Cylindrical battery electrolyte and lithium ion battery

Abstract

The invention discloses a cylindrical battery electrolyte and a lithium ion battery, and belongs to the technical field of lithium ion batteries. The cylindrical battery electrolyte comprises the following components in percentage by mass: 10-14% of electrolyte lithium salt; 68-76% of a non-aqueous organic main solvent; 7-15% of a non-aqueous organic cosolvent; 2-5% of an additive; the non-aqueous organic cosolvent is a weak solvation solvent. According to the cylindrical battery electrolyte provided by the invention, the weak solvation solvent is added into the cylindrical battery electrolyte, so that the solvation energy of the electrolyte can be reduced, a formed solvent shell layer is relatively loose, the viscosity of the electrolyte can be reduced, the fluidity and permeability of the electrolyte in the battery can be improved, and the wettability of the electrolyte can be improved; and meanwhile, lithium ions are allowed to enter the solvation layer through the weak solvation solvent, so that the lithium ions are separated from the solvent shell layer more easily and migrate, and the ionic conductivity of the electrolyte is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a cylindrical battery electrolyte and a lithium ion battery. Background Art

[0002] Compared with lithium-ion soft-pack batteries, lithium-ion cylindrical batteries have higher energy density due to higher internal space utilization and increased content of active materials in the positive and negative electrodes of the battery. However, due to the more compact internal space of the cylindrical battery shell and the influence of the electrolyte's own viscosity, the wettability of the cylindrical battery electrolyte is greatly reduced. In addition, the inner wall of the cylindrical battery's own internal expansion and extrusion structure will reduce the actual effective contact between the positive and negative electrodes and the electrolyte, which will affect the subsequent cycle and capacity performance of the cylindrical battery.

[0003] To address the problem of poor electrolyte wettability in cylindrical batteries, the common solutions currently include: 1) using a bell-shaped cavity for liquid injection; 2) appropriately increasing the vacuum degree during liquid injection and alternating cycles, and other methods to improve the electrolyte wetting effect and shorten the wetting time.

[0004] However, although the use of bell-shaped cavity injection and appropriately increasing the vacuum degree during injection and alternating circulation can improve the electrolyte infiltration effect and shorten the infiltration time, it increases the difficulty of the injection process, requires a more stringent injection environment, and cannot fundamentally solve the problem of poor wettability caused by the high viscosity of the electrolyte with ethylene carbonate as the solvent. It is impossible to ensure that the electrolyte is evenly distributed inside the battery in the later stage of long circulation, resulting in capacity decay of the cylindrical battery. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides an electrolyte with good wettability and suitable for cylindrical batteries; another object of the present invention is to provide a cylindrical battery with excellent cycle performance and rate performance.

[0006] The present invention discloses a cylindrical battery electrolyte, which comprises, by mass percentage:

[0007]

[0008] The non-aqueous organic co-solvent is a weak solvating solvent.

[0009] Furthermore, the weak solvating solvent is tris(2,2,2-trifluoroethyl)borate. The weak solvating tris(2,2,2-trifluoroethyl)borate added to the electrolyte can reduce the solvation energy between lithium salt and lithium ions, allowing anions to enter the solvation layer to form an anion-derived interface layer, which is rich in inorganic components and has low impedance and high ionic conductivity. In addition, the lithium ion migration rate in the weak solvating electrolyte is faster, so that the assembled lithium ion cylindrical battery has better cycle and rate performance.

[0010] Furthermore, the amount of tris(2,2,2-trifluoroethyl)borate added is 11% of the mass of the cylindrical electrolyte. When the amount of tris(2,2,2-trifluoroethyl)borate added is controlled at this value, the cycle retention rate of the lithium-ion cylindrical battery is optimal.

[0011] Further, the non-aqueous organic main solvent includes any one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), or a combination of at least two thereof.

[0012] Furthermore, the non-aqueous organic main solvent is a combination of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate.

[0013] Ethylene carbonate has a high dielectric constant and can effectively dissolve lithium salts. This solubility is essential for forming a stable electrolyte because only when the lithium salt is completely dissolved can it provide enough lithium ions for the charging and discharging process. During the first charging process, ethylene carbonate will decompose on the surface of the negative electrode to form a solid electrolyte interface film (SEI film). This film can prevent further reaction between the electrolyte and the electrode material while allowing lithium ions to pass through. The presence of the SEI film is very important to prevent the continuous consumption of the negative electrode material and improve the cycle life of the battery.

[0014] Compared with ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate have weaker interactions with lithium ions, which is beneficial for providing rate performance of lithium-ion batteries.

[0015] Furthermore, the electrolyte lithium salt includes any one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalatoborate) (LiDFOB), and lithium difluorophosphate (LiPO2F2), or a combination of at least two thereof.

[0016] Furthermore, the electrolyte lithium salt is a combination of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium bis(oxalatoborate).

[0017] Furthermore, the additive is a combination of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propane sultone (PS), and diethyl sulfate (DTD).

[0018] Furthermore, among the additives, the addition amount of fluoroethylene carbonate accounts for 0.95% of the mass of the electrolyte; the addition amount of vinylene carbonate accounts for 0.75% of the mass of the electrolyte; the addition amount of 1,3-propane sultone accounts for 0.75% of the mass of the electrolyte; and the addition amount of vinyl sulfate accounts for 0.55% of the mass of the electrolyte.

[0019] The present invention also provides a cylindrical lithium-ion battery, comprising a shell, a positive electrode sheet, a negative electrode sheet, a separator and the cylindrical battery electrolyte as described above.

[0020] The invention provides a cylindrical battery electrolyte. A weak solvating solvent is added to the cylindrical battery electrolyte, which can reduce the electrolyte solvation energy, form a relatively loose solvent shell layer, reduce the viscosity of the electrolyte, and help improve the fluidity and permeability of the electrolyte inside the battery, and improve the wettability of the electrolyte; at the same time, the weak solvating solvent allows lithium ions to enter the solvation layer, which makes it easier for the lithium ions to detach from the solvent shell layer and migrate, thereby improving the ionic conductivity of the electrolyte. DETAILED DESCRIPTION

[0021] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments.

[0022] Example 1

[0023] In a glove box filled with argon, 21.22g EC, 22.40g EMC, 28.38g DMC, 0.95g FEC, 0.75g VC, 0.75g PS, 0.55g DTD, and 11.00g TFEB were respectively pipetted into a container, and then 12.80g LiPF6, 0.30g LiDFOB, and 0.90g LiFSI were slowly added and stirred until the lithium salt was completely dissolved to obtain an electrolyte.

[0024] Examples 2-3 and Comparative Examples 1-3

[0025] Compared with Example 1, Examples 2-3 and Comparative Examples 1-3 differ only in the materials and quantities added in Table 1, and the preparation steps and methods are the same.

[0026] Table 1 Specific formula of electrolyte of Examples 1-3 and Comparative Examples 1-3

[0027] Content wt% <![CDATA[LiPF6]]> LiFSI LiDFOB EC EMC DMC FEC VC PS DTD TFEB Comparative Example 1 12.80 0.90 0.30 32.22 22.40 28.38 0.95 0.75 0.75 0.55 0 Comparative Example 2 12.80 0.90 0.30 21.22 33.40 28.38 0.95 0.75 0.75 0.55 0 Comparative Example 3 12.80 0.90 0.30 21.22 22.40 39.38 0.95 0.75 0.75 0.55 0 Example 1 12.80 0.90 0.30 21.22 22.40 28.38 0.95 0.75 0.75 0.55 11.00 Example 2 12.80 0.90 0.30 17.22 22.40 28.38 0.95 0.75 0.75 0.55 15.00 Example 3 12.80 0.90 0.30 25.22 22.40 28.38 0.95 0.75 0.75 0.55 7.00

[0028] Wherein TFEB stands for tris(2,2,2-trifluoroethyl)borate.

[0029] Positive electrode sheet: LFP is used as the positive electrode active material. Specifically, 94wt% LFP, 4wt% PVDF and 2wt% conductive agent are homogenized at high speed and evenly coated on aluminum foil. The coating surface density is 120g / m on a single side. 2 , and obtain the positive electrode sheet;

[0030] Negative electrode sheet: Graphite is used as the negative electrode active material. Specifically, 95wt% LFP, 3wt% PVDF and 2wt% conductive agent are homogenized at high speed and evenly coated on copper foil. The coating surface density is 55g / m on a single side. 2 , and obtain the negative electrode sheet;

[0031] Diaphragm: The size of Jinliji membrane is 0.012mm*64mm;

[0032] Assembly: Winding the separator / negative electrode sheet / separator / positive electrode sheet in one direction from bottom to top into a winding core;

[0033] Preparation of lithium-ion battery: Place the core into the shell, then laser weld the upper and lower covers to the shell and the core tabs, inject a proper amount of electrolyte, and seal weld to obtain a cylindrical lithium-ion battery.

[0034] Performance Test:

[0035] The cylindrical lithium-ion batteries assembled from Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests:

[0036] 1. Electrolyte viscosity test: Use a viscometer to measure the viscosity of several electrolytes at room temperature.

[0037] 2.EIS test

[0038] The EIS test was performed using an electrochemical workstation produced by Shanghai Chenhua Instrument Co., Ltd., with a frequency range of 0.01-100 kHz and an amplitude of 10 mV. The impedance fitting was performed using Zview software.

[0039] The test results of viscosity and EIS after formation are shown in Table 2.

[0040] 3. Cycle performance test: voltage range 2.7V-3.65V, +1C / -1C cycle 600 times. The cycle performance test results are shown in Table 3.

[0041] 4. Rate performance test: The voltage range is 2.7V-3.65V, and the charge and discharge cycles are 5 times at 1C / 2C / 5C, respectively, and the temperature is 25°C. The rate performance test results are shown in Table 4.

[0042] Table 2 Viscosity and interfacial film impedance test results of the embodiments and comparative examples

[0043] Viscosity (25℃, unit: mPa.s) After formation Resei(Ω) Comparative Example 1 4.5 53 Comparative Example 2 3.7 44 Comparative Example 3 3.6 47 Example 1 2.8 17 Example 2 1.7 12 Example 3 3.2 26

[0044] Table 3 Cyclic performance test results of embodiments and comparative examples

[0045]

[0046] Table 4 Rate performance of the embodiments and comparative examples

[0047]

[0048] Result analysis:

[0049] From the viscosity test results, it can be seen that the addition of TFEB can reduce the viscosity of the electrolyte and accelerate the wetting effect of the electrolyte on the diaphragm and the electrode;

[0050] From the EIS test results, it can be seen that the SEI impedance of the embodiment is significantly lower than that of the comparative example, indicating that the addition of the weak solvating solvent TFEB generates a low-impedance SEI film;

[0051] From the test results of room temperature and high temperature cycles, it can be seen that the cycle performance of the embodiment is better than that of the comparative example as a whole, especially in high temperature cycles, the embodiment is significantly better than the comparative example, among which the cycle performance of embodiment 1 is the best; from the rate test results of the battery, it can be seen that the rate performance of the embodiment is better than that of the comparative example, among which embodiment 1 is the best. The addition of TFBE significantly improves the electrochemical performance of the battery.

[0052] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A cylindrical battery electrolyte, characterized in that: In terms of mass percentage, it includes: Electrolyte lithium salt 10-14%; Non-aqueous organic main solvent 68-76%; Non-aqueous organic co-solvent 7-15%; Additives 2-5%; The non-aqueous organic co-solvent is a weak solvating solvent.

2. A cylindrical battery electrolyte according to claim 1, characterized in that: The weak solvating solvent is tris(2,2,2-trifluoroethyl)borate.

3. A cylindrical battery electrolyte according to claim 2, characterized in that: The added amount of tris(2,2,2-trifluoroethyl)borate accounts for 11% of the mass of the cylindrical electrolyte.

4. The cylindrical battery electrolyte according to claim 1, characterized in that: The non-aqueous organic main solvent includes any one of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate, or a combination of at least two thereof.

5. A cylindrical battery electrolyte according to claim 4, characterized in that: The non-aqueous organic main solvent is a combination of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate.

6. The cylindrical battery electrolyte according to claim 1, characterized in that: The electrolyte lithium salt includes any one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), and lithium difluorophosphate, or a combination of at least two thereof.

7. A cylindrical battery electrolyte according to claim 6, characterized in that: The electrolyte lithium salt is a combination of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium bis(oxalatoborate).

8. The cylindrical battery electrolyte according to claim 1, characterized in that: The additive is a combination of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone and vinyl sulfate.

9. The cylindrical battery electrolyte according to claim 8, characterized in that: Among the additives, the addition amount of fluoroethylene carbonate accounts for 0.95% of the mass of the electrolyte; the addition amount of vinylene carbonate accounts for 0.75% of the mass of the electrolyte; the addition amount of 1,3-propane sultone accounts for 0.75% of the mass of the electrolyte; and the addition amount of vinyl sulfate accounts for 0.55% of the mass of the electrolyte.

10. A cylindrical lithium-ion battery, characterized in that: The invention comprises a shell, a positive electrode sheet, a negative electrode sheet, a separator and a cylindrical battery electrolyte as claimed in any one of claims 1 to 9.

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