Intrinsic incombustible low-salt-concentration phosphate-based electrolyte and preparation and application thereof

Abstract

The invention relates to the field of lithium ion battery electrolytes, in particular to an intrinsic non-combustible low-salt-concentration phosphate-based electrolyte as well as preparation and application of the intrinsic non-combustible low-salt-concentration phosphate-based electrolyte. The electrolyte contains a non-combustible phosphate organic solvent and a low-concentration lithium salt; the self-extinguishing time is 0s / g, and the electrochemical window is greater than or equal to 4.5 V. The intrinsic non-combustible low-salt-concentration phosphate ester-based electrolyte forms a solvation structure mainly comprising an anionic aggregate, so that the electrolyte can form a stable interface layer rich in inorganic matters such as boron (or aluminum) and lithium fluoride on a positive electrode and a negative electrode, the positive electrode and the negative electrode are protected, side reactions are reduced, and the service life of the electrolyte is prolonged. Furthermore, the high-efficiency compatibility with a high-nickel positive electrode, a lithium cobalt oxide positive electrode, a lithium metal negative electrode, a silicon monoxide negative electrode and the like can be realized. Therefore, the intrinsic non-combustible low-salt-concentration phosphate-based electrolyte is particularly suitable for the fields of high-safety lithium metal batteries, high-safety lithium ion batteries with silicon monoxide as a negative electrode and the like.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery electrolytes, and specifically to an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte and its preparation and application. Background Art

[0002] Lithium-ion batteries have become an indispensable key energy technology in modern society due to their high energy density and excellent cycling performance. However, with the widespread popularity and in-depth application of lithium-ion batteries, their safety issues have become increasingly prominent. Especially in the pursuit of high energy density, the potential risk of thermal runaway always poses a serious threat to practical applications. As a key medium for ion transport in lithium-ion batteries, the performance of the electrolyte plays a crucial role in the overall safety and reliability of the battery. With the rapid development of high-voltage and high-energy-density lithium battery technologies, the safety hazards of electrolytes have become increasingly prominent. For example, traditional organic carbonate-based electrolytes are flammable, which will significantly increase the risk of battery thermal runaway and combustion explosion during battery operation, especially under high-temperature or abusive conditions, posing higher requirements for battery safety. Therefore, developing electrolytes that are intrinsically non-flammable and compatible with positive and negative electrodes is of great significance and urgency for improving the energy density and safety of lithium batteries.

[0003] Phosphate-based electrolytes can more effectively reduce the risk of combustion or explosion compared to carbonate-based electrolytes due to their higher flash points and ignition points. For example, trimethyl phosphate, triethyl phosphate, etc. have been proven to be able to be used as solvents, co-solvents, or flame retardant additives to prepare safer electrolytes. These substances not only have good cost-effectiveness but also excellent environmental sustainability. To achieve the compatibility of this electrolyte with the positive and negative electrodes, researchers have proposed design strategies for high-concentration electrolytes and locally high-concentration electrolytes. (1) By increasing the lithium salt concentration to >3M, this is beneficial for forming a protective solid electrolyte interface derived from anions on the electrode surface (Patents CN108539274A, CN109216763A). However, high-concentration salt electrolytes bring problems such as increased preparation costs, increased viscosity of the electrolyte, and poor wettability of thick electrodes (difficult to achieve high energy density of lithium batteries). (2) Using inert solvents such as fluorinated ethers and fluorinated aromatic diluents in high-concentration lithium salts can reduce the overall concentration of the electrolyte and further improve the stability of the electrolyte (Patents CN114171794A, CN116072975A), but these inert solvents have low boiling points and are extremely volatile. More importantly, lithium salts such as lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide with high solubility are often used in high-concentration salt and locally high-concentration salt electrolytes. The oxidation generated by these lithium salts at high voltages may corrode the aluminum current collector, which poses a new challenge to large-scale applications. Moreover, the fluorinated diluents in locally high-concentration salt electrolytes are very expensive, bringing problems such as a sharp increase in preparation costs and environmental unfriendliness. (3) Mixing with other solvents such as carbonates can improve the compatibility of the electrolyte with the electrode (Patents CN114976246A, CN118919856A), but the introduction of flammable carbonate solvents will volatilize and burn, which will have a negative impact on the safety performance of lithium batteries.

[0004] To construct a high-energy density lithium battery with high efficiency and safety, it is urgent to develop a new type of intrinsically non-flammable low-salt concentration phosphate-based electrolyte with good wettability for thick electrodes, low cost, and low fluorine content, so that it can be well matched with both the positive and negative electrodes at the same time. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide an intrinsically non-flammable low-salt concentration phosphate-based electrolyte with high oxidation stability, high efficiency compatibility with the negative electrode, intrinsic non-flammability, and conducive to the stable operation of lithium batteries, as well as its preparation and application.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] An intrinsically non-flammable low-salt concentration phosphate-based electrolyte, the electrolyte contains a non-flammable phosphate organic solvent and a low-concentration lithium salt; the low-concentration lithium salt has a molar concentration of lithium ions of 0.3 mol / L to 0.75 mol / L.

[0008] The self-extinguishing time of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte is 0 s / g, and the electrochemical window is ≥ 4.5 V.

[0009] The non-flammable phosphate organic solvent is trimethyl phosphate shown in Formula 1,

[0010]

[0011] The lithium salt is lithium difluorooxalate borate, lithium bis(oxalato)borate, lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyanato)borate or lithium perfluorooctylaluminate;

[0012] The lithium salt lithium difluorooxalate borate is shown in Formula 2:

[0013]

[0014] The lithium salt lithium bis(oxalato)borate is shown in Formula 3:

[0015]

[0016] The lithium salt lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyanato)borate is shown in Formula 4:

[0017]

[0018] The lithium salt lithium perfluorooctylaluminate is shown in Formula 5:

[0019]

[0020] Preferably, the concentration of the lithium salt is 0.5 mol / L to 0.75 mol / L in terms of the molar concentration of lithium ions.

[0021] In the electrolyte system with a low salt concentration, both solvent molecules and anions show a tendency to competitively bind with lithium ions. The strong interaction between anions and solvents is not conducive to the participation of anions in the solvation structure; on the contrary, a weak interaction helps to construct a solvation structure involving anions. To quantify the interaction force between solvent molecules and anions, the interaction force between solvent molecules and anions was calculated by the independent gradient model based on Hirshfeld partitioning. δG is used to quantify the sum of all interactions between atom pairs. The larger the δG value, the greater the interaction between the two, and vice versa. The δG value of TMP-TFSI- is 0.3877, and the δG of TMP-DFOB - is 0.3180, and the δG of TMP-BOB - is 0.3038, indicating that compared with TFSI - DFOB- Anions and BOB - The interaction between anions and the solvent is weak, which is beneficial to achieving a solventized structure rich in anions. This anion-rich solventized structure can form a stable interfacial layer rich in inorganic substances such as boron (or aluminum) and lithium fluoride at the positive and negative electrodes. Therefore, the low-concentration electrolyte composed of the above-mentioned specific lithium salt and trimethyl phosphate can achieve high-efficiency compatibility with high-nickel positive electrodes, lithium cobalt oxide positive electrodes, lithium metal negative electrodes, silicon oxide negative electrodes, etc.

[0022] A preparation method of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte described above:

[0023] a) In a glove box filled with argon, use molecular sieves to remove water from the non-flammable phosphate organic solvent.

[0024] b) According to the above ratio, mix the solvent and lithium salt treated in step a) at room temperature with stirring to obtain the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte.

[0025] An application of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte described above, the application of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte in lithium secondary batteries, especially in high-safety lithium metal batteries and high-safety lithium ion batteries with silicon oxide negative electrodes.

[0026] A lithium secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte between the positive and negative electrodes, and the electrolyte is the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte described above.

[0027] The positive electrode is one of lithium iron phosphate positive electrodes, lithium manganese iron phosphate positive electrodes, lithium cobalt oxide positive electrodes, ternary materials (such as lithium nickel cobalt manganese oxide NCM, lithium nickel cobalt aluminum oxide NCA), lithium manganese oxide, lithium nickel manganese oxide, and lithium-rich manganese-based positive electrodes; the negative electrode is one of lithium metal negative electrodes, lithium alloy negative electrodes, lithium tin alloy negative electrodes, lithium silicon alloy negative electrodes, metal tin negative electrodes, silicon negative electrodes, tin-based oxide negative electrodes, silicon oxide negative electrodes, lithium titanate negative electrodes, hard carbon, soft carbon, carbon nanotubes, graphene, and iron oxide.

[0028] The positive electrode of the lithium secondary battery has a high surface loading ≥ 17.0 mg / cm -2 .

[0029] The lithium secondary battery is assembled into a button-type battery, a steel-shell cylindrical battery, an aluminum-shell cylindrical battery, a steel-shell square battery, an aluminum-shell square battery, or a soft-pack battery.

[0030] The advantages and positive effects of the present invention are:

[0031] 1. The intrinsically non-flammable low-salt-concentration phosphate-based electrolyte of the present invention has excellent properties such as intrinsic non-flammability and high oxidation stability.

[0032] 2. The intrinsically non-flammable low-salt-concentration phosphate-based electrolyte can be highly compatible with the positive and negative electrodes, especially with high-nickel cathodes, lithium cobalt oxide cathodes, lithium metal anodes, silicon monoxide anodes, etc. It matches high-voltage cathode materials and high-specific-capacity anode materials, effectively improving the energy density of the battery.

[0033] 3. Due to its low concentration, the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte can effectively infiltrate the electrodes, can match the cathode with a high areal loading (high active loading), which is beneficial to further improving the energy density of the battery.

[0034] 4. The intrinsically non-flammable low-salt-concentration phosphate-based electrolyte has low cost, simple preparation, wide application range, and is conducive to industrial production and popularization. Description of the Drawings

[0035] Figure 1 It is a digital photo of the ignition experiment of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Example 1 of the present invention.

[0036] Figure 2 It is the long-cycle performance curve of the NCM9055 / Li lithium metal battery assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Example 1 of the present invention within the voltage range of 2.75V to 4.3V under the condition of 1C.

[0037] Figure 3 It is the curve obtained by testing the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Example 1 of the present invention through linear sweep voltammetry.

[0038] Figure 4 It is the long-cycle performance curve of the NCM811 / silicon monoxide lithium-ion battery assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Example 2 of the present invention within the voltage range of 2.5V to 4.2V under the condition of 0.2C.

[0039] Figure 5 It is the long-cycle performance curve of the LiFePO4 / Li lithium metal battery assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Example 3 of the present invention within the voltage range of 2.5V to 3.8V under the condition of 1C.

[0040] Figure 6 It is the long-cycle performance curve of the NCM811 / Li lithium metal battery assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Example 4 of the present invention within the voltage range of 2.5V to 4.25V under the condition of 0.2C.

[0041] Figure 7It is the long cycle performance curve of the lithium cobaltate / Li lithium metal battery assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte provided in Embodiment 6 of the present invention within the voltage range of 3V to 4.45V under the condition of 0.2C.

[0042] Figure 8 It is the long cycle performance curve of the NCM9055 / Li lithium metal battery assembled with the electrolyte provided in Comparative Example 1 of the present invention within the voltage range of 2.75V to 4.3V under the condition of 1C.

[0043] Figure 9 It is the digital photo of the ignition experiment of the electrolyte provided in Comparative Example 2 of the present invention.

[0044] Figure 10 It is the curve obtained by testing the electrolyte provided in Comparative Example 2 of the present invention through linear sweep voltammetry.

[0045] Figure 11 It is the long cycle performance curve of the NCM811 / Li lithium metal battery assembled with the electrolyte provided in Comparative Example 2 of the present invention within the voltage range of 2.5V to 4.25V under the condition of 0.2C.

[0046] Figure 12 It is the long cycle performance curve of the NCM622 / silicon monoxide lithium-ion battery assembled with the electrolyte provided in Comparative Example 3 of the present invention within the voltage range of 2.5V to 4.2V under the condition of 0.2C. Detailed Embodiments

[0047] The following further illustrates the detailed embodiments of the present invention in combination with examples. It should be noted that the detailed embodiments described herein are only for illustrating and explaining the present invention and are not limited to the present invention.

[0048] The intrinsically non-flammable low-salt-concentration phosphate-based electrolyte of the present invention forms a solvation structure mainly composed of anion aggregates. Therefore, this electrolyte can form a stable interfacial layer rich in inorganic substances such as boron (or aluminum) and lithium fluoride on the positive and negative electrodes, protect the positive and negative electrodes, reduce side reactions, and thus can achieve efficient compatibility with high-nickel positive electrodes, lithium cobaltate positive electrodes, lithium metal negative electrodes, silicon monoxide negative electrodes, etc. At the same time, this electrolyte has excellent properties such as intrinsically non-flammable, self-extinguishing time of 0 s / g, high flash point, and electrochemical window ≥4.5V. Therefore, this intrinsically non-flammable low-salt-concentration phosphate-based electrolyte is particularly suitable for fields such as high-safety lithium metal batteries and high-safety lithium-ion batteries with silicon monoxide negative electrodes.

[0049] Example 1

[0050] The entire electrolyte preparation process is carried out in an argon - atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, weigh 1.2 g of trimethyl phosphate and add it to a vial. Then, under stirring conditions, add 0.1 g of lithium difluoro(oxalato)borate (the concentration of lithium ions in lithium difluoro(oxalato)borate is 0.7 mol / L). After the lithium salt is completely dissolved, continue stirring for 30 min until uniformly mixed to obtain an intrinsically non - flammable low - salt - concentration phosphate - based electrolyte;

[0051] A lithium battery assembled with a nine - series ternary nickel - cobalt - manganese (NCM9055) as the positive electrode, lithium metal as the negative electrode, and the above - prepared electrolyte is allowed to stand at room temperature for 24 h until the electrolyte is completely infiltrated.

[0052] Lithium battery cycle performance test: The charge - discharge range of the battery is 2.75 V to 4.3 V, the charge - discharge rate is 1C, and the test temperature is 30 °C (see Figure 2 ). The NCM9055 / lithium metal battery assembled based on the intrinsically non - flammable low - salt - concentration phosphate - based electrolyte with a surface loading of 1.0 mg / cm -2 ) exhibits excellent long - cycle stability (the capacity retention rate after 600 cycles is 87%).

[0053] Combustion test: Place 0.5 g of the electrolyte in the positive electrode case and ignite it with an open flame. Record the self - extinguishing time as 0 s / g after removing the heat source (see Figure 1 ).

[0054] Electrochemical window test: Assemble a lithium - on - steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open - circuit voltage of the battery to 6 V at 1 mV / s and record the electrochemical window as 4.90 V (see Figure 3 ).

[0055] Example 2

[0056] The entire electrolyte preparation process is carried out in an argon - atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, weigh 1.2 g of trimethyl phosphate and add it to a vial. Then, under stirring conditions, add 0.085 g of lithium difluoro(oxalato)borate (the concentration of lithium ions in lithium difluoro(oxalato)borate is 0.6 mol / L). After the lithium salt is completely dissolved, continue stirring for 30 min until uniformly mixed to obtain an intrinsically non - flammable low - salt - concentration phosphate - based electrolyte;

[0057] A lithium battery assembled with nickel cobalt manganese (NCM811) of the eight-series ternary system as the positive electrode, silicon monoxide as the negative electrode, and the electrolyte obtained from the above preparation is allowed to stand at room temperature for 24 h until the electrolyte is completely infiltrated.

[0058] Lithium battery cycle performance test: The charge and discharge range of the battery is 2.5 V to 4.2 V, the charge and discharge rate is 0.2 C, and the test temperature is 30 °C. The NCM811 / silicon monoxide lithium-ion battery with a high areal loading (17.3 mg / cm -2 ) assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte exhibits excellent long-cycle stability (the capacity retention rate after 200 cycles is 76%) (see Figure 4 ).

[0059] Combustion test: Place 0.5 g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0 s / g after removing the heat source.

[0060] Electrochemical window test: Assemble a lithium-against-steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open-circuit voltage of the battery to 6 V at 1 mV / s, and record the electrochemical window as 4.92 V.

[0061] Example 3

[0062] The entire electrolyte preparation process is carried out in an argon atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, weigh 1.2 g of trimethyl phosphate and add it to a vial, and then add 0.06 g of lithium difluorooxalate borate (the concentration of lithium ions in lithium difluorooxalate borate is 0.4 mol / L) under stirring conditions. After the lithium salt is completely dissolved, continue stirring for 30 min until evenly mixed to obtain an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte;

[0063] A lithium battery assembled with lithium iron phosphate as the positive electrode, lithium metal as the negative electrode, and the electrolyte obtained from the above preparation is allowed to stand at room temperature for 24 h until the electrolyte is completely infiltrated.

[0064] Lithium battery cycle performance test: The charge and discharge range of the battery is 2.5 V to 3.8 V, the charge and discharge rate is 1 C, and the test temperature is 30 °C. The lithium iron phosphate / Li lithium metal battery with an areal loading (1.0 mg / cm -2 ) assembled with the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte exhibits excellent long-cycle stability (the capacity retention rate after 400 cycles is 97%) (see Figure 5 ).

[0065] Combustion test: Place 0.5 g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0 s / g after removing the heat source.

[0066] Electrochemical window test: Assemble a lithium / steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open-circuit voltage of the battery to 6 V at a rate of 1 mV / s, and record the electrochemical window as 4.92 V.

[0067] Example 4

[0068] The entire electrolyte preparation process was carried out in an argon atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The trimethyl phosphate solvent was dehydrated using 4A molecular sieves. The following steps were taken: Under magnetic stirring conditions, weigh 1.2 g of trimethyl phosphate and add it to a vial. Then, add 0.085 g of lithium bis(oxalato)borate (the concentration of lithium ions in lithium bis(oxalato)borate is 0.45 mol / L) under stirring conditions. After the lithium salt is completely dissolved, continue stirring for 30 min until evenly mixed to obtain an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte.

[0069] Assemble a lithium battery with NCM811 of the eight-series ternary nickel-cobalt-manganese as the positive electrode, lithium metal as the negative electrode, and the above-prepared electrolyte. The battery was left standing at room temperature for 24 h until the electrolyte was completely infiltrated.

[0070] Lithium battery cycle performance test: The charge-discharge range of the battery is 2.5 V to 4.25 V, the charge-discharge rate is 0.2 C, and the test temperature is 30 °C. The NCM811 / Li lithium metal battery assembled based on the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte with a high areal loading (17.3 mg / cm -2 ) showed excellent long-cycle stability (the capacity retention rate after 150 cycles was 86%) (see Figure 6 ).

[0071] Combustion test: Place 0.5 g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0 s / g after removing the heat source.

[0072] Electrochemical window test: Assemble a lithium / steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open-circuit voltage of the battery to 6 V at a rate of 1 mV / s, and record the electrochemical window as 5.03 V.

[0073] Example 5

[0074] The entire electrolyte preparation process is carried out in an argon - atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, weigh 1.2 g of trimethyl phosphate and add it to a vial. Then, under stirring conditions, add 0.105 g of lithium bis(oxalato)borate (the concentration of lithium ions in lithium bis(oxalato)difluoroborate is 0.55 mol / L). After the lithium salt is completely dissolved, continue stirring for 30 min until uniformly mixed to obtain an intrinsically non - flammable low - salt - concentration phosphate - based electrolyte;

[0075] A lithium battery assembled with lithium - nickel - cobalt - manganese oxide (NCM9055) of the nine - series ternary system as the positive electrode, lithium metal as the negative electrode, and the above - prepared electrolyte. The battery is left standing at room temperature for 24 h until the electrolyte is completely infiltrated.

[0076] Lithium battery cycle performance test: The charge - discharge range of the battery is 2.5 V - 4.25 V, the charge - discharge rate is 0.2 C, and the test temperature is 30 °C. The NCM9055 / Li lithium - metal battery with a surface loading of 10.5 mg / cm -2 ) assembled based on the intrinsically non - flammable low - salt - concentration phosphate - based electrolyte exhibits excellent long - cycle stability (the capacity retention rate after 200 cycles is 84%).

[0077] Combustion test: Place 0.5 g of the electrolyte in the positive electrode case and ignite it with an open flame. Record the self - extinguishing time as 0 s / g after removing the heat source.

[0078] Electrochemical window test: Assemble a lithium - on - steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open - circuit voltage of the battery to 6 V at a rate of 1 mV / s, and record the electrochemical window as 5.02 V.

[0079] Example 6

[0080] The entire electrolyte preparation process is carried out in an argon - atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, weigh 1.2 g of trimethyl phosphate and add it to a vial. Then, under stirring conditions, add 0.145 g of lithium bis(oxalato)borate (the concentration of lithium ions in lithium bis(oxalato)difluoroborate is 0.75 mol / L). After the lithium salt is completely dissolved, continue stirring for 30 min until uniformly mixed to obtain an intrinsically non - flammable low - salt - concentration phosphate - based electrolyte;

[0081] A lithium battery assembled with lithium cobaltate as the positive electrode, lithium metal as the negative electrode, and the above - prepared electrolyte. The battery is left standing at room temperature for 24 h until the electrolyte is completely infiltrated.

[0082] Lithium battery cycle performance test: The charge and discharge range of the battery is 3V - 4.45V, the charge and discharge rate is 0.2C, and the test temperature is 30°C. The cobalt oxide lithium / Li lithium metal battery with a surface loading of (10.0mg / cm -2 ) assembled based on the intrinsically non-flammable low-salt-concentration phosphate ester-based electrolyte exhibits excellent long-cycle stability (the capacity retention rate after 150 cycles is 85%) (see Figure 7 ).

[0083] Combustion test: Place 0.5g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0s / g after removing the heat source.

[0084] Electrochemical window test: Assemble a lithium-against-steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open-circuit voltage of the battery to 6V at 1mV / s, and record the electrochemical window as 5.11V.

[0085] Example 7

[0086] The entire electrolyte preparation process is carried out in an argon atmosphere glove box (H2O < 0.5ppm, O2 < 0.5ppm). Use 4A molecular sieve to remove water from the trimethyl phosphate solvent. The steps are as follows: Under magnetic stirring conditions, weigh 1.2g of trimethyl phosphate and add it to a vial, and then add 0.085g of lithium bis(oxalato)borate difluoride (the concentration of lithium bis(oxalato)borate difluoride in terms of the molar concentration of lithium ions is 0.4mol / L) under stirring conditions. After the lithium salt is completely dissolved, continue stirring for 30 minutes until evenly mixed to obtain an intrinsically non-flammable low-salt-concentration phosphate ester-based electrolyte;

[0087] Assemble a lithium battery with lithium metal as the negative electrode, a six-series ternary nickel cobalt manganese (NCM622) as the positive electrode, and the above-prepared electrolyte. The battery is left to stand at room temperature for 24h until the electrolyte is completely infiltrated.

[0088] Lithium battery cycle performance test: The charge and discharge range of the battery is 2.5V - 4.25V, the charge and discharge rate is 0.2C, and the test temperature is 30°C. The NCM622 / Li lithium metal battery with a high surface loading of (17.0mg / cm -2 ) assembled based on the phosphate ester-based electrolyte exhibits excellent long-cycle stability (the capacity retention rate after 400 cycles is 88%).

[0089] Combustion test: Place 0.5g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0s / g after removing the heat source.

[0090] Electrochemical window test: Using the prepared electrolyte, a lithium-steel battery was assembled. Using linear sweep voltammetry, the voltage was scanned from the open-circuit voltage of the battery to 6 V at 1 mV / s, and the recorded electrochemical window was 4.87 V.

[0091] Example 8

[0092] The entire electrolyte preparation process was carried out in an argon atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). 4A molecular sieve was used to remove water from the trimethyl phosphate solvent. The following steps were taken: Under magnetic stirring conditions, 1.2 g of trimethyl phosphate was weighed and added to a vial, and then 0.065 g of lithium bis(oxalato)borate difluoride (the concentration of lithium ions in lithium bis(oxalato)borate difluoride was 0.3 mol / L) was added under stirring conditions. After the lithium salt was completely dissolved, stirring was continued for 30 min until uniformly mixed to obtain an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte.

[0093] A lithium battery assembled with lithium manganese oxide as the positive electrode, lithium metal as the negative electrode, and the above-prepared electrolyte was allowed to stand at room temperature for 24 h until the electrolyte was completely infiltrated.

[0094] Lithium battery cycle performance test: The charge-discharge range of the battery was 2.5 V to 4.2 V, the charge-discharge rate was 0.2 C, and the test temperature was 30 °C. The lithium manganese oxide / Li lithium metal battery with a surface loading of 3.0 mg / cm -2 ) assembled based on the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte exhibited excellent long-cycle stability (the capacity retention rate after 100 cycles was 92%).

[0095] Combustion test: 0.5 g of the electrolyte was placed in the positive electrode case and ignited with an open flame. After removing the heat source, the self-extinguishing time was recorded as 0 s / g.

[0096] Electrochemical window test: Using the prepared electrolyte, a lithium-steel battery was assembled. Using linear sweep voltammetry, the voltage was scanned from the open-circuit voltage of the battery to 6 V at 1 mV / s, and the recorded electrochemical window was 4.91 V.

[0097] Example 9

[0098] The entire electrolyte preparation process is carried out in a glove box under an argon atmosphere (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, 1.2 g of trimethyl phosphate is weighed and added to a vial. Then, 0.35 g of lithium perfluorooctyl aluminate (the concentration of lithium ions in lithium perfluorooctyl aluminate is 0.5 mol / L) is added under stirring conditions. After the lithium salt is completely dissolved, stirring is continued for 30 min until homogeneous mixing is achieved, obtaining an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte;

[0099] A lithium battery assembled with a ternary nickel cobalt manganese of the eighth series (NCM811) as the positive electrode, lithium metal as the negative electrode, and the above-prepared electrolyte is allowed to stand at room temperature for 24 h until the electrolyte is completely infiltrated.

[0100] Lithium battery cycle performance test: The charge-discharge range of the battery is 2.5 V to 4.25 V, the charge-discharge rate is 0.2 C, and the test temperature is 30 °C. The NCM811 / Li lithium metal battery assembled based on the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte with a high areal loading (17.3 mg / cm -2 ) exhibits excellent long-cycle stability (the capacity retention rate after 300 cycles is 80%).

[0101] Combustion test: 0.5 g of the electrolyte is placed in the positive electrode case and ignited with an open flame. After removing the heat source, the self-extinguishing time is recorded as 0 s / g.

[0102] Electrochemical window test: Using the prepared electrolyte, a lithium / steel battery is assembled. Using linear sweep voltammetry, it is scanned from the open-circuit voltage of the battery to 6 V at 1 mV / s, and the electrochemical window is recorded as 4.69 V.

[0103] Example 10

[0104] The entire electrolyte preparation process is carried out in a glove box under an argon atmosphere (H2O < 0.5 ppm, O2 < 0.5 ppm). The organic trimethyl phosphate solvent is dehydrated using 4A molecular sieves. The steps are as follows: Under magnetic stirring conditions, 1.2 g of trimethyl phosphate is weighed and added to a vial. Then, 0.45 g of lithium perfluorooctyl aluminate (the concentration of lithium ions in lithium perfluorooctyl aluminate is 0.65 mol / L) is added under stirring conditions. After the lithium salt is completely dissolved, stirring is continued for 30 min until homogeneous mixing is achieved, obtaining an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte;

[0105] A lithium battery assembled with a ternary nickel cobalt manganese of the sixth series (NCM622) as the positive electrode, silicon suboxide as the negative electrode, and the above-prepared electrolyte is allowed to stand at room temperature for 24 h until the electrolyte is completely infiltrated.

[0106] Lithium battery cycle performance test: The charge-discharge range of the battery is 2.5V - 4.2V, the charge-discharge rate is 0.2C, and the test temperature is 30°C. The high areal loading (17.0mg / cm -2 ) NCM622 / silicon monoxide lithium-ion battery assembled with an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte exhibits excellent long-cycle stability (the capacity retention rate after 300 cycles is 72%).

[0107] Combustion test: Place 0.5g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0 s / g after removing the heat source.

[0108] Electrochemical window test: Assemble a lithium-against-steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open-circuit voltage of the battery to 6V at 1 mV / s, and record the electrochemical window as 4.78V.

[0109] Comparative Example 1

[0110] The entire electrolyte preparation process is carried out in an argon atmosphere glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). Use 4A molecular sieve to remove water from the trimethyl phosphate solvent. The steps are as follows: Under magnetic stirring conditions, weigh 1.2g of trimethyl phosphate and add it to a vial. Then, add 0.14g of lithium bis(fluorosulfonyl)imide (the concentration of lithium bis(fluorosulfonyl)imide in terms of the molar concentration of lithium ions is 0.75 mol / L) to the vial under stirring conditions. After the lithium salt is completely dissolved, continue stirring for 30 min until uniformly mixed to obtain the electrolyte;

[0111] A lithium battery assembled with a nine-series ternary nickel cobalt manganese (NCM9055) as the positive electrode, lithium metal as the negative electrode, and the above-prepared electrolyte is allowed to stand at room temperature for 24 h until the electrolyte is completely infiltrated.

[0112] Lithium battery cycle performance test: The charge-discharge range of the battery is 2.5V - 4.25V, the charge-discharge rate is 0.2C, and the test temperature is 30°C. The areal loading (10.5mg / cm -2 ) NCM9055 / lithium metal battery assembled with this electrolyte exhibits excellent long-cycle stability (the capacity retention rate after 10 cycles is 0%) (see Figure 8 ).

[0113] Combustion test: Place 0.5g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0 s / g after removing the heat source.

[0114] Electrochemical window test: Assemble a lithium-against-steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open-circuit voltage of the battery to 6V at 1 mV / s, and record the electrochemical window as 4.34V.

[0115] Comparative Example 2

[0116] The entire electrolyte preparation process was carried out in an argon - atmosphere glove box (H₂O < 0.5 ppm, O₂ < 0.5 ppm). Under magnetic stirring conditions, 3 g of ethylene carbonate and 7 g of propylene carbonate were weighed and dehydrated using 4A molecular sieves respectively, and then added into a vial. Under stirring conditions, lithium hexafluorophosphate salt (the concentration of lithium ions in the lithium hexafluorophosphate salt was 1.0 mol / L in terms of molar concentration) was added thereto. After the lithium salt was completely dissolved, stirring was continued for 30 min until uniformly mixed to obtain an electrolyte; using ternary nickel - cobalt - manganese of the eighth series (NCM811) as the positive electrode and lithium metal as the negative electrode, the assembled lithium battery was left standing at room temperature for 24 h until the electrolyte was completely infiltrated.

[0117] Lithium battery cycle performance test: The charge - discharge range of the battery was 2.5 V - 4.25 V, the charge - discharge rate was 0.2 C, and the test temperature was 30 °C. The NCM811 / lithium metal battery with a surface loading of (17.3 mg / cm -2 ) assembled based on this electrolyte had very poor cycle stability (the capacity retention rate after 120 cycles was 40%) (see Figure 11 ).

[0118] Combustion test: 0.5 g of the electrolyte was placed in the positive electrode case and ignited with an open flame. After removing the heat source, the self - extinguishing time was recorded as 83 s / g (see Figure 9 ).

[0119] Electrochemical window test: Using the prepared electrolyte, a lithium - to - steel battery was assembled. Using linear sweep voltammetry, scanning from the open - circuit voltage of the battery to 6 V at 1 mV / s, the recorded electrochemical window was 4.09 V (see Figure 10 ).

[0120] Comparative Example 3

[0121] The entire electrolyte preparation process was carried out in an argon - atmosphere glove box (H₂O < 0.5 ppm, O₂ < 0.5 ppm). The trimethyl phosphate solvent was dehydrated using 4A molecular sieves. The steps were as follows: Under magnetic stirring conditions, 1.2 g of trimethyl phosphate was weighed and added into a vial, and then 0.9 g of lithium perfluorooctyl aluminate (the concentration of lithium ions in the lithium perfluorooctyl aluminate was 1.3 mol / L in terms of molar concentration) was added under stirring conditions. After the lithium salt was completely dissolved, stirring was continued for 30 min until uniformly mixed to obtain a phosphate - ester - based electrolyte;

[0122] Using ternary nickel - cobalt - manganese of the sixth series (NCM622) as the positive electrode, silicon suboxide as the negative electrode, and the electrolyte prepared above to assemble a lithium battery, and the battery was left standing at room temperature for 24 h until the electrolyte was completely infiltrated.

[0123] Lithium battery cycle performance test: The charge and discharge range of the battery is 2.5V to 4.2V, the charge and discharge rate is 0.2C, and the test temperature is 30°C. The NCM622 / silicon monoxide lithium-ion battery with a surface loading (17.0mg / cm -2 ) assembled based on the phosphate-based electrolyte has very poor cycle performance (the capacity retention rate is close to 0 after 5 cycles) (see Figure 12 ).

[0124] Combustion test: Place 0.5g of the electrolyte in the positive electrode case, ignite it with an open flame, and record the self-extinguishing time as 0s / g after removing the heat source.

[0125] Electrochemical window test: Assemble a lithium / steel battery using the prepared electrolyte. Using linear sweep voltammetry, scan from the open circuit voltage of the battery to 6V at 1mV / s, and record the electrochemical window as 4.44V.

[0126]

[0127] In summary, the present invention uses a mixture of trimethyl phosphate and a low-concentration specific lithium salt to form an intrinsically non-flammable low-salt-concentration phosphate-based electrolyte. This electrolyte can form a solvation structure mainly composed of anion aggregates. Therefore, this electrolyte can preferentially form a stable inorganic interface layer rich in boron (or aluminum) and lithium fluoride on the positive and negative electrodes, protect the positive and negative electrodes, reduce side reactions, and thus can achieve efficient compatibility with high-nickel positive electrodes, lithium cobalt oxide positive electrodes, lithium metal negative electrodes, silicon monoxide negative electrodes, etc. At the same time, this electrolyte has excellent properties such as intrinsic non-flammability, self-extinguishing time of 0s / g, high flash point, and electrochemical window ≥ 4.5V. Therefore, this intrinsically non-flammable low-salt-concentration phosphate-based electrolyte is particularly suitable for high-safety lithium metal batteries and high-safety lithium-ion batteries with silicon monoxide negative electrodes and other fields.

[0128] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively detailed and specific, but it should not be construed as a limitation on the scope of the present invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent should be subject to the appended claims.

Claims

1. An intrinsically non-flammable low-salt-concentration phosphate-based electrolyte, characterized in that, The electrolyte contains a non-flammable phosphate organic solvent and a low-concentration lithium salt, where the low-concentration lithium salt has a molar concentration of lithium ions of 0.3 mol / L to 0.75 mol / L.

2. The intrinsically non-combustible low-salt-concentration phosphate ester-based electrolyte according to claim 1, wherein: The self-extinguishing time of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte is 0 s / g, and the electrochemical window is ≥4.5 V.

3. The intrinsically non-flammable low-salt-concentration phosphate ester-based electrolyte according to claim 1, wherein: The non-flammable phosphate organic solvent is trimethyl phosphate shown in Formula 1.

4. The inherently non-combustible low-salt-concentration phosphate ester-based electrolyte according to claim 1, wherein: The lithium salt is lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyanato)borate, or lithium perfluorooctylaluminate. The lithium salt lithium difluoro(oxalato)borate is shown in Formula 2. The lithium salt lithium bis(oxalato)borate is shown in Formula 3. The lithium salt lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyanato)borate is shown in Formula 4. The lithium salt lithium perfluorooctylaluminate is shown in Formula 5.

5. A method for preparing the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte according to claim 1, characterized in that: a) In a glove box filled with argon, molecular sieves are used to remove water from the non-flammable phosphate organic solvent. b) According to the above ratio, at room temperature and under stirring, the solvent and the lithium salt treated in step a) are mixed evenly to obtain the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte.

6. Use of the inherently non-flammable low-salt-concentration phosphate ester-based electrolyte according to claim 1, characterized in that: Application of the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte in a lithium secondary battery.

7. A lithium secondary battery, a positive electrode, a negative electrode, a separator, and an electrolytic solution interposed between the positive and negative electrodes, characterized in that: The electrolyte is the intrinsically non-flammable low-salt-concentration phosphate-based electrolyte according to claim 1.

8. The lithium secondary battery according to claim 7, wherein: The positive electrode is one of a lithium iron phosphate positive electrode, a lithium manganese iron phosphate positive electrode, a lithium cobaltate positive electrode, a ternary material (such as lithium nickel cobalt manganese oxide NCM, lithium nickel cobalt aluminate NCA), a lithium manganate, a lithium nickel manganate, a lithium-rich manganese-based positive electrode; the negative electrode is one of a lithium metal negative electrode, a lithium alloy negative electrode, a lithium tin alloy negative electrode, a lithium silicon alloy negative electrode, a metal tin negative electrode, a silicon negative electrode, a tin-based oxide negative electrode, a silicon suboxide negative electrode, a lithium titanate negative electrode, hard carbon, soft carbon, carbon nanotubes, graphene, iron oxide.

9. The lithium secondary battery according to claim 7, wherein: The positive electrode surface loading in the lithium secondary battery is ≥ 17.0 mg / cm -2 .

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