCM523 / NCM622), conductive agent Super P, and binder PVDF are mixed with an appropriate amount of NMP in a mass ratio of 93:3:4 to form a slurry, which is then coated onto aluminum foil. After drying, rolling, and die-cutting, the cathode is formed. Graphite, conductive agent acetylene black, binder CMC, and SBR are mixed in a mass ratio of 94:1:2:3 to form a slurry, which is then coated onto copper foil. After drying, rolling, and die-cutting, the anode is formed. The cathode, anode, and polypropylene separator are then wound together, placed in a casing, injected with the above-mentioned electrolyte, capped, formed, capacity tested, and aged to obtain a lithium-ion battery, which is a single lithium-ion battery cell.

[0064] The aging process is as follows: First stage: immediately after separation, place in an environment of 55℃ and let stand for 48 hours; Second stage: cool down to 20℃ and let stand for 48 hours; Third stage: heat up to 45℃ and let stand for 72 hours.

[0065] After the second stage, perform a small current charge of 0.05C (charge to 50% SOC) before proceeding to the third stage.

[0066] Following the above-described method for preparing lithium-ion batteries, three parallel lithium-ion battery cells were prepared and named Cell 1, Cell 2, and Cell 3, respectively.

[0067] Example 15 The only difference between this embodiment and embodiment 14 is that in the first stage, the environment is placed at 50°C and left to stand for 32 hours, while other conditions are the same as in embodiment 14.

[0068] Example 16 The only difference between this embodiment and embodiment 14 is that in the first stage, the environment is placed at 50°C and left to stand for 48 hours, while other conditions are the same as in embodiment 14.

[0069] Example 17 The only difference between this embodiment and embodiment 14 is that in the second stage, the temperature is lowered to 20°C and left to stand for 72 hours, while other conditions are the same as in embodiment 14.

[0070] Example 18 The only difference between this embodiment and embodiment 14 is that in the second stage, the temperature is lowered to 15°C and left to stand for 48 hours, while other conditions are the same as in embodiment 14.

[0071] Example 19 The only difference between this embodiment and embodiment 14 is that in the second stage, the temperature is lowered to 15°C and left to stand for 72 hours, while other conditions are the same as in embodiment 14.

[0072] Example 20 The only difference between this embodiment and embodiment 14 is that in the third stage, the temperature is raised to 45°C and left to stand for 72 hours, while the other conditions are the same as in embodiment 14.

[0073] Example 21 The only difference between this embodiment and embodiment 14 is that after the second stage, no 0.05C charging is performed, and the third stage is directly entered. All other conditions are the same as in embodiment 14.

[0074] Comparative Example 1 The only difference between this comparative example and Example 14 is that no functional additives were added; all other conditions are the same as in Example 11.

[0075] Comparative Example 2 The only difference between this comparative example and Example 14 is that the functional additive of the present invention is not added, but a conventional cyanide compound, succinic acid, is added. All other conditions are the same as in Example 14.

[0076] Comparative Example 3 The only difference between this comparative example and Example 21 is that segmented aging is not performed, the aging time of the lithium-ion battery is set to 72 hours, the aging temperature is set to 40°C, and other conditions are the same as in Example 21.

[0077] To evaluate the self-discharge rate and consistency of the lithium-ion battery cells prepared in the above embodiments and comparative examples, the present invention calculated the K-value for each embodiment and comparative example. The K-value calculation method is as follows: K-value = (OCV1 - OCV2) / T, where OCV1 is the initial voltage, OCV2 is the voltage remaining after the resting period, and T is the resting time. The testing methods for OCV1 and OCV2 are as follows: at 25°C, the initial voltage of the lithium-ion battery cell was measured using a voltmeter and recorded as OCV1; after resting for 14 days, the voltage of the lithium-ion battery cell was measured using a voltmeter at 25°C and recorded as OCV2. The calculated K-values ​​are shown in Table 1.

[0078] The present invention also conducted a 60°C storage test on the lithium-ion battery cells (cell 1) prepared in various embodiments and comparative examples: the lithium-ion battery cells were charged at 1C constant current and constant voltage to 4.25V at room temperature, and then discharged at 1C constant current to 2.75V, with the initial discharge capacity recorded as C0; then, the lithium-ion battery cells were charged at 1C constant current and constant voltage to 4.25V at room temperature, placed in a 60°C constant temperature chamber, and stored for 28 days. The reversible capacity of the lithium-ion battery cells was tested every 7 days and recorded as Cn, where n is the number of days the lithium-ion battery cells were stored in the 60°C constant temperature chamber. The capacity retention rate of the lithium-ion battery cells after n days is calculated as Cn / C0 × 100%. The capacity retention rates after 28 days of storage are shown in Table 1.

[0079] To further evaluate the consistency of the lithium-ion battery cells prepared in the above embodiments and comparative examples, the initial alternating current resistance (ACR) of the lithium-ion battery cells was tested. The test method was as follows: Under a standard environment of 25℃±2℃, the lithium-ion battery cell under test was fully charged and then left to stand for 4 hours. Using the 1kHz AC small signal injection method, a weak AC current of C / 20 (C represents the rated capacity of the lithium-ion battery cell) was applied to the lithium-ion battery cell using an AC resistance tester. The voltage response was measured and the real part of the impedance was calculated using phase-locked loop detection technology. After repeated measurements, the arithmetic mean was taken as the final initial AC resistance. The test results are shown in Table 1.

[0080] To evaluate the power performance degradation of lithium-ion battery cells during storage, this invention tested the DC internal resistance (DCIR) growth rate of lithium-ion battery cells (cell 1) in some embodiments and comparative examples. The test method was as follows: the lithium-ion battery cell (cell 1) was charged to 50% SOC, allowed to stabilize at room temperature, and then the initial DC internal resistance (DCIR0) was measured using the DC pulse method. Subsequently, the lithium-ion battery cell was stored in a 60°C constant temperature chamber. After 14 days and 28 days of storage, it was removed and allowed to stabilize at room temperature until the temperature and voltage were fully stable. The DC internal resistance (DCIR) at the corresponding time points was then measured under the same pulse conditions. 14 DCIR 28 The DC internal resistance growth rate after 14 days and 28 days of storage at 60℃ was calculated using the formula: (DC internal resistance after storage - initial DC internal resistance) / initial DC internal resistance × 100%, while maintaining consistent test temperature, resting time, and pulse parameters throughout the process. The test results are as follows: Figure 1 and Figure 2 As shown.

[0081] Table 1

[0082] As can be seen from the data in Examples 1-3 in Table 1, increasing the functional additive from 0.2% to 0.6% did not significantly decrease the K value, indicating little difference in self-discharge rate, and the capacity retention rate only increased by 1.3%. This aligns with the monolayer adsorption principle: once the high-energy defect sites on the negative electrode surface are completely covered, further addition of additives cannot further improve the adsorption effect; instead, it may slightly increase the electrolyte viscosity. Therefore, the optimal amount of functional additive is 0.2%.

[0083] Examples 1, 9, and 10 verified the effects of three sulfur-containing additives on lithium-ion batteries. Table 1 shows that the three sulfur-containing additives had little difference in their effects on initial AC internal resistance, K-value, and capacity retention. The lithium-ion battery prepared in Example 10, using vinyl sulfate as the sulfur-containing additive, exhibited the highest capacity retention of 90.6% after storage at 60°C for 28 days. This was higher than that of Example 1, using 1,3-propanesulfonyl lactone as the sulfur-containing additive, and Example 9, using 1,3-propenesulfonyl lactone as the sulfur-containing additive. This may be because vinyl sulfate has better film-forming quality and fewer side reactions at high temperatures, resulting in the highest capacity retention after high-temperature storage.

[0084] Comparing Examples 1 and 12, it can be seen that when the amount of vinylene carbonate added increases from 3% to 0.5%, the initial AC internal resistance of the lithium-ion battery cell increases, the K value increases, and the capacity retention decreases. This is because when the amount of vinylene carbonate added exceeds 0.3% (Example 12), it will cause the SEI film to grow excessively, forming a thick and dense organic layer, which increases the resistance to lithium-ion transport. At the same time, a higher amount of vinylene carbonate is prone to free radical polymerization at high temperatures, which leads to an increase in electrolyte viscosity, thereby reducing the performance of the lithium-ion battery.

[0085] The second stage of the aging process is the core stage for the functional additives. Comparing Examples 14 and 17-19, it can be seen that the lithium-ion battery cell obtained in Example 19 has the lowest K value and the highest capacity retention rate. This is because the lower the temperature and the longer the time in the second stage, the higher the crystallization degree of the SEI film and the fewer defects, which in turn leads to a lower self-discharge rate and a higher capacity retention rate of the lithium-ion battery cell.

[0086] Comparing Example 14 and Example 21, it can be seen that after omitting 50% SOC charging in Example 21, the K value increased by about 13.6%, proving that the low-current charging in Example 14 can activate the active sites on the negative electrode surface, allowing functional additives to bind to the negative electrode surface through chemical bonding rather than physical adsorption, significantly improving the stability of the SEI film and thus reducing the self-discharge rate.

[0087] Comparing Example 14 and Comparative Example 1, it can be seen that in Comparative Example 1 without the functional additive of the present invention, the initial AC internal resistance and K value of the three lithium-ion battery cells (cell 1, cell 2, and cell 3) differed significantly, indicating poor consistency of the lithium-ion battery cells. However, in Example 14, after adding the functional additive, the differences in the initial AC internal resistance and K value of the three lithium-ion battery cells decreased significantly, indicating improved consistency. Furthermore, compared to the lithium-ion battery cells in Comparative Example 1, the K value of the lithium-ion battery cells in Example 14 decreased, while the capacity retention rate increased, indicating a decrease in the self-discharge rate of the lithium-ion battery cells. These results demonstrate that the functional additive introduced in this invention can form a dense and stable SEI film, improving the self-discharge behavior of lithium-ion battery cells, reducing the self-discharge rate, and thus improving the capacity retention rate of lithium-ion battery cells. Simultaneously, the dense and stable SEI film makes the interface structure and internal state between different lithium-ion battery cells more consistent, which is beneficial for improving the consistency of lithium-ion battery cells.

[0088] Comparing Example 14 with Comparative Example 2, it can be seen that the K value of the lithium-ion battery cell prepared with the functional additive of the present invention is lower than that of the lithium-ion battery cell prepared with cyanobutylene as an additive. This indicates that the functional additive of the present invention can better reduce the self-discharge rate of the lithium-ion battery cell and improve the capacity retention rate. This is because, on the one hand, the rigid planar conjugated framework in the functional additive of this invention can be tightly bonded to the negative electrode surface through π-π stacking, forming a continuous, dense, and stable adsorption layer that fully covers the highly active sites of the negative electrode, thus isolating the electrolyte from direct contact with the negative electrode and suppressing redox side reactions. On the other hand, the fixedly arranged polycyano groups in the functional additive of this invention can form strong multidentate chelate coordination with transition metal ions, with coordination ability far superior to the freely rotating flexible dicyano groups in succinate. This not only anchors the transition metal ions in the negative electrode to inhibit their dissolution but also complexes the dissolved transition metal ions in the electrolyte to block the shuttle effect, preventing damage to the SEI film of the negative electrode. Furthermore, the regular rigid planar structure of the functional additive of this invention can achieve orderly pre-assembly on the negative electrode surface, forming a uniform, dense, and mechanically strong stable SEI film after reduction. This eliminates the continuous side reactions and irreversible consumption of active lithium caused by SEI film damage during charging and discharging. Combined with the superior electrochemical stability of the conjugated framework itself, this further reduces interfacial side reactions, ultimately resulting in a significant reduction in the self-discharge rate of lithium-ion battery cells.

[0089] Table 1 also shows that, compared with the lithium-ion battery cells obtained by the single-temperature aging process in Comparative Example 3, the initial AC internal resistance and K-value differences of the three lithium-ion battery cells obtained by the segmented aging process in Example 21 are all reduced. Furthermore, compared with Comparative Example 3, the K-value of the lithium-ion battery cells obtained in Example 21 is lower and the capacity retention rate is higher. This is because Comparative Example 3 uses a 40°C isothermal aging process, where all additives react simultaneously, resulting in a chaotic film formation process and uneven distribution of organic and inorganic phases in the SEI film, leading to high self-discharge rates and poor consistency in the lithium-ion battery cells. These results demonstrate that the segmented aging process of this invention results in a complete and stable SEI film with reduced interface defects, thereby significantly suppressing the self-discharge behavior of the lithium-ion battery. The interface structure and internal state between different lithium-ion battery cells become more consistent, thus significantly improving the self-discharge rate and consistency of the lithium-ion battery cells.

[0090] from Figure 1 and Figure 2 It can be seen that during high-temperature storage, the DC internal resistance growth rate of the lithium-ion battery cells obtained by adding functional additives and undergoing segmented aging in Examples 1, 5, 14, 16 and 18 is significantly lower than that in Comparative Examples 1 and 2. This indicates that the power performance degradation of the lithium-ion battery cells in the embodiments of the present invention is reduced during high-temperature storage, thereby extending the life of lithium-ion batteries. This is due to the synergistic effect between various additives and between segmented aging.

[0091] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that, include: The lithium salt electrolyte, organic solvent, sulfur-containing additive, carbonate additive, lithium salt additive, and functional additive, wherein the functional additive is selected from any one of the following compounds: 。 2. The electrolyte according to claim 1, characterized in that, The functional additive has a mass percentage of 0.2% to 0.6% in the electrolyte.

3. The electrolyte according to claim 1, characterized in that, The sulfur-containing additive is 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, or vinyl sulfate.

4. The electrolyte according to claim 1, characterized in that, The carbonate additive is vinylene carbonate.

5. The electrolyte according to claim 1, characterized in that, The lithium salt additive is lithium bis(oxalato)borate or lithium difluorophosphate.

6. The electrolyte according to claim 1, characterized in that, The lithium salt electrolyte is lithium hexafluorophosphate.

7. The electrolyte according to claim 1, characterized in that, The organic solvents include cyclic carbonates and chain carbonates.

8. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the electrolyte is the electrolyte according to any one of claims 1 to 7.

9. A method for preparing a lithium-ion battery as described in claim 8, characterized in that, A lithium-ion battery is obtained by assembling a positive electrode, a negative electrode, an electrolyte, and a separator, followed by formation, capacity testing, and aging. The aging process includes three stages: the first stage involves placing the battery in an environment of 50-55°C for 36-48 hours; the second stage involves cooling the battery to 15-20°C and allowing it to stand for 48-72 hours; and the third stage involves heating the battery to 40-45°C and allowing it to stand for 48-72 hours.

10. The method for preparing a lithium-ion battery according to claim 9, characterized in that, A charge is performed after the second phase ends and before the third phase begins.

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