Electrolyte and lithium ion battery

Abstract

The present application relates to the technical field of lithium ion battery, especially relates to an electrolyte and a lithium ion battery. The electrolyte provided by the present application comprises a lithium salt, an organic solvent and an additive; the number of the types of the additive satisfying a / b >= 1 / 2 accounts for more than 60% of the total number of the types of the additive; the mass of the additive satisfying a / b >= 1 / 2 accounts for 60%-80% of the total mass of the additive; wherein a is the number of atoms other than carbon, oxygen and hydrogen in each additive, and b is the number of carbon atoms in each additive. The electrolyte provided by the present application can realize low low-temperature DCR, high conductivity and high energy retention rate at low temperature of the electrolyte by regulating the type and amount of the additive, and can also balance the normal temperature and high-temperature cycle performance by the mutual cooperation of each component; when the electrolyte is used in a lithium ion battery, it is beneficial to the balanced improvement of the low-temperature performance and the normal and high-temperature performance of the lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to an electrolyte and a lithium-ion battery. Background Technology

[0002] Currently, electric vehicles are rapidly gaining market share and popularity among users due to their advantages such as low energy consumption and zero carbon emissions. With technological advancements, issues such as short driving range and long charging times are gradually being resolved. Currently, low-temperature performance is a major bottleneck restricting the application of electric vehicles in cold regions, and improving battery charging and discharging capabilities at low temperatures has become a key focus of technological research in the industry.

[0003] In lithium-ion batteries, the electrolyte plays a crucial role in the battery's low-temperature performance. The electrolyte consists of a solvent, lithium salt, and additives. For a battery to exhibit good low-temperature performance, the electrolyte needs to possess the following properties: high lithium-ion conductivity at low temperatures, a good lithium-ion conduction capacity of the SEI film, and rapid charge transfer between the electrolyte and electrode surfaces.

[0004] In existing industry technologies, low-temperature electrolytes are generally designed using a combination of low-melting-point solvents, highly dissociable lithium salts, and low-resistance film-forming additives. Low-melting-point solvents are typically carboxylic acid esters such as EA and EP; lithium salts often use LiFSI as a highly dissociable lithium salt; and film-forming additives include boron-containing additives such as LiBF4, as well as other additives containing sulfur and phosphorus.

[0005] Carboxylic acid esters exhibit superior low-temperature performance, but suffer from poor electrode compatibility, particularly exhibiting poor reduction stability on the negative electrode surface. Due to the extensive use of carboxylic acid esters and their simplistic solvent structure, electrolyte stability is low, and their conductivity is limited by the amount of carboxylic acid ester used, making it difficult to achieve a balance between low-temperature performance and other properties. To address this, the industry often improves high-temperature performance by increasing the amount of high-temperature film-forming additives, but this results in a loss of low-temperature performance, often leaving a situation where low-temperature and high-temperature performance cannot be simultaneously optimized.

[0006] In view of this, this invention is hereby proposed. Summary of the Invention

[0007] The primary objective of this invention is to provide an electrolyte that combines low-temperature DCR, high conductivity and high energy retention at low temperatures, and excellent room-temperature and high-temperature cycling performance.

[0008] The second objective of this invention is to provide a lithium-ion battery with excellent low-temperature performance and normal-temperature performance.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0010] In a first aspect, the present invention provides an electrolyte comprising a lithium salt, an organic solvent, and additives;

[0011] The number of additives that satisfy a / b≥1 / 2 accounts for more than 60% of the total number of additives; the mass of additives that satisfy a / b≥1 / 2 accounts for 60% to 80% of the total mass of additives.

[0012] Where a represents the number of atoms other than carbon, oxygen, and hydrogen in each additive, and b represents the number of carbon atoms in each additive.

[0013] Furthermore, the additive includes a first additive and a second additive;

[0014] The first additive includes at least one of vinyl sulfate, vinylene carbonate, tris(trimethylsilane) phosphate, 1,3-propane sulphol, methane disulfonate, and fluorovinyl carbonate.

[0015] The second additive includes lithium salt additives.

[0016] Furthermore, the number of types of the first additive is ≤4.

[0017] Furthermore, in the electrolyte, the mass percentage of the first additive is 1% to 2%.

[0018] Furthermore, the lithium salt additive includes at least one of lithium difluorophosphate, lithium difluorodioxarate phosphate, lithium difluorooxarate borate, and lithium tetrafluoroborate.

[0019] Furthermore, in the electrolyte, the mass percentage of the second additive is 0.2% to 2%.

[0020] Furthermore, the lithium salt comprises lithium hexafluorophosphate and / or lithium difluorosulfonylimide;

[0021] And / or, the content of the lithium salt in the electrolyte is 10wt% to 20wt%.

[0022] Furthermore, the organic solvent includes at least one of cyclic carbonates, linear carbonates, and carboxylic acid esters.

[0023] Secondly, the present invention also provides a lithium-ion battery comprising the electrolyte as described above.

[0024] Furthermore, the lithium-ion battery also includes a nickel-cobalt-manganese ternary cathode material, wherein the mass percentage of nickel in the nickel-cobalt-manganese ternary cathode material is ≥60%.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The electrolyte provided by this invention, by controlling the type and amount of additives in the electrolyte, fully leverages the synergistic effect of each component in the electrolyte. It does not require the addition of high-impedance high-temperature additives, and can achieve low-temperature DCR, high conductivity and high energy retention rate of the electrolyte at low temperatures, while also taking into account the cycling performance at room temperature and high temperature. Using the electrolyte of this invention in lithium-ion batteries is beneficial to the balanced improvement of the low-temperature performance and room-temperature performance of lithium-ion batteries. Detailed Implementation

[0027] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0028] The following is a detailed description of an electrolyte and a lithium-ion battery according to an embodiment of the present invention.

[0029] In some embodiments of the present invention, an electrolyte is provided, comprising a lithium salt, an organic solvent, and an additive;

[0030] The number of additives that satisfy a / b≥1 / 2 accounts for more than 60% of the total number of additives; the mass of additives that satisfy a / b≥1 / 2 accounts for 60% to 80% of the total mass of additives.

[0031] Where a represents the number of atoms other than carbon, oxygen, and hydrogen in each additive, and b represents the number of carbon atoms in each additive.

[0032] The electrolyte provided by this invention is a low-temperature electrolyte. By adjusting the type and amount of additives in the electrolyte, the synergistic effect of each component in the electrolyte can be fully utilized. It does not require the addition of high-impedance high-temperature additives. It can achieve low-temperature DCR, high conductivity and high energy retention rate at low temperatures, while also taking into account the cycling performance at room temperature and high temperature.

[0033] In some embodiments of the invention, the number of additives satisfying a / b≥1 / 2 accounts for 60% to 75% of the total number of additives; typically, but not limitingly, for example, the number of additives satisfying a / b≥1 / 2 accounts for 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 75%, or any combination thereof of the total number of additives.

[0034] In some embodiments of the invention, typically but not limitingly, for example, the mass of the additive satisfying a / b≥1 / 2 accounts for 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 76%, 78%, 80% of the total mass of the additive, or any combination thereof.

[0035] The composition of the SEI (Sediment Ion Exchange) has a decisive impact on its performance. Inorganic components exhibit Arrhenius law-like ion conductivity, resulting in high conductivity at low temperatures, while organic components show increased viscosity, higher activation energy, and slower ion transport at low temperatures. Furthermore, inorganic components also possess higher stability, which is beneficial for ensuring the stability and normal function of the SEI at high temperatures. However, excessive inorganic content can lead to brittleness of the SEI film, necessitating the addition of organic components to adjust and enhance its flexibility. By appropriately adjusting the ratio of inorganic to organic components, high lithium-ion permeability and mechanical stability of the SEI film can be achieved, thereby ensuring a balanced improvement in both low-temperature performance and ambient-high-temperature cycling performance of the battery.

[0036] The ratio of inorganic to organic components significantly influences the properties of the SEI (Sediment Ion), and the composition of the SEI is primarily determined by the types and quantities of elements in the additives. The ratio a / b (number of inorganic atoms excluding C, O, and H in the additive / number of C atoms) is used as a parameter to measure the inorganic content of the additive. Additives with a / b ≥ 1 / 2 are considered inorganic additives, such as DTD, while those with a / b < 1 / 2 are considered organic additives, such as VC. By adjusting the type and quantity ratio of inorganic and organic additives in the electrolyte to meet the scope of this invention, an SEI structure can be formed where inorganic components are dominant and uniformly dispersed within an organic matrix. This structure can utilize the high lithium-ion conductivity of inorganic components to achieve high lithium-ion permeability of the SEI film, while the flexibility of organic components ensures sufficient mechanical stability of the SEI as a whole, thereby guaranteeing a balanced improvement in the battery's low-temperature performance and high-temperature cycling performance.

[0037] Inorganic additives and organic additives are defined in this invention. For example, by definition, the inorganic atom in DTD is S, a is 1, b is 2, and a / b = 1 / 2, so DTD belongs to inorganic additives.

[0038] In some embodiments of the present invention, the additives include a first additive and a second additive;

[0039] The first additive includes at least one of vinyl sulfate (DTD), vinylene carbonate (VC), tris(trimethylsilane) phosphate (TMSP), 1,3-propane sulpholol (PS), methane disulfonate (MMDS), and fluoroethylene carbonate (FEC);

[0040] The second additive includes lithium salt additives.

[0041] In some embodiments of the present invention, the number of types of the first additive is ≤4; typically, but not limitingly, for example, the number of types of the first additive can be 1, 2, 3 or 4; preferably, the number of types of the first additive is 3.

[0042] In some embodiments of the present invention, the mass percentage of the first additive in the electrolyte is 1% to 2%; typically, but not limitingly, for example, the mass percentage of the first additive in the electrolyte may be a range of 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any combination thereof.

[0043] In the electrolyte of this invention, an excessively high content of the first additive will lead to an increase in battery impedance and a weakening of low-temperature discharge capability; an insufficient content of the first additive will make it difficult for the SEI to be fully repaired during cycling, resulting in deterioration of cycle performance.

[0044] In some embodiments of the present invention, the mass percentage of each first additive in the electrolyte is ≥0.1%.

[0045] If the mass percentage of each primary additive in the electrolyte is low, it will have an adverse effect on the cycle performance of the battery.

[0046] In some specific embodiments of the present invention, the first additive includes DTD, VC and TMSP; and in the electrolyte, the mass percentage of DTD, VC and TMSP is ≥0.1%; preferably, the mass ratio of DTD, VC and TMSP is (5-10):(1-5):(1-5).

[0047] In some embodiments of the present invention, the lithium salt additive includes at least one of lithium difluorophosphate (LiPO2F2), lithium difluorodioxophosphate (LiODFP), lithium difluorooxoborate (LiODFB), and lithium tetrafluoroborate (LiBF4).

[0048] In some embodiments of the present invention, the mass percentage of the second additive in the electrolyte is 0.2% to 2%; typically, but not limitingly, for example, the mass percentage of the second additive in the electrolyte may be a range of 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any combination thereof.

[0049] In some embodiments of the present invention, the lithium salt includes lithium hexafluorophosphate (LiPF6) and / or lithium bisfluorosulfonylimide (LiFSI).

[0050] In some embodiments of the present invention, the lithium salt content in the electrolyte is 10 wt% to 20 wt%; typically, but not limitingly, for example, the lithium salt content in the electrolyte may be a range of 10 wt%, 12 wt%, 14 wt%, 16 wt%, 18 wt%, 20 wt%, or any combination thereof.

[0051] In some embodiments of the present invention, the organic solvent includes at least one of cyclic carbonates, linear carbonates, and carboxylic acid esters.

[0052] In some specific embodiments of the present invention, the organic solvents include ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0053] In some embodiments of the present invention, a method for preparing the above-mentioned electrolyte is also provided, comprising the following steps:

[0054] The electrolyte is obtained by mixing lithium salt, organic solvent and additives evenly.

[0055] In some embodiments of the present invention, a lithium-ion battery comprising the above-described electrolyte is also provided.

[0056] Using the electrolyte of this invention in lithium-ion batteries is beneficial for simultaneously and evenly improving the low-temperature performance and normal-temperature performance of lithium-ion batteries.

[0057] In some embodiments of the present invention, the lithium-ion battery further includes a nickel-cobalt-manganese (NCM) ternary cathode material, wherein the mass percentage of nickel in the NCM ternary cathode material is ≥60%.

[0058] In some embodiments of the present invention, the lithium-ion battery further includes a negative electrode material, which includes, but is not limited to, graphite.

[0059] In some embodiments of the present invention, the upper limit of the operating voltage of the lithium-ion battery is no higher than 4.4V.

[0060] Examples 1-7

[0061] The electrolyte preparation method provided in this embodiment includes the following steps:

[0062] In an argon atmosphere glove box with a water content of <10ppm, battery-grade ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 2:4.5:3.5 to form an organic solvent; then lithium salt and additives were added and mixed evenly to obtain an electrolyte; wherein the lithium salt was lithium hexafluorophosphate, and the lithium salt content in the electrolyte was 12wt%; the composition of the additives and the content of each additive component in the electrolyte are shown in Table 1.

[0063] In Table 1, the content of each component is the percentage of the mass of each component to the total mass of the electrolyte.

[0064] Table 1

[0065]

[0066]

[0067] Comparative Examples 1-6

[0068] The electrolyte preparation method provided in this comparative example includes the following steps:

[0069] In an argon atmosphere glove box with a water content of <10ppm, battery-grade ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 2:4.5:3.5 to form an organic solvent; then lithium salt and additives were added and mixed evenly to obtain an electrolyte; wherein the lithium salt was lithium hexafluorophosphate, and the lithium salt content in the electrolyte was 12wt%; the composition of the additives and the content of each additive component in the electrolyte are shown in Table 2.

[0070] In Table 2, the content of each component is the percentage of the mass of each component to the total mass of the electrolyte.

[0071] Table 2

[0072]

[0073]

[0074] Test case

[0075] Lithium-ion batteries were assembled using the electrolytes from Examples 1-7 and Comparative Examples 1-6, respectively, and the electrochemical performance of each battery was tested. The results are shown in Table 3.

[0076] The preparation method of lithium-ion batteries includes the following steps:

[0077] The positive electrode active material (ternary NCM, LiNi) 0.6 Co 0.2 Mn 0.2O2), polyvinylidene fluoride as a binder, and Super P as a conductive agent are mixed in a weight ratio of 98:1:1. N-methylpyrrolidone (NMP) is added, and the mixture is stirred under vacuum until the system is homogeneous and transparent to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated onto aluminum foil. After the aluminum foil is dried at room temperature, it is transferred to an oven for drying. Then, it is cold-pressed and slit to obtain the positive electrode sheet.

[0078] Artificial graphite was used as the negative electrode active material, Super P as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) as the thickener, and styrene-butadiene rubber (SBR) as the binder. They were mixed at a mass ratio of 96:1:1:2, and deionized water was added. The mixture was stirred in a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto a copper foil for the negative electrode current collector. After the copper foil was dried at room temperature, it was transferred to an oven for drying. Then, it was cold-pressed and slit to obtain the negative electrode sheet.

[0079] Using a 12μm thick polypropylene film (PP) as the separator, the above-prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. Then, an aluminum-plastic film is wrapped around the separator, and the separator is dried in a vacuum oven at 120°C. After injecting 3.0g / Ah of electrolyte, the separator is sealed and liquefied to produce a 1Ah soft-pack battery (i.e., a lithium-ion battery).

[0080] The electrochemical performance testing methods are as follows:

[0081] Secondary battery cycle test: In an oven at a specified temperature (room temperature 25℃ or high temperature 45℃), perform cyclic charging and discharging at a current of 1C within a specified potential range, record the discharge capacity of each cycle, and end the test when the battery capacity reaches 80% of the capacity of the first cycle.

[0082] Secondary battery DC resistance (DCR) test: At a specified temperature, the battery is discharged at a 1C current to 50% SOC (state of charge, reflecting the battery's remaining capacity). The current is then increased to 4C and maintained for 30 seconds. The difference between the updated stable voltage and the original plateau voltage is measured. The ratio of this difference to the 4C current value is the battery's DC resistance. The DCR test result performed after the battery's first full charge is the battery's initial DCR.

[0083] Low-temperature discharge capacity retention test of secondary batteries: After fully charging the secondary battery, place it in a constant temperature chamber at -20℃. After it is fully cooled, discharge it at a rate of 1C to the cutoff voltage and compare its capacity as a percentage of the initial discharge capacity.

[0084] The battery's charge / discharge cutoff voltage is 2.8–4.4V.

[0085] Table 3

[0086]

[0087]

[0088] As can be seen from Table 3, a comparison of Examples 1, 2 and Comparative Example 1 shows that the number of additives with a high proportion of inorganic components has a significant impact on low-temperature performance. The higher the proportion of such additives, the better the low-temperature performance. However, excessive inorganic components can cause the SEI to become brittle, which will adversely affect the corresponding cycle performance.

[0089] A comparison of Examples 1, 3, 4, 2, and 3 shows that, in addition to the type of additive, the content of the additive also affects the level of inorganic components in the SEI. When the content of additives with a high proportion of inorganic components is high, the low-temperature performance of the battery is improved. When the proportion of inorganic components is low, the low-temperature performance deteriorates, but the cycle performance is improved. When the content of additives with a high proportion of inorganic components exceeds the limits of this invention (≤60% or ≥80%), both low-temperature performance and ambient and high-temperature cycle performance cannot be achieved.

[0090] A comparison of Examples 1, 5, 6, 4, and 5 shows that the total amount of the first additive also needs to be controlled within a reasonable range. Too high a content of the first additive will lead to an increase in battery impedance and a weakening of low-temperature discharge capability. Too low a content of the first additive will make it difficult for the SEI to be fully repaired during cycling, resulting in deterioration of cycle performance.

[0091] A comparison of Examples 1, 7 and Comparative Example 6 shows that using three types of first additives can achieve better results, while reducing the number of first additives will affect the cycle performance of the battery.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte, characterized in that, Including lithium salts, organic solvents, and additives; The additives include: a first additive and a second additive; The number of additives that satisfy a / b≥1 / 2 accounts for more than 70% of the total number of additives; the mass of additives that satisfy a / b≥1 / 2 accounts for 74% to 80% of the total mass of additives. Where a is the number of atoms other than carbon, oxygen and hydrogen in each additive, and b is the number of carbon atoms in each additive; The first additive is composed of vinyl sulfate, vinylene carbonate and tris(trimethylsilane) phosphate in a mass ratio of (5~10):(1~5):(1~5); In the electrolyte, the mass percentage of the first additive is 1% to 2%; The second additive includes at least one of lithium difluorophosphate, lithium difluorodioxarate phosphate, lithium difluorooxarate borate, and lithium tetrafluoroborate. In the electrolyte, the mass percentage of the second additive is 0.2% to 2%.

2. The electrolyte according to claim 1, characterized in that, The lithium salt includes lithium hexafluorophosphate and / or lithium difluorosulfonylimide salt; And / or, the content of the lithium salt in the electrolyte is 10wt%~20wt%.

3. The electrolyte according to claim 1, characterized in that, The organic solvent includes at least one of cyclic carbonates, linear carbonates, and carboxylic acid esters.

4. A lithium-ion battery, characterized in that, Includes the electrolyte as described in any one of claims 1 to 3.

5. The lithium-ion battery according to claim 4, characterized in that, The lithium-ion battery also includes a nickel-cobalt-manganese ternary cathode material, wherein the mass percentage of nickel in the nickel-cobalt-manganese ternary cathode material is ≥60%.

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