Propylene carbonate-based electrolyte and application thereof

Abstract

The application provides a propylene carbonate-based electrolyte and application, and belongs to the technical field of lithium ion batteries. The electrolyte comprises a lithium salt, a non-aqueous organic solvent taking propylene carbonate as a main body, and an additive. The additive is a cyclic sulfamic acid ester type heterocyclic compound or an N-carbamate-based-3-sulfoxy-substituted pyrrolidine compound, and the mass percentage of the additive in the electrolyte is not less than 2%. The electrolyte of the application is a propylene carbonate system, the additive reacts with the solvent at the graphite interface first and forms a dense and stable interface film, the interface film is mainly composed of inorganic salt components such as sulfates and sulfites and is accompanied by nitrogen-containing organic residues to participate in the construction, thereby inhibiting the co-intercalation side reaction of propylene carbonate, reducing the damage to the graphite structure, and improving the cycle stability and long-term service life of the secondary battery. The electrolyte of the application is applied to lithium batteries and other secondary batteries, and the problem that propylene carbonate is prone to co-intercalation on the graphite negative electrode to cause graphite peeling and cannot be cycled is well solved.

Description

Technical Field

[0001] This application relates to an electrolyte based on propylene carbonate and its application, belonging to the field of lithium-ion battery technology. Background Technology

[0002] Lithium-ion batteries (LIBs) have become the preferred energy storage device for electric vehicles and portable electronic devices due to their high specific energy, low self-discharge, good cycle characteristics, no memory effect, and environmental friendliness. Graphite, on the other hand, possesses a relatively high theoretical specific capacity (372 mAh / g) and a relatively low operating potential (0.1 V~0.2 V vs. Li / Li). + Due to its good structural stability, graphite is currently the most widely used commercial anode material. However, the stability of the graphite interface is highly dependent on the solid electrolyte interphase (SEI) film formed in the first pass of the electrolyte. Most existing commercial electrolytes use ethylene carbonate (EC) systems, where EC can be preferentially reduced on the graphite surface to form a relatively stable SEI, thus enabling reversible lithium intercalation. However, the high melting point and limited stability of EC at high voltages restrict its further application in wide-temperature-range and high-nickel cathode systems.

[0003] Propylene carbonate (PC) possesses a wide liquidus temperature range and good compatibility with high-nickel cathodes, making it a promising candidate solvent for achieving wide-temperature-range electrolytes. However, PC often struggles to form an effective SEI on graphite anodes, readily co-intercalating with lithium ions and triggering graphite layer stripping, leading to a significant decrease in first-cycle efficiency and cycle life. To address the PC co-intercalation issue, existing technologies attempt to control the interface by adjusting electrolyte salt concentration, introducing co-solvents, or using additives. While these strategies improve the compatibility of PC and graphite to some extent, they still suffer from the following drawbacks:

[0004] 1) Poor SEI quality: The SEI film generated by existing additive reduction has a low content of inorganic components with high ionic conductivity and high strength (such as LiF, Li2O, etc.), and is consumed quickly during the cycle.

[0005] 2) Simple structural design: Most additive molecules have limited reaction sites, making it difficult to construct multi-component, multi-functional composite SEI structures simultaneously during the reduction process. Summary of the Invention

[0006] To address the shortcomings of existing technologies, particularly the specific problem of propylene carbonate (PC) easily co-intercalating on graphite anodes and causing structural stripping, the applicant's primary objective is to propose an electrolyte based on propylene carbonate. This electrolyte can not only further enrich the composition and structural characteristics of the SEI, but also achieve anode stabilization, effectively solving the problem of propylene carbonate easily co-intercalating on graphite anodes, leading to graphite stripping and decreased cycle performance.

[0007] Specifically, this application is implemented through the following scheme:

[0008] An electrolyte comprising a lithium salt, a solvent, and an additive, wherein the solvent is a non-aqueous organic solvent mainly composed of propylene carbonate, and the additive is a cyclic thiocyanate-type heterocyclic compound or an N-carbamate-3-thioyloxy-substituted pyrrolidine compound, wherein the mass percentage of the additive in the electrolyte is not less than 2%.

[0009] The structural formula of the cyclic thiosulfate ester type heterocyclic compound satisfies:

[0010] .

[0011] R1 and R2 are selected from any one of H atoms, haloalkyl, halogen atoms, and halophenyl; R3 is selected from any one of alkyl, branched alkyl, haloalkyl, phenyl, halophenyl, and alkenyl.

[0012] The structural formula of the N-carbamate-3-thioyloxy-substituted pyrrolidine compounds satisfies:

[0013] .

[0014] R4 is selected from any one of alkyl, branched alkyl, haloalkyl, phenyl, halophenyl, and alkenyl; R5 is selected from any one of alkyl, branched alkyl, haloalkyl, phenyl, halophenyl, and alkenyl.

[0015] This application focuses on solvent systems primarily composed of propylene carbonate. An additive with multiple polar bonds and capable of generating various reaction sites during reduction is designed and developed. This additive is a nitrogen-containing heterocycle with a cyclic sulfur-containing skeleton and multiple polar bonds. It introduces urethane linking units (-NC(=O)-OR) onto the nitrogen-containing heterocycle and retains CO bond sites that can trigger cleavage. The sulfur-containing polar bonds and urethane groups work synergistically, making the additive more easily activated by electron injection at the graphite interface and reacting before PC. In Li + Under coordination and interfacial electronic interactions, additives tend to undergo selective cleavage / ring-opening to form a complex solid electrolyte interfacial phase (SEI) dominated by sulfate / sulfite inorganic salts and accompanied by nitrogen-containing organic salt residues. This allows for the rapid establishment of a stable passivation layer before co-intercalation side reactions occur, significantly inhibiting the migration of PC molecules with Li. + By penetrating into the graphite interlayer, the risk of irreversible expansion and peeling is reduced, thereby improving the long-term cycle stability of PC-based lithium-ion batteries under half-cell and practical battery conditions.

[0016] Furthermore, as a preferred option:

[0017] The halogen in the above-mentioned alkyl halogroups, halogen atoms, and phenyl halogroups is F, Cl, or Br.

[0018] The additives are 1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, 5-(trifluoromethyl)-1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, (S)-4-(2,5-difluorophenyl)-1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, (S)-4-(3,4-difluorophenyl)-1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, 4-(4-bromobenzyl)-1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, and (S)-4-(2-tert-butoxy-2-oxo) At least one of the following: (Ethyl)-1,2,3-oxathiazoline-3-carboxylic acid tert-butyl ester-2,2-dioxide, (R)-3-(tert-butoxycarbonyl)-1,2,3-oxathiazoline-4-carboxylic acid methyl ester-2,2-dioxide, (S / R)-4-(4-bromo-2-pyridyl)-1,2,3-oxathiazoline-3-carboxylic acid tert-butyl ester-2,2-dioxide, 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine, 1-(tert-butoxycarbonyl)-3-(trifluoromethanesulfonyloxy)pyrrolidine, 3-(methanesulfonyloxy)pyrrolidine-1-carboxylic acid methyl ester, and 3-[(4-chlorophenyl)sulfonyloxy]pyrrolidine-1-carboxylic acid tert-butyl ester. More preferably, it is selected from at least one of 1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, 5-(trifluoromethyl)-1,2,3-oxazolidine-3-carboxylic acid tert-butyl ester 2,2-dioxide, and 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine.

[0019] The additive has a mass percentage of 2-10% in the electrolyte, with 3-7% being preferred.

[0020] The lithium salt is any one of inorganic anionic lithium salts and organic anionic lithium salts, such as lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium oxalate borate (LiBOB), lithium difluorooxalate borate (LiDFOB), lithium hexafluorophosphate (LiPF6), and lithium tetrafluoroborate (LiBF4). More preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0021] The lithium salt concentration is 0.2~3.0 mol / L. More preferably, the lithium salt concentration in the electrolyte is 0.6~1.5 mol / L, and preferably 1.0~1.5 mol / L.

[0022] The propylene carbonate accounts for no less than 10% of the mass percentage in the electrolyte.

[0023] The electrolyte may be a propylene carbonate single solvent system, and may further include a carbonate co-solvent, wherein the carbonate co-solvent is selected from at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC).

[0024] The applicant's second objective is to provide a secondary battery comprising the aforementioned electrolyte, wherein the secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The active material of the positive electrode is selected from at least one of layered oxide positive electrode materials, spinel positive electrode materials, or olivine positive electrode materials. Preferably, the layered oxide positive electrode material is lithium nickel cobalt manganese oxide (NCM) or lithium nickel cobalt aluminum oxide (NCA) type materials; more preferably, it is a high-nickel layered oxide positive electrode material. Further preferably, the positive electrode active material is NCM811. The active material of the negative electrode is artificial graphite, natural graphite, or composite graphite; preferably, the negative electrode active material is artificial graphite.

[0025] The applicant's third objective is to provide an electrical device that includes a battery with the aforementioned features.

[0026] When the electrolyte of this application is used in lithium-ion batteries, its advantages and technical effects are as follows:

[0027] To address the failure of PC in graphite anodes, this application introduces a class of nitrogen-containing heterocyclic additives with multipolar synergistic triggering characteristics, consisting of sulfur-containing polar bonds and urethane groups. The molecules simultaneously possess strong polar bonds such as S=O, C=O, and S–N, thereby exhibiting stronger polarization response and more efficient "precursor reaction" capabilities under external electric field or interfacial electron injection conditions. Through free radical transfer, nitrogen-containing carbonyl ester fragments are removed, ultimately resulting in the directional generation of stable inorganic salts such as lithium sulfite, while retaining nitrogen-containing organic residues to participate in the construction of the interfacial film.

[0028] This application further recognizes that, compared to additives containing only a single polar bond structure, primarily relying on a single functional group for their function, or solely relying on a cyclic sulfur-containing framework for film formation, introducing multiple polar bonds such as S=O, C=O, S–N, and C–N within the same molecule is beneficial for forming a richer polarity distribution and multi-site synergistic effects, thereby enhancing the overall reactivity of the molecule at the graphite anode interface. Specifically, S=O and C=O bonds are beneficial for increasing the overall polarity of the molecule and its response capability under the influence of the interfacial electric field, enhancing the interaction between the molecule and Li. +The interaction between the electrode interface and the S–N and C–N bonds, along with their adjacent structural units, helps to regulate the local electron distribution of the molecule, enhancing its tendency to preferentially transform under the influence of interfacial electrons. Through the synergistic effect of the above-mentioned multiple polar bonds, the additive described in this application constructs a SEI on the graphite surface that is different from the traditional cyclic sulfate ester "single inorganic salt dominant" type—characterized by the enrichment of inorganic components such as sulfate / sulfite and their synergistic effect with nitrogen-containing organic components. Mechanistically, this provides a new molecular design direction for faster and stronger passivation of interface protection in the PC system. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 The graph shows the charge-discharge curves of graphite||Li coin cells with different electrolytes. In the graph, (a) is the electrolyte of Example 1; and (b) is the electrolyte of Comparative Example 1.

[0031] Figure 2 The graph shows the electrochemical performance of graphite||Li coin cells with different electrolytes. Part (a) is a comparison of the cyclic voltammetry (CV) curves of the cells corresponding to the electrolyte of Example 1; part (b) is a magnified view of the CV curve of the cells corresponding to the electrolyte of Example 1; and part (c) is the CV curve of the cells corresponding to the electrolyte of Comparative Example 1.

[0032] Figure 3 The images show scanning electron microscope (SEM) images of the graphite anode of graphite||Li coin cells with different electrolytes. Part (a) in the images shows the electrolyte of Example 1, and part (b) shows the electrolyte of Comparative Example 1.

[0033] Figure 4 High-resolution transmission electron microscopy (HRTEM) images of the graphite anode of graphite||Li coin cells with different electrolytes. Part (a) in the figure is the electrolyte of Example 1; part (b) is the electrolyte of Comparative Example 1.

[0034] Figure 5 The image shows the X-ray photoelectron spectroscopy (XPS) image of the graphite anode of the graphite||Li coin cell corresponding to the electrolyte of Example 1. Parts (a), (b), (c), (d), and (e) in the image are C 1s, F 1s, Li 1s, N 1s, and S 2p spectra, respectively.

[0035] Figure 6 The figure shows the electrochemical performance of the electrolyte in Example 1 in different batteries. Part (a) is the long-cycle performance graph for graphite||Li coin cell; part (b) is the charge-discharge curve for NCM811||Gr pouch cell; and part (c) is the long-cycle performance graph for NCM811||Gr pouch cell.

[0036] Figure 7 This is a schematic diagram of the decomposition principle of the additive in Example 1. Part (a) in the figure is the decomposition path of the additive; part (b) is the energy evidence of the multi-step decomposition path of the additive.

[0037] Figure 8 The graphite||Li coin cell charge-discharge curves for different electrolytes are shown in the figure. Part (a) is the electrolyte of Example 2; part (b) is the electrolyte of Comparative Example 2.

[0038] Figure 9 The graphite||Li coin cell shows the charge-discharge performance of different electrolytes. Part (a) of the figure is the charge-discharge curve of the electrolyte in Example 3; part (b) is the long-cycle performance of the battery corresponding to the electrolyte in Example 3; and part (c) is the charge-discharge curve of the battery corresponding to the electrolyte in Comparative Example 3.

[0039] Figure 10 The figure shows the electrochemical performance of batteries corresponding to different electrolytes. Part (a) is the charge-discharge curve of the graphite||Li coin cell corresponding to the electrolyte of Example 4; part (b) is the long-cycle performance of the graphite||Li coin cell corresponding to the electrolyte of Example 4; and part (c) is the charge-discharge curve of the graphite||Li coin cell corresponding to the electrolyte of Comparative Example 4.

[0040] Figure 11 The graphite||Li coin cells corresponding to the electrolytes of Comparative Examples 5, 6, 7 and 8 are shown in the graph. Part (a) is the electrolyte of Comparative Example 5; part (b) is the electrolyte of Comparative Example 6; part (c) is the electrolyte of Comparative Example 7; and part (d) is the electrolyte of Comparative Example 8. Detailed Implementation

[0041] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the technical solutions in the embodiments of this application will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit the technical solutions of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without creative effort are within the scope of protection of this application.

[0042] Example 1

[0043] This embodiment provides an electrolyte based on propylene carbonate, which has the following composition:

[0044] Lithium salt: LiPF6, concentration 1.1 mol / L.

[0045] Solvent: propylene carbonate.

[0046] Additive: 1,2,3-oxathiazolidin-3-carboxylic acid tert-butyl ester 2,2-dioxide, accounting for 5% of the total mass of the electrolyte.

[0047] The preparation method of the above electrolyte is as follows:

[0048] Propylene carbonate was mixed with LiPF6 and stirred at room temperature until the lithium salt was completely dissolved. Then, 1,2,3-oxathiazolidin-3-carboxylic acid tert-butyl ester 2,2-dioxide was added, and stirring was continued until the electrolyte became clear, thus obtaining the electrolyte solution.

[0049] Comparative Example 1

[0050] This comparative example has the same setup as Example 1, except that it does not contain any additives, and its composition is as follows:

[0051] Lithium salt: LiPF6, concentration 1.1 mol / L.

[0052] Solvent: propylene carbonate.

[0053] The preparation method of the above electrolyte is as follows:

[0054] Add an appropriate amount of LiPF6 to propylene carbonate and stir until the lithium salt is completely dissolved and a clear electrolyte is obtained.

[0055] The electrolytes of Example 1 and Comparative Example 1 were subjected to electrical performance tests and morphological characterization tests, and the results are as follows: Figures 1 to 6 As shown.

[0056] Figure 1 The graph shows the charge-discharge curves of a graphite||Li battery. Part (a) of the graph shows the application of the electrolyte prepared in Example 1 to a graphite||Li battery. The charge-discharge curve of this battery was obtained by performing 3 charge-discharge cycles at a test rate of 0.1 C and 0.2 C. It can be seen that the electrolyte of Example 1 can achieve efficient and reversible charge-discharge cycling. Part (b) of the graph shows the charge-discharge curves of the electrolyte of Comparative Example 1 applied to a graphite||Li battery under the same test conditions: reversible charge-discharge cycling is not possible. The first discharge curve shows an extremely long voltage plateau at approximately 0.8 V. This is attributed to the continuous co-intercalation of PC solvent molecules, which leads to irreversible damage to the graphite structure.

[0057] Figure 2 The figure shows the cyclic voltammetry (CV) curves of the graphite||Li coin cell. Part (a) of the figure shows the CV curve of the electrolyte from Example 1 used in the graphite||Li cell. In the first cycle, as shown in the magnified view in part (b), an irreversible reduction peak attributable to the additive molecules appears in the 1.2~1.5 V range, which is much higher than the PC decomposition potential. This indicates that the additive is preferentially reduced over the PC solvent. This preferential reaction forms a stable SEI film on the graphite surface, effectively protecting the graphite anode and laying the foundation for subsequent stable and reversible lithium insertion / extraction behavior. More importantly, this process exhibits significant one-time, self-limiting characteristics: the reduction peak almost completely disappears in the second scan, and the CV curves of the second and third cycles highly overlap. The CV curve of the electrolyte from Comparative Example 1 used in the graphite||Li cell is shown below. Figure 2 As shown in section (c), a broad and sustained reduction current band appears starting at approximately 0.80 V. Subsequent cycles failed to establish stable lithium insertion / extraction characteristic peaks, indicating that an effective SEI could not be formed on the electrode surface. In the subsequent second and third scans, although the reduction current weakened somewhat, it did not disappear, indicating that the PC co-intercalation reaction continued.

[0058] Figure 3 Scanning electron microscope (SEM) images of the graphite anode after 60 cycles of different electrolytes used in graphite||Li batteries are shown. Part (a) of the image corresponds to the electrolyte of Example 1: the graphite particles maintain a smooth and dense surface morphology, with no obvious cracks or peeling, and the structural integrity of the graphite electrode is well protected. Part (b) of the image corresponds to the electrolyte of Comparative Example 1: the structure of the graphite anode is severely damaged, the surface of the graphite particles is rough, and obvious lamellar peeling, cracking, and pores appear. This is a typical morphology of structural collapse caused by PC solvent co-intercalation.

[0059] Figure 4 After using different electrolytes in graphite||Li batteries, the high-resolution transmission electron microscope (HRTEM) images of the graphite anode after 60 cycles are shown. Part (a) in the figure corresponds to the electrolyte of Example 1: the layered structure of graphite remains regular and orderly, and the C(002) interplanar spacing d is maintained at 0.334 nm, which is almost identical to the original graphite. More importantly, a uniformly thick SEI film is formed on the surface of the graphite particles. Part (b) corresponds to the electrolyte of Comparative Example 1: the (002) interplanar spacing d of graphite is irreversibly increased to 0.452 nm, which is much larger than the intrinsic 0.334 nm of graphite, and the layered stacked structure also exhibits severe bending and dislocation.

[0060] Figure 5The X-ray photoelectron spectroscopy (XPS) image of the graphite anode when the electrolyte of Example 1 is used in a graphite||Li battery: The S 2p spectrum mainly contains sulfate ions (SO4). 2- and sulfite SO3 2- The N 1s spectrum showed N=C and NC signals, indicating that nitrogen-containing organic fragments were retained and embedded in the SEI layer after the SN bond breakage, constituting the organic phase component. Besides elements from additive sources, the SEI also contained common inorganic components from the decomposition of the host electrolyte. The C 1s, F 1s, and Li 1s spectra all showed obvious Li₂CO₃ and LiF characteristic peaks, indicating that the system formed an inorganic framework rich in carbonates and lithium fluoride.

[0061] Figure 6 Example 1 illustrates the effect of electrolyte on the electrochemical performance of different batteries. Part (a) shows the long-cycle performance of the electrolyte used in a graphite||Li battery. The test voltage range was 0.005 V to 1.0 V, and the test procedure was three cycles of charge-discharge at 0.1 C followed by a long cycle at 0.2 C. It can be seen that the battery exhibits significantly improved cycle stability. After 355 cycles at 0.2 C, the capacity retention rate still reached 71.16%, and the average coulombic efficiency remained close to 100%. Parts (b) and (c) show the effects of the electrolyte used in LiNi batteries. 0.8 Co 0.1 Mn 0.1 The charge-discharge test curves and long-cycle performance graphs for O2|| graphite pouch batteries, including LiNi 0.8 Co 0.1 Mn 0.1 The areal capacity of the O2 electrode is 3.40 mAh·cm³. -2 The areal capacity of the graphite anode is 2.769 mAh·cm³. -2 The test voltage range was 2.8 V to 4.3 V, and the test rate was 0.1 C for 3 cycles and 0.5 C for long cycles. It can be seen that the curves overlapped significantly with each cycle, and the voltage polarization change was not obvious, indicating that the interface impedance remained stable during long cycles. In the long cycle test in part (c), the pouch cell still maintained a capacity retention of 72.37% after 226 cycles, demonstrating good structural and chemical stability.

[0062] The decomposition principle of the additive introduced in this embodiment was characterized, and the results are as follows: Figure 7 As shown: Additive molecules bind Li under electron injection. +The process involves the formation of a four-membered ring transition state; subsequently, the CO bonds in the four-membered ring transition state undergo homolytic cleavage to form C radicals; finally, the nitrogen-containing carboxylic acid ester is removed via radical transfer to form lithium sulfite. Energetic evidence for the multi-step decomposition pathway in part (b) shows that its energy evolution was simulated using DFT calculations. The energy profile also validates the feasibility of the decomposition pathway. The initial CO bond cleavage reaction energy is -5.99 eV; the subsequent steps involving structural rearrangement to generate lithium sulfite have an energy of -14.31 eV. The final optimized structure clearly identifies Li₂SO₃ and nitrogen-containing organic residues, consistent with the decomposition pathway. This indicates that the additive molecules decompose via CO bond cleavage, generating a composite SEI on the graphite surface consisting of stable inorganic substances such as sulfates / sulfites and nitrogen-containing organic components.

[0063] Example 2

[0064] This embodiment provides an electrolyte based on propylene carbonate, which has the following composition:

[0065] Lithium salt: LiFSI, concentration 0.9 mol / L.

[0066] Solvent: Propylene carbonate and dimethyl carbonate were mixed in a volume ratio of 9:1.

[0067] Additive: 5-(trifluoromethyl)-1,2,3-oxathiazolidin-3-carboxylic acid tert-butyl ester-2,2-dioxide, accounting for 4% of the total mass of the electrolyte.

[0068] The preparation method of the above electrolyte is as follows:

[0069] Propylene carbonate and dimethyl carbonate were mixed in a volume ratio, and then the mixed electrolyte was mixed with LiFSI. The mixture was stirred at room temperature until the lithium salt was completely dissolved. Then, 5-(trifluoromethyl)-1,2,3-oxathiazolidin-3-carboxylic acid tert-butyl ester-2,2-dioxide was added, and the mixture was stirred until the electrolyte became clear, thus obtaining the electrolyte solution.

[0070] Comparative Example 2

[0071] The setup for this comparative example is the same as that for Example 2, except that the composition of the additives is different:

[0072] Lithium salt: LiFSI, concentration 0.9 mol / L.

[0073] Solvent: Propylene carbonate and dimethyl carbonate were mixed in a volume ratio of 9:1.

[0074] Additive: Ethylene carbonate, accounting for 4% of the total mass of the electrolyte.

[0075] The electrolyte preparation method is as follows:

[0076] Propylene carbonate and dimethyl carbonate were mixed in a volume ratio, and then the mixed electrolyte was mixed with LiFSI. Ethylene carbonate was then added, and the mixture was stirred until the electrolyte became clear to obtain the electrolyte solution.

[0077] The electrolytes of Example 2 and Comparative Example 2 were used in a graphite||Li asymmetric battery. Tests were performed at 0.1 C charge-discharge for 3 cycles and 0.2 C charge-discharge for extended cycles. The results are as follows: Figure 8 As shown, the battery using the electrolyte of Example 2 can achieve efficient and reversible charge-discharge cycles. In contrast, the battery using the electrolyte of Comparative Example 2 achieves a lower efficiency at 0.8 V (vs. Li / Li). + The presence of a significant and persistent reduction decomposition voltage plateau near the 0.05 ohm (PC) indicates that the system failed to effectively suppress the co-intercalation behavior of propylene carbonate (PC) molecules, resulting in a large irreversible capacity loss. Vinylene carbonate is a commonly used film-forming additive, but it lacks the sulfur-containing polar structure, nitrogen-containing heterocyclic carbamate structure, and the synergistic arrangement of multiple polar bonds described in this application. Therefore, it cannot construct an interfacial film with sufficient passivation capability in a PC-based system, and its electrochemical protection effect is significantly inferior to the novel additive described in this invention.

[0078] Example 3

[0079] This embodiment describes an electrolyte based on propylene carbonate, the composition of which is as follows:

[0080] Lithium salt: LiPF6, concentration 1.1 mol / L.

[0081] Solvent: propylene carbonate.

[0082] Additives: A mixture of 1,2,3-oxathiazolidin-3-carboxylic acid tert-butyl ester, 2,2-dioxide, and 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine in a 1:1 ratio, with the total mass of the additives accounting for 5% of the total mass of the electrolyte.

[0083] The preparation method of the above electrolyte is as follows:

[0084] Propylene carbonate was mixed with an appropriate amount of LiPF6 and stirred at room temperature until the lithium salt was completely dissolved. Then a mixture of 1,2,3-oxathiazolidin-3-carboxylic acid tert-butyl ester 2,2-dioxide and 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine was added, and stirring was continued until the electrolyte was clear, thus obtaining the electrolyte solution.

[0085] Comparative Example 3

[0086] The setup for this comparative example is the same as that for Example 3, except that the composition of the additives is different:

[0087] Lithium salt: LiPF6, concentration 1.1 mol / L.

[0088] Solvent: propylene carbonate.

[0089] Additive: 1-Butylsulfonyl fluoride, accounting for 5% of the total mass of the electrolyte.

[0090] The preparation method is as follows:

[0091] An appropriate amount of LiPF6 was added to propylene carbonate and stirred until the lithium salt was completely dissolved and a clear electrolyte was obtained. Then, 1-butylsulfonyl fluoride was added and stirred until clear to obtain the electrolyte.

[0092] Figure 9 The figures show the electrochemical performance of batteries used in Example 3 and Comparative Example 3, respectively, applied to graphite||Li batteries. Figure (a) shows the charge-discharge curves of batteries using the electrolyte of Example 3, with a test rate of 0.1 C for 3 charge-discharge cycles and a long charge-discharge cycle of 0.2 C. It can be seen that the battery can achieve efficient and reversible charge-discharge cycles. Figure (b) shows the cycle life curves of batteries using the electrolyte of Example 3, with a test voltage range of 0.001 V to 1.0 V and a test procedure of 0.1 C for 3 charge-discharge cycles and a long cycle of 0.2 C. It can be seen that the capacity retention rate of the above electrolyte is 92.84% after 50 cycles. Figure (c) shows the charge-discharge images of batteries using the electrolyte of Comparative Example 3. It can be seen that graphite anodes cannot achieve reversible charge-discharge in the aforementioned electrolyte. While 1-butylsulfonyl fluoride has a sulfur-containing structure with S=O and S–O bonds, it lacks the nitrogen-containing heterocyclic carbamate structure and the molecular characteristics of multiple polar bonds synergistically set as described in this application. This indicates that 1-butylsulfonyl fluoride, as an additive for PC-based electrolytes, cannot make them compatible with graphite anode materials. Therefore, introducing only a single sulfur-containing polar structure is insufficient to achieve effective interface protection.

[0093] Example 4

[0094] This embodiment describes an electrolyte based on propylene carbonate, the composition of which is as follows:

[0095] Lithium salt: LiPF6, concentration 1.5 mol / L.

[0096] Solvent: propylene carbonate.

[0097] Additive: 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine, accounting for 2% of the total mass of the electrolyte.

[0098] The preparation method of the above electrolyte is as follows:

[0099] Propylene carbonate was mixed with an appropriate amount of LiPF6 and stirred at room temperature until the lithium salt was completely dissolved. Then, 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine was added, and stirring was continued until the electrolyte became clear, thus obtaining the electrolyte solution.

[0100] Comparative Example 4

[0101] The setup for this comparative example is the same as that for Example 3, except for the different additives. Its composition is as follows:

[0102] Lithium salt: LiPF6, concentration 1.5 mol / L.

[0103] Solvent: propylene carbonate.

[0104] Additive: Ethyl ethanesulfonate, accounting for 2% of the total mass of the electrolyte.

[0105] The preparation method of the above electrolyte is as follows:

[0106] Add an appropriate amount of LiPF6 to propylene carbonate and stir until the lithium salt is completely dissolved. Then add ethyl ethanesulfonate to obtain a clear electrolyte.

[0107] The electrolytes of Example 4 and Comparative Example 4 were applied to graphite||Li batteries, and their electrochemical performance was tested. The results are as follows: Figure 10 As shown in the figure. Part (a) is the charge-discharge curve of the battery corresponding to the electrolyte of Example 4, with a test rate of 0.1 C charge-discharge for 3 cycles and 0.2 C charge-discharge long cycle. It can be seen that the above electrolyte can achieve efficient and reversible charge-discharge cycling. Part (b) is the cycle life curve of the battery corresponding to the electrolyte of Example 4, with a test voltage range of 0.001 V to 1.0 V and a test procedure of 0.1 C charge-discharge for 3 cycles and 0.2 C long cycle. It can be seen that the electrolyte of this application retains 99.86% of its capacity after 50 cycles at a 0.2 C rate. Part (c) is the charge-discharge curve of the electrolyte of Comparative Example 4 used in a graphite||Li asymmetric battery, with a test procedure of 0.1 C charge-discharge for 3 cycles followed by 0.2 C long cycle. It can be seen that the electrolyte failed to effectively suppress the co-intercalation behavior of propylene carbonate into graphite, resulting in a huge irreversible capacity loss. Its electrochemical protection effect is not as good as that of the electrolyte of this invention. The additive ethyl ethanesulfonate in Comparative Example 4 has a sulfur-containing structure with S=O and S–O bonds, but it lacks the molecular characteristics of the nitrogen-containing heterocyclic carbamate structure and the synergistic arrangement of multiple polar bonds described in this application. Its electrochemical performance indicates that simply introducing a common sulfur-containing structure is insufficient to effectively protect the graphite interface of the PC system; without the synergistic effect of multiple polar bonds, the additive struggles to simultaneously achieve both preferential interfacial reactivity and stable interfacial film construction, thus failing to achieve the technical effects described in this application. In contrast, the novel additive of this invention, due to the synergistic arrangement of sulfur-containing polar structures, carbamate-related structures, and multiple polar sites within the same molecule, is more conducive to preferentially participating in interfacial reactions and forming an effective protective film before the co-intercalation side reaction of propylene carbonate, thereby exhibiting a superior graphite anode protection effect.

[0108] Comparative Example 5

[0109] The electrolyte preparation method for this comparative example is as follows:

[0110] An appropriate amount of LiFSI was added to propylene carbonate and stirred until the lithium salt was completely dissolved and a clear electrolyte was obtained. Then, 5 wt% of 1-Boc-tetrahydropyrrole was added, and the concentration of LiFSI in the prepared electrolyte was 1.0 mol / L.

[0111] Comparative Example 6

[0112] The electrolyte preparation method for this comparative example is as follows:

[0113] An appropriate amount of LiPF6 was added to propylene carbonate and stirred until the lithium salt was completely dissolved and a clear electrolyte was obtained. Then, 8 wt% of fluoroethylene carbonate was added, and the concentration of LiPF6 in the prepared electrolyte was 1.1 mol / L.

[0114] Comparative Example 7

[0115] The electrolyte preparation method for this comparative example is as follows:

[0116] An appropriate amount of LiPF6 was added to propylene carbonate and stirred until the lithium salt was completely dissolved and a clear electrolyte was obtained. Then, 10 wt% of 1,3-propanesulfonyl lactone was added, and the concentration of LiPF6 in the prepared electrolyte was 1.0 mol / L.

[0117] Comparative Example 8

[0118] The electrolyte preparation method for this comparative example is as follows:

[0119] An appropriate amount of LiPF6 was added to propylene carbonate and stirred until the lithium salt was completely dissolved and a clear electrolyte was obtained. Then, 5 wt% of 3-ethyl-1,2,3-oxathiazolidinedione 2,2-dioxide was added, and the concentration of LiPF6 in the prepared electrolyte was 1.1 mol / L.

[0120] Electrolytes from Comparative Examples 5, 6, 7, and 8 were used in the batteries for electrical performance testing. The test procedure was: three cycles of charge-discharge at 0.1 C followed by a long cycle at 0.2 C. The results are as follows: Figure 11 As shown: The introduction of 1-Boc-tetrahydropyrrole containing urethane functional groups into the electrolyte cannot suppress the co-intercalation behavior of propylene carbonate into graphite (see...). Figure 11(See part (a)). Introducing fluoroethylene carbonate containing C=O and C–F bonds, but lacking sulfur polarity and urethane structures, into the electrolyte resulted in a large irreversible plateau at approximately 0.8 V in the first discharge curve of the corresponding battery's graphite anode. Even with higher addition levels (such as 8% in Comparative Example 6), fluoroethylene carbonate failed to effectively suppress unfavorable side reactions and co-intercalation-related processes at the graphite anode interface in the PC system (see...). Figure 11 (Part (b)). When 1,3-propanesulfonyl lactone containing S=O and S–O bonds but lacking a carbamate structure is introduced into the electrolyte, the first-cycle discharge curve of the corresponding graphite anode still exhibits a large irreversible plateau at approximately 0.8 V. Even with a higher addition amount (such as 10 wt% in Comparative Example 7), the PC-based electrolyte still shows a significant irreversible plateau characteristic of approximately 0.8 V during the first cycle of the graphite anode discharge (see...). Figure 11 (Part (c)). Introducing 3-ethyl-1,2,3-oxathiazoline 2,2-dioxide into the electrolyte, containing a sulfur- and nitrogen-containing heterocyclic skeleton and polar structures such as S=O, SN, and SO, but lacking a carbamate structure, still could not effectively suppress the co-intercalation behavior of propylene carbonate on the graphite anode (see...). Figure 11 (d) in the middle.

[0121] The results of Comparative Examples 5 to 8 above show that introducing urethane functional groups alone, having only polar structures such as C=O and C–N but lacking sulfur-containing polar structural units, introducing only sulfur-containing and nitrogen-containing heterocyclic skeletons and some polar structures, or even increasing their addition ratio, cannot effectively suppress the co-intercalation behavior of PC.

[0122] Examples 3 and 4 further illustrate that, within the structural framework of sulfur-containing polar units and nitrogen-containing skeletons defined in this application, different subclasses of additives can also establish passivating interfacial films and improve reversible lithium intercalation / deintercalation behavior in the PC system. In contrast, Comparative Example 1 shows that the system containing only LiPF6-PC exhibits a sustained reduction plateau at approximately 0.8 V, leading to severe graphite exfoliation, demonstrating that PC co-intercalation is difficult to terminate in the absence of effective prior film formation. Comparative Examples 2 and 6 show that conventional vinylene carbonate and fluoroethylene carbonate film-forming additives still cannot provide sufficient passivation capability in the PC system. Comparative Examples 3, 4, 5, and 7, from a counterexample perspective, verify the necessity of "key structural units / key reaction sites."

[0123] The above embodiments and comparative results collectively demonstrate that the technical effect of this application does not originate from the simple film-forming effect of conventional additives, nor can it be achieved through a single functional group or a single sulfur-containing structure. Rather, it stems from the synergistic effect of the sulfur-containing polar structure, urethane-related structure, and multiple polar bonds or polar structural units in the additive molecule. This synergistic effect makes the additive more likely to participate preferentially in the reaction process at the graphite anode interface and form an effective protective film before the co-intercalation side reaction of propylene carbonate occurs. This improves the interfacial compatibility between the PC-based electrolyte and the graphite anode, reduces graphite structural damage, and improves the reversible charge-discharge performance and cycle stability of the battery. This application, without significantly altering the PC main solvent system and without requiring complex process modifications, introduces functional additives with specific sulfur-containing polar structures and controllable fracture sites. These additives react before PC at the graphite interface and rapidly establish a passivation layer, thus completing interfacial coverage before co-intercalation occurs. This achieves effective suppression of PC co-intercalation and a significant improvement in interfacial stability, significantly enhancing the interfacial compatibility between the PC-based electrolyte and graphite. Additives, represented by cyclic thiosulfate ester-type heterocyclic compounds, exhibit a one-time, self-limiting preferential reduction behavior, enabling the formation of a uniform and dense SEI film. This film is rich in sulfur-containing inorganic species such as sulfates / sulfites, and synergistically forms a composite protective layer with inorganic frameworks such as LiF and Li₂CO₃, as well as a small amount of nitrogen-containing organic components. This allows graphite to maintain its intact morphology and regular layered structure after cycling, resulting in more stable polarization and better long-cycle retention in both half-cells and pouch cells. Experimental results further demonstrate that the additives described in this application exhibit good interface protection and long-cycle stability in both half-cells and pouch cells, showing promising application prospects.

[0124] The above-described embodiments are merely illustrative of several feasible implementations of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention, nor are the embodiments intended to limit the scope of protection in the claims of the present invention. For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention. All equivalent implementations or changes that do not depart from the present invention should be included in the technology of the present invention.

Claims

1. An electrolyte based on propylene carbonate, comprising a lithium salt, a non-aqueous organic solvent mainly composed of propylene carbonate, and additives, characterized in that: The additive has a mass percentage of not less than 2% in the electrolyte, and the additive is an N-carbamate-3-thioyloxy-substituted pyrrolidine compound. The structural formula of the N-carbamate-3-thioyloxy-substituted pyrrolidine compounds satisfies: ， R4 is selected from any one of alkyl, haloalkyl, phenyl, halophenyl, and alkenyl; R5 is selected from any one of alkyl, haloalkyl, phenyl, halophenyl, and alkenyl.

2. The electrolyte based on propylene carbonate according to claim 1, characterized in that: In R4, the alkyl group is a branched alkyl group; in R5, the alkyl group is a branched alkyl group.

3. The electrolyte based on propylene carbonate according to claim 1, characterized in that: The halogen in the haloalkyl or halophenyl group is F, Cl, or Br.

4. The electrolyte based on propylene carbonate according to claim 1, characterized in that: The additive is at least one of 1-(tert-butoxycarbonyl)-3-methanesulfonyloxypyrrolidine, 1-(tert-butoxycarbonyl)-3-(trifluoromethanesulfonyloxy)pyrrolidine, methyl 3-(methanesulfonyloxy)pyrrolidine-1-carboxylate, and tert-butyl 3-[(4-chlorophenyl)sulfonyloxy]pyrrolidine-1-carboxylate.

5. The electrolyte based on propylene carbonate according to claim 1, characterized in that: The additive has a mass percentage of 2-10% in the electrolyte.

6. The electrolyte based on propylene carbonate according to claim 1, characterized in that: The lithium salt is any one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonyl)imide, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium hexafluorophosphate, and lithium tetrafluoroborate.

7. The electrolyte based on propylene carbonate according to claim 1, characterized in that: The lithium salt concentration is 0.2 ~ 3.0 mol / L.

8. The electrolyte based on propylene carbonate according to claim 1, characterized in that: The electrolyte also contains a carbonate co-solvent, which is selected from at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

9. The application of the electrolyte according to any one of claims 1 to 8 in a secondary battery, characterized in that: The secondary battery includes a positive electrode and a negative electrode. The active material of the positive electrode is selected from at least one of layered oxide positive electrode material, spinel positive electrode material or olivine positive electrode material, and the active material of the negative electrode is artificial graphite, natural graphite or composite graphite.

10. An electrical device comprising a secondary battery as described in claim 9.

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