A non-aqueous electrolyte and a sodium-ion battery

Abstract

The present invention discloses a non-aqueous electrolyte and a sodium-ion battery. The non-aqueous electrolyte includes a sodium salt, a non-aqueous organic solvent, and an additive, and the additive includes bixa orellana di-sodium. The bixa orellana di-sodium additive reduces the consumption of sodium ions in the non-aqueous electrolyte during the formation and film-forming stage. Moreover, the SEI film and CEI film formed by bixa orellana di-sodium are not easily decomposed at high temperatures. In addition, bixa orellana di-sodium can also react with the active oxygen free radicals released by the positive electrode material during the cycling of the sodium-ion battery. Therefore, the sodium-ion battery of the present invention has a high initial capacity and initial Coulombic efficiency, good cycling performance, and good high-temperature storage performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a non-aqueous electrolyte and a sodium-ion battery. Background Art

[0002] Secondary batteries, also known as rechargeable batteries, are batteries that can be repeatedly charged and discharged and used multiple times. Currently, the main secondary battery technologies include lead-acid batteries, nickel-chromium batteries, nickel-metal hydride batteries, and lithium-ion batteries. Lithium-ion batteries have been widely used in the energy storage field due to their high energy density and long lifespan. However, the low natural abundance and uneven distribution of lithium have led to a continuous increase in the cost of lithium-ion batteries. Sodium is directly below lithium in the periodic table and has the closest chemical properties to metallic lithium. Moreover, the sodium reserves in the earth's crust are abundant, thousands of times that of lithium. Therefore, sodium-ion batteries are expected to become a new generation of high-performance and low-cost energy storage technologies. The working principle of sodium-ion batteries is similar to that of lithium-ion batteries, and energy storage and release are achieved through the redox reaction of sodium ions at the positive and negative electrodes. The core components of sodium-ion batteries include a positive electrode, a negative electrode, and an electrolyte. During charging, sodium ions are removed from the positive electrode active material and embedded in the negative electrode active material; during discharging, sodium ions are removed from the negative electrode active material and embedded in the positive electrode active material. Compared with that in lithium-ion batteries, the diffusion coefficient of sodium ions in sodium-ion batteries has a lower diffusion rate. The main reason is that compared with lithium-ion batteries, in sodium-ion batteries, sodium ions with a larger volume and weight perform ion migration to complete the redox reaction during the charge and discharge process. Therefore, during long-term cycling, the capacity decay of sodium-ion batteries is faster than that of currently mature lithium-ion batteries.

[0003] Over the years, extensive research has been conducted on various positive electrode materials for sodium-ion batteries. It has been found that layered transition metal oxides have the advantage of relatively high volumetric energy density. Therefore, layered transition metal oxides have become the preferred positive electrode materials for sodium-ion batteries. However, phase transitions exist in the transition metal oxides themselves. During long-term cycling, the phase transition of the positive electrode material causes the positive electrode material to release active oxygen free radicals. After the active oxygen free radicals come into contact with the electrolyte, they will oxidize the electrolyte and generate gas, which is what we call "cycling gas generation". Therefore, the cycle stability of this layered transition metal oxide positive electrode material is relatively poor; moreover, when this material is in direct contact with the electrolyte at high temperatures, its oxidizing property towards the electrolyte is extremely strong, which will also cause the electrolyte to oxidize and decompose to generate gas. Therefore, the high-temperature stability of this positive electrode material is relatively poor; in addition, since film formation consumes some sodium ions in the electrolyte, the first Coulombic efficiency of sodium-ion batteries will also be relatively low.

[0004] Therefore, there is an urgent need for a non-aqueous electrolyte and a sodium-ion battery to solve the deficiencies of the existing technology. Summary of the Invention

[0005] The object of the present invention is to provide a non-aqueous electrolyte, which is applicable to conventional sodium ion batteries, can improve the first capacity and first Coulombic efficiency of sodium ion batteries, and can also enhance the high-temperature storage and cycling performance of sodium ion batteries.

[0006] Another object of the present invention is to provide a sodium ion battery, which has a relatively high first capacity and first Coulombic efficiency, good cycling performance and good high-temperature storage.

[0007] To achieve the above objects, the present invention provides a non-aqueous electrolyte, which includes a sodium salt, a non-aqueous organic solvent and an additive, and the additive includes bixa orellana disodium.

[0008] The structural formula of bixa orellana disodium (CAS: 33261-81-3) is shown as follows:

[0009]

[0010] Compared with the prior art, the present invention for the first time uses bixa orellana disodium, a sodium salt of a natural pigment, as an additive in the non-aqueous electrolyte. Specifically, when bixa orellana disodium is added to the non-aqueous electrolyte, a part of bixa orellana disodium can replace part of the solvent to participate in the formation of the SEI film and CEI film in the first formation stage. Since the additive itself has a relatively high sodium ion content, the consumption of sodium ions in the non-aqueous electrolyte during the film formation stage of formation is reduced, thereby improving the first capacity and first Coulombic efficiency of the sodium ion battery; at the same time, bixa orellana disodium has strong high-temperature stability due to its unique long-chain structure, so the SEI film and CEI film formed by bixa orellana disodium are not easily decomposed at high temperatures, which blocks the direct contact between the positive electrode material and the electrolyte, and significantly improves the high-temperature storage performance of the sodium ion battery; and another part of bixa orellana disodium that does not participate in film formation can react with the active oxygen free radicals released by the positive electrode material during the cycling of the sodium ion battery due to the presence of numerous unsaturated double bonds, thereby consuming the oxygen free radicals released by the positive electrode, and thus enhancing the cycling performance of the sodium ion battery. Therefore, when bixa orellana disodium is added to the non-aqueous electrolyte, the non-aqueous electrolyte is applicable to conventional sodium ion batteries, can significantly improve the first capacity and first Coulombic efficiency of the sodium ion battery, and can also significantly enhance the cycling performance and high-temperature storage performance of the sodium ion battery.

[0011] Preferably, the weight percentage of bixaorellin disodium of the present invention in the non-aqueous electrolyte is less than or equal to 0.1% and not zero. Specifically, it can be but is not limited to 0.005%, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%. That is, bixaorellin disodium can improve the first Coulombic efficiency of sodium-ion batteries, inhibit the high-temperature gas generation of sodium-ion batteries, and enhance the cycling performance of sodium-ion batteries when added in a very small amount (≤0.1%), and it is a three-functional electrolyte additive for sodium-ion batteries.

[0012] Preferably, the sodium salt of the present invention is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium difluorophosphate (NaDFP), sodium bis(oxalato)borate (NaBOB), sodium difluoro(oxalato)borate (NaDFOB), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).

[0013] Preferably, the non-aqueous organic solvent of the present invention is selected from at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), ethyl acetate (EA), butyl acetate (BA), γ-butyrolactone (GBA), propyl propionate (PP), difluoroethyl acetate (DFEA), and 2,2,2-trifluoroethyl acetate (TFEA).

[0014] Preferably, the weight percentage of the sodium salt of the present invention in the non-aqueous electrolyte is 8-20%. Specifically, it can be but is not limited to 8%, 10%, 12%, 14%, 16%, 18%, 20%.

[0015] Preferably, the weight percentage of the non-aqueous organic solvent of the present invention in the non-aqueous electrolyte is 70-90%. Specifically, it can be but is not limited to 70%, 72%, 75%, 77%, 80%, 82%, 85%, 88%, 90%.

[0016] Preferably, the present invention further includes an additive, which is selected from at least one of tris(trimethylsilyl) borate (TMSB), tris(trimethylsilyl) phosphate (TMSP), 4,4'-bi-1,3-dioxolane-2,2'-dione (BDC), 3,3-bis(vinylsulfonyl) ethylene (BDTD), triallyl phosphate (TAP), tripropargyl phosphate (TPP), vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), diethyl pyrocarbonate (DEPC), 1,3-propane sultone (PS), divinyl sulfone (DTD), and 1,3-propylene glycol cyclic sulfate (PCS). The additive can form a stable passivation film on the surface of the positive electrode material, prevent the oxidative decomposition of the electrolyte on the positive electrode surface, inhibit the dissolution of transition metal ions from the positive electrode, and improve the stability of the structure and interface of the positive electrode material. Therefore, the additive can further improve the high-temperature storage performance and cycling performance of the sodium-ion battery.

[0017] Preferably, the weight percentage of the additive of the present invention in the non-aqueous electrolyte is 0.1-8%. Specifically, it can be but is not limited to 0.1%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5%, 6.5%, 7%, 7.5%, 8%.

[0018] To achieve the above objectives, the present invention also provides a sodium-ion battery, which includes a positive electrode material, a negative electrode material, and the non-aqueous electrolyte mentioned above.

[0019] Compared with the prior art, the non-aqueous electrolyte of the sodium-ion battery of the present invention includes bixa orellana disodium additive. Bixa orellana disodium reduces the consumption of sodium ions in the non-aqueous electrolyte during the formation film stage, and the SEI film and CEI film formed by bixa orellana disodium are not easily decomposed at high temperatures. In addition, bixa orellana disodium can also react with the active oxygen free radicals released by the positive electrode material during the cycling of the sodium-ion battery. Therefore, the sodium-ion battery of the present invention has a higher initial capacity and initial Coulomb efficiency, better cycling performance, and better high-temperature storage performance.

[0020] Preferably, the positive electrode material of the present invention is Na x CoO2, Na x MnO2, NaNi 0.33 Fe 0.33 Mn 0.33 O2, NaFePO4, NaCoPO4, and Na3V2(PO4)3, where 0 < x ≤ 1. Since NaNi 0.33 Fe 0.33 Mn 0.33 O2 has the advantages of convenient synthesis, simple structure, and relatively high voltage platform, the positive electrode material of the present invention is preferably NaNi 0.33 Fe 0.33Mn 0.33 O2, more specifically, it can be pure NaNi 0.33 Fe 0.33 Mn 0.33 O2, or it can also be doped and / or coated NaNi 0.33 Fe 0.33 Mn 0.33 O2.

[0021] Preferably, the negative electrode material of the present invention is one or more of soft carbon, hard carbon, sodium titanate, and metals that can form alloys with sodium. Specifically, the metals that can form alloys with sodium can be, but are not limited to, potassium, aluminum, copper, molybdenum, etc. Detailed implementation manners

[0022] To better illustrate the purpose, technical solution, and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the methods described in the following embodiments are further explanatory descriptions of the present invention and should not be regarded as limitations on the present invention.

[0023] Example 1

[0024] In a glove box filled with nitrogen (O2 < 1 ppm, H2O < 1 ppm), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed evenly according to a mass ratio of 5:4:1 to obtain 84.9 g of non-aqueous organic solvent. Then, 0.1 g of euxanthone disodium (EATD) was added as an additive to obtain a mixed solution. The mixed solution was sealed and packed and placed in a freezer (-4°C) for 2 h and then taken out. In a glove box filled with nitrogen (O2 < 1 ppm, H2O < 1 ppm), 15 g of sodium hexafluorophosphate was slowly added to the mixed solution. After mixing evenly, the electrolyte was prepared.

[0025] The non-aqueous electrolyte formulations of Examples 2 to 14 and Comparative Example 1 are shown in Table 1. The steps for preparing the non-aqueous electrolyte are the same as those in Example 1.

[0026] Table 1 Non-aqueous electrolyte formulations of Examples 1 to 14 and Comparative Example 1

[0027] Group Non-aqueous organic solvent / mass (g) Sodium salt / mass (g) Additive / mass (g) Auxiliary agent / mass (g) Example 1 PC / EMC / DEC(5:4:1) / 84.9 <![CDATA[NaPF6 / 15]]> EATD / 0.1 --- Example 2 PC / EMC / DEC(5:4:1) / 84.8 <![CDATA[NaPF6 / 15]]> EATD / 0.2 --- Example 3 PC / EMC / DEC(5:4:1) / 84.95 <![CDATA[NaPF6 / 15]]> EATD / 0.05 --- Example 4 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 VC / 0.5 Example 5 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 FEC / 0.5 Example 6 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 PS / 0.5 Example 7 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 PCS / 0.5 Example 8 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 DTD / 0.5 Example 9 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 TMSB / 0.5 Example 10 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 TMSP / 0.5 Example 11 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 BDC / 0.5 Example 12 PC / EMC / DEC(5:4:1) / 84.4 <![CDATA[NaPF6 / 15]]> EATD / 0.1 BDTD / 0.5 Example 13 EC / DEC / PC / DFEA(1:3:5:1) / 84.9 <![CDATA[NaPF6 / 14.5+NaDFP / 0.5]]> EATD / 0.1 --- Example 14 EC / EMC / PC / DFEA(1:4:4:1) / 84.9 <![CDATA[NaPF6 / 14.5+NaFSI / 0.5]]> EATD / 0.1 --- Comparative Example 1 PC / EMC / DEC(5:4:1) / 85 <![CDATA[NaPF6 / 15]]> --- ---

[0028] Using NaNi 0.33 Fe 0.33 Mn 0.33 O2 with a maximum charging voltage of 4.0 V as the positive electrode material and hard carbon as the negative electrode material, sodium ion batteries with a theoretical capacity of 1000 mAh were prepared with the non-aqueous electrolytes of Examples 1 to 14 and Comparative Example 1 with reference to the conventional sodium ion battery preparation method, and the first Coulomb efficiency, room temperature cycle performance, high temperature cycle performance, and high temperature storage performance were tested respectively. The test conditions are as follows, and the test results are shown in Table 2.

[0029] First Coulomb efficiency test:

[0030] Prepare a sodium-ion battery with electrolyte injected through steps such as aging, formation, sealing, and grading by conventional means, and record the formation capacity C1, continuous charge capacity C2, and grading capacity C3 during the production process of the sodium-ion battery. Calculate the first Coulomb efficiency according to the following formula:

[0031] First Coulomb efficiency = grading capacity C3 / (formation capacity C1 + continuous charge capacity C2) × 100%.

[0032] Room temperature cycle performance test:

[0033] Place the sodium-ion battery in an environment at 25°C, constantly charge it at a current of 1C until 4.0V, then constantly charge it under a constant voltage until the current drops to 0.05C, and then constantly discharge it at a current of 1C until 3.0V. Repeat this cycle, record the discharge capacity of the first cycle and the last cycle, and calculate the capacity retention rate of the room temperature cycle according to the following formula:

[0034] Capacity retention rate = discharge capacity of the last cycle / discharge capacity of the first cycle × 100%.

[0035] High temperature cycle performance test:

[0036] Place the sodium-ion battery in an environment at 45°C, constantly charge it at a current of 1C until 4.0V, then constantly charge it under a constant voltage until the current drops to 0.05C, and then constantly discharge it at a current of 1C until 3.0V. Repeat this cycle, record the discharge capacity of the first cycle and the last cycle, and calculate the capacity retention rate of the high temperature cycle according to the following formula.

[0037] Capacity retention rate = discharge capacity of the last cycle / discharge capacity of the first cycle × 100%

[0038] High temperature storage test

[0039] Place the sodium-ion battery in an environment at 25°C and charge it at a constant current of 0.5C until it reaches 4.0V, then charge it at a constant voltage until the current drops to 0.05C, and then discharge it at a constant current of 0.5C until it reaches 3.0V. Record the discharge capacity at this time as C0. Then charge the battery at a constant current of 0.5C until it reaches 4.0V and then charge it at a constant voltage until the current drops to 0.05C, and record the battery thickness at this time as T0. Then place the battery in a constant-temperature oven at 60°C for 30 days, take out the battery, and record the battery thickness at this time as T1. Then leave the battery in an environment at 25°C for 2 hours, and discharge it at a constant current of 0.5C in an environment at 25°C until it reaches 3.0V. Record the discharge capacity at this time as C1. Then charge it at a constant current of 0.5C until it reaches 4.0V and then charge it at a constant voltage until the current drops to 0.05C, and discharge it at a constant current of 0.5C until it reaches 3.0V. Record the discharge capacity at this time as C2. Calculate the capacity retention rate, capacity recovery rate, and thickness change rate according to the following formulas:

[0040] Capacity retention rate = C1 / C0 × 100%;

[0041] Capacity recovery rate = C2 / C0 × 100%;

[0042] Thickness change rate = (T1 - T0) / T0 × 100%.

[0043] Table 2 Performance test results of sodium-ion batteries in Examples 1 to 14 and Comparative Example 1

[0044]

[0045] As can be seen from the results in Table 2, the first Coulombic efficiency, normal temperature cycling, high temperature cycling, and high temperature storage performance of Examples 1 to 14 are all better than those of Comparative Example 1. This is because when sodium honokiolate is added to the non-aqueous electrolyte, a part of sodium honokiolate can replace part of the solvent to participate in the formation of the SEI film and CEI film during the first formation stage. Since the additive itself has a relatively high sodium ion content, the consumption of sodium ions in the non-aqueous electrolyte during the film formation stage of formation is reduced, thereby improving the first capacity and first Coulombic efficiency of the sodium ion battery. At the same time, due to its unique long-chain structure, sodium honokiolate has strong high temperature stability. Therefore, the SEI film and CEI film formed by sodium honokiolate are not easily decomposed at high temperatures, which blocks the direct contact between the positive electrode material and the electrolyte, significantly improving the high temperature storage performance of the sodium ion battery. And another part of the sodium honokiolate that does not participate in film formation contains many unsaturated double bonds and can react with the active oxygen free radicals released by the positive electrode material during the cycling of the sodium ion battery, thereby consuming the oxygen free radicals released by the positive electrode, thus improving the cycling performance of the sodium ion battery. Therefore, adding sodium honokiolate to the non-aqueous electrolyte, this non-aqueous electrolyte is suitable for conventional sodium ion batteries, can significantly improve the first capacity and first Coulombic efficiency of the sodium ion battery, and can also significantly improve the cycling performance and high temperature storage performance of the sodium ion battery.

[0046] As can be seen from Table 2, when comparing Example 1 with Example 2, it can be found that when the addition amount of sodium honokiolate exceeds 0.1%, the improvement of the performance of the sodium ion battery is very limited. This may be because there are no more oxygen free radicals precipitated, so the increase in content has a relatively limited effect on improving the performance of the sodium ion battery. This also indicates that the content of sodium honokiolate around 0.1% can already play its best role.

[0047] As can be seen from Table 2, when comparing Example 1 with Examples 4 to 12, it can be found that by adding VC, FEC, PS, DTD, PCS, TMSB, TMSP, BDC, BDTD additives on the basis of the sodium honokiolate additive, the cycling performance and high temperature storage performance of the prepared sodium ion battery are further improved. This indicates that the additives can further improve the high temperature storage performance and cycling performance of the sodium ion battery.

[0048] As can be seen from Table 2, when comparing Example 1 with Examples 13 to 14, it can be found that by adding other sodium salts such as NaDFP and NaFSI on the basis of NaPF6 sodium salt, the cycling performance and high temperature storage performance of the prepared sodium ion battery are further improved. This indicates that mixing two different types of sodium salts can further improve the high temperature storage performance and cycling performance of the sodium ion battery.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A non-aqueous electrolyte, comprising a sodium salt, a non-aqueous organic solvent, and an additive, characterized in that, The additive includes bixin disodium salt, and the weight percentage of bixin disodium salt in the non-aqueous electrolyte is less than or equal to 0.1% and not zero.

2. The non-aqueous electrolyte according to claim 1, characterized in that, The sodium salt is selected from at least one of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(fluorosulfonyl)imide.

3. The non-aqueous electrolyte according to claim 1, characterized in that, The non-aqueous organic solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, ethyl acetate, butyl acetate, γ-butyrolactone, propyl propionate, difluoroethyl acetate, and 2,2,2-trifluoroethyl acetate.

4. The non-aqueous electrolyte according to claim 1, wherein The weight percentage of the sodium salt in the non-aqueous electrolyte is 8-20%.

5. The non-aqueous electrolyte according to claim 1, characterized in that, The weight percentage of the non-aqueous organic solvent in the non-aqueous electrolyte is 70-90%.

6. The non-aqueous electrolyte according to claim 1, wherein It further includes an auxiliary agent, and the auxiliary agent is selected from at least one of tris(trimethylsilyl) borate, tris(trimethylsilyl) phosphate, 4,4'-bi-1,3-dioxolane-2,2'-dione, 3,3'-bi(vinylenesulfate), triallyl phosphate, tripropargyl phosphate, vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, diethyl pyrocarbonate, 1,3-propane sultone, ethylene sulfate, and 1,3-propylene glycol cyclic sulfate.

7. The non-aqueous electrolyte according to claim 6, wherein, The weight percentage of the auxiliary agent in the non-aqueous electrolyte is 0.1-8%.

8. A sodium-ion battery, comprising a positive electrode material and a negative electrode material, characterized in that, It further includes the non-aqueous electrolyte according to any one of claims 1-7.

9. The sodium ion battery according to claim 8, characterized in that, The positive electrode material is Na x CoO2, Na x MnO2, NaNi 0.33 Fe 0.33 Mn 0.33 O2, NaFePO4, NaCoPO4, and at least one of Na3V2(PO4)3, where 0 < x ≤ 1.

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