A lithium-ion battery for outdoor low-temperature applications

Abstract

This application relates to an outdoor low-temperature lithium-ion battery, comprising a negative electrode material, a positive electrode material, an electrolyte, and a separator. The electrolyte includes a lithium salt, a solvent, and additives. The lithium salt is a fluorinated lithium salt, the solvent is a combination of acetate and carbonate, and the additives are tetraethyl orthosilicate and p-hydroxyanisole. By introducing tetraethyl orthosilicate and p-hydroxyanisole into the electrolyte, this application can dilute the electrolyte at low temperatures and prevent liquefaction of some components at higher temperatures. Furthermore, tetraethyl orthosilicate can prevent the generation of HF from the fluorinated lithium salt, thus protecting the battery performance. P-hydroxyanisole has antioxidant properties and can inhibit the dissolution of positive electrode metal ions at low temperatures, thereby protecting the positive electrode.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, and in particular to a lithium-ion battery for outdoor low-temperature use. Background Technology

[0002] Lithium-ion batteries have been widely used due to their advantages of being lightweight, having high energy density, low self-discharge, long lifespan, high discharge power, and being environmentally friendly. A lithium-ion battery mainly consists of four parts: the positive electrode, the negative electrode, the separator, and the electrolyte. The materials of these four parts directly affect the performance of the lithium-ion battery. However, current lithium-ion batteries are limited by the inherent properties of these four materials. Especially at low temperatures below -20°C, as the electrolyte viscosity increases and the conductivity decreases, the impedance at the electrolyte-electrode interface and the charge transfer impedance increase. Particularly during high-rate charging at low temperatures, lithium metal will precipitate and deposit at the negative electrode. The deposited lithium metal easily undergoes irreversible reactions with the electrolyte, consuming a large amount of electrolyte and significantly increasing the battery's internal resistance. This will greatly impair the battery's low-temperature performance, cycle life, and safety performance. Therefore, improving the low-temperature, high-rate performance of lithium-ion batteries has become a crucial and urgent problem that researchers in the field of lithium-ion batteries need to solve.

[0003] To address the low-temperature rate performance of lithium-ion batteries, improvements are made to the positive electrode material, negative electrode material, and electrolyte. The most common improvement is in the electrolyte. For example, in CN113594544A, "An Electrolyte for Low-Temperature Lithium Batteries and Its Preparation Method," a mixture of low-temperature additives, potassium bromide, phenylcyclohexane, succinate, and fluoroethylene carbonate is added to a conventional electrolyte. However, the addition of potassium bromide introduces potassium ions, which can affect battery life. Phenylcyclohexane has a melting point of around 5°C, and its dilution effect on the solvent is limited at lower temperatures. Succinate, at low temperatures, not only complexes with metal ions in the positive electrode but also affects metal ions in the electrolyte, thus impacting electrolyte performance while protecting the positive electrode. For example, in patent publication number CN109860711A "A low-temperature electrolyte for lithium-ion batteries and its application", acetate, which has a very low melting point, is used as a solvent and boron-containing lithium salt is used as the lithium salt, which can effectively improve the low-temperature performance of lithium batteries. Although the electrolyte does not easily become viscous at low temperatures when acetate is used as a solvent, it is prone to liquefaction at higher temperatures, which affects the formation of the interface film and thus affects the use of the battery.

[0004] Regarding the aforementioned technologies, the inventors believe that while existing solutions can address the issue of lithium-ion batteries operating at low temperatures, they can easily affect other performance characteristics of the lithium battery. Summary of the Invention

[0005] To address the issue of low-temperature use of lithium-ion batteries, this application provides an outdoor low-temperature lithium-ion battery. By adjusting the composition of the battery electrolyte, the low-temperature use of the battery can be effectively addressed without affecting other battery performance characteristics.

[0006] This application provides an outdoor low-temperature lithium-ion battery, which is composed of a negative electrode material, a positive electrode material, an electrolyte, and a separator; the electrolyte includes lithium salt, solvent, and additives.

[0007] The lithium salt is a fluorinated lithium salt, the solvent is a combination of acetate and carbonate, and the additives are tetraethyl orthosilicate and p-hydroxyanisole.

[0008] This application includes tetraethyl orthosilicate as an additive. Tetraethyl orthosilicate has a low melting point and a high boiling point, and under the action of p-hydroxyanisole, it can fully fuse with acetate and carbonate to form a stable mixture. At low temperatures, tetraethyl orthosilicate acts as a diluent, mitigating the increased electrolyte viscosity. Its high boiling point, combined with p-hydroxyanisole, prevents liquefaction of acetate at higher temperatures, promoting the formation of a stable interfacial film and improving the cycle stability of the lithium-ion battery. p-hydroxyanisole also possesses antioxidant properties, inhibiting the dissolution of cathode metal ions at low temperatures and protecting the cathode.

[0009] Preferably, the fluorinated lithium salt is one or a combination of lithium difluorooxalate phosphate (LiBC2O4F2), lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), and lithium trifluoromethanesulfonate (LiCF3SO3); the concentration of the fluorinated lithium salt in the electrolyte is 0.1–3 mol / L.

[0010] The use of fluorinated lithium salts in this application is primarily because they can effectively suppress the decomposition of the electrolyte on the negative electrode surface at low temperatures, while also reducing the impedance of the SEI film, thus exhibiting better cycle stability at low temperatures. Furthermore, fluorinated lithium salts have low association with solvents such as acetate and carbonate, making them easy to dissociate, resulting in higher ionic conductivity of the electrolyte at low temperatures. To address the issue that fluorinated lithium salts may react with trace amounts of water in the electrolyte to form HF acid, thereby affecting battery stability, tetraethyl orthosilicate is added in this application. A small amount of HF acid will interact with the silicon-oxygen bonds, preventing the generated HF acid from affecting battery stability.

[0011] Preferably, the acetate is one or more of methyl acetate, ethyl acetate, butyl acetate, and vinyl acetate; the carbonate is one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate, and the volume ratio of acetate to carbonate is (0.5-1.5):(0.5-1.5).

[0012] Acetate has a relatively low melting point, which can ensure that the electrolyte does not become viscous quickly at low temperatures. The combination of carbonate and acetate can ensure the stability of the SEI film formed at low temperatures. At the same time, the addition of carbonate can also reduce the liquefaction of acetate at higher temperatures to some extent.

[0013] Preferably, the volume ratio of tetraethyl orthosilicate to solvent is (25-45):1, and the concentration of p-hydroxyanisole in the electrolyte is 0.3-0.5 mg / mL.

[0014] In this application, although tetraethyl orthosilicate and p-hydroxyanisole can play a good role, excessive addition will affect the conductivity of the solution and the stability of the battery. Within the above ratio range, the stability of battery performance can be guaranteed and the performance of the battery will not be affected.

[0015] Preferably, the negative electrode material is a graphite negative electrode, the separator is a PP or PE separator, and the positive electrode material is lithium iron phosphate.

[0016] Preferably, the cathode material is carbon-coated lithium iron phosphate.

[0017] In this application, carbon-coated lithium iron phosphate material is used. Carbon coating can reduce the surface resistance of the cathode material, enhance its stability and conductivity, and thus improve the low-temperature performance of the battery.

[0018] Preferably, the method for preparing the carbon-coated lithium iron phosphate material includes the following steps:

[0019] 1) Add humic acid and ferrous chloride to water, add hydrogen peroxide solution under stirring to carry out complexation reaction. After the reaction is complete, add dilute hydrochloric acid to adjust the pH to 3-5, filter, and obtain humic acid-iron complex.

[0020] 2) In step 1), the humic acid-iron complex is added to water, followed by the addition of phosphate and organic acid to adjust the pH of the solution to 1-4. The reaction is then carried out under stirring. After the reaction is complete, precipitation is performed. After precipitation, the solution is filtered and washed to obtain the precipitate.

[0021] 3) Mix the precipitate with the lithium source, then dry it. After drying, calcine it under an inert atmosphere to obtain carbon-coated lithium iron phosphate material.

[0022] In the preparation of the carbon-coated cathode material in this application, humic acid is used as the carbon source. Humic acid has a good complexing effect with iron ions, which can fix iron inside the humic acid. Furthermore, during the complexation process, hydrogen peroxide is used to oxidize ferrous ions to generate new iron ions, which increases the activity of the iron ions, thereby enhancing the complexation effect. Moreover, the carbon bond structure of humic acid itself is a loose network structure. After calcination and carbonization, its specific surface area is large. A larger specific surface area provides more active sites for electrons and ions during charge and discharge, thereby improving the specific capacitance of the battery.

[0023] In the preparation process of this application, iron ions are first complexed, then reacted with phosphate and precipitated, and finally mixed with lithium source and calcined. The iron ions are completely complexed in the carbon source. During the calcination and reaction process, the generated lithium iron phosphate can be coated in the carbon material. Moreover, because the pores in the carbon source are relatively small, the particle size of the generated iron phosphate can be controlled and the dispersibility is high.

[0024] Preferably, in step 1), the mass ratio of humic acid to ferrous chloride is 1:(1.5-3.0); the concentration of ferrous chloride in water is 0.3-1.0 g / mL; the concentration of hydrogen peroxide is 25-30%, and the mass ratio of ferrous chloride to hydrogen peroxide is 1:(0.5-0.8); the complexation reaction temperature is 30-40℃, and the complexation reaction time is 1-2 h.

[0025] Under the above reaction conditions, humic acid and iron ions can be better complexed, ensuring the complexation effect of iron ions.

[0026] Preferably, in step 2), the mass-to-volume ratio of humic acid-iron complex to water is (5-8) g:(20-30) mL, the phosphate is one of ammonium hydrogen phosphate and ammonium dihydrogen phosphate, and the organic acid is one of citric acid, malic acid and malonic acid; the molar ratio of iron to phosphate in the humic acid-iron complex is 1:(1-2); the reaction temperature is 50-70℃, the reaction time is 1-3 h, and the precipitation time is 2-5 h.

[0027] The use of phosphates and organic acids in this application ensures that the phosphorus source and carbon source in the system will not introduce new impurities during the calcination process.

[0028] Preferably, in step 3), the lithium source is lithium hydroxide, the molar ratio of iron in the humic acid-iron complex to lithium in the lithium source is 1:1 to 1.1, the calcination temperature is 800 to 1000°C, and the calcination time is 4 to 6 hours.

[0029] The humic acid in this application has higher stability compared to other carbon sources, therefore its calcination temperature needs to be slightly higher than that of other carbon sources in order to achieve better carbonization.

[0030] In summary, this application includes at least one of the following beneficial technical effects:

[0031] 1. In this application, by introducing the additives tetraethyl orthosilicate and p-hydroxyanisole into the electrolyte, the electrolyte can be diluted at low temperatures and the liquefaction of some components in the electrolyte can be avoided at higher temperatures. Moreover, tetraethyl orthosilicate can prevent the generation of HF from fluorinated lithium salts from affecting battery performance, and p-hydroxyanisole has antioxidant properties that can inhibit the dissolution of positive electrode metal ions at low temperatures, thus protecting the positive electrode.

[0032] 2. In this application, the cathode material uses carbon-coated lithium iron phosphate material, which can reduce the surface resistance of the cathode material, enhance the stability and conductivity of the cathode material, and thus improve the low-temperature performance of the battery.

[0033] 3. The outdoor low-temperature lithium-ion battery provided in this application has good electrochemical performance at both -50℃ and 40℃. Attached Figure Description

[0034] Figure 1 This is the XRD pattern of the carbon-coated lithium iron phosphate material prepared in Example 1; Detailed Implementation

[0035] The technical solution of this application will be described in detail below through the following preparation examples and embodiments.

[0036] Preparation Example 1:

[0037] 1) Add 40g of humic acid and 80g of ferrous chloride to 200mL of water. Add 48g of 30% hydrogen peroxide solution under stirring. Then, carry out the complexation reaction at 35℃ and under stirring for 1.5h. After the reaction is complete, add dilute hydrochloric acid to adjust the pH to 3-4. Filter to obtain humic acid-iron complex. The total filtrate is 253mL. The iron ion concentration is 0.71mmol / mL. The mass of iron in the humic acid-iron complex is 25.2g.

[0038] 2) In step 1), all the humic acid-iron complex was added to 200 mL of water, then 60 g of ammonium hydrogen phosphate was added, and citric acid was added to adjust the pH of the solution to 2. The mixture was then stirred and reacted at 60 °C for 2 h. After the reaction was completed, the mixture was allowed to stand for 3 h to settle, then filtered and washed to obtain the precipitate.

[0039] 3) Mix all the precipitate from step 2) with 10.8g of lithium hydroxide, then dry it. After drying, calcine it at 900℃ for 5 hours under a nitrogen atmosphere to obtain carbon-coated lithium iron phosphate material.

[0040] The XRD pattern of the carbon-coated lithium iron phosphate material prepared in this example is shown below. Figure 1 As shown, its absorption peak is basically consistent with that of lithium iron phosphate material. However, due to carbon coating, its absorption peak has shifted upwards due to the influence of the carbon peak.

[0041] Preparation Example 2

[0042] 1) Add 40g of humic acid and 60g of ferrous chloride to 200mL of water. Add 30g of 30% hydrogen peroxide solution under stirring. Then, carry out the complexation reaction at 40℃ and under stirring for 2h. After the reaction is complete, add dilute hydrochloric acid to adjust the pH to 3-4. Filter to obtain humic acid-iron complex. The total filtrate is 235mL. The iron ion concentration is 0.49mmol / mL. The mass of iron in the humic acid-iron complex is 20.0g.

[0043] 2) In step 1), all the humic acid-iron complex was added to 250 mL of water, then 41.5 g of ammonium dihydrogen phosphate was added, and citric acid was added to adjust the pH of the solution to 2. The mixture was then stirred and reacted at 50 °C for 3 h. After the reaction was completed, the mixture was allowed to stand for 2 h to settle, then filtered and washed to obtain the precipitate.

[0044] 3) Mix all the precipitate from step 2) with 8.6g of lithium hydroxide, then dry it. After drying, calcine it at 800℃ for 6 hours under a nitrogen atmosphere to obtain carbon-coated lithium iron phosphate material.

[0045] Preparation Example 3

[0046] 1) Add 40g of humic acid and 100g of ferrous chloride to 300mL of water. Add 80g of 30% hydrogen peroxide solution under stirring. Then, carry out the complexation reaction at 30℃ and under stirring for 2h. After the reaction is complete, add dilute hydrochloric acid to adjust the pH to 3-4. Filter to obtain humic acid-iron complex. The total filtrate is 392mL. The iron ion concentration is 0.76mmol / mL. The mass of iron in the humic acid-iron complex is 27.4g.

[0047] 2) In step 1), all the humic acid-iron complex was added to 250 mL of water, then 58.2 g of ammonium dihydrogen phosphate was added, and citric acid was added to adjust the pH of the solution to 3. The mixture was then stirred and reacted at 70 °C for 1 h. After the reaction was completed, the mixture was allowed to stand for 5 h to settle, then filtered and washed to obtain the precipitate.

[0048] 3) Mix all the precipitate from step 2) with 11.7g of lithium hydroxide, then dry it. After drying, calcine it for 5 hours under a nitrogen atmosphere and at 1000℃ to obtain carbon-coated lithium iron phosphate material.

[0049] Comparative Preparation Example 1

[0050] The main steps are similar to step 1) in Example 1, except that in this comparative example, ferric chloride is added directly. The specific steps are as follows: 40g of humic acid and 102.4g of ferric chloride are added to 200mL of water, and then a complexation reaction is carried out at 35℃ with stirring for 1.5h. After the reaction is completed, dilute hydrochloric acid is added to adjust the pH to 3-4, and the mixture is filtered to obtain humic acid-iron complex. The total filtrate is 202mL. The iron ion concentration is 1.22mmol / mL, and the mass of iron in the humic acid-iron complex is 21.5g.

[0051] A comparison of the preparation examples in Example 1 and Example 1 shows that, for the same molar concentration of iron ions, the addition of ferrous ions and hydrogen peroxide for activation results in a greater amount of complexed iron ions.

[0052] Example 1

[0053] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of commercially available lithium iron phosphate, and a PP separator.

[0054] The electrolyte composition is as follows: the solvent is methyl acetate and ethylene carbonate; the lithium salt is lithium hexafluorophosphate; the additives are tetraethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of methyl acetate, ethylene carbonate and tetraethyl orthosilicate is 20:20:1; the concentration of lithium hexafluorophosphate in the electrolyte is 1 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.4 g / L.

[0055] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0056] Example 2

[0057] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of commercially available lithium iron phosphate, and a PP separator.

[0058] The electrolyte composition is as follows: the solvent is vinyl acetate and dimethyl carbonate; the lithium salt is lithium tetrafluoroborate; the additives are tetraethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of vinyl acetate, dimethyl carbonate and tetraethyl orthosilicate is 20:15:1; the concentration of lithium tetrafluoroborate in the electrolyte is 1 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.3 g / L.

[0059] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0060] Example 3

[0061] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of commercially available lithium iron phosphate, and a PP separator.

[0062] The electrolyte composition is as follows: the solvent is butyl acetate and propylene carbonate; the lithium salt is lithium trifluoromethanesulfonate; the additives are tetraethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of butyl acetate, propylene carbonate and tetraethyl orthosilicate is 25:20:1; the concentration of lithium trifluoromethanesulfonate in the electrolyte is 1 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.5 g / L.

[0063] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0064] Comparative Example 1

[0065] It is basically the same as Example 1, except that tetraethyl orthosilicate is not added to the electrolyte.

[0066] Comparative Example 2

[0067] It is basically the same as Example 1, except that p-hydroxyanisole is not added to the electrolyte.

[0068] Comparative Example 3

[0069] It is basically the same as Example 1, except that tetraethyl orthosilicate and p-hydroxyanisole are not added to the electrolyte.

[0070] The electrochemical performance of the batteries in Examples 1-3 and Comparative Examples 1-3 was tested at -50°C, 25°C and 40°C, and the ionic conductivity of the electrolyte at -50°C and 25°C was tested. The results are shown in Table 1.

[0071] As can be seen from Table 1, in Examples 1-3, the ionic conductivity of the electrolyte at -50℃ decreases compared to 25℃. This is mainly due to the increased viscosity of the electrolyte as the temperature decreases, but the decrease is not significant, ranging from 0.6 to 1.1 mS / cm. The charge / discharge capacity performance of the batteries in Examples 1-3 at -50℃ decreases compared to 25℃, with a decrease of 8.2% to 8.9%. Although there is a certain decrease, it does not affect the normal use of the batteries. The charge / discharge capacity performance of the batteries in Examples 1-3 at 40℃ decreases compared to 25℃, with a decrease of 2.8% to 3.6%. The decrease is relatively small, mainly due to the liquefaction of acetate in the electrolyte. However, because of the addition of tetraethyl orthosilicate and p-hydroxyanisole, the change is relatively small.

[0072] In Comparative Examples 1-3, the ionic conductivity of the electrolyte at -50°C decreased compared to 25°C, with a decrease of 1.1-1.3 ms / cm, which is relatively large compared to Example 1 (0.6 ms / cm). The charge / discharge capacity performance of the batteries in Comparative Examples 1-3 at -50°C decreased significantly compared to 25°C, with a decrease of 14.2-21.6%, which is relatively large compared to Example 1 (8.6%). This indicates that the addition of tetraethyl orthosilicate and p-hydroxyanisole in this application can improve the low-temperature electrochemical performance of the battery. The charge / discharge capacity performance of the batteries in Comparative Examples 1-3 at 40°C decreased to some extent compared to 25°C, with a decrease of 8.2-13.8%, which is relatively large compared to Example 1 (2.8%). This indicates that tetraethyl orthosilicate and p-hydroxyanisole in this application can improve the liquefaction phenomenon of acetate ester and ensure the battery's performance at higher temperatures.

[0073] Table 1 shows the electrochemical performance of the batteries in Examples 1-3 and Comparative Examples 1-3 at -50°C, 25°C, and 40°C, as well as the ionic conductivity of the electrolytes at -50°C and 25°C.

[0074]

[0075] Example 4

[0076] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of carbon-coated lithium iron phosphate prepared in Preparation Example 1, and a PP separator.

[0077] The electrolyte composition is as follows: the solvent is methyl acetate and ethylene carbonate; the lithium salt is lithium hexafluorophosphate; the additives are tetraethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of methyl acetate, ethylene carbonate and tetraethyl orthosilicate is 20:20:1; the concentration of lithium hexafluorophosphate in the electrolyte is 1 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.4 g / L.

[0078] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0079] Example 5

[0080] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of carbon-coated lithium iron phosphate prepared in Preparation Example 2, and a PP separator.

[0081] The electrolyte composition is as follows: the solvent is vinyl acetate and dimethyl carbonate; the lithium salt is lithium tetrafluoroborate; the additives are tetraethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of vinyl acetate, dimethyl carbonate and tetraethyl orthosilicate is 20:15:1; the concentration of lithium tetrafluoroborate in the electrolyte is 1 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.3 g / L.

[0082] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0083] Example 6

[0084] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of carbon-coated lithium iron phosphate prepared in Preparation Example 2, and a PP separator.

[0085] The electrolyte composition is as follows: the solvent is butyl acetate and propylene carbonate; the lithium salt is lithium trifluoromethanesulfonate; the additives are tetraethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of butyl acetate, propylene carbonate and tetraethyl orthosilicate is 25:20:1; the concentration of lithium trifluoromethanesulfonate in the electrolyte is 1 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.5 g / L.

[0086] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0087] The electrochemical performance of the batteries in Examples 4 to 6 was tested at -50°C, 25°C, and 40°C, and the results are shown in Table 2.

[0088] As shown in Table 2, the charge / discharge capacity performance of the batteries in Examples 4-6 decreases at -50℃ compared to 25℃, with a reduction of 5.7% to 7.3%. Although there is a certain reduction, it does not affect the normal use of the batteries. The charge / discharge capacity performance of the batteries in Examples 4-6 decreases at 40℃ compared to 25℃, with a reduction of 1.9% to 2.2%, which is relatively small. Compared with Examples 1-3, it can be seen that the charge / discharge capacity of the carbon-coated lithium iron phosphate is significantly improved, and the corresponding decrease in charge / discharge capacity at higher and ultra-low temperatures is also less than that of Examples 1-3. This indicates that the carbon-coated lithium iron phosphate in this application can further improve the low-temperature and high-temperature performance of the battery.

[0089] Table 2 shows the electrochemical performance of the batteries in Examples 4-6 at -50°C, 25°C, and 40°C.

[0090]

[0091] Example 7

[0092] An outdoor low-temperature lithium-ion battery comprises a negative electrode material of natural graphite, a positive electrode material of carbon-coated lithium iron phosphate prepared in Preparation Example 1, and a PP separator.

[0093] The electrolyte composition is as follows: the solvent is methyl acetate and ethylene carbonate; the lithium salt is lithium difluorodioxanol phosphate; the additives are ethyl orthosilicate and p-hydroxyanisole; wherein the volume ratio of ethyl acetate, propylene carbonate and ethyl orthosilicate is 20:20:1; the concentration of lithium hexafluorophosphate in the electrolyte is 2 mol / L, and the concentration of p-hydroxyanisole in the solvent is 0.5 g / L.

[0094] By assembling the components as described above, an outdoor low-temperature lithium-ion battery can be obtained.

[0095] Example 8

[0096] It is basically the same as Example 7, except that the volume ratio of ethyl acetate, propylene carbonate and tetraethyl orthosilicate is 15:15:1.

[0097] Example 9

[0098] It is basically the same as Example 7, except that the volume ratio of ethyl acetate, propylene carbonate and tetraethyl orthosilicate is 20:15:1.

[0099] The electrochemical performance of the batteries in Examples 7 to 9 was tested at -50°C, 25°C, and 40°C, and the results are shown in Table 3.

[0100] As can be seen from Table 3, in terms of the charge and discharge capacity of the batteries in Examples 7 to 9, within the range of addition ratios in this application, changing the content of tetraethyl orthosilicate does not have a significant impact on the electrochemical performance of the batteries. However, overall, the charge and discharge capacity is the largest when the ratio of solvent to tetraethyl orthosilicate is 35:1.

[0101] Table 3 shows the electrochemical performance of the batteries in Examples 7-9 at -50°C, 25°C, and 40°C.

[0102]

[0103] Example 10

[0104] It is basically the same as Example 7, except that the concentration of p-hydroxyanisole is 0.4 g / L.

[0105] Example 11

[0106] It is basically the same as Example 7, except that the concentration of p-hydroxyanisole is 0.3 g / L.

[0107] The electrochemical performance of the batteries in Examples 10 and 11 was tested at -50°C, 25°C, and 40°C, and the results are shown in Table 4.

[0108] As can be seen from Table 4, considering the charge and discharge capacities of the batteries in Examples 7, 10, and 11, within the addition ratio range of this application, changing the content of tetraethyl orthosilicate does not have a significant impact on the electrochemical performance of the batteries. However, overall, the electrochemical performance is optimal when the concentration of p-hydroxyanisole is 0.3 g / L.

[0109] Table 4 shows the electrochemical performance of the batteries in Examples 7, 10, and 11 at -50°C, 25°C, and 40°C.

[0110]

[0111] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. An outdoor low-temperature lithium-ion battery, characterized by, It is composed of negative electrode material, positive electrode material, electrolyte and diaphragm; the electrolyte includes lithium salt, solvent and additive; The lithium salt is fluorine-containing lithium salt, the solvent is a combination of acetate and carbonate, and the additive is tetraethyl orthosilicate and p-hydroxyanisole; The negative electrode material is graphite negative electrode, and the diaphragm is PP or PE diaphragm; the positive electrode material is carbon-coated lithium iron phosphate material; The preparation method of the carbon-coated lithium iron phosphate material comprises the following steps: 1) humic acid and ferrous chloride are added to water, hydrogen peroxide solution is added under stirring condition, complexation reaction is carried out, after the reaction is completed, dilute hydrochloric acid is added to adjust the pH to 3-5, filtration is carried out, and a humic acid-iron complex is obtained; 2) the humic acid-iron complex in step 1) is added to water, then phosphate is added, and organic acid is added to adjust the pH of the solution to 1-4, then stirring is carried out, reaction is carried out, after the reaction is completed, sedimentation is carried out, after the sedimentation is completed, filtration and washing are carried out, and a precipitate is obtained; 3) the precipitate is mixed with a lithium source, then drying is carried out, after the drying is completed, calcination is carried out under inert atmosphere, and the carbon-coated lithium iron phosphate material is obtained.

2. The outdoor low-temperature lithium-ion battery according to claim 1, characterized by The fluorine-containing lithium salt is one or a combination of two or more of lithium difluorophosphate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium difluorophosphate and lithium triflate; the concentration of the fluorine-containing lithium salt in the electrolyte is 0.1-3 mol / L.

3. The outdoor low temperature lithium-ion battery of claim 1, wherein, The acetate is one or a combination of two or more of methyl acetate, ethyl acetate, butyl acetate and vinyl acetate; the carbonate is one of vinyl carbonate, dimethyl carbonate, methyl ethyl carbonate and propylene carbonate; the volume ratio of the acetate to the carbonate is (0.5-1.5):(0.5-1.5).

4. The outdoor low temperature lithium-ion battery of claim 1, wherein, The volume ratio of the tetraethyl orthosilicate to the solvent is (25-45):1, and the concentration of the p-hydroxyanisole in the electrolyte is 0.3-0.5 mg / mL.

5. The outdoor low temperature lithium-ion battery of claim 1, wherein, In step 1), the mass ratio of the humic acid to the ferrous chloride is 1:(1.5-3.0); the concentration of the ferrous chloride in water is 0.3-1.0 g / mL; the concentration of the hydrogen peroxide is 25-30%, and the mass ratio of the ferrous chloride to the hydrogen peroxide is 1:(0.5-0.8); the complexation reaction temperature is 30-40℃, and the complexation reaction time is 1-2 h.

6. The outdoor low temperature lithium-ion battery of claim 5, wherein, In step 2), the mass-volume ratio of the humic acid-iron complex to water is (5-8) g:(20-30) mL, the phosphate is one of ammonium hydrogen phosphate and diammonium hydrogen phosphate, and the organic acid is one of citric acid, malic acid and malonic acid; the molar ratio of iron in the humic acid-iron complex to the phosphate is 1:(1-2); the reaction temperature is 50-70℃, the reaction time is 1-3 h, and the sedimentation time is 2-5 h.

7. The outdoor low temperature lithium-ion battery of claim 5, wherein, In step 3), the lithium source is lithium hydroxide, the molar ratio of iron in the humic acid-iron complex to lithium in the lithium source is 1:1-1.1, the calcination temperature is 800-1000℃, and the calcination time is 4-6 h.

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