Flame-retardant electrolyte capable of being solidified in situ and sodium ion secondary battery

Abstract

The invention provides a flame-retardant electrolyte capable of being solidified in situ and a sodium ion secondary battery. Specifically, the flame-retardant electrolyte capable of being solidified in situ comprises the following components: sodium salt, a solvent, a functional additive, a three-functional-group acrylate monomer and an initiator, the solvent comprises a primary solvent and an optional secondary solvent; wherein the main solvent is triethyl phosphate; the secondary solvent is selected from at least one of a carbonic ester solvent, a carboxylic ester solvent and an ether solvent; the main solvent accounts for 70%-100% of the volume fraction of the solvent; the trifunctional acrylate monomer accounts for 3%-15% of the mass of the electrolyte. The invention also provides an in-situ solidified sodium ion secondary battery which comprises a positive electrode material, a negative electrode material and the flame-retardant electrolyte capable of being solidified in situ. The electrolyte is low in cost, high in safety and compatible with a carbon negative electrode.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology. Specifically, this invention relates to a flame-retardant electrolyte that can be solidified in situ and an in-situ solidified sodium-ion secondary battery. Background Technology

[0002] In this critical period of energy transition, high-performance rechargeable batteries, as efficient and convenient energy storage and conversion devices, play a vital role in building a clean energy system and realizing large-scale energy storage. Sodium-ion batteries, with their abundant resources, wide distribution, and low cost, are widely recognized as a powerful complement to lithium-ion batteries, especially in the field of large-scale energy storage, demonstrating enormous application potential and becoming a hot research topic for research teams worldwide.

[0003] However, the safety of sodium-ion batteries has always been a key bottleneck restricting their development. Currently, most sodium-ion batteries use flammable carbonate and / or ether solvent systems as electrolytes. Under abnormal operating conditions such as overcharging, short circuits, or heating, the battery is highly susceptible to fire, combustion, or even explosion, leading to serious safety accidents. Although the development of flame-retardant electrolytes has alleviated some safety issues to a certain extent, the inherent leakage risk of liquid electrolytes remains, undoubtedly casting a shadow over the widespread application of sodium-ion batteries.

[0004] Therefore, there is an urgent need for a safe, high-performance electrolyte (such as one with good cycle stability) that is free from leakage risks. Summary of the Invention

[0005] The purpose of this invention is to provide a flame-retardant electrolyte and sodium-ion secondary battery that can be solidified in situ, so as to solve the safety hazards and leakage risks of liquid electrolyte in existing sodium-ion batteries, improve the safety performance and cycle stability of sodium-ion batteries, and make them more suitable for large-scale energy storage and other application scenarios.

[0006] The above-mentioned objective of the present invention is achieved through the following technical solution.

[0007] In a first aspect, the present invention provides a flame-retardant electrolyte that can be solidified in situ, comprising the following components: sodium salt, solvent, trifunctional acrylate monomer and initiator;

[0008] The solvent comprises a primary solvent and an optional secondary solvent; wherein the primary solvent is triethyl phosphate; the secondary solvent is selected from at least one of carbonate solvents, carboxylic acid ester solvents, and ether solvents; and the primary solvent accounts for 70%-100% of the volume fraction of the solvent.

[0009] The trifunctional acrylate monomer accounts for 3%-15% of the mass fraction of the electrolyte.

[0010] The inventors of this application unexpectedly discovered that adding triethyl phosphate, trifunctional acrylate monomers, and an initiator to the electrolyte can solve the electrolyte safety problem and improve the cycle stability of the battery.

[0011] The inventors of this application also unexpectedly discovered that phosphate ester solvents with similar structures exhibit significant performance differences in actual battery use. For example, trimethyl phosphate with a high phosphorus content, while possessing high flame retardant efficiency, cannot solve battery safety issues due to interface problems and high-temperature stability. In contrast, the triethyl phosphate of this invention, with a moderate phosphorus content, can effectively suppress the release of heat generated by the positive electrode material at high temperatures. Combined with acrylate monomers containing trifunctional groups, it can achieve over 1000 cycles in quasi-solid-state sodium batteries.

[0012] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonate), sodium trifluoromethanesulfonate, and sodium perchlorate.

[0013] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the concentration of the sodium salt in the electrolyte is 0.1-2 mol / L.

[0014] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the trifunctional acrylate monomer is trimethylolpropane triacrylate (TMPTA) and / or pentaerythritol triacrylate (PETA).

[0015] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the mass ratio of the trifunctional acrylate monomer to the initiator is trifunctional acrylate monomer: initiator = (5~500): 1.

[0016] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the initiator includes at least one of azobisisobutyronitrile (AIBN), di(2,4-dichlorobenzoyl) peroxide (DCBP), diacetyl peroxide, dioctyl peroxide, dilauryl peroxide, dicarbonate peroxide, diisopropyl peroxide, diisobutyl peroxide, dicyclohexyl peroxide, and di(p-tert-butylcyclohexyl) peroxide.

[0017] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the carbonate solvent is a cyclic carbonate solvent and / or a chain carbonate solvent.

[0018] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the cyclic carbonate is ethylene carbonate and / or propylene carbonate.

[0019] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the chain carbonate is a carbonate synthesized from a straight-chain or branched aliphatic monool with 3-8 carbon atoms and carbonic acid.

[0020] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate and methyl ethyl carbonate.

[0021] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the ether solvent is selected from one or more of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethoxymethane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0022] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the carboxylic acid ester solvent is a cyclic carboxylic acid ester solvent and / or a chain carboxylic acid ester solvent.

[0023] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the cyclic carboxylic acid ester is γ-butyrolactone, and the chain carboxylic acid ester is a chain carboxylic acid ester with 3-8 carbon atoms.

[0024] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the chain carboxylic acid ester is selected from one or more of methyl acetate, ethyl acetate, propyl acetate, propyl propionate and ethyl propionate.

[0025] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the electrolyte further comprises functional additives.

[0026] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the functional additive is selected from one or more of ethylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, propylene-1,3-sulfonate lactone, cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, vinyl sulfate, dimethyl sulfite, diethyl sulfite, and succinate.

[0027] Preferably, in the flame-retardant electrolyte that can be solidified in situ according to the present invention, the functional additive has a mass fraction of 0.5-10% in the electrolyte.

[0028] The flame-retardant electrolyte of the present invention, which can be solidified in situ, can be solidified in situ by a method including the following steps:

[0029] (1) Under an inert atmosphere, the sodium salt, triethyl phosphate, functional additive and secondary solvent are mixed and stirred to dissolve, resulting in a homogeneous mixture.

[0030] (2) Add trifunctional acrylate monomer and initiator to the mixture obtained in step (1), mix well, and obtain electrolyte;

[0031] (3) Inject the electrolyte into the battery casing and seal it;

[0032] (4) The battery after being injected with electrolyte is heated and solidified at 60°C to 80°C for 1-10 hours to initiate the polymerization of the trifunctional acrylate monomers to form a gel electrolyte, thereby realizing the in-situ solidification of the electrolyte.

[0033] Secondly, the present invention provides an in-situ solidified sodium-ion secondary battery, which includes a positive electrode material, a negative electrode material, and the flame-retardant electrolyte of the present invention that can be solidified in situ.

[0034] Preferably, in the in-situ solid-state sodium-ion secondary battery of the present invention, the positive electrode material is a layered metal oxide, a polyanion-based positive electrode material, or a Prussian blue-based material, and the negative electrode material is a carbon-based material.

[0035] Preferably, in the in-situ solid-state sodium-ion secondary battery of the present invention, the positive electrode material is a layered metal oxide, such as O3-Na[Cu] 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 O2; the negative electrode material is hard carbon.

[0036] The present invention has the following beneficial effects:

[0037] (1) Safety performance is significantly improved

[0038] Excellent flame retardant effect: The flame retardant triethyl phosphate (TEP) in the electrolyte significantly improves the battery's flame retardant performance. In cases of battery misuse or accidental injury (such as puncture), it can effectively reduce or prevent the risk of fire.

[0039] The leakage problem has been solved: In-situ solidification technology is adopted to form a quasi-solid structure of electrolyte inside the battery, which effectively avoids the leakage risk of traditional liquid electrolytes and further enhances the safety and reliability of the battery.

[0040] (2) Excellent electrochemical performance

[0041] Excellent cycle performance: By rationally selecting and proportioning flame retardants, acrylate monomers, initiators and other components, a long cycle life of sodium-ion batteries is achieved.

[0042] High compatibility: This flame-retardant, in-situ solidifiable electrolyte is compatible with a variety of positive electrode materials (such as layered metal oxides, polyanionic positive electrode materials, and Prussian blue materials) and negative electrode materials (such as carbon materials), especially with hard carbon negative electrodes. This broadens the range of materials to be selected for sodium-ion batteries and provides more possibilities for battery design and optimization.

[0043] (3) High cost-effectiveness

[0044] Low raw material costs: The selected flame retardants, such as triethyl phosphate and acrylate monomers, are widely available and relatively inexpensive, which reduces the production cost of the electrolyte and makes sodium-ion batteries more competitive in cost-sensitive applications such as large-scale energy storage.

[0045] (4) Simple production process: The preparation process of in-situ solidifiable electrolyte is relatively simple, without the need for complex equipment and complicated process steps, which is conducive to large-scale production and application, and further reduces the manufacturing cost of batteries.

[0046] (5) Strong environmental adaptability

[0047] Good high-temperature stability: Under high-temperature conditions, the flame retardants and additives in this electrolyte can effectively suppress the heat release of the positive electrode material and reduce the risk of thermal runaway inside the battery.

[0048] It is adaptable to a variety of solvent systems: In addition to triethyl phosphate, it can also be used as a secondary solvent in various other solvents such as carbonates, ethers, and carboxylic esters. The battery cycle performance is stable and the capacity retention rate is high, which enhances the adaptability of the electrolyte to different environmental conditions and battery systems. Attached Figure Description

[0049] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0050] Figure 1 The first charge-discharge curve of the battery of Example 1 at room temperature is shown;

[0051] Figure 2 The performance graph of the battery of Example 1 after 1000 cycles at room temperature is shown;

[0052] Figure 3 A photograph of the in-situ solidified electrolyte gas production in Comparative Example 8 is shown;

[0053] Figure 4 Examples 1 and 3 illustrate the heat generation behavior of the electrolyte with the positive electrode at high temperatures. Detailed Implementation

[0054] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0055] O3-Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 O2 is used as the positive electrode active material, acetylene black as the conductive agent, and polytetrafluoroethylene (PTFE) as the binder, and they are mixed evenly in a mass ratio of 90:6:4. Then, N-methylpyrrolidone is added and the mixture is prepared into a slurry. The slurry is coated onto aluminum foil. It is then vacuum-dried overnight at 120°C. After rolling, the positive electrode sheet is obtained. Hard carbon is used as the negative electrode active material, acetylene black as the conductive agent, and PTFE as the binder, and they are mixed evenly in a mass ratio of 90:5:5. Then, N-methylpyrrolidone is added and the mixture is prepared into a slurry. The slurry is coated onto aluminum foil and vacuum-dried overnight at 120°C. After rolling, the negative electrode sheet is obtained.

[0056] Electrolyte preparation: In an argon-filled glove box, weigh the sodium salts according to the data in Table 1. Then, add a certain volume of solvent and additives. After thorough stirring, the prepared electrolyte is obtained (note that a small amount of salt from the 15% HFE group precipitates; use the supernatant). After mixing the prepared electrolyte with a certain amount of monomer, add the initiator, and then inject it into the battery cell (18650 cylindrical cell). After curing at 70℃ for 4 hours, perform composition testing and then test the cycle performance and safety performance. The needle penetration test is conducted under full charge.

[0057] The English abbreviations in Table 1 are as follows:

[0058] TMP: Trimethyl phosphate; TEP: Triethyl phosphate; TPP: Tripropyl phosphate; DEC: Diethyl carbonate; EC: Ethyl carbonate; EP: Propyl acetate; DEGDME: Diethylene glycol dimethyl ether; THF: Tetrahydrofuran; HFE: 1,1,2,2-Tetrafluoroethyl-2,2,3,3-Tetrafluoropropyl ether; VC: Vinylene carbonate; TMPTA: Trimethylolpropane triacrylate; AA: Acrylic acid; MMA: Methyl methacrylate; PETA: Pentaerythritol triacrylate; PETTA: Pentaerythritol tetraacrylate; AIBN: Azobisisobutyronitrile; DCBP: Di(2,4-dichlorobenzoyl) peroxide; FEC: Fluorinated vinyl carbonate.

[0059]

[0060]

[0061] Figure 1 This is the first charge-discharge curve of the battery in Example 1 at room temperature. Figure 1 This shows the hard carbon / O3-Na[Cu 1 / 9Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 The initial charge-discharge curves of an O2 full cell at a 0.1C rate (1C = 100 mA / g). From... Figure 1 It can be seen from this that: the hard carbon / O3-Na[Cu] used 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 The specific capacity of the O2 full cell is approximately 120 mAh / g. The flame-retardant in-situ solid-state electrolyte and carbon anode are compatible, and the capacity is utilized normally.

[0062] Figure 2 This is a cycle performance graph of the battery in Example 1 at room temperature. From Figure 2 As can be seen, the battery of Example 1 can be stably cycled 1000 times at room temperature with a capacity retention of 93.5%. The flame-retardant in-situ solid electrolyte and carbon anode are compatible and have good cycle performance.

[0063] The electrolytes from Examples 1-9, Comparative Examples 1 and 2 were injected into cylindrical battery cells (capacity 1.2Ah). After two cycles of cell circulation, a needle penetration test was performed with the cells fully charged. During the penetration test, the batteries from Examples 1-9 did not smoke, catch fire, or explode. The batteries from Comparative Example 1 (a traditional carbonate electrolyte formulation) and Comparative Example 2 produced thick smoke during the penetration test, posing a safety hazard. This comparison demonstrates that batteries with higher TEP content (70% or more of the solvent by volume) have higher battery safety performance, while batteries with lower content (less than or equal to 65% of the solvent by volume) have lower battery safety performance.

[0064] Examples 1-8 show that, in addition to TEP, the second solvent for flame retardant can be carbonate, carboxylic acid ester, or ether solvent, resulting in stable battery cycle performance and high capacity retention.

[0065] Examples 1, 9, 10, and 11 show that the selected combinations of sodium salts, or NaPF6, NaBF4, and NaFSI, exhibit stable cycle performance and high capacity retention.

[0066] Examples 1, 12, 13 and Comparative Examples 8, 9 illustrate that the ratio of monomer to initiator needs to be appropriate. Too low an initiator amount (600:1) leads to insufficient polymerization of the electrolyte and high electrolyte fluidity. Too high an initiator amount (4:1) causes initiator gas production, which degrades battery performance.

[0067] Examples 1 and 3 illustrate the importance of flame retardant selection. TMP generates a significant amount of heat at high temperatures with the positive electrode, while TEP generates less heat in comparison. This may cause the TMP-based flame-retardant electrolyte to fail the puncture test. In Comparative Example 5, the high viscosity of the TPP flame retardant may have led to poor battery cycle performance.

[0068] Examples 1, 14, 15, 16, and Comparative Examples 6 and 7 illustrate that too few monomers (2%) cannot be used to prepare an in-situ solid electrolyte, while too many monomers (20%) result in poor battery cycle performance.

[0069] Examples 1 and 18 illustrate that by selecting VC and FEC additives, the battery cycle performance is stable and the capacity retention rate is high.

[0070] Examples 1 and 19 illustrate that DCBP batteries using the selected initiator exhibit stable cycle performance and high capacity retention.

[0071] Examples 1 and 17, and Comparative Examples 10, 11, and 12, demonstrate that the polymerization of trifunctional TMPTA and TEPA can form highly cross-linked polymer structures, resulting in better electrochemical performance of the batteries. AA, MMA, or other acrylates such as pentaerythritol tetraacrylate exhibit poorer performance.

Claims

1. A flame-retardant electrolyte that can be solidified in situ, comprising the following components: sodium salt, solvent, trifunctional acrylate monomer and initiator; The solvent comprises a primary solvent and an optional secondary solvent; wherein, The primary solvent is triethyl phosphate; the secondary solvent is selected from at least one of carbonate solvents, carboxylic acid ester solvents, and ether solvents; and the primary solvent accounts for 70%-100% of the volume fraction of the solvent. The trifunctional acrylate monomer accounts for 3%-15% of the mass fraction of the electrolyte.

2. The flame-retardant electrolyte that can be solidified in situ according to claim 1, wherein, The sodium salt is selected from one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonate), sodium trifluoromethanesulfonate, and sodium perchlorate. Preferably, the concentration of the sodium salt in the electrolyte is 0.1-2 mol / L.

3. The flame-retardant electrolyte that can be solidified in situ according to claim 1, wherein, The trifunctional acrylate monomer is trimethylolpropane triacrylate and / or pentaerythritol triacrylate.

4. The flame-retardant electrolyte that can be solidified in situ according to claim 1, wherein, The mass ratio of the trifunctional acrylate monomer to the initiator is trifunctional acrylate monomer: initiator = (5~500): 1; Preferably, the initiator comprises at least one of azobisisobutyronitrile, bis(2,4-dichlorobenzoyl) peroxide, diacetyl peroxide, dioctyl peroxide, dilauryl peroxide, dicarbonate peroxide, diisopropyl peroxide, diisobutyl peroxide, dicyclohexyl peroxide, and di(p-tert-butylcyclohexyl) peroxide.

5. The flame-retardant electrolyte that can be solidified in situ according to claim 1, wherein, The carbonate solvent is a cyclic carbonate solvent and / or a chain carbonate solvent; Preferably, the cyclic carbonate is ethylene carbonate and / or propylene carbonate; Preferably, the chain carbonate is a carbonate synthesized from a straight-chain or branched aliphatic monool with 3-8 carbon atoms and carbonic acid. Preferably, the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate and methyl ethyl carbonate.

6. The flame-retardant electrolyte that can be solidified in situ according to claim 1, wherein, The ether solvent is selected from one or more of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethoxymethane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. Preferably, the carboxylic acid ester solvent is a cyclic carboxylic acid ester solvent and / or a chain carboxylic acid ester solvent; More preferably, the cyclic carboxylic acid ester is γ-butyrolactone, and the chain carboxylic acid ester is a chain carboxylic acid ester with 3-8 carbon atoms; More preferably, the chain carboxylic ester is selected from one or more of methyl acetate, ethyl acetate, propyl acetate, propyl propionate and ethyl propionate.

7. The flame-retardant electrolyte that can be solidified in situ according to claim 1, wherein, The electrolyte also contains functional additives; Preferably, the functional additive is selected from one or more of ethylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, propenyl-1,3-sulfonate lactone, cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, vinyl sulfate, dimethyl sulfite, diethyl sulfite, and succinate.

8. The flame-retardant electrolyte that can be solidified in situ according to claim 7, wherein, The functional additive has a mass fraction of 0.5-10% in the electrolyte.

9. An in-situ solidified sodium-ion secondary battery, comprising a positive electrode material, a negative electrode material, and a flame-retardant electrolyte capable of being solidified in situ according to any one of claims 1-8.

10. The in-situ solid-state sodium-ion secondary battery according to claim 9, wherein, The positive electrode material is a layered metal oxide, a polyanionic positive electrode material, or a Prussian blue material, and the negative electrode material is a carbon material. Preferably, the positive electrode material is a layered metal oxide, and the negative electrode material is hard carbon.

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