Low-temperature electrolyte, positive electrode slurry and low-temperature sodium ion battery thereof

Abstract

The application provides a low-temperature electrolyte, a positive electrode slurry and a low-temperature sodium ion battery, and belongs to the technical field of battery materials.The low-temperature electrolyte comprises sodium hexafluorophosphate, propylene carbonate, ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, fluoroethylene carbonate, 2,2,3,4,4,4-hexafluorobutyl acrylate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.The battery comprises the low-temperature electrolyte.The low-temperature electrolyte and the low-temperature sodium ion battery can not only be compatible with blended positive electrode materials, but also take into account the low-temperature performance, ensure that the prepared low-temperature sodium ion battery has good reaction kinetics and cycle performance, and can also improve the energy density of the electrode and reduce the powder resistivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery materials, and relates to a low-temperature electrolyte, a positive electrode slurry and a low-temperature sodium ion battery thereof, in particular to a low-temperature electrolyte, a layered oxide doped polyanion positive electrode slurry and a low-temperature sodium ion battery thereof. BACKGROUND

[0002] Commercial batteries do not work well at low temperatures (usually below 0℃, usually between -20℃ and -40℃). Even at 0℃, the range of electric vehicles is often short. When the temperature drops below freezing, the capacity, voltage, power and life of the battery are greatly reduced. The viscosity of the electrolyte in the electrolyte increases at low temperature, and the ionic conductivity decreases significantly, even the electrolyte freezes at -40℃, hindering the shuttling of sodium ions between the anode and the cathode, increasing the resistance during charging, slowing down the sodium ion insertion kinetics, causing Na to be electroplated at the negative electrode, which can cause side reactions with the electrolyte, causing the electrode / electrolyte interface layer to thicken, and possibly forming large Na dendrites or "dead Na" on the anode surface, causing internal short circuits and thermal runaway safety problems. The sodium ion insertion and extraction at low temperature is limited, which reduces the coulombic efficiency, complicates the charging and discharging, and further reduces the life of the sodium ion battery.

[0003] There are surface coating, lattice doping, structure optimization and other ways to modify the positive electrode at low temperature, and the purpose of improvement is to increase the diffusion of ions, improve the electronic conductivity, and ensure the cycle stability, but there are problems such as high energy consumption, harsh preparation conditions, poor consistency, etc., which bring great challenges to commercial production. In addition, the NFPP electrode has good reaction kinetics and excellent cycle performance, but has problems such as low energy density and low electronic conductivity. O3-type layered compounds have the advantages of high energy density and high compaction, but O3-type layered compounds with high Na content are usually unstable and prone to crystal distortion or multiple phase transitions during electrochemical cycling, with poor reaction kinetics. Blending the two types of materials is a good response measure, which can not only retain good reaction kinetics and cycle performance, but also improve the energy density of the electrode and reduce the powder resistivity. However, due to the different composition elements and structures of the materials, the working voltage range is different, and the electrolyte compatible with it faces great challenges, especially the low-temperature performance. Especially at low temperature, on the one hand, the ionic conductivity decreases and the viscosity increases, slowing down the Na + transport speed in the electrolyte; on the other hand, the desolvation process of solvated Na + is hindered, and the transmission energy barrier of Na + in the SEI increases, thereby reducing the Na +The speed through the interface. Although low-temperature electrolytes suitable for various positive electrode materials have been developed, electrolytes that are actually compatible with mixed positive electrodes of layered oxides-polyanions are currently almost nonexistent.

[0004] Therefore, it is still essential and urgent to develop an electrolyte that can be compatible with mixed positive electrodes while also withstanding extremely low temperatures and a low-temperature sodium ion battery. SUMMARY

[0005] To achieve the above-mentioned purposes, the present application provides a low-temperature electrolyte, a positive electrode slurry and a low-temperature sodium ion battery, which not only can be compatible with blended positive electrode materials, but also can take into account the low-temperature performance, ensure that the prepared low-temperature sodium ion battery has good reaction kinetics and cycle performance, and also can improve the energy density of the electrode and reduce the powder resistivity.

[0006] To achieve the above-mentioned purposes, the technical solutions of the present application are as follows:

[0007] In a first aspect, the present application provides a low-temperature electrolyte, which comprises sodium hexafluorophosphate, a solvent and an additive.

[0008] Further, the solvent is selected from at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC).

[0009] Further, the additive is selected from at least one of fluoroethylene carbonate (FEC), 2,2,3,4,4,4-hexafluorobutyl acrylate (HFA), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TFTFE), tris(trimethylsilyl) phosphite (TMSPi), vinylene carbonate (VC) and 1,3,2-dioxazolothiophene-2,2-dioxide (DTD).

[0010] Further, in the low-temperature electrolyte, the concentration of sodium hexafluorophosphate in the low-temperature electrolyte is 0.1 mol / L-1 mol / L, preferably 0.1 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.8 mol / L or 1 mol / L.

[0011] Further, in the low-temperature electrolyte, the ethylene carbonate (EC) accounts for 10-30%, the propylene carbonate (PC) accounts for 5-20%, the dimethyl carbonate (DMC) accounts for 15-30% and the methyl ethyl carbonate (EMC) accounts for 20-55% by weight percentage.

[0012] Further, in the low-temperature electrolyte, the fluorinated ethylene carbonate (FEC) accounts for 0-15%, the 2,2,3,4,4,4-hexafluorobutyl acetate (HFA) accounts for 0-15%, and the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TFTFE) accounts for 0-15% by weight percentage.

[0013] Further, in the low-temperature electrolyte, the sodium hexafluorophosphate accounts for 12-15%, the ethylene carbonate accounts for 16-18%, the propylene carbonate accounts for 8-9%, the dimethyl carbonate accounts for 16-18%, the methyl ethyl carbonate accounts for 40-44%, the fluorinated ethylene carbonate (FEC) accounts for 0-5%, the 2,2,3,4,4,4-hexafluorobutyl acetate accounts for 0-5%, and the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TFTFE) accounts for 0-5% by weight percentage.

[0014] Further, in the low-temperature electrolyte, the sodium hexafluorophosphate accounts for 12.5-13.5%, the ethylene carbonate accounts for 16-17.5%, the propylene carbonate accounts for 8-9%, the dimethyl carbonate accounts for 16-17.5%, the methyl ethyl carbonate accounts for 41-43.5%, and the fluorinated ethylene carbonate accounts for 4.5-5% by weight percentage.

[0015] Further, in the low-temperature electrolyte, the sodium hexafluorophosphate accounts for 12.5-13.5%, the ethylene carbonate accounts for 16-17.5%, the propylene carbonate accounts for 8-9%, the dimethyl carbonate accounts for 16-17.5%, the methyl ethyl carbonate accounts for 41-43.5%, and the 2,2,3,4,4,4-hexafluorobutyl acetate accounts for 4.5-5% by weight percentage.

[0016] Further, in the low-temperature electrolyte, the sodium hexafluorophosphate accounts for 12.5-13.5%, the ethylene carbonate accounts for 16-17.5%, the propylene carbonate accounts for 8-9%, the dimethyl carbonate accounts for 16-17.5%, the methyl ethyl carbonate accounts for 41-43.5%, and the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether accounts for 4.5-5% by weight percentage.

[0017] Further, in the low-temperature electrolyte, the sodium hexafluorophosphate accounts for 12.5-13.5%, the ethylene carbonate accounts for 16-17.5%, the propylene carbonate accounts for 8-9%, the dimethyl carbonate accounts for 16-17.5%, the methyl ethyl carbonate accounts for 41-43.5%, the fluorinated ethylene carbonate accounts for 2-3%, and the 2,2,3,4,4,4-hexafluorobutyl acetate accounts for 2-3% by weight percentage.

[0018] Further, the low-temperature electrolyte, by weight percentage, sodium hexafluorophosphate accounts for 12.5-13.5%, ethylene carbonate 16-17.5%, propylene carbonate accounts for 8-9%, dimethyl carbonate accounts for 16-17.5%, methyl ethyl carbonate accounts for 41-43.5%, fluoroethylene carbonate accounts for 2-3%, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether accounts for 2-3%.

[0019] Further, the low-temperature electrolyte, by weight percentage, sodium hexafluorophosphate accounts for 12.5-13.5%, ethylene carbonate 16-17.5%, propylene carbonate accounts for 8-9%, dimethyl carbonate accounts for 16-17.5%, methyl ethyl carbonate accounts for 41-43.5%, 2,2,3,4,4,4-hexafluorobutyl accounts for 2-3%, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether accounts for 2-3%.

[0020] Further, the low-temperature electrolyte, by weight percentage, sodium hexafluorophosphate accounts for 12.5-13.5%, ethylene carbonate 16-17.5%, propylene carbonate accounts for 8-9%, dimethyl carbonate accounts for 16-17.5%, methyl ethyl carbonate accounts for 41-43.5%, fluoroethylene carbonate accounts for 1.5-2%, 2,2,3,4,4,4-hexafluorobutyl accounts for 1.5-2%, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether accounts for 1.5-2%.

[0021] Further, in the low-temperature electrolyte, by weight percentage, ethylene carbonate (EC) accounts for 10%-30%, preferably 10%, 15%, 20%, 25%, or 30%.

[0022] Further, in the low-temperature electrolyte, by mass percentage, propylene carbonate accounts for 5%-20%, preferably 5%, 8%, 10%, 15%, or 20%.

[0023] Further, in the low-temperature electrolyte, by mass percentage, dimethyl carbonate accounts for 15%-30%, preferably 15%, 17%, 20%, 25%, or 30%.

[0024] Further, in the low-temperature electrolyte, by mass percentage, methyl ethyl carbonate accounts for 20%-55%, preferably 20%, 25%, 30%, 40%, 45%, 50%, or 55%.

[0025] Further, in the low-temperature electrolyte, by mass percentage, sodium hexafluorophosphate accounts for 5-15%, preferably 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%.

[0026] Further, the content of fluoroethylene carbonate in the low-temperature electrolyte is 0-15% by mass, preferably 0-5%, preferably 1%, 1.5%, 2.0%, 2.5%, 3%, 3.5%, 4.0%, 4.5%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0027] Further, the content of 2,2,3,4,4,4-hexafluorobutyl acrylate in the low-temperature electrolyte is 0-15% by mass, preferably 0-5%, preferably 1%, 1.5%, 2.0%, 2.5%, 3%, 3.5%, 4.0%, 4.5%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0028] Further, the content of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the low-temperature electrolyte is 0-15% by mass, preferably 0-5%, preferably 1%, 1.5%, 2.0%, 2.5%, 3%, 3.5%, 4.0%, 4.5%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0029] In a second aspect, the present application provides a preparation method of a low-temperature electrolyte, comprising: uniformly mixing various solvents to form a multi-component solvent; dissolving sodium hexafluorophosphate in the multi-component solvent to obtain a base electrolyte; and adding additives one by one to the base electrolyte, and after adding each additive, stirring and mixing uniformly before adding the next additive, to obtain the low-temperature electrolyte.

[0030] Further, after adding each additive, stirring and mixing uniformly for 1-3 hours.

[0031] In a third aspect, the present application provides a sodium-ion low-temperature battery comprising the low-temperature electrolyte of the first aspect of the present application or the low-temperature electrolyte prepared by the preparation method of the second aspect of the present application.

[0032] In a fourth aspect, the present application provides a positive electrode slurry comprising a positive electrode active material, a binder, and conductive carbon, wherein the positive electrode active material is a mixture of sodium ferric pyrophosphate (NFPP) and sodium nickel manganese phosphate.

[0033] Further, the weight ratio of the NFM and the NFPP is 1:9-9:1, preferably 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 2:1, 2:3, 2:7, 2:9, 3:1, 3:2, 3:4, 3:5, 4:1, 5:1, 6:1, 7:1, 8:1, or 9:1.

[0034] Further, the binder is polyvinylidene fluoride, and the polyvinylidene fluoride is preferably PVDF5130 or PVDF HSV-900.

[0035] Further, the conductive carbon is selected from at least two of acetylene black, ketjen black, graphite KS-6, graphite KS-15, carbon nanotube, graphene, SP carbon black, and carbon nanotube, and is preferably a mixture of graphene and carbon nanotube at a mass ratio of 1:1.

[0036] Further, in the positive electrode slurry, the positive electrode active material accounts for 80-95%, the binder accounts for 0.5-15%, and the conductive carbon accounts for 0.5-10% by weight percentage.

[0037] Further, in the positive electrode slurry, the positive electrode active material accounts for 80%, 82%, 84%, 86%, 88%, 90%, 92%, 93%, 94%, or 95% by weight percentage.

[0038] Further, in the positive electrode slurry, the binder accounts for 0.5%, 1%, 2%, 3%, 4%, 6%, 7%, 8%, 10%, 12%, 14%, or 15% by weight percentage.

[0039] Further, in the positive electrode slurry, the conductive carbon accounts for 0.5%, 1%, 2%, 3%, 4%, 6%, 7%, 8%, or 10% by weight percentage.

[0040] Further, in the positive electrode slurry, the positive electrode active material accounts for 80%, the binder accounts for 10%, and the conductive carbon accounts for 10% by weight percentage.

[0041] In a fifth aspect, the present application provides a preparation method of the positive electrode slurry of the fourth aspect, comprising the following steps:

[0042] S1: pretreating the binder, the conductive carbon, and the positive electrode material;

[0043] S2: dry powder pre-mixing the conductive carbon and the positive electrode material to obtain a mixture;

[0044] S3: adding the binder solution to the mixture and stirring uniformly to obtain the positive electrode slurry.

[0045] Further, the binder, the conductive carbon, and the positive electrode material all need to be subjected to baking pretreatment. Further, the baking temperature is 110-125℃, and the baking time is 4-6h. Preferably, the baking temperature is 110℃, 115℃, 120℃, or 125℃, and the baking time is 4h, 4.5h, 5h, 5.5h, or 6h.

[0046] Further, the binder needs to be prepared into a binder solution with a solvent. Further, the mass fraction of the binder solution is 6-8%, preferably 6%, 7%, or 8%.

[0047] Further, the binder solution is a PVDF solution with a mass fraction of 6-8% (such as 6%, 7%, or 8%) prepared with N-methyl pyrrolidone (NMP) as a solvent.

[0048] Further, in S3, a solvent needs to be added.

[0049] Further, the device used for the premixing of S2 is a planetary dispersion vacuum stirrer, and the stirring revolution speed used for the premixing of S2 is 1-5 r / min, and the stirring time is 5-10 min.

[0050] Further, the stirring after the addition of the binder in S3 includes stirring at a revolution speed of 10-15 r / min.

[0051] Further, the revolution speed of the stirring after the addition of the binder in S3 is 10-15 r / min.

[0052] Further, the stirring time of the stirring after the addition of the binder in S3 is 5-20 min.

[0053] Further, each of the solvents is independently at least one of N-methyl pyrrolidone, N,N-dimethylformamide, water, ethanol, and toluene.

[0054] In a sixth aspect, the present application provides a preparation method of a positive electrode tab, including the following steps: coating the positive electrode slurry of the present application or the positive electrode slurry prepared by the preparation method of the present application on a current collector, drying, tabbing, and obtaining.

[0055] Further, the current collector is at least one of an aluminum foil, a composite aluminum foil, a copper foil, a steel wire mesh, and a foamed nickel.

[0056] In a seventh aspect, the present application provides a sodium ion battery including the positive electrode slurry of the present application or the positive electrode tab of the present application or the low-temperature electrolyte of the present application.

[0057] In an eighth aspect, the present application provides a use of a sodium ion battery in an energy storage device.

[0058] Beneficial effects:

[0059] In view of the problems of rapid decline of discharge voltage platform, low discharge capacity, poor rate performance, dendrite growth, and serious reduction of the service life of the battery when the temperature is lower than 0℃, improvements are made from two aspects of the positive electrode design and the research on the electrolyte formula, which are specifically shown as follows:

[0060] 1. The ester-based electrolyte is widely used in various electrochemical systems, including half-cell and full-cell tests, which exhibit excellent performance in low-temperature environments. The electrolyte prepared in the present invention is a modified ester-based electrolyte, which enhances electrode stability by using fluorides (fluorinated ethylene carbonate FEC, 2,2,3,4,4,4-hexafluorobutyl acrylate HFA); the addition of fluorides can reduce the chemical and structural heterogeneity of the battery and the influence of hydrogenation in the electrode on the chemical-mechanical coupling of the oxygen structure in the positive electrode layer, forming a thinner NaF-rich anion-dominated CEI film in the positive electrode, further improving the electrode reaction kinetics at low temperature, improving the migration ability of sodium ions at the positive electrode electrolyte film interface, and thus improving the charge and discharge capacity; the addition of low-temperature cosolvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether TFTFE builds a weak solvation structure, and the molecules reduce the Na + -PF6 − Coulombic interaction, intermolecular forces between solvents, resulting in very high Na + mobility, and thus improving the low-temperature performance of the battery. The solvation structure in these electrolytes can spontaneously convert at low temperatures to avoid salt precipitation, thus making the electrolyte temperature-adaptive.

[0061] 2. Although low-temperature electrolytes suitable for various positive electrode materials have been developed in the prior art, no electrolyte has been found that can actually be compatible with mixed positive electrodes of layered oxides-polyanions. The positive electrode prepared from the positive electrode slurry and the battery prepared from the electrolyte provided in the present invention are not only compatible with each other, but also compatible with hard carbon negative electrodes at low temperature-40℃. The discharge efficiency of the developed low-temperature electrolyte at-40℃ is 86.01%, which shows good low-temperature characteristics. In the present invention, "wt%" means weight percentage. BRIEF DESCRIPTION OF DRAWINGS

[0062] Figure 1 The electrolyte flowability test chart of the electrolyte prepared in Example 1 at different temperatures, Figure (a) is the electrolyte flowability test chart at 25℃; Figures (b) and (c) are both electrolyte flowability test charts at-40℃; Figure (b) is a positive and Figure (c) is an inverted chart.

[0063] Figure 2 The dQ / dV chart of the battery in Example 2 during charging and discharging.

[0064] Figure 3 The Nyquist curve chart of the battery in Example 2 at room temperature 25℃.

[0065] Figure 4 The Nyquist curve chart of the battery in Example 2 at room temperature-20℃.

[0066] Figure 5 Figure 3. High and low temperature discharge efficiency plot for the battery in Example 3.

[0067] Figure 6 Figure 4. AC internal resistance plot after high and low temperature discharge for the battery in Example 3.

[0068] Figure 7 Figure 5. Constant current charge capacity ratio plot at -20°C for the battery in Example 3.

[0069] The application is further described in conjunction with specific examples. These examples are intended to be illustrative only and are not intended to limit the scope of the application. DETAILED DESCRIPTION

[0070] The application is further described in conjunction with specific examples. These examples are intended to be illustrative only and are not intended to limit the scope of the application.

[0071] I. Chemicals and Instruments

[0072] Sodium hexafluorophosphate NaPF6 (99.9%), 2,2,3,4,4,4-hexafluorobutyl acrylate HFA (95%), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether TFTFE (99.5%), ethyl methyl carbonate EMC (98%), anhydrous oxalic acid (99%) were purchased from Shanghai Aldrich Biochemical Technology Co., Ltd.; ethylene carbonate EC (99%), propylene carbonate PC (99.5%), fluoroethylene carbonate FEC (99%) were purchased from Shanghai Yinn Chemical Technology Co., Ltd.; dimethyl carbonate DMC (99.9%) was purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd.; N-methyl pyrrolidone NMP (99.5%) was provided by Chongqing Zhongrun New Material Co., Ltd.; 4Å molecular sieve (analytical pure) was purchased from Tianjin Kemio Chemical Reagent Co., Ltd. The positive electrode material sodium ferric pyrophosphate NFPP (NFPP-100), layered oxides (NFM, sodium nickel-iron-manganese acid, MS-XN-33S), and negative electrode hard carbon (type-2) were provided by Shenzhen Keyi Zhida Technology Co., Ltd.

[0073] Planetary dispersion vacuum mixer (XFZ-05L) was purchased from Liuzhou Haogejie Chemical Machinery Co., Ltd.; battery tester (CT-4008-5VA-SI) was purchased from Shenzhen Xinweier Electronics Co., Ltd.; high and low temperature test box (LRHS-800B-L) was purchased from Shanghai Linpian Instrument Co., Ltd.; electrochemical workstation (CHI660E) was purchased from Shanghai Chenhua Instrument Technology Co., Ltd.; vacuum oven (DZF-6020BZ) was purchased from Shanghai Yixi Scientific Instrument Co., Ltd.; pipette gun (Discovery-E+) was purchased from Shanghai Lichen Instrument Technology Co., Ltd.; internal resistance tester (RK2517) was purchased from Shenzhen Meiruike Electronics Technology Co., Ltd.

[0074] Preparation

[0075] 1) Solvent dehydration: In the glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the multi-solvent was mixed in proportion by weight, wherein, calculated based on the total mass of the multi-solvent, vinyl carbonate EC accounted for 10-30 wt%, propylene carbonate PC accounted for 5-20 wt%, dimethyl carbonate DMC accounted for 20-30 wt%, and methyl ethyl carbonate EMC accounted for 20-55 wt%.

[0076] Fluorinated ethylene carbonate FEC, 2,2,3,4,4,4-hexafluorobutyl HFA, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether TFTFE were respectively taken in an appropriate amount of 4 Å molecular sieve and dehydrated for 3-4 days.

[0077] 2) Preparation of electrolyte

[0078] NaPF6 electrolyte: 1 mol of NaPF6 powder was dissolved in 1 L of multi-solvent prepared by EC, PC, DMC and EMC as a reference electrolyte.

[0079] The additives were added to the reference electrolyte as needed, and the added FEC accounted for 0-15%, HFA accounted for 0-15%, and TFTFE accounted for 0-15% based on the total mass of the reference electrolyte.

[0080] 3) Preparation before experiment: The positive electrode materials sodium iron phosphate pyrophosphate NFPP, layered oxide O3 (NFM) and conductive carbon were vacuum baked at 110-130 ℃ for 6-8 h to dry the surface moisture of the materials and keep them dry; hard carbon HC was dried at 90-120 ℃ under vacuum for 3-5 h. Polyvinylidene fluoride PVDF (PVDF5130 or PVDFHSV-900) was dried at 80-110 ℃ for 3-4 h.

[0081] Electrochemical test

[0082] Electrolyte flowability test: The required electrolyte was prepared in a glove box, 1 ml was taken into a glass reagent bottle with a pipette, sealed, and placed in a long-term dehumidification (relative humidity less than 15%) environment at room temperature 25℃, and photographed on the experimental table. Then the electrolyte was placed in a high and low test box at -40℃ for 48h, taken out, and placed in a long-term dehumidification (relative humidity less than 15%) environment at room temperature 25℃, and photographed on the experimental table.

[0083] Button type full cell

[0084] Charge-discharge test: After standing for 4h, the battery was charged at a current density of 0.1C, and the cutoff voltage was 3.9V. After standing for 10 min, the battery was discharged at a current density of 0.1C, and the cutoff voltage was 1.5V. According to NFPP: NFM=6:4, the calculated standard gram capacity was 102 mAh / g.

[0085] Example 1

[0086] Preparation of electrolyte:

[0087] In the glove box, the base electrolyte: NaPF6 electrolyte, NaPF6 powder was dissolved in EC, PC, DMC and EMC mass ratio 2:1:2:5 multi-solvent as the base electrolyte, named electrolyte 1;

[0088] FEC as an additive, the specific preparation process is: take the proportion of FEC in electrolyte 1, stir for 1 h. The addition method of only adding one additive HFA or TFTFE is the same as FEC. Named electrolyte 2-4.

[0089] According to the mass ratio FEC: HFA = 1:1 as an additive, the specific preparation process is: take the proportion of FEC in electrolyte 1, stir for 1 h, then add HFA, stir for 1.5 h. The addition method of two additives is the same as FEC and HFA. Named electrolyte 5-7.

[0090] According to the mass ratio FEC: HFA: TFTFE = 1:1:1 as an additive, the specific preparation process is: take the proportion of FEC in electrolyte 1, stir for 1 h, then add HFA and TFTFE in turn, and the stirring time increases by 0.5 h each time. The obtained electrolyte is named electrolyte 8.

[0091] Table 1: The amount of each electrolyte (unit g)

[0092]

[0093] The prepared electrolytes 1-8 were placed at -40℃ for 48h, taken out from the high and low temperature box, and placed in a long-term dehumidification environment (relative humidity less than 15%)Figure 1 The electrolyte flowability test chart of the electrolyte prepared in Example 1 at different temperatures. As a control, Figure 1 (a) is the state of the electrolyte at room temperature.

[0094] It can be found that, as Figure 1 (b) shows that the electrolyte 1-8 is still clear, and a thin layer of water mist appears on the outside of the glass bottle since it is taken out from the low temperature -40℃ to room temperature; as Figure 1 (c) shows that the electrolyte does not scrape the wall or form a cluster of dendrites due to the inversion, and still has flowability, indicating that the electrolyte 1-8 has good low temperature adaptability.

[0095] Example 2: Full-electric electrochemical test of 2032# button cell

[0096] Preparation before experiment: The positive electrode materials sodium iron phosphate pyrophosphate NFPP, layered oxide O3 (NFM) and conductive carbon are vacuum baked at 120℃ for 7h to dry the surface moisture of the materials and keep them dry; the hard carbon HC is dried at 100℃ in vacuum for 5h. The polyvinylidene fluoride PVDF (PVDF5130 or PVDF HSV-900) is dried at 80℃ for 4h.

[0097] (1) Preparation of positive electrode slurry: the positive electrode active material, conductive carbon and binder are weighed according to the mass ratio of 8:1:1, the ratio of sodium iron phosphate pyrophosphate NFPP to layered oxide (sodium nickel iron manganese acid) in the positive electrode active material is 6:4, and the ratio of carbon nanotube CNTC to graphene GO in the conductive carbon is 1:1. These components are mixed, the mixing equipment used is a planetary dispersion vacuum stirrer, the revolution stirring speed is 3r / min, and the stirring time is 10min; the required amount of PVDF in the formula is added to the PVDF solution (PVDF is prepared into a PVDF solution with a mass fraction of 7% using N-methyl pyrrolidone (NMP) (i.e. polyvinylidene fluoride PVDF5130 binder)), the revolution speed used is 15r / min, the rotation speed is 10r / min, and the stirring time is 20min, to obtain a mixture. An appropriate amount of NMP is added to make the solid content of the mixture 30wt%, and the slurry is stirred until it is uniform to obtain the positive electrode slurry.

[0098] (2) Preparation of negative electrode slurry: the negative electrode active material (hard carbon), conductive carbon, CMC and SBR are weighed according to the mass ratio of 8:1:0.35:0.65, and the ratio of CNTC to GO in the conductive carbon is 1:1. These components are mixed and ground for 12min to obtain a mixture, and the slurry is stirred until it is uniform to obtain the negative electrode slurry.

[0099] (3) Preparation of button cell: the positive electrode slurry and the negative electrode slurry were coated on the aluminum foil respectively by using a doctor blade, dried and cut into several round pieces, and then electrolyte 1-8 was dripped to prepare button full cells. The cells were named according to the electrolyte injected. The cell with electrolyte 1 was named as No. 1 cell, and the others were named similarly.

[0100] (4) The button cells were subjected to charge-discharge at a current density of 0.1 C at room temperature 25℃ in the voltage range of 1.5-3.9 V. Four parallel samples were set for each test, and the median of the most was taken to obtain Table 2. Since the button cell itself has a large ohmic internal resistance, in order to reduce the resistance, the amount of conductive carbon and glue in the positive and negative electrodes is relatively large, which further leads to the fact that the large specific surface area of the conductive carbon consumes more sodium ions during the first charge-discharge, resulting in a low initial efficiency. Therefore, the data obtained are only for comparison on this basis, and do not represent the most optimized formula.

[0101] Table 2 0.1C cycle table of button cell

[0102]

[0103] From the data in Table 2, it can be seen that the best reversible capacity, initial efficiency and 100-week capacity retention rate is No. 8 cell (containing electrolyte 8), which has the lowest reversible capacity of 46 mAhg -1 , the lowest initial efficiency of 45.68%, and the lowest cycle capacity retention rate, with a capacity retention rate of 56.23% after 100 weeks.

[0104] From the data in Table 2, it can be seen that with the addition of different types of low-temperature additives, the reversible capacity, initial efficiency, and charge-discharge capacity retention rate all increase to varying degrees. This may be due to the use of fluoride (fluorinated ethylene carbonate FEC, 2,2,3,4,4,4-hexafluorobutyl acrylate HFA) to enhance the stability of the electrode. The added fluoride can reduce the chemical and structural heterogeneity of the battery and the influence of hydrogenation on the chemical-mechanical coupling of the oxygen structure in the positive electrode layer, allowing the positive electrode to form a thinner CEI film dominated by anions containing NaF, further improving the electrode reaction kinetics at low temperatures and improving the migration ability of sodium ions at the positive electrode electrolyte film interface, thereby improving the charge-discharge capacity. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether TFTFE can construct a weak solvation structure, and the molecule reduces the Na + -PF6 − coulombic interaction between the molecules, the molecular interaction between the solvents, and produces a very high Na +The migration rate is further improved, and the electrochemical performance of the battery is improved. Moreover, the additive in the electrolyte 5-7 is the additive of the two effects (battery No. 5-7), which is better than the effect of the single additive in the electrolyte 2-4 (battery No. 2-4), but the effect of the battery No. 8 is the best, and the electrolyte of the battery No. 8 simultaneously adds three kinds of additives. It may be because the film-forming additive in the electrolyte 2-4 is not enough to make the battery No. 2-4 generate a stable interface film, and the reversible capacity, the initial efficiency and the cycle capacity retention rate are improved compared with the battery No. 1, but there is still a large room for improvement. The electrolyte 5, the electrolyte 6 and the electrolyte 7 are mixed with two kinds of film-forming additives, and the effect is improved, but it is still not as good as the electrolyte 8 mixed with three kinds of additives, which shows that there is a synergistic film-forming effect of three kinds of additives.

[0105] (5) In order to further explore the influence of low-temperature additives on the charge and discharge capacity of the battery, the dQ / dV curves of the button batteries prepared by using eight kinds of electrolytes are taken, and the curve is obtained by drawing the data of the battery prepared by using each kind of electrolyte at room temperature and with a current density of 0.1C at the 100th cycle. Figure 2 .

[0106] It can be seen from Figure 2 that there is a sharp oxidation peak at 3.11V and a sharp reduction peak at 3.10V, which is from the battery No. 8. The good symmetry of the diffraction peak shows that the chemical reaction in the battery has good reversibility, and the sharp peak type shows that the chemical reaction is fast. It is verified again that the electrolyte 8 has an effective gain. In sharp contrast to the battery No. 1 prepared by using the electrolyte 1, the peak shape is hardly seen, and the area surrounded by the oxidation and reduction curves is the smallest, indicating that the capacity at the 100th cycle is very small, and it can be seen from Figure 2 that the capacity is electrolyte 1 < electrolyte 2, electrolyte 3, electrolyte 4 < electrolyte 5, electrolyte 6, electrolyte 7 < electrolyte 8, indicating that the effect of adding three kinds of additives is the best, and the additive of two kinds is better than the additive of only one kind, and the additive without film-forming agent is the worst. The data of the reversible capacity and the cycle capacity retention rate obtained in Table 1 are corresponding.

[0107] (6) In order to further explore the influence factors of the low-temperature additives on the charge and discharge capacity of the battery, the battery which has been cycled for 100 weeks is simultaneously subjected to electrochemical impedance testing, and the Nyquist curve of the battery at room temperature 25℃ is obtained. As shown in Figure 3 , the semi-circle of the battery No. 1 is the smallest, indicating that it has the smallest charge transfer resistance. It may be due to the fact that there is no film-forming additive, the interface film of the electrode is thin and not dense, leading to the dissolution of the transition metal elements of the positive electrode, and the capacity attenuation. The second smallest semi-circle is the battery No. 8, indicating that the reaction speed of the electrode surface is very fast, and the reaction activity of the active material on the electrode surface is good.

[0108] (7) In order to further explore the effect of low-temperature additives on the battery at low temperature, the 1-8 batteries were reassembled, and the electrochemical impedance test was carried out on the battery at-20°C for 50 cycles, and the results are shown in Figure 4 Figure 4 The starting point of the graph shown in

[0109] Preparation and electrochemical performance test of 26650 cylindrical sodium ion battery of example 3

[0110] Battery preparation:

[0111] According to the conventional sodium ion cylindrical battery preparation procedure, slurry, discharging, coating, rolling, cutting, welding lug, winding, baking, liquid injection, formation and container were prepared. The cylindrical battery preparation process is a conventional technology, which will not be described here. The slurry preparation process is as follows:

[0112] The positive electrode slurry was prepared by dry mixing slurry. The total mass of the positive electrode material, solid conductive carbon, anhydrous oxalic acid and conductive carbon solution was calculated. The total proportion of the positive electrode material in the total mass was 94wt%, the ratio of NFM and NFPP was 4:6, and the positive electrode material NFM and NFPP were added in two batches. After adding the positive electrode material, the revolution speed was 5 r / m. Then 1.5wt% of solid conductive carbon (calculated based on the total mass of the positive electrode material, solid conductive carbon, anhydrous oxalic acid and conductive carbon solution) was added for dry powder pre-mixing. The ratio of carbon nanotube CNTC to graphene GO in the conductive carbon was 1:1 (mass ratio), and 0.5wt% of anhydrous oxalic acid was added under normal pressure for 10 min. Then the dry powder was mixed, 2.5wt% (calculated based on the total mass of the positive electrode material, solid conductive carbon, anhydrous oxalic acid and conductive carbon solution) of solid adhesive PVDFHSV-900 was added, and stirred for 10 min under normal pressure. The revolution speed was 10 r / m, then the rotation was started, the speed was 15 r / m, and the duration was 20 min.

[0113] ​Homogenization: Add conductive carbon solution (1.5wt% of the total mass (calculated based on the total mass of the positive material, solid conductive carbon, anhydrous oxalic acid and conductive carbon solution) to the dry powder, the conductive carbon solution is NMP and conductive carbon with a mass ratio of 1:1, the powder is kneaded, scraped after 10 minutes to prevent the powder adhering to the stirring paddle from being unable to be fully infiltrated due to the large wetting angle of the solid-liquid interface. At this time, each material is subjected to high-speed shearing of the stirring paddle and the dispersion blade, and each material rubs against each other. The battery slurry is a pseudoplastic fluid in non-Newtonian fluid, which has shear thinning characteristics. The kneaded slurry is in a high-viscosity and non-flowing state like paste. The homogenization is set to 10 r / min of revolution speed, supplemented by 30 r / min of rotation speed, and the time is 4.5h, and the vacuum degree is greater than 90 Bar.

[0114] Viscosity adjustment: Add 5wt% of NMP solvent based on the total mass of the positive material, solid conductive carbon, anhydrous oxalic acid and conductive carbon solution, and continue the original shearing speed for 0.5h. Finally, vacuum degassing is performed at 15 r / min of revolution speed only for 0.3h.

[0115] The negative electrode slurry is prepared by wet milling. A 2% CMC solution (calculated based on the total mass of the CMC glue, negative hard carbon, conductive carbon, SBR and water, the CMC solution accounts for 2wt% of the total mass) is prepared with water as the solvent. The negative hard carbon (calculated based on the total mass of the CMC solution, negative hard carbon, conductive carbon, SBR and water, the mass of the negative hard carbon accounts for 93wt%) and the conductive carbon (calculated based on the total mass of the CMC solution, negative hard carbon, conductive carbon, SBR and water, the mass of the conductive carbon accounts for 2wt%) are added in sequence, wherein the mass ratio of carbon nanotube CNTC to graphene GO in the conductive carbon is 1:1. The homogenization is set to 10-15 r / min of revolution speed, supplemented by 30-40 r / min of rotation speed, and the time is 3-4h, and the vacuum degree is greater than 90 Bar. SBR and water (weight ratio of 1:1) are added (calculated based on the total mass of the CMC glue, negative hard carbon, conductive carbon, SBR and water, the mass of SBR and water accounts for 3wt% in total), and they are dispersed together for 0.5h. Finally, vacuum degassing is performed at 15 r / min of revolution speed only for 0.3h. SBR is styrene-butadiene rubber.

[0116] Electrochemical performance test

[0117] (1) The prepared 26650 cylindrical sodium ion battery is subjected to formation and capacity test (0.2C charging and discharging in the voltage range of 1.5-3.9V), and it is found that the capacity of the battery is 2500±60 mAh. Then the battery is placed in a constant temperature and humidity test box, and is named as battery 1, battery 2, battery 3, battery 4, battery 5, battery 6, battery 7 and battery 8 according to the injected electrolyte. Each electrolyte is injected into 5 batteries.

[0118] (2) Set high and low temperature charge-discharge process: at 25°C, constant current and constant voltage charging to 3.9V at 0.5C rate, static 10min, set temperature gradient to 55°C / 0°C / -10°C / -20°C / -40°C, each temperature battery static time is 4h, constant current discharge at 0.2 rate Figure 5 .

[0119] As Figure 5 shown, at this temperature of -40°C, the best discharge efficiency is 86.10% of battery 8. The discharge efficiency at -20°C is 93.5%, which is also the best among the same batch of batteries. It is worth noting that this experimental result shows that under the premise of layer oxide doped poly anion, equipped with 1M sodium hexafluorophosphate electrolyte (electrolyte 1), itself also has good low temperature discharge capacity, the discharge efficiency at -40°C is 68.14%, and the discharge efficiency at -20°C is 80.87%. It can be seen that the addition of FHT indeed has a beneficial effect on the low temperature of the battery, which may be because the use of fluoride (fluorinated carbonate FEC, 2,2,3,4,4,4-hexafluorobutyl acrylate HFA) can enhance the stability of the electrode, the added fluoride can reduce the chemical and structural heterogeneity of the battery and the influence of hydrogenation in the electrode on the chemical-mechanical coupling of the layer oxide structure of the positive electrode, so that the positive electrode forms a thinner CEI film dominated by anions with NaF, further improving the electrode reaction kinetics at low temperature, improving the migration ability of sodium ions at the positive electrode electrolyte film interface, and thus improving the charge and discharge capacity; the addition of low temperature cosolvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether TFTFE, the weak solvation structure of the molecule reduces the Na + -PF6 − coulombic interaction, the molecular interaction between solvents, produces very high Na + mobility, thereby improving the low temperature performance of the battery. The solvation structure in these electrolytes can spontaneously convert at low temperature to avoid salt precipitation, thereby making the electrolyte temperature adaptive.

[0120] (3) The cylindrical battery was also tested for AC internal resistance, as shown in Figure 6 . In the process of internal resistance change of a single battery after high and low temperature discharge of the battery, the lowest internal resistance is at 0°C. Among the temperatures from 25°C downward, the internal resistance of battery 8 shows a lower temperature sensitivity and grows the least. The largest growth is from 25°C to 55°C, which increases by 0.85mΩ, which may be caused by the formation of a thicker interface film at high temperature by the low temperature film former. Battery 1 with electrolyte 1 has the largest internal resistance throughout the process, indicating that the additives of the electrolyte can effectively reduce the internal resistance (the interface internal resistance of the electrode and the electrolyte, the migration internal resistance of the ions, and the charge transfer internal resistance of the electrode surface).

[0121] (4) We also tested the low-temperature charge-discharge capability of the cylindrical battery at -20°C. The first charge capacity of the battery at -20°C is as follows: Figure 7 As shown, the battery with the highest charging capacity is 2291.9 mAh, with a constant current charging capacity of 2280.5 mAh, a constant current ratio of 99.4%, and a constant voltage ratio of 0.6%. Since constant voltage charging is a charging method where the current gradually decreases under a certain voltage, it uses a smaller charging current compared to constant current charging. Therefore, a high constant voltage ratio during charging exacerbates concentration polarization and electrochemical polarization. Clearly, at low temperatures, cylindrical battery 1 has a charging capacity of 1937.2 mAh, a constant current charging capacity of 1689.2 mAh, a constant current ratio of 87.2%, and a constant voltage ratio of 12.8%, which is very detrimental to cycling. Figure 7 The blue area represents the constant current ratio of a single battery during charging, and the orange area represents the constant voltage ratio. It is clear that the constant current ratio of batteries 2-4 with one low-temperature additive is greater than that of battery 1 without additive. The constant current ratio of batteries 5-7 with two low-temperature additives is better than that of batteries 2-4 with one additive. The mixed effect of three additives in battery 8 is the best, with the largest charging constant current ratio at a low temperature of -20℃.

[0122] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A sodium-ion battery, characterized in that, Including low-temperature electrolyte and positive electrode plate; The low-temperature electrolyte, by weight percentage, comprises: 10-20% sodium hexafluorophosphate, 10-30% ethylene carbonate, 5-20% propylene carbonate, 15-30% dimethyl carbonate, 20-55% methyl ethyl carbonate, 1-15% fluoroethylene carbonate, 1-15% 2,2,3,4,4,4-hexafluorobutyl acrylate, and 1-15% 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The positive electrode slurry is coated onto the current collector, dried, and then formed into a sheet to obtain the positive electrode sheet. The positive electrode slurry includes a positive electrode active material, a binder, and conductive carbon, wherein the positive electrode active material is a mixture of sodium iron phosphate pyrophosphate and sodium nickel iron manganese oxide.

2. The sodium-ion battery according to claim 1, characterized in that, The low-temperature electrolyte, by weight percentage, comprises 12-15% sodium hexafluorophosphate, 16-18% ethylene carbonate, 8-9% propylene carbonate, 16-18% dimethyl carbonate, 40-44% methyl ethyl carbonate, 1-5% fluoroethylene carbonate, 1-5% 2,2,3,4,4,4-hexafluorobutyl acrylate, and 1-5% 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

3. The sodium-ion battery according to claim 1, characterized in that, The low-temperature electrolyte, by weight percentage, comprises 12.5-13.5% sodium hexafluorophosphate, 16-17.5% ethylene carbonate, 8-9% propylene carbonate, 16-17.5% dimethyl carbonate, 41-43.5% methyl ethyl carbonate, 1.5-2% fluoroethylene carbonate, 1.5-2% 2,2,3,4,4,4-hexafluorobutyl acrylate, and 1.5-2% 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

4. The sodium-ion battery according to claim 1, characterized in that, The method for preparing the low-temperature electrolyte includes: mixing the solvents evenly to prepare a multi-component solvent; dissolving sodium hexafluorophosphate in the multi-component solvent to prepare a basic electrolyte; adding the additives one by one to the basic electrolyte, stirring and mixing before adding the next additive after each addition to obtain the low-temperature electrolyte. Each time an additive is added, it needs to be stirred for 1-3 hours to mix thoroughly before adding the next additive; the solvent is propylene carbonate, ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate; The additives are fluoroethylene carbonate, 2,2,3,4,4,4-hexafluorobutyl acrylate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

5. The sodium-ion battery according to claim 1, characterized in that, The weight ratio of sodium iron phosphate pyrophosphate to sodium nickel iron manganese oxide is 1:9-9:1; the conductive carbon is selected from at least two of acetylene black, Ketjen black, graphite KS-6, graphite KS-15, carbon nanotubes, graphene, SP carbon black, and carbon nanotubes; the binder is polyvinylidene fluoride.

6. The sodium-ion battery according to claim 5, characterized in that, The weight ratio of sodium ferric pyrophosphate to sodium nickel iron manganate is 6:

4. The polyvinylidene fluoride is PVDF5130 or PVDFHSV-900; The conductive carbon is a mixture of graphene and carbon nanotubes, with a mass ratio of graphene to carbon nanotubes of 1:

1.

7. The sodium-ion battery according to claim 1, characterized in that, The positive electrode slurry comprises, by weight percentage: 80-95% positive electrode active material, 0.5-15% binder, and 0.5-10% conductive carbon.

8. The sodium-ion battery according to claim 1, characterized in that, In the positive electrode slurry, by weight percentage, the positive electrode active material accounts for 80%, the binder accounts for 10%, and the conductive carbon accounts for 10%.

9. The sodium-ion battery according to claim 1, characterized in that, The method for preparing the positive electrode slurry includes the following steps: S1: Dry the binder, conductive carbon, and positive electrode material separately; S2: The conductive carbon and positive electrode material pretreated in step S1 are dry powder premixed to obtain a mixture; S3: Add the pretreated adhesive solution from step S1 to the mixture obtained in step S2 and stir until homogeneous.

10. The sodium-ion battery according to claim 9, characterized in that, The adhesive solution refers to an adhesive solution prepared with N-methylpyrrolidone to a mass fraction of 7%.

11. The sodium-ion battery according to claim 9, characterized in that, The equipment used for premixing in step S2 is a planetary dispersion vacuum mixer. The revolution stirring speed for premixing is 1-5 r / min, and the stirring time is 5-10 min.

12. The sodium-ion battery according to claim 9, characterized in that, After adding the binder solution, the revolution speed is 10-15 r / min, the rotation speed is 10-15 r / min, and the stirring time is 5-20 min.

13. Use of the sodium-ion battery of claim 1 in an energy storage device.

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