A low-temperature electrolyte, a low-temperature lithium-ion battery and its preparation method

Abstract

This invention belongs to the field of lithium-ion battery technology, and particularly relates to a low-temperature lithium-ion battery electrolyte, a low-temperature lithium-ion battery, and a method for preparing the same. The low-temperature lithium-ion battery electrolyte comprises an electrolyte solvent, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, ethylene fluorocarbonate, lithium difluorophosphate, and ethylene sulfate. This invention provides a low-temperature lithium-ion battery electrolyte, a low-temperature lithium-ion battery, and a method for preparing the same to improve the low-temperature charge-discharge performance of lithium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a low-temperature lithium-ion battery electrolyte, a low-temperature lithium-ion battery, and a method for preparing the same. Background Technology

[0002] Existing technology:

[0003] Since entering the market, lithium-ion batteries have gained widespread application due to their advantages such as long lifespan, high specific capacity, and no memory effect. Especially in the power battery sector, the market has experienced breakthrough growth driven by both policy support and subsidies. However, with the continued development of the lithium-ion battery industry, its poor environmental adaptability has gradually attracted market attention. For example, lithium-ion batteries suffer from low capacity, severe degradation, poor cycle rate performance, significant lithium plating, and unbalanced lithium insertion / extraction when used at low temperatures. As application areas continue to expand, the constraints caused by the poor low-temperature performance of lithium-ion batteries are becoming increasingly apparent: reduced range in winter and the reluctance to use heating to maintain driving range have become major pain points for electric vehicle owners, also hindering the development of the electric vehicle industry. In high-latitude and high-altitude regions, the application of lithium-ion batteries is limited due to their poor low-temperature performance.

[0004] Currently, the main methods to address the poor low-temperature performance of lithium-ion batteries include introducing heating devices, insulation layers, and low-temperature electrolyte technology. Low-temperature electrolyte technology can effectively improve the low-temperature performance of lithium-ion batteries. However, current low-temperature lithium-ion electrolyte technology primarily improves the low-temperature discharge performance of lithium-ion batteries, lacking research on optimizing the low-temperature charging performance of low-temperature lithium-ion batteries, especially those operating at extremely low temperatures (-40℃ and below).

[0005] Furthermore, current technologies primarily focus on improving the low-temperature ionic conductivity of electrolytes and the low-temperature discharge capacity retention of batteries, without addressing the improvement of low-temperature charging capacity. Low-temperature charging, the process of lithium intercalation at the negative electrode at low temperatures, carries the risk of lithium plating at the negative electrode, placing higher demands on the battery's kinetic characteristics. Reducing mass transfer impedance is a crucial means to enhance its lithium intercalation capability.

[0006] The difficulty and significance of the above-mentioned technical issues:

[0007] Therefore, based on these issues, providing a low-temperature lithium-ion battery electrolyte, a low-temperature lithium-ion battery, and its preparation method to improve the low-temperature charge-discharge performance of lithium-ion batteries has important practical value. Summary of the Invention

[0008] The purpose of this application is to provide a low-temperature lithium-ion battery electrolyte, a low-temperature lithium-ion battery, and a method for preparing the same, in order to solve the technical problems in the prior art and improve the low-temperature charge and discharge performance of lithium-ion batteries.

[0009] The technical solution adopted in this application embodiment to solve the technical problems existing in the prior art is as follows:

[0010] A low-temperature lithium-ion battery electrolyte, the low-temperature lithium-ion battery electrolyte comprising an electrolyte solvent, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, ethylene fluorocarbonate, lithium difluorophosphate, and ethylene sulfate.

[0011] The embodiments of this application may also employ the following technical solutions:

[0012] In the aforementioned low-temperature lithium-ion battery electrolyte, the electrolyte solvent accounts for 48.5%-85.5% of the electrolyte mass percentage, and the electrolyte solvent comprises the following components by mass percentage: 5%-35% ethylene carbonate, 10%-30% ethylene glycol propyl ether, 8%-28% methyl ethyl carbonate, and 5%-20% dimethyl carbonate.

[0013] In the aforementioned low-temperature lithium-ion battery electrolyte, the lithium difluorosulfonyl imide component accounts for 3%-20% by mass, the lithium difluorooxalate borate component accounts for 1.0%-2% by mass, the fluoroethylene carbonate component accounts for 1.0%-5% by mass, the lithium difluorophosphate component accounts for 0.5%-5% by mass, and the ethylene sulfate component accounts for 0.5%-8% by mass.

[0014] A low-temperature lithium-ion battery, the low-temperature lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and a low-temperature lithium-ion battery electrolyte as described in any one of the above.

[0015] A method for preparing a low-temperature lithium-ion battery, wherein the method is used to prepare the low-temperature lithium-ion battery.

[0016] In the above-mentioned method for preparing a low-temperature lithium-ion battery, the method further includes the following steps:

[0017] Step 1: Mix the graphite anode material, conductive agent, binder, and NMP solvent, and stir to obtain the anode slurry;

[0018] Step 2: Coat the negative electrode slurry onto the surface of the current collector, dry, roll, and punch to obtain the negative electrode sheet;

[0019] Step 3: Mix the ternary cathode material, conductive agent, binder, and NMP solvent, and stir to obtain the cathode slurry;

[0020] Step 4: Coat the positive electrode slurry onto the surface of the current collector, dry, roll, and punch to obtain the positive electrode sheet;

[0021] Step 5: The positive electrode, separator, and negative electrode are stacked in a Z-shape, placed into the casing, dried, and then injected with liquid in a vacuum environment at a dew point of -40°C. The resulting low-temperature lithium-ion battery is then encapsulated.

[0022] One or more technical solutions provided in the embodiments of this application have at least the following beneficial effects:

[0023] 1. The electrolyte of this invention has a conductivity σ > 4 mS / cm at 20℃, a conductivity σ > 1 mS / cm at -40℃, and a conductivity σ > 0.7 mS / cm at -50℃, which means it has good charge and discharge performance at -40℃.

[0024] 2. In the prior art, lithium difluorosulfonylimide is corrosive to the current collector and is not used as the only lithium salt. The solvated lithium salt structure formed by the solvent component of the present invention inhibits the corrosion of the current collector by lithium difluorosulfonylimide. Therefore, the present invention uses a high content of lithium difluorosulfonylimide lithium salt to improve the low-temperature conductivity of the electrolyte.

[0025] 3. The use of fluoroethylene carbonate and ethylene glycol propyl ether in this invention facilitates the desolvation process of lithium salts and reduces charge transfer resistance. Fluoroethylene carbonate can also be used as a film-forming additive in this invention. While not a commonly used film-forming additive in graphite systems, fluoroethylene carbonate can form a stable, low-impedance thin SEI film, which similarly helps reduce charge transfer resistance, improves the battery's low-temperature charge-discharge capability, and enhances the battery's charge-discharge capacity retention under extremely low-temperature conditions. On the one hand, it can improve the conductivity of the electrolyte; on the other hand, it can not only improve the battery's discharge capacity retention rate at low temperatures but also improve the battery's charge capacity retention rate at low temperatures.

[0026] 3. This invention uses lithium difluorosulfonylimide as the lithium salt, and the solvent components are 5%-35% ethylene carbonate, 10%-30% ethylene glycol propyl ether, 8%-28% methyl ethyl carbonate, and 5%-20% dimethyl carbonate. The electrolyte additives are lithium difluorooxalate borate, fluoroethylene carbonate, lithium difluorophosphate, and ethylene sulfate. Through the design of the lithium salt, solvent, and film-forming additives in the electrolyte, the desolvation process of the lithium salt on the negative electrode surface is facilitated, the SEI film impedance on the negative electrode surface is reduced, and the low-temperature conductivity of the electrolyte is improved. Compared with ordinary electrolytes, the electrolyte of this invention can effectively reduce mass transfer impedance and improve the dynamic performance of the battery.

[0027] 4. Currently, lithium bis(fluorosulfonyl)imide exhibits better low-temperature performance. However, in existing technologies, to reduce the corrosion of foils by lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate is commonly used as the lithium salt, or a small amount of lithium bis(fluorosulfonyl)imide is added to lithium hexafluorophosphate, which limits the low-temperature conductivity of the electrolyte. This invention, by adjusting the electrolyte formulation, specifically adds ethylene glycol propyl ether to the solvent components (5%-35% ethylene carbonate, 10%-30% ethylene glycol propyl ether, 8%-28% methyl ethyl carbonate, and 5%-20% dimethyl carbonate) in the patented electrolyte formulation. This prevents lithium bis(fluorosulfonyl)imide from corroding the foil. Therefore, this solution can use lithium bis(fluorosulfonyl)imide with a high content as the lithium salt, without adding lithium hexafluorophosphate, significantly improving the low-temperature conductivity of the electrolyte compared to ordinary electrolytes.

[0028] 5. Ethylene glycol propyl ether is commonly used as a solvent in the coatings industry. This invention applies it to the field of electrochemistry, using ethylene glycol propyl ether as an electrolyte solvent to reduce the low-temperature viscosity of the electrolyte and effectively improve the low-temperature conductivity of the electrolyte.

[0029] 6. This invention can generate a tough SEI film. Ethylene fluorocarbonate is commonly used in silicon-carbon anode systems. This invention utilizes its thin film and low film impedance to apply it to low-temperature graphite systems, effectively reducing interfacial impedance and improving the low-temperature performance of the electrolyte.

[0030] 7. The electrolyte formulation used in this invention solves the problem of lithium bis(fluorosulfonyl)imide corroding foil. It can be used as the sole lithium salt in the electrolyte, which greatly improves the low-temperature conductivity (the conductivity is still above 0.001 S / cm at -30℃). Combined with the use of low-viscosity mixed solvent components (e.g., ethylene glycol propyl ether) and low-resistance film-forming additives (e.g., FEC), the low-temperature performance of the electrolyte is comprehensively improved, enabling the battery to be charged and discharged stably at -40℃. Detailed Implementation

[0031] The lithium salt for use in this invention is lithium difluorosulfonylimide, with solvent components of 5%-35% ethylene carbonate, 10%-30% ethylene glycol propyl ether, 8%-28% methyl ethyl carbonate, and 5%-20% dimethyl carbonate, and electrolyte additives of lithium difluorooxalate borate, fluoroethylene carbonate, lithium difluorophosphate, and ethylene sulfate.

[0032] To better understand the above technical solution, the following will provide a detailed explanation of the technical solution in conjunction with specific implementation methods.

[0033] Example 1

[0034] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 80% electrolyte solvent, 10% lithium difluorosulfonyl imide, 2% lithium difluorooxalate borate, 3% fluoroethylene carbonate, 3% lithium difluorophosphate, and 2% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 35% ethylene carbonate, 30% ethylene glycol propyl ether, 15% methyl ethyl carbonate, and 20% dimethyl carbonate.

[0035] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a soft-pack lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Example 1.

[0036] Example 2

[0037] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 85% electrolyte solvent, 10% lithium difluorosulfonyl imide, 2% lithium difluorooxalate borate, 1% fluoroethylene carbonate, 1% lithium difluorophosphate, and 1% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 30% ethylene carbonate, 30% ethylene glycol propyl ether, 20% methyl ethyl carbonate, and 20% dimethyl carbonate.

[0038] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a pouch lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Example 2.

[0039] Example 3

[0040] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 80% electrolyte solvent, 12% lithium difluorosulfonyl imide, 2% lithium difluorooxalate borate, 2% fluoroethylene carbonate, 2% lithium difluorophosphate, and 2% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 30% ethylene carbonate, 30% ethylene glycol propyl ether, 20% methyl ethyl carbonate, and 20% dimethyl carbonate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 25% ethylene carbonate, 30% ethylene glycol propyl ether, 25% methyl ethyl carbonate, and 20% dimethyl carbonate.

[0041] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a soft-pack lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Example 3.

[0042] Example 4

[0043] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 85% electrolyte solvent, 11% lithium difluorosulfonyl imide, 1% lithium difluorooxalate borate, 1% fluoroethylene carbonate, 1% lithium difluorophosphate, and 1% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 35% ethylene carbonate, 25% ethylene glycol propyl ether, 20% methyl ethyl carbonate, and 20% dimethyl carbonate.

[0044] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a soft-pack lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Example 4.

[0045] Example 5

[0046] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 80% electrolyte solvent, 15% lithium difluorosulfonyl imide, 1% lithium difluorooxalate borate, 2% fluoroethylene carbonate, 1% lithium difluorophosphate, and 1% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 30% ethylene carbonate, 30% ethylene glycol propyl ether, 25% methyl ethyl carbonate, and 15% dimethyl carbonate.

[0047] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a soft-pack lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Example 5.

[0048] Example 6

[0049] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 85% electrolyte solvent, 15% lithium difluorosulfonyl imide, 1% lithium difluorooxalate borate, 1% fluoroethylene carbonate, 1% lithium difluorophosphate, and 2% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 30% ethylene carbonate, 30% ethylene glycol propyl ether, 28% methyl ethyl carbonate, and 12% dimethyl carbonate.

[0050] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a soft-pack lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Example 6.

[0051] Comparative Example 1

[0052] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 90% electrolyte solvent, 11% lithium hexafluorophosphate, 0.1% lithium difluorooxalate borate, 0.5% fluoroethylene carbonate, 0.5% lithium difluorophosphate, and 1.9% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 50% ethylene carbonate, 20% methyl ethyl carbonate, and 30% dimethyl carbonate.

[0053] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a pouch lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Comparative Example 1.

[0054] Comparative Example 2

[0055] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 90% electrolyte solvent, 15% lithium hexafluorophosphate, 3% lithium difluorooxalate borate, 0.2% fluoroethylene carbonate, 0.3% lithium difluorophosphate, and 1.5% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 75% ethylene carbonate, 10% methyl ethyl carbonate, and 15% dimethyl carbonate.

[0056] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a pouch lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Comparative Example 2.

[0057] Comparative Example 3

[0058] The low-temperature electrolyte of this embodiment is composed of the following components in the following proportions: 88% electrolyte solvent, 2% lithium difluorosulfonyl imide, 5% lithium difluorooxalate borate, 0.1% fluoroethylene carbonate, 2% lithium difluorophosphate, and 2.9% ethylene sulfate. The electrolyte solvent in the electrolyte is composed of the following components by mass percentage: 80% ethylene carbonate, 10% methyl ethyl carbonate, and 10% dimethyl carbonate.

[0059] Using graphite as the negative electrode and ternary material as the positive electrode, and adding the aforementioned low-temperature electrolyte, a pouch lithium-ion battery with a capacity of 2Ah was fabricated. This yielded Comparative Example 3.

[0060] Experiment 1

[0061] The conductivity of the low-temperature electrolytes prepared in Examples 1-6 and Comparative Examples 1-3 was measured at 20℃, -40℃, and -50℃, respectively. The test results are shown in Table 1.

[0062] Table 1. Electrolyte conductivity at different temperatures

[0063] serial number Conductivity at 20℃ (mS / cm) Conductivity at -40℃ (mS / cm) Conductivity at -50℃ (mS / cm) Example 1 4.6 1.3 0.74 Example 2 5.1 1.5 0.77 Example 3 4.4 1.2 0.73 Example 4 4.5 1.3 0.74 Example 5 4.5 1.2 0.74 Example 6 4.8 1.4 0.76 Comparative Example 1 2.1 0.5 0.41 Comparative Example 2 1.9 0.4 0.33 Comparative Example 3 2.0 0.6 0.41

[0064] The test results show that the conductivity of the low-temperature lithium-ion battery electrolyte prepared using this method is significantly improved compared with that of the comparative examples. Comparative Examples 1 and 2 both used lithium hexafluorophosphate with a high component content as the lithium salt, and the solvent component did not contain ethylene glycol propyl ether. The low-temperature conductivity of the electrolyte was significantly lower than that of the example. Comparative Example 3 used lithium difluorosulfonyl imide with a low component content as the lithium salt, and the solvent component did not contain ethylene glycol propyl ether. The low-temperature conductivity of the electrolyte was significantly lower than that of the example.

[0065] Experiment 2

[0066] The lithium-ion batteries prepared in Examples 1-6 and Comparative Examples 1-3 were tested according to the following test plan:

[0067] 1) Let stand at 25℃ for 4 hours;

[0068] 2) Discharge at a constant current of 1Ah to 2.8V;

[0069] 3) Let stand for 0.5 hours;

[0070] 4) Charge at 1Ah constant current to 4.2V, then charge at 4.2V constant voltage to 100mAh;

[0071] 5) Let stand for 10 minutes;

[0072] 6) Let stand at -40℃ for 8 hours;

[0073] 7) 1Ah constant current discharge to 2.8V, obtaining the discharge capacity retention rate at -40℃;

[0074] 8) Let stand at 25℃ for 4 hours;

[0075] 9) 1Ah constant current discharge to 2.8V;

[0076] 10) Let stand for 10 minutes;

[0077] 11) Let stand at -40℃ for 8 hours;

[0078] 12) Charged at a constant current of 0.4Ah to 4.2V, and then charged at a constant voltage of 4.2V to 100mAh to obtain the capacity retention rate at -40℃.

[0079] 13) Let stand at 25℃ for 4 hours;

[0080] 14) Discharge at 0.5C constant current to 1.5V

[0081] Table 2 Low-Temperature Charge-Discharge Capacity Retention Rate

[0082] serial number -40℃ discharge capacity retention rate -40℃ charging capacity retention rate Example 1 72％ 60％ Example 2 75％ 61％ Example 3 70％ 54％ Example 4 70％ 55％ Example 5 71％ 55％ Example 6 73％ 58％ Comparative Example 1 32％ 20％ Comparative Example 2 28％ 15％ Comparative Example 3 30％ 20％

[0083] The test results are shown in Table 2. The results show that the low-temperature discharge capacity and low-temperature charging capacity retention of the lithium-ion battery prepared using this method are significantly improved compared to the comparative examples. Comparative Examples 1 and 2 both used lithium hexafluorophosphate with a high component content as the lithium salt, and the solvent component did not contain ethylene glycol propyl ether; therefore, the low-temperature charge-discharge capability of the corresponding lithium-ion batteries was significantly lower than that of the examples. Comparative Example 3 used lithium difluorosulfonylimide with a low component content as the lithium salt, and the solvent component did not contain ethylene glycol propyl ether; therefore, the low-temperature charge-discharge capability of the corresponding lithium-ion battery was significantly lower than that of the examples.

[0084] In summary, this invention provides a low-temperature lithium-ion battery electrolyte, a low-temperature lithium-ion battery, and a method for preparing the same, which improve the low-temperature charge-discharge performance of lithium-ion batteries.

[0085] The above embodiments have provided a detailed description of the present invention, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent variations and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A low-temperature lithium-ion battery electrolyte, characterized in that: The low-temperature lithium-ion battery electrolyte comprises an electrolyte solvent, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, fluoroethylene carbonate, lithium difluorophosphate, and ethylene sulfate. Lithium hexafluorophosphate is not added to the electrolyte. The lithium bis(fluorosulfonyl)imide component accounts for 3%-20% of the electrolyte mass, and the electrolyte solvent accounts for 48.5%-85.5% of the electrolyte mass. Based on the total mass of the electrolyte solvent, the mass percentage of ethylene carbonate in the electrolyte solvent is 5%-35%, the mass percentage of ethylene glycol propyl ether in the electrolyte solvent is 10%-30%, the mass percentage of methyl ethyl carbonate in the electrolyte solvent is 8%-28%, and the mass percentage of dimethyl carbonate in the electrolyte solvent is 5%-20%. The solvated lithium salt structure formed by the electrolyte solvent inhibits the corrosion of the current collector by lithium bis(fluorosulfonyl)imide, and the addition of ethylene glycol propyl ether prevents lithium bis(fluorosulfonyl)imide from corroding the foil.

2. The low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The lithium difluorooxalate borate component accounts for 1.0% to 2% of the electrolyte by mass, the fluoroethylene carbonate component accounts for 1.0% to 5% of the electrolyte by mass, the lithium difluorophosphate component accounts for 0.5% to 5% of the electrolyte by mass, and the ethylene sulfate component accounts for 0.5% to 8% of the electrolyte by mass.

3. A low-temperature lithium-ion battery, characterized in that: The low-temperature lithium-ion battery includes a positive electrode, a negative electrode, a separator, and the low-temperature lithium-ion battery electrolyte according to any one of claims 1-2.

4. A method for preparing a low-temperature lithium-ion battery, characterized in that: The method for preparing the low-temperature lithium-ion battery is used to prepare the low-temperature lithium-ion battery according to claim 3; The method for preparing the low-temperature lithium-ion battery includes the following steps: Step 1: Mix the graphite anode material, conductive agent, binder, and NMP solvent, and stir to obtain the anode slurry; Step 2: Coat the negative electrode slurry onto the surface of the current collector, dry, roll, and punch to obtain the negative electrode sheet; Step 3: Mix the ternary cathode material, conductive agent, binder, and NMP solvent, and stir to obtain the cathode slurry; Step 4: Coat the positive electrode slurry onto the surface of the current collector, dry, roll, and punch to obtain the positive electrode sheet; Step 5: The positive electrode, separator, and negative electrode are stacked in a Z-shape, placed into the casing, dried, and then injected with liquid in a vacuum environment at a dew point of -40°C. The resulting low-temperature lithium-ion battery is then encapsulated.