High-voltage electrolyte additive, electrolyte and sodium ion battery

Abstract

A high voltage electrolyte additive, an electrolyte, and a sodium ion battery, the high voltage electrolyte additive comprising a cyano group and a trifluoromethyl group (or a fluorine group), and optionally comprising an amino group. The high-voltage electrolyte comprises sodium salt, an organic solvent and the high-voltage electrolyte additive. The high-voltage electrolyte additive disclosed by the invention not only can effectively inhibit the dissolution of transition metal ions of a layered oxide positive electrode of a sodium-ion battery under a high-voltage condition, but also can promote the generation of an inorganic positive electrode electrolyte interface (CEI) film containing fluorine and nitrogen, so that the stability of a layered positive electrode / electrolyte interface is further improved; and the cycle performance and the rate capability of the sodium ion battery under high voltage are improved. The high-voltage electrolyte additive is novel in molecular structure, simple in electrolyte formula, low in cost and wide in applicability. Therefore, the electrochemical performance of the sodium-ion battery under high voltage is improved, and the practical application of the sodium-ion battery is further promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a high-voltage electrolyte additive, an electrolyte and a sodium ion battery. Background Art

[0002] As a new type of secondary battery, sodium ion battery has attracted extensive attention in recent years due to its advantages such as low cost and abundant resources. The cathode material is its key component, which affects the energy density, cycle life and safety of the battery. The research on cathode materials of sodium ion batteries mainly focuses on layered transition metal oxides, polyanion compounds and Prussian blue compounds. Layered oxide cathode materials have become the most promising cathode candidates due to their high operating voltage, high reversible capacity, long service life, low production cost and practicality. The advantages of high operating voltage are also accompanied by many problems. Under high voltage working conditions, the solvent in the electrolyte will undergo an excessive oxidation process. This reaction will generate an unstable interface film on the surface of the cathode material, which will significantly affect the electrochemical performance of the battery, such as reducing the battery's charge and discharge efficiency and increasing the internal resistance. More seriously, this unstable interface film may further aggravate the instability of the internal structure of the battery, leading to a series of problems such as the dissolution of transition metal ions and the collapse of the electrode material structure, thereby greatly shortening the battery's service life and increasing safety hazards.

[0003] Electrolyte additive technology has been widely proven to effectively improve the interfacial stability of layered oxide cathode materials under high voltage. The main feature of high-voltage electrolyte additives is that they can be preferentially oxidized and decomposed to form a stable cathode electrolyte interface (CEI) film on the cathode surface, thereby inhibiting the decomposition of other components in the electrolyte, thereby improving the oxidation stability of the electrolyte and improving the electrochemical performance of the battery.

[0004] At present, commonly used high-voltage electrolyte additives are mainly divided into the following categories: (1) F-containing additives: F-containing additives usually play a film-forming role during use. This type of additive is represented by fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), etc., which generate stable CEI containing NaF on the positive electrode surface to inhibit further oxidation of solvent molecules. (2) B-containing additives: B-containing additives mainly include sodium bis(oxalyl)borate (NaBOB), sodium bis(fluoro)oxalyl)borate (NaDFOB), sodium tetrafluoroborate (NaBF4), etc., which decompose to generate B-containing CEI film to protect the positive electrode under high voltage, thereby improving the cycle performance of the battery. (3) Nitrile additives: mainly include succinonitrile (SN), adiponitrile (ADN), sulfonyl dipropionitrile (SDPN), etc. Nitrile additives show electrochemical stability under high voltage and inhibit electrolyte decomposition. They form chemical complexes with metal ions on the surface of the positive electrode material, reduce the generation of strong oxidizing ions, improve the stability of the battery interface, reduce corrosion, and thus improve the safety and stability of the battery.

[0005] Although significant progress has been made in the research of high-voltage electrolyte additives, there are still some challenges. One major problem is that many existing additives have relatively single functions and can usually only solve specific battery problems, such as inhibiting aluminum foil corrosion, improving membrane density, or extending cycle life. This means that in practical applications, a variety of different additives are often required to comprehensively improve battery performance. However, multiple additives increase the complexity of the electrolyte and may affect other battery properties. Therefore, the development of multi-functional additives has become the focus of current research. Summary of the invention

[0006] The purpose of the present invention is to design a high-voltage electrolyte additive that can improve the high-voltage resistance of the electrolyte by means of multi-group collaboration. The additive can form a stable CEI film on the surface of the positive electrode of the battery, complex and reduce the dissolution of transition metal ions, and improve the stability of the interface between the positive electrode and the electrolyte. At the same time, the additive can also optimize the solvation structure of the electrolyte system, reduce the aggregation of excessive solvent molecules under high-voltage conditions, and improve the high-voltage performance of the battery. Finally, the present invention provides an excellent high-voltage electrolyte additive and a high-voltage electrolyte, which helps to improve the cycle and rate performance of sodium ion batteries at high voltage.

[0007] To achieve the above objectives, the present invention provides a high voltage electrolyte additive, an electrolyte and a sodium ion battery.

[0008] Furthermore, the high voltage electrolyte additive is at least one of the structural formula (1);

[0009]

[0010] The R group is one of -F or -CF3 groups, one of the R1 and R2 groups is a -CN group, and the other is one of -F or -CF3 groups.

[0011] Furthermore, the high voltage electrolyte includes an organic solvent, an electrolyte salt and an additive.

[0012] Furthermore, the organic solvent is at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0013] Furthermore, the sodium salt is at least one of sodium hexafluorophosphate (NaPF6), sodium bis(oxalatoborate) (NaBOB), sodium bis(fluorooxalatoborate) (NaDFOB), and sodium tetrafluoroborate (NaBF4).

[0014] Furthermore, the concentration of sodium salt in the electrolyte is 0.2-5 mol / L, preferably 0.5-3 mol / L.

[0015] Furthermore, the mass percentage of the electrolyte additive to the mass percentage of the sodium salt is 0.2%-10%, preferably 1%-5%.

[0016] The invention also provides a high-voltage sodium ion battery.

[0017] The positive electrode material of the sodium ion battery is selected from sodium transition metal layered oxides, the molecular formula of which is Na x TMO2 (x≤1, TM can be one or more 3d transition metals such as Ni, Mn, Fe, Co, Cu, etc.) The negative electrode material includes but is not limited to one of hard carbon and sodium sheet.

[0018] Advantages and beneficial effects of the present invention:

[0019] The present invention provides a modified electrolyte with low cost, simple preparation process and excellent effect. The additive molecule contains cyano, trifluoromethyl or fluorine groups, and may selectively contain amino groups. After the introduction of the additive, the electrochemical stability window of the electrolyte is improved, the cyano group complexes with the transition metal ions to inhibit their dissolution, and reduces the excessive oxidation of the solvent on the positive electrode side, inhibiting the decomposition of the electrolyte. The thin and uniform fluorine-containing / nitrogen-containing interface film formed by the additive effectively prevents the precipitation of transition metal ions and enhances the rapid conduction and diffusion of sodium ions. The electrolyte improves the high voltage stability and cycle performance of the battery, and exhibits excellent electrochemical performance. In the voltage range of 2-4.2V and the current density of 200mA / g, NaNi 1 / 3 Fe 1 / 3Mn 1 / 3The 100-cycle capacity retention rate of the O2 (NFM111) positive electrode material increased from 44.13% to 82.55%; in the 2-4.3V voltage range, at a current density of 100mA / g, the 170-cycle capacity retention rate was 78.99%. At a high current of 10C, the capacity increased from 46.3mAh / g to 77.9mAh / g, and when returning to a low current of 0.5C, the capacity retention rate increased from 86.45% to 91.19%. This electrolyte system effectively improves the capacity, rate performance and cycle life of the battery, has a good competitive advantage, and has practical application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : Electrochemical stability window test of two electrolytes;

[0021] Figure 2 :Screening diagram of different additive dosages, first activated at 20mA / g current density for five cycles, then cycled at 100mA / g current density for 100 cycles, the test voltage window is 2.0-4.2V (vs.Na + / Na). Finally, the optimal performance dosage was selected according to the capacity retention rate. Figure 2 The results show that 3% is the best dosage;

[0022] Figure 3 : Rate performance diagram of the comparative example and the embodiment at different current densities;

[0023] Figure 4 : 2-4.2V (200mA / g) cycle comparison of the two systems;

[0024] Figure 5 : Scanning electron microscope (SEM) images of the positive electrode after 100 cycles: (a) Example 1; (b) Comparative Example 1; The example maintains the original structure after the cycle, proving that the interface derived from the additive effectively protects the positive electrode material, while the comparative example has no obvious layered structure and is full of cracks;

[0025] Figure 6 : Transmission electron microscopy (TEM) images of the interface after 15 cycles: (a) Example 1; (b) Comparative Example 1; Compared with the thick and uneven interface film CEI of the comparative example, the CEI containing the additive is thinner and more uniform; DETAILED DESCRIPTION

[0026] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0027] Embodiment 1:

[0028] 1. Preparation of electrolyte: EC and DEC were mixed in a volume ratio of 1:1, and then a corresponding amount of NaPF6 was added to obtain a 1 mol / L electrolyte. After the sodium salt was dissolved, 3% of the additive 4-amino-2-trifluoromethylbenzonitrile (ATMBN) was added.

[0029] 2. Preparation of positive electrode sheet: Mix the nickel iron sodium manganese oxide ternary material NFM111, the conductive agent SuperP, the adhesive PVDF and NMP in a mass ratio of 8:1:1, apply it on aluminum foil, and vacuum dry it at 100°C for 8h. Cut the electrode sheet into a working electrode with a diameter of 10mm.

[0030] 3. Roll the sodium block into a thinner sodium sheet, and cut it into 12mm round sodium sheets as the counter electrode.

[0031] 4. Cut the glass fiber diaphragm into 12mm, place it in a vacuum oven, and vacuum dry it at 80℃ for 12h.

[0032] 5. Sodium ion battery assembly: Assemble into CR2032 button half-cells in a glove box filled with argon.

[0033] Embodiments 2 to 3:

[0034] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet and the assembly of the sodium ion battery are the same as in Example 1, but the contents of the additives are 1.0% and 5.0% respectively.

[0035] Embodiment 4:

[0036] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet and the assembly of the sodium ion battery are the same as in Example 1, but the additive is the commonly used commercial additive fluoroethylene carbonate.

[0037] Embodiment 5:

[0038] The preparation of the electrolyte, the negative electrode sheet and the assembly of the sodium ion battery are the same as those in Example 1, except that one side of the positive electrode sheet is replaced with a stainless steel sheet.

[0039] Comparative Example 1:

[0040] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet and the assembly of the sodium ion battery are the same as those in Examples 1 to 4, except that no additive is added to the electrolyte.

[0041] Comparative Example 2:

[0042] The preparation of the electrolyte, the negative electrode sheet and the assembly of the sodium ion battery are the same as those in Example 5, except that no additive is added to the electrolyte.

[0043] The electrolyte components and battery systems of the above examples and comparative examples are shown in Table 1.

[0044] Table 1: Electrolyte compositions and battery systems of the embodiments and comparative examples

[0045] Electrolyte composition Battery system Example 1 <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% ATMBN additive]]> NFM111 / Na Example 2 <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 1% ATMBN additive]]> NFM111 / Na Example 3 <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 5% ATMBN additive]]> NFM111 / Na Example 4 <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% FEC additive]]> NFM111 / Na Example 5 <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% ATMBN additive]]> Stainless steel / Na Comparative Example 1 <![CDATA[1MNaPF6inEC:DEC=1:1]]> NFM111 / Na Comparative Example 2 <![CDATA[1MNaPF6inEC:DEC=1:1]]> Stainless steel / Na

[0046] Test Example 1: Electrochemical Cycle Performance Test

[0047] After the battery was left at 25°C for 6 hours, it was tested with the Land system and charged to 4.2V at a rate of 0.2C (1C = 100mA / g), and then discharged to 2.0V at a current of 0.2C. The charging and discharging steps were repeated for 5 weeks for activation, and then charged to 4.2V at currents of 0.5C, 1C, and 2C, respectively, and then discharged to 2.0V at currents of 0.5C, 1C, and 2C, respectively. The discharge capacity was recorded as C0, and the discharge capacity at the 100th week was obtained as C 100 , capacity retention rate = C 100 / C0*100%.

[0048] After the battery was left at 25°C for 6 hours, it was tested with the Land system, charged to 4.3V at a rate of 0.2C (1C = 100mA / g), and then discharged to 2.0V at a current of 0.2C. The charging and discharging steps were repeated for 5 weeks for activation, and then charged to 4.3V at a current of 1C, and then discharged to 2.0V at a current of 1C. The discharge capacity was recorded as C0, and the discharge capacity at the 170th week was obtained. 170 , capacity retention rate = C 170 / C0*100%.

[0049] Test results:

[0050] Table 2: Cycle test results

[0051] Electrolyte composition Current density Voltage range Capacity retention rate <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% ATMBN additive]]> 100mA / g 2.0-4.2V 100 laps: 80.8% <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% ATMBN additive]]> 200mA / g 2.0-4.2V 100 laps: 82.55% <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% ATMBN additive]]> 100mA / g 2.0-4.3V 170 laps: 78.99% <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% ATMBN additive]]> 50mA / g 2.0-4.2V 100 laps: 79.09% <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 1% ATMBN additive]]> 100mA / g 2.0-4.2V 100 laps: 82.22% <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 5% ATMBN additive]]> 100mA / g 2.0-4.2V 100 laps: 62.64% <![CDATA[1 M NaPF6 in EC:DEC = 1:1, 3% FEC additive]]> 100mA / g 2.0-4.2V 100 laps: 46.6% <![CDATA[1MNaPF6inEC:DEC=1:1]]> 100mA / g 2.0-4.2V 100 laps: 55.3% <![CDATA[1MNaPF6inEC:DEC=1:1]]> 200mA / g 2.0-4.2V 100 laps: 44.13% <![CDATA[1MNaPF6inEC:DEC=1:1]]> 100mA / g 2.0-4.3V 170 laps: 32.08% <![CDATA[1MNaPF6inEC:DEC=1:1]]> 50mA / g 2.0-4.2V 100 laps: 49.31%

[0052] Test Example 2: Electrochemical rate performance test

[0053] The battery was left at 25°C for 6 hours and then charged and discharged using the Land system. During the test, the battery was charged to 4.2V at 0.2C and then discharged to 2.0V, repeated for 5 weeks, followed by 0.5C, 1C, 2C, 5C and 10C, each rate was repeated for 5 weeks. Finally, the battery was charged and discharged at 1C and 0.5C, respectively, for 5 weeks.

[0054] Test Example 3: Linear Sweep Voltammetry Test

[0055] After the Na-stainless steel battery was left at room temperature for 6 hours, a linear sweep voltammetry test was performed using a Chenhua electrochemical workstation. The scan voltage range was from open circuit voltage to 6.0 V, and the scan rate was 1 mV / s.

[0056] The electrochemical stability window test results of the two electrolytes in Example 5 and Comparative Example 2 are as follows: Figure 1 shown.

[0057] The screening results of different additive dosages are as follows Figure 2 As shown, the optimal dosage for performance was screened according to the capacity retention rate, indicating that 3% is the optimal dosage.

[0058] The rate performance diagrams of Example 1 and Comparative Example 1 at different current densities are shown in FIG. Figure 3 shown.

[0059] The 2-4.2V (200mA / g) cycle comparison diagram of the two systems of Example 1 and Comparative Example 1 is shown in the figure below: Figure 4 shown.

[0060] The positive electrode scanning electron microscope (SEM) images of Example 1 and Comparative Example 1 after 100 cycles are as follows: Figure 5 As shown, Example 1 maintains its original structure after cycling, proving that the interface derived from the additive effectively protects the positive electrode material, while Comparative Example 1 has no obvious layered structure and is covered with cracks.

[0061] The interface transmission electron microscope (TEM) images of Example 1 and Comparative Example 1 after 15 cycles are as follows: Figure 6 As shown, compared with the thick and non-uniform interface film CEI of Comparative Example 1, the CEI containing additives in Example 1 is thinner and more uniform.

[0062] It should be further explained that the above embodiments are only used to understand the technical solution of the present invention, and are not used to limit the protection scope of the present invention. Any obvious adjustments and modifications made to the technical solution of the present invention that belong to the technical concept of the present invention should also fall within the protection scope of the present invention.

Claims

1. A high voltage electrolyte additive, characterized in that: The additive is one of the following structural formula (1), wherein the R group is one of -F or -CF3 groups, one of the R1 and R2 groups is a -CN group, and the other is one of -F or -CF3 groups; 2. A high voltage electrolyte, characterized in that: The invention comprises a sodium salt, an organic solvent and the high voltage electrolyte additive as claimed in claim 1.

3. A high voltage electrolyte according to claim 2, characterized in that: The mass percentage of the high voltage electrolyte additive to the mass percentage of the sodium salt is 0.2%-10%.

4. A high voltage electrolyte according to claim 2, characterized in that: The sodium salt is at least one of sodium hexafluorophosphate NaPF6, sodium bis(oxalatoborate) NaBOB, sodium bis(fluorooxalatoborate) NaDFOB, and sodium tetrafluoroborate NaBF4.

5. A high voltage electrolyte according to claim 2, characterized in that: The concentration of the sodium salt in the electrolyte is 0.2-5 mol / L.

6. A high voltage electrolyte according to claim 2, characterized in that: The organic solvent is selected from one or more of chain and cyclic carbonates. The cyclic carbonate solvent refers to ethylene carbonate EC, fluoroethylene carbonate FEC or propylene carbonate PC; the chain carbonate solvent refers to dimethyl carbonate DMC, diethyl carbonate DEC or ethyl methyl carbonate EMC.

7. A high voltage sodium ion battery, characterized in that: The positive electrode material of the sodium ion battery is selected from layered transition metal oxides of sodium, the molecular formula of which is Na x TMO2, wherein x≤1, TM is one or more of 3d transition metals among Ni, Mn, Fe, Co, and Cu; the negative electrode material includes but is not limited to one of hard carbon or sodium sheet.

8. A method for preparing a high voltage sodium ion battery according to claim 7, characterized in that: The following steps are involved: 1) Preparation of electrolyte: one or more organic solvents of chain and cyclic carbonates are mixed at a volume ratio of 1:1, and a corresponding amount of sodium salt is added to obtain a 1 mol / L electrolyte. After the sodium salt is dissolved, 0.2%-10% by mass fraction of the additive described in claim 1 is added; 2) Preparation of positive electrode: The ternary material Na x TMO2, conductive agent SuperP, adhesive PVDF and NMP are mixed evenly at a mass ratio of 8:1:1, coated on aluminum foil or stainless steel sheet, and vacuum dried at 100°C for 8 hours. The electrode sheet is cut into a working electrode with a diameter of 10 mm, wherein x≤1, TM is one or more 3d transition metals among Ni, Mn, Fe, Co and Cu; 3) Cut one of the hard carbon or sodium sheets into 12 mm circles as the counter electrode; 4) Cut the glass fiber diaphragm into 12 mm pieces, place it in a vacuum oven, and vacuum dry it at 80°C for 12 h. 5) Sodium ion battery assembly: Assemble into CR2032 type button half-cells in a glove box filled with argon.

9. The method for preparing a high voltage sodium ion battery according to claim 8, characterized in that: The cyclic carbonate solvents refer to ethylene carbonate EC, fluoroethylene carbonate FEC or propylene carbonate PC; the chain carbonate solvents refer to dimethyl carbonate DMC, diethyl carbonate DEC or ethyl methyl carbonate EMC.

10. The method for preparing a high voltage sodium ion battery according to claim 8, characterized in that: The sodium salt is at least one of sodium hexafluorophosphate NaPF6, sodium bis(oxalatoborate) NaBOB, sodium bis(fluorooxalatoborate) NaDFOB, and sodium tetrafluoroborate NaBF4.

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