Non-aqueous electrolyte and secondary battery thereof

Abstract

The present invention provides a non-aqueous electrolyte and a secondary battery thereof. The non-aqueous electrolyte includes a lithium salt, a non-aqueous organic solvent, and an additive, and the additive includes compound A represented by structural formula I or compound B represented by structural formula II. The additive of the electrolyte of the present invention includes compound A or compound B, which is a natural ginger extract. When the positive electrode material undergoes a phase change and oxygen free radicals are precipitated during the formation and initial cycle of the secondary battery, compound A or compound B can be dehydrogenated and oxidized to provide electron free radicals, thereby effectively capturing the oxygen free radicals generated by the phase change of the positive electrode material to terminate the chain reaction of the oxygen free radicals generated by the positive electrode material, thereby inhibiting the adverse phase change of the positive electrode material, and further inhibiting the dissolution of transition metal ions in the positive electrode material. Therefore, the electrochemical performance such as the cycle of the secondary battery can be improved.

Description

Technical Field

[0001] The present invention relates to the field of energy storage devices, and particularly to a non-aqueous electrolyte and a secondary battery thereof. Background Art

[0002] Secondary batteries are widely used in fields such as 3C digital, power tools, aerospace, energy storage, and electric vehicles due to their advantages of high specific energy, no memory effect, and long cycle life. The rapid development of electronic information technology and consumer products has put forward higher requirements for the high voltage and high energy density of secondary batteries.

[0003] However, a higher voltage means a greater amount of lithium deintercalation from the positive electrode material, which further means the instability of the structure of the positive electrode material. Once the structure of the positive electrode material is unstable, oxygen evolution is likely to occur during long-term cycling. For example, high-voltage lithium cobalt oxide materials will undergo a phase change from O3 to O1, structure collapse, and oxygen evolution during long-term cycling with a charging voltage exceeding 4.5V. High-nickel ternary systems such as NCM811 or NCA systems will also undergo a phase change from H2 to H3 during long-term cycling with a charging voltage exceeding 4.15V, which will then lead to structure collapse and oxygen evolution of the positive electrode. Therefore, to pursue high energy density and increase the voltage of secondary batteries, oxygen evolution is an inevitable problem. However, oxygen evolution will accelerate the oxidative decomposition of the electrolyte by oxygen, further causing the failure of additives and solvents, and ultimately resulting in a battery voltage drop. Moreover, too much oxygen content in the electrolyte is more likely to cause battery explosion under thermal runaway conditions.

[0004] At present, there are many means in the biological and food industries to capture oxygen free radicals and singlet oxygen, but such means are rarely used in electrolytes. Summary of the Invention

[0005] The purpose of the present invention is to provide a non-aqueous electrolyte and a secondary battery thereof. The additive in this non-aqueous electrolyte can capture oxygen free radicals to avoid oxygen evolution from accelerating the oxidative decomposition of the electrolyte, thereby improving the electrochemical performance of the secondary battery. This electrolyte is particularly suitable for secondary batteries with high-nickel positive electrode material systems.

[0006] To achieve the above purpose, in the first aspect of the present invention, a non-aqueous electrolyte is provided, which includes a lithium salt, a non-aqueous organic solvent, and an additive. The additive includes compound A with the structural formula I or compound B shown in the structural formula II.

[0007]

[0008] Compared with the prior art, the additive of the electrolyte of the present invention includes compound A or compound B, which is a natural ginger extract. When the cathode material undergoes a phase change and oxygen free radicals are precipitated during the formation and initial cycle of the secondary battery, compound A or compound B can dehydrogenate and provide electron free radicals, thereby effectively capturing the oxygen free radicals generated by the phase change of the cathode material to terminate the chain reaction of oxygen free radical generation in the cathode material, thereby inhibiting the adverse phase change in the cathode material, and further inhibiting the dissolution of transition metal ions in the cathode material. Therefore, the electrochemical performance such as the cycle of the secondary battery can be improved.

[0009] Preferably, the mass percentage of compound A or compound B in the non-aqueous electrolyte is 0.1 to 0.5%. Specifically but not limited to 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%.

[0010] Preferably, the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiDFP), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluoro bis(oxalato)phosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium tetrafluoro(oxalato)phosphate (LiTFOP), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI). The content of the lithium salt accounts for 6 to 16% of the mass of the non-aqueous electrolyte, and the concentration of the lithium salt in the non-aqueous electrolyte is 0.5 to 1.5 mol / L.

[0011] Preferably, the non-aqueous organic solvent is selected from chain carbonates, cyclic carbonates or carboxylic acid esters. The non-aqueous organic solvent is selected from at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), n-butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), n-propyl propionate (n-PP), ethyl propionate (EP), ethyl butyrate (EB), fluoroethylene carbonate (FEC), 2,2-difluoroethyl acetate (2,2-DFEA), and 2,2,2-trifluoroethyl acetate (2,2,2-TFEA). Preferably, the non-aqueous organic solvent is at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), 2,2-difluoroethyl acetate (2,2-DFEA), and 2,2,2-trifluoroethyl acetate (2,2,2-TFEA). The non-aqueous organic solvent accounts for 65 to 85% of the weight of the non-aqueous electrolyte.

[0012] Preferably, it further includes an additive accounting for 0.5-10% of the mass of the electrolyte, and the additive is selected from at least one of ethyl methyl 2,2,2-trifluorocarbonate (TFEMC), diethyl 2,2,2-trifluorocarbonate (TFDEC), ethyl propyl 2,2,2-trifluorocarbonate (TFPC), vinylene carbonate (VC), difluoroethylene carbonate (DFEC), 1,3-propane sultone (PS), divinyl sulfate (DTD), tris(trimethylsilyl) phosphate (TMSP), 4,4'-bioxolane-2,2'-dione (BDC), 3,3'-bioxolane-2,2'-dione (BDTD), and tripropargyl phosphate (TPP). These additives can form a stable passivation film on the surface of the positive electrode, prevent the oxidative decomposition of the electrolyte on the surface of the positive electrode, inhibit the dissolution of transition metal ions from the positive electrode, improve the stability of the structure and interface of the positive electrode material, and thus significantly improve the high-temperature performance and cycling performance of the battery. Preferably, the additives are selected from vinylene carbonate (VC), 1,3-propane sultone (PS), divinyl sulfate (DTD), tris(trimethylsilyl) phosphate (TMSP), 4,4'-bioxolane-2,2'-dione (BDC), 3,3'-bioxolane-2,2'-dione (BDTD), and their contents are 0.1-2%, 0.2-6%, 0.2-2%, 0.2-2%, 0.1-1.5%, and 0.1-1.5% respectively. Among them, when divinyl sulfate (DTD) is added as an additive to the electrolyte, it can modify the components of the SEI film on the surface of the negative electrode of the lithium battery, increase the relative contents of sulfur atoms and oxygen atoms. Sulfur atoms and oxygen atoms have lone pairs of electrons, which can attract lithium ions, accelerate the shuttle of lithium ions in the SEI film, reduce the battery interface impedance, and thus effectively improve the low-temperature charge and discharge performance of high-voltage secondary batteries. 1,3-Propane sultone (PS) has good film-forming performance as an additive, can form a large number of CEI films containing sulfonic acid groups at the positive electrode interface, inhibit the decomposition and gas generation of FEC at high temperatures, reduce the capacity loss of the secondary battery during the first charge and discharge, and thus is beneficial to improving the reversible capacity of the secondary battery, and further improves the high-temperature performance and long-term cycling performance of the secondary battery. Tris(trimethylsilyl) phosphate (TMSP) can absorb moisture and free acid, improving the cycling performance of the battery.

[0013] The second aspect of the present invention provides a secondary battery, including a positive electrode material, a negative electrode material, and an electrolyte, and the maximum charging voltage is 4.3V. The chemical formula of the positive electrode material is LiNi x Co y Mn z M (1-x-y-z) O2 or LiNi x Co y Al z N (1-x-y-z)O₂, where M is any one of Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, N is any one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, V, and Ti, 0.5 ≤ x < 1, 0 < y ≤ 1, 0 < z ≤ 1, and x + y + z ≤ 1. It is a high-nickel ternary cathode material. The anode material is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite, and silicon monoxide. The electrolyte is the aforementioned non-aqueous electrolyte. The additive of the non-aqueous electrolyte of the secondary battery of the present invention includes compound A or compound B, which can effectively capture the oxygen free radicals generated by the phase change of the cathode material, thereby inhibiting the adverse phase change of the cathode material, and further inhibiting the dissolution of transition metal ions in the cathode material. Therefore, the electrochemical performance such as the cycle of the secondary battery can be improved. Detailed Embodiments

[0014] The purpose, technical solution, and beneficial effects of the present invention will be further described below through specific embodiments, but it does not constitute any limitation to the present invention. For those not specifying specific preparation conditions in the embodiments, they can be carried out according to conventional conditions or the conditions recommended by the manufacturer. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial channels.

[0015] Example 1

[0016] (1) Preparation of the non-aqueous electrolyte of the secondary battery: In a glove box filled with nitrogen (O₂ < 1 ppm, H₂O < 1 ppm), a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) was used as the organic solvent and mixed evenly according to a mass ratio of 2:5:1 to obtain 84.7 g of non-aqueous organic solvent. 0.3 g of compound A was added. 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 lithium hexafluorophosphate was slowly added to the mixed solution and mixed evenly to prepare the non-aqueous electrolyte of the secondary battery.

[0017] (2) Preparation of the cathode: The ternary material lithium nickel 0.9 manganese 0.05 cobalt 0.05 O₂, binder PVDF, and conductive agent Super P were mixed evenly according to a mass ratio of 95:1:4 to prepare a secondary battery cathode slurry with a certain viscosity. The prepared slurry was coated on both sides of the aluminum foil, dried, and rolled to obtain the cathode sheet.

[0018] (3) Preparation of the negative electrode: Artificial graphite, conductive agent SuperP, thickening agent CMC, and binder SBR (styrene-butadiene rubber emulsion) are made into a slurry in a mass ratio of 95:1.5:1:2.5, mixed evenly. After coating both sides of the copper foil with the prepared slurry, it is dried and roll-pressed to obtain the negative electrode sheet.

[0019] (4) Preparation of the secondary battery: The positive electrode, separator, and negative electrode are made into a square battery cell in a stacked manner, packaged with a polymer, filled with the non-aqueous electrolyte of the secondary battery prepared above, and made into a secondary battery with a capacity of 1000 mAh after processes such as formation and grading.

[0020] The electrolyte formulations of Examples 2 to 12 and Comparative Example 1 are shown in Table 1, and the steps for preparing the electrolyte are the same as those in Example 1.

[0021] Table 1 Electrolyte components of each example

[0022]

[0023] The secondary batteries prepared in Examples 1 to 12 and Comparative Example 1 are respectively tested for room temperature cycle performance, high temperature cycle performance, and nickel and manganese ion dissolution. The test conditions are as follows, and the test results are shown in Table 2.

[0024] Room temperature cycle test: Place the secondary battery in an environment of 25°C, charge it at a constant current of 1C to 4.3V, then charge it at a constant voltage until the current drops to 0.05C, and then discharge it at a constant current of 1C to 3.0V. Repeat this cycle, and record the discharge capacity of the first cycle and the last cycle. Calculate the capacity retention rate of the high temperature cycle according to the following formula.

[0025] Capacity retention rate = Discharge capacity of the last cycle / Discharge capacity of the first cycle × 100%

[0026] High temperature cycle test: Place the secondary battery in an environment of 45°C, charge it at a constant current of 1C to 4.3V, then charge it at a constant voltage until the current drops to 0.05C, and then discharge it at a constant current of 1C to 3.0V. Repeat this cycle, and record the discharge capacity of the first cycle and the last cycle. Calculate the capacity retention rate of the high temperature cycle according to the following formula.

[0027] Capacity retention rate = Discharge capacity of the last cycle / Discharge capacity of the first cycle × 100%

[0028] Nickel and manganese ion dissolution test:

[0029] Use scissors to cut open one side of the secondary battery after the battery has been divided and cycled at 0.5C / 0.5C for 10 weeks. Use ceramic tweezers to peel off the aluminum-plastic film of the battery packaging, inject 10 times the amount of dichloromethane in the secondary battery, seal the injection port, and let it stand for 20 hours with the injection port facing upward. After standing, cut the injection port again, and use ICP-OES to analyze the content of transition metal ions in the electrolyte of the extracted dichloromethane-electrolyte mixture.

[0030] Table 2 Performance test results of secondary batteries

[0031]

[0032]

[0033] As can be seen from the results in Table 2, compared with Comparative Example 1, the room temperature cycle performance and high temperature cycle performance of Examples 1 to 12 are better, and the dissolution of nickel and manganese ions is lower. The present invention uses compound A or compound B as an electrolyte additive, which is a natural ginger extract. In the initial stage of secondary battery formation and circulation, when the positive electrode material undergoes a phase change and precipitates oxygen free radicals, compound A or compound B can be deoxidized and provided to electronic free radicals, thereby effectively capturing the oxygen free radicals generated by the phase change of the positive electrode material, so as to terminate the chain reaction of the positive electrode material to produce oxygen free radicals, thereby inhibiting the adverse phase change of the positive electrode material, and then inhibiting the dissolution of transition metal ions in the positive electrode material, so the cycle performance of the secondary battery can be improved.

[0034] By comparing Example 1 with Examples 5 to 10, it can be seen that by adding additives such as VC, PS, DTD, TMSP, BDC, and BDTD to Example 1, the cycle performance of the obtained battery is better, but the dissolution amount of nickel and manganese ions is not reduced, which proves that the way in which additives such as VC, PS, DTD, TMSP, BDC, and BDTD improve the cycle performance is different from that of Compound A and Compound B, and they do not improve the cycle performance of the secondary battery by inhibiting the dissolution of transition metal ions.

[0035] By comparing Example 1 with Examples 11 to 12, it can be seen that by adding other lithium salts to lithium hexafluorophosphate, the cycle performance of the secondary battery obtained is better, but the amount of transition metal ion dissolution is not reduced, which proves that the way of adding other lithium salts to improve the cycle performance is different from that of Compound A and Compound B.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A secondary battery, comprising a positive electrode material, a negative electrode material, and an electrolyte, characterized in that, The maximum charging voltage is 4.3 V. The electrolyte includes a lithium salt, a non-aqueous organic solvent, and an additive. The additive includes compound A with the structural formula I or compound B shown in the structural formula II. The mass percentage of compound A or compound B in the non-aqueous electrolyte is 0.1-0.5%.

2. The secondary battery according to claim 1, wherein The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro bis(oxalato)phosphate, lithium tetrafluoroborate, lithium tetrafluoro(oxalato)phosphate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(fluorosulfonyl)imide.

3. The secondary battery according to claim 1, wherein The non-aqueous organic solvent is selected from chain carbonates, cyclic carbonates, or carboxylic acid esters.

4. The secondary battery according to claim 3, wherein The non-aqueous organic solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate, ethyl butyrate, fluoroethylene carbonate, difluoroethyl acetate, and 2,2,2-trifluoroethyl acetate.

5. The secondary battery according to claim 1, wherein It further includes an auxiliary agent, and the auxiliary agent is selected from at least one of 2,2,2-trifluoroethyl methyl carbonate, 2,2,2-trifluoroethyl ethyl carbonate, 2,2,2-trifluoroethyl propyl carbonate, vinylene carbonate, difluoroethylene carbonate, 1,3-propane sultone, ethylene sulfate, tris(trimethylsilyl) phosphate, 4,4'-bi-1,3-dioxolane-2,2'-dione, 3,3'-biethylene disulfate, and triallyl phosphate.

6. The secondary battery according to claim 1, wherein The chemical formula of the positive electrode material is LiNi x Co y Mn z M (1-x-y-z) O2 or LiNi x Co y Al z N (1-x-y-z) O2, where M is any one of Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, N is any one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, V, and Ti, 0.5 ≤ x < 1, 0 < y ≤ 1, 0 < z ≤ 1, and x + y + z ≤ 1.

7. The secondary battery according to claim 1, characterized in that, The negative electrode material is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material, and silicon monoxide.

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