Non-aqueous electrolyte and secondary battery thereof

Abstract

The present invention provides a non-aqueous electrolyte and a secondary battery thereof, wherein the non-aqueous electrolyte includes a lithium salt, a non-aqueous organic solvent and an additive, and the additive includes a cyclic amide compound as shown in Structural Formula I, Structural Formula II or Structural Formula III, wherein R1 to R8 are each independently selected from hydrogen, a halogen, a substituted or unsubstituted methyl group, a substituted or unsubstituted C2 to C9 hydrocarbon group. The cyclic amide compound of the present invention can be reduced to form a silicon-containing amide compound, which has good thermal stability and can isolate the direct contact between the electrolyte and the positive electrode material at high temperature, thereby inhibiting the decomposition of the electrolyte. In addition, by introducing the -Si-N-CO- structure, the toughness of the chemical bond can be increased to avoid the aminoacyl group from being unstable and producing gas under high voltage and high pressure. At the same time, the cyclic amide compound contains a Si-N bond structure, which can remove free water and HF in the battery system and improve the performance of the positive electrode material.

Description

Technical Field

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

[0002] As the requirements for secondary battery capacity in pure electric vehicles, hybrid electric vehicles and portable energy storage devices continue to increase, people are looking forward to developing secondary batteries with higher energy density and power density to achieve energy storage and long-term endurance.

[0003] In addition to improvements in existing materials and battery manufacturing processes, high-voltage (4.35-5V) positive electrode materials are one of the more popular research directions, which achieve high energy density of the battery by increasing the charging depth of the positive electrode active material. Among them, high-nickel positive electrode materials (nickel content higher than 0.6) are more commonly used positive electrode materials due to their higher capacity. Under high-voltage systems, conventional electrolytes are prone to side reactions that deteriorate the performance of positive electrode materials and affect the electrochemical performance of secondary batteries. At the same time, secondary batteries under high-voltage systems also have stricter control over moisture. Under high voltage, excess moisture easily reacts with lithium salts to generate HF, thereby destroying the formation of the interface film and causing deterioration of battery performance.

[0004] Therefore, it is necessary to develop an electrolyte that can withstand high voltage, so as to achieve excellent electrical performance of secondary batteries with high nickel positive electrode material systems. Summary of the Invention

[0005] The present invention aims to provide a non-aqueous electrolyte and a secondary battery thereof. The additive in the non-aqueous electrolyte can effectively remove moisture and HF in the battery system, form a stable interface film, thereby effectively exerting the performance of the positive electrode material and improving the cycle and storage performance of the secondary battery under high voltage and high temperature.

[0006] To achieve the above-mentioned objectives, the first aspect of the present invention provides a non-aqueous electrolyte comprising a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive comprises a cyclic amide compound as shown in Structural Formula I, Structural Formula II or Structural Formula III, wherein R1 to R8 are each independently selected from hydrogen, halogen, substituted or unsubstituted methyl, or substituted or unsubstituted C2 to C9 hydrocarbon groups.

[0007]

[0008] Compared with the prior art, the electrolyte additive of the present invention includes a cyclic amide compound, which is reduced at the electrode / electrolyte interface to form a silicon-containing amide compound. This compound has good thermal stability and can isolate the direct contact between the electrolyte and the positive electrode material at high temperatures, thereby inhibiting the decomposition of the electrolyte, and therefore has better high-temperature performance. In addition, by introducing the -Si-N-CO- structure into the cyclic amide compound, the toughness of the chemical bond can be increased to avoid the aminoacyl group from being unstable and producing gas under high voltage and high pressure, thereby effectively improving high-temperature storage performance. At the same time, the cyclic amide compound contains a Si-N bond structure, which can remove free water and HF in the battery system, improve the performance of the positive electrode material, and thus improve the electrochemical performance of the secondary battery cycle.

[0009] Preferably, R1 and R2 are each independently selected from hydrogen or a C1-C6 hydrocarbon group, and R3-R8 are methyl groups. The mass percentage of the cyclic amide compound in the non-aqueous electrolyte is 0.01-4.0%. Specific amounts include, but are not limited to, 0.01%, 0.2%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3%, 3.5%, and 4.0%. The cyclic amide compound is selected from at least one of Compounds A to F.

[0010]

[0011]

[0012] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalatoborate) (C4BLiO8), lithium difluorophosphate (LiPO2F2), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonylimide) (LiN(CF3SO2)2), lithium perchlorate (LiClO4), lithium difluorooxalatoborate (C2BF2LiO4), lithium difluorobis(oxalatophosphate) (LiDFBP) and lithium bis(fluorosulfonylimide) (LiFSI).

[0013] Preferably, 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), butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), propyl propionate (n-Pp), ethyl propionate (EP) and ethyl butyrate (Eb).

[0014] Preferably, it further includes 0.01-2% of an auxiliary agent, specifically but not limited to 0.01%, 0.2%, 0.5%, 1%, 1.5%, 2.0%. The auxiliary agent is selected from at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), 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. The positive electrode material is a high-nickel positive electrode material, and the chemical formula of the high-nickel positive electrode material 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, Ti, Mg, and Zr. The negative electrode material is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material, and silicon monoxide. Its maximum charging voltage can be 4.4V, and the electrolyte is the aforementioned non-aqueous electrolyte. The additive of the non-aqueous electrolyte of the secondary battery of the present invention includes a cyclic amide compound, which can improve the high-temperature storage and cycling performance of the secondary battery. Specific Embodiments

[0016] The following further illustrates the purpose, technical solution, and beneficial effects of the present invention through specific embodiments, but does not constitute any limitation to the present invention. For those not specifying specific preparation conditions in the embodiments, conventional conditions or conditions recommended by the manufacturer can be followed. For those reagents or instruments not 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 < 2 ppm, H2O < 3 ppm), a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) was used as an organic solvent and mixed evenly according to a mass ratio of 1:0.5:0.5 to obtain 87 g of a non-aqueous organic solvent. 0.5 g of Compound A was added. The 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 < 2 ppm, H2O < 3 ppm), 12.5 g of lithium hexafluorophosphate was slowly added to the mixed solution and mixed evenly to obtain the non-aqueous electrolyte of the secondary battery.

[0019] (2) Preparation of the positive electrode: The ternary material of lithium nickel cobalt manganate LiNi 0.8 Mn 0.1 Co 0.1O2, adhesive PVDF and conductive agent SuperP are evenly mixed in a mass ratio of 98:1:1 to prepare a secondary battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of an aluminum foil, dried and rolled to obtain a positive electrode sheet.

[0020] (3) Preparation of negative electrode: artificial graphite, conductive agent SuperP, thickener CMC, and adhesive SBR (styrene-butadiene rubber latex) are mixed in a mass ratio of 97:1:1:1 to prepare a slurry, mix them evenly, apply the mixed slurry on both sides of the copper foil, dry and roll-press to obtain the negative electrode sheet.

[0021] (4) Preparation of secondary batteries: The positive electrode, separator and negative electrode are stacked to form a square battery cell, which is packaged with a polymer and filled with the secondary battery non-aqueous electrolyte prepared above. After the formation and capacity separation processes, the secondary battery is made.

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

[0023] Table 1 Electrolyte components of various examples

[0024]

[0025]

[0026]

[0027] The secondary batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 6 were tested for room temperature cycle performance, high temperature cycle performance, and high temperature storage performance. The specific test conditions are as follows. The performance test results are shown in Table 2.

[0028] Normal temperature cycle test: At room temperature (25°C), the secondary battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0) with an upper limit voltage of 4.4V, and then charged and discharged at 1.0C / 1.0C for 500 cycles at room temperature (the battery discharge capacity is C1).

[0029] Capacity retention rate = (C1 / C0)*100%.

[0030] High temperature cycle test: Under high temperature (45°C) conditions, the secondary battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0) with an upper limit voltage of 4.4V, and then charged and discharged at 1.0C / 1.0C for 300 cycles (the battery discharge capacity is C1) under high temperature (45°C) conditions.

[0031] Capacity retention rate = (C1 / C0)*100%

[0032] High-temperature storage performance test: At room temperature (25°C), the secondary battery is charged and discharged once at 0.3C / 0.3C (the discharge capacity is recorded as C0), with an upper limit voltage of 4.4V; the secondary battery is placed in a 60°C high-temperature box for 15 days, and the secondary battery is taken out; it is discharged at 0.3C at 25°C (the discharge capacity is recorded as C1); and the secondary battery is further charged and discharged once at room temperature (25°C) at 0.3C / 0.3C (the discharge capacity is recorded as C2), with an upper limit voltage of 4.4V. The capacity retention rate and capacity recovery rate of the secondary battery are calculated using the following formula.

[0033] Capacity retention rate = C1 / C0*100%

[0034] Capacity recovery rate = C2 / C0*100%

[0035] Table 2 Secondary battery performance test results

[0036]

[0037]

[0038] As can be seen from the results in Table 2, Examples 1 to 13 have better high and low temperature and cycle performance than Comparative Examples 1 to 6. This is because the electrolyte of the present invention includes a cyclic amide compound, which is reduced at the electrode / electrolyte interface to form a silicon-containing amide compound. The compound has good thermal stability and can isolate the direct contact between the electrolyte and the positive electrode material at high temperatures, thereby inhibiting the decomposition of the electrolyte, and therefore has better high temperature performance. In addition, by introducing the -Si-N-CO- structure into the cyclic amide compound, the toughness of the chemical bond can be increased to avoid the aminoacyl group from being unstable and producing gas under high voltage and high pressure, and free water and HF in the battery system can also be removed, which can improve the performance of the positive electrode material and thus improve the electrochemical performance of the secondary battery cycle.

[0039] Even when Compound G and Compound H are combined in Comparative Example 4, their performance is still inferior to that of the embodiment, especially the high-temperature storage performance. This is because the absence of the -Si-N-CO- structure in the system fails to increase the toughness of the chemical bond and cannot prevent the aminoacyl group from being unstable and generating gas under high voltage and high pressure.

[0040] Combined with Examples 2, 12-13 and Comparative Examples 5-6, it can be seen that when auxiliary agents such as VC or PS are added on the basis of the additive cyclic amide compound, the secondary battery performance is better.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery comprising a positive electrode material, a negative electrode material and a non-aqueous electrolyte, characterized in that: The upper limit voltage is 4.4 V, the positive electrode material is a high-nickel positive electrode material, and the chemical formula of the high-nickel positive electrode material 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, M is at least one of Al, Ti, Mg, and Zr, the non-aqueous electrolyte includes a lithium salt, a non-aqueous organic solvent, and an additive, and the additive includes a cyclic amide compound represented by Structural Formula II or Structural Formula III Wherein, R1 to R8 are each independently selected from hydrogen, halogen, substituted or unsubstituted methyl, and substituted or unsubstituted C2 to C9 hydrocarbon groups.

2. The secondary battery according to claim 1, wherein R1 and R2 are each independently selected from hydrogen or a C1-C6 hydrocarbon group, and R3-R8 are methyl groups.

3. The secondary battery according to claim 1, wherein The cyclic amide compound is selected from at least one of Compound D to Compound F, 4. The secondary battery according to claim 1, wherein The mass percentage of the cyclic amide compound in the non-aqueous electrolyte is 0.01-4.0%.

5. The secondary battery according to claim 1, wherein The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl imide), lithium perchlorate, lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate) and lithium bis(fluorosulfonyl imide).

6. The secondary battery according to claim 1, wherein The non-aqueous organic solvent is at least one selected from the group consisting of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate and ethyl butyrate.

7. The secondary battery according to claim 1, wherein The invention further comprises an auxiliary agent, which is selected from at least one of fluoroethylene carbonate, vinylene carbonate, vinyl carbonate, vinyl sulfite, 1,3-propane sultone and vinyl sulfate.

8. The secondary battery according to claim 1, 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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