Electrolyte adaptive to graphite negative electrode for lithium ion battery and application of electrolyte

Abstract

The invention provides an electrolyte suitable for a graphite negative electrode of a lithium ion battery, a preparation method of the electrolyte and application of the lithium ion battery. The electrolyte is composed of a lithium salt, an organic solvent and an additive. The electrolyte prepared by the invention has good compatibility with a graphite negative electrode, and meanwhile, the rate capability of the lithium ion battery is improved. The preparation method is simple and effective, is easy for large-scale production, and has commercial popularization value.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium secondary batteries, and in particular relates to a lithium ion battery electrolyte adapted to a graphite negative electrode, and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have been widely used in consumer electronics, electric vehicles, energy storage and other fields. Lithium-ion batteries are composed of positive electrode materials, negative electrode materials, separators, electrolytes and auxiliary materials. Existing lithium-ion batteries often use graphitized carbon as the negative electrode material, with a theoretical specific capacity of 372mAh / g. The application of carbon materials maintains the high voltage characteristics of lithium-ion batteries and also improves the safety of lithium batteries. Current commercial carbon negative electrode materials mainly include artificial graphite and natural graphite.

[0003] Graphite carbon material is a layered structure compound formed by stacking sp2 hybridized carbon atoms, and the layers are connected by van der Waals forces. The basic principle of graphite lithium storage is that lithium ions are embedded between graphite sheets to form graphite intercalation compounds with the chemical formula LiC6. The embedding and extraction of lithium ions between graphite sheets are highly reversible, with a potential of about 0.1V vsLi / Li + , therefore, the stable charge and discharge characteristics and low cost make graphite the most widely used negative electrode material for lithium-ion batteries. Natural graphite is a more ideal negative electrode material due to its lower cost and higher reversible specific capacity. However, the electrochemical properties of natural graphite are greatly affected by the electrolyte, because a solid electrolyte membrane (SEI membrane) will be generated on the surface of the graphite negative electrode when the battery is charged for the first time, and the formation of the SEI membrane requires the consumption of lithium ions, resulting in a decrease in the initial coulombic efficiency. Secondly, the structure and composition of the SEI membrane will affect the reversibility of lithium ion embedding and extraction. For example, when using propylene carbonate (PC) electrolyte, PC molecules will be embedded in graphite at the same time as lithium ions, resulting in the peeling of graphite sheets, thereby causing the destruction of the graphite material and affecting the electrochemical performance of the battery.

[0004] In order to solve the above problems, ethylene carbonate (EC) is usually used as a solvent to form a dense and stable SEI film on the surface of the graphite negative electrode to ensure good reversibility of the battery. However, EC has a high melting point, which makes the viscosity of the electrolyte larger, so low-viscosity, low-melting-point linear carbonates are generally required for use. Compared with carbonate solvents, ether solvents have lower viscosity, stronger dissolving ability, and lower cost, but ether electrolytes are often not compatible with graphite negative electrodes because ether solvent molecules are easily embedded in the graphite interlayer with lithium ions, resulting in collapse and destruction of the graphite structure. By adjusting the solvation structure between the solvent molecules and the lithium salt, the electrolyte forms an efficient and stable SEI film on the surface of the graphite negative electrode, thereby improving the electrochemical performance of the battery, which is one of the effective ways to solve the compatibility problem between ether electrolytes and graphite negative electrodes. Summary of the invention

[0005] The purpose of the present invention is to provide an electrolyte adapted to a graphite negative electrode. By adjusting the electrolyte solvent, lithium salt and additives, the electrolyte is made compatible with the graphite negative electrode, so that the graphite negative electrode can exert excellent electrochemical properties, thereby improving the rate performance and cycle performance of lithium-ion batteries.

[0006] The present invention is achieved through the following technical solutions:

[0007] In one aspect, the present invention provides a lithium-ion battery electrolyte, which can be compatible with a graphite negative electrode, and the raw materials for preparing the electrolyte include a lithium salt, an organic solvent and an additive;

[0008] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, and lithium hexafluorophosphate;

[0009] The organic solvent comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, dibutyl ether, ethylene glycol diethyl ether, 1,3-dioxolane, and 1,4-dioxane;

[0010] The additive includes one or more of fluoroethylene carbonate, difluoroethylene carbonate, vinylene carbonate, and vinyl sulfite;

[0011] A second aspect of the present invention provides a method for preparing the above-mentioned electrolyte, comprising the following steps:

[0012] The lithium salt, organic solvent and additives are mixed.

[0013] According to some embodiments of the present invention, the concentration of the lithium salt in the electrolyte is 0.1M to 3M, and a preferred concentration is 1M to 2M;

[0014] According to some embodiments of the present invention, the preferred organic solvent is a mixed solvent of 1,4-dioxane and ethylene glycol diethyl ether, and the further preferred volume ratio is 0.2 to 0.8:1;

[0015] According to some embodiments of the present invention, the preferred additive is one or both of fluoroethylene carbonate and vinylene carbonate, and the further preferred mass fraction of the additive in the electrolyte is 5% to 20%;

[0016] According to a third aspect of the present invention, a lithium-ion battery is provided, wherein the raw materials for preparing the battery include the electrolyte prepared above.

[0017] According to some embodiments of the present invention, the negative electrode material of the lithium-ion battery is graphite;

[0018] According to some embodiments of the present invention, the specific capacity of the graphite is 320 to 360 mAh / g;

[0019] According to some embodiments of the present invention, the graphite material is a mixture of natural graphite and artificial graphite, and a further preferred ratio is 0.2 to 0.8:1;

[0020] According to some embodiments of the present invention, the positive electrode material of the lithium-ion battery is lithium iron phosphate.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] Compared with the existing ether electrolyte, the electrolyte prepared by the present invention shows good compatibility with both the graphite negative electrode and the lithium iron phosphate positive electrode, and improves the cycle stability of the graphite negative electrode. The existing commercial lithium-ion battery electrolyte is mainly carbonate electrolyte. In comparison, the electrolyte prepared by the present invention has higher ion conductivity and low cost, and improves the rate performance of lithium iron phosphate batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0024] Figure 1 The first charge and discharge curve of the artificial graphite negative electrode using the electrolyte of Comparative Example 1 and Comparative Example 2 at 0.2C.

[0025] Figure 2 The first charge and discharge curves of the artificial graphite negative electrode using the electrolyte of Example 1 and Comparative Example 1 at 0.2C.

[0026] Figure 3 The first charge and discharge curves of the artificial graphite negative electrode using the electrolytes of Example 2 and Comparative Example 1 at 0.2C.

[0027] Figure 4 The first charge and discharge curves of the artificial graphite negative electrode using the electrolytes of Example 3 and Comparative Example 1 at 0.2C.

[0028] Figure 5 The graph is a cycle performance diagram of the artificial graphite negative electrode using the electrolytes of Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2 at 0.2C.

[0029] Figure 6 This is a rate performance diagram of the lithium iron phosphate positive electrode using the electrolyte of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0030] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0031] In the embodiments and comparative examples of the present invention:

[0032] EC refers to ethylene carbonate;

[0033] DMC refers to dimethyl carbonate;

[0034] FEC refers to fluoroethylene carbonate;

[0035] DX refers to 1,4-dioxane;

[0036] DEE refers to ethylene glycol diethyl ether;

[0037] VC refers to vinylene carbonate;

[0038] LiPF6 refers to lithium hexafluorophosphate;

[0039] LiFSI refers to lithium bis(fluorosulfonyl)imide;

[0040] LiTFSI refers to lithium bistrifluoromethanesulfonimide.

[0041] Comparative Example 1

[0042] This comparative example provides an electrolyte solution, which is composed of EC and DMC as solvents in a volume ratio of 1:1, the lithium salt is LiPF6, and the molar concentration of the lithium salt is 1M.

[0043] The preparation steps of the electrolyte of this comparative example are as follows: in a glove box filled with argon, EC and DMC are first mixed evenly, and LiPF6 is slowly added at room temperature to dissolve and mix evenly.

[0044] Comparative Example 2

[0045] This comparative example provides an electrolyte solution, wherein the electrolyte solution is composed of DEE as a solvent, LiFSI as a lithium salt, and a molar concentration of the lithium salt of 1M.

[0046] The preparation steps of the electrolyte of this comparative example are: in a glove box filled with argon, slowly add LiFSI into DEE at room temperature to dissolve and mix evenly.

[0047] Example 1

[0048] This embodiment provides an electrolyte, which is composed of DX and DEE in a volume ratio of 1:1 as solvents, the lithium salt is LiFSI, the molar concentration of the lithium salt is 1M, and FEC is used as an additive.

[0049] The steps for preparing the electrolyte of this embodiment are as follows: in a glove box filled with argon, DX and DEE are first mixed evenly, LiFSI is slowly added to dissolve at room temperature, and then 20% by mass of FEC is added and mixed evenly.

[0050] Example 2

[0051] This embodiment provides an electrolyte solution, wherein DEE is used as a solvent, the lithium salt is LiFSI, and the molar concentration of the lithium salt is 1M.

[0052] The steps for preparing the electrolyte of this embodiment are: in a glove box filled with argon, slowly add LiFSI to dissolve at room temperature, and then add 10% by mass of FEC and mix evenly.

[0053] Example 3

[0054] This embodiment provides an electrolyte, wherein the composition DX is used as a solvent, the lithium salt is LiFSI, and the molar concentration of the lithium salt is 1M.

[0055] The steps for preparing the electrolyte of this embodiment are: in a glove box filled with argon, slowly add LiFSI to dissolve at room temperature, and then add 10% by mass of FEC and mix evenly.

[0056] The electrolytes prepared in Comparative Examples 1 and 2 and Examples 1 to 3 were assembled into batteries, and the electrochemical performance was tested.

[0057] The graphite electrode is made of 95% by mass of artificial graphite and 5% by mass of polyvinylidene fluoride binder, which are evenly coated on copper foil and then vacuum dried.

[0058] The lithium iron phosphate positive electrode is made of 80% by mass of lithium iron phosphate powder, 10% by mass of conductive carbon black and 10% by mass of polyvinylidene fluoride, which are uniformly mixed and then coated on aluminum foil and vacuum dried.

[0059] The assembly steps of the graphite half-cell are: using a metal lithium sheet as a reference electrode and a graphite electrode as a working electrode, separated by a polypropylene diaphragm, adding the above electrolyte, and making a button cell;

[0060] The assembly steps of the lithium iron phosphate half-cell are: using a metal lithium sheet as a reference electrode and a lithium iron phosphate electrode as a working electrode, separated by a polypropylene diaphragm, adding the above electrolyte, and making a button cell;

[0061] The above assembled battery was subjected to relevant electrochemical tests.

[0062] Specific test items include: constant current charge and discharge test, cycle performance test and rate performance test. The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which does not affect the essential content of the present invention.

Claims

1. A method for preparing a lithium ion battery electrolyte, characterized in that: The electrolyte is an electrolyte solution uniformly mixed with lithium salt, organic solvent and additives.

2. The electrolyte according to claim 1, characterized in that The lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

3. The electrolyte according to claim 1, characterized in that The organic solvent includes one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, dibutyl ether, ethylene glycol diethyl ether, 1,3-dioxolane, and 1,4-dioxane.

4. The electrolyte according to claim 1, characterized in that The additive includes one or more of fluoroethylene carbonate, difluoroethylene carbonate, vinylene carbonate, and vinyl sulfite.

5. The electrolyte according to claim 1, characterized in that The lithium salt concentration is 0.1-3M.

6. The method for preparing an electrolyte according to any one of claims 1 to 5, characterized in that: The following steps are involved: The lithium salt and the additive are dissolved in the organic solvent and mixed evenly.

7. A lithium ion battery, characterized in that: The preparation raw materials include the electrolyte described in claims 1 to 5.

8. The lithium-ion battery according to claim 7, characterized in that: The negative electrode material used in the lithium-ion battery is one or both of artificial graphite and natural graphite.

9. The lithium-ion battery according to claim 7, characterized in that: The positive electrode material of the lithium-ion battery is lithium iron phosphate.

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