Amide-based co-crystal electrolyte, preparation method thereof and lithium metal secondary battery

Abstract

The application relates to an amide-based eutectic electrolyte, a preparation method thereof and a lithium metal secondary battery, and belongs to the technical field of lithium metal secondary batteries. The amide-based eutectic electrolyte comprises a lithium salt, an organic solvent and an amide-based eutectic solvent; the amide-based eutectic solvent comprises an amide-based compound shown in structural formula (I). In the amide-based eutectic electrolyte, the amide-based eutectic solvent shown in structural formula (I) can produce a eutectic effect with the organic solvent, the hydrogen bond formed by the eutectic effect greatly improves the oxidation resistance of the cluster, can stabilize the positive electrode of the lithium metal secondary battery, forms a CEI rich in Li3N, and the introduction of the amide-based solvent can slightly solubilize the lithium salt, which is beneficial to the formation of a stable SEI of the amide-based eutectic electrolyte on the negative electrode side of the lithium metal secondary battery, so that the lithium metal secondary battery has good normal-temperature cycle performance and high-temperature cycle performance under high voltage.

Description

Technical Field

[0001] This invention relates to the field of lithium metal secondary battery technology, and in particular to an amide-based eutectic electrolyte, its preparation method, and a lithium metal secondary battery. Background Technology

[0002] In recent years, with the popularization and development of portable electronic devices, power tools, and electric vehicles, lithium metal batteries have received widespread attention as a new type of high-energy power source. Currently, the main cathode materials for commercially available lithium metal secondary batteries include lithium manganese oxide, lithium cobalt oxide, ternary materials, and lithium iron phosphate. With the development and popularization of lithium metal secondary batteries with higher voltage systems, conventional electrolytes, in addition to being prone to self-oxidation and decomposition, will also undergo oxidation and decomposition on the surface of the cathode material (especially on the surface of cathode materials containing transition metal elements), affecting the high-temperature and ambient-temperature cycling performance of lithium metal secondary batteries.

[0003] Nickel-rich NCM (LiNi) x Mn y Co 1-x-y O2, and x>0.6) lithium metal secondary batteries, especially LiNi 0.8 Mn 0.1 Co 0.1 O2 (NCM 811) lithium metal secondary batteries are promising due to their enhanced capacity when combined with an increased upper cutoff voltage. However, maintaining high performance at high voltages remains a challenge, as these batteries suffer from degradation through multiple pathways, such as irreversible phase transitions, oxygen release, dissolution and crosstalk effects of transition metals (TMs), stress corrosion cracking, and continuous growth of the cathode-electrolyte interface (CEI).

[0004] In related technologies, the use of electrolytes containing perfluorinated compounds is expected to improve the high-voltage LiNi. 0.8 Mn 0.1 Co 0.1 The stability of the interfacial chemistry in O2 (NCM 811) lithium metal secondary batteries is a concern. However, perfluorinated compounds exhibit extreme stability and are difficult to degrade in the natural environment, potentially leading to environmental pollution. Furthermore, the recycling and treatment of electrolytes containing perfluorinated compounds require more complex processes, increasing the difficulty and cost of handling them.

[0005] Therefore, determining which high-voltage resistant electrolyte to use to achieve stable cycling of high-energy-density, high-voltage lithium metal secondary batteries is a problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] Based on this, the purpose of this invention is to provide an amide-based eutectic electrolyte, its preparation method, and a lithium metal secondary battery. The amide-based eutectic electrolyte is used in a lithium metal secondary battery, enabling the lithium metal secondary battery to have good room temperature cycling performance and high temperature cycling performance at high voltage (e.g., 4.6-4.8V).

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] The first aspect of the present invention is to provide an amide-based eutectic electrolyte, comprising a lithium salt, an organic solvent, and an amide-based eutectic solvent; wherein the organic solvent is at least one selected from chain carbonates, cyclic carbonates, carboxylic acid esters, ethers, and heterocyclic compounds; and the amide-based eutectic solvent comprises an amide compound represented by structural formula (I).

[0009]

[0010] In structural formula (Ⅰ), R1 is selected from one of amino, methyl, ethyl, methoxy, phenyl, difluoromethyl, trifluoromethyl, nitrile, and trimethylsilyl; R2 and R3 are each independently selected from one of H element, amino, methyl, ethyl, methoxy, phenyl, difluoromethyl, trifluoromethyl, nitrile, and trimethylsilyl.

[0011] The amide-based eutectic electrolyte of the present invention comprises a lithium salt, the aforementioned organic solvent, and an amide-based eutectic solvent as shown in structural formula (I). The organic solvent is at least one of chain carbonates, cyclic carbonates, carboxylic acid esters, and ethers. By adding the amide-based eutectic solvent as shown in structural formula (I), the present invention enables the amide-based eutectic solvent to undergo eutectic interaction with the organic solvent in the amide-based eutectic electrolyte. The hydrogen bonds formed (e.g., NHO) significantly enhance the antioxidant capacity of the clusters, stabilizing the positive electrode of lithium metal secondary batteries (e.g., NCM811 lithium metal secondary batteries, LCO lithium metal secondary batteries, LR lithium metal secondary batteries, etc.), forming a Li3N-rich CEI. Furthermore, the introduction of the amide-based solvent can slightly solubilize the lithium salt (e.g., lithium nitrate), which is beneficial for the formation of a stable SEI on the negative electrode side of the lithium metal secondary battery by the amide-based eutectic electrolyte. The amide-based eutectic electrolyte of this invention, when used in lithium metal secondary batteries, enables the lithium metal secondary batteries to exhibit good room-temperature cycling performance and high-temperature cycling performance at high voltages (e.g., 4.6-4.8V). Furthermore, the amide-based eutectic solvent shown in structural formula (I) of this invention is biodegradable, conforming to the principles of green chemistry.

[0012] As a preferred embodiment, the amide compound represented by structural formula (I) is selected from at least one of the compounds represented by structural formulas (I-1), (I-2), (I-3), (I-4), (I-5), and (I-6);

[0013]

[0014]

[0015] As a preferred embodiment, the mass of the amide-based eutectic solvent accounts for 10-35% of the total mass of the amide-based eutectic electrolyte, specifically 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, and 35%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0016] As a preferred embodiment, the lithium salt is selected from at least one of lithium dioxaburate, lithium difluorooxaburate, lithium difluorophosphate, lithium difluorobis(oxaburate) phosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, and lithium nitrate.

[0017] As a preferred embodiment, the lithium salt accounts for 5-30% of the total mass of the amide-based eutectic electrolyte, specifically 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, and 30%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0018] As a preferred embodiment, the organic solvent is selected from at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), fluoroethylene carbonate (FEC), butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), propyl propionate (n-PP), ethyl propionate (EP), and ethyl butyrate (Eb).

[0019] As a preferred embodiment, the organic solvent accounts for 45-75% of the total mass of the amide-based eutectic electrolyte, specifically 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 71%, 77%, 73%, 74%, and 75%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0020] As a preferred embodiment, the amide-based eutectic electrolyte further includes an additive selected from at least one of vinylene carbonate (VC), 1,3-propanesulfonate lactone (PS), 1,4-butanesulfonate lactone (BS), acrylate lactone (RPS), ethylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), and lithium difluorooxalate borate (LiDFOB).

[0021] As a preferred embodiment, the additive accounts for 0.1-8% of the total mass of the amide-based eutectic electrolyte, specifically, but not limited to, 0.1%, 0.5%, 1.5%, 2%, 2.5%, 3%, 4%, 4.5%, 5%, 5.5%, 6.0%, 7%, and 8%. Preferably, the additive accounts for 0.5-2% of the total mass of the amide-based eutectic electrolyte. The addition of the additive can further improve the room temperature cycle performance and high temperature storage performance of lithium metal secondary batteries.

[0022] A second aspect of the present invention is to provide a method for preparing the amide-based eutectic electrolyte as described above, comprising the following steps: mixing the various substances uniformly to obtain the amide-based eutectic electrolyte.

[0023] A third aspect of the present invention is to provide a lithium metal secondary battery, comprising a positive electrode, a negative electrode, and an amide-based eutectic electrolyte as described above. The lithium metal secondary battery containing the amide-based eutectic electrolyte of the present invention can still achieve good room temperature cycling performance and high temperature cycling performance at a maximum charging voltage of 4.8V.

[0024] As a preferred embodiment, the active material of the positive electrode includes lithium iron phosphate (LiFePO4). 4、 LiCoO2 and Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b At least one of them; Li (1+a) Ni x Co y M z N 1-x-y-z O2+b In this context, M represents Mn or Al, N represents any one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, Ba, V, and Ti, and -0.10 ≤ a ≤ 0.50. <x<1,0<y<1,0<z<1,0.7<x+y+z≤1,-0.05≤b≤0.10。

[0025] As a preferred embodiment, the negative electrode of a lithium metal secondary battery is lithium metal. Commonly used negative electrode materials include at least one of artificial graphite, natural graphite, Si and its alloys, Sn and its alloys, lithium metal and its alloys, transition metal oxides, and lithium titanate. Among these negative electrode materials, lithium metal negative electrodes have a high theoretical specific capacity (mAh / g), but they are prone to forming lithium dendrites during cycling, leading to a decrease in battery cycle stability. The amide-based eutectic solvent shown in structural formula (I) of this invention is compatible with lithium metal and can solubilize lithium salts (e.g., lithium nitrate) to a certain extent, enabling the amide-based eutectic electrolyte to form a better SEI layer on the lithium metal negative electrode during the formation stage, allowing the lithium metal secondary battery to cycle stably under high voltage conditions.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] The amide-based eutectic electrolyte of this invention uses an amide-based eutectic solvent as shown in structural formula (Ⅰ), which can undergo eutectic reaction with the organic solvent. The hydrogen bonds (NHO) formed significantly enhance the antioxidant capacity of its clusters, stabilizing the positive electrode of lithium metal secondary batteries (e.g., NCM811 lithium metal secondary batteries, LCO lithium metal secondary batteries, LR lithium metal secondary batteries, etc.) and forming a Li3N-rich CEI. Furthermore, the introduction of the amide-based solvent can slightly solubilize lithium salts (e.g., lithium nitrate), which is beneficial for the formation of a stable SEI on the negative electrode side of the lithium metal secondary battery. When the amide-based eutectic electrolyte of this invention is used in lithium metal secondary batteries, it enables the lithium metal secondary batteries to have good room temperature cycling performance and high temperature cycling performance at high voltages (e.g., 4.6-4.8V). Attached Figure Description

[0028] To better understand and implement this invention, the following detailed description is provided in conjunction with the accompanying drawings.

[0029] Figure 1 The rate performance diagram of Li metal / LCO high-voltage lithium metal secondary battery;

[0030] Figure 2 For the electrolytes of Comparative Example 1, Example 4 and Example 5 1 HNMR spectrum;

[0031] Figure 3For the electrolytes of Comparative Example 1, Example 3 and Example 5 1 HNMR image. Detailed Implementation

[0032] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0033] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0034] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0035] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0036] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0037] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below with reference to specific embodiments. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it.

[0038] Example 1

[0039] This embodiment provides an amide-based eutectic electrolyte, comprising a lithium salt, an organic solvent, an amide-based eutectic solvent, and additives.

[0040] The lithium salts include lithium bis(fluorosulfonyl)imide (LiFSI) and lithium nitrate (LiNO3); the organic solvents include ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); the amide-based eutectic solvent is an amide compound represented by structural formula (Ⅰ); and the additive is lithium difluorooxalate borate (LiDFOB). For the specific composition and dosage of each component in the amide-based eutectic electrolyte of this embodiment, please refer to Tables 1 and 2.

[0041] This embodiment also provides a method for preparing an amide-based eutectic electrolyte, comprising the following steps:

[0042] According to the proportions in Table 1, the organic solvent and lithium salt are mixed evenly, and then the additives and amide eutectic solvent are added and mixed evenly to obtain the final product.

[0043] Example 2-12

[0044] Examples 2-12 provide an amide-based eutectic electrolyte, comprising a lithium salt, an organic solvent, an amide-based eutectic solvent, and an additive.

[0045] The lithium salts include lithium bis(fluorosulfonyl)imide (LiFSI) and lithium nitrate (LiNO3); the organic solvents include ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); the amide eutectic solvent is an amide compound of structural formula (Ⅰ); and the additive is lithium difluorooxalate borate (LiDFOB). For the specific composition and dosage of each component in the amide eutectic electrolytes of Examples 2-12, please refer to Tables 1 and 2.

[0046] The preparation methods of the amide-based eutectic electrolytes in Examples 2-12 are the same as those in Example 1.

[0047] Comparative Example 1

[0048] This comparative example provides an electrolyte comprising a lithium salt and an organic solvent.

[0049] The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvents include ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC). For the specific composition and dosage of each component in the electrolyte of this comparative example, please refer to Table 1.

[0050] The preparation method of the electrolyte in this comparative example includes the following steps: the organic solvent and lithium salt are mixed evenly according to the proportions in Table 1.

[0051] Comparative Example 2

[0052] This comparative example provides an electrolyte comprising a lithium salt, an organic solvent, and additives.

[0053] The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvents include ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); and the additive is lithium difluorooxalate borate (LiDFOB). For the specific composition and dosage of each component in the electrolyte of this comparative example, please refer to Table 1.

[0054] The preparation method of the electrolyte in this comparative example includes the following steps: according to the proportions in Table 1, the organic solvent and lithium salt are mixed evenly, and then the additives are added and mixed evenly to obtain the electrolyte.

[0055] Comparative Example 3

[0056] This comparative example provides an electrolyte comprising a lithium salt and an organic solvent.

[0057] The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvents include ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC). For the specific composition and dosage of each component in the electrolyte of this comparative example, please refer to Table 1.

[0058] The preparation method of the electrolyte in this comparative example includes the following steps: the organic solvent and lithium salt are mixed evenly according to the proportions in Table 1.

[0059] Comparative Example 4

[0060] This comparative example provides an electrolyte comprising a lithium salt, an organic solvent, and additives.

[0061] The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvents include ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); and the additive is lithium difluorooxalate borate (LiDFOB). For the specific composition and dosage of each component in the electrolyte of this comparative example, please refer to Table 1.

[0062] The preparation method of the electrolyte in this comparative example includes the following steps: according to the proportions in Table 1, the organic solvent and lithium salt are mixed evenly, and then the additives are added and mixed evenly to obtain the electrolyte.

[0063] Table 1 Electrolyte composition of Examples 1-12 and Comparative Examples 1-4

[0064]

[0065]

[0066] The structural formulas of the compounds in Table 1 and Table 2 are listed below.

[0067]

[0068] Examples 13-24, Comparative Examples 5-8

[0069] Examples 13-24 and Comparative Examples 5-8 each provide a lithium metal secondary battery, specifically an LCO lithium metal secondary battery.

[0070] The LCO lithium metal secondary batteries of Examples 13-24 were prepared from the amide-based eutectic electrolytes of Examples 1-12, respectively.

[0071] The LCO lithium metal secondary batteries of Comparative Examples 5-8 were prepared from the electrolytes of Comparative Examples 1-4, respectively.

[0072] Preparation methods of LCO lithium metal secondary batteries in Examples 13-24 and Comparative Examples 5-8:

[0073] The amide-based eutectic electrolytes of Examples 1-12 and the electrolytes of Comparative Examples 1-4 were injected into button lithium metal batteries with LiCoO2 (LCO) as the positive electrode and lithium metal as the negative electrode. The batteries were charged three times with a constant current of 0.1C, and then charged and discharged to 4.6V with a constant current of 0.5C, and discharged to 2.8V with a constant current of 0.5C.

[0074] Examples 25-36, Comparative Examples 9-12

[0075] Examples 25-36 and Comparative Examples 9-12 each provide a lithium metal secondary battery, specifically an NCM811 lithium metal secondary battery.

[0076] The NCM811 lithium metal secondary batteries of Examples 25-36 were prepared from the amide-based eutectic electrolytes of Examples 1-12, respectively.

[0077] The NCM811 lithium metal secondary batteries of Comparative Examples 9-12 were prepared from the electrolytes of Comparative Examples 1-4, respectively.

[0078] Preparation methods of NCM811 lithium metal secondary batteries in Examples 13-24 and Comparative Examples 9-12:

[0079] The amide-based eutectic electrolytes of Examples 1-12 and the electrolytes of Comparative Examples 1-4 were respectively injected into a ternary material LiNi 0.8 Co 0.1 Mn 0.1 In a button lithium metal battery with O2 (NCM811) as the positive electrode and lithium metal as the negative electrode, it is charged and discharged three times with a constant current of 0.2C, then charged to 4.7V with a constant current of 0.5C, and discharged to 2.8V with a constant current of 0.5C.

[0080] Examples 37-48, Comparative Examples 13-16

[0081] Examples 37-48 and Comparative Examples 13-16 each provide a lithium metal secondary battery, specifically an LR lithium metal secondary battery.

[0082] The LR lithium metal secondary batteries of Examples 37-48 were prepared from the amide-based eutectic electrolytes of Examples 1-12, respectively.

[0083] The LR lithium metal secondary batteries of Comparative Examples 13-16 were prepared from the electrolytes of Comparative Examples 1-4, respectively.

[0084] Preparation methods of LR lithium metal secondary batteries in Examples 37-48 and Comparative Examples 13-16:

[0085] The amide-based eutectic electrolytes of Examples 1-12 and the electrolytes of Comparative Examples 1-4 were respectively injected into Li 1.12 Ni 0.13 Co 0.3 Mn 0.54 In a button lithium metal battery with O2(LR) as the positive electrode and lithium metal as the negative electrode, it is charged and discharged three times with a constant current of 0.2C, then charged to 4.8V with a constant current of 0.33C, and discharged to 2V with a constant current of 5C.

[0086] Performance testing

[0087] The LCO lithium metal secondary batteries of Examples 13-24, NCM811 lithium metal secondary batteries of Examples 25-36, and LR lithium metal secondary batteries of Examples 37-48, prepared with the amide-based eutectic electrolytes of Examples 1-12, and the LCO lithium metal secondary batteries of Comparative Examples 5-8, NCM811 lithium metal secondary batteries of Comparative Examples 9-12, and LR lithium metal secondary batteries of Comparative Examples 13-16, prepared with the electrolytes of Comparative Examples 1-4, were subjected to room temperature cycling tests and high temperature cycling tests under the following test conditions:

[0088] (1) Room temperature cycling performance test:

[0089] Under normal temperature (25℃) storage conditions:

[0090] The LCO lithium metal secondary battery was charged to 4.6V at a constant current of 0.5C and discharged to 2.8V at a constant current of 0.5C. The cycle performance was evaluated after 100 cycles.

[0091] The formed NCM811 lithium metal secondary battery was charged to 4.7V at a constant current of 0.5C and discharged to 2.8V at a constant current of 0.5C. The cycle performance was evaluated after 100 cycles.

[0092] The formed LR lithium metal secondary battery was charged to 4.8V at a constant current of 0.33C and discharged to 2V at a constant current of 5C. The cycle performance was evaluated after 100 cycles.

[0093] Cyclic performance is calculated using the capacity retention rate formula below:

[0094] Capacity retention rate (%) = (Discharge capacity of the 100th cycle / Initial discharge capacity) × Discharge capacity per cycle.

[0095] (2) High-temperature cycling performance test:

[0096] Under high temperature (60℃) storage conditions:

[0097] The LCO lithium metal secondary battery was charged to 4.6V at a constant current of 0.5C and discharged to 2.8V at a constant current of 0.5C. The cycle performance was evaluated after 100 cycles.

[0098] The NCM811 lithium metal secondary battery was charged to 4.7V at a constant current of 0.5C and discharged to 2.8V at a constant current of 0.5C. The cycle performance was evaluated after 100 cycles.

[0099] The LR lithium metal secondary battery was charged to 4.8V at a constant current of 0.33C and discharged to 2V at a constant current of 5C. The cycle performance was evaluated after 100 cycles.

[0100] Cyclic performance is calculated using the capacity retention rate formula below:

[0101] Capacity retention rate (%) = (Discharge capacity of the 100th cycle / Initial discharge capacity) × Discharge capacity per cycle.

[0102] The test results are shown in Tables 3 and 4:

[0103] Table 3. Cyclic performance test results of lithium metal secondary batteries at room temperature (25℃)

[0104]

[0105]

[0106] Table 4. High-temperature (60℃) cycle performance test results of lithium metal secondary batteries

[0107]

[0108]

[0109] As can be seen from Tables 3 and 4, the lithium metal secondary batteries prepared with the amide-based eutectic electrolytes of Examples 1-12 of the present invention (LCO lithium metal secondary batteries of Examples 13-24, NCM811 lithium metal secondary batteries of Examples 25-36, and LR lithium metal secondary batteries of Examples 37-48) showed significant improvements in both room temperature cycling performance and high temperature cycling performance.

[0110] This is because the amide-based eutectic electrolytes in Examples 1-12 of this invention use amide-based compounds (specifically, amino compounds shown in structural formulas (I-1), (I-2), (I-3), (I-4), (I-5), and (I-6)) as the eutectic solvent for the electrolyte. This amide-based eutectic solvent can undergo eutectic reaction with the organic solvent to form hydrogen bonds (NHO), which greatly enhances the antioxidant capacity of its clusters and stabilizes the positive electrode of lithium metal secondary batteries (NCM811 lithium metal secondary batteries, LCO lithium metal secondary batteries, LR lithium metal secondary batteries, etc.), forming a CEI rich in Li3N. Furthermore, the introduction of the amide-based eutectic solvent can slightly solubilize lithium nitrate, which is beneficial for the formation of a stable SEI on the negative electrode side of the lithium metal secondary battery by the amide-based eutectic electrolyte, enabling the lithium metal secondary battery to have good room temperature cycling performance and high temperature cycling performance at high voltage (4.6-4.8V).

[0111] Furthermore, test data from the LCO lithium metal secondary battery of Example 17, the NCM811 lithium metal secondary battery of Example 29, and the LR lithium metal secondary battery of Example 41, prepared with the amide-based eutectic electrolyte of Example 5, show that their room temperature cycling performance and high temperature cycling performance are excellent. The amide-based eutectic electrolyte of Example 5 uses an amide compound with structural formula (Ⅰ-5) as the amide-based eutectic solvent. Compared to other amide-based eutectic solvents mentioned in other embodiments of the present invention, the amide compound with structural formula (Ⅰ-5) has -NH-CH3 and C≡N in its structure. The -NH-CH3 can co-crystallize with organic solvents, forming hydrogen bonds (NHO) to enhance the antioxidant capacity of its clusters; while C≡N can form atypical hydrogen bonds with CH3 in the organic solvents (FEC, EMC), resulting in the most superior antioxidant performance of the clusters.

[0112] The test data of lithium metal secondary batteries prepared with the amide-based eutectic electrolytes in Comparative Examples 1-6 also show that the lithium metal secondary batteries prepared using the amide compound shown in structural formula (I-3) as the amide-based eutectic solvent have slightly worse room temperature and high temperature cycling performance. This may be because the amide compound shown in structural formula (I-3) has a -NH2 group in its structure, resulting in a lower impedance and stability of the SEI film. In addition, due to the presence of its -NH2 group structure, it is very easy to generate gas. The lithium metal secondary batteries prepared with the amide compound shown in structural formula (I-3) as the amide-based eutectic solvent have slightly worse high temperature cycling performance than those prepared with the amide compound shown in structural formula (I-2) as the amide-based eutectic solvent. Furthermore, because the two H atoms in the -NH2 group can rotate under other forces in the electrolyte, the reactivity of -NH2 is greatly increased, resulting in poorer performance. In addition, the test data of lithium metal secondary batteries prepared with the amide eutectic electrolytes of Comparative Examples 1-3 show that although the -NH2 group can also form hydrogen bonds with organic solvent molecules, it is easy to decompose at high temperature due to its high reactivity. As the -NH2 group gradually decreases in Compound I, its electrochemical performance is continuously enhanced.

[0113] The lithium metal secondary batteries prepared using the electrolytes of Comparative Examples 1-4 (LCO lithium metal secondary batteries of Comparative Examples 5-8, NCM811 lithium metal secondary batteries of Comparative Examples 9-12, and LR lithium metal secondary batteries of Comparative Examples 13-16) showed significantly poorer performance in both room temperature and high temperature cycling. Furthermore, data from Comparative Examples 1-4 indicate that adding an additive (lithium difluorooxalate borate (LiDFOB)) to the electrolyte can further improve the room temperature and high temperature cycling performance of the lithium metal secondary batteries.

[0114] The test data of the amide-based eutectic electrolytes and the corresponding lithium metal secondary batteries obtained from Comparative Examples 1-6 show that introducing different side groups onto the amide-based eutectic solvent shown in structural formula (I) has a significant impact on its eutectic effect and the antioxidant performance of the solvent itself. Among them, the compounds shown in structural formula (I-4) of Example 4 are trifluoromethyl groups introduced into structural formula (I), the compounds shown in structural formula (I-5) of Example 5 are nitrile groups introduced into structural formula (I), and the compounds shown in structural formula (I-6) of Example 6 are trimethylsilyl groups introduced into structural formula (I). All of these can improve the antioxidant capacity of the amide-based eutectic solvent to a certain extent. However, the trifluoromethyl and trimethylsilyl groups have large steric hindrances and their eutectic ability with organic solvent molecules is weak, so their performance is slightly worse.

[0115] This invention also compares the rate performance of the LCO lithium metal secondary battery prepared with the amide-based eutectic electrolyte of Example 5 and the LCO lithium metal secondary battery prepared with the electrolyte of Comparative Example 1. Please refer to [link to relevant documentation]. Figure 2 The performance of the LCO lithium metal secondary battery prepared by the amide-based eutectic electrolyte in Example 5 after multiple cycles was significantly better than that of the LCO lithium metal secondary battery prepared by the electrolyte in Comparative Example 1.

[0116] The present invention also obtained the amide-based eutectic electrolytes of Examples 4 and 5, and the electrolyte of Comparative Example 1. 1 Please refer to the HNMR spectrum. Figure 2 and Figure 3 , Figure 2 This indicates that hydrogen bonds were formed in the amide eutectic electrolytes of Examples 4 and 5. Figure 3 This indicates that the amide-based eutectic electrolyte of Example 5 also forms atypical hydrogen bonds, and the coupling between typical and atypical hydrogen bonds strengthens the hydrogen bonds.

[0117] In summary, the amide-based eutectic electrolyte of this invention can form NHO hydrogen bonds with O in the electrolyte, thereby increasing the antioxidant capacity of the cluster molecules and improving the oxidation stability of the electrolyte. The high-voltage lithium metal secondary battery assembled with it can achieve long-term stable cycling and improve the rate performance of the high-voltage lithium metal secondary battery.

[0118] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An amide-based co-crystal electrolyte, characterized in that: The lithium salt comprises lithium nitrate; the organic solvent comprises methyl ethyl carbonate and fluoroethylene carbonate; and the amide-based co-crystal solvent is an amide-based compound shown in structural formula (I-5). The mass of the lithium salt accounts for 5-30% of the total mass of the amide-based co-crystal electrolyte; the mass of the organic solvent accounts for 45-75% of the total mass of the amide-based co-crystal electrolyte; and the mass of the amide-based co-crystal solvent accounts for 10-35% of the total mass of the amide-based co-crystal electrolyte.

2. The amide co-crystal electrolyte of claim 1, wherein: The lithium salt further comprises at least one of lithium bis(oxalato)borate, lithium bis(difluoro oxalato)borate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methylsulfonate, lithium trifluoromethylsulfonate, lithium bisfluorosulfonylimide and lithium bistrifluoromethylsulfonylimide.

3. The amide co-crystal electrolyte of claim 1, wherein: The organic solvent further comprises at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

4. The amide co-crystal electrolyte of claim 1, wherein: The auxiliary agent is selected from at least one of vinylene carbonate, 1,3-propane sulfone lactone, 1,4-butane sulfone lactone, acrylic acid lactone, vinyl ethylene carbonate, fluoroethylene carbonate, lithium difluorophosphate, lithium bis(difluoro oxalato)borate.

5. The amide co-crystal electrolyte of claim 4, wherein: The auxiliary agent accounts for 0.1-8.0% of the total mass of the amide-based co-crystal electrolyte.

6. A process for the preparation of the amide co-crystal electrolyte according to any one of claims 1-5, characterized in that: The substances are mixed uniformly to obtain the amide-based co-crystal electrolyte.

7. A lithium metal secondary battery, characterized by: The amide-based co-crystal electrolyte of any one of claims 1-5.

8. The lithium metal secondary battery according to claim 7, characterized by: Also included are a positive electrode and a negative electrode; the active material of the positive electrode includes at least one of lithium iron phosphate LiFePO4, LiCoO2, Li （1+a） Ni x Co y M z N 1-x-y-z O 2+b Li （1+a） Ni x Co y M z N 1-x-y-z O 2+b wherein M is Mn or Al, N is any one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, Ba, V, and Ti, -0.10≤a≤0.50, 0

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