CN113346138A

CN113346138A - Electrolyte and lithium battery

Info

Publication number: CN113346138A
Application number: CN202110593771.6A
Authority: CN
Inventors: 马建民; 戚世瀚
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2021-05-28
Filing date: 2021-05-28
Publication date: 2021-09-03

Abstract

The invention discloses an electrolyte and a lithium battery, wherein the electrolyte comprises: a lithium salt, a non-aqueous organic solvent, and an additive comprising an organic compound represented by the following structural formula:

wherein R is₁、R₂、R₃Each independently selected from an alkane group having 1 to 3 carbon atoms. The invention at least can solve the problem of how to improve the working voltage of the positive electrode of the lithium battery while inhibiting the growth of the dendritic crystal of the lithium of the negative electrode of the lithium battery, and further can solve the problem of how to simultaneously optimize the performances of the positive electrode and the negative electrode of the lithium battery, thereby improving the electrochemical performance of the lithium battery under the high-pressure working condition.

Description

Electrolyte and lithium battery

Technical Field

The present invention relates to the field of electrochemistry. More particularly, the present invention relates to an electrolyte and a lithium battery.

Background

Lithium ion batteries are favored because of their advantages of no memory effect, environmental friendliness, long cycle life, and the like. However, in recent years, due to rapid development of communication, consumer electronics, and electric vehicles, the energy density of conventional lithium ion batteries has been difficult to meet the market demand. In order to increase the energy density of the battery, it is necessary to improve the performance of both the positive and negative electrodes. However, the conventional positive and negative electrodes always have a lot of defects, and in the field of lithium batteries, it is difficult to simultaneously consider the electrochemical properties of the positive and negative electrodes.

Specifically, the conventional lithium metal negative electrode has an ultra-high theoretical capacity (3860mAh g)^-1) And an extremely low electrochemical potential (-3.04V vs standard hydrogen electrode), but side reactions are caused due to excessively high reactivity, and dendrite growth occurs during lithium deposition, so that Lithium Metal Batteries (LMBs) assembled from the LMBs suffer from problems of low coulombic efficiency, poor safety, short cycle life, and the like.

Similarly, has a high theoretical specific capacity (275mAh g)^-1) Lithium cobaltate (LiCoO)₂) The positive electrode also has a drawback that cannot be ignored. It is below 4.35V cut-off voltage, and only has no more than 170mAh g^-1The capacity of the lithium battery can be utilized, and once the lithium battery operates in a high-pressure environment exceeding 4.4V, the problems of cobalt ion loss, irreversible phase change increase, positive electrode material particle fragmentation and the like can be caused, so that the capacity of the lithium battery made of the lithium cobaltate material is rapidly reduced.

Disclosure of Invention

An object of the present invention is to solve at least the above problems and to provide at least the advantages described later.

In summary, it is also an object of the present invention to provide an electrolyte and a lithium battery; the problem of how to promote lithium battery positive pole operating voltage when inhibiting lithium dendrite growth of lithium battery negative pole at least can be solved, and then can solve the problem of how to optimize lithium battery positive pole and negative polarity performance simultaneously to can improve the electrochemical performance of lithium battery under high pressure operating condition.

Specifically, the invention is realized by the following technical scheme:

< first aspect of the invention >

A first aspect provides an electrolyte comprising: a lithium salt, a non-aqueous organic solvent, and an additive comprising an organic compound represented by the following structural formula:

wherein R is₁、R₂、R₃Each independently selected from an alkane group having 1 to 3 carbon atoms.

After the additive is added into the electrolyte provided by the application, a CEI (CEI, the English name of CEI is totally called solid electrolyte interphase) with higher mechanical strength can be formed, and the CEI film is wrapped on the surface of lithium cobaltate positive electrode particles to prevent the positive electrode particles from being cracked; in addition, the CEI film can block cobalt ions in the crystal structure on the surface of the anode material from contacting with the electrolyte, and prevent the cobalt ions from being dissolved in the electrolyte, so that the occurrence of irreversible phase change is reduced; therefore, the electrolyte provided by the application can optimize the CEI of the lithium cobaltate anode in the lithium battery, slow down the breakage of lithium cobaltate particles, increase the working voltage of the lithium cobaltate anode material, and increase the cut-off voltage to be more than 4.7V, thereby increasing the energy density of the lithium metal battery.

Meanwhile, the SEI film formed by reducing the organic compound in the electrolyte contains Li₃N, the substance is a fast ion conductor, can improve the lithium ion diffusion rate of the SEI film, reduce the non-uniform deposition of lithium, inhibit the growth of lithium dendrites and optimize the SEI of the negative electrode.

In addition, the isocyanate group (-N ═ C ═ O) in the additive can react with hydrofluoric acid to generate a (-NH-C-F) functional group, so that the hydrofluoric acid is prevented from corroding the cathode material, and therefore hydrofluoric acid (HF) impurities in the electrolyte can be removed, and the performances of the lithium battery such as charge-discharge, cycle efficiency and the like are improved. When the content of hydrofluoric acid in the lithium battery exceeds a certain concentration, limited lithium ions are consumed, so that the irreversible capacity of the battery is increased, the generated lithium oxide and lithium fluoride are not beneficial to improving the electrochemical performance of the electrode, and the gas generated by the reaction can cause the pressure in the battery to increase. As the content of hydrofluoric acid continues to increase, the performance of lithium batteries, such as charge and discharge, cycle efficiency, etc., is remarkably reduced and even completely destroyed. Therefore, the removal of hydrofluoric acid (HF) impurities in the electrolyte is of great help to improve the working performance of the lithium battery. Therefore, the electrolyte provided by the application can be used for simultaneously optimizing the performances of the positive electrode and the negative electrode of the lithium battery.

In some embodiments, R₁、R₂、R₃Are the same group.

In some embodiments, R₁、R₂、R₃Is methyl, and the organic compound is trimethylsilyl isocyanate, and has the following structure:

more excellent effects can be achieved in the electrolytic solution.

In some technical schemes, the content of the additive is 0.2 wt% -2.0 wt% based on the electrolyte.

In some technical schemes, the content of the additive is 0.2 wt% -1.0 wt% based on the electrolyte.

In some embodiments, the lithium salt may be selected from LiPF₆、LiBF₄、LiBOB、LiDFOB、LiSbF₆、LiAsF₆、LiN(SO₂CF₃)₂、LiN(SO₂C₂F₅)₂、LiC(SO₂CF₃)₃Or LiN (SO)₂F)₂One or more than two of them.

In some embodiments, the lithium salt is present at a concentration of 0.9 to 1.1M.

In some embodiments, the lithium salt is at a concentration of 1M.

In some embodiments, the non-aqueous organic solvent is selected from a mixture of cyclic carbonates and chain carbonates. The cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate or butylene carbonate, and the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or propyl methyl carbonate. The mixed solution of the cyclic carbonate organic solvent with high dielectric constant and the chain carbonate organic solvent with low viscosity is used as the solvent of the lithium ion battery electrolyte, so that the mixed solution of the organic solvent has high ionic conductivity, high dielectric constant and low viscosity.

In some embodiments, the non-aqueous organic solvent is Ethylene Carbonate (EC) and Ethyl Methyl Carbonate (EMC).

In some technical schemes, the volume ratio of the Ethylene Carbonate (EC) to the Ethyl Methyl Carbonate (EMC) is 1:1, so that the performance of the electrolyte can be further improved.

The technical effects of the embodiment of the invention at least comprise:

after the additive is added into the electrolyte provided by the embodiment of the application, the CEI of the lithium cobaltate anode in the lithium battery can be optimized, the crushing of lithium cobaltate particles is slowed down, the working voltage of a lithium cobaltate anode material is increased, the cut-off voltage is increased to be more than 4.7V, and the energy density of the lithium metal battery is further increased. Meanwhile, the electrolyte containing the organic compound can inhibit the growth of lithium dendrites on the negative electrode of the battery and optimize the SEI of the negative electrode. In addition, the electrolyte can also remove hydrofluoric acid (HF) impurities in the electrolyte, and the performances of the lithium battery such as charge-discharge efficiency, cycle efficiency and the like are improved. Therefore, the electrolyte provided by the application can improve the performance of the lithium battery from multiple aspects, and comprehensively optimizes the performances of the positive electrode and the negative electrode of the lithium battery.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

Drawings

Fig. 1 is a scanning electron micrograph of the lithium negative electrode surface of a lithium battery prepared from an electrolyte 1;

fig. 2 is a scanning electron micrograph of the lithium negative electrode surface of a lithium battery prepared from the electrolyte 2;

fig. 3 is a scanning electron micrograph of the lithium negative electrode surface of a lithium battery prepared from the electrolyte 3;

FIG. 4 is a scanning electron micrograph of the lithium negative surface of a lithium battery prepared from the control electrolyte;

FIG. 5 is a three-week charge-discharge curve diagram of a Li | | LCO lithium metal battery in electrolyte 2;

FIG. 6 is a graph comparing the cycle performance of symmetric lithium batteries in electrolytes 1-3 and a control electrolyte.

Detailed Description

The present invention is further described in detail below with reference to the attached drawings so that those skilled in the art can implement the invention by referring to the description text.

The terms "comprises" and "comprising," as well as any variations thereof, in the embodiments of the present application, are intended to cover a non-exclusive inclusion. For example, a process, method, system, article, or apparatus that comprises a list of steps or elements is not limited to only those steps or elements listed, but may alternatively include other steps or elements not listed, or inherent to such process, method, article, or apparatus.

In addition to the foregoing, it should be emphasized that reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. It is explicitly and implicitly understood by one skilled in the art that the embodiments described herein can be combined with other embodiments.

< electrolyte solution >

The invention provides an electrolyte for assembling a lithium battery, which comprises the following components: a lithium salt, a non-aqueous organic solvent, and an additive comprising an organic compound represented by the following structural formula:

In the application, after the additive is added into the electrolyte, not only can the HF impurities in the electrolyte be removed, but also the CEI of the lithium cobaltate anode in the lithium battery can be optimized, the crushing of lithium cobaltate particles is slowed down, the working voltage of a lithium cobaltate anode material is increased, the cut-off voltage is increased to be more than 4.7V, and further the energy density of the lithium metal battery is increased; meanwhile, the growth of lithium dendrites can be inhibited at the negative electrode of the battery, and the SEI of the negative electrode is optimized. Therefore, the electrolyte provided by the invention can be used for simultaneously optimizing the performances of the positive electrode and the negative electrode of the lithium battery.

Further, R₁、R₂、R₃Are the same group.

Further, R₁、R₂、R₃Being a methyl group, the organic compound has the following structure:

more excellent effects can be achieved in the electrolytic solution.

Further, based on the electrolyte, the additive is 0.2-2.0 wt%; preferably, the additive is 0.2 wt% to 1.0 wt% based on the electrolyte.

The lithium salt may be selected from LiPF₆、LiBF₄、LiBOB、LiDFOB、LiSbF₆、LiAsF₆、LiN(SO₂CF₃)₂、LiN(SO₂C₂F₅)2、LiC(SO₂CF₃)₃Or LiN (SO)₂F)₂One or more than two of them.

The concentration of the lithium salt in the electrolyte may be set to 0.9 to 1.1M; preferably, the concentration of the lithium salt is 1M, which can further improve the performance of the electrolyte.

As for the non-aqueous organic solvent, it can be realized by common general knowledge; or a mixture of cyclic carbonates and chain carbonates. The cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate or butylene carbonate, and the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or propyl methyl carbonate. The mixed solution of the cyclic carbonate organic solvent with high dielectric constant and the chain carbonate organic solvent with low viscosity is used as the solvent of the lithium ion battery electrolyte, so that the mixed solution of the organic solvent has high ionic conductivity, high dielectric constant and low viscosity.

Furthermore, the nonaqueous organic solvent is Ethylene Carbonate (EC) and Ethyl Methyl Carbonate (EMC), the volume ratio of the EC to EMC is 1:1, and the performance of the electrolyte can be further improved.

< lithium cell >

On the basis of the electrolyte, the invention also provides a lithium battery, which comprises: a positive electrode, a negative electrode and the electrolyte. Further, the positive electrode includes lithium cobaltate; further, the positive electrode is mainly prepared from a lithium cobaltate material. Further, the cut-off voltage of the lithium battery can reach more than 4.7V.

Example 1

This example illustrates the electrolyte disclosed in the present invention.

In a glove box (H)₂O<0.1ppm，O₂<0.1ppm), an appropriate amount of lithium hexafluorophosphate (LiPF) was weighed₆) And dissolving the electrolyte in an organic solution to obtain a base electrolyte.

Lithium salt concentration: 1M lithium hexafluorophosphate (LiPF)₆)；

Non-aqueous organic solvent: ethylene Carbonate (EC), a mixed solvent of dimethyl carbonate (DMC) 1:1(v: v);

adding an organic compound with the mass fraction of 0.2% into the base electrolyte, and uniformly stirring to obtain an electrolyte 1, wherein the organic compound has the structure:

example 2

An electrolytic solution was prepared in accordance with the method of example 1, except that an organic compound was added to the base electrolytic solution in a mass fraction of 0.5%, to obtain an electrolytic solution 2.

Example 3

An electrolytic solution was prepared in accordance with the method of example 1, except that an organic compound was added to the base electrolytic solution in a mass fraction of 1%, to obtain an electrolytic solution 3.

Example 4

An electrolytic solution was prepared in accordance with the method of example 1, except that an organic compound was added to the base electrolytic solution in a mass fraction of 2%, to obtain an electrolytic solution 4.

Comparative example 1

The base electrolyte prepared in the manner of example 1 was used as a control electrolyte.

Performance test of electrolyte HF content and lithium negative electrode morphology

Measuring and recording the content of HF in each electrolyte, preparing lithium batteries by respectively adopting the prepared electrolytes 1-4 and a control group electrolyte, and observing the appearance of lithium cathodes after different electrolytes are circulated by adopting a Hitachi S4800 scanning electron microscope. The test results are shown in table 1 and fig. 1 to 4, in which fig. 1 is a scanning electron microscope image of a lithium negative electrode of a lithium battery prepared from the electrolyte 1; fig. 2 is a scanning electron micrograph of a lithium negative electrode of a lithium battery prepared from the electrolyte 2, fig. 3 is a scanning electron micrograph of a lithium negative electrode of a lithium battery prepared from the electrolyte 3, and fig. 4 is a scanning electron micrograph of a lithium negative electrode of a lithium battery prepared from a control electrolyte.

The preparation method of the lithium battery comprises the following steps:

lithium symmetrical cell: in a glove box (H)₂O<0.1ppm，O₂<0.1ppm), sequentially assembling the positive electrode shell → the lithium sheet → the electrolyte → the diaphragm → the lithium sheet → the stainless steel gasket → the negative electrode shell from bottom to top, and then transferring to a tablet press for stamping and packaging to obtain the finished lithium symmetric battery.

All-battery: in a glove box (H)₂O<0.1ppm，O₂<0.1ppm), the positive electrode can → NCM622 (LiNi) in that order_0.6Co_0.2Mn_0.2O₂) The pole piece → electrolyte → diaphragm → lithium piece → stainless steel gasket → spring plate → negative electrode shell is assembled from bottom to top, then transferred to a tablet press for punching and packaging, and the manufactured full battery is obtained.

TABLE 1

Serial number	Morphology of battery cathode	HF content
			Example 1	The surface is smooth and has only a few lithium dendrites	Hardly any
Example 2	The surface is smooth, and no lithium dendrite exists;	hardly any
			Example 3	The surface is smooth, and only a few lithium dendrites exist;	hardly any
Example 4	The surface is smooth, and only a few lithium dendrites exist;	hardly any
			Comparative example 1	Is quite rough and the surface is filled with a large number of lithium dendrites and voids.	A large number of

As can be seen from table 1, the electrolytes 1 to 4 provided in the examples of the present invention can remove HF well compared to the control electrolyte. More importantly, the electrolytes 1 to 4 provided by the embodiment of the invention can well remove lithium dendrites of the negative electrode, wherein the battery negative electrode appearances of the electrolytes 1 to 3 can be sequentially shown in fig. 1 to 3. However, the comparative electrolyte cannot play a good role in removing lithium dendrites due to the lack of the organic compound provided in the present invention, so that there is a gap in the corresponding battery negative electrode, and the appearance of the battery negative electrode of the comparative electrolyte can be seen in fig. 4.

Furthermore, one result that can also be derived from table 1 is: the effect of the content of organic compounds on the removal of lithium dendrites. Specifically, when the content of the organic compound is 0.5%, the electrolyte can exert the best effect, in other words, when the content of the organic compound is too large or too small, the effect of the electrolyte is affected.

Electrochemical performance test

Electrochemical performance tests were performed on the assembled cells using novalr test equipment. The specific experimental process is as follows: assembling a lithium symmetric battery by taking the lithium sheets as positive and negative electrodes to perform constant current charge and discharge test; lithium sheet as negative electrode, NCM622 (LiNi)_0.6Co_0.2Mn_0.2O₂) Is a positive active material, and is matched and assembled into a full cell for constant current charge and discharge test. The results are shown in Table 2 and FIGS. 5-6; fig. 5 is a three-cycle charge-discharge curve diagram of a Li | | LCO lithium metal battery in an electrolyte 2 (the Li | | LCO lithium metal battery refers to a battery in which a metal lithium electrode is adopted as a negative electrode and a lithium cobaltate electrode is adopted as a positive electrode), and fig. 6 is a comparison diagram of cycle performance of lithium symmetric batteries in electrolytes 1 to 3 and a control group electrolyte.

TABLE 2

As can be seen from table 2, in the full cell, the performance of the cell assembled using the electrolytes 1 to 5 of the present invention was greatly improved; under the cut-off voltage of 4.7V, the specific capacity of the first three weeks exceeds 80 percent and far exceeds that of the electrolyte of a control group. Similarly, the specific capacities of electrolytes 1-4 also greatly exceeded that of electrolyte 5, and the contribution to this difference should be attributed to the synergistic effect of dimethyl carbonate (DMC) in the non-aqueous organic solvent in combination with other components of the electrolyte.

In addition, the specific capacity change trend of the electrolyte 1-3 is observed, and the specific capacity is gradually maintained to be stable after the content of the organic compound is increased to 0.5 percent and is continuously maintained until the content of the organic compound reaches 1.0 percent, and then the content of the organic compound is continuously increased, so that the specific capacity is not increased any more and is reduced instead. It can be seen from this that, beyond a content of 1.0%, the positive advantage of the organic compound with respect to the specific capacity is no longer increased.

It should be noted that the specific capacity of the electrolyte 2 can be shown in the charge-discharge curve before three weeks of the Li | | LCO lithium metal battery shown in fig. 5, and the charge-discharge curves of other electrolytes are not shown.

With continued reference to fig. 6, it was found that the cycle life of the electrolyte 1-3 assembled cells was greatly extended, wherein the cycle life of the electrolyte 1 assembled cells reached approximately 80 hours and the cycle life of both electrolyte 2 and 3 assembled cells reached 90 hours. The battery assembled with the electrolyte of the control group has serious polarization and short circuit after about 60 hours.

In conclusion, the electrolyte provided by the invention can inhibit the growth of lithium dendrite on the negative electrode of the battery, optimize the SEI of the negative electrode, optimize the CEI of the positive electrode of lithium cobaltate in the lithium battery, slow down the breakage of lithium cobaltate particles, increase the working voltage of a lithium cobaltate positive electrode material, increase the cut-off voltage to be more than 4.7V, and further increase the energy density of the lithium metal battery; therefore, the electrolyte provided by the invention can simultaneously optimize the performances of the positive electrode and the negative electrode of the lithium battery.

In addition, it should be emphasized that the electrolyte solution 2 provides the best effect in the electrolyte solution provided by the present invention, and to maximize the advantages of the electrolyte solution of the present invention, it is necessary to consider not only the combination of the components of the lithium salt, the nonaqueous organic solvent and the organic compound, but also the content of each component. For example, after the content of the organic compound exceeds 0.5%, the appearance of the SEI film of the negative electrode is not greatly improved, but before the content of the organic compound does not exceed 1.0%, the battery can still keep better specific capacity, and the improvement of the working voltage of the lithium cobaltate positive electrode material is greatly facilitated. Therefore, if the performance of the positive and negative electrodes of the lithium battery is to be optimized while taking overall consideration, the content of the organic compound should preferably be 0.5% to 1.0%.

While embodiments of the invention have been described above, it is not limited to the applications set forth in the description and the embodiments, which are fully applicable in various fields of endeavor to which the invention pertains, and further modifications may readily be made by those skilled in the art, it being understood that the invention is not limited to the details shown and described herein without departing from the general concept defined by the appended claims and their equivalents.

Claims

1. An electrolyte, characterized by comprising: a lithium salt, a non-aqueous organic solvent, and an additive comprising an organic compound represented by the following structural formula:

2. The electrolyte of claim 1, wherein R is₁、R₂、R₃Being a methyl group, the organic compound has the following structure:

3. the electrolyte of claim 1, wherein the additive is present in an amount of 0.2 wt% to 2.0 wt% based on the electrolyte.

4. The electrolyte of claim 3, wherein the additive is present in an amount of 0.2 wt% to 1.0 wt% based on the electrolyte.

5. The electrolyte of claim 1, wherein the lithium salt is selected from LiPF₆、LiBF₄、LiBOB、LiDFOB、LiSbF₆、LiAsF₆、LiN(SO₂CF₃)₂、LiN(SO₂C₂F₅)₂、LiC(SO₂CF₃)₃Or LiN (SO)₂F)₂One or more than two of them.

6. The electrolyte of claim 1, wherein the lithium salt is present at a concentration of 0.9-1.1M.

7. The electrolyte of claim 1, wherein the non-aqueous organic solvent is selected from a mixture of cyclic carbonates and chain carbonates.

8. The electrolyte according to claim 7, wherein the non-aqueous organic solvent is ethylene carbonate and ethyl methyl carbonate, and the volume ratio of the ethylene carbonate to the ethyl methyl carbonate is 1: 1.

9. A lithium battery, comprising: a positive electrode, a negative electrode and an electrolyte as claimed in any one of claims 1 to 8.

10. The lithium battery of claim 9, wherein a cutoff voltage of the lithium battery is above 4.7V.