Multi-component synergistic siloxane electrolyte system and SEI membrane in-situ construction method

Abstract

The invention discloses a multi-component synergistic siloxane electrolyte system and an SEI membrane in-situ construction method, and belongs to the technical field of electrochemical energy. According to the specific scheme, the multi-component synergistic siloxane electrolyte system comprises a main solvent, a coordination diluent and a film forming additive, the main solvent is a siloxane solvent, the coordination diluent is an ether solvent, and the film forming additive is vinylene carbonate and fluoroethylene carbonate. The SEI in-situ construction method based on the multi-component synergetic siloxane electrolyte system comprises the following steps: step 1, low-temperature discharge activation; 2, step-by-step heating activation is carried out; through a two-step activation process, the densification advantage of low-temperature film formation and the mechanical stability advantage of high-temperature film formation are creatively combined, so that the high-quality SEI film with two excellent characteristics is constructed on the surface of the TNO / WNO negative electrode in situ.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy technology, specifically relating to an electrolyte system for lithium-ion batteries and a method for in-situ construction of a solid electrolyte interface (SEI) membrane, particularly suitable for low-temperature fast-charging lithium-ion batteries using titanium niobium oxide (TNO) or tungsten niobium oxide (WNO) as the negative electrode. Background Technology

[0002] Lithium-ion batteries, due to their high energy density and long cycle life, have become the mainstream energy storage technology for portable electronic devices and electric vehicles in today's society. However, with the expansion of application scenarios, their performance at extreme temperatures, especially low temperatures, faces severe challenges. In sub-zero environments, the viscosity of the electrolyte in traditional lithium-ion batteries (usually composed of lithium salt LiPF6 and carbonate solvent) increases significantly, and the ionic conductivity decreases sharply, leading to a sharp increase in internal resistance and a significant degradation in charge-discharge performance. In addition, under low-temperature fast charging conditions, the insertion and extraction kinetics of lithium ions on the negative electrode surface are hindered, which easily triggers irreversible metallic lithium precipitation, forming lithium dendrites. This not only consumes the active lithium source, leading to capacity decay, but may also puncture the separator, causing internal short circuits and serious safety accidents. To solve these problems, researchers and industry have explored various aspects. Among them, the construction of the SEI film is recognized as a key factor affecting the long-term stability of the battery. The SEI film is an interfacial film formed by the reduction and decomposition of the electrolyte on the negative electrode surface during the first charge of a lithium-ion battery. An ideal SEI film should possess electronic insulation and high ionic conductivity, effectively passivating the electrode surface and preventing the continuous decomposition of the electrolyte, thereby ensuring the long-term stable operation of the battery.

[0003] Traditional graphite anodes have a low operating potential (approximately 0.1V for Li / Li). + SEI formation is an inevitable and crucial process. However, for high-potential lithium intercalation anode materials, such as titanium niobium oxide (TNO) and tungsten niobium oxide (WNO), their operating potentials are relatively high (TNO is approximately 1.5V for Li / Li). + High-potential anodes, which can prevent lithium metal precipitation to some extent, are considered to have excellent safety characteristics and fast-charging potential. However, research has found that even these high-potential anodes still form an SEI film on their surface, and the quality of this film directly determines the battery's electrochemical performance. Under the harsh conditions of low temperature and fast charging, how to construct an SEI film with both density and ion conductivity in situ for these high-potential anodes has become a pressing technical challenge.

[0004] Existing methods for constructing SEIs have significant limitations. Traditional single-solvent electrolytes have a limited solvation structure, making it difficult to precisely control SEI formation under complex and variable electrochemical environments. For example, while single siloxane solvents (such as dimethylsiloxane, DMS) possess high least unoccupied molecular orbitals (LUMOs) and weak polarity, which facilitate preferential reduction and film formation at lower voltages and can form flexible SEIs with Si-O bonds, their film-forming behavior and stability in complex systems still require optimization.

[0005] Furthermore, contradictory findings exist regarding SEI activation processes. Studies indicate that SEI films formed at low temperatures (e.g., -20°C) are generally denser, more stable, and exhibit lower impedance. This is attributed to the limited electrochemical reaction kinetics at low temperatures, which facilitates the formation of a more controllable and uniform initial SEI layer. However, other studies have found that SEI films formed at higher temperatures (e.g., 60°C) are thicker, possess a more uniform structure, and exhibit higher crystallinity, including, for example, a crystalline Li₂O layer. This results in greater mechanical stability during subsequent cycles, effectively inhibiting the continued growth of the SEI. Therefore, a single temperature-based activation method cannot simultaneously achieve both SEI film density (low-temperature advantage) and mechanical stability (high-temperature advantage), leading to a compromise in performance. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-component synergistic siloxane electrolyte system, combined with a stepwise gradient temperature activation method, to solve the technical problem in existing technologies where traditional electrolytes and activation processes cannot effectively construct high-performance SEI films for high-potential anodes such as TNO / WNO under low-temperature fast-charging conditions. This invention provides a multi-component synergistic siloxane electrolyte system and a method for in-situ SEI construction through temperature control. Without sacrificing high-temperature performance, a high-quality SEI film with both density and mechanical stability, as well as excellent ion conductivity, can be constructed in-situ on the anode surface, thereby significantly improving the low-temperature fast-charging performance and long cycle life of lithium-ion batteries.

[0007] This invention achieves the above objectives through a combination of an innovative electrolyte formulation and a unique stepwise gradient temperature activation method. Specifically, the technical solution is as follows:

[0008] A multi-component synergistic siloxane electrolyte system includes a main solvent, a coordinating diluent, and a film-forming additive, wherein the main solvent is a siloxane solvent, the coordinating diluent is an ether solvent, and the film-forming additive is vinylene carbonate and fluoroethylene carbonate.

[0009] The siloxane solvents include one or more combinations of dimethyldimethoxysilane (DMDMS), dimethyldiethoxysilane, cyclohexylmethyldimethoxysilane, trimethylsiloxane alkanes, linear siloxanes, and cyclic siloxanes. The siloxane solvents listed above have high LUMO and weak polarity, enabling them to preferentially undergo reduction reactions when the electrode potential reaches their reduction potential, forming an SEI film with a Si-O bond framework. This structure imparts a certain degree of flexibility to the SEI film.

[0010] The ether solvents include one or more combinations of tetraethylene glycol dimethyl ether (TTE), dimethoxyethane, 1,2-diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether. The ether solvents listed above, acting as coordination diluents, do not primarily function as simple dilution agents, but rather regulate the content of siloxane solvent molecules in the first solvation shell of lithium ions through their coordination ability with lithium ions. In the electrolyte, different solvents have different coordination abilities with lithium ions.

[0011] The siloxane solvent accounts for 70%-95% of the total volume of the mixed solvent (siloxane solvent + ether solvent), preferably 80%-90%.

[0012] This invention utilizes the synergistic effect of ether solvents and siloxane solvents to precisely control the proportion of siloxane solvents in the solvated sheath layer under specific temperature conditions (-20℃ to -50℃), thereby achieving their preferential decomposition and film formation. Ether solvents themselves possess good reduction stability, high ionic conductivity, and low freezing point, which helps to reduce the low-temperature viscosity of the electrolyte and improve ion transport efficiency.

[0013] The decomposition potentials of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are higher than those of ordinary carbonate solvents, allowing them to decompose preferentially during the initial charging process, thereby inhibiting the continued decomposition of the main solvent and forming a protective layer. The decomposition mechanism of VC is believed to be free radical polymerization, with decomposition products mainly being Li₂CO₃ and polymers, which helps to form a stable passivation film. The decomposition of FEC, through free radical polymerization and defluorination, forms a highly cross-linked polymer and a LiF-rich SEI. The LiF-rich SEI possesses excellent mechanical strength and ionic conductivity, effectively withstanding the minute volume changes that may occur during lithium insertion / extraction processes in electrodes such as TNO / WNO, preventing SEI film rupture, and thus inhibiting the continued progress of side reactions.

[0014] The mass fraction of the vinylene carbonate in the electrolyte system is 0.1-5 wt%, preferably 1-1.5% wt, and the mass fraction of the fluoroethylene carbonate in the electrolyte system is 0.1-10 wt%, preferably 1-3.0% wt.

[0015] The electrolyte system of this invention organically combines the above three components, achieving multiple synergistic effects of solvation structure regulation, SEI component optimization, and enhanced mechanical properties.

[0016] The siloxane electrolyte system is applied to low-temperature fast-charging lithium-ion batteries with titanium oxide niobium or tungsten niobium oxide as the negative electrode.

[0017] The in-situ SEI construction method based on a multi-component synergistic siloxane electrolyte system upgrades the traditional single-temperature activation process to a step-by-step, gradient-temperature activation process. This method aims to leverage the differences in SEI film formation kinetics and product structure at different temperatures to achieve synergistic optimization of SEI film performance.

[0018] Step 1: Low-temperature discharge activation

[0019] The lithium-ion battery assembled using the above-mentioned multi-component synergistic siloxane electrolyte system was activated for 1-5 cycles within the range of -60℃ to -40℃ with a lower limit of the voltage window of 0.3V-1.2V and an upper limit of the voltage window of 2.8-3.2V; the activation rate range was 0.05C to 0.5C.

[0020] Objective and Principle: Under these low-temperature conditions, the electrolyte viscosity is high, limiting ion migration, but the solvation structure tends to be stable. By controlling the discharge to a lower voltage, the coordination content of siloxane solvent molecules in the first solvation shell of lithium ions can be increased, promoting their preferential decomposition into a film over other solvent molecules. This step utilizes controlled electrochemical reaction kinetics at low temperature to construct an initial, dense, and stable SEI film, laying a solid foundation for subsequent activation.

[0021] Step 2: Step-by-step heating activation

[0022] Parameters: After low-temperature discharge activation, the battery is transferred to a higher temperature for activation and cycled at a higher voltage. That is, the temperature is gradually increased to the range of -40℃ to -10℃, and activation is continued for 1-5 cycles with a lower limit of the voltage window of 0.5V-1.5V and an upper limit of the voltage window of 2.8~3.2V; the activation rate range is 0.05C~0.5C; and the lower limit of the voltage window in step two is higher than the lower limit of the voltage window in step one.

[0023] Purpose and Principle: As temperature increases, both ion mobility and electrochemical reaction rate significantly improve. This activation stage aims to promote further densification and crystallization of the SEI film. Studies have shown that SEI films formed at higher temperatures exhibit higher crystallinity, stronger mechanical stability, and better resistance to electrode stress during cycling, preventing repeated rupture and regeneration of the SEI film.

[0024] Preferably, the activation temperature in step one is at least two temperature values ​​between -60℃ and -40℃, and at the lower temperature value, the lower limit of the voltage window is lower than the lower limit of the voltage window corresponding to the higher temperature value. The activation temperature in step two is at least two temperature values ​​between -40℃ and -10℃, and at the lower temperature value, the lower limit of the voltage window is lower than the lower limit of the voltage window corresponding to the higher temperature value.

[0025] Preferably, the activation temperatures in step one are -50℃ and -40℃, with the lower limit of the voltage window corresponding to -50℃ being 0.4V and the lower limit of the voltage window corresponding to -40℃ being 0.6V. In step two, the activation temperatures are -30℃ and -20℃, with the lower limit of the voltage window corresponding to -30℃ being 0.8V and the lower limit of the voltage window corresponding to -20℃ being 1.0V.

[0026] Preferably, in step one: 2 cycles of 0.1C charge / discharge at -50℃, voltage window 0.4V-3.0V; then 2 cycles of 0.1C charge / discharge at -40℃, voltage window 0.6V-3.0V; and the lower limit of the voltage window corresponding to -50℃ is lower than the lower limit of the voltage window corresponding to -40℃. In step two: 2 cycles of 0.1C charge / discharge at -30℃, voltage window 0.8V-3.0V; 2 cycles of 0.1C charge / discharge at -20℃, voltage window 1.0V-3.0V; and the lower limit of the voltage window corresponding to -30℃ is lower than the lower limit of the voltage window corresponding to -20℃; and the lower limit of the voltage window corresponding to -40℃ in step one is lower than the lower limit of the voltage window corresponding to -30℃.

[0027] -50℃, 0.4V-3.0V: Siloxane solvents at 0.4V-0.6V (V Vs Li / Li + Within the range of ), it is most easily reduced, and under this condition, siloxanes are preferentially decomposed to construct an initial inner SEI composed of Si-O-Si.

[0028] -40℃, 0.6V-3.0V: The reduction potential of FEC is higher than that of siloxane solvents. Under these conditions, FEC is promoted to participate in local film formation. At the same time, the temperature is increased to -40℃ to increase the reaction rate and promote the deep reduction of siloxane solvents, making the SEI denser.

[0029] -30℃, 0.8V-3.0V: FEC and FSI - LiF is generated by decomposition near a potential of 0.8V, which enhances the formation of the LiF enrichment layer.

[0030] -20℃, 1.0V-3.0V: The decomposition potential of VC is close to 1.0V, which promotes the formation of the outer VC polymer film and improves the stability of the SEI film.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] This invention creatively combines the "densification" advantage of low-temperature film formation and the "mechanical stability" advantage of high-temperature film formation through a two-step activation process, thereby constructing a high-quality SEI film with both excellent properties in situ on the TNO / WNO anode surface. Attached Figure Description

[0033] Figure 1 It is Nb 14 W3O 44 XRD patterns;

[0034] Figure 2 These are state diagrams of the electrolyte prepared in Example 1 at room temperature and -50°C;

[0035] Figure 3 This is a graph showing the results of a cyclic charge-discharge test at -50°C for the battery prepared in Example 2. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0037] Example 1:

[0038] Electrode preparation: WNO(Nb) 14 W3O 44 Super P and PVDF were mixed in a mass ratio of 7:2:1, and an appropriate amount of NMP (N-methylpyrrolidone) was added dropwise to prepare a negative electrode slurry. After stirring for 10 hours, the slurry was coated onto the surface of copper foil to a thickness of 100 μm. The coated electrode was then transferred to a vacuum oven and dried at 80°C for 10 hours. The dried electrode was then cut into 14 mm diameter plates and held under 10 MPa for 5 minutes. The XRD pattern of the WNO material is shown below. Figure 1 As shown.

[0039] Electrolyte preparation: In an Ar glove box, dissolve LiFSI in a mixed solution of DMDMS and TTE (DMDMS to TTE volume ratio 4:1), then add 1.5% wt VC and 3% wt FEC to prepare a LiFSI concentration of 1.5 mol / L. -1 The solution was stirred for 2 hours. The electrolyte state at room temperature and -50℃ is as follows: Figure 2 As shown.

[0040] Using the WNO negative electrode and lithium metal prepared above, CR2025 coin cells were assembled using the above-described electrolyte. The cells were activated by sequentially performing two charge-discharge cycles at 0.1C at -50℃ (0.4-3.0V), -40℃ (0.6-3.0V), -30℃ (0.8-3.0V), and -20℃ (1.0-3.0V). Afterward, the cells underwent a 0.2C charge-discharge cycle test at -50℃. The results of the -50℃ cycle charge-discharge test are as follows: Figure 3 As shown, the battery operates stably at this temperature, maintaining a coulombic efficiency of 100% and achieving a charge / discharge specific capacity of up to 155 mAh / g.

[0041] Example 2:

[0042] Electrode preparation: WNO(Nb) 14 W3O 44 Super P and PVDF were mixed in a mass ratio of 7:2:1, and an appropriate amount of NMP (N-methylpyrrolidone) was added dropwise to prepare a negative electrode slurry. After stirring for 10 hours, the slurry was coated onto the surface of copper foil with a coating thickness of 100 μm. The coated electrode was then transferred to a vacuum oven and dried at 80°C for 10 hours. The dried electrode was then cut into 14 mm diameter electrodes and held under pressure at 10 MPa for 5 minutes.

[0043] Electrolyte preparation: In an Ar glove box, dissolve LiFSI in a mixed solution of dimethyldiethoxysilane and dimethoxyethane (volume ratio of dimethyldiethoxysilane to dimethoxyethane is 7:3), then add 0.1% wt VC and 10% wt FEC to prepare a LiFSI concentration of 1.5 mol / L. -1 The solution was stirred for 2 hours.

[0044] Using the WNO anode and lithium metal prepared above, CR2025 coin cells were assembled using the above-mentioned electrolyte. The cells were activated by sequentially performing one charge-discharge cycle at -60℃ (0.3-2.8V), -50℃ (0.5-2.8V), -40℃ (0.7-2.8V), and -30℃ (0.9-2.8V) at a current of 0.05C. After that, the cells were subjected to a 0.2C charge-discharge cycle test at -50℃.

[0045] Example 3:

[0046] Electrode preparation: WNO(Nb) 14 W3O 44Super P and PVDF were mixed in a mass ratio of 7:2:1, and an appropriate amount of NMP (N-methylpyrrolidone) was added dropwise to prepare a negative electrode slurry. After stirring for 10 hours, the slurry was coated onto the surface of copper foil with a coating thickness of 100 μm. The coated electrode was then transferred to a vacuum oven and dried at 80°C for 10 hours. The dried electrode was then cut into 14 mm diameter electrodes and held under pressure at 10 MPa for 5 minutes.

[0047] Electrolyte preparation: In an Ar glove box, dissolve LiFSI in a mixed solution of cyclohexylmethyldimethoxysilane and triethylene glycol dimethyl ether (volume ratio of cyclohexylmethyldimethoxysilane to triethylene glycol dimethyl ether is 19:1), then add 5% wt VC and 0.1% wt FEC to prepare a LiFSI concentration of 1.5 mol / L. -1 The solution was stirred for 2 hours.

[0048] Using the WNO anode and lithium metal prepared above, CR2025 coin cells were assembled using the above-mentioned electrolyte. The cells were activated by sequentially performing one charge-discharge cycle at 0.5C at -40℃ (1.0-3.2V), -30℃ (1.2-3.2V), -20℃ (1.4-3.2V), and -10℃ (1.5-3.2V). After that, the cells were subjected to a 0.2C charge-discharge cycle test at -50℃.

[0049] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A multi-component synergistic siloxane electrolyte system, characterized in that: It includes a main solvent, a coordinating diluent, and a film-forming additive. The main solvent is a siloxane solvent, the coordinating diluent is an ether solvent, and the film-forming additive is vinylene carbonate and fluoroethylene carbonate.

2. The multi-component synergistic siloxane electrolyte system according to claim 1, characterized in that: The siloxane solvent includes one or more combinations of dimethyldimethoxysilane, dimethyldiethoxysilane, cyclohexylmethyldimethoxysilane, trimethylsiloxane alkanes, linear siloxanes, and cyclic siloxanes; the ether solvent includes one or more combinations of tetraethylene glycol dimethyl ether, dimethoxyethane, 1,2-diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether.

3. The multi-component synergistic siloxane electrolyte system according to claim 2, characterized in that: The siloxane solvent accounts for 70%-95% of the total volume of the siloxane solvent and ether solvent, the vinylene carbonate has a mass fraction of 0.1-5 wt% in the electrolyte system, and the fluoroethylene carbonate has a mass fraction of 0.1-10 wt% in the electrolyte system.

4. The multi-component synergistic siloxane electrolyte system according to claim 1, characterized in that: The siloxane electrolyte system is applied to low-temperature fast-charging lithium-ion batteries with titanium oxide niobium or tungsten niobium oxide as the negative electrode.

5. A method for in-situ construction of an SEI based on the multi-component synergistic siloxane electrolyte system according to any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Low-temperature discharge activation The lithium-ion battery assembled using the above-mentioned multi-component synergistic siloxane electrolyte system was activated for 1-5 cycles within the range of -60℃ to -40℃ with a lower limit of the voltage window of 0.3V-1.2V and an upper limit of the voltage window of 2.8~3.2V; the activation rate range was 0.05C~0.5C. Step 2: Step-by-step heating activation Gradually increase the temperature to the range of -40℃ to -10℃, and continue activation for 1-5 cycles with a voltage window of 0.5V-1.5V at the lower limit and 2.8-3.2V at the upper limit; the activation rate range is 0.05C to 0.5C. Furthermore, the lower limit of the voltage window in step two is higher than the lower limit of the voltage window in step one.

6. The SEI in-situ construction method according to claim 5, characterized in that: The activation temperature in step one is at least two of the temperature values ​​between -60℃ and -40℃, and at the lower temperature value, the lower limit of the voltage window is lower than the lower limit of the voltage window corresponding to the higher temperature value.

7. The SEI in-situ construction method according to claim 5, characterized in that: The activation temperature in step two is at least two of the temperature values ​​between -40℃ and -10℃, and at the lower temperature value, the lower limit of the voltage window is lower than the lower limit of the voltage window corresponding to the higher temperature value.

8. The SEI in-situ construction method according to claim 6, characterized in that: In step one, the activation temperature values ​​are selected as -50℃ and -40℃. The lower limit of the voltage window corresponding to -50℃ is 0.4V, and the lower limit of the voltage window corresponding to -40℃ is 0.6V.

9. The SEI in-situ construction method according to claim 7, characterized in that: In step two, the activation temperature values ​​are selected as -30℃ and -20℃. The lower limit of the voltage window corresponding to -30℃ is 0.8V, and the lower limit of the voltage window corresponding to -20℃ is 1.0V.

10. The SEI in-situ construction method according to claim 5, characterized in that: In step one: charge and discharge at 0.1C for 2 cycles at -50℃, with a voltage window of 0.4V-3.0V; then charge and discharge at 0.1C for 2 cycles at -40℃, with a voltage window of 0.6V-3.0V; and the lower limit of the voltage window corresponding to -50℃ is lower than the lower limit of the voltage window corresponding to -40℃. In step two, the voltage window is 0.8V-3.0V after two cycles of 0.1C charge and discharge at -30℃ and 1.0V-3.0V after two cycles of 0.1C charge and discharge at -20℃; and the lower limit of the voltage window corresponding to -30℃ is lower than the lower limit of the voltage window corresponding to -20℃. Furthermore, the lower limit of the voltage window corresponding to -40℃ in step one is lower than the lower limit of the voltage window corresponding to -30℃.

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