Electrolyte and secondary battery

Abstract

The invention discloses an electrolyte and a secondary battery. The electrolyte comprises a cyclic phosphate compound, and the cyclic phosphate compound comprises one or a combination of more of compounds shown in the formula 1. The beneficial effects of the invention are as follows: by adding the cyclic phosphate compound with a P = O bond, the cyclic phosphate compound is subjected to ring opening polymerization on the surface of a positive electrode; a compact and stable CEI film containing a P-O-Li network is formed through preferential oxidation, the content of active oxygen is efficiently reduced, oxygenolysis of the electrolyte is inhibited, gas production is reduced, and then the high-temperature performance of the battery is improved.

Description

Technical Field

[0001] This application belongs to the field of new energy technology, and in particular relates to an electrolyte and a secondary battery. Background Technology

[0002] Cathode lithium replenishment agents are compounds added to the cathode material of lithium-ion batteries. Their main function is to release additional active lithium ions (Li+) through an irreversible oxidation-decomposition reaction during the first charge of the battery. + This is to compensate for lithium loss during battery cycling. These materials typically have high lithium content and specific electrochemical activity, and can decompose within the positive electrode potential range.

[0003] However, in some related technologies, the positive electrode lithium replenishment agent causes serious gas generation in the battery due to its own material characteristics. Summary of the Invention

[0004] In view of this, this application provides an electrolyte and a secondary battery, which aim to solve the aforementioned technical problems.

[0005] As a first aspect of this application, embodiments of this application provide an electrolyte comprising a cyclic phosphoric acid compound, said cyclic phosphoric acid compound comprising one or more combinations of compounds shown in [Formula 1]:

[0006] Formula 1

[0007] R1 is selected from any one of the following conditions:

[0008] a. Halogen substituents;

[0009] b. Alkyl or alkoxy groups having 1 to 6 carbon atoms, substituted or unsubstituted;

[0010] c. Alkenyl, ynyl, alkenyloxy, and ynyloxy groups having 3 to 9 carbon atoms, substituted or unsubstituted;

[0011] d. Any one of an aromatic group having 6 to 10 carbon atoms, whether substituted or unsubstituted;

[0012] f. A cyano group having 3 to 9 carbon atoms, substituted or unsubstituted;

[0013] R2 and R3 each independently satisfy any one of the following conditions:

[0014] (1) Selected from hydrogen atoms and halogen substituents;

[0015] (2) Alkyl groups having 1 to 6 carbon atoms, whether substituted or unsubstituted;

[0016] (3) Alkenyl or ynyl groups having 3 to 9 carbon atoms, whether substituted or unsubstituted;

[0017] (4) Any one of an aromatic group having 6 to 10 carbon atoms, whether substituted or unsubstituted;

[0018] (5) R2 and R3 bond together to form a ring with 3 to 6 carbon atoms.

[0019] Optionally, in some embodiments of this application, a benzene ring is formed between R2 and R3 of the cyclic phosphoric acid compound.

[0020] Optionally, in some embodiments of this application, the compound represented by [Formula 1] is any one of the following compounds:

[0021]

[0022] Optionally, in some embodiments of this application, the cyclic phosphoric acid compound R1, R2, and R3 contain at least one halogen substituent.

[0023] Optionally, in some embodiments of this application, the compound represented by [Formula 1] is any one of the following compounds:

[0024]

[0025]

[0026] Optionally, in some embodiments of this application, R1 of the cyclic phosphate compound comprises a trifluoromethyl group.

[0027] Optionally, in some embodiments of this application, the compound represented by [Formula 1] is any one of the following compounds:

[0028]

[0029] Optionally, in some embodiments of this application, the R1 of the cyclic phosphate compound contains at least one unsaturated bond.

[0030] Optionally, in some embodiments of this application, the compound represented by [Formula 1] is any one of the following compounds:

[0031]

[0032]

[0033] Optionally, in some embodiments of this application, the cyclic phosphate compound is bonded between R2 and R3 to form a ring having 3 to 6 carbon atoms, wherein the ring having 3 to 6 carbon atoms includes at least one unsaturated bond.

[0034] Optionally, in some embodiments of this application, the compound represented by [Formula 1] is any one of the following compounds:

[0035]

[0036] Optionally, in some embodiments of this application, the ratio of the cyclic phosphoric acid compound to the total mass of the electrolyte ranges from 0.1 wt% to 2.0 wt%.

[0037] As a second aspect of this application, embodiments of this application provide a secondary battery, including the aforementioned electrolyte.

[0038] The beneficial effects of this application are as follows: It provides a method that, through the addition of a cyclic phosphoric acid compound with P=O bonds, allows the cyclic phosphoric acid compound to undergo ring-opening polymerization on the surface of the positive electrode, preferentially oxidizing to form a dense and stable CEI film containing a PO-Li network, effectively reducing the content of active oxygen, inhibiting the oxidative decomposition of the electrolyte, reducing gas production, and thus improving the high-temperature performance of the battery. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram showing the content of cyclic phosphate compounds and the capacity retention rate after 800 cycles at 25°C in Examples 7, 9 to 14, Comparative Example 1, and Comparative Example 2 of this application.

[0041] Figure 2 This is a schematic diagram showing the curves of the content of cyclic phosphoric acid compounds in Examples 7, 9 to 14, Comparative Example 1, and Comparative Example 2 of this application versus the capacity retention rate after 500 cycles at 45°C.

[0042] Figure 3 This is a schematic diagram showing the curves of the content of cyclic phosphoric acid compounds and the volume expansion rate at 60°C during high-temperature storage in Examples 7, 9 to 14, Comparative Example 1, and Comparative Example 2 of this application. Detailed Implementation

[0043] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. In addition, it should be understood that the specific embodiments described herein are only for illustration and explanation of this application and are not intended to limit this application.

[0044] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0045] In this application, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in its actual use or operating state, specifically the drawing directions in the accompanying drawings; while "inner" and "outer" refer to the outline of the device. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". The terms first, second, third, etc., are used merely as illustrative purposes and do not impose numerical requirements or establish a numerical order.

[0046] In this application, "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural.

[0047] In this application, "at least one" means one or more, and "more than one" means two or more. "One or more", "at least one of the following", or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0048] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0049] In a first aspect, the electrolyte of this application includes a cyclic phosphoric acid compound, which includes one or more combinations of compounds shown in [Formula 1]:

[0050] Formula 1

[0051]

[0052] R1 is selected from any one of the following conditions:

[0053] a. Halogen substituents;

[0054] b. Alkyl or alkoxy groups having 1 to 6 carbon atoms, substituted or unsubstituted;

[0055] c. Alkenyl, ynyl, alkenyloxy, and ynyloxy groups having 3 to 9 carbon atoms, substituted or unsubstituted;

[0056] d. Any one of an aromatic group having 6 to 10 carbon atoms, whether substituted or unsubstituted;

[0057] f. A cyano group having 3 to 9 carbon atoms, substituted or unsubstituted;

[0058] R2 and R3 each independently satisfy any one of the following conditions:

[0059] (1) Selected from hydrogen atoms and halogen substituents;

[0060] (2) Alkyl groups having 1 to 6 carbon atoms, whether substituted or unsubstituted;

[0061] (3) Alkenyl or ynyl groups having 3 to 9 carbon atoms, whether substituted or unsubstituted;

[0062] (4) Any one of an aromatic group having 6 to 10 carbon atoms, whether substituted or unsubstituted;

[0063] (5) R2 and R3 bond together to form a ring with 3 to 6 carbon atoms.

[0064] During the first charge of the battery, the lithium replenishing agent at the positive electrode undergoes an irreversible reaction, releasing Li. + This process replenishes the active lithium lost due to the formation of the SEI film on the negative electrode side, thereby increasing battery capacity and extending battery life. However, this reaction process is accompanied by the generation of reactive oxygen free radicals. These reactive oxygens not only nucleophilically attack the positive electrode but also shuttle to the negative electrode interface, reacting with the electrolyte to produce gas, which in turn leads to poor battery interface performance.

[0065] When the lithium supplement is Li5FeO4 (LFO), the Fe in LFO is in a high oxidation state (Fe2+). 3+ During charging (especially at high potentials), oxygen in the crystal lattice may be released as reactive oxygen species (such as O2). This can trigger oxidative decomposition reactions, producing gases such as CO2 and CO, especially in the presence of the electrolyte. Simultaneously, during the delithiation process of LFO (Li5FeO4→LiFeO2+Li),... + +e - Irreversible phase transitions may occur, leading to lattice collapse, release of reactive oxygen species, or direct reaction with the electrolyte; LFO is charged to a high potential (>4.5V vs. Li). + When LiPF6 is in contact with Fe, its strong oxidizing properties will cause carbonate electrolytes (such as EC and DMC) to undergo oxidative decomposition, generating CO2 and alkane gases (such as CH4 and C2H4). Trace amounts of water in the electrolyte react with LiPF6 to generate HF, which will corrode the LFO surface, accelerate Fe dissolution and oxygen release, and further aggravate gas production (such as H2 and PF5 hydrolysis products).

[0066] Using the above scheme, a cyclic phosphate compound is added to the electrolyte of this application. The P=O bond in the cyclic phosphate compound is a strong electrophilic center, which is easily attacked by the nucleophilic free radicals. It undergoes ring-opening polymerization on the surface of the positive electrode and preferentially oxidizes to form a dense and stable CEI film containing a PO-Li network. This effectively reduces the content of reactive oxygen, inhibits the oxidative decomposition of the electrolyte, reduces gas production, and thus improves the high-temperature performance of the battery.

[0067] The electrolyte described in this application is suitable for secondary batteries in which the positive electrode contains a positive electrode lithium replenishing agent.

[0068] In some embodiments of this application, the compound shown in Formula 1 is any one of the compounds of structural formulas 1 to 17:

[0069]

[0070] In some embodiments of this application, the cyclic phosphoric acid compound R1, R2, R3 contains at least one halogen substituent.

[0071] In some examples, the halogen substituents include one or more of F, Cl, Br, I, and At.

[0072] Understandably, the high electronegativity of fluorine substituents can lower the HOMO energy level, improve the oxidation stability of additives, and the strong CF bond can resist free radical attacks. The formation of LiF can optimize the inorganic components of CEI, thereby improving the cycle performance and high-temperature cycle performance of the battery.

[0073] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0074]

[0075] In some embodiments of this application, a benzene ring is formed between R2 and R3 of the cyclic phosphate compound.

[0076] It is understandable that benzene rings are formed by the bonding between R2 and R3 in cyclic phosphoric acid compounds. Generally speaking, the lithium replenishment stage of the positive electrode requires a voltage of 4.1V or higher. The conjugated structure of the benzene ring has higher oxidation stability, and the aromatic polymer film formed by polymerization has stronger high voltage resistance. The benzene ring is adsorbed on the surface of the positive electrode active material through π-π stacking or van der Waals forces to form a physical isolation layer, which reduces the direct contact between the electrolyte and the positive electrode active material, inhibits the dissolution of transition metals in the positive electrode active material, and thus improves the cycle performance and high temperature cycle performance of the battery.

[0077] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0078]

[0079] In some embodiments of this application, the cyclic phosphoric acid compounds R1, R2, and R3 contain trifluoromethyl groups.

[0080] Understandably, compared to monofluorine atoms, trifluoromethyl groups exhibit steric hindrance and stronger electron-withdrawing capabilities, enabling the formation of a denser CEI film and stronger HF trapping ability, thereby further improving the battery's cycle performance and high-temperature cycle performance.

[0081] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0082]

[0083] In some embodiments of this application, the cyclic phosphoric acid compounds R1, R2, and R3 contain at least one to three carbon atoms of a trifluoroalkoxy group.

[0084] Understandably, the strong electron-withdrawing property of fluorine atoms in trifluoroalkoxy groups with 1 to 3 carbon atoms lowers the LUMO energy level of cyclic phosphate molecules, preferentially reducing and decomposing them on the negative electrode surface over organic solvents, thus forming a stable, dense, and inorganic (LiF)-rich SEI film.

[0085] In some examples, trifluoroalkoxy groups with 1 to 3 carbon atoms include trifluoroethoxy groups.

[0086] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0087]

[0088] In some embodiments of this application, the R1 of the cyclic phosphate compound contains at least one unsaturated bond.

[0089] Understandably, the unsaturated bonds on the cyclic phosphate compound R1 preferentially decompose on the negative electrode surface to form SEI, thereby improving the stability of the system.

[0090] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0091]

[0092]

[0093] In some embodiments of this application, the cyclic phosphate compounds R1, R2, and R3 contain at least a cyano group having 3 to 9 carbon atoms, either substituted or unsubstituted.

[0094] It is understandable that the cyano group and the P=O bond of the cyclic phosphate ester work together to form an inorganic-organic composite SEI / CEI film containing Li3PO4 and lithium cyanide (LiCN), which has both high ionic conductivity and mechanical strength, thereby improving the cycle performance of the battery.

[0095] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0096]

[0097] In some embodiments of this application, the R2 and R3 of the cyclic phosphate compound are bonded together to form a ring having 3 to 6 carbon atoms, wherein the ring having 3 to 6 carbon atoms includes at least one unsaturated bond.

[0098] It is understandable that in cyclic phosphate compounds, the R2 and R3 bonds form a ring with 3 to 6 carbon atoms, and this ring includes at least one unsaturated bond, i.e., an unsaturated carbon ring. When the unsaturated carbon ring is not a benzene ring, under high battery voltage (e.g., exceeding 4.1V), the electrolyte preferentially undergoes oxidation, and the unsaturated bond undergoes ring-opening polymerization to form a CEI film, reducing the dissolution of transition metals.

[0099] In some embodiments of this application, the compound represented by Formula 1 is any one of the following compounds:

[0100]

[0101]

[0102] In some embodiments of this application, the ratio of the cyclic phosphoric acid compound to the total mass of the electrolyte ranges from 0.1 wt% to 2.0 wt%. The ratio of the cyclic phosphoric acid compound to the total mass of the electrolyte can be any value or a range of any two values ​​from 0.1, 0.2, 0.5, 1.0, 1.5, 2.0.

[0103] Understandably, excessive addition of cyclic phosphate compounds will increase the viscosity of the electrolyte, significantly increase the impedance, and increase the risk of corrosion of the positive electrode current collector aluminum foil, thus degrading battery performance, especially when the amount exceeds 2%. Insufficient addition will prevent the formation of a dense and stable protective film on the positive electrode surface, thus failing to effectively suppress battery gas production.

[0104] In some embodiments of this application, the electrolyte further includes auxiliary additives, which include one or more of the following: vinylene carbonate, vinyl sulfate, tris(trimethylsilane) phosphite, fluoroethylene carbonate, 1,3-propane sulpholone, methylene disulfonate, or tris(trimethylsilane) borate. The auxiliary additives comprise one or more of the following: vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, 1,3-propane sulpholone, and methylene disulfonate. The mass of the auxiliary additives accounts for 0.1 wt% to 3.0 wt% of the total mass of the electrolyte. The mass of the auxiliary additives as a percentage of the total mass of the electrolyte can be any value or a range of any two values ​​from 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0.

[0105] It is understandable that the auxiliary additives are mainly negative electrode film-forming additives, which can work synergistically with the cyclic phosphoric acid compounds of this application to better protect the positive and negative electrodes, suppress gas production, and improve the high-temperature performance of the battery.

[0106] In some embodiments of this application, the electrolyte further includes a lithium salt, which includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium bis(oxalate borate), lithium difluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide; wherein the molar concentration of the lithium salt is from 0.5M to 1.5M. The molar concentration of the lithium salt can be any value or a range of any two values ​​from 0.1, 0.2, 0.5, 0.8, 1.0, 1.2, and 1.5.

[0107] In some embodiments of this application, the electrolyte further includes an organic solvent, which includes one or more of ethylene carbonate, propylene carbonate, butene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, or ethyl butyrate.

[0108] Secondly, embodiments of this application provide a secondary battery, including the aforementioned electrolyte.

[0109] In some embodiments of this application, the secondary battery further includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0110] In some embodiments of this application, the positive electrode sheet includes a positive electrode active material and a positive electrode lithium supplement.

[0111] In some embodiments of this application, the negative electrode sheet includes a negative electrode active material. The negative electrode active material includes one or a combination of carbon-based negative electrode materials, silicon-based negative electrode materials, tin-based negative electrode materials, silicon-containing alloy negative electrode materials, and tin-containing alloy negative electrode materials.

[0112] In some examples, the negative electrode active material includes graphite and / or silicon.

[0113] In some embodiments of this application, the secondary battery further includes a positive electrode active material, which comprises one or more of the following chemical formulas:

[0114] LiN i(1-a-b) Co a Mn b O2, where 0≤a≤1, 0≤b≤1, and a+b≤1;

[0115] LiMn c Fe 1-c PO4, where 0 ≤ c ≤ 1.

[0116] In some embodiments of this application, the secondary battery further includes a lithium replenishing agent, which includes a binary lithium-containing compound or a multi-component lithium-containing compound.

[0117] In some embodiments of this application, the binary lithium-containing compound includes one or more of Li2O and Li2O2.

[0118] In some embodiments of this application, the multi-component lithium-containing compound includes Li b M c O d M includes one or more of Fe, Co, and Ni, 0 < d ≤ 6, 0 < c ≤ 1, and 0 < f ≤ 4.

[0119] In some examples, the multi-component lithium-containing compound is Li5FeO4 (LFO).

[0120] Based on the above solutions, the battery's cycle performance, high-temperature gas generation, and high-temperature cycle performance can be balanced.

[0121] The present application will be further described below through specific embodiments.

[0122] Example 1

[0123] Prepare the electrolyte according to the following steps:

[0124] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a weight ratio of EC:DMC:EMC = 3:2:5 to obtain a mixed organic solvent system. 1.2 M lithium hexafluorophosphate (LiPF6) was added to the system, and after the lithium salt was completely dissolved, 1.0 wt% of a cyclic phosphoric acid compound was added. (i.e., the compound of structural formula 2), 2.0 wt% vinylene carbonate (VC).

[0125] The battery is prepared according to the following steps:

[0126] Preparation of positive electrode:

[0127] Step 1: Lithium iron phosphate (LFP), lithium iron phosphate (Li5FeO4), conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a weight ratio of 94:2.5:1.5:1:1 to prepare a lithium-ion battery positive electrode slurry with a solid content of 63±1% and a viscosity of 5000~7000mPa.s.

[0128] Step 2: Coat onto current collector aluminum foil, with a double-sided areal density of 412±6.2 g / m³. 2 ;

[0129] Step 3: After drying at 85℃, cold pressing is performed, and the thickness of the electrode sheet after roll pressing is 177.8±2.0μm;

[0130] Step 4: Then, the strips and slices are cut.

[0131] Step 5: Bake in a vacuum oven at 85°C for 4 hours to produce the positive electrode sheet for lithium-ion batteries.

[0132] Preparation of negative electrode:

[0133] Step 6: Mix artificial graphite with conductive agent SuperP, thickener CMC and binder SBR in a weight ratio of 96:1.2:1.3:1.5 to form a slurry with a solid content of 53±2.0% and a viscosity of 3000~6000mPa.s.

[0134] Step 8: Coat both sides of the copper foil current collector, with a double-sided areal density of 198.0±3.0 g / m². 2 ;

[0135] Step 9: Drying;

[0136] Step 10: Roll forming; the thickness of the electrode sheet after roll forming is 133.8±2.0μm.

[0137] Step 11: Cut the sheet to make the negative electrode sheet for lithium-ion batteries.

[0138] Preparation of lithium-ion batteries:

[0139] Step 12: The above positive electrode, negative electrode and separator are stacked to form a soft pack battery with a capacity of 2Ah;

[0140] Step 13: Then vacuum bake at 85℃ for 48 hours;

[0141] Step 14: Inject electrolyte at a rate of 4 g / Ah to complete battery fabrication.

[0142] Example 2

[0143] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 3).

[0144] Example 3

[0145] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 5).

[0146] Example 4

[0147] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 7).

[0148] Example 5

[0149] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 10).

[0150] Example 6

[0151] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 11).

[0152] Example 7

[0153] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 14).

[0154] Example 8

[0155] The difference from Example 1 is that the cyclic phosphoric acid compound is (i.e., the compound with structural formula 16).

[0156] Example 9

[0157] The difference from Example 7 is that the amount of compound of structural formula 14 added is 0.2 wt%.

[0158] Example 10

[0159] The difference from Example 7 is that the amount of compound of structural formula 14 added is 0.5 wt%.

[0160] Example 11

[0161] The difference from Example 7 is that the amount of compound of structural formula 14 added is 1.5 wt%.

[0162] Example 12

[0163] The difference from Example 7 is that the amount of compound of structural formula 14 added is 2.0 wt%.

[0164] Comparative Example 1

[0165] The difference from Example 1 is that no cyclic phosphoric acid compound is added.

[0166] Comparative Example 2

[0167] The difference from Example 7 is that the amount of compound of structural formula 14 added is 2.5 wt%.

[0168] Battery performance test:

[0169] Lithium replenishment activation process: At 45°C, the above lithium-ion batteries were charged to 3.4V with constant current at 0.05C / 0.1C, then charged to 3.65V with constant current and constant voltage at 0.2C, then charged to 4.1V with constant current at 0.02C, and discharged to 2.5V with constant current.

[0170] 25℃ Cycling Performance: Under 25℃ conditions, the above lithium-ion battery was charged to 3.65V at 1C constant current and constant voltage, and then discharged to 2.5V at 1C constant current. This charge / discharge cycle was repeated 800 times, and the capacity retention rate of the lithium-ion battery was measured.

[0171] 45℃ Cycling Performance: The above lithium-ion batteries were placed at a high temperature of 45℃ for 3 hours, then charged at 1C constant current and constant voltage to 3.65V, and discharged at 1C constant current to 2.5V. This charge / discharge cycle was repeated 500 times, and the capacity retention rate of the lithium-ion batteries was measured.

[0172] Volume expansion rate after 30 days of storage at 60℃: The lithium-ion battery was charged to a full charge of 3.65V at 1C constant current at 25℃, and then placed in a high and low temperature chamber at 60℃ for 30 days. The volume expansion rate of the lithium-ion battery after 30 days of high-temperature storage was calculated using the water displacement method.

[0173] The battery performance of the embodiments and comparative examples in this application is detailed in Table 1.

[0174] Table 1 shows the battery performance of the embodiments and comparative examples in this application.

[0175]

[0176]

[0177]

[0178] As can be seen from the comparison between Examples 1 to 12 of this application and Comparative Example 1, the addition of cyclic phosphate compounds to the electrolyte of this application results in the P=O bond in the cyclic phosphate compound being a strong electrophilic center, which is easily attacked by the nucleophilic free radicals. It undergoes ring-opening polymerization on the surface of the positive electrode and preferentially oxidizes to form a dense and stable CEI film containing a PO-Li network. This effectively reduces the content of reactive oxygen species, inhibits the oxidative decomposition of the electrolyte, reduces gas production, and thus improves the high-temperature performance of the battery.

[0179] As can be seen from the comparison between Example 2 and Example 1 of this application, the unsaturated bonds on the cyclic phosphate compound R1 are preferentially reduced and decomposed on the negative electrode surface to form SEI, which improves the stability of the system and enhances the performance of the battery.

[0180] A comparison of Example 3 and Example 2 of this application shows that the R2 and R3 bonds in the cyclic phosphoric acid compound form a ring with 3 to 6 carbon atoms, wherein the ring with 3 to 6 carbon atoms includes at least one unsaturated bond, i.e., an unsaturated carbon ring. When the unsaturated carbon ring is not a benzene ring, under high battery voltage (e.g., exceeding 4.1V), the electrolyte preferentially oxidizes, and the unsaturated bond undergoes ring-opening polymerization to form a CEI film, reducing the dissolution of transition metals and thus improving battery performance.

[0181] A comparison of Examples 4 and 3, and Examples 6 and 5 of this application shows that a benzene ring is formed between R2 and R3 of the cyclic phosphoric acid compound. The positive electrode lithium supplementation stage requires a voltage of 4.1V or higher. The conjugated structure of the benzene ring has higher oxidative stability, resulting in a more robust aromatic polymer film with stronger high-voltage resistance and a more significant reduction in gas production. The benzene ring adheres to the surface of the positive electrode active material through π-π stacking or van der Waals forces, forming a physical isolation layer that reduces direct contact between the electrolyte and the positive electrode active material, inhibiting the dissolution of iron from the positive electrode active material and the lithium supplementation agent. However, due to the relatively high impedance generated by the benzene ring, the cycle performance and high-temperature cycle performance of Example 6 are lower than those of Example 5.

[0182] As can be seen from the comparison between Example 5 and Example 3 of this application, when R1, R2, and R3 of the cyclic phosphoric acid compound contain at least one halogen substituent, such as a fluorine substituent, the high electronegativity of the fluorine substituent can reduce the HOMO energy level, improve the oxidation stability of the cyclic phosphoric acid compound additive, and the strong CF bond can resist free radical attack. The formation of LiF can optimize the inorganic components of CEI, thereby improving the cycle performance and high-temperature cycle performance of the battery.

[0183] As can be seen from the comparison between Example 7 and Example 6 of this application, the cyclic phosphoric acid compounds R1, R2, and R3 contain at least a trifluoroethoxy group. The strong electron-withdrawing property of the fluorine atom in the trifluoroethoxy group lowers the LUMO energy level of the cyclic phosphoric acid compound molecule, which preferentially decomposes on the negative electrode surface over organic solvents, forming a stable, dense, and inorganic (LiF)-rich SEI film.

[0184] As can be seen from the comparison between Example 8 and Example 4 of this application, the cyclic phosphoric acid compounds R1, R2, and R3 contain at least a cyano group with 3 to 9 carbon atoms, either substituted or unsubstituted. The cyano group works in concert with the P=O bond of the cyclic phosphate ester to form an inorganic-organic composite SEI / CEI film containing Li3PO4 and lithium cyanide (LiCN), which has both high ionic conductivity and mechanical strength, thereby improving the cycle performance of the battery.

[0185] Depend on Figure 1 , Figure 2 , Figure 3It is evident that insufficient addition of cyclic phosphate compounds fails to form a dense and stable protective film on the positive electrode surface, thus failing to effectively suppress battery gas production and resulting in poor electrochemical performance. With the increase of cyclic phosphate compound additives, the P=O bond in the compound of formula 14, being a strongly electrophilic center, is susceptible to nucleophilic attack by reactive oxygen free radicals. This leads to ring-opening polymerization on the positive electrode surface, preferentially oxidizing to form a dense and stable CEI film containing a PO-Li network. This effectively reduces reactive oxygen content, inhibits electrolyte oxidative decomposition, reduces gas production, and consequently improves the battery's high-temperature performance. Benzene rings adhere to the surface of the positive electrode active material via π-π stacking or van der Waals forces, forming a physical isolation layer that reduces direct contact between the electrolyte and the positive electrode active material, inhibiting the dissolution of iron from the positive electrode active material and lithium supplement. The strong electron-withdrawing property of fluorine atoms in the trifluoroethoxy group lowers the LUMO energy level of the molecule, preferentially reducing and decomposing it on the graphite negative electrode surface before the solvent, forming a stable, dense, and inorganic (LiF)-rich SEI film. This additive, through the synergistic effect of the P=O bonds on the ring, the benzene ring, and the trifluoroethoxy group, plays multiple protective functions in the positive electrode, negative electrode, and electrolyte, thereby effectively improving the cycle life of the battery. However, when the amount of cyclic phosphate compound additive is too large, the viscosity and impedance of the electrolyte increase, and the risk of corrosion of the positive electrode current collector aluminum foil also increases. When the amount exceeds 2%, the battery performance drops sharply.

[0186] The technical solutions provided by the embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. An electrolyte, characterized in that, This includes cyclic phosphoric acid compounds, which comprise one or more combinations of compounds shown in Formula 1: Formula 1 R1 is selected from any one of the following conditions: a. Halogen substituents; b. Alkyl or alkoxy groups having 1 to 6 carbon atoms, substituted or unsubstituted; c. Alkenyl, ynyl, alkenyloxy, and ynyloxy groups having 3 to 9 carbon atoms, substituted or unsubstituted; d. Any one of an aromatic group having 6 to 10 carbon atoms, whether substituted or unsubstituted; f. A cyano group having 3 to 9 carbon atoms, substituted or unsubstituted; R2 and R3 each independently satisfy any one of the following conditions: (1) Selected from hydrogen atoms and halogen substituents; (2) Alkyl groups having 1 to 6 carbon atoms, whether substituted or unsubstituted; (3) Alkenyl or ynyl groups having 3 to 9 carbon atoms, whether substituted or unsubstituted; (4) Any one of an aromatic group having 6 to 10 carbon atoms, whether substituted or unsubstituted; (5) R2 and R3 bond together to form a ring with 3 to 6 carbon atoms.

2. The electrolyte according to claim 1, characterized in that, The cyclic phosphate compound forms a benzene ring by bonding between R2 and R3.

3. The electrolyte according to claim 1 or 2, characterized in that, The compound represented by Formula 1 is any one of the following compounds:

4. The electrolyte according to claim 1, characterized in that, The cyclic phosphoric acid compound R1, R2, and R3 contain at least one halogen substituent.

5. The electrolyte according to claim 1 or 4, characterized in that, The compound represented by Formula 1 is any one of the following compounds:

6. The electrolyte according to claim 1, characterized in that, R1 of the cyclic phosphate compound contains a trifluoromethyl group.

7. The electrolyte according to claim 1 or 6, characterized in that, The compound represented by Formula 1 is any one of the following compounds:

8. The electrolyte according to claim 1, characterized in that, The R1 of the cyclic phosphate compound contains at least one unsaturated bond.

9. The electrolyte according to claim 1 or 8, characterized in that, The compound represented by Formula 1 is any one of the following compounds:

10. The electrolyte according to claim 1, characterized in that, The cyclic phosphate compound forms a ring with 3 to 6 carbon atoms by bonding between R2 and R3, wherein the ring with 3 to 6 carbon atoms includes at least one unsaturated bond.

11. The electrolyte according to claim 1 or 10, characterized in that, The compound represented by Formula 1 is any one of the following compounds:

12. The electrolyte according to claim 1, characterized in that, The ratio of the cyclic phosphate compound to the total mass of the electrolyte ranges from 0.1 wt% to 2.0 wt%.

13. A secondary battery, characterized in that, The electrolyte includes any one of claims 1 to 12.

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