An electrolyte and a lithium secondary battery

Abstract

The present invention belongs to the field of batteries and discloses an electrolyte, which comprises a lithium salt, a solvent and a first additive. The first additive has the structure shown in the following structural formula (Ⅰ), and the dosage of the first additive is equivalent to 0.1 wt% - 5.0 wt% of the total amount of the electrolyte; this additive contains three effective groups, namely a thiazolyl group, a phenyl group and a thiol group. The thiazolyl group has weak basicity, so it can effectively reduce HF in the electrolyte; the thiol group has strong oxidizing property, so it causes the additive to be oxidized earlier than the conventional electrolyte carbonate solvent and form a protective film on the surface of the positive electrode; the interfacial film containing a phenyl group has strong Li + ion conduction ability. Therefore, this additive can improve the cycle performance and high and low temperature performance of high-voltage system batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to an electrolyte and a lithium-ion battery. Background Art

[0002] At present, lithium-ion batteries have broad application prospects in the fields of portable electronic products, electric vehicles, hybrid electric vehicles, and smart grids. Among the existing cathode materials, the layered ternary material LiNi x Co y Mn 1-x-y O2 (hereinafter referred to as NCM) has a high specific capacity, a working voltage platform, and good rate performance, and occupies an important position in the field of electric vehicles. However, the current rapid development of electric vehicles has put forward higher requirements for the battery energy density. Increasing the operating voltage of the ternary material (>4.3V) can oxidize Co 3+ , Ni 3+ to Co 4+ , Ni 4+ , releasing more capacity, and the energy density = voltage × specific capacity. Therefore, the high working voltage greatly improves the energy density of the ternary lithium-ion battery, thus meeting the cruising range requirements of electric vehicles. However, the traditional carbonate electrolyte has a narrow electrochemical window (<4.3V vs. Li + / Li). After the voltage increases, on the one hand, the electrolyte itself will decompose; on the other hand, the side reaction between the cathode and the electrolyte intensifies, and the transition metal in NCM dissolves out, further causing the electrolyte to decompose. A large amount of gas is generated, which is extremely likely to cause combustion and explosion.

[0003] Adding a film-forming additive to a lithium-ion battery to improve the stability between the cathode and the interface in a high-voltage battery system, thereby suppressing electrolyte decomposition and metal ion dissolution, is a simple, easy-to-operate, and low-cost method to improve battery performance. Generally speaking, additives containing a thiazole group have a weak basicity, so they have the effect of reducing the HF content under high voltage, improving the high-temperature storage and high-voltage performance of the battery; film-forming additives containing a phenyl group can improve the Li + -conducting ability of the interface film, and ultimately improve the low-temperature performance and cycling performance of the battery; additives containing a mercapto group, due to their strong oxidizing property, are oxidized earlier than the conventional electrolyte carbonate solvent, forming a protective film on the cathode surface, thereby better improving the high-voltage resistance, high-temperature resistance, cycling, and low-temperature performance of the battery.

[0004] Based on this background, it is of great significance to develop an additive with better high-voltage resistance, high-temperature resistance, cycling, and excellent low-temperature performance by utilizing the synergistic effect of the above three groups. Summary of the Invention

[0005] One of the objectives of the present invention is to provide an electrolyte with high-voltage resistance, high-temperature resistance, cycling performance, and low-temperature performance. At the same time, a lithium secondary battery based on this electrolyte is also disclosed.

[0006] To achieve the above objective, the present invention provides 1. An electrolyte comprising a lithium salt, a solvent, and a first additive. The first additive has the structure shown in the following structural formula (Ⅰ), and the dosage of the first additive is 0.1 wt% - 5.0 wt% of the total amount of the electrolyte;

[0007]

[0008] In the above electrolyte, the dosage of the first additive is 0.5 wt% - 5.0 wt% of the total amount of the electrolyte.

[0009] In the above electrolyte, the dosage of the first additive is 0.5 wt% - 2 wt% of the total amount of the electrolyte.

[0010] In the above electrolyte, the lithium salt is selected from the conductive lithium salt lithium hexafluorophosphate, and the concentration of the lithium salt in the electrolyte is 0.5 M - 1.5 M.

[0011] In the above electrolyte, the solvent is one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propyl acetate, ethyl acetate, and propyl propionate.

[0012] In the above electrolyte, a second additive is further included. The second additive is vinylene carbonate; the dosage of the second additive is 0 wt% - 2 wt% of the total amount of the electrolyte.

[0013] Meanwhile, the present invention also discloses a lithium secondary battery, which includes a positive electrode, a negative electrode, and the electrolyte as described in any one of the above. Among them: the positive electrode material is a ternary material of lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Mn 0.2 O2; the negative electrode material is selected from at least one of graphite, silicon-carbon composite material, and lithium titanate.

[0014] The advantages of the present invention are as follows:

[0015] Compared with the prior art, since the electrolyte additive of the present invention contains a thiazole group, and the thiazole group has weak basicity, it has the effect of reducing the HF content under high voltage, improving the high-temperature storage and high-voltage performance of the battery; secondly, it contains a phenyl group, which can improve the Li conduction of the interface film. +The ability ultimately improves the low-temperature performance and cycling performance of the battery; moreover, it contains an additive with a thiol group, which, due to its strong oxidizing property, is oxidized earlier than the conventional electrolyte carbonate solvent and forms a protective film on the positive electrode surface, thereby better improving the high-voltage resistance, high-temperature resistance, cycling, and low-temperature performance of the battery. Detailed implementation mode

[0016] The following is a further description of the present invention in combination with embodiments, but it does not constitute any limitation to the present invention. Any limited modifications made within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0017] In order to elaborate on the technical content of the present invention, the following further explanation is provided in combination with the implementation mode.

[0018] Embodiment 1

[0019] 1. Preparation of the electrolyte: Ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are mixed in a mass ratio of EC:DEC:EMC = 1:1:1. After mixing, 1 mol of lithium hexafluorophosphate (LiPF6) is added. After the lithium salt is completely dissolved, 0.5% of the first additive (a compound having the structure shown in Structural Formula I, the same below) is added.

[0020] 2. Preparation of the positive electrode sheet: The ternary material lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Mn 0.2 O2, conductive agent SuperP, binder PVDF, and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 97.5:1.5:1:1 to form a lithium-ion battery positive electrode slurry with a certain viscosity. The slurry is coated on an aluminum foil used as a current collector, and the coating amount is 324 g / m 2 . After drying at 85°C, cold pressing is carried out; then trimming, slitting, and striping are performed. After striping, it is dried at 85°C for 4 hours under vacuum conditions, and the tab is welded to make a lithium-ion battery positive electrode sheet that meets the requirements.

[0021] 3. Preparation of the negative electrode sheet: Artificial graphite, conductive agent SuperP, thickener CMC, and binder SBR (styrene-butadiene rubber latex) are made into a slurry in a mass ratio of 95:1.5:1.0:2.5, mixed evenly, and the mixed slurry is coated on both sides of a copper foil, dried, and rolled to obtain a negative electrode sheet, making a lithium-ion battery negative electrode sheet that meets the requirements.

[0022] 4. Preparation of the lithium-ion battery: The positive electrode sheet, negative electrode sheet, and separator prepared according to the above process are made into a lithium-ion battery with a thickness of 4.7 mm, a width of 55 mm, and a length of 60 mm through a stacking process. The capacity is 1800 mAh, vacuum baked at 85°C for 48 hours, and the above electrolyte is injected to complete the battery production.

[0023] Examples 2 to 4

[0024] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 1, but the contents of the additives are 1.0%, 2.0%, and 5.0% respectively.

[0025] Example 5

[0026] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 1, but it contains the first additive and the second additive;

[0027] The first additive is a compound with the structure shown in Structural Formula Ⅰ; the second additive is vinylene carbonate;

[0028] The dosage of the first additive is 0.5% of the total amount of the electrolyte; the dosage of the second additive is 0.5% of the total amount of the electrolyte.

[0029] Example 6

[0030] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 5, but the dosage of the first additive is 0.7% of the total amount of the electrolyte; the dosage of the second additive is 0.3% of the total amount of the electrolyte.

[0031] Example 7

[0032] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 5, but the dosage of the first additive is 0.9% of the total amount of the electrolyte; the dosage of the second additive is 0.1% of the total amount of the electrolyte.

[0033] Example 8

[0034] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 5, but the dosage of the first additive is 0.3% of the total amount of the electrolyte; the dosage of the second additive is 0.7% of the total amount of the electrolyte.

[0035] Example 9

[0036] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 5, but the dosage of the first additive is 1.4% of the total amount of the electrolyte; the dosage of the second additive is 0.6% of the total amount of the electrolyte.

[0037] Example 10

[0038] The preparation of the electrolyte, the positive electrode sheet, the negative electrode sheet, and the preparation of the lithium ion battery are the same as those in Example 5, but the dosage of the first additive is 2.1% of the total amount of the electrolyte; the dosage of the second additive is 0.9% of the total amount of the electrolyte.

[0039] Comparative Examples 1 to 4

[0040] The electrolyte preparation method and the battery preparation method refer to Example 1, and the additives are no additive, Comparative Compounds 1 to 3 respectively, and the addition amount of the additive is 1.0% for all.

[0041] Comparative Example 5

[0042] The electrolyte preparation method and the battery preparation method refer to Example 1, Comparative Compounds 1 to 3 are added, and the additives are all 1.0%, with a total of 3%.

[0043] Comparative Example 6

[0044] The electrolyte preparation method and the battery preparation method refer to Example 1, the first additive is not added, and the second additive is added, and the dosage of the second additive is 1% of the total amount of the electrolyte.

[0045] Comparative Example 7

[0046] The electrolyte preparation method and the battery preparation method refer to Example 1, the first additive is not added, and the second additive is added, and the dosage of the second additive is 2% of the total amount of the electrolyte.

[0047] Comparative Example 8

[0048] The electrolyte preparation method and the battery preparation method refer to Example 1, the first additive is not added, and the second additive is added, and the dosage of the second additive is 5% of the total amount of the electrolyte.

[0049] Comparative Compounds 1 to 3 are as follows:

[0050]

[0051] The electrolyte compositions and battery systems of the above examples and comparative examples are shown in Table 1.

[0052] Table 1: Electrolyte Compositions and Battery Systems of Examples and Comparative Examples

[0053]

[0054]

[0055] Application Experiments of Examples and Comparative Examples

[0056] 25°C 1.0C / 1.0C Normal Temperature Cycle Test: At 25°C, charge at a constant current of 1.0C to 4.5V, charge at a constant voltage of 4.5V until the cut-off current is 0.05C, and then discharge the battery at a constant current of 1.0C. The discharge capacity is recorded as C0, and repeat the charge and discharge steps for 1000 cycles to obtain the discharge capacity C at the 1000th cycle 1000, Capacity retention rate = C 1000 / C0 * 100%.

[0057] 45°C 1.0C / 1.0C high-temperature cycle test: Charge the battery at a constant current of 1.0C to 4.5V at 45°C, then charge at a constant voltage until the cut-off current of 0.05C, and then discharge the battery at a constant current of 1.0C. The discharge capacity is recorded as C0. Repeat the charge and discharge steps for 1000 cycles to obtain the discharge capacity C at the 1000th cycle 1000 , Capacity retention rate = C 1000 / C0 * 100%

[0058] Capacity retention rate after storing at 60°C for 14 days: After the battery is cycled 3 times at a charge and discharge rate of 1C, it is stored at a high temperature of 60°C for 14 days in a fully charged state and then subjected to a discharge test. The obtained discharge capacity divided by the discharge capacity of the first cycle gives the capacity retention rate after high-temperature storage

[0059] -20°C low-temperature discharge test: Charge the battery at a constant current of 1.0C to 4.5V at 25°C, then charge at a constant voltage of 4.5V until the cut-off current of 0.05C, and then discharge the battery at a constant current of 0.5C. The discharge capacity is recorded as C0. At 25°C, charge the battery at a constant current of 1.0C to 4.5V, then charge at a constant voltage of 4.5V until the cut-off current of 0.05C, and then transfer the battery to -20°C and let it stand for 240 min, and then discharge the battery at a constant current of 0.5C. The discharge capacity is recorded as C1, and the -20°C discharge rate = C_{1} / C_{0} * 100%.

[0060] After the electrolytes in the above examples and comparative examples are made into lithium-ion batteries, the room-temperature cycle energy absorption, high-temperature cycle performance, high-temperature storage performance, and low-temperature discharge performance of the lithium-ion batteries are tested. The results are shown in Table 2

[0061] Table 2: Performance test results of lithium-ion batteries

[0062]

[0063]

[0064] In the high-voltage system, through Examples 1 to 4, it is found that the optimal dosage of the first additive is 1.0%. When the addition amount is 0.5%, due to the too small addition amount, the film density formed on the positive electrode is not enough, so the overall performance is worse than that of 1.0%. When the addition amount exceeds 1.0%, due to the too large addition amount, the film density formed on the positive electrode is too dense, so the low-temperature performance is worse than that of 1.0% but other performances are the same as those of 1.0%.

[0065] Through examples and comparative examples, by making comparisons, it is found that the performance of the additive containing a single thiazolyl group, phenyl group and mercapto group is better than that without the additive. The additive containing a thiazolyl group mainly improves the normal-temperature, high-temperature cycle performance and high-temperature storage performance of the battery, but has little positive effect on the low-temperature performance; the additive containing a phenyl group mainly improves the low-temperature performance of the battery, but has little influence on the normal-temperature, high-temperature cycle performance and high-temperature storage performance of the battery; the additive containing a mercapto group can improve the overall performance of the battery, but the improvement amplitude is not large. When the three are used simultaneously, the overall performance of the battery is improved, but there is still a large gap compared with the first additive, which proves that an additive containing three groups simultaneously will better play a synergistic effect and improve the overall performance of the battery.

[0066] Meanwhile, through Examples 5 to 10 and Comparative Examples 6 to 8, when the first additive and the second additive of the present invention are mutually compounded, the normal-temperature and high-temperature cycle, high-temperature storage and low-temperature performance of the first additive of the present invention can be better exerted. The optimal addition amounts of the two are 0.7% and 0.3% respectively, and the ratio is 7:3.

Claims

1. An electrolyte, comprising a lithium salt, a solvent, a first additive and a second additive, characterized in that, The first additive has the structure shown in the following structural formula (I), and the dosage of the first additive is equivalent to 0.5 wt% - 2.0 wt% of the total amount of the electrolyte; The second additive is vinylene carbonate, and the dosage of the second additive is equivalent to 0.1 wt% - 0.9 wt% of the total amount of the electrolyte.

2. The electrolyte according to claim 1, wherein The lithium salt is lithium hexafluorophosphate, and the concentration of the lithium salt in the electrolyte is 0.5 M - 1.5 M.

3. The electrolyte according to claim 1, wherein The solvent is one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propyl acetate, ethyl acetate, and propyl propionate.

4. A lithium secondary battery, characterized in that, The lithium secondary battery includes a positive electrode, a negative electrode, and an electrolyte as described in any one of claims 1 to 3, wherein: the positive electrode material is a ternary material of lithium nickel cobalt manganate LiNi 0.6 Co 0.2 Mn 0.2 O2; the negative electrode material is selected from at least one of graphite, silicon-carbon composite material, and lithium titanate.

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