Electrolyte and secondary battery thereof

Abstract

This invention provides an electrolyte and a secondary battery, comprising: a non-aqueous organic solvent, an electrolyte salt, a film-forming additive, and a functional additive; wherein the non-aqueous organic solvent includes cyclic carbonates, linear carbonates, and fluorinated solvents; the functional additive is ethyltrimethylsilyl difluoroacetate; the fluorinated solvent has a mass content of 10%-25% in the electrolyte, and the functional additive has a mass content of 0.5%-2% in the electrolyte. The electrolyte of this invention, through the synergistic effect of the groups of ethyltrimethylsilyl difluoroacetate and the fluorinated solvent, forms a highly stable, low-impedance, and highly adaptable composite SEI / CEI film at the positive and negative electrodes, respectively, achieving dual stability at the positive and negative electrode interfaces. This solves the core interface pain point of electrolytes under high voltage and fast charging, and improves the performance of the battery under high voltage and high rate.

Description

Technical Field

[0002] This invention belongs to the field of battery technology, specifically relating to an electrolyte and its secondary battery. Background Technology

[0004] With the development of the new energy industry, the comprehensive performance of lithium-ion batteries has become a core requirement. However, the electrolytes currently used in lithium batteries often encounter the following technical problems: First, during fast charging, lithium ions rapidly insert from the positive electrode into the negative electrode, leading to increased polarization. At high charging rates, ion accumulation and lithium plating can easily occur, and lithium dendrites pose a risk of puncturing the separator. When charging above 3C, the cell temperature can surge by 15-20°C within 10 minutes. Excessive temperature rise can cause side reactions in the electrolyte, producing HF that corrodes the SEI film, thus increasing electrolyte consumption and severely affecting the cell's cycle performance. Second, traditional SEI / CEI films are prone to oxidation and decomposition under high voltage. After the film fails, the electrolyte directly contacts the positive electrode surface, triggering continuous oxidation and decomposition, forming a vicious cycle of "film rupture-re-decomposition-film thickening," resulting in a sharp increase in interfacial impedance. Therefore, there is an urgent need to develop an electrolyte with excellent electrical performance under high-voltage fast charging. Summary of the Invention

[0006] To address the aforementioned technical problems in existing methods, this invention proposes an electrolyte comprising: a non-aqueous organic solvent, an electrolyte salt, a film-forming additive, and a functional additive; wherein the non-aqueous organic solvent comprises cyclic carbonates, linear carbonates, and fluorinated solvents; the functional additive is ethyltrimethylsilyl difluoroacetate; the fluorinated solvent has a mass content of 10%-25% in the electrolyte, and the functional additive has a mass content of 0.5%-2% in the electrolyte.

[0007] Furthermore, the fluorinated solvent includes at least one of methyltrifluoroethyl carbonate (FEMC), 2,2-difluoroethyl acetate (DFEA), and ethyl trifluoroacetate (TFAE).

[0008] Furthermore, the cyclic carbonate is a mixed solvent of propylene carbonate (PC) and ethylene carbonate (EC).

[0009] Furthermore, the linear carbonate is at least one of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0010] Further, the film-forming additive is at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), and vinyl sulfate (DTD).

[0011] Furthermore, the film-forming additive is a mixture of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), and vinyl sulfate (VC).

[0012] Furthermore, the electrolyte lithium salt includes at least one of lithium hexafluorophosphate (LiPF6) and lithium difluorooxalate borate (LiODFB).

[0013] Furthermore, the electrolyte lithium salt is a mixture of lithium hexafluorophosphate (LiPF6) and lithium difluorooxalate borate (LiODFB).

[0014] Another aspect of the present invention provides a secondary battery, comprising a positive electrode, a negative electrode separator, and any of the electrolytes described above.

[0015] The electrolyte and secondary battery provided by this invention can provide a multifunctional composite electrolyte interface membrane within the battery. This interface membrane is formed by combining ethyltrimethylsilyl difluoroacetate (ETMSDFA) and methyltrifluoroethyl carbonate (FEMC), forming a siloxane crosslinked-LiF-rich-flexible carbonate composite SEI film at the negative electrode of the lithium-ion battery. Through synergistic group formation, matching film formation timing, and complementary interface components, the two achieve dual stability of the positive and negative electrode interfaces. Compared to existing electrolytes, the electrolyte of this invention, when added to the battery, can form a composite film layer that possesses high mechanical strength, flexibility, and ionic conductivity, while simultaneously suppressing negative electrode dendrite / volume expansion and positive electrode transition metal dissolution. It is an excellent interface control combination suitable for high-voltage, fast-charging electrolytes, and exhibits superior electrochemical performance under high voltage and fast charging conditions. Attached Figure Description

[0017] 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 only one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0018] Figure 1 LSV curves of lithium-ion batteries prepared according to various embodiments of the electrolyte of the present invention;

[0019] Figure 2 EIS curves of lithium-ion batteries prepared according to various embodiments of the electrolyte of this invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Preparation of electrolyte

[0023] The electrolyte is prepared mainly according to the following steps:

[0024] (1) Weigh each substance according to the following proportions: the mass fraction of the non-aqueous organic solvent is A1; the mass fraction of the electrolyte lithium salt is A2; the mass fraction of the film-forming additive is A3; and the mass fraction of the functional additive is A4.

[0025] (2) Then the electrolyte lithium salt is dissolved in a non-aqueous organic solvent and stirred to form a homogeneous solution 1;

[0026] (3) Then add the negative electrode film-forming additive and the functional additive one by one to the solution 1 obtained in step (2) to obtain the electrolyte.

[0027] The electrolytes used in Examples 1-32 and Comparative Examples 1-3 were prepared according to the steps described above. The specific mass percentages of each substance in the electrolyte in each example are shown in Table 1.

[0028] Example 1 12 0.5 15 54.5 15 / 0.5 0.7 0.5 0.8 0.5 Example 2 12 0.5 15 54 15 / 0.5 0.7 0.5 0.8 1 Example 3 12 0.5 15 53.5 15 / 0.5 0.7 0.5 0.8 1.5 Example 4 12 0.5 15 53 15 / 0.5 0.7 0.5 0.8 2 Example 5 12 0.5 15 59.5 10 / 0.5 0.7 0.5 0.8 0.5 Example 6 12 0.5 15 59 10 / 0.5 0.7 0.5 0.8 1 Example 7 12 0.5 15 58.5 10 / 0.5 0.7 0.5 0.8 1.5 Example 8 12 0.5 15 58 10 / 0.5 0.7 0.5 0.8 2 Example 9 12 0.5 15 50 20 / 0.5 0.7 0.5 0.8 0.5 Example 10 12 0.5 15 50 20 / 0.5 0.7 0.5 0.8 1 Example 11 12 0.5 15 50 20 / 0.5 0.7 0.5 0.8 1.5 Example 12 12 0.5 15 50 20 / 0.5 0.7 0.5 0.8 2 Example 13 12 0.5 15 44.5 25 / 0.5 0.7 0.5 0.8 0.5 Example 14 12 0.5 15 44 25 / 0.5 0.7 0.5 0.8 1 Example 15 12 0.5 15 43.5 25 / 0.5 0.7 0.5 0.8 1.5 Example 16 12 0.5 15 43 25 / 0.5 0.7 0.5 0.8 2 Example 17 12 0.5 15 / 10 59.5 0.5 0.7 0.5 0.8 0.5 Example 18 12 0.5 15 / 10 59 0.5 0.7 0.5 0.8 1 Example 19 12 0.5 15 / 10 58.5 0.5 0.7 0.5 0.8 1.5 Example 20 12 0.5 15 / 10 58 0.5 0.7 0.5 0.8 2 Example 21 12 0.5 15 / 15 54.5 0.5 0.7 0.5 0.8 0.5 Example 22 12 0.5 15 / 15 54 0.5 0.7 0.5 0.8 1 Example 23 12 0.5 15 / 15 53.5 0.5 0.7 0.5 0.8 1.5 Example 24 12 0.5 15 / 15 53 0.5 0.7 0.5 0.8 2 Example 25 12 0.5 15 / 20 50 0.5 0.7 0.5 0.8 0.5 Example 26 12 0.5 15 / 20 50 0.5 0.7 0.5 0.8 1 Example 27 12 0.5 15 / 20 50 0.5 0.7 0.5 0.8 1.5 Example 28 12 0.5 15 / 20 50 0.5 0.7 0.5 0.8 2 Example 29 12 0.5 15 / 25 44.5 0.5 0.7 0.5 0.8 0.5 Example 30 12 0.5 15 / 25 44 0.5 0.7 0.5 0.8 1 Example 31 12 0.5 15 / 25 43.5 0.5 0.7 0.5 0.8 1.5 Example 32 12 0.5 15 / 25 43 0.5 0.7 0.5 0.8 2 Comparative Example 1 12 0.5 15 70 / / 0.5 0.7 0.5 0.8 / Comparative Example 2 12 0.5 15 55 15 / 0.5 0.7 0.5 0.8 / Comparative Example 3 12 0.5 15 70 / / 0.5 0.7 0.5 0.8 1.0%

[0029] Preparation of secondary batteries

[0030] A lithium battery was prepared using ternary NCM613 as the positive electrode, polyethylene as the separator, graphite as the negative electrode, and the electrolyte prepared according to the above embodiments and comparative examples, with all other conditions being the same.

[0031] Battery cycle performance test

[0032] The prepared batteries were tested using a Xinwei cycle test cabinet connected to a high and low temperature oven. The oven temperatures were set to 25℃ and 45℃ respectively, and the battery cells were placed in the ovens for capacity retention testing. The specific test steps are as follows: S1: First, let the prepared batteries rest for 10 minutes; S2: Then, charge the batteries at a 4C rate using constant current and constant voltage until the upper limit voltage is 4.4V; S3: Let the charged batteries rest for another 10 minutes; S4: Then, discharge the batteries at a 1C rate using constant current and constant voltage until 3V; The corresponding batteries were cycled 1000 times at 25℃ and 800 times at 45℃ according to the above steps, and the relevant data were recorded in Table 2.

[0033] Example 1 81.2% 83.4% Example 2 83.6% 85.1% Example 3 82.0% 83.3% Example 4 80.2% 82.1% Example 5 78.8% 80.5% Example 6 80.6% 82.7% Example 7 78.1% 80.9% Example 8 77.0% 78.4% Example 9 78.8% 80.7% Example 10 80.7% 82.6% Example 11 78.6% 81.3% Example 12 75.9% 79.9% Example 13 71.7% 75.6% Example 14 73.3% 78.4% Example 15 72.2% 74.6% Example 16 69.8% 70.9% Example 17 74.2% 75.9% Example 18 75.8% 77.8% Example 19 73.3% 76.1% Example 20 71.5% 72.6% Example 21 74.4% 76.2% Example 22 77.1% 79.1% Example 23 73.2% 75.3% Example 24 70.8% 71.3% Example 25 70.6% 73.3% Example 26 73.5% 77.8% Example 27 69.4% 72.8% Example 28 66.5% 67.6% Example 29 68.1% 70.3% Example 30 69.8% 72.2% Example 31 67.8% 71.1% Example 32 66.5% 69.8% Comparative Example 1 42.2% 45.4% Comparative Example 2 65.8% 70.6% Comparative Example 3 63.2% 66.4%

[0034] As can be seen from the test results in Table 2, compared with the comparative example, adding the functional additive ethyltrimethylsilyl difluoroacetate (ETMSDFA) to the electrolyte can significantly improve the cycle life of the battery. In particular, when the functional additive ethyltrimethylsilyl difluoroacetate (ETMSDFA) and the fluorinated solvent methyltrifluoroethyl carbonate are added to the electrolyte at the same time, the cycle life of the corresponding battery is improved more significantly, and the battery has the best cycle life.

[0035] LSV test

[0036] To further determine the performance of the electrolyte of this invention, given the improved battery cycle performance, several electrolytes with good cycle performance, namely the formulations of Examples 1-4 and Comparative Examples 1-3, were selected for LSV testing. The testing methods are as follows:

[0037] Li / Al coin cells were assembled, with each cell containing 70 μL of electrolyte. Three cells were assembled for each sample. Open-circuit voltage (OPV) tests were performed using an electrochemical workstation. The LSV scan rate was set to 0.5 mV / s, and the voltage range was set to an open-circuit voltage of 6.5 V. The test data were summarized and compared. The test results are as follows: Figure 1 As shown.

[0038] EIS test

[0039] Several electrolytes with good cycle performance, namely those prepared in Examples 1-4 and Comparative Examples 1-3, were selected for EIS testing of the corresponding batteries. The testing procedure is as follows: NCM613 was used as the positive electrode, and graphite was used to fabricate coin cells. The electrolyte injection volume was 70 μL. Three batteries were assembled for each sample. The batteries were activated in a test cabinet and charged to 50% SOC. EIS testing was performed using an electrochemical workstation, with the detection frequency set to 0.1-100000 Hz and the detection amplitude to 10 mV. The test data were summarized and compared. The test results are as follows. Figure 2 .

[0040] The test results of LSV and EIS tests of the battery above show that adding ethyltrimethylsilyl difluoroacetate to the electrolyte can effectively improve the electrochemical window of the electrolyte. With the increase of the addition amount, the EIS gradually increases, and there is a significant increase when the addition is 2%, indicating that adding too much has a great impact on impedance. In terms of rate cycling retention, compared with the comparative example, the 25℃ 4C / 1C cycle performance improved from 42.2% retention rate after 1000 cycles to 81.2%, and the 45℃ 4C / 1C cycle performance improved from 45.4% retention rate after 800 cycles to 83.4%.

[0041] The combination of ETMSDFA and FEMC is a multifunctional interface regulation combination that achieves a synergistic effect ("1+1>2"). Its core value lies in the formation of highly stable, low-impedance, and highly adaptable composite SEI / CEI films at the positive and negative electrodes through group synergy, achieving dual stability at the positive and negative electrode interfaces. This solves the core interface pain points of high-voltage, fast-charging electrolytes (dendrite growth, volume expansion, transition metal dissolution, increased interface impedance, etc.). The combination has a simplified formulation and excellent performance, making it a high-quality compounding solution for next-generation high-voltage, high-rate lithium-ion battery electrolytes.

[0042] The main mechanism by which the electrolyte of the present invention improves the performance of the battery under high voltage and high rate conditions may be as follows: Ethyltrimethylsilyl difluoroacetate (ETMSDFA) and fluorinated solvent form a composite system that can form a composite SEI / CEI film with high stability, low impedance and strong adaptability on the positive and negative electrode surfaces.

[0043] Specifically, it may involve the formation of an SEI film at the negative electrode, primarily a siloxane crosslinked-LiF-rich phase-flexible carbonate composite film (core). This can be further categorized as follows:

[0044] 1. Formation of a dense inner layer (in direct contact with the negative electrode): The difluoroacetate group of ETMSDFA breaks the CF bond, and the fluorocarbonate group of FEMC is reduced and decomposed, synergistically generating a high content of LiF (far higher than that of a single additive system). At the same time, the organic lithium salt (ROCO2Li) decomposed by ETMSDFA and the methyl carbonate lithium salt of FEMC form a thin and dense organic-inorganic mixed layer, preventing the electrolyte from directly contacting the negative electrode.

[0045] 2. Formation of a porous middle layer (ion conduction channel): The residual trimethylsilyl (TMS) in ETMSDFA reacts with trace amounts of HF / water in the electrolyte to form a Si-O-Si siloxane cross-linked network. At the same time, the ethyl / methyl groups of FEMC regulate the porosity of the membrane layer, and the ethyl groups of ETMSDFA further optimize the pore distribution, constructing an ion channel adapted to Li⁺ transport.

[0046] 3. Thin outer layer (in contact with electrolyte): The incompletely decomposed FEMC oligomers and the siloxane oligomers of ETMSDFA form a flexible thin outer layer, which alleviates the film stretching caused by negative electrode volume changes (such as silicon-based expansion and lithium metal dendrite growth).

[0047] The electrolyte of this invention also forms a CEI film at the positive electrode, namely a thin and dense LiF / fluorocarbonate polymer composite film. The core components of this film are mainly: fluorocarbonate polymer generated by the oxidative decomposition of FEMC, trace amounts of LiF, and the TMS groups of ETMSDFA undergo only slight oxidation due to steric hindrance, forming a thin siloxane layer without the accumulation of large amounts of inorganic products. The positive electrode film structure is a single-component thin and dense film (thickness <10nm), closely attached to the surface of the positive electrode active material, without pores or cracks, effectively preventing electrolyte solvent molecules from embedding into the positive electrode lattice, while not affecting the insertion / extraction transport of Li⁺.

[0048] The ethyltrimethylsilyl difluoroacetate (ETMSDFA) and methyltrifluoroethyl carbonate (FEMC) in this invention combine to form a siloxane crosslinked-LiF rich phase-flexible carbonate composite SEI film at the negative electrode of a lithium-ion battery. Through group synergy, film formation timing matching, and complementary interface components, the two achieve dual stability of the positive and negative electrode interfaces. Compared with single additives, the film layer of the composite system has high mechanical strength, flexibility, and ion conductivity, and can also simultaneously suppress negative electrode dendrite / volume expansion and positive electrode transition metal dissolution. It is a high-quality interface regulation combination suitable for high-voltage and fast-charging electrolytes.

Claims

1. An electrolyte, characterized by, include: The electrolyte contains a non-aqueous organic solvent, an electrolyte salt, a film-forming additive, and a functional additive; wherein the non-aqueous organic solvent includes cyclic carbonates, linear carbonates, and fluorinated solvents; the functional additive is ethyltrimethylsilyl difluoroacetate; the fluorinated solvent has a mass content of 10%-25% in the electrolyte, and the functional additive has a mass content of 0.5%-2% in the electrolyte.

2. The electrolyte of claim 1, wherein The fluorinated solvent includes at least one of methyltrifluoroethyl carbonate, 2,2-difluoroethyl acetate, and ethyl trifluoroacetate.

3. The electrolyte of claim 2, wherein The cyclic carbonate is a mixed solvent of propylene carbonate and ethylene carbonate.

4. The electrolyte of claim 2, wherein The linear carbonate is at least one of methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate.

5. The electrolyte of claim 1, wherein The film-forming additive is at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, and vinyl sulfate.

6. The electrolyte as described in claim 5, characterized in that, The film-forming additive is a mixture of vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, and vinyl sulfate.

7. The electrolyte as described in claim 1, characterized in that, The electrolyte lithium salt includes at least one of lithium hexafluorophosphate and lithium difluorooxalate borate.

8. The electrolyte as described in claim 7, characterized in that, The electrolyte lithium salt is a mixture of lithium hexafluorophosphate and lithium difluorooxalate borate.

9. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1-8.

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