A high-nickel lithium-ion secondary battery

Abstract

The application relates to the field of electrochemistry, in particular to a high-nickel lithium ion secondary battery. The application comprises a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the electrolyte comprises a solvent, a lithium salt and a lithium salt additive, the lithium salt additive comprises at least one salt anion, and the structural general formula of the salt anion is (B 12 F x Z 12‑x ) 2‑ , wherein Z is an alkoxy group and 0

Description

TECHNICAL FIELD

[0001] The application relates to the field of electrochemistry, in particular to a high-nickel lithium ion secondary battery. BACKGROUND

[0002] With the continuous increase of human activities, the demand for energy is becoming more and more vigorous, and the dependence on traditional fossil energy causes the continuous deterioration of the environment and climate. In order to cope with climate change, European countries have successively introduced the timetable for banning fuel vehicles, and China has also formulated the timetable for carbon peak and carbon neutralization. New energy vehicles replacing traditional fuel vehicles are the inevitable trend of history. In new energy vehicles, lithium ion batteries become the first choice of new energy vehicle power due to the mature industry chain, stable and reliable products.

[0003] At present, new energy vehicles are facing three major bottlenecks, one of which is the problem of endurance mileage. The reaction to the lithium ion battery is the improvement of the energy density, mainly adopting two schemes, one is to improve the capacity of the positive electrode material, and the other is to improve the voltage of the positive electrode material. However, the two will bring new problems, the nature of the positive electrode material is active and unstable, which causes the decomposition of the electrolyte and further deteriorates the performance of the battery, affecting the user experience. Therefore, it is very important to develop a high-nickel lithium ion secondary battery. SUMMARY

[0004] In order to solve the above technical problems, the application provides a high-nickel lithium ion secondary battery, which can improve the high-temperature storage and cycle performance of the lithium ion battery.

[0005] The application provides a high-nickel lithium ion secondary battery, which comprises a positive electrode, a negative electrode, a separator and an electrolyte, the electrolyte comprises a solvent, a lithium salt and a lithium salt additive, the lithium salt additive comprises at least one salt anion, and the structure general formula of the salt anion is (B 12 F x Z 12-x ) 2- , wherein Z is alkoxy and 0

[0006] The inventors of the application find through a large number of experiments that free acid HF is easy to cause the thickness expansion of the battery after high-temperature cycle and the large increase of lactone, and there is a risk of aggravating the metal ion dissolution of the high-voltage positive electrode, which further deteriorates the battery performance. In view of the above, in order to reduce the free HF generated after the decomposition of the lithium salt additive, the salt anion (B 12 F x Z 12-x ) 2-The lithium salt additive contains more boron elements in the salt anion, which can be used to stabilize the nickel ions in the positive electrode, avoid the valence state conversion of the nickel ions, and reduce the catalytic activity of the charged positive electrode material. At the same time, the salt anion with low fluorine atom number can form different SEI film groups on the surface of the carbon anode, so as to improve the conductivity of the overall SEI film and further improve the battery performance.

[0007] In some embodiments of the present application, the mass percentage of the lithium salt additive is A, satisfying 0

[0008] Preferably, the mass percentage of the lithium salt additive is 0.5% in the total mass of the electrolyte.

[0009] In some embodiments of the present application, the electrolyte further comprises one or more of a sulfate compound, a sulfonate compound, and a carbonate compound.

[0010] In some embodiments of the present application, the sulfate compound comprises one or more of vinyl sulfate, methyl vinyl sulfate, and vinyl sulfite.

[0011] The sulfonate compound comprises one or more of 1,3-propane sulfonate lactone, 1,4-butane sulfonate lactone, and 1,3-propylene sulfonate lactone.

[0012] The carbonate compound comprises one or more of vinylene carbonate, vinyl ethylene carbonate, methylene vinyl carbonate, fluorinated vinyl carbonate, trifluoromethyl vinyl carbonate, and difluorinated vinyl carbonate.

[0013] In some embodiments of the present application, the mass percentage of the sulfate compound is 0.1%-5.0%, the mass percentage of the sulfonate compound is 0.1%-5.0%, and the mass percentage of the carbonate compound is 0.1%-5.0% in the total mass of the electrolyte.

[0014] Preferably, the electrolyte comprises a carbonate compound, and the mass percentage of a mixture of vinylene carbonate and fluorinated vinyl carbonate is 1% in the total mass of the electrolyte. More preferably, the mass percentage of the vinylene carbonate is 0.5%, and the mass percentage of the fluorinated vinyl carbonate is 0.5%.

[0015] In some embodiments of the present application, the lithium salt is selected from at least one of LiPF6, LiPO2F2, LiBF4, LiBOB, LiClO4, LiSbF6, LiAsF6, LiCF3SO3, LiDFOB, LiFSI, LiTFSI, LiN(SO2CF3)2, LiC(SO2CF3)3, LiN(SO2C2F5)2, and LiN(SO2F)2.

[0016] In some embodiments of the present application, the solvent comprises at least one of cyclic carbonate, chain carbonate, carboxylic acid ester.

[0017] In some embodiments of the present application, the cyclic carbonate comprises at least one of ethylene carbonate, propylene carbonate, or butylene carbonate.

[0018] The chain carbonate comprises at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, or methyl propyl carbonate.

[0019] The carboxylic acid ester comprises at least one of methyl formate, methyl acetate, ethyl acetate, ethyl propionate, propyl propionate, or ethyl butyrate.

[0020] Preferably, the solvent is a mixture of ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate. More preferably, the volume ratio of ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate is 20:60:20.

[0021] In some embodiments of the present application, the positive electrode comprises a positive electrode active material selected from at least one of LiNi x Co y M 1-x-y O2, wherein M is selected from at least one element of Mn and Al, 0.8≤x<1, and 0

[0022] In some embodiments of the present application, the negative electrode comprises a negative electrode active material comprising one or more of carbon-based negative electrode, silicon-based negative electrode, tin-based negative electrode, and lithium negative electrode.

[0023] Compared with the prior art, the present application has the following advantages:

[0024] (1) The present application adds a low-fluorine lithium salt additive in the electrolyte, wherein the salt anion with a low number of fluorine atoms forms different SEI film groups on the surface of the carbon anode, which improves the conductivity of the overall SEI film and is more beneficial to the improvement of battery performance.

[0025] (2) The low-fluorine atom number salt anion additive contains more boron elements, the boron elements can stabilize the metal ions such as nickel ions in the positive electrode, avoid the valence state conversion of the nickel ions, improve the stability of the battery positive electrode material at high temperature, and thus ensure that the high-nickel lithium ion battery has excellent electrochemical performance. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments in the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0027] Embodiment 1

[0028] 1) Preparation of electrolyte:

[0029] Vinyl carbonate (abbreviated as EC), diethyl carbonate (abbreviated as DEC) and methyl ethyl carbonate (abbreviated as EMC) are mixed in a volume ratio of EC:DEC:EMC = 20:60:20, lithium hexafluorophosphate (LiPF6) is added to a molar concentration of 1 mol / L, and components containing mass percentage contents shown in Table 1 are added, with the total mass of the electrolyte being 100%.

[0030] 2) Preparation of positive electrode plate:

[0031] The positive electrode active material lithium nickel cobalt manganese oxide LiNi 0.8 Co 0.1 Mn 0.1 O2, conductive carbon black Super-P and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 93:4:3, and then they are dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry is uniformly coated on both sides of an aluminum foil, dried, calendered and vacuum dried, and then an aluminum lead wire is welded on the positive electrode plate by using an ultrasonic welding machine to obtain the positive electrode plate, and the thickness of the plate is between 120-150 μm.

[0032] 3) Preparation of negative electrode plate:

[0033] The negative electrode active material artificial graphite, conductive carbon black Super-P, binder styrene-butadiene rubber (abbreviated as SBR) and carboxymethyl cellulose (abbreviated as CMC) are mixed in a mass ratio of 94:1:2.5:2.5, and then they are dispersed in deionized water to obtain a negative electrode slurry. The slurry is coated on both sides of a copper foil, dried, calendered and vacuum dried, and then a nickel lead wire is welded on the negative electrode plate by using an ultrasonic welding machine to obtain the negative electrode plate, and the thickness of the plate is between 120-150 μm.

[0034] 4) The battery assembly method is as follows: a three-layer separator with a thickness of 20 μm is placed between the positive plate and the negative plate, then the sandwich structure composed of the positive plate, the negative plate and the separator is wound, and then the wound body is flattened and placed in an aluminum foil packaging bag, vacuum baking at 85°C for 24h to obtain the battery cell to be injected with electrolyte; the electrolyte prepared above is injected into the battery cell, vacuum packaged, and left for 24h;

[0035] Then the first charge of the conventional formation is carried out according to the following steps: 0.05C constant current charging for 180min, 0.1C constant current charging to 3.95V, vacuum sealing twice, standing at 45°C for 48h, then further constant current charging to 4.35V at a current of 0.2C, cutting off the current at 0.01C, and then constant current discharging to 3.0V at a current of 0.2C. The lithium ion battery of the present example is obtained.

[0036] 5) Normal temperature cycle performance test

[0037] At 25°C, constant current charging to 4.2V at a current of 0.5C and then constant voltage charging until the current drops to 0.05C, then constant current discharging to 3.0V at a current of 1C, and so on for 200 cycles, the discharge capacity of the first week and the discharge capacity of the 200th week are recorded, and the capacity retention rate of the normal temperature cycle is calculated according to the following formula:

[0038] Capacity retention rate = discharge capacity of the 200th week / discharge capacity of the first week x 100%.

[0039] 6) Normal temperature direct current impedance (DCIR) performance test

[0040] At 25°C, the battery after formation is charged to SOC=50% at 1C, and then charged and discharged at 0.1C, 0.2C, 0.5C, 1C and 2C respectively for ten seconds, and the cut-off voltage of charging and discharging is recorded respectively. Then, the charging and discharging current at different rates is taken as the abscissa (unit: A), and the cut-off voltage corresponding to the charging and discharging current is taken as the ordinate, and a linear relationship graph (unit: mV) is drawn.

[0041] Discharge DCIR value = the slope value of the linear graph of different discharge currents and the corresponding cut-off voltage.

[0042] 7) High temperature storage performance test

[0043] The lithium ion battery is charged to 4.2V at 1C constant current and constant voltage after formation at normal temperature, the initial discharge capacity and the initial battery thickness of the battery are measured, then the battery is stored at 60°C for 7 days, and then discharged to 3V at 1C, the retention capacity and the recovery capacity of the battery and the battery thickness after storage are measured. The calculation formula is as follows:

[0044] Thickness expansion rate (%) = (battery thickness after storage-initial battery thickness) / initial battery thickness x 100%.

[0045] Battery capacity retention rate (%) = retained capacity / initial capacity x 100%;

[0046] Battery capacity recovery rate (%) = recovered capacity / initial capacity x 100%.

[0047] Example 2-4

[0048] Example 2-4 includes most of the operation steps in Example 1, the difference is that:

[0049] In the preparation process of the electrolyte, the components shown in Table 1 in Example 2-4 are added in the mass percentage shown in Table 1, and the test results are filled in Table 1.

[0050] Comparative Example 1-2

[0051] As shown in Table 1, in the preparation process of the electrolyte, the components shown in Table 1 in Comparative Example 1-2 are added in the mass percentage shown in Table 1, and the test results are filled in Table 1.

[0052] Table 1

[0053]

[0054] From the data in Table 1, it can be seen that the battery prepared by adding the low-fluorine lithium salt additive (Li2B 12 F x Z 12-x , x≥3 and Z is alkoxy) has better room temperature cycle performance and high temperature storage performance, and the corresponding DCIR is lower. 12 F x Z 12-x , 0

[0055] Example 5-10

[0056] Example 5-10 includes most of the operation steps in Example 1, the difference is that:

[0057] In the preparation process of the electrolyte, the components shown in Table 2 in Example 5-10 are added in the mass percentage shown in Table 2, and the test results are filled in Table 2.

[0058] Comparative Example 3

[0059] As shown in Table 2, except that no lithium salt additive is added in the preparation of the electrolyte, the others are the same as Example 1, and the test results are filled in Table 2.

[0060] Table 2

[0061]

[0062] From the data in Table 2, it can be seen that when the content of the lithium salt additive is gradually increased from 0.1% to 0.5%, the room temperature cycle performance and high temperature storage performance of the battery are significantly improved, but when the content of the lithium salt additive is continuously added to 1%, the room temperature cycle performance and high temperature storage performance of the battery are slightly decreased, and when the content of the lithium salt additive is continuously added to 5%, the room temperature cycle performance and high temperature storage performance of the battery are obviously decreased, but the room temperature cycle performance and high temperature storage performance of the battery are still obviously better than that of the battery without adding the lithium salt additive.

[0063] Examples 11-15

[0064] Examples 11-15 include most of the operation steps in Example 1, the difference is that:

[0065] In the preparation process of the electrolyte, the components in Table 3 in Examples 11-15 are added in the mass percentage, and the test results are filled in Table 3.

[0066] Comparative Example 4

[0067] As shown in Table 3, except that no lithium salt additive is added in the preparation of the electrolyte, only conventional additives are added, and the other is the same as Example 1, and the test results are filled in Table 3.

[0068] Table 3

[0069]

[0070] From the data in Table 3, it can be seen that on the basis of using VC and / or FEC, further adding a low-fluorine lithium salt additive can make the battery obtain better room temperature cycle performance and high temperature storage performance, and more preferably, when the two conventional additives are mixed, for example, a mixture of VC and FEC, and combined with the low-fluorine lithium salt additive in a certain proportion, the room temperature cycle performance and high temperature storage performance of the battery obtained are better than that of the battery obtained by using only one conventional additive, such as VC or FEC, combined with the low-fluorine lithium salt additive; Furthermore, when the content of the conventional mixed additive is increased from 0.5% to 1% in combination with the low-fluorine lithium salt additive, adding a certain mass percentage of the low-fluorine lithium salt additive can significantly improve the room temperature cycle performance and high temperature storage performance of the battery prepared, but when the content of the conventional mixed additive is continuously added to 5%, the room temperature cycle performance and high temperature storage performance of the battery show a downward trend, but it is still better than that of the battery using only the conventional additive.

[0071] Examples 16-17

[0072] Examples 16-17 include most of the operation steps in Example 1, the difference is that the positive plate is prepared by adding the positive active material shown in Table 4 in Examples 16-17, and the test results are filled in Table 4.

[0073] Comparative Examples 5-6

[0074] As shown in Table 4, except that no lithium salt additive is added in the preparation of the electrolyte, and the positive plate is prepared by adding the positive active material shown in Table 4 in Comparative Examples 5-6, the rest is the same as Example 1, and the test results are filled in Table 4.

[0075] Table 4

[0076]

[0077] From the data in Table 4, it can be seen that in the lithium ion battery with LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.3 Co 0.4 Mn 0.3 O2 as the positive active material, the addition of lithium salt additive can improve the high-temperature storage performance of the battery, at the same time, it can also improve the room temperature cycle performance of the battery, and the corresponding DCIR is also reduced; Specifically, in the LiNi 0.8 Co 0.1 Mn 0.1 O2 system, the difference between Comparative Example 1 and Example 1 is significant, in the LiNi 0.5 Co 0.2 Mn 0.3 O2 system, the difference between Comparative Example 5 and Example 16 is not large, and in the LiNi 0.3 Co 0.4 Mn 0.3 O2 system, the difference between Comparative Example 6 and Example 17 is small, it can be seen that the higher the Ni content, the more obvious the advantage of adding lithium salt additive in improving the room temperature cycle performance and high-temperature storage performance of the battery.

[0078] Examples 18-20

[0079] Examples 18-20 include most of the operation steps in Example 1, the difference is that:

[0080] The solvent of the components shown in Table 5 in Examples 18-20 is added in the preparation of the electrolyte, and the test results are filled in Table 5.

[0081] Table 5

[0082]

[0083] From the data in Table 5, it can be seen that the room temperature cycle performance and high temperature storage performance of the battery prepared by using a single solvent are not as good as the room temperature cycle performance and high temperature storage performance of the battery prepared by using a plurality of solvents mixed according to a certain proportion, especially, selecting any three of the carbonates (ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate and methyl propyl carbonate) as the solvent of the electrolyte according to a certain volume ratio can achieve good use effect.

[0084] In summary, the high-voltage positive electrode material (high-nickel material) is used to improve the energy density of the lithium ion battery, and a certain proportion of low-fluorine lithium salt additive is added in the preparation process of the electrolyte. Compared with adding the same proportion of high-fluorine lithium salt additive, the low-fluorine atom number forms different SEI film compositions, and improves the conductivity of the overall SEI film. It should be noted that the high-nickel material releases O2 during the high-temperature charging and discharging of the battery, and the carbonate solvent decomposes to produce a large amount of gas under the action of O2, resulting in the deterioration of the high-temperature performance of the battery. Therefore, in order to avoid the release of oxygen by the high-nickel material during the high-temperature charging and discharging of the battery, the present inventors add the low-fluorine lithium salt additive in the preparation process of the electrolyte. First, the B element in the low-fluorine lithium salt additive can stabilize the positive electrode material, reduce the oxygen release of the positive electrode, and inhibit the decomposition of the carbonate solvent. At the same time, F is easy to decompose under the catalytic action of O2 due to its strong electronegativity, and the higher the F content, the more obvious the decomposition. Since F itself does not conduct electricity, the higher the F content, the worse the conductivity of the generated SEI film. Therefore, from the perspective of stabilizing the positive electrode and improving the conductivity of the negative electrode SEI film, the low-fluorine lithium salt additive has good performance, and is therefore more beneficial to the improvement of the performance of the battery. Second, vinylene carbonate and fluoroethylene carbonate are added. The unsaturated carbide vinylene carbonate has a double bond structure, which makes it have lower energy and be more easily reduced. The halogen-containing organic compound additive is also helpful to the improvement of the performance of the lithium ion battery. The addition of fluoroethylene carbonate in the silicon negative electrode lithium ion battery is found to promote the formation of LiF and polycarbonate compounds, and reduce the impedance of the SEI film on the silicon surface, thereby improving the cycle performance of the lithium ion battery. In addition, the salt anion additive with low fluorine atom number contains more B elements, which can stabilize the positive electrode nickel ions and other metal ions, avoid the valence change of nickel ions, and improve the stability of the positive electrode material of the battery at high temperature, thereby ensuring that the high-nickel lithium ion battery has excellent electrochemical performance.

[0085] The application is further described in detail with specific examples, but it should be understood that the specific description herein should not be construed to limit the scope of the application, and various modifications made by those skilled in the art after reading the description are within the scope of the application.

Claims

1. A high-nickel lithium-ion secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte comprises a solvent, a lithium salt, and a lithium salt additive, wherein the lithium salt additive includes at least one salt anion with the general structural formula (B0). 12 F x Z 12-x ) 2- Where Z is an alkoxy group and 0 <x<3。 2. The high-nickel lithium-ion secondary battery according to claim 1, characterized in that, With the total mass of the electrolyte being 100%, the mass percentage of the lithium salt additive is A, satisfying 0. <A≤5%。 3. The high-nickel lithium-ion secondary battery according to claim 1, characterized in that, The electrolyte also includes one or more of sulfate esters, sulfonates, and carbonates.

4. The high-nickel lithium-ion secondary battery according to claim 3, characterized in that, The sulfate ester compounds include one or more of vinyl sulfate, vinyl methyl sulfate, and vinyl sulfite; The sulfonate compounds include one or more of 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, and 1,3-propenesulfonate lactone. The carbonate compounds include one or more of the following: vinylene carbonate, ethylene ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, and difluoroethylene carbonate.

5. The high-nickel lithium-ion secondary battery according to claim 3 or 4, characterized in that, Based on the total mass of the electrolyte (100%), the mass percentage of the sulfate ester compounds is 0.1%-5.0%, the mass percentage of the sulfonate ester compounds is 0.1%-5.0%, and the mass percentage of the carbonate compounds is 0.1%-5.0%.

6. The high-nickel lithium-ion secondary battery according to claim 1, characterized in that, The lithium salt is selected from at least one of LiPF6, LiPO2F2, LiBF4, LiBOB, LiClO4, LiSbF6, LiAsF6, LiCF3SO3, LiDFOB, LiFSI, LiTFSI, LiC(SO2CF3)3 and LiN(SO2C2F5)2.

7. The high-nickel lithium-ion secondary battery according to claim 1, characterized in that, The solvent includes at least one of cyclic carbonates, chain carbonates, and carboxylic acid esters.

8. The high-nickel lithium-ion secondary battery according to claim 7, characterized in that: The cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate, or butene carbonate. The chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate or methyl propyl carbonate; The carboxylic acid ester includes at least one of methyl formate, methyl acetate, ethyl acetate, ethyl propionate, methyl propionate, or ethyl butyrate.

9. The high-nickel lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode includes a positive electrode active material, which is selected from LiNi. x Co y M 1-x-y At least one of O2, wherein M is selected from at least one element selected from Mn and Al, 0.8≤x<1, 0<y≤0.

2.

10. The high-nickel lithium-ion secondary battery according to claim 1, characterized in that, The negative electrode includes a negative electrode active material, which includes one or more of carbon-based negative electrode, silicon-based negative electrode, tin-based negative electrode, and lithium negative electrode.

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