阅读说明：本技术 一种电解液添加剂及其电解液和锂金属电池 (Electrolyte additive, electrolyte and lithium metal battery ) 是由 程萌 李谦 刘成勇 郭永胜 胡波兵 付佳玮 梁成都 于 2019-04-10 设计创作，主要内容包括：本发明属于电池技术领域,更具体地涉及一种改善锂金属表面的SEI膜的组成来提升电池循环寿命的电解液添加剂及其电解液和锂金属电池,由于添加了含有硝酸锂的电解液添加剂,硝酸锂在该电解液添加剂中溶解性能优良,被均匀地引入锂金属电池体系,使锂金属表面形成了稳定的SEI层,有效改善了SEI层的离子电导率及稳定性,从而有效防止电解液和锂金属之间副反应的发生,保证了活性锂的有效含量,使锂金属电池兼具高能量密度和长循环寿命。(The invention belongs to the technical field of batteries, and particularly relates to an electrolyte additive for improving the composition of an SEI film on the surface of a lithium metal to prolong the cycle life of the battery, an electrolyte and a lithium metal battery.)

1. An electrolyte additive comprises a first solvent and lithium nitrate, and is characterized in that the first solvent is an amide compound with a chemical structure shown as a general formula I,

wherein R is₁、R₂、R₃Are respectively selected from H or alkyl with 1-6 carbon atoms.

2. The electrolyte additive according to claim 1, wherein the lithium nitrate has a solubility in the amide compound of 1 to 6mol/L, preferably 2 to 5 mol/L.

3. The electrolyte additive as claimed in claim 1, wherein the amide compound is selected from any one or more of compounds represented by chemical formulas 1 to 6.

4. An electrolyte comprising a second solvent, an additive and a lithium salt, wherein: the additive is an electrolyte additive according to any one of claims 1 to 3.

5. The electrolyte of claim 4, wherein the second solvent is fluoroethylene carbonate and/or linear ethylene carbonate.

6. The electrolyte of claim 5, wherein the fluoroethylene carbonate is present in an amount of 30% to 50% based on the total mass of the electrolyte.

7. The electrolyte of claim 4, wherein the lithium salt is one or more of lithium bistrifluoromethylsulfonyl imide (LiTFSI), lithium bistrifluoromethylsulfonyl imide (LiFSI), lithium bis (oxalato) borate (LiBOB), and lithium difluoro (oxalato) borate (LiDFOB).

8. The electrolyte of claim 4, wherein the additive is present in an amount of 3% to 8% based on the total mass of the electrolyte.

9. A lithium metal battery, comprising:

a lithium metal negative electrode;

a diaphragm;

a positive electrode and an electrolyte as claimed in any one of claims 4 to 8.

10. The lithium metal battery of claim 9, wherein: the lithium metal surface further comprises a polymeric protective layer, and the protective layer comprises lithium nitrate.

Technical Field

The invention belongs to the technical field of batteries, and particularly relates to an electrolyte additive for improving the composition of an SEI (solid electrolyte interphase) film on the surface of lithium metal to prolong the cycle life of a battery, an electrolyte of the electrolyte additive and the lithium metal battery.

Background

As a lithium metal battery for upgrading a lithium ion battery, the lithium metal battery is expected to solve the problem of insufficient capacity of the conventional lithium ion battery due to the capability of storing more energy, and is increasingly favored in the field of commercial batteries.

Lithium metal batteries using lithium metal as the negative electrode have very high energy density due to the very high theoretical specific capacity (3860mAh/g) and very low electrochemical potential of lithium metal. However, the current lithium metal battery has a serious side reaction with lithium metal due to an electrolyte, so that active lithium is rapidly consumed, and the cycle life of the lithium metal battery is deteriorated. In lithium metal batteries, the solid electrolyte interface layer on the surface of lithium metal has an important influence on the deposition behavior of lithium metal, and is related to the life and safety of the battery. The composition of the solid electrolyte interface layer (SEI layer) that improves the surface of lithium metal has a crucial impact on the cycle life of the lithium metal battery.

Present research shows that SEI layer contains LiF and Li₃N,Li₂And inorganic molecules such as O and the like can effectively improve the ionic conductivity and stability of the interface and improve the cycling stability of the lithium metal battery. How to effectively improve the surface interface layer of the lithium metal battery is a very meaningful work for improving the cycle stability of the lithium metal battery.

Disclosure of Invention

In view of the above, it is desirable to use Li₃N and the like are introduced into an SEI layer, and Li is introduced into the invention₃The N component is mainly formed by lithium salt-lithium nitrate LiNO₃Decomposed, but lithium nitrate is not dissolved in a carbonate solvent, and solubility in an ether solvent is also low, thereby providing an electrolyte additive, which is required to have excellent ability to dissolve lithium nitrate and to uniformly introduce lithium nitrate into a lithium metal battery system, and an electrolyte and a lithium metal battery using the sameAnd a stable SEI layer is generated on the surface of the lithium metal, so that the purpose of prolonging the cycle life of the lithium metal battery is achieved.

In order to achieve the above object, in a first aspect of the present invention, the inventors provide an electrolyte additive comprising a first solvent and lithium nitrate, wherein the first solvent is an amide-based compound having a chemical structure represented by general formula i,

wherein R is₁、R₂、R₃Are respectively selected from H or alkyl with 1-6 carbon atoms.

In a second aspect of the present invention, the inventors provide an electrolyte comprising a second solvent, an additive and a lithium salt, wherein the additive is the electrolyte additive according to the first aspect of the present invention.

In a third aspect of the present invention, the inventors provide a lithium metal battery, wherein the lithium metal battery comprises: a lithium metal negative electrode, a separator, a positive electrode and an electrolyte according to the second aspect of the invention.

Compared with the prior art, the technical scheme of the invention at least has the following beneficial technical effects:

because the electrolyte additive containing lithium nitrate is added, the lithium nitrate has excellent solubility in the electrolyte additive and is uniformly introduced into a lithium metal battery system, so that a stable SEI layer is formed on the surface of lithium metal, the ionic conductivity and stability of the SEI layer are effectively improved, side reactions between the electrolyte and the lithium metal are effectively prevented, the effective content of active lithium is ensured, and the lithium metal battery has high energy density and long cycle life.

Detailed Description

The electrolyte additive of the present invention, the electrolyte thereof, and the lithium metal battery will be described in detail below.

First, an electrolyte additive according to a first aspect of the present invention is described, which comprises a first solvent and lithium nitrate, wherein the first solvent is an amide compound having a chemical structure represented by general formula i,

wherein R is₁、R₂、R₃Are respectively selected from H or alkyl with 1-6 carbon atoms.

Lithium nitrate is insoluble in carbonate solvents due to strong ion-ion interactions, and also has low solubility in ether solvents. The inventors have conducted extensive studies and found that, in an amide-based solvent, the unshared electron pair on the amino nitrogen in the molecule forms a conjugated system with the pi electron of the carbonyl group, and the electron cloud density on the nitrogen is reduced, so that the ability to accept a proton is weakened, and at this time, the C — N bond exhibits a certain degree of double bonding. At the same time, the electron cloud density on the nitrogen decreases, but the polarity of the N-H bond increases. R₁、R₂、R₃The short-chain amide compound is respectively selected from H or alkyl with 1-6 carbon atoms, and can fully exert the double bond property of a C-N bond and the polarity of an N-H bond, so that the lithium nitrate has good solubility in the amide compound.

Furthermore, lithium nitrate is used as a component of the electrolyte additive, so that the lithium metal battery can effectively inhibit the lithium metal from being converted into lithium dendrites in the circulation process of the lithium metal battery, inhibit the increase of the surface roughness of the lithium metal negative electrode and protect the lithium metal negative electrode structure. In addition, the electrolyte component and an inorganic film (SEI film) formed by a chemical reaction of the lithium metal negative electrode isolate the contact of the electrolyte and the metal lithium, inhibit the side reaction between the lithium metal negative electrode and the electrolyte, and greatly improve the charge-discharge capacity and the cycle performance of the lithium metal battery.

Therefore, the solution of the electrolyte additive to uniformly introduce lithium nitrate into a lithium metal battery system includes two solutions: (1) using the electrolyte additive as an electrolyte component; (2) the electrolyte additive is used for polymer film formation and is used as a lithium metal protective layer.

Further, the solubility of lithium nitrate in the amide-based compound in the electrolyte additive has an important influence on the improvement of the ionic conductivity and stability of the SEI layer, and although a smaller dissolving power can also improve the stability of the SEI layer to some extent, the inventors have found that the electrolyte additive has a better effect on the cycle life and energy density maintenance of the electrolyte and the lithium metal battery thereof when the dissolving power of lithium nitrate in the amide-based compound is 1-6mol/L, preferably 2-5 mol/L. The performance of the lithium nitrate in the additive is enhanced and then tends to be stable along with the increase of the lithium nitrate in the additive, and when the concentration of the lithium nitrate reaches 6mol/L, the additive is continuously added, and the lithium nitrate in the electrolyte is separated out.

Preferably, the dissolving power of the lithium nitrate in the amide compound is 1-6mol/L, preferably 2-5 mol/L.

Further, although R is₁、R₂、R₃The amide compounds respectively selected from H or a hydrocarbon group having 1 to 6 carbon atoms have a good solubility for lithium nitrate, but have a higher polarity at a lower electron cloud density on N, and have a better solubility for lithium nitrate, and the amide solvent has a lower stability for lithium metal as the number of carbon atoms increases.

Preferably, the amide-based compound is selected from any one of the compounds represented by chemical formulas 1 to 6 or a mixture of two or more thereof.

Next, an electrolyte solution according to a second aspect of the present invention is an electrolyte solution containing a second solvent, an additive, and a lithium salt, wherein: the additive is an electrolyte additive according to the first aspect of the invention.

In the electrolyte of the second aspect of the present invention, the second solvent and the lithium salt are both selected from solvents and lithium salts commonly used in lithium metal battery electrolytes, and specifically, the second solvent may be one or more mixed solvents selected from dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, fluoroethylene carbonate, propylene carbonate, ethyl methyl trifluorocarbonate, ethyl difluoroacetate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether. The lithium salt may be selected from lithium hexafluorophosphate (LiP)F₆) One or a mixture of two or more of lithium bistrifluoromethylsulfonyl imide (LiTFSI), lithium bistrifluoromethylsulfonyl imide (LiFSI), lithium bis (oxalato) borate (LiBOB), and lithium bis (oxalato) borate (lidob). The additive is the electrolyte additive in the first aspect of the invention, and can enable the electrolyte system to form a stable SEI layer on the surfaces of the positive electrode and the negative electrode, thereby ensuring excellent ionic conductivity and stability.

Further, although other common solvents for lithium metal batteries can be used to provide the electrolyte with better ionic conductivity and stability by combining with the electrolyte additive provided in the first aspect of the present invention, a great deal of research has revealed that when fluoroethylene carbonate and/or linear ethylene carbonate is used as the solvent in the electrolyte for lithium metal batteries, the ionic conductivity and stability of the electrolyte are the best.

Preferably, the second solvent is fluoroethylene carbonate and/or linear ethylene carbonate.

Furthermore, the mass of the second solvent in the electrolyte has a certain influence on the performance of the electrolyte, the fluoro-carbonic acid solvent is beneficial to the stability of the SEI layer, although the stability of the SEI layer is enhanced under the condition of high content, the content is too much, the increase of the viscosity of the electrolyte can cause the conductivity of the electrolyte to be reduced, and meanwhile, the wettability of the battery cell can be influenced.

Preferably, the fluoroethylene carbonate is contained in an amount of 30 to 50% based on the total mass of the electrolyte.

Preferably, the lithium salt is one or a mixture of two or more of lithium bistrifluoromethylsulfonyl imide (LiTFSI), lithium bistrifluoromethylsulfonyl imide (LiFSI), lithium dioxalate borate (LiBOB) and lithium difluorooxalate borate (lidob).

In the electrolyte solution of the second aspect of the present invention, the additive amount of the additive has an important influence on the performance of the electrolyte solution, and although an increase in the additive amount of the additive will form a stable SEI film first, when the additive amount is too large, the absolute content of the amide-based solvent in the electrolyte solution increases, which in turn deteriorates the performance of the electrolyte solution and the lithium metal battery.

Preferably, the additive is contained in an amount of 3% to 8% based on the total mass of the electrolyte.

Finally, a lithium metal battery according to a third aspect of the present invention is described, comprising a lithium metal negative electrode, a separator, a positive electrode and the electrolyte according to the second aspect of the present invention.

Preferably, the lithium metal surface further comprises a polymeric protective layer, and the protective layer comprises lithium nitrate.

In order to explain technical contents, structural features, and objects and effects of the technical means in detail, the following detailed description is given with reference to specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present application.

The preparation method of all examples and comparative lithium metal batteries was as follows: