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. The additive includes compound A shown in structural formula I, wherein R1 and R2 each independently selected from hydrogen, halogen, substituted or unsubstituted methyl, substituted or unsubstituted C2-C8 hydrocarbon group, substituted or unsubstituted aryl group, ether group or isocyanate group. Compound A is reduced at the positive electrode / electrolyte interface to form an interfacial film containing sulfur, nitrogen, and oxygen. This interfacial film has good thermal stability, isolates the direct contact between the electrolyte and the positive electrode material at high temperatures, and inhibits the decomposition of the electrolyte, so the high-temperature performance can be improved. At the same time, this interfacial film has relatively low impedance and good lithium-ion transport channels. The lithium-ion transport channels are not easily shrunk at low temperatures and are not easily collapsed and closed during the cycling process. Therefore, the low-temperature discharge and cycling performance of the secondary battery are 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] With the continuous improvement of the requirements for the capacity of secondary batteries in pure electric vehicles, hybrid electric vehicles, portable energy storage devices, etc., people expect to develop secondary batteries with higher energy density and power density to achieve energy storage and long-term endurance.

[0003] In addition to the improvement of the production processes of existing materials and batteries, high-voltage (4.35 - 5V) cathode materials are one of the popular research directions, which achieve high energy density of the battery by increasing the charging depth of the cathode active material. Among them, nickel-cobalt-manganese ternary cathode materials are the most commonly used ternary cathode materials due to their high capacity. However, nickel-cobalt-manganese ternary cathode materials are prone to irreversible phase transformation from H2 to H3 at high voltage and high temperature, resulting in the precipitation of oxygen, leading to instability of the electrolyte and the electrode interface. The secondary battery faces problems such as poor high-temperature storage and serious cyclic gas generation. Currently, conventional electrolytes will oxidize and decompose on the surface of the cathode at a high voltage of 4.4V, especially at high temperature, which will accelerate this reaction and also exacerbate the side reactions of the electrolyte, deteriorating the performance of nickel-cobalt-manganese ternary cathode materials and ultimately affecting the performance of the battery.

[0004] Therefore, it is necessary to develop an electrolyte that can withstand a high voltage of 4.4V, so as to achieve excellent performance of the secondary battery based on the nickel-cobalt-manganese ternary cathode material system. 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 form a low-impedance and stable interfacial film, thereby improving the high and low temperature and cyclic performance of the secondary battery based on the nickel-cobalt-manganese ternary cathode material system at a high voltage of 4.4V.

[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 a compound A represented by Structural Formula I,

[0007]

[0008] wherein, R1 and R2 are each independently selected from hydrogen, halogen, substituted or unsubstituted methyl, substituted or unsubstituted C2 - C8 hydrocarbon group, substituted or unsubstituted aryl group, ether group, or isocyanate group.

[0009] Compared with the prior art, the additive of the electrolyte of the present invention includes compound A shown in structural formula I. This compound A is reduced at the positive electrode / electrolyte interface to form an interfacial film containing sulfur, nitrogen, and oxygen. This interfacial film has good thermal stability, isolates the direct contact between the electrolyte and the positive electrode material at high temperatures, inhibits the decomposition of the electrolyte, and thus can improve the high-temperature performance. At the same time, this interfacial film has relatively low impedance and good lithium-ion transport channels. At low temperatures, the lithium-ion transport channels are not easily constricted, and during the cycling process, the lithium-ion transport channels are not easily collapsed and closed. Therefore, the low-temperature discharge and cycling performance of the secondary battery are improved. In summary, the above additive of the present invention can improve the high and low temperature and cycling performance of the secondary battery.

[0010] Preferably, R1 and R2 are each independently selected from C1-C4 alkyl, substituted phenyl, C1-C3 ether group, or linear isocyanate group. Preferably, it is a C1-C3 ether group. The ether group can significantly reduce the dissociation energy of the lithium salt in the non-aqueous electrolyte and improve the low-temperature performance of the secondary battery. At the same time, due to the relatively excellent interfacial stability of the interfacial film containing sulfur, nitrogen, and oxygen, it can make up for the deficiency of the poor oxidation resistance of the ether group. The mass percentage of compound A in the non-aqueous electrolyte is 0.1-5.0%. Specifically, it can be but is not limited to 0.1%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5.0%. Compound A is selected from at least one of compound I to compound VI

[0011]

[0012] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (C4BLiO8), lithium difluorophosphate (LiPO2F2), lithium difluoro(oxalato)borate (C2BF2LiO4), lithium difluoro(dioxalato)phosphate (LiDFBP), and lithium bis(fluorosulfonyl)imide (LiFSI). The concentration of the lithium salt is 0.5-1.5 M. Preferably, the lithium salt is LiPF6 or a mixture of LiPF6 and other lithium salts.

[0013] Preferably, the non-aqueous organic solvent is selected from at least one of linear carbonates, cyclic carbonates, and carboxylic acid esters. More preferably, the non-aqueous organic solvent can be 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), and ethyl butyrate (Eb).

[0014] Preferably, it further includes 0.1-5% of an auxiliary agent, specifically but not limited to 0.1%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5.0%. The auxiliary agent is selected from at least one of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), ethylene sulfite (ES), 1,3-propane sultone (1,3-PS), and divinyl sulfate (DTD).

[0015] 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 4.4V. The positive electrode material includes nickel cobalt manganese oxide, and the chemical formula of the nickel cobalt manganese oxide is LiNi x Co y Mn (1-x-y) M z O2, where 0.6≤x≤0.9, x + y<1, 0≤z<0.08, and M is at least one of Al, Mg, Zr, and Ti. The negative electrode material is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material, 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 shown in structural formula I, which can improve the high and low temperature and cycling performance of the secondary battery. It can meet the usage requirements of nickel cobalt manganese oxide system secondary batteries with high energy density and high voltage. Specific Embodiments

[0016] The purpose, technical solution, and beneficial effects of the present invention will be further illustrated below through specific embodiments, but it does not constitute any limitation to the present invention. For those not specified in the embodiments regarding specific preparation conditions, 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.

[0017] Example 1

[0018] (1) Preparation of the non-aqueous electrolyte of the secondary battery: In a glove box filled with nitrogen (O2<2ppm, H2O<3ppm), a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) is used as the organic solvent, and they are mixed evenly according to a mass ratio of 1:1:1 to obtain 86.5 g of the non-aqueous organic solvent. Then 0.5 g of compound A is added. The solution is sealed and packaged and placed in a freezer (-4℃) for 2 h and then taken out. In a glove box filled with nitrogen (O2<2ppm, H2O<3ppm), 13 g of lithium hexafluorophosphate is slowly added to the mixed solution, and after mixing evenly, the non-aqueous electrolyte of the secondary battery is prepared.

[0019] (2) Preparation of the positive electrode: The ternary material lithium nickel cobalt manganese oxide LiNi 0.6 Mn 0.2 Co 0.2 O2, the binder PVDF, and the conductive agent SuperP are mixed evenly in a mass ratio of 97:1.5:1.5 to prepare a secondary battery positive electrode paste with a certain viscosity. After the mixed paste is coated on both sides of the aluminum foil, it is dried and roll-pressed to obtain a positive electrode sheet.

[0020] (3) Preparation of the negative electrode: Artificial graphite, the conductive agent SuperP, the thickener CMC, and the binder SBR (styrene-butadiene rubber latex) are made into a paste in a mass ratio of 96:1:1.5:1.5, mixed evenly, and the mixed paste is coated on both sides of the copper foil. After drying and roll-pressing, a negative electrode sheet is obtained.

[0021] (4) Preparation of the secondary battery: The positive electrode, the separator, and the 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 after processes such as formation and grading.

[0022] The electrolyte formulations of Examples 2 to 13 and Comparative Examples 1 to 6 are shown in Table 1, and the steps for preparing the electrolyte are the same as those in Example 1.

[0023] Table 1 Electrolyte components of each example

[0024] Group Non-aqueous organic solvent / mass (g) Lithium salt / mass (g) Additive / mass (g) Auxiliary agent (g) Example 1 EC / DEC / EMC (1:1:1) / 86.5 <![CDATA[LiPF6 / 13]]> Compound Ⅰ / 0.5 Example 2 EC / DEC / EMC (1:1:1) / 86.5 <![CDATA[LiPF6 / 13]]> Compound Ⅱ / 0.5 Example 3 EC / DEC / EMC (1:1:1) / 86.5 <![CDATA[LiPF6 / 13]]> Compound Ⅲ / 0.5 Example 4 EC / DEC / EMC (1:1:1) / 86.5 <![CDATA[LiPF6 / 13]]> Compound Ⅳ / 0.5 Example 5 EC / DEC / EMC (1:1:1) / 86.5 <![CDATA[LiPF6 / 13]]> Compound Ⅴ / 0.5 Example 6 EC / DEC / EMC (1:1:1) / 86.5 <![CDATA[LiPF6 / 13]]> Compound Ⅵ / 0.5 Example 7 EC / DEC / EMC (1:1:1) / 86.4 <![CDATA[LiPF6 / 13.5]]> Compound Ⅲ / 0.1 Example 8 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> Compound Ⅲ / 1.5 Example 9 EC / DEC / EMC (1:1:1) / 85 <![CDATA[LiPF6 / 13]]> Compound Ⅲ / 2 Example 10 EC / DEC / EMC (1:1:1) / 84.5 <![CDATA[LiPF6 / 10.5]]> Compound Ⅲ / 5 Example 11 DMC / DEC / EMC (3:5:2) / 86 <![CDATA[LiPF6+LiBF4 / 12+1.8]]> Compound Ⅰ / 0.2 Example 12 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> Compound Ⅲ / 0.5 VEC / 1 Example 13 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> Compound Ⅲ / 0.5 DTD / 1 Comparative Example 1 EC / DEC / EMC (1:1:1) / 87 <![CDATA[LiPF6 / 13]]> Comparative Example 2 EC / DEC / EMC (1:1:1) / 85 <![CDATA[LiPF6 / 13]]> Compound Ⅶ / 2 Comparative Example 3 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> VEC / 1.5 Comparative Example 4 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> DTD / 1.5 Comparative Example 5 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> Compound Ⅶ / 0.5 VEC / 1 Comparative Example 6 EC / DEC / EMC (1:1:1) / 85.5 <![CDATA[LiPF6 / 13]]> Compound Ⅶ / 0.5 DTD / 1

[0025]

[0026] The synthetic route of Compound VII is as follows.

[0027]

[0028] The secondary batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 6 were respectively tested for their room temperature cycle performance, high temperature cycle performance, high temperature storage, and low temperature discharge performance. The specific test conditions are as follows, and the performance test results are shown in Table 2.

[0029] Room temperature cycle test: Under room temperature (25 °C) conditions, the secondary battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0), the upper limit voltage is 4.4V, and then it is charged and discharged at 1.0C / 1.0C for 500 cycles under room temperature conditions (the battery discharge capacity is C1).

[0030] Capacity retention rate = (C1 / C0) * 100%.

[0031] High-temperature cycling test: Under the condition of high temperature (45 °C), a secondary battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0), the upper limit voltage is 4.4V, and then it is charged and discharged at 1.0C / 1.0C for 300 cycles under the condition of high temperature (45 °C) (the battery discharge capacity is C1).

[0032] Capacity retention rate = (C1 / C0) * 100%

[0033] High-temperature storage performance test: Under the condition of normal temperature (25 °C), a secondary battery is charged and discharged at 0.3C / 0.3C once (the discharge capacity is recorded as C0), the upper limit voltage is 4.4V; the secondary battery is placed in a high-temperature oven at 60 °C for 15 days, and then the secondary battery is taken out; it is discharged at 0.3C at 25 °C (the discharge capacity is recorded as C1); continue to charge and discharge the secondary battery at 0.3C / 0.3C once under the condition of normal temperature (25 °C) (the discharge capacity is recorded as C2), the upper limit voltage is 4.4V, and the capacity retention rate and capacity recovery rate of the secondary battery are calculated using the following formula.

[0034] Capacity retention rate = C1 / C0 * 100%

[0035] Capacity recovery rate = C2 / C0 * 100%

[0036] Low-temperature discharge performance test: The formed secondary battery is charged at a constant current and voltage of 1C to 4.4V at normal temperature (the discharge capacity is recorded as C0), then the battery is placed in a low-temperature environment at -20 °C for 4 hours, and discharged at 0.3C to 3.0V (the discharge capacity is recorded as C1), the discharge rate of the battery is measured, and the discharge rate of the secondary battery is calculated using the following formula.

[0037] Discharge rate (%) = C1 / C0 * 100%

[0038] Table 2 Test results of secondary battery performance

[0039]

[0040] As can be seen from the results in Table 2, compared with Comparative Examples 1-6, Examples 1-13 have better high and low temperature and cycling performance. This is because the additive of the electrolyte of the present invention includes compound A shown in structural formula I, and this compound A is reduced at the positive electrode / electrolyte interface to form an interfacial film containing sulfur, nitrogen, and oxygen. This interfacial film has good thermal stability, isolates the direct contact between the electrolyte and the positive electrode material at high temperature, and inhibits the decomposition of the electrolyte, so the high-temperature performance can be improved. At the same time, this interfacial film has relatively low impedance and good lithium-ion transmission channels. At low temperature, the lithium-ion transmission channels are not easy to shrink, and during the cycling process, the lithium-ion transmission channels are not easy to collapse and close. Therefore, the low-temperature discharge and cycling performance of the secondary battery have been improved.

[0041] In Comparative Example 2 and Comparative Examples 5 to 6, although Compound VII was also used as an additive, due to the polymerization of the double bond within the ring in the system, the impedance of the secondary battery was large and the low-temperature discharge performance was not ideal. At the same time, the interfacial film formed was not as thermally stable as the interfacial film formed by Compound A, so the high-temperature performance was also poor.

[0042] Combined with Examples 8, 12 to 13 and Comparative Examples 3 to 4, it can be seen that on the basis of the additive Compound A, when additives such as VEC or DTD are further added, the comprehensive performance of cycling and storage is better.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to 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 lithium salt, a non-aqueous organic solvent, and an additive, characterized in that, The additive includes compound A shown in structural formula I, and the mass percentage of compound A in the non-aqueous electrolyte is 0.1-5.0%. Wherein, R1 and R2 are each independently selected from hydrogen, halogen, substituted or unsubstituted methyl, substituted or unsubstituted C2-C8 hydrocarbon group, substituted or unsubstituted aryl group, ether group or isocyanate group.

2. The non-aqueous electrolyte according to claim 1, wherein R1 and R2 are each independently selected from C1-C4 alkyl group, substituted phenyl, C1-C3 ether group or linear isocyanate group.

3. The non-aqueous electrolyte according to claim 1, characterized in that, Compound A is selected from at least one of compound I to compound VI.

4. The non-aqueous electrolyte according to claim 1, wherein The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium difluoro(dioxalato)phosphate and lithium bis(fluorosulfonyl)imide.

5. The non-aqueous electrolyte according to claim 1, wherein The non-aqueous organic solvent is selected from at least one of chain carbonates, cyclic carbonates and carboxylic acid esters.

6. The non-aqueous electrolyte according to claim 1, wherein It also includes an auxiliary agent, and the auxiliary agent is selected from at least one of vinylene carbonate, vinyl ethylene carbonate, fluorinated ethylene carbonate, ethylene sulfite, 1,3-propane sultone and ethylene sulfate.

7. A secondary battery, comprising a positive electrode material, a negative electrode material, and an electrolyte, characterized in that, The electrolyte is the non-aqueous electrolyte according to any one of claims 1-6, and the maximum charging voltage is 4.4V.

8. The secondary battery according to claim 7, characterized in that, The positive electrode material includes nickel cobalt manganese oxide, and the chemical formula of the nickel cobalt manganese oxide is LiNi x Co y Mn (1-x-y) M z O2, where 0.6 ≤ x ≤ 0.9, x + y < 1, 0 ≤ z < 0.08, and M is at least one of Al, Mg, Zr, and Ti.

9. The secondary battery according to claim 7, wherein 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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