Amido eutectic electrolyte, preparation method thereof and lithium metal secondary battery

Abstract

The invention relates to an amido eutectic electrolyte, a preparation method thereof and a lithium metal secondary battery, and belongs to the technical field of lithium metal secondary batteries. The amido eutectic electrolyte comprises a lithium salt, an organic solvent and an amido eutectic solvent, the amido eutectic solvent comprises an amido compound as shown in a structural formula (I). In the amido eutectic electrolyte disclosed by the invention, the amido eutectic solvent shown in the structural formula (I) can generate eutectic action with the organic solvent, and the formed hydrogen bond greatly improves the cluster oxidation resistance, so that the anode of the lithium metal secondary battery can be stabilized, and CEI rich in Li3N is formed; and the introduction of the amide-based solvent can solubilize a small amount of lithium salt, so that the formation of stable SEI on the negative electrode side of the lithium metal secondary battery by the amide-based eutectic electrolyte is facilitated, and the lithium metal secondary battery has relatively good normal-temperature cycle performance and high-temperature cycle performance under high voltage.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium metal secondary batteries, and in particular to an amide-based eutectic electrolyte, a preparation method thereof, and a lithium metal secondary battery. Background Art

[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. At present, the positive electrode materials of commercial lithium metal secondary batteries mainly include lithium manganese oxide, lithium cobalt oxide, ternary materials, lithium iron phosphate, etc. With the development and popularization of lithium metal secondary batteries with higher voltage systems, conventional electrolytes are not only prone to self-oxidation and decomposition reactions, but also oxidative decomposition on the surface of the positive electrode material (especially on the surface of the positive electrode material containing transition metal elements), affecting the normal high temperature cycle 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, it remains a challenge to maintain high performance of such batteries at high voltages, where they suffer from multiple degradation 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 performance of high-voltage LiNi 0.8 Mn 0.1 Co 0.1 O2 (NCM 811) The stability of the interfacial chemistry of lithium metal secondary batteries. However, perfluorinated compounds are extremely stable and difficult to degrade in the natural environment, which may cause environmental pollution problems, and the recovery and treatment of electrolytes containing these perfluorinated compounds require more complex processes, which increases the difficulty and cost of processing.

[0005] Therefore, what kind of high-voltage resistant electrolyte to use to achieve stable cycling of high-energy-density and high-voltage lithium metal secondary batteries is a difficult problem that technicians in this field need to solve. Summary of the invention

[0006] Based on this, the purpose of the present invention is to provide an amide-based eutectic electrolyte and a preparation method thereof and a lithium metal secondary battery. The amide-based eutectic electrolyte is used in a lithium metal secondary battery, so that the lithium metal secondary battery can have good room temperature cycle performance and high temperature cycle performance under high voltage (such as 4.6-4.8V).

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[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; the organic solvent is at least one of a chain carbonate, a cyclic carbonate, a carboxylic acid ester, an ether and a heterocyclic compound; the amide-based eutectic solvent comprises an amide-based 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 organic solvent, and the amide-based eutectic solvent shown in structural formula (I). The organic solvent is at least one of chain carbonates, cyclic carbonates, carboxylates and ethers. The present invention adds the amide-based eutectic solvent shown in structural formula (I) so that in the amide-based eutectic electrolyte of the present invention, the amide-based eutectic solvent shown in structural formula (I) can produce a eutectic effect with the organic solvent, and the hydrogen bond (for example: NHO) formed by the amide-based eutectic solvent greatly improves the antioxidant capacity of the cluster, and can stabilize the positive electrode of the lithium metal secondary battery (for example, NCM811 lithium metal secondary battery, LCO lithium metal secondary battery, LR lithium metal secondary battery, etc.), forming a CEI rich in Li3N; and the introduction of the amide-based solvent can solubilize a small amount of lithium salt (for example: lithium nitrate), which is conducive to the formation of a stable SEI of the amide-based eutectic electrolyte on the negative electrode side of the lithium metal secondary battery. The amide-based eutectic electrolyte of the present invention is used in a lithium metal secondary battery, which can enable the lithium metal secondary battery to have good room temperature cycle performance and high temperature cycle performance at a high voltage (such as 4.6-4.8V). In addition, the amide-based eutectic solvent shown in the structural formula (I) of the present invention is biodegradable, which complies with the principle 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 scheme, the mass of the amide eutectic solvent accounts for 10-35% of the total mass of the amide 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%, 35%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0016] As a preferred embodiment, the lithium salt is selected from at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethylsulfonate, lithium bis(fluorosulfonyl imide), lithium bis(trifluoromethylsulfonyl imide) and lithium nitrate.

[0017] As a preferred scheme, the mass of 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%, 30%, but is not limited to the listed values, and other values ​​not listed within the numerical 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 scheme, the mass of 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%, 75%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0020] As a preferred solution, the amide-based eutectic electrolyte also includes an auxiliary agent, and the auxiliary agent is selected from at least one of vinylene carbonate (VC), 1,3-propane sultone (PS), 1,4-butane sultone (BS), propylene glycol (RPS), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), and lithium difluorooxalatoborate (LiDFOB).

[0021] As a preferred solution, the auxiliary agent 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%, 8%. Preferably, the auxiliary agent accounts for 0.5-2% of the total mass of the amide-based eutectic electrolyte. The addition of the auxiliary agent can further improve the room temperature cycle performance and high temperature storage performance of the lithium metal secondary battery.

[0022] The second aspect of the present invention is to provide a method for preparing any of the above-mentioned amide-based eutectic electrolytes, comprising the following steps: mixing the various substances evenly to obtain the amide-based eutectic electrolyte.

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

[0024] As a preferred solution, the active material of the positive electrode includes lithium iron phosphate LiFePO 4、 LiCoO2 and Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b At least one of Li (1+a) Ni x Co y M z N 1-x-y-z O2+b In the formula, 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 <x<1,0<y<1,0<z<1,0.7<x+y+z≤1,-0.05≤b≤0.10。

[0025] As a preferred solution, the negative electrode of the lithium metal secondary battery is lithium metal. Commonly used negative electrode materials are at least one of artificial graphite, natural graphite, Si and its alloys, Sn and its alloys, metallic lithium and its alloys, transition metal oxides and lithium titanate. Among these negative electrode materials, the lithium metal negative electrode has a higher theoretical specific capacity (mAh / g), but it is very easy to produce lithium dendrites during the cycle process, resulting in a decrease in the battery cycle stability. The amide-based eutectic solvent shown in the structural formula (I) of the present invention is compatible with lithium metal and can solubilize lithium salts (for example: lithium nitrate) to a certain extent, so that the amide-based eutectic electrolyte can form a good SEI layer on the lithium metal negative electrode during the formation stage, so that the lithium metal secondary battery can be stably cycled under high voltage conditions.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The amide-based eutectic electrolyte of the present invention adopts the amide-based eutectic solvent shown in the structural formula (I), which can produce a eutectic effect with the organic solvent, and the hydrogen bond (NHO) formed by it greatly improves the antioxidant capacity of its cluster, and can stabilize the positive electrode of the lithium metal secondary battery (such as NCM811 lithium metal secondary battery, LCO lithium metal secondary battery, LR lithium metal secondary battery, etc.), forming a CEI rich in Li3N; and the introduction of the amide-based solvent can solubilize a small amount of lithium salt (such as lithium nitrate), which is conducive to the formation of a stable SEI by the amide-based eutectic electrolyte on the negative electrode side of the lithium metal secondary battery. The amide-based eutectic electrolyte of the present invention is used in lithium metal secondary batteries, which can make the lithium metal secondary batteries have good room temperature cycle performance and high temperature cycle performance at high voltage (such as 4.6-4.8V). BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For better understanding and implementation, the present invention is described in detail below with reference to the accompanying drawings.

[0029] Figure 1 This is the rate performance diagram of Li metal / LCO high voltage lithium metal secondary battery;

[0030] Figure 2 The electrolyte of Comparative Example 1, Example 4 and Example 5 1 HNMR spectra;

[0031] Figure 3The electrolyte of Comparative Example 1, Example 3 and Example 5 1 HNMR spectrum. DETAILED DESCRIPTION

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

[0033] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0034] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0035] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

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

[0037] In order to better illustrate the purpose, technical scheme and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the following implementation method is a further explanation of the present invention and should not be used as a limitation of the present invention.

[0038] Example 1

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

[0040] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium nitrate (LiNO3); the organic solvent includes ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); the amide eutectic solvent is an amide compound shown in structural formula (I); and the auxiliary agent is lithium difluorooxalatoborate (LiDFOB). Please refer to Tables 1 and 2 for the specific composition and dosage of each component in the amide eutectic electrolyte of this embodiment.

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

[0042] According to the ratio in Table 1, the organic solvent and the lithium salt are mixed evenly, and then the auxiliary agent and the amide-based eutectic solvent are added and mixed evenly to obtain a eutectic solution.

[0043] Example 2-12

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

[0045] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium nitrate (LiNO3); the organic solvent includes ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); the amide eutectic solvent is an amide compound of structural formula (I); and the auxiliary agent is lithium difluorooxalatoborate (LiDFOB). For the specific composition and dosage of each component of the amide eutectic electrolyte of Examples 2-12, please refer to Table 1 and Table 2.

[0046] The preparation method of the amide eutectic electrolyte of Example 2-12 is the same as that of 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), and the organic solvent includes ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC). Please refer to Table 1 for the specific composition and dosage of each component in the electrolyte of this comparative example.

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

[0051] Comparative Example 2

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

[0053] The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvent includes ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); and the auxiliary agent is lithium difluorooxalatoborate (LiDFOB). Please refer to Table 1 for the specific composition and dosage of each component in the electrolyte of this comparative example.

[0054] The preparation method of the electrolyte of this comparative example comprises the following steps: according to the proportions in Table 1, the organic solvent and the lithium salt are mixed evenly, and then the auxiliary agent is 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), and the organic solvent includes ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC). Please refer to Table 1 for the specific composition and dosage of each component in the electrolyte of this comparative example.

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

[0059] Comparative Example 4

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

[0061] The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvent includes ethyl methyl carbonate (EMC) and fluoroethylene carbonate (FEC); and the auxiliary agent is lithium difluorooxalatoborate (LiDFOB). Please refer to Table 1 for the specific composition and dosage of each component in the electrolyte of this comparative example.

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

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

[0064]

[0065]

[0066] Table 2 Structural formula of each compound in Table 1

[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 are 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 of Examples 13-24 and Comparative Examples 5-8:

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

[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 a NCM811 lithium metal secondary battery.

[0076] The NCM811 lithium metal secondary batteries of Examples 25-36 are 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 method of NCM811 lithium metal secondary battery of 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 injected into the 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 was charged and discharged three times at a constant current of 0.2C, then charged to 4.7V at a constant current of 0.5C, and discharged to 2.8V at 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 a LR lithium metal secondary battery.

[0082] The LR lithium metal secondary batteries of Examples 37-48 are 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 of Examples 37-48 and Comparative Examples 13-16:

[0085] The amide eutectic electrolytes of Examples 1-12 and the electrolytes of Comparative Examples 1-4 were injected into the electrolyte solution containing 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 was charged and discharged three times at a constant current of 0.2C, then charged to 4.8V at a constant current of 0.33C, and discharged to 2V at a constant current of 5C.

[0086] Performance Testing

[0087] The LCO lithium metal secondary batteries of Examples 13-24, the NCM811 lithium metal secondary batteries of Examples 25-36, and the LR lithium metal secondary batteries of Examples 37-48 prepared by the amide-based eutectic electrolytes of Examples 1-12, and the LCO lithium metal secondary batteries of Comparative Examples 5-8, the NCM811 lithium metal secondary batteries of Comparative Examples 9-12, and the LR lithium metal secondary batteries of Comparative Examples 13-16 prepared by the electrolytes of Comparative Examples 1-4 were respectively subjected to normal temperature cycle tests and high temperature cycle tests on lithium ion batteries according to the following test conditions, and the test items are as follows:

[0088] (1) Normal temperature cycle performance test:

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

[0090] The formed LCO lithium metal secondary battery was charged to 4.6 V at a constant current of 0.5 C, and discharged to 2.8 V at a constant current of 0.5 C. 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] The cycle performance is calculated by the capacity retention rate as follows:

[0094] Capacity retention rate (%) = (discharge capacity at the 100th cycle / initial discharge capacity) × second discharge capacity.

[0095] (2) High temperature cycle performance test:

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

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

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

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

[0100] The cycle performance is calculated by the capacity retention rate as follows:

[0101] Capacity retention rate (%) = (discharge capacity at the 100th cycle / initial discharge capacity) × second discharge capacity.

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

[0103] Table 3 Cycling performance test results of lithium metal secondary batteries at room temperature (25°C)

[0104]

[0105]

[0106] Table 4 High temperature (60°C) cycle performance test results of lithium metal secondary batteries

[0107]

[0108]

[0109] It can be seen from Tables 3 and 4 that the lithium metal secondary batteries prepared corresponding to 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, LR lithium metal secondary batteries of Examples 37-48) have achieved significant improvements in both room temperature cycle performance and high temperature cycle performance.

[0110] This is because the amide eutectic electrolyte of Examples 1-12 of the present invention uses an amide compound shown in structural formula (I) (specifically an amino compound shown in structural formula (I-1), (I-2), (I-3), (I-4), (I-5), and (I-6)) as the eutectic solvent of the electrolyte, and the amide eutectic solvent can produce a eutectic effect with the organic solvent to form a hydrogen bond (NHO) to greatly improve the antioxidant capacity of its cluster, and can stabilize the positive electrode of the lithium metal secondary battery (NCM811 lithium metal secondary battery, LCO lithium metal secondary battery, LR lithium metal secondary battery, etc.), forming a CEI rich in Li3N; and the introduction of the amide eutectic solvent can solubilize a small amount of lithium nitrate, which is conducive to the amide eutectic electrolyte to form a stable SEI on the negative electrode side of the lithium metal secondary battery, so that the lithium metal secondary battery can have good room temperature cycle performance and high temperature cycle performance at high voltage (4.6-4.8V).

[0111] In addition, from the test data of 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 corresponding to the amide-based eutectic electrolyte of Example 5, it can be seen that its room temperature cycle performance and high temperature cycle performance are very excellent. The amide-based eutectic electrolyte of Example 5 uses an amide-based compound shown in structural formula (I-5) as an amide-based eutectic solvent. Compared with amide-based eutectic solvents of other structures mentioned in other embodiments of the present invention, the amide-based compound shown in structural formula (I-5) has -NH-CH3 and C≡N in its structure, wherein -NH-CH3 can produce a eutectic effect with an organic solvent to form a hydrogen bond (NHO) to improve the antioxidant capacity of its cluster; and C≡N can form atypical hydrogen bonds with CH3 in the organic solvent (FEC, EMC), so that the antioxidant performance of the cluster is the most superior.

[0112] From the test data of the lithium metal secondary battery obtained by using the amide-based eutectic electrolyte of Comparative Examples 1-6, it can also be known that the room temperature cycle performance and high temperature cycle performance of the lithium metal secondary battery obtained by using the amide-based compound shown in the structural formula (Ⅰ-3) as the amide-based eutectic solvent are slightly worse. It may be that the amide-based compound shown in the structural formula (Ⅰ-3) has an -NH2 group in its structure, and the impedance and high stability of the formed SEI film are lower. In addition, due to the presence of its -NH2 group structure, it is very easy to produce gas. The lithium metal secondary battery obtained by using the amide-based compound shown in the structural formula (Ⅰ-3) as the amide-based eutectic solvent is slightly worse than the lithium metal secondary battery obtained by using the amide-based compound shown in the structural formula (Ⅰ-2) as the amide-based eutectic solvent in terms of high temperature cycle performance; and because the two H atoms in the -NH2 group can rotate in the electrolyte under other forces, thereby greatly increasing the reaction activity of -NH2, the performance is poor. In addition, from the test data of the lithium metal secondary batteries prepared corresponding to the amide-based eutectic electrolytes of Comparative Examples 1-3, it can be found that although the -NH2 group can also form hydrogen bonds with organic solvent molecules, it is easily decomposed at high temperatures due to its high reaction activity. As the -NH2 group gradually decreases in Compound I, its electrochemical performance continues to increase.

[0113] The lithium metal secondary batteries prepared by 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) are significantly inferior in room temperature cycle performance and high temperature cycle performance. In addition, it can be seen from the data of Comparative Examples 1-4 that adding an additive (lithium difluorooxalate borate (LiDFOB)) to the electrolyte can further improve the room temperature cycle performance and high temperature cycle performance of the lithium metal secondary battery.

[0114] From the test data of the amide eutectic electrolytes of comparative examples 1-6 and the corresponding lithium metal secondary batteries prepared therefrom, it can be seen that the introduction of different side groups on the amide eutectic solvent represented by structural formula (I) has a great influence on its eutectic effect and the antioxidant properties of the solvent itself. Among them, the compound represented by structural formula (I-4) in Example 4 is a trifluoromethyl group introduced into structural formula (I), the compound represented by structural formula (I-5) in Example 5 is a nitrile group introduced into structural formula (I), and the compound represented by structural formula (I-6) in Example 6 is a trimethylsilyl group introduced into structural formula (I), all of which can improve the antioxidant capacity of the amide eutectic solvent to a certain extent. However, the steric hindrance of the trifluoromethyl and trimethylsilyl groups is large, and their eutectic ability with organic solvent molecules is weak, so their performance is slightly worse.

[0115] The present invention also compares the rate performance of the LCO lithium metal secondary battery prepared by the amide-based eutectic electrolyte of Example 5 and the LCO lithium metal secondary battery prepared by the electrolyte of Comparative Example 1, please refer to Figure 2 The performance of the LCO lithium metal secondary battery prepared by the amide-based eutectic electrolyte of Example 5 after multiple cycles is significantly better than that of the LCO lithium metal secondary battery prepared by the electrolyte of Comparative Example 1.

[0116] The present invention also obtains the amide-based eutectic electrolytes of Examples 4 and 5, and the electrolyte of Comparative Example 1. 1 HNMR diagram, see Figure 2 and Figure 3 , Figure 2 This indicates that hydrogen bonds are formed in the amide-based 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 of typical hydrogen bonds and atypical hydrogen bonds strengthens the hydrogen bonds.

[0117] In summary, the amide-based eutectic electrolyte described in the present invention and the amide-based eutectic solvent can generate NHO hydrogen bonds with O in the electrolyte, thereby increasing the antioxidant property of the cluster molecules and improving the oxidative stability of the electrolyte. The high-voltage lithium metal secondary battery assembled therefrom 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 solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. An amide-based eutectic electrolyte, characterized in that: It includes lithium salt, organic solvent and amide eutectic solvent; the organic solvent is at least one of chain carbonate, cyclic carbonate, carboxylic acid ester and ether; the amide eutectic solvent includes an amide compound shown in structural formula (I); 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.

2. The amide-based eutectic electrolyte according to claim 1, characterized in that: The amide compound represented by structural formula (I) is selected from at least one of the amide compounds represented by structural formulas (I-1), (I-2), (I-3), (I-4), (I-5) and (I-6); 3. The amide-based eutectic electrolyte according to claim 1, characterized in that: The mass of the lithium salt accounts for 5-30% of the total mass of the amide-based eutectic electrolyte; the mass of the organic solvent accounts for 45-75% of the total mass of the amide-based eutectic electrolyte; the mass of the amide-based eutectic solvent accounts for 10-35% of the total mass of the amide-based eutectic electrolyte.

4. The amide-based eutectic electrolyte according to claim 1, characterized in that: The lithium salt is selected from at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethylsulfonate, lithium bis(fluorosulfonyl imide), lithium bis(trifluoromethylsulfonyl imide) and lithium nitrate.

5. The amide-based eutectic electrolyte according to claim 1, characterized in that: The organic solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, fluoroethylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

6. The amide-based eutectic electrolyte according to claim 1, characterized in that: The invention also includes an auxiliary agent, which is selected from at least one of vinylene carbonate, 1,3-propane sultone, 1,4-butane sultone, acrylic acid lactone, vinyl ethylene carbonate, fluoroethylene carbonate, lithium difluorophosphate, and lithium difluorooxalatoborate.

7. The amide-based eutectic electrolyte according to claim 6, characterized in that: The auxiliary agent accounts for 0.1-8.0% of the total mass of the amide-based eutectic electrolyte.

8. A method for preparing an amide-based eutectic electrolyte according to any one of claims 1 to 7, characterized in that: The various substances are mixed uniformly to obtain the amide-based eutectic electrolyte.

9. A lithium metal secondary battery, characterized in that: It comprises the amide-based eutectic electrolyte described in any one of claims 1-7.

10. The lithium metal secondary battery according to claim 9, characterized in that: It also includes a positive electrode and a negative electrode; the active material of the positive electrode includes lithium iron phosphate LiFePO4, LiCoO2, Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b At least one of Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b In the formula, 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 <x<1,0<y<1,0<z<1,0.7<x+y+z≤1,-0.05≤b≤0.10。

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