Non-aqueous electrolyte and secondary battery

Abstract

The invention discloses a non-aqueous electrolyte and a secondary battery. The non-aqueous electrolyte comprises a solvent, a metal salt and an additive; the additive comprises a benzo structure additive with at least one of the following structures and TMP; and in the formula 1, R1, R2, R3 and R4 are respectively selected from one of H, F, Cl, Br, hydroxyl and C1-10 alkyl groups. According to the electrolyte, trimethyl phosphite is used, meanwhile, an additive combination with a specific structure is introduced, an NH or = N structure in a benzo structure can be coupled with an-O-structure in TMP, and after benzo dehydrogenation or bond opening occurs,-N-O acid salt or-N-O-P phosphorus nitrogen acid salt is formed with TMP, an SEI film can be repaired more effectively for a long time, and the service life of the electrolyte is prolonged. The cycle and storage performance of a long life cycle are maintained, and gas production in the long-term use process of the battery is inhibited; and the overall performance of the battery cell in a long life cycle is comprehensively improved.

Description

Technical Field

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

[0002] Secondary batteries have the characteristics of large capacity, fast charging speed, high energy density, long cycle life, etc., and have been widely used in daily life and production equipment, such as consumer electronics products, new energy electric vehicles, and household and industrial and commercial energy storage products. A large number of academic and industrial research findings show that the static storage decay and dynamic cycle decay of batteries at high temperatures are strongly related to the side reactions of residual water content in the batteries, the stability of the SEI film on the negative electrode, and the side reactions between the fully intercalated negative electrode and the electrolyte. This will cause the loss of active lithium, the damage of the SEI film, and the aggravation of side reactions. At the same time, it may be accompanied by gas release, resulting in the swelling of the battery cell, thereby deteriorating the battery performance and significantly reducing the overall service life of the battery.

[0003] The prior art usually uses various additives to improve the construction of the SEI film and maintain the long cycle performance of the battery. Using different additives can significantly improve the composition of the SEI / CEI and enhance the cycle life, such as vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, 1,3-propane sultone, methylene methanedisulfonate, and so on. However, the SEI films formed by the above-mentioned various additives still have some defects, and cannot effectively inhibit the peeling and decomposition of the SEI, nor can they inhibit the side reactions caused by residual water molecules. The long-term cycle performance and high-temperature performance are still poor. Therefore, it is crucial to develop additives with excellent high-temperature performance and long cycle performance and their non-aqueous electrolytes.

[0004] After searching the existing patent literature, it is found that CN1819324A provides an electrolyte for a lithium-ion battery. The functional additives in the electrolyte include benzotriazole compounds and at least one of vinyl ethylene carbonate, vinylene carbonate, cyclic sulfonic acid, cyclic sulfate ester, cyclic sulfite ester, cyclic anhydride, and cyclic imide. Using the electrolyte to prepare a lithium-ion battery can improve the mechanism of the electrolyte during battery formation, reduce the gas evolution amount, improve the storage life of the battery, and show high capacity and excellent charge / discharge cycle performance. However, pure benzotriazole compounds, or the combinations in the above-mentioned CN1819324A, mainly reduce the gas production amount during the formation stage. During the long-term cycle use and storage of the battery, the effect of continuously inhibiting gas production is limited, and it is not easy to repair the peeled and damaged SEI film during cycling, making it difficult to maintain the overall performance during the long life cycle. Summary of the Invention

[0005] Aiming at the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a non-aqueous electrolyte and a secondary battery.

[0006] The object of the present invention is achieved by the following technical solutions:

[0007] The present invention provides a non-aqueous electrolyte for a secondary battery, which comprises a solvent, a metal salt, and an additive; the additive comprises a benzo-structured additive, which is at least one of the following structures:

[0008]

[0009] In the above structures, R1, R2, R3, and R4 may be one of H, F, Cl, Br, hydroxyl group, and methyl to decyl.

[0010] As an embodiment, the additive further comprises trimethyl phosphite. That is, in the electrolyte, the additive includes trimethyl phosphite (TMP) and at least one of Structure 1, Structure 2, Structure 3, and Structure 4.

[0011] As an embodiment, in the electrolyte, the content of the benzo-structured additive is 0.01-1 wt.%.

[0012] As an embodiment, in the electrolyte, the content of trimethyl phosphite is 0.01-1 wt.%. The preferred content is 0.02-0.5 wt.%.

[0013] As an embodiment, in the electrolyte, the dosage ratio of the benzo-structured additive to trimethyl phosphite is 1:100-100:1.

[0014] As an embodiment, the metal salt is a lithium salt, and the content of the lithium salt in the electrolyte is 7-18 wt.%.

[0015] As an embodiment, the metal salt is a lithium salt, and the lithium salt comprises LiPF 6 , LiPO 2 F 2 , LiFSI, LiFSA, LiBF 4 , LiODFP, LiODFB, LiAsF 6 , LiTFSI, or any combination of at least two of them, and the content is 7-18 wt.%.

[0016] As an embodiment, the metal salt is a sodium salt, and the content of the sodium salt in the electrolyte is 5-18 wt.%.

[0017] As an embodiment, the metal salt is a sodium salt, and the sodium salt comprises at least one of NaPF6, NaPO 2 F 2 , NaFSI, NaBF 4 , NaODFP, NaODFB, and NaTFSI.

[0018] As an embodiment, the metal salt is a potassium salt, and the content of the potassium salt in the electrolyte is 3-18 wt.%.

[0019] As an embodiment, the metal salt is a potassium salt, and the potassium salt includes at least one of KPF6, KPO 2 F 2 , KFSI, KBF 4 , KODFP, KODFB, KTFSI.

[0020] As an embodiment, the solvent includes a combination of at least two of EC, PC, DMC, EMC, DEC, EA (ethyl acetate), EP, PP, MA, and MP.

[0021] As an embodiment, in the electrolyte, the content of the solvent is 70-88 wt.%.

[0022] As an embodiment, the electrolyte may further contain a negative electrode film-forming additive, including at least one or a combination of more than one of VC, FEC (fluoroethylene carbonate), DTD (ethylene sulfate), MMDS (methyl methanedisulfonate), and PS (1,3-propane sultone).

[0023] As an embodiment, the electrolyte may further contain a positive electrode film-forming additive, including any one or a combination of at least two of TMSP (tris(trimethylsilyl) phosphate) and TMSB (TMSB).

[0024] On the other hand, a secondary battery using the aforementioned non-aqueous electrolyte also falls within the protection scope of the present invention.

[0025] As an embodiment, the secondary battery is a lithium battery system; the active material in the positive electrode is any one or a combination of at least two of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium cobalt phosphate, and lithium nickel cobalt manganese aluminum oxide. At the same time, it can be correspondingly used for sodium battery, potassium battery systems, etc., as long as the corresponding lithium salt is replaced with a sodium salt or a potassium salt.

[0026] As an embodiment, the secondary battery is a lithium battery system; the active material in the negative electrode is any one or a combination of at least two of graphite and silicon-containing materials.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1) The present invention introduces a specific structural additive combination into the electrolyte. The N in the benzene structure and the O and P in TMP can complex with the transition metal dissolution on both the positive electrode and the negative electrode side, further reducing the deposition of transition metals during long-term cycling and storage, and improving the stability of the negative electrode SEI. On the positive electrode side, under high voltage, it will complex with the positive electrode metal and then form a stable CEI, further protecting the positive electrode material and preventing deterioration.

[0029] 2) The present invention introduces a specific structural additive combination into the electrolyte. After ring opening, the structure can generate film components containing composite nitrate, nitrite, phosphate, and N-P organic substances, improving the original SEI components, improving the negative electrode interface, and inhibiting lithium deposition.

[0030] 3) Trimethyl phosphite is used in the electrolyte of the present invention. There are lone pairs of electrons on the phosphorus of trimethyl phosphite. At the same time, a specific structural additive combination is introduced. The NH or =N structure in its benzene structure can couple with the -O- structure in TMP. After benzene dehydrogenation or bond opening, it forms -N-O salts or -N-O-P phosphonitrogen salts with TMP, which can improve the SEI composition during the long-term cycling and storage of the battery, more effectively repair the SEI film in the long term, maintain the cycling and storage performance of the long life cycle, and inhibit gas generation during the long-term use of the battery; comprehensively improve the overall performance of the battery cell within the long life cycle. Detailed implementation manners

[0031] The present invention will be described in detail below with reference to the embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several adjustments and improvements can still be made. These all belong to the protection scope of the present invention.

[0032] Example 1

[0033] A lithium iron phosphate battery system is used. The electrolyte formula is 1.1M LiPF6, EC, DMC, and EMC with a volume ratio of 2:3:3, and the additives are 3.5 wt.% VC, 0.5 wt.% FEC, 0.3 wt.% TMP, and additive (benzotriazole) of structure 4

[0034] with a content of 0.3 wt.%.

[0035] Example 2

[0036] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6 in EC / DMC / EMC (2:3:3 v / v / v), and the additives are 3.5 wt.% VC, 0.5 wt.% FEC, 0.3 wt.% TMP, and an additive of Structure 2, where R2, R3, and R4 are all methyl groups. That is, the additive is 1-(trimethylsilyl)benzotriazole, with a content of 0.3 wt.%.

[0037] Example 3

[0038] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6 in EC / DMC / EMC (2:3:3 v / v / v), and the additives are 3.5 wt.% VC, 0.5 wt.% FEC, 0.3 wt.% TMP, and an additive of Structure 1.

[0039] Where R1 is a hydroxyl group; that is, the additive is 1-hydroxybenzotriazole, with a content of 0.3 wt.%.

[0040] Example 4

[0041] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6 in EC / DMC / EMC (2:3:3 v / v / v), and the additives are 3.5 wt.% VC, 0.5 wt.% FEC, 0.3 wt.% trimethyl phosphate, and an additive of Structure 4 (benzotriazole), with a content of 0.3 wt.%.

[0042] Example 5

[0043] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6 in EC / DMC / EMC (2:3:3 v / v / v), and the additives are 3.5 wt.% VC, 0.5 wt.% FEC, 0.3 wt.% TMP, and an additive with the following structure, where R2 is H and R1 is a phenyl group. The specific substance is: 1-benzyl-1H-benzotriazole (CAS: 4706-43-8), with a content of 0.3 wt.%.

[0044] Example 6

[0045] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6, with EC, DMC, and EMC in a volume ratio of 2:3:3, and the additives are 3.5 wt.% VC, 0.5 wt.% FEC, and an additive of Structure 4 (benzotriazole), with a content of 0.3 wt.%.

[0046] Example 7

[0047] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6, with a volume ratio of 2:3:3 of EC, DMC, and EMC, and additives of 3.5 wt.% VC, 0.5 wt.% FEC, and 0.3 wt.% TMP.

[0048] Example 8

[0049] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6, with a volume ratio of 2:3:3 of EC, DMC, and EMC, and additives of 3.5 wt.% VC, 0.5 wt.% FEC, 0.1 wt.% TMP, and Structure 4

[0050] of the additive (benzotriazole), with a content of 0.1 wt.%.

[0051] Example 9

[0052] Using a lithium iron phosphate battery system, the electrolyte formulation is 1.1 M LiPF6, with a volume ratio of 2:3:3 of EC, DMC, and EMC, and additives of 3.5 wt.% VC, 0.5 wt.% FEC, 1 wt.% TMP, and Structure 4

[0053] of the additive (benzotriazole), with a content of 1 wt.%.

[0054] Apply the electrolytes of Examples 1 - 9 to the lithium iron phosphate battery system for performance testing, and the performance comparisons are shown in Table 1 below. Among them, the positive electrode slurry ratio is lithium iron phosphate: conductive carbon black (Super P): binder (polyvinylidene fluoride PVDF_5130): dispersant (polyvinylpyrrolidone PVP_30) = 97.15:1.1:1.6:0.15.

[0055] The negative electrode slurry ratio is graphite: conductive agent (Super P): binder (styrene - butadiene rubber SBR): binder (lithium carboxymethyl cellulose CMC_Li) = 96.1:1:1.8:1.1.

[0056] Then perform coating, coat the positive electrode slurry on aluminum foil and the negative electrode slurry on copper foil, dry and wind up. Then perform rolling, and then die - cutting. After that, fabricate the stacked electrode core, using the fabricated positive and negative electrode sheets and separator. The separator uses a PE separator (7μm base film + 2μm Al2O3 ceramic layer + 1μm PVDF adhesive layer). After hot - pressing the stacked electrode core, weld the tabs, and then assemble it into an aluminum - plastic film to make a soft - packaged dry battery cell.

[0057] After baking the dry battery cells, electrolyte injection is carried out. The injection volume is calculated according to the injection coefficient * the designed capacity of the battery cell. The injection coefficient is 3.6 g / Ah, and the designed capacity of the battery cell is 8.7 Ah. Therefore, the injection volume per single battery cell is 31.32 ± 0.5 g. After soaking, the battery is formed and capacity-fractionated to obtain the finished battery cells.

[0058] High-temperature cycling test method:

[0059] 1. Place the battery in a constant-temperature cabinet at 55 °C until the temperature is balanced and the battery temperature rises to the specified temperature;

[0060] 2. Charge at 1C CCCV to 3.65 V and cut off the current at 0.05C;

[0061] 3. Leave it standing for 30 min;

[0062] 4. Discharge at 1C to 2.0 V;

[0063] 5. Leave it standing for 30 min;

[0064] 6. Repeat the above steps for cyclic testing;

[0065] 7. Check the capacity retention rate after 500 cycles.

[0066] High-temperature storage test method:

[0067] 1. Under the condition of a constant-temperature cabinet at room temperature of 25 °C, charge the battery at 1C CCCV to 3.65 V and cut off the current at 0.05C; 2. Place the battery in an oven at 60 °C for storage for 30 days;

[0068] 3. Place the battery in a constant-temperature cabinet at room temperature of 25 °C and leave it standing to cool down for 4 h;

[0069] 4. Test the gas generation amount of the battery;

[0070] 5. Discharge at 1C to 2.0 V, which is recorded as the capacity retention rate.

[0071] 2.2C fast charging and lithium plating determination method:

[0072] 1. Place the battery in a constant-temperature cabinet at 25 °C until the temperature is balanced and the battery temperature rises to the specified temperature;

[0073] 2. Discharge at 1C to 2.0 V;

[0074] 3. Leave it standing for 30 min;

[0075] 4. Charge at 2.2C CC to 3.65 V;

[0076] 5. Leave it standing for 30 min;

[0077] 6. Repeat the above steps for cyclic testing;

[0078] When cycling reaches 50 laps, the battery is taken out and disassembled in a drying room. The electrode sheets are unfolded, and the lithium deposition situation on the fully embedded negative electrode sheet is observed.

[0079] Table 1

[0080]

[0081]

[0082] As can be seen from Table 1, compared with Example 1, in Example 4, the gas production at 60 °C with 100% SOC stored for 30 days increased significantly, the retention rate of high-temperature cycling at 55 °C for 500 laps and the capacity retention rate of 60 °C with 100% SOC stored for 30 days decreased significantly, and large-area lithium deposition occurred during 2.2C fast charging for 50 laps. The reason may be that the additive trimethyl phosphate has a P=O structure and it is difficult to form an electron-donating group. Compared with Example 1, in Example 5, the gas production at 60 °C with 100% SOC stored for 30 days also increased significantly, the retention rate of high-temperature cycling at 55 °C for 500 laps and the capacity retention rate of 60 °C with 100% SOC stored for 30 days decreased significantly, and large-area lithium deposition occurred during 2.2C fast charging for 50 laps. The reason may be that in the additive structural formula of Example 5, N is connected to the benzene ring, which increases the difficulty of coupling with the -O- structure in TMP. The performance comparison between Examples 6 and 7 and Example 1 shows that in the system of the present invention, by introducing a specific structural additive combination while using trimethyl phosphite, the SEI film can be effectively repaired in the long term during the long-term cycling use and storage of the battery, maintaining the cycling and storage performance of a long life cycle, and suppressing gas production during the long-term use of the battery.

[0083] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A non-aqueous electrolyte for a secondary battery, comprising a solvent, a metal salt and an additive; the additive comprises a benzo structure additive, which is at least one of the following structures: in, R1, R2, R3 and R4 are each selected from one of H, F, Cl, Br, hydroxyl and C1-10 alkyl.

2. The non-aqueous electrolyte according to claim 1, characterized in that The additive further comprises trimethyl phosphite.

3. The non-aqueous electrolyte according to claim 1 or 2, characterized in that In the electrolyte, the content of the benzo structure additive is 0.01-1wt.%, and the content of trimethyl phosphite is 0.01-1wt.%.

4. The non-aqueous electrolyte according to claim 3, characterized in that In the electrolyte, the dosage ratio of the benzo structure additive to trimethyl phosphite is 1:100-100:

1.

5. The non-aqueous electrolyte according to claim 1 or 2, characterized in that: It also includes at least one of the following technical features: A. The metal salt is a lithium salt, and the content of the lithium salt in the electrolyte is 7-18wt.%; B. The metal salt is a lithium salt, and the lithium salt comprises at least one of LiPF6, LiPO2F2, LiFSI, LiFSA, LiBF4, LiODFP, LiODFB, LiAsF6, and LiTFSI; C. The metal salt is a sodium salt, and the content of the sodium salt in the electrolyte is 5-18wt.%; D. The metal salt is a sodium salt, and the sodium salt comprises at least one of NaPF6, NaPO2F2, NaFSI, NaBF4, NaODFP, NaODFB, and NaTFSI; E. The metal salt is potassium salt, and the content of potassium salt in the electrolyte is 3-18wt.%; F. The metal salt is a potassium salt, and the potassium salt comprises at least one of KPF6, KPO2F2, KFSI, KBF4, KODFP, KODFB, and KTFSI; G. The solvent includes a combination of at least two of EC, PC, DMC, EMC, DEC, EA, EP, PP, MA, and MP; H. The content of the solvent in the electrolyte is 70-88wt.%.

6. The non-aqueous electrolyte according to claim 1 or 2, characterized in that: The additives also include negative electrode film-forming additives; the negative electrode film-forming additives include at least one of VC, FEC, DTD, MMDS, and PS.

7. The non-aqueous electrolyte according to claim 1 or 2, characterized in that: The additives also include positive electrode film-forming additives; the positive electrode film-forming additives include at least one of TMSP and TMSB.

8. A secondary battery using the nonaqueous electrolyte according to claim 1 or 2.

9. The secondary battery according to claim 8, characterized in that: The active material in the negative electrode of the secondary battery is at least one of graphite and a silicon-containing material.

10. The secondary battery according to claim 8, characterized in that: The secondary battery is a lithium battery system; the active material in the positive electrode is at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium cobalt phosphate, and nickel cobalt manganese aluminum lithium.

Images