Lithium ion battery and low temperature application

Abstract

The invention provides a lithium ion battery. The lithium ion battery comprises a negative electrode, a positive electrode, a diaphragm and electrolyte, wherein the negative electrode is a surface-modified graphite material, and the work function of the modified material is less than 6 eV; the electrolyte is an organic solution of lithium salt, the oxidation voltage of the electrolyte is not less than 4 V, and the melting point of the electrolyte is less than-40 DEG C. On the other hand, the invention also provides an application of the lithium ion battery as a low-temperature lithium ion battery, and the working temperature of the low-temperature lithium ion battery is less than-20 DEG C.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrode materials for lithium-ion batteries, and in particular relates to a lithium-ion battery and low-temperature applications. Background Art

[0002] In low temperature environments, lithium-ion batteries have problems such as capacity attenuation and difficulty in charging. Studies have found that the defects in their low-temperature performance are largely related to the binding of the graphite negative electrode to the electrolyte. In related technologies, the most widely used lithium-ion battery is a lithium-ion battery based on ethylene carbonate (EC) electrolyte and graphite anode. However, due to the high viscosity and melting point of the EC solvent, the viscosity of the electrolyte increases and the conductivity decreases at low temperatures (≤-20°C), which in turn affects the migration rate of lithium ions and the charge and discharge performance of the battery, and the power and energy density of the battery will drop sharply. In addition, low temperatures will also aggravate electrode polarization, reduce battery capacity, and affect the overall performance of the battery.

[0003] At present, the research and development of lithium-ion batteries is limited to electrolyte systems based on EC solvents with high melting points. The low-temperature performance of lithium-ion batteries is improved by adding different electrolyte additives or introducing external heating systems. For example, the electrolyte system using 1-methyl-3-[(3-trimethoxysilyl)-propyl] imidazole bis(trifluoromethanesulfonamide) as a low-temperature additive and adding other functional additives achieves a 76% discharge capacity of a lithium iron phosphate graphite full battery at -20°C; the electrolyte system including vinylene carbonate and pentaerythritol bicyclic sulfate and thiophene compound combined additives achieves a discharge capacity of 80% of the full battery of NCM523 and graphite at -20°C; however, these additives still cannot achieve charging at low temperatures below -20°C. Studies have found that the use of additives or other complex lithium salt systems will inevitably affect the performance of the positive or negative electrode side. Related technologies also include devices that heat and insulate battery modules by capturing feedback braking energy that cannot be absorbed by the battery modules, thereby enabling lithium-ion batteries to be charged at low temperatures in winter; however, the use of a heating system will have a negative impact on the energy density of battery cells and battery packs.

[0004] Therefore, it is of far-reaching significance to find a simpler electrolyte system to achieve good low-temperature performance of lithium-ion batteries with reduced or even no additives and the assistance of an external heating system. Summary of the invention

[0005] In view of this, in order to at least partially solve at least one of the above-mentioned technical problems, the present disclosure provides a lithium-ion battery, including a negative electrode, a positive electrode, a separator and an electrolyte. The negative electrode is a surface-modified graphite material, wherein the work function of the modified material is less than 6 eV; the electrolyte is an organic solution of a lithium salt, wherein the oxidation voltage of the electrolyte is not less than 4 V, and the melting point of the electrolyte is less than -40 °C.

[0006] According to an embodiment of the present disclosure, the viscosity of the electrolyte is less than 4.5 mPa s at -40°C.

[0007] According to an embodiment of the present disclosure, the solvent includes a first type of solvent, and the first type of solvent includes at least one of propylene carbonate, butylene carbonate, and fluoropropyl carbonate.

[0008] According to an embodiment of the present disclosure, in the organic solution, the volume fraction of the first type of solvent is 10%-100%.

[0009] According to an embodiment of the present disclosure, the solvent also includes a second type of solvent, and the second type of solvent includes at least one of dimethyl sulfite, methyl acetate, ethyl acetate, ethyl propionate, methyl propionate, ethyl butyrate, methyl propyl carbonate, ethyl methyl carbonate, diethyl carbonate, diethyl sulfite, 1,3-dioxolane, ethylene glycol dimethyl ether, diethyl ether, acetonitrile, tetrahydrofuran, tetrahydropyran, and 2-methyltetrahydrofuran.

[0010] According to an embodiment of the present disclosure, in the organic solution, the volume fraction of the first type of solvent is greater than 10%, and the volume fraction of the second type of solvent is greater than 10%.

[0011] According to an embodiment of the present disclosure, the dielectric constant of the first type of solvent is greater than 40, and the dielectric constant of the second type of solvent is less than 30.

[0012] According to an embodiment of the present disclosure, the lithium salt includes at least one of lithium hexafluorophosphate, lithium perchlorate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoro(oxalate)phosphate, lithium difluorophosphate, lithium monofluorophosphate, lithium bis(oxalate)borate, and lithium tetrafluoroborate; and the concentration of the lithium salt is 0.5 to 2 mol / L.

[0013] According to an embodiment of the present disclosure, the modified material is a lithium ion fast ion conductor, and the lithium ion fast ion conductor includes at least one of lithium oxide, sulfide, hydride, halide, borate, phosphate and lithium phosphorus oxygen nitrogen.

[0014] In another aspect of the present disclosure, an application of the aforementioned lithium ion battery as a low-temperature lithium ion battery is also proposed, wherein the operating temperature of the low-temperature lithium ion battery is less than -20°C.

[0015] According to the embodiments of the present disclosure, the present disclosure proposes a low-temperature electrolyte system for use with a modified graphite negative electrode, wherein the surface of the modified graphite negative electrode can promote the desolvation process of lithium ions and increase the transfer rate of lithium ions between the negative electrode and the electrolyte interface; the electrolyte is an ester solvent with a relatively low melting point, or is composed of an ester solvent and other types of solvents. The electrolyte system takes into account the characteristics of high oxidation decomposition voltage, low viscosity (-40 ℃, <4.5 mPa s) and high conductivity (-40 ℃, >1.5 mS / cm) at low temperatures. With the cooperation of the modified graphite negative electrode, good low-temperature performance can be achieved without electrolyte additives and external heating systems. The optimization of the electrolyte formula can reduce the need for an external heating system for the battery in a low-temperature use environment, thereby optimizing the battery module structure and improving energy density and working performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a charge and discharge curve diagram of the lithium half-battery at room temperature (25° C.) in Examples 1-3 and Comparative Examples 1 and 2 of the present disclosure;

[0017] Figure 2 1 is a charge and discharge curve diagram of the lithium half-cell at low temperature (-20° C.) in Examples 1-3 and Comparative Examples 1 and 2 of the present disclosure. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0019] The endpoints and any values ​​of the ranges disclosed in this disclosure are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this disclosure.

[0020] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0021] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0022] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the present disclosure shall have the usual meanings understood by persons with ordinary skills in the field to which the present disclosure belongs. If the full text involves descriptions such as "first", "second", etc., the descriptions such as "first", "second", etc. are only used to distinguish similar objects, and cannot be understood as indicating or implying their relative importance, order of precedence, or implicitly indicating the number of technical features indicated. It should be understood that the data described by "first", "second", etc. can be interchangeable under appropriate circumstances.

[0023] In order to make the purpose, technical solution and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of them.

[0024] Similarly, in order to simplify the present disclosure and help understand one or more of the various disclosed aspects, in the above description of the exemplary embodiments of the present disclosure, the various features of the present disclosure are sometimes grouped together into a single embodiment, figure, or description thereof. The description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0025] In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by this disclosure.

[0026] In the present disclosure, the term "low-temperature lithium-ion battery" means a lithium-ion battery that maintains good performance in a low-temperature environment (usually below 0°C). By optimizing the electrolyte formulation, electrode materials or battery structure, low-temperature lithium-ion batteries can still discharge effectively under severe cold conditions and have a high capacity retention rate. Low-temperature lithium-ion batteries are widely used in electric vehicles, portable devices, aerospace vehicles and military equipment in cold regions. For example: as a power source for aviation, aerospace, and deep-sea diving equipment, as a power source for polar scientific research and exploration equipment, as a power source for cold-zone rescue and disaster relief, and as a power source for cold-proof clothing and cold-proof shoes.

[0027] In the present disclosure, the term "additive" is used to mean an additional component added to the electrolyte with a volume fraction of less than 10%.

[0028] In the present disclosure, the terms "low temperature" and "low temperature environment" are used to refer to an environment temperature below -20°C.

[0029] In the process of the present disclosure, it was found that under low temperature (≤-20 °C), the viscosity of the electrolyte increased linearly due to the influence of the high melting point solvent EC (melting point is about 37 °C), which reduced the wettability and ionic conductivity of the electrolyte and hindered the transmission of ions in the electrolyte. The study compared electrolytes with different ratios and found that the EC concentration was negatively correlated with the capacity retention rate of the graphite half-cell at low temperature. Ester and ether solvents usually have a low melting point, but their complexing ability with lithium ions is strong or the components are thermodynamically unstable, which makes it difficult for the solvent and lithium ions to desolvate and co-embed graphite, destroying the graphite structure. In addition, low-melting-point ester solvents such as propylene carbonate (PC) have a high viscosity, while the oxidation potential of ether solvents with lower viscosity is generally lower than 4 V at room temperature, making it difficult to apply them to high-energy-density lithium-ion batteries.

[0030] Therefore, the purpose of the present disclosure is to propose a new electrolyte design system and its application to address the key problems that graphite negative electrodes have poor compatibility with most low-melting-point electrolytes, require many additional additives or a complex electrolyte system in low-temperature application scenarios, and require external heating devices for the battery cells, resulting in high costs.

[0031] The present disclosure provides a lithium-ion battery, comprising a negative electrode, a positive electrode, a separator and an electrolyte. The negative electrode is a surface-modified graphite material, wherein the work function of the modified material is less than 6 eV; the electrolyte is an organic solution of a lithium salt, wherein the oxidation voltage of the electrolyte is not less than 4 V, and the melting point of the electrolyte is less than -40 °C.

[0032] According to the embodiments of the present disclosure, the present disclosure proposes a low-temperature electrolyte system for use with a modified graphite negative electrode, wherein the surface of the modified graphite negative electrode can promote the desolvation process of lithium ions and increase the transfer rate of lithium ions between the negative electrode and the electrolyte interface and the electrolyte; the electrolyte is an ester solvent with a relatively low melting point, or is composed of an ester solvent and other types of solvents. The electrolyte system takes into account the characteristics of high oxidation decomposition voltage, low viscosity (-40 ° C, <4.5mPa s) and high conductivity (-40 ° C, >1.5 mS / cm) at low temperatures, and can be compatible with the modified graphite negative electrode in the present disclosure. With the cooperation of the modified graphite negative electrode, good low-temperature performance can be achieved without electrolyte additives and external heating systems. The optimization of the electrolyte formula can reduce the need for an external heating system for the battery in a low-temperature use environment, thereby optimizing the battery module structure and improving energy density and working performance.

[0033] According to an embodiment of the present disclosure, the viscosity of the electrolyte is less than 4.5 mPas at -40°C.

[0034] According to the embodiments of the present disclosure, by selecting a suitable electrolyte viscosity, the lithium-ion battery can still maintain good wettability to the electrode at low temperatures, which is beneficial for improving ion conductivity.

[0035] According to an embodiment of the present disclosure, the solvent includes a first type of solvent, and the first type of solvent includes at least one of propylene carbonate (PC, melting point -48.8 ° C), butylene carbonate (BC, -53 ° C), and fluoropropyl carbonate (FPC, -51.38 ° C).

[0036] According to the embodiments of the present disclosure, under the premise that the interface-modified graphite negative electrode promotes charge transfer, the excellent properties of the first type of carbonate solvents, such as the strong chemical stability, low viscosity and high electrical conductivity of carbonates, can ensure the ion transfer rate of the electrolyte at low temperatures, allowing lithium-ion batteries to be used in low-temperature environments.

[0037] According to an embodiment of the present disclosure, in the organic solution, the volume fraction of the first type of solvent is 10%-100%, for example, it can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, etc.

[0038] According to an embodiment of the present disclosure, the solvent also includes a second type of solvent, which includes dimethyl sulfite (DMS, -141 °C), methyl acetate (MA, -98 °C), ethyl acetate (EA, -84 °C), ethyl propionate (EP, -73.6 °C), methyl propionate (MP, -88 °C), ethyl butyrate (EB, -93 °C), methyl propyl carbonate (MPC, -49 °C), ethyl methyl carbonate (EMC, -53 °C), diethyl carbonate (DEC, -43 °C), diethyl sulfite (DES, -112 °C), 1,3-dioxolane (DOL, -95 °C), ethylene glycol dimethyl ether (DME, -58 °C), diethyl ether (DEE, -117 °C), acetonitrile (AN, -45 °C), tetrahydrofuran (THF, -109 °C), 2-methyltetrahydrofuran (MeTHF, -136 °C), and 1,3-dioxolane (DOL, -95 °C). ℃), tetrahydropyran (THP, -109 ℃), at least one of the following:

[0039] According to the embodiments of the present disclosure, the addition of the second type of solvent can further optimize the properties of the electrolyte and increase the oxidation voltage. The second type of solvent includes esters, alkanes, ethers and furan solvents. Among them, when other solvents such as easily oxidized ether groups that exhibit a negative electric field are combined with carbonate-based solvents that have strong electron-withdrawing ability and exhibit a positive electric field to the outside, the charge distribution of the electrolyte solvent structure can be adjusted, thereby increasing the oxidation voltage of the electrolyte; at the same time, low-melting-point solvents with high dielectric constants (25°C, >40) and strong coordination ability can ensure the transmission speed of lithium ions at low temperatures.

[0040] According to an embodiment of the present disclosure, when the organic solution includes both the first type of solvent and the second type of solvent, the volume fraction of the first type of solvent is greater than 10%, and the volume fraction of the second type of solvent is greater than 10%.

[0041] According to an embodiment of the present disclosure, when the organic solution includes both the first type of solvent and the second type of solvent, the volume fractions of the first type of solvent and the second type of solvent are both greater than 10%, and there is no need to introduce additional additives into the organic solution (the volume fraction is less than 10%), thereby avoiding a complex electrolyte system.

[0042] According to an embodiment of the present disclosure, the dielectric constant of the first type of solvent is greater than 40, and the dielectric constant of the second type of solvent is less than 30.

[0043] According to the embodiments of the present disclosure, in practical applications, since a single solvent is often difficult to meet all requirements (such as high dielectric constant but high viscosity, low viscosity but low dielectric constant, low oxidation potential, etc.), a mixed solvent can be used to optimize the composition and properties of the electrolyte. By mixing a solvent with a large dielectric constant and a high oxidation voltage with a solvent with a small viscosity and a low oxidation potential, an electrolyte with excellent performance can be obtained. The difference in organic functional groups between the first type of solvent and the second type of solvent defined in the present disclosure is of great significance for adjusting the oxidation potential and viscosity of the electrolyte. The organic functional group of the first type of solvent is hydroxyl oxygen, which has a greater electronegativity than carbon and has an electron-withdrawing inductive effect, so the carbonyl carbon has a stronger positive charge. Mixing it with the second type of solvent can make the charge distribution more reasonable, thereby improving the oxidation stability of the electrolyte, that is, improving the oxidation voltage of the electrolyte. In addition, mixing a solvent with a high dielectric constant but high viscosity with a solvent with a low dielectric constant but low viscosity in a certain proportion can balance the various properties of the electrolyte and ultimately optimize the oxidation potential and ionic conductivity of the electrolyte, thereby improving the energy density, cycle life and safety performance of the battery.

[0044] According to an embodiment of the present disclosure, the lithium salt includes: at least one of lithium hexafluorophosphate, lithium perchlorate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoro(oxalate)phosphate, lithium difluorophosphate, lithium monofluorophosphate, lithium bis(oxalate)borate, and lithium tetrafluoroborate; the concentration of the lithium salt is 0.5 to 2 mol / L, for example, it can be 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, etc.

[0045] According to an embodiment of the present disclosure, the modified material is a lithium ion fast ion conductor, and the lithium ion fast ion conductor includes at least one of lithium oxide, sulfide, hydride, halide, borate, phosphate and lithium phosphorus oxygen nitrogen.

[0046] According to the embodiments of the present disclosure, the lithium ion fast ion conductor changes the graphite surface structure and electron distribution, improves the ionic conductivity of the graphite surface, promotes the desolvation of lithium ions on the graphite surface, and improves the electrochemical performance of the battery. Under low temperature conditions, the charge transfer rate is greatly reduced. It is known that the work function is a physical quantity that describes the minimum energy required for electrons to escape from the surface of a material. A material with a low work function means that its electrons are more likely to escape from the surface, that is, it can act as an electron donor and effectively adsorb lithium ions on the negative electrode surface, thereby weakening the binding force between lithium ions and solvent molecules, thereby promoting the desolvation process of lithium ions, stabilizing the interface between the negative electrode and the electrolyte, and promoting the transfer of lithium ions at the interface between the negative electrode and the electrolyte, so that the surface-modified graphite can be compatible with a variety of electrolyte solvents and achieve excellent low-temperature performance. The surface-modified graphite is matched with different electrolyte systems to assemble a lithium half-battery, and its electrochemical behavior that contributes mainly to capacity during the charge and discharge process is the insertion or extraction of lithium ions.

[0047] In another aspect of the present disclosure, an application of the aforementioned lithium ion battery as a low-temperature lithium ion battery is also proposed, wherein the operating temperature of the low-temperature lithium ion battery is less than -20°C.

[0048] According to the embodiments of the present disclosure, the discharge capacity of the lithium-ion full battery proposed in the present disclosure at room temperature is higher than 90% of the theoretical capacity. Without the need for a heating system, a capacity retention rate of higher than 70% can be achieved in a low temperature environment of -20°C, and no lithium is precipitated during low-temperature fast charging.

[0049] It should be noted that the described embodiments are only part of the embodiments of the present disclosure, rather than all of the embodiments. Based on the embodiments of the present disclosure, other embodiments obtained by ordinary technicians in the field without creative work are all within the scope of protection of the present disclosure.

[0050] Example 1

[0051] In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, the first type of solvent propylene carbonate (PC) and the second type of solvent ethylene glycol dimethyl ether (DME) were mixed in a volume ratio of 1:1, and then 1 mol / L lithium hexafluorophosphate (LiPF 6 ), and stirred thoroughly until it dissolved to obtain electrolyte 1 (PC / DME), with a viscosity of 3.3 cp measured at -40 °C.

[0052] Example 2

[0053] In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, the first type of solvent fluoropropyl carbonate (FPC) and the second type of solvent 1,3-dioxolane (THF) were mixed in a volume ratio of 3:7, and then 1 mol / L lithium hexafluorophosphate (LiPF 6 ), and stirred thoroughly until it dissolved to obtain electrolyte 2 (FPC / THF), the viscosity of which was measured at -40 °C to be 3.9 cp.

[0054] Comparative Example 1

[0055] In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, 1 mol / L lithium hexafluorophosphate (LiPF 6 ), and stirred thoroughly until it dissolved to obtain electrolyte 3 (100% PC), the viscosity of which was measured at 20°C to be 7.9 cp.

[0056] Comparative Example 2

[0057] In a glove box filled with argon with an oxygen content of ≤1ppm and a water content of ≤1ppm, ester solvents ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1, and then 1 mol / L lithium hexafluorophosphate (LiPF 6 ), and stirred thoroughly until it dissolved to obtain electrolyte 4 (EC / DEC), with a viscosity of 5.2 cp measured at -40 °C.

[0058] Comparative Example 3

[0059] In a glove box filled with argon with an oxygen content of ≤1 ppm and a water content of ≤1 ppm, 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to the ether solvent ethylene glycol dimethyl ether (DME) and stirred thoroughly until it dissolved to obtain electrolyte 5 (DME). The viscosity was measured to be 2.3 cp at -40 °C.

[0060] The electrolytes obtained in Examples 1 and 2 and Comparative Examples 1-3 were matched with positive and negative electrodes to prepare lithium ion batteries, and performance tests were performed. The preparation methods of the positive and negative electrodes of the lithium ion batteries are as follows:

[0061] Conventional commercial cathode: LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) powder, conductive agent Ketjen black, and binder polyvinylidene fluoride were fully stirred and mixed in an N-methylpyrrolidone solvent system in a weight ratio of 94:3:3, and then coated on A1 foil, dried and rolled to obtain a positive electrode sheet.

[0062] Interface modified graphite negative electrode: Red phosphorus and graphite powder were treated by mechanical vibration at a mass ratio of 3:97 in an argon-filled atmosphere at a frequency of 20 Hz for 21 hours to obtain a phosphorus-coated graphite negative electrode material. The negative electrode active material, conductive agent, and binder were fully stirred and mixed in a co-solvent system at a weight ratio of 94:3:3, and then coated on Cu foil, dried, and rolled to obtain an interface modified graphite negative electrode sheet.

[0063] Test Example 1

[0064] The NCM523 positive electrode and the electrolytes obtained in Examples 1, 2 and Comparative Examples 1-3 were respectively assembled into button cells for constant current charge and discharge tests on lithium half-cells. The test temperature was room temperature (25°C), the test voltage range was 2.8-4.2 V, and the test rate was 0.1C.

[0065] Figure 1 1 and 2 are charge and discharge curves of the lithium half-cell at room temperature (25° C.) in Examples 1 and 2 and Comparative Examples 1-3 of the present disclosure.

[0066] like Figure 1 As shown, the NCM523 half-cells in Examples 1 and 2 can be reversibly charged and discharged above 4 V, and their charge and discharge capacities are similar to those of conventional electrolyte 4 (Comparative Example 2), both above 145 mAh / g, that is, their reversible capacity is more than 90% of the theoretical capacity. In contrast, when the NCM523 half-cell in Comparative Example 3 is charged to less than 4 V, electrolyte 5 decomposes, which shows that the electrolytes of the system of Examples 1 and 2 can achieve reversible charge and discharge of ether electrolytes matching high-voltage positive electrodes at room temperature at a lower lithium salt concentration without using any additives.

[0067] Test Example 2

[0068] The interface-modified graphite negative electrode and NCM523 positive electrode were assembled into a button cell with different electrolytes for constant current and constant voltage charge and discharge tests. The test temperature was -20 ℃, the test voltage range was 2.8~4.3 V, the test rate was 0.1C, and the charge cut-off current was 0.05C.

[0069] Figure 2 It is the charge and discharge curve of the NCM523 full battery in Examples 1 and 2 and Comparative Examples 1-3 of the present disclosure at low temperature (-20°C).

[0070] like Figure 2 As shown, the results show that the NCM523 full batteries in Examples 1 and 2 can be reversibly charged and discharged at -20 °C, and their capacity retention rates relative to room temperature (~155 mAh / g) are 91% and 86%, respectively. In contrast, the capacity retention rate of the full battery in the conventional electrolyte 4 (Comparative Example 2) at -20 °C is only 49%, which shows that the electrolyte of the system of Examples 1 and 2 can achieve high capacity retention rate in a low temperature environment without using any additives and external heating devices. In addition, when pure first-class solvents or second-class solvents are used (Comparative Examples 1 and 2), their capacity retention rates relative to room temperature are less than 60%, indicating that the viscosity and dielectric constant of the solvent have a great influence on the low-temperature performance of the electrolyte system.

[0071] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present disclosure. It should be understood that the above description is only a specific embodiment of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A lithium-ion battery comprising: A negative electrode, wherein the negative electrode is a surface-modified graphite material, wherein the work function of the modified material is less than 6 eV; An electrolyte, wherein the electrolyte is a lithium salt solution, wherein the oxidation voltage of the electrolyte is not less than 4 V and the melting point of the electrolyte is less than -40°C; and Positive electrode and separator.

2. The lithium ion battery according to claim 1, wherein The viscosity of the electrolyte is less than 4.5 mPa s at -40°C.

3. The lithium ion battery according to claim 1, wherein The solvent includes a first type of solvent, and the first type of solvent includes at least one of propylene carbonate, butylene carbonate, and fluoropropyl carbonate.

4. The lithium ion battery according to claim 3, wherein: In the organic solution, the volume fraction of the first type of solvent is 10%-100%.

5. The lithium ion battery according to claim 3, wherein: The solvent also includes a second type of solvent, which includes at least one of dimethyl sulfite, methyl acetate, ethyl acetate, ethyl propionate, methyl propionate, ethyl butyrate, methyl propyl carbonate, ethyl methyl carbonate, diethyl carbonate, diethyl sulfite, 1,3-dioxolane, ethylene glycol dimethyl ether, diethyl ether, acetonitrile, tetrahydrofuran, tetrahydropyran, and 2-methyltetrahydrofuran.

6. The lithium ion battery according to claim 5, wherein: In the organic solution, the volume fraction of the first type of solvent is greater than 10%, and the volume fraction of the second type of solvent is greater than 10%.

7. The lithium ion battery according to claim 6, wherein: The dielectric constant of the first type of solvent is greater than 40, and the dielectric constant of the second type of solvent is less than 30.

8. The lithium ion battery according to claim 1, wherein: The lithium salt includes at least one of lithium hexafluorophosphate, lithium perchlorate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoro(oxalate)phosphate, lithium difluorophosphate, lithium monofluorophosphate, lithium bis(oxalate)borate, and lithium tetrafluoroborate; the concentration of the lithium salt is 0.5 to 2 mol / L.

9. The lithium ion battery according to claim 1, wherein: The modified material is a lithium ion fast ion conductor, and the lithium ion fast ion conductor includes at least one of lithium oxide, sulfide, hydride, halide, borate, phosphate and lithium phosphorus oxygen nitrogen.

10. Use of the lithium ion battery according to any one of claims 1 to 9 as a low-temperature lithium ion battery, wherein: The operating temperature of the low-temperature lithium-ion battery is less than -20°C.

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