Lithium battery low-concentration / ultra-low-concentration electrolyte and lithium battery

Abstract

The invention relates to a lithium ion battery low-concentration / ultra-low-concentration electrolyte and a lithium ion battery, the lithium ion battery low-concentration / ultra-low-concentration electrolyte comprises a lithium salt, an organic solvent and an additive, the total molar concentration of the lithium salt is 0.1-0.5 mol / L, and the total content of the additive in the electrolyte is 5-10 wt%; the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis (trifluoromethylsulfonyl) amide (LiFSI) and lithium tetrafluoroborate (LiBF4); the organic solvent is selected from one or more of propylene carbonate (PC), methyl ethyl carbonate (EMC) and fluoromethyl ethyl carbonate (FEMC); the additive is one or more of fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium difluoro (oxalato) borate (LiDFOB) and ethyl sulfate (DTD). Compared with the prior art, the transmission rate of lithium ions across an electrode / electrolyte interface can be improved, and the cycle life and the capacity retention rate of the battery under the conditions of room temperature and low temperature are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and particularly relates to a lithium battery low-concentration / ultra-low-concentration electrolyte and a lithium battery. BACKGROUND

[0002] Lithium ion batteries are the mainstream energy storage devices in the fields of electric vehicles, 3C electronic products and military space, and have the characteristics of high energy density, long cycle life, environmental friendliness and the like. However, with the increasing application requirements of lithium batteries in high altitudes or latitudes and certain national defense and space applications, the performance degradation of lithium batteries at low temperatures is increasingly prominent. The lower limit of the working temperature of the power supply of consumer electronic equipment is usually 20 DEG C, which is basically consistent with the limit working temperature of conventional lithium ion batteries; however, in order to adapt to the temperature difference of different regions and seasons, the power supply of EV / HEV usually needs to work stably at a temperature of 30 DEG C or even lower for a long time, which poses a challenge to the current mainstream lithium ion battery electrolyte system.

[0003] In order to enable lithium ion batteries (LIB) to normally operate at low temperatures, the current most LIB battery systems usually use internal or external thermal management systems. External heating is to preheat the battery by gas, liquid or electric heating wire to achieve the purpose of improving the charge and discharge capacity. However, external heating may cause uneven heating of different single batteries, rapid local temperature rise, thermal runaway in the battery pack, and ultimately lead to serious safety accidents. In addition, external heating usually requires redesigning the layout of the battery and increasing the total mass of the battery. Internal heating uses the internal resistance of the battery itself, and through the excitation of the battery internal chemicals by alternating current, the battery generates heat. This method has the advantages of high efficiency, uniform heating, high energy density of battery mass, but is not very mature in practical application. The thermal management system not only has certain safety hazards itself, but also causes additional costs, makes the battery system complex and reduces the energy density of the battery. And the main solvent used in the current electrolyte system, ethylene carbonate (EC), has a very high melting point (36 DEG C), and the viscosity of the electrolyte increases at low temperatures, and even part of it becomes a solid state, which significantly reduces the ionic conductivity. At the same time, the negative electrode is easy to lithiumize at low temperature, and the lithium extracted reacts with the electrolyte to cause the thickness of SEI / CEI to increase. Based on this background, it is urgent to develop a new type of electrolyte system with high ionic conductivity, small viscosity and high electrochemical stability under low temperature conditions.

[0004] In lithium-ion battery systems, the concentration of the electrolyte is one of the key factors affecting its ion transport characteristics, interfacial chemical behavior, and final electrochemical performance. Traditional electrolytes typically use a lithium salt concentration of approximately 1 mol / L to achieve high ionic conductivity and moderate viscosity at room temperature, thereby ensuring good cycle stability and rate performance. For example, patent application CN113299995A describes an electrolyte, a lithium-ion battery, its preparation method, and its application. This electrolyte includes a lithium salt, an organic solvent, and a first additive; wherein the lithium salt includes lithium hexafluorophosphate; the concentration of lithium hexafluorophosphate is 0.8–1.5 mol / L; the flash point at room temperature can be increased to 38–50°C; it exhibits excellent wetting performance relative to positive and negative materials and separators; and the ionic conductivity can reach 7.42–8.75 mS·cm. -1 However, under low-temperature conditions (such as below 0°C), the limitations of conventional concentration electrolytes gradually become apparent: their viscosity increases significantly, ion migration resistance rises sharply, leading to a significant decrease in electrolyte ionic conductivity; at the same time, the lithium-ion desolvation barrier increases at high concentrations, the interfacial charge transfer process is hindered, resulting in severe polarization and capacity decay, which limits the practical application of batteries in low-temperature environments.

[0005] In contrast, low-concentration electrolyte systems, such as 0.5 mol / L or ultra-low-concentration systems, such as 0.1 mol / L, exhibit multiple potential advantages at low temperatures. First, low salt concentration significantly reduces electrolyte viscosity, improving its low-temperature fluidity and thus enhancing ion migration rate and overall electrolyte ionic conductivity, helping to mitigate kinetic limitations at low temperatures. Second, lower lithium salt concentration reduces the size of solvated clusters formed between lithium ions and solvent molecules, lowering the desolvation barrier at the electrode interface, promoting interfacial charge transfer, and improving charge-discharge performance at low temperatures. Furthermore, low-concentration electrolytes help suppress salt precipitation and excessive lithium dendrite growth that may occur under high-concentration conditions, improving the battery's low-temperature cycling safety and stability. Simultaneously, low salt usage also reduces electrolyte costs and mitigates side reactions caused by the decomposition of high-concentration salts, contributing to the long-term stability of the electrode / electrolyte interface.

[0006] Therefore, for low-temperature applications, developing low-concentration or ultra-low-concentration electrolytes with lithium salt concentrations of 0.1~0.5 mol / L is expected to not only solve the key problems of sluggish ion conduction and increased interfacial impedance of traditional electrolytes at low temperatures, but also provide a new technical path for achieving next-generation lithium-ion batteries that combine high and low temperature adaptability, high safety and low cost. Summary of the Invention

[0007] The purpose of this invention is to overcome the defects of the prior art and provide a low-concentration / ultra-low-concentration electrolyte and lithium battery with excellent low-temperature resistance.

[0008] The objective of this invention can be achieved through the following technical solution: a low-concentration / ultra-low-concentration electrolyte for lithium-ion batteries, comprising lithium salt, organic solvent and additives, wherein the total molar concentration of lithium salt is 0.1-0.5 mol / L and the total content of additives in the electrolyte is 5-10 wt%. The lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)amino (LiFSI), and lithium tetrafluoroborate (LiBF4); The organic solvent is selected from one or more of propylene carbonate (PC), ethyl methyl carbonate (EMC), and fluoroethyl methyl carbonate (FEMC); The additive is one or more of the following: fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD).

[0009] Furthermore, the lithium salt is a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)amino in a molar ratio of 1:(0.25~1.5).

[0010] Furthermore, the lithium salt is a mixture of lithium hexafluorophosphate and lithium tetrafluoroborate in a molar ratio of 1:(0.5~1).

[0011] Further, the lithium salt is a mixture of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)amino and lithium tetrafluoroborate in a molar ratio of 1:(0.5~1):(0.5~1).

[0012] Furthermore, the organic solvent is a mixture of methyl ethyl carbonate and propylene carbonate in a volume ratio of 1:(0.2~0.5).

[0013] Furthermore, the organic solvent is a mixture of propylene carbonate and methyl ethyl fluorocarbonate in a volume ratio of 1:(0.5~2).

[0014] Further, the organic solvent is a mixture of methyl ethyl carbonate, propylene carbonate and methyl ethyl fluorocarbonate in a volume ratio of 1:(0.5~2):(0.5~2).

[0015] Further, the additives fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD) are a mixture in a mass fraction of (3~1):(2~1):(0.5~1).

[0016] The present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous lithium-ion battery electrolyte as described above (i.e., a low / ultra-low concentration lithium-ion battery electrolyte).

[0017] Furthermore, the cathode material is selected from lithium transition metal oxides such as LiCoO2, LiNiO2, and LiNi. x Co y Mn 1-x- y One of the components of O2.

[0018] Furthermore, the negative electrode material is lithium metal.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a specific ratio of lithium salt composition, such as lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)amino, or lithium hexafluorophosphate and lithium tetrafluoroborate, with a total lithium salt concentration of low or ultra-low concentration of 0.1~0.5 mol / L, which is different from the conventional concentration of 1~1.5 mol / L; through the synergistic function of different lithium salts, the strong dependence of performance and concentration under a single salt system is broken, thereby reducing the lithium salt concentration in the electrolytic cell. Its principle includes: (1) In a single LiPF6 system, to achieve a stable SEI film, a sufficient concentration of PF6 is required. - To provide decomposition products. Lowering the concentration will lead to incomplete film formation and a sharp decline in performance. In dual-salt systems, such as the LiPF6 / LiFSI combination, even at a low concentration of LiPF6, a passivation layer can be formed on the aluminum foil; while the SEI film is synergistically constructed from the decomposition products of the two salts. Therefore, excellent interfaces can be achieved without high concentrations of one salt, creating conditions for reducing the total concentration; (2) A major challenge of low-concentration electrolytes is the increase in ion pairs and aggregates, leading to a decrease in effective charge carriers (free Li + As the concentration of Li decreases, its conductivity decreases. Introducing bulky, weakly coordinated anions, or small anions, can "dilute" or "reconstruct" Li. + The solvated sheath structure helps reduce ion pair binding and increase the free Li +The amount of [amount] is increased, thus maintaining considerable ionic conductivity at a low total salt concentration. Furthermore, by incorporating additives with good film-forming properties, such as one or more of fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD), and using a specific ratio of organic solvent, preferably methyl ethyl carbonate and propylene carbonate, or a mixture of propylene carbonate and methyl ethyl fluorocarbonate, a non-aqueous lithium-ion battery electrolyte can be formulated. The low concentration of dibasic salts provides the basic ionic conductivity and initial interfacial framework, achieving high-voltage compatibility and thermal stability. Functional additives preferentially react on the electrode surface, constructing a robust and highly ion-conducting interfacial protective film (SEI / CEI). Solvent composition optimization improves the bulk ion transport performance and low-temperature fluidity of the electrolyte by regulating the electrolyte solvation structure, while also assisting in interfacial stabilization. Therefore, the resulting electrolyte exhibits good low-temperature performance without affecting room-temperature performance, improving the battery's low-temperature capacity retention and room-temperature cycling stability.

[0020] (2) The lithium-ion battery electrolyte of the present invention comprises a lithium salt, an organic solvent, and additives at a concentration of 0.1~0.5 mol / L. The lithium salt comprises one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)amino, and lithium tetrafluoroborate. The organic solvent is a low-melting-point solvent such as propylene carbonate, ethyl methyl carbonate, and fluoroethyl methyl carbonate as a single solvent or a mixed solvent. The additives are one or more of unsaturated ester additives, lithium salt additives, and sulfur-containing additives. When the electrolyte is used in a lithium-ion battery, it can improve the lithium-ion transport rate across the electrode / electrolyte interface, reduce battery polarization, and improve the cycle life and capacity retention of the battery under low-temperature conditions. Attached Figure Description

[0021] Figure 1 Raman spectra of the electrolytes of Comparative Example 1 and Examples 1-3 of this invention, as well as the solvents of ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC). Figure 2 The first-cycle capacity retention rate of Examples 1-7 and Comparative Examples 1 and 2 relative to ambient temperature 25°C under conditions of -20°C and 0.1°C; Figure 3 The capacity retention rates of Examples 1-7 and Comparative Examples 1 and 2 are given under conditions of 25°C and 0.1°C. Detailed Implementation

[0022] To make the purpose, technical solution, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the embodiments described in this specification are merely for explaining this application and are not intended to limit it.

[0023] This invention provides a low / ultra-low concentration electrolyte for a low-temperature lithium-ion battery, wherein the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)amino (LiFSI), and lithium tetrafluoroborate (LiBF4); the organic solvent is selected from one or more of propylene carbonate (PC), ethyl methyl carbonate (EMC), and ethyl fluorocarbonate (FEMC); and the additive is one or more of ethylene fluorocarbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD).

[0024] This invention selects three lithium salts: LiPF6, LiFSI, and LiBF4. LiPF6 is widely used in lithium-ion battery electrolytes due to its excellent conductivity and balanced electrochemical performance. Besides improving the ionic conductivity of the electrolyte, LiPF6 can also form a dense protective film of aluminum fluoride (AlF3) and organoaluminum salts on the surface of the aluminum current collector at the cathode. LiFSI has extremely high dissociation and excellent chemical stability. Furthermore, fluorosulfonylimide anions preferentially undergo solvation with lithium ions before hexafluorophosphate, but with lower binding energy, resulting in faster desolvation. Therefore, LiFSI can accelerate the transport kinetics of lithium ions across the cathode / electrolyte interface, reduce battery polarization, and achieve good low-temperature performance. LiBF4 has higher thermal stability than LiPF6, is insensitive to water, does not easily generate HF, has low charge transfer impedance at low temperatures, and is inexpensive, making it suitable for constructing low-temperature electrolytes.

[0025] Ethyl methyl carbonate (EMC) has a wide liquid temperature range, from -50°C to up to 80°C. This allows it to maintain good fluidity and ionic conductivity at low temperatures; moreover, EMC has low polarization characteristics, which means it can provide low polarization resistance, which is beneficial for the low-temperature operation of batteries.

[0026] Ethyl fluorocarbonate (FEMC) also has the physical properties of low viscosity, low melting point and good solubility, and the added fluorinated groups are conducive to the formation of interfacial film, thereby improving the cycle life and stability of the battery.

[0027] Compared to linear ester compounds, propylene carbonate (PC) not only has a lower melting point but also a stronger solvation ability. It can preferentially solvate lithium ions, promote the dissociation of lithium salts, and ensure the ionic conductivity of the electrolyte.

[0028] LiPO2F2 has a low LUMO energy, is easily reduced, and has strong film-forming ability, readily forming SEI layers rich in LiF and phosphorus-containing compounds. The high LiF content promotes the formation of Li... +The migration kinetics process improves the conductivity of the SEI layer, which can form a stable SEI film with high ion conductivity on the negative electrode surface, thereby improving the stability of the electrolyte; FEC and lithium ions have lower binding energy, which can accelerate the desolvation process on the negative electrode side during charging, promote the transport dynamics of lithium ions across the negative electrode / electrolyte interface, and form an SEI film with higher ionic conductivity and more stable chemical properties, thereby improving the cycle performance of the battery.

[0029] The oxalate-based borate polymer and LiF formed by the decomposition of LiDFOB can stabilize the electrode interface, improve the stability of the electrolyte, and form an interfacial film with relatively low impedance compared to other additives. LiDFOB can also form a passivation layer on the positive electrode, reducing the decomposition of the electrolyte under high voltage, thereby enhancing the structural stability of the oxide and reducing the dissolution of transition metal elements.

[0030] DTD can optimize the composition of the SEI film, effectively suppress the decline in the initial capacity of the battery, increase the initial discharge capacity, and provide good benefits to the low-temperature performance, cycle performance, and even high-temperature performance of the battery.

[0031] Furthermore, the lithium salt is preferably a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)amino in a molar ratio of 1:(0.25~1.5); or the lithium salt is preferably a mixture of lithium hexafluorophosphate and lithium tetrafluoroborate in a molar ratio of 1:(0.5~1); or the lithium salt is preferably a mixture of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)amino and lithium tetrafluoroborate in a molar ratio of 1:(0.5~1):(0.5~1).

[0032] Due to its small anionic radius and easy association, LiBF4 has low solubility and low conductivity in organic carbonate solvents. Furthermore, LiBF4 has poor film-forming properties and poor compatibility with electrodes. Using LiPF6 in combination with it can improve the ionic conductivity of the electrolyte.

[0033] LiFSI can corrode aluminum foil under certain conditions, while the presence of LiPF6 can help stabilize the aluminum foil. In other words, LiPF6 can passivate the positive current collector to a certain extent, thereby inhibiting the destructive effect of LiFSI.

[0034] Furthermore, the total molar concentration of the lithium salt is 0.1~0.5 mol / L.

[0035] Reducing the lithium salt concentration helps to lower the viscosity of the electrolyte, improve wettability, and facilitate lithium ion transport at low temperatures; furthermore, lowering the salt concentration also helps to reduce costs.

[0036] Furthermore, the organic solvent is preferably a mixture of methyl ethyl carbonate and propylene carbonate in a volume ratio of 1:(0.2~0.5); or the organic solvent is preferably a mixture of propylene carbonate and methyl ethyl fluorocarbonate in a volume ratio of 1:(0.5~2); or the organic solvent is a mixture of methyl ethyl carbonate, propylene carbonate and methyl ethyl fluorocarbonate in a volume ratio of 1:(0.5~2):(0.5~2).

[0037] Furthermore, the amount of the additive in the electrolyte is 5-10 wt%; the additive is preferably 10% fluoroethylene carbonate (FEC); or the additive is preferably 5% lithium difluorooxalate borate (LiDFOB); or the additive is preferably a mixture of fluoroethylene carbonate (FEC), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD) in a mass fraction of (3~1):(2~1):(0.5~1).

[0038] On the other hand, the present invention provides a lithium-ion battery, which includes a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte is the low-concentration / ultra-low-concentration electrolyte of the present invention described above.

[0039] Furthermore, the cathode material is selected from lithium transition metal oxides such as LiCoO2, LiNiO2, and LiNi. x Co y Mn 1xy O2 ( x + y + z =1) is one of them, and the negative electrode material is lithium metal.

[0040] The disclosure of this application is described in more detail below through specific embodiments. These embodiments are merely illustrative, as various modifications and variations within the scope of the disclosure of this application will be apparent to those skilled in the art. All reagents used in the embodiments are commercially available or synthesized using conventional methods and can be used directly without further processing. Similarly, the instruments used in the embodiments are commercially available.

[0041] Example 1: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 10% by mass of fluoroethylene carbonate (FEC) additive was added to ethyl methyl carbonate (EMC) solvent and mixed thoroughly. Then, 0.5 mol / L of LiPF6 was added and the mixture was stirred slowly until completely dissolved.

[0042] Example 2: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 10% by mass of fluoroethylene carbonate (FEC) additive was added to ethyl methyl carbonate (EMC) solvent and mixed thoroughly. Then, 0.25 mol / L of LiPF6 was added and the mixture was stirred slowly until completely dissolved.

[0043] Example 3: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 10% by mass of fluoroethylene carbonate (FEC) additive was added to ethyl methyl carbonate (EMC) solvent and mixed thoroughly. Then, 0.1 mol / L of LiPF6 was added and the mixture was stirred slowly until completely dissolved.

[0044] Example 4: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 0.5 mol / L of LiPF6 salt and 5% by mass of LiDFOB additive were added to ethyl methyl carbonate (EMC) solvent and stirred slowly until completely dissolved.

[0045] Example 5: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 0.5 mol / L of LiPF6 and LiFSI double salts (molar ratio 1:1) were added to a solvent containing ethyl methyl carbonate (EMC) solvent and 10% fluoroethylene carbonate (FEC) additive, and stirred slowly until completely dissolved.

[0046] Example 6: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 0.5 mol / L LiPF6 and LiBF4 double salts (molar ratio 1:1) were added to a solvent containing a homogeneous mixture of ethyl methyl carbonate (EMC) and 10% fluoroethylene carbonate (FEC) additive, and the mixture was stirred slowly until completely dissolved.

[0047] Example 7 Preparation of Electrolyte In a glove box filled with argon gas and containing water and oxygen content of less than 1 ppm, ethyl methyl carbonate (EMC), propylene carbonate (PC), and fluoroethyl methyl carbonate (FEMC) were weighed and mixed evenly in a volume ratio of 4:3:3 to obtain a mixed solvent. 2% fluoroethylene carbonate (FEC), 2% lithium difluorooxalate borate (LiDFOB), and 1% ethyl sulfate (DTD) additives were added and mixed evenly. Subsequently, lithium salts LiPF6, LiFSI, and LiBF4 in a molar ratio of 5:4:1 were added and stirred evenly to obtain an electrolyte with a total lithium salt concentration of 0.1 mol / L.

[0048] Example 8: Preparation of Electrolyte In a glove box filled with argon gas and containing water and oxygen content of less than 1 ppm, ethyl methyl carbonate (EMC), propylene carbonate (PC), and fluoroethyl methyl carbonate (FEMC) were weighed and mixed evenly in a volume ratio of 4:3:3 to obtain a mixed solvent. 2% fluoroethylene carbonate (FEC), 2% lithium difluorooxalate borate (LiDFOB), and 1% ethyl sulfate (DTD) additives were added and mixed evenly. Subsequently, lithium salts LiPF6, LiFSI, and LiBF4 in a molar ratio of 2:2:1 were added and stirred evenly to obtain an electrolyte with a lithium salt concentration of 0.5 mol / L, and the concentrations of the three lithium salts were 0.2, 0.2, and 0.1 mol / L, respectively.

[0049] Preparation of electrolyte in Comparative Example 1 In a glove box filled with argon gas and containing water and oxygen content of less than 1 ppm, 0.5 mol / L LiPF6 was added to ethyl methyl carbonate (EMC) and stirred slowly until completely dissolved.

[0050] Comparative Example 2: Preparation of Electrolyte In a glove box filled with argon gas and containing less than 1 ppm of both water and oxygen, 0.1 mol / L LiPF6 was added to ethyl methyl carbonate (EMC) and stirred slowly until completely dissolved.

[0051] The electrolytes prepared in Examples 1-7 and Comparison 1-2 were tested. The main test methods are as follows: LiNi 0.8 Co 0.1 Mn 0.1 The preparation method of O2 positive electrode is as follows: Active material, acetylene black, and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 8:1:1, using N-methylpyrrolidone (NMP) as the solvent. After the slurry is thoroughly mixed, it is coated onto aluminum foil and dried in an 80℃ drying oven. After completion, it is cut into 12mm electrode sheets, with a positive electrode active material concentration of 3.5 mg / cm³. 2 .

[0052] The prepared electrolyte was used for LiNi 0.8 Co 0.1 Mn 0.1 Cyclic performance testing was conducted on O2 / Li batteries (CR2032 type). The batteries were activated by two charge-discharge cycles at 0.1C and 2.8V to 4.4V under constant temperature of 25℃, followed by 100 charge-discharge cycles at 0.3C.

[0053] The battery was activated by two charge-discharge cycles at -20℃ with a rate of 0.1C and a voltage of 2.8V to 4.4V, followed by charge-discharge cycles at a rate of 0.3C. The battery's capacity retention at low temperatures was calculated as (first discharge specific capacity at -20℃ / first discharge specific capacity at 25℃) * 100%.

[0054] Figure 1 The images show the Raman spectra of the electrolytes in Comparative Example 1 and Examples 1-3 of this invention, as well as the EMC and FEC solvents. The high-frequency region in the spectra represents the characteristic peaks of FEC and EMC, reflecting the degree of solvent involvement in the solvation shell, while the low-frequency region around 7250 cm⁻¹... -1 and 743 cm -1 The peaks that appeared were those of unsolvated and solvated PF6, respectively. - Characteristic peak. It is important to note that the peak is at 730 cm⁻¹. -1 The peaks are characteristic peaks of FEC. For example, no low-frequency peaks were observed in the commercial carbonate (1M LiPF6) system, indicating that all salt anions participated in solvation. Comparing the spectra of low-concentration and ultra-low-concentration electrolytes reveals that FEC also enters the solvation shell in the low-concentration electrolyte system. This means that lithium ions can solvate with FEC in addition to EMC, which is beneficial for FEC decomposition at the interface, thereby improving battery cycle stability. Furthermore, as the concentration increases, the salt anion content in the solvation shell gradually increases, indicating that all salt anions participate in solvation in the low-concentration system.

[0055] Figure 2 The capacity retention rate of the nickel-cobalt-manganese lithium oxide batteries in Examples 1-2 and 1-8 of this invention was measured at -20°C during the first cycle. It can be seen that the low-concentration electrolyte system with added additives exhibited a significantly higher low-temperature capacity retention rate than the comparative examples. Furthermore, compared to simple low-concentration / ultra-low-concentration electrolyte systems, optimizing the composition and ratio of the three lithium salts, three solvents, and three additives is beneficial for improving the low-temperature capacity retention rate. This indicates that the low-temperature characteristics of low-concentration / ultra-low-concentration electrolytes need to consider the electrolyte viscosity at low temperatures, the oxidation and reduction stability of the electrolyte solvent, and the effectiveness of the film-forming additives at the positive and negative electrode interfaces.

[0056] Figure 3 The figures show the long-cycle capacity retention of the nickel-cobalt-manganese lithium oxide batteries in Comparative Examples 1-2 and Examples 1-8 of this invention at room temperature. It can be seen that simply reducing the lithium salt concentration is not beneficial to long-cycle stability. However, by introducing film-forming additives for the positive and negative electrodes, the interface stability of metallic lithium and high-nickel ternary cathodes can be improved. In addition, using mixed lithium salts and optimizing the component ratio can achieve the effect of taking the best of both worlds and compensating for the shortcomings, thereby achieving stable battery operation.

[0057] The electrolytes obtained in Examples 1-8 and Comparative Examples 1-2 were used to assemble batteries, and their performance was tested. The results are shown in the table below: As shown in the table above, compared to traditional electrolyte systems, a low-concentration / ultra-low-concentration salt strategy can reduce electrolyte viscosity and battery cost, while potentially improving electrolyte wettability to the separator. This opens a new avenue for cryogenic electrolytes. However, the main factors affecting battery performance at low and room temperatures include the electrolyte's ionic conductivity, viscosity, interfacial impedance (low-temperature impedance of SEI / CEI), and charge transfer impedance. To balance the electrolyte's oxidation-reduction stability and ionic conductivity, precise control of the low-concentration electrolyte's composition and solvation structure is necessary, rather than simply reducing the lithium salt concentration. For highly active positive and negative electrodes, interfacial stability is the primary factor determining cycle life. For example, NMC811 requires a robust, dense, ionically conductive CEI to suppress transition metal dissolution and oxygen release; LiFSI has an irreplaceable advantage in constructing a high-quality CEI. LiBF4 salts, however, cannot form a stable CEI, thus exhibiting lower capacity retention. For lithium metal anodes, a uniform, LiF-rich, and mechanically strong SEI layer is also required to guide uniform lithium deposition and reduce side reactions. PC solvents exhibit poor compatibility with lithium metal anodes, while EMC and FEMC solvents show better compatibility. Furthermore, to ensure stable battery operation at low temperatures, in addition to balancing electrolyte ionic conductivity and viscosity, it is necessary to optimize the SEI and CEI compositions to reduce interfacial impedance. This electrolyte contains various film-forming additives to ensure the formation of stable SEI and CEI. For example, FEC promotes uniform deposition of lithium metal anodes, LiDFOB can inhibit aluminum corrosion and improve the cathode interface, while DTD can inhibit electrolyte oxidation and transition metal dissolution. Therefore, low-concentration electrolytes need to be paired with good film-forming additives to suppress interfacial side reactions. As shown in Examples 7 and 8, by using an electrolyte formulation that combines three lithium salts, three solvents, and three additives, the advantages of each lithium salt, additive, and solvent are combined to achieve better low-temperature performance and cycle stability.

Claims

1. A low / ultra-low concentration electrolyte for lithium-ion batteries, characterized in that, It includes lithium salt, organic solvent and additives, wherein the total molar concentration of lithium salt is 0.1~0.5 mol / L and the content of additives in the electrolyte is 5-10 wt%. The lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)amino (LiFSI), and lithium tetrafluoroborate (LiBF4); The organic solvent is selected from one or more of propylene carbonate (PC), ethyl methyl carbonate (EMC), and fluoroethyl methyl carbonate (FEMC); The additive is one or more of the following: fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD).

2. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The lithium salt is a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)amino in a molar ratio of 1:(0.25~1.5).

3. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The lithium salt is a mixture of lithium hexafluorophosphate and lithium tetrafluoroborate in a molar ratio of 1:(0.5~1).

4. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The lithium salt is a mixture of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)amino, and lithium tetrafluoroborate in a molar ratio of 1:(0.5~1):(0.5~1), with a total lithium salt concentration of 0.1~0.5 mol / L.

5. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The organic solvent is a mixture of methyl ethyl carbonate and propylene carbonate in a volume ratio of 1:(0.2~0.5).

6. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The organic solvent is a mixture of propylene carbonate and methyl ethyl fluorocarbonate in a volume ratio of 1:(0.5~2).

7. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The organic solvent is a mixture of methyl ethyl carbonate, propylene carbonate, and fluoromethyl ethyl carbonate in a volume ratio of 1:(0.5~2):(0.5~2).

8. The low / ultra-low concentration electrolyte for lithium-ion batteries according to claim 1, characterized in that, The additive is a mixture of fluoroethylene carbonate (FEC), lithium difluorooxalate borate (LiDFOB), and ethyl sulfate (DTD) in a mass fraction of (3~1): (2~1): (0.5~1).

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and a low / ultra-low concentration electrolyte as described in any one of claims 1-8.

10. A lithium-ion battery according to claim 9, characterized in that, The cathode material is selected from lithium transition metal oxides LiCoO2, LiNiO2, and LiNi x Co y Mn 1-x-y O2 ( x + y + z =1) One of them; the negative electrode material is lithium metal.

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