A high-voltage lithium-ion battery electrolyte and its preparation method and application

Abstract

The present invention discloses an electrolyte for a high-voltage lithium-ion battery, its preparation method, and application. The electrolyte comprises a lithium salt, an organic solvent, and a zwitterionic salt additive; the zwitterionic salt additive is N-methyl-N-(butylsulfonic acid)pyrrolidinium. The zwitterionic salt in the electrolyte of the present invention interacts with the organic solvent propylene carbonate molecules, reducing the number of freely mobile PC molecules in the electrolyte. This in turn reduces the redox decomposition of the solvent PC molecules on the electrode surface, thereby increasing the electrochemical stability of the electrolyte, improving the high-voltage performance of the electrolyte, and providing high safety performance.

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, a preparation method thereof, and an application thereof. Background Art

[0002] Propylene carbonate (PC) has an electrochemical window close to that of EC, approximately 4.8V, a lower melting point (-48.8°C), and a lower density than EC. Replacing EC with PC not only improves the physicochemical properties of the electrolyte but also helps increase the energy density of the battery. Therefore, PC has long been considered a good alternative to EC. However, PC has poor compatibility with both positive and negative electrode materials. For example, in the commonly used graphite anode, PC molecules form solvated lithium ions with lithium ions and co-intercalate into the graphite layer, causing the graphite layer structure to collapse. On the positive electrode surface, especially the high-voltage positive electrode surface, the electrolyte undergoes a strong electrochemical reaction on the electrode surface, resulting in very low initial coulombic efficiency and poor cycling performance. These problems are mainly due to the difficulty of PC-based electrolytes in forming an effective solid electrolyte film on the electrode surface, which prevents 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 lithium salt concentration. However, these methods are still not ideal in terms of safety and electrochemical cycling performance, especially under high voltage (5V) conditions. For example, adding film-forming additives to the electrolyte can form an effective solid electrolyte film on the electrode surface, but the electrolyte has poor safety performance. 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 cycling and poor rate performance.

[0003] Zwitterion salts (Ziwitterions, 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, and a wide electrochemical window, but they also have their own unique advantages: (1) Some zwitterion salts have higher thermal stability and electrochemical stability than ionic liquids, and exhibit 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. Therefore, in an electrostatic field, they will not move along the potential gradient, 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 a commercially used EC-based electrolyte. The EC molecules in the electrolyte will not co-embed the graphite layer with the lithium ions, resulting in damage to the graphite electrode structure. Therefore, the role of zwitterionic salts is to provide complexation points with lithium ions through the polyether chains in the molecules, thereby increasing 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. Therefore, the zwitterionic salts are more reflected in their 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] Embodiments of the present invention provide an electrolyte for a high-voltage lithium-ion battery, a preparation method, and applications thereof. By utilizing the interaction between a highly polar zwitterionic salt and the polar solvent molecule propylene carbonate, the free movement of propylene carbonate molecules is restrained, reducing the free solvent molecules in the electrolyte. This primarily addresses the problem of intense redox reactions of PC solvent molecules on electrode surfaces in propylene carbonate-based (PC) electrolytes.

[0005] To achieve the above object, 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 propylene carbonate molecules, reduces the free solvent molecules in the electrolyte, and weakens the redox reaction of 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 to 10 wt % of the total mass of the lithium salt and the organic solvent.

[0016] Optionally, the added amount of the ionic 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 described above, comprising: dissolving a stoichiometric amount of lithium salt in an organic solvent, adding a zwitterionic salt additive, shaking, and allowing to stand 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 lithium-ion battery electrolyte. 0.5 Mn 1.5 Application in O4 / Li half-cells.

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

[0020] (1) The present invention uses zwitterionic salts (ZI) as additives to PC-based electrolytes. On the one hand, the dissociation function of the zwitterionic salts is utilized to improve the solubility of lithium salts in the solvent without increasing the lithium salt concentration, which is equivalent to increasing the lithium salt concentration. On the other hand, the interaction between the zwitterionic salts and PC molecules is utilized to restrain the PC molecules, preventing a large number of PC molecules from migrating toward the electrodes under the action of the electric field, thereby preventing the electrolyte from undergoing strong electrochemical reactions 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 regulating 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 This is 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 with reference to 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. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered 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 long-term cycling performance of a 2032 button-type half-cell was tested on a Blue Electric test system at 25°C for 200 cycles. The test conditions were: positive electrode material LNMO, test voltage 3-5V, 5 cycles at a 0.1C rate, and continued cycling at 4-5V at a 0.2C rate for up to 200 cycles.

[0032] Example 1

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

[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 the zwitterionic salt so as to be 7 wt% 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 an LNMO / Li half-cell. The test voltage was 3-5V, and the formation rate was 0.1C for 5 cycles. The cycle was continued at 0.2C at 4-5V for up to 200 cycles. The initial coulombic efficiency was 90.1%, and the capacity retention rate after 200 cycles was 89%.

[0038] Reference Figures 1 to 4 The interaction between the zwitterionic salt and PC molecules 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: 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 (position "1") is 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 was prepared in a glove box. The main components include: an organic solvent, a lithium salt, and a zwitterionic salt additive. The organic solvent was propylene carbonate (PC), the lithium salt was LiPF6, the molar concentration of the lithium salt in the organic solvent was 1.5 mol / L, and the zwitterionic salt additive was MBPyS, added in an amount of 9 wt%.

[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 the zwitterionic salt so as to be 9 wt% 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 an LNMO / Li half-cell. The test voltage was 3-5V, and the formation rate was 0.1C for 5 cycles. The cycle was continued at 0.2C for 4-5V up to 200 cycles. The initial coulombic efficiency was 85%, and the capacity retention rate after 200 cycles was 87.4%.

[0045] Comparative Example 1

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

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

[0048] Weigh the 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 an LNMO / Li half-cell. The test voltage was 3-5V, and the formation rate was 0.1C for 5 cycles. The cycle was continued at 0.2C for 4-5V up to 200 cycles. The initial coulombic efficiency was 56%, and the capacity retention rate after 200 cycles was 70%.

[0050] Comparative Example 2

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

[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 the zwitterionic salt so as to be 3 wt% 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 an LNMO / Li half-cell. The test voltage was 3-5V, and the formation rate was 0.1C for 5 cycles. The cycle was continued at 0.2C at 4-5V for 200 cycles. The initial coulombic efficiency was 65.4%, and the capacity retention rate after 200 cycles was 74%.

[0056] Comparative Example 3

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

[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 the zwitterionic salt so as to be 5 wt% 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 an LNMO / Li half-cell. The test voltage was 3-5V, and the formation rate was 0.1C for 5 cycles. The cycle was continued at 0.2C for 4-5V up to 200 cycles. The initial coulombic efficiency was 77%, and 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 skilled in 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 one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A lithium-ion battery, characterized in that: Including LiNi 0.5 Mn 1.5 An O4 positive electrode and an electrolyte, wherein the electrolyte comprises a lithium salt, an organic solvent, and a zwitterionic salt additive, wherein the lithium salt is selected from one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; the organic solvent is propylene carbonate; and the amount of the zwitterionic salt added is 7-10 wt% of the total mass of the lithium salt and the organic solvent; The structural formula of the zwitterionic salt additive is: 。 2. The lithium-ion battery according to claim 1, wherein The molar concentration of the lithium salt in the electrolyte is 1-2 mol / L.

3. The lithium-ion battery according to claim 2, wherein: The molar concentration of the lithium salt in the electrolyte is 1.5 mol / L.

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