Electrolyte additive and application

Abstract

The invention provides an electrolyte additive and application, and relates to the technical field of lithium ion batteries. Specifically, the electrolyte additive comprises lithium difluoroborate sulfate and 5-phenyl-1, 3, 2, 4-dioxathiazole 2-oxide, and the electrolyte additive comprises lithium difluoroborate sulfate and 5-phenyl-1, 3, 2, 4-dioxathiazole 2-oxide. The two components provided by the invention have a strong synergistic effect when being used as the lithium battery electrolyte additive; wherein the lithium difluoroborate sulfate constructs a negative electrode solid electrolyte membrane with more inorganic components, so that the impedance is reduced, and meanwhile, the negative electrode protection is realized; 5-phenyl-1, 3, 2, 4-dioxathiazole 2-oxide reacts at the positive electrode, so that the effect of providing electrons can be achieved, the actual lithium intercalation amount of the negative electrode can be increased, the lithium supplementing effect of the positive electrode is further achieved, the lithium loss of the negative electrode is reduced by increasing the capacity of the positive electrode, and the cycle performance is improved; according to the invention, the electronic impedance of an electrode interface is improved on the premise of not increasing the dosage of the additive, and the electrochemical performance of the battery under high-low temperature and high-power conditions is maintained.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to an electrolyte additive and application thereof. Background Art

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and no memory effect, have shown broad application prospects in consumer electronics, transportation, energy storage, industry, and many other fields. Specifically, in the current field of power tools, especially new energy vehicles, whether the cycle life of lithium-ion batteries can be further improved is a key performance that restricts their use as a power source. The attenuation of the cycle life of lithium-ion batteries often comes from the destruction of the positive and negative active materials and the loss of active lithium during the cycle process; among them, the former is related to the selection of positive and negative electrode materials, and the latter is related to the redox side reactions of the electrolyte at the positive and negative electrodes. For the latter, for example, the organic solvent molecules in the electrolyte may be oxidized and generate gas or decomposition products of lithium salts, and the decomposition products of lithium salts further consume the solvent in the electrolyte, react and consume active lithium ions, resulting in a decrease in battery capacity.

[0003] Currently, the main approach to reducing active lithium loss is to introduce film-forming additives into the electrolyte. During the first charge of the battery, the film-forming additives will decompose on the surface of the positive and negative electrodes, and the products of their decomposition will form a solid electrolyte interface film (SEI film) on the electrode surface, thereby increasing the electronic impedance at the interface, preventing the transfer of electrons from the electrode surface to the electrolyte, and also stabilizing the electrode interface, thereby reducing further redox reactions. Common film-forming additives in the industry, such as diethylenetriaminetetradecanoate (DTD), fluoroethylene carbonate (FEC), and vinylene carbonate (VC), can all achieve the above effects. However, if a significant increase in electronic impedance is required, a high content of conventional film-forming additives will need to be added. However, when the battery impedance increases sharply, it will affect the performance of lithium-ion batteries under low temperature conditions or high power requirements.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The first object of the present invention is to provide an electrolyte additive, which is mainly used to solve the defect that the performance of lithium-ion batteries is reduced under low temperature or high power conditions when the electronic impedance at the electrode interface is sharply increased by adding conventional electrolyte additives.

[0006] A second object of the present invention is to provide an electrolyte.

[0007] The third object of the present invention is to provide a method for preparing the electrolyte.

[0008] A fourth object of the present invention is to provide a lithium ion battery.

[0009] A fifth object of the present invention is to provide an electrical device.

[0010] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted: An electrolyte additive comprises lithium difluoroborate sulfate and 5-phenyl-1,3,2,4-dioxathiazole 2-oxide.

[0011] An electrolyte comprises the electrolyte additive.

[0012] Preferably, the mass ratio of the lithium difluoroborate sulfate to the electrolyte is 0.2% to 2%.

[0013] Preferably, the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.1% to 2%.

[0014] More preferably, the mass ratio of the lithium difluoroborate sulfate to the electrolyte is 0.5% to 1.5%.

[0015] More preferably, the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.5% to 1%.

[0016] Preferably, the electrolyte comprises an organic solvent, a lithium salt and the electrolyte additive.

[0017] More preferably, the electrolyte further comprises a second electrolyte additive; the second electrolyte additive comprises at least one of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propane sultone, vinyl ethylene carbonate or ethylene sulfate.

[0018] More preferably, the organic solvent comprises cyclic carbonates and chain acid esters; The cyclic carbonate includes at least one of ethylene carbonate, vinylene carbonate, propylene carbonate or butylene carbonate, and the chain acid ester includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate or propyl acetate.

[0019] More preferably, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate or lithium perchlorate.

[0020] More preferably, the mass ratio of the lithium salt to the electrolyte is 2.5% to 16%.

[0021] A method for preparing the electrolyte comprises the following steps: fully mixing the raw material components of the electrolyte to obtain the electrolyte.

[0022] A lithium-ion battery comprises the electrolyte.

[0023] An electrical device comprises the lithium-ion battery.

[0024] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides an additive for lithium-ion battery electrolytes, comprising a combination of lithium difluoroborate sulfate and 5-phenyl-1,3,2,4-dioxathiazole 2-oxide. The present invention finds that the two additive components exhibit a strong synergistic effect. The lithium difluoroborate sulfate constructs a negative electrode solid electrolyte membrane with a relatively high inorganic content, thereby reducing impedance while simultaneously protecting the negative electrode and reducing side reactions. The 5-phenyl-1,3,2,4-dioxathiazole 2-oxide reacts at the positive electrode, providing electrons and thereby increasing the actual amount of lithium inserted into the negative electrode, thereby achieving a positive electrode lithium replenishment effect, reducing negative electrode lithium loss by increasing positive electrode capacity, and improving cycle performance. Consequently, while negative electrode lithium loss is reduced, the amount of negative electrode lithium insertion increases, but the amount of negative electrode side reactions also increases. However, the lithium difluoroborate sulfate acting on the negative electrode can further reduce the rate of negative electrode side reactions. When the degree of negative electrode side reactions is reduced, the negative electrode lithium replenishment effect of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide is more effective, further improving the cycle life. Therefore, by having the two additive components in the positive and negative electrodes respectively, the performance of the lithium-ion battery is improved, and a synergistic enhancement effect of 1+1>2 is achieved on a macro scale. DETAILED DESCRIPTION

[0025] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments, but those skilled in the art will understand that the embodiments described below are part of embodiments of the present invention, rather than 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 in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially. In addition, the terms "first", "second" and "third" are only used for descriptive purposes and are not to be understood as indicating or implying relative importance.

[0026] The first aspect of the present invention is to provide an electrolyte additive, specifically, an additive composition comprising at least two components; the electrolyte additive comprises lithium difluoroborate sulfate and 5-phenyl-1,3,2,4-dioxathiazole 2-oxide.

[0027] During the initial charge and discharge process of a lithium-ion battery, the lithium difluoroborate sulfate (LiSO4BF2) of the present invention decomposes on the electrode surface and participates in the formation of an SEI film. Compared to conventional SEI film compositions, the resulting SEI film is denser and more stable, effectively preventing further reaction between the electrolyte and the electrode, reducing the loss of active lithium, and thereby improving the battery's initial Coulombic efficiency and cycle life. Furthermore, at high temperatures, lithium difluoroborate sulfate can inhibit electrolyte decomposition and phase transitions in electrode materials, reducing the increase in the battery's internal resistance and capacity decay. At low temperatures, lithium difluoroborate sulfate can improve the electrolyte's ionic conductivity, promote lithium ion transport, and enhance the battery's charge and discharge performance, thereby effectively improving the lithium-ion battery's electrochemical performance at both low and high temperatures.

[0028] The 5-phenyl-1,3,2,4-dioxathiazole 2-oxide (C7H5NO3S) of the present invention has a CAS number of 19279-25-5 and a chemical structure as shown below: .

[0029] It is known in the art that there are two types of reduction reactions at the negative electrode when a lithium-ion battery is charged: one is that lithium ions are embedded in the graphite negative electrode to produce LiC6, which can contribute reversible capacity during discharge; the other is that the solvents and additives in the electrolyte gain electrons and decompose on the surface of the negative electrode to form solid or soluble by-products; the latter cannot contribute reversible capacity during discharge, so it is also called active lithium loss, which is the main reason why the battery cycle coulombic efficiency is less than 100%.

[0030] In the present invention, the aforementioned electrolyte additive is applied to a lithium-ion battery environment. Under charging conditions, 5-phenyl-1,3,2,4-dioxathiazole 2-oxide in the electrolyte can oxidize at the positive electrode, donating electrons, and then enter the negative electrode through an external circuit for a reduction reaction. This process effectively increases the positive electrode capacity and triggers more intercalation reactions and side reactions at the negative electrode. Simply increasing these two negative electrode processes proportionally will not improve the battery's long-term performance: while more intercalation reactions can ensure greater reversible capacity, more side reactions will lead to rapid electrolyte consumption and a significant decline in long-term performance.

[0031] Therefore, in order to make the additional capacity contribution of 5-phenyl-1,3,2,4-dioxathiazole 2-oxide at the positive electrode have a positive effect, it is necessary to improve the electrolyte composition to achieve the improvement of the disadvantages of 5-phenyl-1,3,2,4-dioxathiazole 2-oxide, thereby simultaneously reducing the electrolyte decomposition side reaction at the negative electrode. In addition, because 5-phenyl-1,3,2,4-dioxathiazole 2-oxide contains multiple double bonds, it can undergo reduction polymerization reaction at the negative electrode, which will also offset its effectiveness in providing electrons at the positive electrode.

[0032] Summarizing the above two defect requirements together, it is necessary to synergistically introduce another electrolyte additive that can effectively reduce the electronic conductivity of the negative electrode interface, so that the reduction of the solvent and 5-phenyl-1,3,2,4-dioxathiazole 2-oxide at the negative electrode can be weakened, thereby strengthening the positive effect of 5-phenyl-1,3,2,4-dioxathiazole 2-oxide oxidation at the positive electrode. The present invention has found through experimental testing that lithium difluoroborate sulfate decomposes at the negative electrode, which helps to increase the content of inorganic components in the negative electrode SEI, can effectively reduce the electronic conductivity of the negative electrode SEI, and prevent the reduction of solvents and 5-phenyl-1,3,2,4-dioxathiazole 2-oxide. Therefore, controlled growth of SEI can be achieved, thereby reducing the total amount of additives in the electrolyte, reducing the SEI film thickness, reducing the interface ion conduction impedance, and achieving improvements in the long cycle and low temperature performance of the battery.

[0033] The second aspect of the present invention is to provide an electrolyte, wherein the electrolyte comprises the electrolyte additive as described in the first aspect.

[0034] As a preferred embodiment, the electrolyte includes an organic solvent, a lithium salt and the electrolyte additive; in some more preferred embodiments, the electrolyte further includes a second electrolyte additive.

[0035] As a preferred embodiment, the mass ratio of the lithium difluoroborate sulfate to the electrolyte is 0.2% to 2%, and the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.1% to 2%.

[0036] As an optional embodiment, the mass ratio of the lithium difluoroborate sulfate to the electrolyte includes but is not limited to 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2% or any one of them, or a numerical range consisting of any two of them, and the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2% or any one of them, or a numerical range consisting of any two of them.

[0037] As a more preferred embodiment, the mass ratio of the lithium difluoroborate sulfate to the electrolyte is 0.5% to 1.5%, and the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.5% to 1%.

[0038] As a more preferred embodiment, the organic solvent includes a cyclic carbonate and a chain acid ester; wherein the cyclic carbonate includes at least one of ethylene carbonate, vinylene carbonate, propylene carbonate or butylene carbonate, and the chain acid ester includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate or propyl acetate. It is understood that the cyclic carbonate and the chain acid ester can be selected from a single component or a combination of several components, such as some typical but non-limiting examples: a combination of ethylene carbonate and propylene carbonate, a combination of propylene carbonate and butylene carbonate, a combination of dimethyl carbonate and diethyl carbonate, a combination of ethyl methyl carbonate and methylpropyl carbonate, a combination of ethylpropyl carbonate and methyl formate, a combination of methyl formate and ethyl formate, a combination of propyl formate and methyl acetate, a combination of ethyl acetate and propyl acetate, etc. In some optional embodiments, the volume ratio of the cyclic carbonate to the linear acid ester is (10-50): (50-90).

[0039] As a more preferred embodiment, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate or lithium perchlorate; in some optional embodiments, the mass ratio of the lithium salt to the electrolyte is 2.5%~16%.

[0040] As a more preferred embodiment, the second electrolyte additive includes at least one of vinylene carbonate (VC), diethylene sulfate (DTD), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), vinyl ethylene carbonate (VEC), or ethylene sulfate (DTD). In some optional embodiments, the mass ratio of the second electrolyte additive to the electrolyte is 1% to 5%.

[0041] The third aspect of the present invention is to provide a method for preparing the electrolyte as described in the second aspect, which mainly comprises the following steps: fully mixing the raw material components of the electrolyte to obtain the electrolyte.

[0042] It is understood that the thorough mixing can be assisted by oscillation, stirring, shaking, centrifugation, ultrasound, heating, etc., which helps to accelerate dispersion and obtain a relatively uniform dispersion system.

[0043] A fourth aspect of the present invention is to provide a lithium-ion battery comprising the electrolyte as described in the second aspect.

[0044] It is understood that the lithium-ion battery includes a positive electrode, a negative electrode, the electrolyte, a separator, and other necessary or non-essential functional elements or packaging components, etc., which can be arbitrarily selected and combined by those skilled in the art; wherein the active material of the positive electrode includes but is not limited to lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, ternary materials, lithium iron phosphate, etc., the active material of the negative electrode includes but is not limited to carbon-based materials, titanium-based materials, silicon-based materials, tin-based materials, etc., and the separator includes but is not limited to polyethylene separators, polypropylene separators, ethylene-propylene copolymer films, or polyimide separators, etc. When the electrolyte of the present invention is included in the lithium-ion battery, whether or not other electrolytes are also used in combination in the lithium-ion battery can be regarded as an embodiment of the present invention.

[0045] A fifth aspect of the present invention is to provide an electrical device comprising the lithium-ion battery as described in the fourth aspect.

[0046] It can be understood that the electrical equipment can be any equipment or device that relies on electric energy to work or operate, including but not limited to new energy vehicles, building electrical equipment, industrial electrical appliances, household and agricultural appliances, etc.; when the secondary battery is included, any electrical equipment equipped with the secondary battery can belong to an embodiment of the present invention.

[0047] Examples and Comparative Examples In an argon atmosphere glove box with a water content of <10 ppm, battery-grade ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3:7 to form an organic solvent. Other components were quantitatively added according to the electrolyte composition shown in Table 1 and stirred until uniformly mixed to obtain the respective electrolytes. The content of each component in Table 1 is calculated as weight percentage based on the total weight of the electrolyte. In Table 1, "DTD" stands for ethylene sulfate, "VC" stands for vinylene carbonate, "LiPF6" stands for lithium hexafluorophosphate, "A" stands for 5-phenyl-1,3,2,4-dioxathiazole 2-oxide, and "LiSO4BF2" stands for lithium difluoroborate sulfate.

[0048] Table 1

[0049] Test example The electrolytes of the above embodiments and comparative examples were used to prepare corresponding lithium-ion batteries for testing, as follows: S1. Preparation of lithium iron phosphate positive electrode: Mix the positive electrode active material LiFePO4, the binder polyvinylidene fluoride, and the conductive agent SuperP in a weight ratio of 97:1:2, add N-methylpyrrolidone NMP, and stir under the action of a vacuum mixer until the system becomes uniform and transparent to obtain a positive electrode slurry; evenly coat the positive electrode slurry on aluminum foil; dry the aluminum foil at room temperature and then transfer it to an oven for drying, and then cold press and cut it to obtain the positive electrode sheet.

[0050] S2. Preparation of graphite negative electrode: The negative electrode active material artificial graphite, conductive agent SuperP, thickener sodium carboxymethyl cellulose CMC-Na, and binder styrene-butadiene rubber SBR are mixed in a mass ratio of 96:1:1:2, deionized water is added, and the negative electrode slurry is obtained under the action of a vacuum mixer; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil; the copper foil is dried at room temperature and then transferred to an oven for drying, and then cold pressed and cut to obtain the negative electrode sheet.

[0051] S3: Use polypropylene film as the isolation film with a thickness of 12 μm.

[0052] S4. Lithium-ion battery assembly: The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed between the positive and negative electrodes to serve as an isolation layer. The sheets are then wrapped with aluminum-plastic film and transferred to a vacuum oven for drying at 120°C. 3.0 g / Ah of the electrolyte of each embodiment or comparative example is injected into the sheets, sealed, and the electrolyte is formed to finally prepare a soft-pack battery for testing with a capacity of 1 Ah.

[0053] The lithium-ion batteries corresponding to the examples and comparative examples were tested. The test items are as follows, and the test results of each item are recorded in Table 2.

[0054] 1) Secondary battery cycle test: In a high temperature (45°C) oven, perform cyclic charge and discharge at a current of 1C within a specified potential range (2.0V-3.8V). Measure the discharge capacity of each cycle. End the test when the battery capacity reaches 80% of the initial cycle capacity, and record the number of cycles at this point.

[0055] 2) Secondary Battery DC Resistance (DCR) Test: At room temperature, discharge the battery at 1C to 50% SOC. Then, increase the current to 4C and maintain it for 30 seconds. Measure the difference between the updated stable voltage and the original platform voltage. The ratio of this value to the 3C current value is the battery's DC resistance. Record the DCR test result after the battery is fully charged for the first time as the battery's initial DCR.

[0056] 3) Secondary battery low-temperature discharge capacity retention test: After fully charging the battery, place it in a constant temperature box at 0°C. After sufficient cooling, discharge it at a rate of 1C to the cut-off voltage. Compare its capacity to the percentage of the initial discharge capacity and record it as the low-temperature capacity retention rate.

[0057] Table 2

[0058] As can be seen from Table 2, the use of lithium difluoroborate sulfate in the present invention helps to reduce impedance and improve circulation, but too high an amount thereof will bring certain adverse effects; 5-phenyl-1,3,2,4-dioxathiazole 2-oxide helps to improve the high-temperature cycle of the battery, but it will worsen the impedance and will not improve the low-temperature discharge rate much; the simultaneous use of the above two electrolyte additives in the present invention can improve the cycle, initial impedance and low-temperature discharge, but if the amount of 5-phenyl-1,3,2,4-dioxathiazole 2-oxide exceeds a certain range, it will lead to excessive oxidation of the positive electrode and excessive reduction of the negative electrode, causing lithium precipitation, resulting in a reduced cycle life.

[0059] Although the present invention has been illustrated and described using specific embodiments, it should be appreciated that the above embodiments are merely intended to illustrate the technical solutions of the present invention rather than to limit them. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and scope of the present invention. However, these modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. An electrolyte additive, characterized in that The electrolyte additives include lithium sulfate difluoroborate and 5-phenyl-1,3,2,4-dioxathiazole 2-oxide.

2. An electrolyte, characterized in that The electrolyte comprises the electrolyte additive according to claim 1 .

3. The electrolyte according to claim 2, characterized in that The mass ratio of the lithium difluoroborate sulfate to the electrolyte is 0.2% to 2%; And / or, the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.1% to 2%.

4. The electrolyte according to claim 3, characterized in that The mass ratio of the lithium difluoroborate sulfate to the electrolyte is 0.5% to 1.5%; And / or, the mass ratio of the 5-phenyl-1,3,2,4-dioxathiazole 2-oxide to the electrolyte is 0.5% to 1%.

5. The electrolyte according to claim 2, characterized in that The electrolyte comprises an organic solvent, a lithium salt and the electrolyte additive; Preferably, the electrolyte further comprises a second electrolyte additive; the second electrolyte additive comprises at least one of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propane sultone, vinyl ethylene carbonate or ethylene sulfate.

6. The electrolyte according to claim 5, characterized in that The organic solvent includes cyclic carbonate and chain acid ester; The cyclic carbonate includes at least one of ethylene carbonate, vinylene carbonate, propylene carbonate or butylene carbonate, and the chain acid ester includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate or propyl acetate.

7. The electrolyte according to claim 5, characterized in that The lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate or lithium perchlorate; Preferably, the mass ratio of the lithium salt to the electrolyte is 2.5% to 16%.

8. The method for preparing an electrolyte according to any one of claims 2 to 7, wherein: The method comprises the following steps: fully mixing various raw material components of the electrolyte to obtain the electrolyte.

9. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the electrolyte according to any one of claims 2 to 7.

10. An electrical device, characterized in that: The electric device comprises the lithium-ion battery as claimed in claim 9.

Images