Lithium ion battery electrolyte and lithium ion battery

Abstract

The invention relates to a lithium ion battery electrolyte and a lithium ion battery, aims to solve the problem that a lithium ion battery electrolyte in the prior art cannot give consideration to normal-temperature cycle performance, high-temperature shelving safety and low-temperature discharge efficiency of the lithium ion battery at the same time, and provides the lithium ion battery electrolyte which comprises a solvent, a lithium salt and an additive, wherein the additive comprises a nitrobenzonitrile derivative, the nitrobenzonitrile derivative is one or more of compounds shown in a general formula (1), the general formula (1) is # imgabs0 #, R1 and R2 are independently H, F, alkyl, fluoroalkyl, oxyalkyl, sulfanyl, nitro or cyano, and the nitrobenzonitrile derivative does not comprise p-nitrobenzonitrile. According to the lithium ion battery electrolyte disclosed by the invention, the normal-temperature cycle performance, the high-temperature shelving performance and the low-temperature discharge performance of the lithium ion battery are obviously improved.

Description

Technical Field

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

[0002] Lithium-ion batteries have the advantages of high energy density, high working voltage, no memory effect, fast charge and discharge, and little environmental pollution, so they are widely used. However, with the higher requirements for the performance of commercial lithium-ion batteries, such as energy density, safety performance and service life, it is an urgent need for the industry to develop lithium-ion batteries with high energy density, good cycling performance and high safety.

[0003] During the first charging process of a lithium-ion battery, the electrolyte will react with the carbon negative electrode to form a passivation film (SEI film). During the charge and discharge process, lithium ions must pass through the SEI film. Therefore, the SEI film has an important impact on the main performance of lithium-ion batteries (such as cycling, high temperature and power performance, etc.). The SEI film can prevent the further decomposition of the electrolyte and reduce the side reactions on the negative electrode interface. However, as the charge and discharge proceed, the SEI film may dissolve, rupture, decompose, recombine or thicken, resulting in negative effects such as increased internal resistance or gas generation of the battery, thus affecting the cycle life, capacity utilization and safety performance of the battery. These negative effects will be more serious when the battery is stored or cycled at high temperature. In addition, as the voltage of the lithium-ion battery increases, a good passivation film (CEI film) also needs to be formed on the positive electrode side during the cycling process. Since different additives or different amounts of the same additive in the electrolyte will result in different qualities of the formed SEI film and different film impedances. Therefore, it is very necessary to improve the quality of the SEI / CEI film by selecting appropriate additives and electrolyte formulations to achieve high-performance lithium-ion batteries.

[0004] The increase in operating voltage can provide lithium-ion batteries with higher energy density. However, the resulting capacity decay problem hinders its commercial application. On the one hand, a large number of scholars believe that the commercial carbonate-based electrolytes currently used cannot withstand higher voltages. Although, in the redox test of the inert electrode, the commercial carbonate-based electrolyte showed a high electrochemical window. However, in practical applications, the catalytic effect of transition metals on the electrolyte must also be considered, which makes the current commercial carbonate-based electrolyte unable to maintain high-voltage battery cycles for a long time. On the other hand, when the cut-off voltage of the lithium cobalt oxide battery is as high as 4.5V, the CEI interface formed by the traditional carbonate-based electrolyte is very fragile. When this fragile interface is damaged, the Co ions will detach from the material and enter the electrolyte, and transfer to the negative electrode side through electromigration, where a reduction reaction occurs, destroying the SEI of the negative electrode and causing the battery to collapse. In addition, the catalytic transition metal ions will further catalyze the decomposition of the electrolyte in the electrolyte, resulting in an irreversible vicious cycle. At the same time, the electrochemical stability of the electrolyte deteriorates under high voltage, and it is easy to decompose and produce HF, which will corrode the SEI film and further affect Li + 's disembedding.

[0005] Silicon anode has serious volume expansion problem, which needs to be suppressed by stable SEI film.

[0006] When high-voltage lithium cobalt oxide is matched with silicon negative electrode batteries, as the voltage increases, carbonates and lithium hexafluorophosphate in the electrolyte further react to produce HF. High voltage will catalyze this hydrolysis reaction. The main reason is PF 6 – After combining with water, the structural oxidation stability decreases, and the decomposition energy barrier decreases, which eventually leads to the intensification of the hydrolysis reaction. HF corrodes the SEI film on the surface of the silicon negative electrode, and the volume expansion problem of the silicon negative electrode becomes more obvious, which in turn leads to a significant degradation of the full battery performance of the high-voltage lithium cobalt oxide and silicon negative electrode battery. At the same time, during the charging process, as the voltage increases, more lithium ions are embedded in the silicon negative electrode, but due to the uneven embedding of Li, extreme volume expansion and contraction occur in local areas of the Si electrode, generating cumulative stress in these areas, causing the Si electrode to disintegrate and the cycle performance to deteriorate.

[0007] Compared with high-voltage lithium cobalt oxide / graphite batteries, the performance degradation of the same formulation on high-voltage lithium cobalt oxide / silicon-based anode batteries is more obvious. The influence of the transition metal Co crosstalk effect on the SEI chemistry and performance of silicon-based batteries and the battery aging behavior may be very different from that of traditional graphite-based batteries. The surface chemistry of the anode material affects the electrolyte, resulting in different SEI chemistries of the Si / graphite composite electrode with the change of Si content, or Si can easily form various more stable alloys or Zintl phases with a variety of metal ions. Compared with graphite, the significantly lower electronic conductivity of Si may be another reason, and the lower electronic conductivity kinetically hinders the reduction reaction of transition metal ions. For example, adiponitrile (ADN) and 1,3,6-hexanetricarbonitrile (HTCN) can significantly improve the cycling performance of high-voltage lithium cobalt oxide / graphite batteries, but they cannot improve the performance on high-voltage lithium cobalt oxide / silicon-based anode batteries. This may be because the cyano group can complex with the metal ion Co to avoid the degradation of the graphite anode by Co, but Co has no effect on the silicon anode, instead, it will lead to an increase in the impedance of the silicon anode and degrade the performance of the silicon anode battery. Therefore, new additives need to be found to compensate for the performance degradation of the battery. Summary of the Invention

[0008] The object of the present invention is to provide a lithium-ion battery electrolyte that can take into account the high-voltage room-temperature cycling performance, high-temperature storage safety, and low-temperature discharge efficiency of lithium-ion batteries, and a lithium-ion battery using this electrolyte.

[0009] To achieve the above object, the technical solution adopted by the present invention is:

[0010] A lithium-ion battery electrolyte, which includes a solvent, a lithium salt, and an additive. The additive includes a nitrobenzonitrile derivative, and the nitrobenzonitrile derivative is one or more of the compounds represented by the general formula (1). The general formula (1) is:

[0011]

[0012] R 1 、R 2 Independently are H, F, alkyl, fluoroalkyl, oxyalkyl, thioalkyl, nitro, or cyano, and the nitrobenzonitrile derivative does not include p-nitrobenzonitrile.

[0013] Preferably, R 1 、R 2 Independently are H, F, alkyl with 1 to 3 carbon atoms, fluoroalkyl with 1 to 3 carbon atoms, oxyalkyl with 1 to 3 carbon atoms, thioalkyl with 1 to 3 carbon atoms, nitro, or cyano, and the nitrobenzonitrile derivative does not include p-nitrobenzonitrile.

[0014] The alkyl mentioned in the present invention can be a straight-chain alkyl or a branched-chain alkyl.

[0015] Preferably, the nitrobenzonitrile derivative is one or more of the following compounds A - N:

[0016]

[0017] Preferably, the mass of the nitrobenzonitrile derivative accounts for 1% - 8% of the total mass of the lithium - ion battery electrolyte.

[0018] More preferably, the mass of the nitrobenzonitrile derivative accounts for 1% - 5% of the total mass of the lithium - ion battery electrolyte.

[0019] Even more preferably, the mass of the nitrobenzonitrile derivative accounts for 2% - 4% of the total mass of the lithium - ion battery electrolyte.

[0020] Preferably, the solvent is one or more of carbonates, carboxylates, and ethers.

[0021] More preferably, the carbonate is one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate.

[0022] More preferably, the carboxylate is one or more of propyl propionate, ethyl acetate, ethyl butyrate, and methyl propionate.

[0023] More preferably, the ether is ethylene glycol dimethyl ether and / or 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether.

[0024] More preferably, the solvent is a mixture of carbonates and carboxylates.

[0025] More preferably, the solvent at least includes dimethyl carbonate and carboxylate, and the sum of the mass of dimethyl carbonate and the carboxylate accounts for 10% - 40% of the total mass of the solvent, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%.

[0026] Even more preferably, the sum of the mass of dimethyl carbonate and the carboxylate accounts for 25% - 35% of the total mass of the solvent.

[0027] Preferably, the lithium salt is LiClO 4 , LiPF 6 , LiTFSI, LiFSI or one or more of them.

[0028] Preferably, the molar concentration of the lithium salt in the lithium - ion battery electrolyte is 0.001 - 2 mol / L.

[0029] Further preferably, the molar concentration of the lithium salt in the lithium battery electrolyte is 0.1 - 2 mol / L.

[0030] Even more preferably, the molar concentration of the lithium salt in the lithium battery electrolyte is 1 - 1.5 mol / L.

[0031] Preferably, the additive further includes other additives, and the other additives are one or more of fluoroethylene carbonate, lithium tetrafluoroborate, and lithium difluorophosphate.

[0032] In the present invention, it is preferred that the other additives account for 3% - 15% of the total mass of the lithium ion battery electrolyte, and more preferably 5% - 10%.

[0033] The present invention also provides a lithium ion battery, including a positive electrode, a negative electrode, and an electrolyte, and the electrolyte is the above-mentioned lithium ion battery electrolyte.

[0034] In the present invention, the positive electrode of the lithium ion battery is a lithium cobaltate positive electrode material, and the negative electrode is a negative electrode containing a silicon material, not limited to nano-silicon, silicon monoxide, or silicon carbide.

[0035] In the present invention, the upper working voltage of the lithium cobaltate positive electrode material of the lithium ion battery ≥ 4.55V.

[0036] In the lithium battery electrolyte of the present invention, by selecting a nitrobenzonitrile derivative having the general formula (1) and optimizing the formulation of the electrolyte, the quality of the SEI / CEI film is improved, and the performance of the lithium ion battery is significantly enhanced. The capacity retention rate of the lithium ion battery of the present invention can reach up to 93% after 300 cycles at 1.0C at 25°C, and the swelling rate is at least 19% after standing at 60°C for 30 days, achieving good technical effects.

[0037] Due to the application of the above technical solutions, the present invention has the following advantages compared with the prior art:

[0038] The lithium battery electrolyte of the present invention can improve the performance of the lithium battery. The lithium ion battery prepared with the electrolyte of the present invention has the advantages of good high-voltage cycling performance at room temperature, low swelling rate during high-temperature storage, and high low-temperature discharge efficiency, and has great application prospects. Specific Embodiments

[0039] The present invention will be further described below in conjunction with embodiments. However, the present invention is not limited to the following embodiments. The implementation conditions adopted in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in each implementation manner of the present invention can be combined with each other as long as they do not conflict with each other.

[0040] For the experimental methods in the following examples and comparative examples, unless otherwise specified, they are all conventional methods; for the experimental materials used, unless otherwise specified, they are all obtained from conventional biochemical reagent manufacturers.

[0041] Examples 1 to 27 and Comparative Examples 1 to 9 provide different electrolytes, and the specific formulations are shown in Table 1.

[0042] Table 1

[0043]

[0044]

[0045] EC: ethylene carbonate; PC: propylene carbonate; DMC: dimethyl carbonate; EMC: ethyl methyl carbonate; EA: ethyl acetate; LiPF6: lithium hexafluorophosphate; FEC: fluoroethylene carbonate; LiODFB: lithium difluorooxalate borate; LiPO 2 F 2 : lithium difluorophosphate; LiNO 3 : lithium nitrate; SBN: sebaconitrile; A: B: C: D: E: F: G: H: I: J: K: L: M: N:

[0046] The structural formula of p-nitrobenzonitrile used in Comparative Examples 2 to 5 and Comparative Example 7 is:

[0047]

[0048] Performance test:

[0049] The electrolytes of Examples 1 to 27 and Comparative Examples 1 to 9 were assembled into lithium-ion batteries with the positive electrode and negative electrode according to the conventional process. The negative electrode is C material doped with 20% Si, and the positive electrode is 4.55V LCO material.

[0050] The lithium-ion batteries prepared above were tested at 25°C with a constant current charge of 1.0C to 4.55V, and then a constant current discharge of 1.0C to 2.75V. The battery capacity retention rate after 300 cycles of such charge and discharge was measured. The 300-week cycle capacity retention rate (%) = the discharge capacity of the lithium-ion battery after 300 weeks of cycling / the average value of the discharge capacity of the lithium battery in the first 5 weeks of cycling × 100.

[0051] The swelling rate of the lithium-ion batteries prepared in each example and comparative example was tested after being stored at 60 °C for 30 days. The swelling rate (%) = (the thickness of the lithium-ion battery after 30 days of storage - the thickness of the lithium-ion battery before storage) / the thickness of the lithium-ion battery before storage × 100.

[0052] The lithium-ion batteries prepared above were charged at a constant current of 1.0 C to 4.55 V at room temperature (25 °C), and the discharge efficiency at -20 °C when discharged at 0.2 C to 2.75 V was tested. The discharge efficiency at -20 °C at 0.2 C (%) = the discharge capacity of the lithium-ion battery at 0.2 C at -20 °C / the charge capacity of the lithium-ion battery at 0.2 C at room temperature × 100.

[0053] The above performance test results are shown in Table 2.

[0054] Table 2

[0055]

[0056]

[0057] Combining Table 1 and Table 2, it can be seen that in a full battery with a cobalt lithium oxide at a high voltage of 4.55 V and a silicon-carbon negative electrode, compared with the basic formulation (Comparative Example 6), the room-temperature cycling performance of the lithium-ion batteries with different nitrobenzonitrile derivatives added is improved, and the gas generation of the lithium-ion batteries after being stored at 60 °C for 30 days is inhibited; whether the nitrobenzonitrile derivatives used in the examples are used alone or in combination with other additives, their improvement effects on the room-temperature cycling performance, high-temperature gas generation performance, and low-temperature discharge performance of the lithium-ion batteries are better than those of SS, SBN, or LiNO 3 , combining the nitrobenzonitrile derivatives with FEC, LiODFB, and LiPO 2 F 2 has a more obvious improvement effect on the battery.

[0058] The above examples are only used to illustrate the technical concept and characteristics of the present invention, and their purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. Any equivalent changes made according to the spirit and essence of the present invention.

Claims

1. A lithium ion battery electrolyte comprising a solvent, a lithium salt and an additive, characterized in that: The additive includes a nitrobenzonitrile derivative, and the nitrobenzonitrile derivative is one or more compounds represented by the general formula (1), wherein the general formula (1) is: R1 and R2 are independently H, F, alkyl, fluoroalkyl, oxyalkyl, sulfanyl, nitro or cyano, and the nitrobenzonitrile derivative does not include p-nitrobenzonitrile.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that: R1 and R2 are independently H, F, an alkyl group having 1 to 3 carbon atoms, a fluoroalkyl group having 1 to 3 carbon atoms, an oxyalkyl group having 1 to 3 carbon atoms, a sulfanyl group having 1 to 3 carbon atoms, a nitro group or a cyano group, and the nitrobenzonitrile derivative does not include p-nitrobenzonitrile.

3. The lithium ion battery electrolyte according to claim 1, characterized in that: The nitrobenzonitrile derivative is one or more of the following compounds A to N:

4. The lithium-ion battery electrolyte according to claim 1, characterized in that: The mass of the nitrobenzonitrile derivative accounts for 1% to 8% of the total mass of the lithium ion battery electrolyte.

5. The lithium-ion battery electrolyte according to claim 1, characterized in that: The solvent is one or more of carbonates, carboxylates and ethers.

6. The lithium ion battery electrolyte according to claim 5, characterized in that: The carbonate is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and propylene carbonate; the carboxylic acid ester is one or more of propyl propionate, ethyl acetate, ethyl butyrate, and methyl propionate; and the ether is ethylene glycol dimethyl ether and / or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

7. The lithium ion battery electrolyte according to claim 5, characterized in that: The solvent at least comprises dimethyl carbonate and carboxylic acid ester, and the sum of the mass of the dimethyl carbonate and the carboxylic acid ester accounts for 10% to 40% of the total mass of the solvent.

8. The lithium-ion battery electrolyte according to claim 1, characterized in that: The lithium salt is one or more of LiClO4, LiPF6, LiTFSI, and LiFSI; the molar concentration of the lithium salt in the lithium-ion battery electrolyte is 0.001 to 2 mol / L.

9. The lithium-ion battery electrolyte according to claim 1, characterized in that: The additives also include other additives, and the other additives are one or more of fluoroethylene carbonate, lithium tetrafluoroborate, lithium difluorooxalatoborate, and lithium difluorophosphate.

10. A lithium ion battery comprising a positive electrode, a negative electrode and an electrolyte, characterized in that: The electrolyte is the lithium ion battery electrolyte according to any one of claims 1 to 9.

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