Electrolyte of high-voltage lithium ion battery as well as preparation method and application of electrolyte

Abstract

The invention discloses an electrolyte of a high-voltage lithium ion battery as well as a preparation method and application of the electrolyte. The electrolyte comprises a lithium salt, an organic solvent and a zwitterionic salt additive, the zwitterionic salt additive is N-methyl-N-(butyl sulfonic acid) pyrrolidinium. The zwitterionic salt in the electrolyte can interact with organic solvent propylene carbonate molecules, so that freely moving PC molecules in the electrolyte are reduced, redox decomposition of the solvent molecules PC on the surface of an electrode is further weakened, the electrochemical stability of the electrolyte is improved, the high-voltage performance of the electrolyte is improved, and the safety performance is relatively high.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to an electrolyte for a high-voltage lithium ion battery and a preparation method and application thereof. Background Art

[0002] The electrochemical window of propylene carbonate (PC) is close to that of EC, about 4.8V, with a lower melting point (-48.8℃) and a smaller density than EC. Replacing EC with PC can not only improve the physicochemical properties of the electrolyte, but also help to improve the energy density of the battery. Therefore, PC has always been considered a good substitute for EC. However, PC has poor compatibility with positive and negative electrode materials. For example, for the commonly used graphite negative electrode, PC molecules will form solvated lithium ions with lithium ions and co-embed the graphite layer, causing the graphite layer structure to collapse. On the surface of the positive electrode, especially the high-voltage positive electrode surface, the electrolyte will undergo a strong electrochemical reaction on the electrode surface, resulting in a very low first coulomb efficiency and poor cycle performance. These problems are mainly because it is difficult for PC-based electrolytes to form an effective solid electrolyte film on the electrode surface, preventing the electrolyte from continuously reacting on the electrode surface. In view of this, many researchers have improved the compatibility of PC-based electrolytes with various electrode materials by adding film-forming additives to PC-based electrolytes, mixing PC with functional organic solvents, or increasing the concentration of lithium salts. However, these methods are still not ideal in terms of safety and electrochemical cycle performance, especially under high voltage (5V). For example, adding film-forming additives to the electrolyte can form an effective solid electrolyte film on the electrode surface, but the safety performance of the electrolyte is poor. Although high-concentration electrolytes make up for the poor safety performance of conventional electrolytes, the high lithium salt concentration leads to low capacity retention during long-term cycles and poor rate performance.

[0003] Zwitterion salts (Ziwitterion, ZI) are formed by combining the anions and cations of ionic liquids through covalent bonds. Compared with conventional ionic liquids, they have many similar properties, such as high thermal stability, low vapor pressure, wide electrochemical window, etc., but they also have their own unique advantages: (1) Some zwitterion salts have higher thermal stability and electrochemical stability than ionic liquids, and show a wider electrochemical window; (2) Zwitterion salts have the function of dissociating lithium salts; (3) Zwitterion salts combine anions and cations through covalent bonds, and are electrically neutral as a whole, so they will not move along the potential gradient in the electrostatic field, and can effectively inhibit the movement of other ions or polar molecules that interact with them along the potential gradient. In the prior art, for example, the electrolyte used in CN 102723528 B is an EC-based electrolyte for commercial applications. The EC molecules in the electrolyte will not be co-embedded in the graphite layer with lithium ions, resulting in the destruction of the graphite electrode structure. Therefore, the role of zwitterionic salts is to provide complexing points with lithium ions through the polyether chains in the molecules, thereby improving the mobility of lithium ions and improving the electrochemical performance of lithium-ion batteries. However, the zwitterionic salts in this electrolyte are chain structures and contain electronegative oxygen in the structure, so the zwitterionic salts are more reflected in the effect on lithium ions rather than on organic solvent molecules. In particular, they have no regulatory effect on electrolytes composed of propylene carbonate (PC) solvents that will be co-embedded in the graphite layer during the charge and discharge process, and will still cause the graphite electrode structure to collapse, resulting in poor electrochemical cycle performance and low capacity retention. Summary of the invention

[0004] The embodiment of the present invention provides an electrolyte for a high-voltage lithium-ion battery, a preparation method and an application thereof. Through the interaction between a zwitterionic salt with a strong polarity and a polar solvent molecule propylene carbonate, the free movement of propylene carbonate molecules is restrained, and the free solvent molecules in the electrolyte are reduced. The problem of a strong redox reaction of PC solvent molecules on the electrode surface in a propylene carbonate-based (PC) electrolyte is mainly solved.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] In one aspect, the present invention provides an electrolyte for a high-voltage lithium-ion battery, the electrolyte comprising a lithium salt, an organic solvent and a zwitterionic salt additive;

[0007] The structural formula of the zwitterionic salt additive is:

[0008]

[0009] In the present invention, the zwitterionic salt additive is N-methyl-N-(butyl sulfonate)pyrrolidinium (MBPyS), which combines anions and cations in one, has strong polarity, and can generate electrostatic interactions with organic solvent molecules in the electrolyte.

[0010] Specifically, the zwitterionic salt with strong polarity of the present invention interacts with the polar solvent molecule propylene carbonate, restrains the free movement of the propylene carbonate molecules, reduces the free solvent molecules in the electrolyte, and weakens the redox reaction of the propylene carbonate molecules on the electrode surface.

[0011] As a further embodiment of the present invention, the lithium salt is selected from one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide.

[0012] As a further solution of the present invention: the molar concentration of the lithium salt in the electrolyte is 1 to 2 mol / L.

[0013] As a further solution of the present invention: the molar concentration of the lithium salt in the electrolyte is 1.5 mol / L.

[0014] As a further embodiment of the present invention: the organic solvent is propylene carbonate.

[0015] As a further solution of the present invention: the addition amount of the zwitterionic salt is 7-10 wt % of the total mass of the lithium salt and the organic solvent.

[0016] Optionally, the added amount of the lithium ion salt is 7%, 8%, 9%, or 10% of the total mass of the lithium salt and the organic solvent.

[0017] On the other hand, the present invention provides a method for preparing the electrolyte of the high-voltage lithium-ion battery, comprising: dissolving a stoichiometric amount of lithium salt in an organic solvent, adding a zwitterionic salt additive, shaking and standing to obtain the electrolyte of the high-voltage lithium-ion battery.

[0018] The third aspect of the present invention also provides a method for preparing a high-voltage LiNi 0.5 Mn 1.5 Application in O4 / Li half-cell.

[0019] The beneficial effects of the present invention are:

[0020] (1) The present invention uses zwitterionic salt (ZI) as an additive for PC-based electrolyte. On the one hand, the dissociation function of zwitterionic salt is used to improve the solubility of lithium salt in solvent without increasing the concentration of lithium salt, which is equivalent to increasing the concentration of lithium salt. On the other hand, the interaction between zwitterionic salt and PC molecules is used to restrain PC molecules, thereby preventing a large number of PC molecules from moving toward the electrode under the action of the electric field and preventing the electrolyte from having a strong electrochemical reaction on the electrode surface. The preparation method of the present invention can not only improve the safety performance of the electrolyte, but also improve the electrochemical cycle performance of the electrolyte.

[0021] (2) In the present invention, although the zwitterionic salt still plays the role of an additive, it does not participate in the electrochemical reaction. Instead, it changes the solution structure of the electrolyte through its own advantages (strong dissociation ability, interaction with PC molecules and immobility in the electric field), thereby adjusting the electrochemical properties of the electrolyte.

[0022] (3) The electrolyte provided by the present invention has a simple composition, is easy to prepare, and has good performance. 0.5 Mn 1.5 The compatibility of O4 positive electrode has been significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the molecular structure of propylene carbonate (PC);

[0024] Figure 2 This is the structure of the zwitterionic salt MBPyS;

[0025] Figure 3 The PC-MBPyS structure before optimization in Example 1 of the present invention;

[0026] Figure 4 This is the optimized PC-MBPyS structure of Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with specific examples and comparative examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of protection of the present invention.

[0028] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0029] Performance testing method in the present invention

[0030] Electrochemical performance test:

[0031] The 200-cycle long-term cycle performance of 2032 button half-cell was tested on the Blue Electric test system at a test temperature of 25°C. The test conditions were: positive electrode material LNMO, test voltage of 3-5V, 5 cycles at a rate of 0.1C, and continued to cycle up to 200 cycles at a rate of 4-5V at 0.2C.

[0032] Example 1

[0033] The electrolyte of the present invention is prepared in a glove box, and its main components are: organic solvent, lithium salt, and zwitterionic salt additive. The organic solvent is propylene carbonate (PC), the lithium salt is LiPF6, the molar concentration of the lithium salt in the organic solvent is 1.5 mol / L, and the zwitterionic salt additive is MBPyS, and the addition amount is 7wt%.

[0034] Preparation method of the above lithium ion battery electrolyte:

[0035] Step 1: weigh a stoichiometric amount of LiPF6 according to the molar concentration of the electrolyte and dissolve it in a certain volume of PC solvent.

[0036] Step 2: Weigh the mass of zwitterionic salt according to 7wt% of the total mass of the electrolyte, add it to the electrolyte in step 1, shake it thoroughly, and let it stand for at least 24 hours.

[0037] The lithium-ion battery electrolyte obtained in this example was assembled into a LNMO / Li half-cell, and the test voltage was 3-5V, 5 cycles were formed at a rate of 0.1C, and the cycle was continued to 200 cycles at a rate of 0.2C at 4-5V, with an initial coulombic efficiency of 90.1%. The capacity retention rate after 200 cycles was 89%.

[0038] Reference Figure 1 to Figure 4 The interaction between the zwitterionic salt and the PC molecule in the present invention is obtained by density functional theory in Gaussian software, such as Figure 3 and 4 , before optimization (such as Figure 3 ) The bond lengths of the three S=O bonds of the sulfonic acid group in the zwitterionic salt are as follows according to the numbers in the figure: After optimization (such as Figure 4 )S=O bond lengths are The bond length of the S=O bond near the methyl position of the PC molecule (at the number "1" position) becomes significantly longer, which is caused by the interaction between the sulfonic acid group in the zwitterionic salt and the PC molecule.

[0039] Example 2

[0040] The electrolyte of the present invention is prepared in a glove box, and its main components are: organic solvent, lithium salt, and zwitterionic salt additive. The organic solvent is propylene carbonate (PC), the lithium salt is LiPF6, the molar concentration of the lithium salt in the organic solvent is 1.5 mol / L, and the zwitterionic salt additive is MBPyS, and the addition amount is 9wt%.

[0041] Preparation method of the above lithium ion battery electrolyte:

[0042] Step 1: weigh a stoichiometric amount of LiPF6 according to the molar concentration of the electrolyte and dissolve it in a certain volume of PC solvent.

[0043] Step 2: Weigh the mass of zwitterionic salt according to 9wt% of the total mass of the electrolyte, add it to the electrolyte in step 1, shake it thoroughly, and let it stand for at least 24 hours.

[0044] The lithium-ion battery electrolyte obtained in this example was assembled into a LNMO / Li half-cell, and the test voltage was 3-5V, 5 cycles were formed at a rate of 0.1C, and the cycle was continued to 200 cycles at a rate of 0.2C at 4-5V, with an initial coulombic efficiency of 85%. The capacity retention rate after 200 cycles was 87.4%.

[0045] Comparative Example 1

[0046] The electrolyte of the present invention is prepared in a glove box, and its main components are: organic solvent and lithium salt. The organic solvent is propylene carbonate (PC), the lithium salt is LiPF6, and the molar concentration of the lithium salt in the organic solvent is 1.5 mol / L.

[0047] Preparation method of the above lithium ion battery electrolyte:

[0048] Weigh a stoichiometric amount of LiPF6 according to the molar concentration of the electrolyte, dissolve it in a certain volume of PC solvent, shake it thoroughly, and let it stand for at least 24 hours.

[0049] The lithium-ion battery electrolyte obtained in this example was assembled into a LNMO / Li half-cell, and the test voltage was 3-5V, 5 cycles were formed at a rate of 0.1C, and the cycle was continued to 200 cycles at a rate of 0.2C at 4-5V, with an initial coulombic efficiency of 56%. The capacity retention rate after 200 cycles was 70%.

[0050] Comparative Example 2

[0051] The electrolyte of the present invention is prepared in a glove box, and its main components are: organic solvent, lithium salt, and zwitterionic salt additive. The organic solvent is propylene carbonate (PC), the lithium salt is LiPF6, the molar concentration of the lithium salt in the organic solvent is 1.5 mol / L, and the zwitterionic salt additive is MBPyS, and the addition amount is 3wt%.

[0052] Preparation method of the above lithium ion battery electrolyte:

[0053] Step 1: weigh a stoichiometric amount of LiPF6 according to the molar concentration of the electrolyte and dissolve it in a certain volume of PC solvent.

[0054] Step 2: Weigh the mass of zwitterionic salt according to 3wt% of the total mass of the electrolyte, add it to the electrolyte in step 1, shake it thoroughly, and let it stand for at least 24 hours.

[0055] The lithium-ion battery electrolyte obtained in this example was assembled into a LNMO / Li half-cell, and the test voltage was 3-5V, 5 cycles were formed at a rate of 0.1C, and the cycle was continued to 200 cycles at a rate of 0.2C at 4-5V, with an initial coulombic efficiency of 65.4% and a capacity retention rate of 74% after 200 cycles.

[0056] Comparative Example 3

[0057] The electrolyte of the present invention is prepared in a glove box, and its main components are: organic solvent, lithium salt, and zwitterionic salt additive. The organic solvent is propylene carbonate (PC), the lithium salt is LiPF6, the molar concentration of the lithium salt in the organic solvent is 1.5 mol / L, and the zwitterionic salt additive is MBPyS, and the addition amount is 5wt%.

[0058] Preparation method of the above lithium ion battery electrolyte:

[0059] Step 1: weigh a stoichiometric amount of LiPF6 according to the molar concentration of the electrolyte and dissolve it in a certain volume of PC solvent.

[0060] Step 2: Weigh the mass of zwitterionic salt according to 5wt% of the total mass of the electrolyte, add it to the electrolyte in step 1, shake it thoroughly, and let it stand for at least 24 hours.

[0061] The lithium-ion battery electrolyte obtained in this example was assembled into a LNMO / Li half-cell, and the test voltage was 3-5V, 5 cycles were formed at a rate of 0.1C, and the cycle was continued to 200 cycles at a rate of 0.2C at 4-5V, with an initial coulombic efficiency of 77%. The capacity retention rate after 200 cycles was 86.3%.

[0062] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. An electrolyte for a high-voltage lithium-ion battery, characterized in that: The electrolyte comprises a lithium salt, an organic solvent and a zwitterionic salt additive; The structural formula of the zwitterionic salt additive is:

2. The electrolyte of the high-voltage lithium-ion battery according to claim 1, characterized in that: The lithium salt is selected from one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide.

3. The electrolyte of the high-voltage lithium-ion battery according to claim 1, characterized in that: The molar concentration of the lithium salt in the electrolyte is 1-2 mol / L.

4. The electrolyte of the high-voltage lithium-ion battery according to claim 3, characterized in that: The molar concentration of the lithium salt in the electrolyte is 1.5 mol / L.

5. The electrolyte of the high-voltage lithium-ion battery according to claim 1, characterized in that: The organic solvent is propylene carbonate.

6. The electrolyte of the high-voltage lithium-ion battery according to claim 1, characterized in that: The amount of the zwitterionic salt added is 7-10 wt % of the total mass of the lithium salt and the organic solvent.

7. A method for preparing an electrolyte for a high-voltage lithium-ion battery according to any one of claims 1 to 6, characterized in that: The method comprises: dissolving a stoichiometric lithium salt in an organic solvent, adding a zwitterionic salt additive, shaking well, and standing to obtain the electrolyte of the high-voltage lithium-ion battery.

8. An electrolyte for a high-voltage lithium-ion battery as claimed in any one of claims 1 to 6 in the preparation of a high-voltage LiNi 0.5 Mn 1.5 Application in O4 / Li half-cell.

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