Pre-lithiated negative electrode with spontaneously formed lithium-containing compound interface layer

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a pre-lithiation negative electrode with a spontaneously formed lithium-containing compound interface layer. The pre-lithiation negative electrode comprises a negative electrode, a lithium-containing compound interface layer positioned on the negative electrode and a metal lithium layer for pre-lithiation. And the lithium-containing compound interface layer is generated through spontaneous reaction of metal lithium and gas adsorbed on the surface of the negative electrode. One part of the active material layer and the metal lithium layer are in direct contact to form an electron channel, the other part of the active material layer and the metal lithium layer are separated by a lithium-containing compound interface layer, and the interface layer serves as a lithium ion channel in the pre-lithiation process. According to the method, manual introduction of a negative electrode pre-lithiation interface layer is avoided, and meanwhile, deep and uniform pre-lithiation of the negative electrode can be realized. The production cost is greatly reduced, and meanwhile, the first-circle coulombic efficiency and the cycle performance of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and particularly to a prelithiated anode with a spontaneously formed lithium - containing compound interface layer. Background Art

[0002] In the development process of lithium - ion batteries, the performance of the anode material plays a key role in the overall performance of the battery. With the continuous improvement of requirements for battery energy density, cycle life, etc., traditional anode materials face many challenges. During the first charge - discharge process, complex electrochemical reactions occur on the anode surface to form a solid electrolyte interface (SEI) film. However, the existing SEI films have some deficiencies, such as limited stability and low ionic conductivity, which can lead to problems such as rapid battery capacity decay and low charge - discharge efficiency. At the same time, in order to compensate for the irreversible lithium loss during the first charge - discharge process of the battery and improve the actual capacity and energy density of the battery, prelithiation technology has become a research hotspot. Compared with cathode prelithiation, the anode prelithiation technology directly performs prelithiation treatment on the anode side, avoiding the introduction of inactive substances into the electrode, having less negative impact on the electrode, and being able to maximize the improvement of the first - cycle Coulomb efficiency and energy density of the battery.

[0003] The methods of anode prelithiation mainly include electrochemical prelithiation, anode prelithiation additives, and chemical prelithiation, etc. Electrochemical prelithiation is a process in which the anode reacts electrochemically with metallic lithium in the presence of an electrolyte. This method can precisely control the prelithiation rate, uniformity, and degree of prelithiation, but has limitations such as complex operation, high cost, and the need for subsequent battery manufacturing processes under inert gas conditions. Anode prelithiation additives are another commonly used prelithiation method. These additives mainly include passivated lithium powder (SLMP) and lithium alloy compounds, etc. SLMP has a high prelithiation capacity and can effectively prelithiate carbon and silicon anodes. However, SLMP is unstable in the solvents of common polyvinylidene fluoride (PVDF) binders, resulting in SLMP not being able to be uniformly added to the anode through the current slurry - coated electrode preparation process. Therefore, SLMP can generally only be dispersed on the anode surface by spraying or dropping hydrocarbon - based or some ether - based dispersions of SLMP. Recently, precisely covering metallic lithium on the anode surface is an excellent prelithiation method. However, it is necessary to artificially prepare a layer of discontinuous covering electrode (solid electrolyte, oxide, nitride, phosphide, sulfide, etc.) compound before covering metallic lithium. Current prelithiation methods often have problems such as complex processes, high costs, and possible damage to the electrode structure. Therefore, it is of great significance to develop a technology that can spontaneously form a stable and excellent anode interface layer and achieve effective prelithiation. Summary of the Invention

[0004] In view of this, the object of the present invention is to provide a prelithiated negative electrode with a spontaneously formed lithium-containing compound interface layer to solve the problem of lithium loss in the first charge and discharge process of existing negative electrode materials.

[0005] Based on the above object, the present invention provides a prelithiated negative electrode with a spontaneously formed lithium-containing compound interface layer, which is prepared by the following steps:

[0006] (1) After drying and heating the negative electrode, it is placed in a dry gas environment so that its surface adsorbs gas;

[0007] (2) A layer of metallic lithium is covered on the surface of the negative electrode adsorbed with gas and left for 0.5 - 1 h. The gas adsorbed on the surface of the negative electrode and the metallic lithium react spontaneously to form a lithium-containing compound interface layer;

[0008] (3) The negative electrode with the lithium-containing compound interface layer is placed in an inert gas or vacuum environment for a period of time to complete the negative electrode prelithiation process, and a prelithiated negative electrode is obtained.

[0009] Preferably, in the step (1), the negative electrode includes a current collector and an active material layer.

[0010] Preferably, the active material layer includes an active material, a conductive agent, and a binder.

[0011] Preferably, the current collector is one or more of copper foil, copper-plated Al foil, and stainless steel foil.

[0012] Preferably, the active material includes one or more of natural graphite, artificial graphite, silicon, silicon monoxide, porous silicon-carbon composite, tin, and germanium.

[0013] Preferably, the conductive agent includes one or more of conductive carbon black, Ketjen black, carbon fiber, carbon nanotube, and graphene.

[0014] Preferably, the binder includes one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyamide, polyimide, polymethyl methacrylate, carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, and polyvinyl alcohol.

[0015] Preferably, in the active material layer, the mass ratio of the active material is 70% - 97%, the mass ratio of the conductive agent is 1.5% - 15%, and the mass ratio of the binder is 1.5% - 15%; the thickness of the active material layer is 10 μm - 300 μm.

[0016] Preferably, the drying method in the step (1) is vacuum drying or forced-air drying, and the drying temperature is 30°C - 110°C.

[0017] Preferably, the dry gas environment in step (1) consists of one or more of nitrogen, oxygen, carbon dioxide, and dry air.

[0018] Preferably, the method for covering the surface of the negative electrode in step (2) with a layer of metallic lithium includes one or more combinations of vacuum resistance heating evaporation, vacuum electron beam evaporation, vacuum laser heating evaporation, magnetron sputtering, and vacuum scraping coating, and the thickness of the metallic lithium is 0.5 μm - 20 μm.

[0019] Preferably, the composition of the lithium-containing compound interface layer in step (2) consists of one or more of Li3N, Li2O, and Li2CO3, and the coverage of this interface layer on the surface of the negative electrode is between 20% and 80%.

[0020] Preferably, the inert gas in step (3) includes one or more of argon, carbon dioxide, nitrogen, and helium, and the temperature of the negative electrode prelithiation process is 10°C - 80°C, and the time is 1 - 72 h.

[0021] Advantages of the present invention: The interface layer composed of one or more lithium-containing compounds such as lithium nitride, lithium oxide, and lithium carbonate prepared by the present invention is spontaneously formed by the chemical reaction between metallic lithium and the negative electrode (due to the strong reducibility of metallic lithium, its standard electrode potential is -3.04 V, which means that lithium is prone to losing electrons in chemical reactions, so metallic lithium can react with components such as N2, O2, CO2, and H2O adsorbed on the electrode surface to generate Li3N, Li2O, and Li2CO3). The process is simple and easy for large-scale production. This interface layer has a unique chemical composition and microstructure, and the interface layer composed of one or more of lithium nitride, lithium oxide, and lithium carbonate has good ionic conductivity, chemical stability, and mechanical stability. This interface layer can accelerate the prelithiation reaction between metallic lithium and the negative electrode as the lithium ion channel during the prelithiation process. There is no need for additional equipment and complex processes to artificially manufacture a discontinuous pre-lithiated interface layer on the surface of the negative electrode, reducing production costs. At the same time, the prelithiation process is milder and will not damage the structure of the negative electrode material, which is beneficial to improving the electrochemical performance of the negative electrode. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0023] Figure 1 are the SEM and elemental mapping distribution diagrams of the prelithiated negative electrode prepared in the embodiment;

[0024] Figure 2 is the TEM diagram of the prelithiated negative electrode prepared in the embodiment;

[0025] Figure 3XPS diagram of the prelithiated anode prepared in the example;

[0026] Figure 4 XRD diagrams of the prelithiated anodes prepared in the example and Comparative Example 1;

[0027] Figure 5 First charge-discharge curves of the half-cells of the example, Comparative Example 1 and Comparative Example 2;

[0028] Figure 6 First charge-discharge curves of the full-cells of the example, Comparative Example 1 and Comparative Example 2;

[0029] Figure 7 Long-term cycling performance of the full-cells of the example, Comparative Example 1 and Comparative Example 2. Detailed implementation manners

[0030] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to specific examples.

[0031] Example:

[0032] (1) Graphite, silicon monoxide, acetylene black, and sodium alginate were added to water in a mass ratio of 80:10:4:4 and mixed evenly to obtain the anode slurry. Then the slurry was coated on a copper foil, first dried in a blast oven at 60 °C for 4 h and then dried under vacuum at 80 °C for 12 h to obtain the anode electrode;

[0033] (2) The prepared anode electrode was placed in a mixed gas of nitrogen and carbon dioxide (volume ratio 4:1) for 4 h to adsorb nitrogen and carbon dioxide gases on the anode surface;

[0034] (3) A metal lithium layer with a thickness of 3.5 μm was deposited on the surface of the gas-adsorbed anode electrode by vacuum electron beam evaporation, and left for 30 min. A lithium nitride and lithium carbonate interface layer was spontaneously formed on the surface of the anode material;

[0035] (4) The anode electrode with the lithium nitride and lithium carbonate interface layer was left standing at 45 °C in an argon atmosphere for 24 h to complete the prelithiation process of the anode electrode, and a prelithiated anode with a spontaneously formed lithium compound interface layer was obtained.

[0036] Cathode electrode: Lithium cobaltate (LiCoO2), conductive carbon black, and binder PVDF were added to N-methylpyrrolidone in a mass ratio of 95:3:2, stirred evenly to obtain the cathode slurry, and then doctor-bladed on the surface of an aluminum foil, dried and cut into pieces to prepare the cathode electrode.

[0037] The prelithiated anode with a spontaneously formed lithium-containing compound interface layer was used as the working electrode, and a lithium metal sheet and the positive electrode were used as the counter electrodes to assemble the anode half-cell and the full cell, respectively. A solution of 1 mol / L LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) / dimethyl carbonate (DMC) (volume ratio 1:1:1) was used as the electrolyte, and a button cell was assembled in a glove box filled with argon. Then, the battery was subjected to charge-discharge tests. The charge-discharge voltage range of the anode half-cell was 0.01 - 1.5 V, and the voltage range of the full cell was 3.0 - 4.45 V. The current density of the half-cell was 50 mA / g. The initial cycle current density of the full cell was 0.05 C, and the subsequent cycles were 0.2 C.

[0038] Comparative Example 1:

[0039] (1) Graphite, silicon monoxide, acetylene black, and sodium alginate were added to water in a mass ratio of 80:10:4:4 and mixed evenly to obtain the anode slurry. Then, the slurry was coated on a copper foil, first dried in a blast oven at 60 °C for 4 h and then dried under vacuum at 80 °C for 12 h to obtain the anode electrode.

[0040] (2) A metal lithium layer with a thickness of 3.5 μm was deposited on the surface of the anode electrode by vacuum electron beam evaporation.

[0041] (3) The anode electrode with the metal lithium layer was left standing at 45 °C in an argon atmosphere for 24 h to complete the prelithiation process of the anode electrode, obtaining the prelithiated anode.

[0042] Positive electrode: Lithium cobaltate (LiCoO2), conductive carbon black, and binder PVDF were added to N-methylpyrrolidone in a mass ratio of 95:3:2, stirred evenly to obtain the positive electrode slurry, and then scraped and coated on the surface of an aluminum foil, dried, and cut into pieces to prepare the positive electrode.

[0043] The prelithiated anode was used as the working electrode, and a lithium metal sheet and the positive electrode were used as the counter electrodes to assemble the anode half-cell and the full cell, respectively. A solution of 1 mol / L LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) / dimethyl carbonate (DMC) (volume ratio 1:1:1) was used as the electrolyte, and a button cell was assembled in a glove box filled with argon. Then, the battery was subjected to charge-discharge tests. The charge-discharge voltage range of the anode half-cell was 0.01 - 1.5 V, and the voltage range of the full cell was 3.0 - 4.45 V. The current density of the half-cell was 50 mA / g. The initial cycle current density of the full cell was 0.05 C, and the subsequent cycles were 0.2 C.

[0044] Comparative Example 2:

[0045] Negative electrode: Graphite, silicon suboxide, acetylene black, and sodium alginate were added to water in a mass ratio of 80:10:4:4 and mixed evenly to obtain a negative electrode slurry. Then, the slurry was coated on a copper foil, dried in a blast oven at 60 °C for 4 h, and then dried under vacuum at 80 °C for 12 h to obtain the negative electrode;

[0046] Positive electrode: Lithium cobaltate (LiCoO2), conductive carbon black, and binder PVDF were added to N-methylpyrrolidone in a mass ratio of 95:3:2, stirred evenly to obtain a positive electrode slurry, and then blade-coated on the surface of an aluminum foil, dried, and cut into pieces to prepare the positive electrode.

[0047] Using the negative electrode as the working electrode, a negative half-cell and a full cell were assembled with a lithium metal sheet and the positive electrode as the counter electrodes, respectively. A 1 mol / L LiPF6 solution of ethylene carbonate (EC) / diethyl carbonate (DEC) / dimethyl carbonate (DMC) (volume ratio 1:1:1) was used as the electrolyte, and a button cell was assembled in a glove box filled with argon. Then, the battery was subjected to charge and discharge tests. The charge and discharge voltage range of the negative half-cell was 0.01 - 1.5 V, and the voltage range of the full cell was 3.0 - 4.45 V. The current density of the half-cell was 50 mA / g. The initial cycle current density of the full cell was 0.05 C, and the subsequent cycles were 0.2 C.

[0048] Table 1 Initial charge and discharge data of the half-cell

[0049]

[0050] Data analysis:

[0051] From Figure 1 the SEM and EDS elemental mapping images of the example (pre-lithiated negative electrode), it can be seen that the three elements C, N, and O are discontinuously distributed on the surface of the negative electrode, indicating that the interfacial layers Li3N, Li2O, and Li2CO3 formed spontaneously by metallic lithium and the gas adsorbed on the surface of the negative electrode are still distributed on the surface of the negative electrode after the pre-lithiation is completed. Figure 2 is the high-resolution TEM image of the example. It can be seen that there is an interfacial layer composed of Li3N, Li2O, and Li2CO3 on the surface of the negative electrode, and its thickness is several tens of nanometers. Combining with X-ray photoelectron spectroscopy analysis ( Figure 3 ), the characteristic peak of the N 1s spectrum at 397 eV can be attributed to Li3N, and the characteristic peaks of the O 1s spectrum at 528.5 eV and 531.5 eV can be attributed to Li2O and Li2CO3, respectively. The above results prove that metallic lithium and the negative electrode can spontaneously form an interfacial layer composed of Li3N, Li2O, and Li2CO3. Figure 4XRD of Example (pre-lithiated anode) and Comparative Example 2 (original anode). Compared with Comparative Example 2, there is an additional diffraction peak at ~26° in the Example, corresponding to the pre-lithiated product LiC of graphite. 18 , Si and its pre-lithiated products in the anode are amorphous structures and cannot be detected. In summary, the presence of LiC 18 proves that the pre-lithiation process of the anode has been completed.

[0052] From Figure 5 and the data in Table 1, it can be seen that after pre-lithiation without a lithium-containing compound interface layer, the first-cycle Coulombic efficiency of the graphite / silicon monoxide anode battery increased from 77.8% to 84.1% (Comparative Example 1). The utilization rate of the corresponding metallic lithium can be calculated to be 53.4% by the ratio of the irreversible capacity loss compensated to the theoretical capacity of metallic lithium. After introducing an in-situ formed lithium-containing compound interface layer and pre-lithiating by this method, the first charge-discharge efficiency of the pre-lithiated graphite / silicon monoxide anode battery increased to 89.2%. The utilization rate of metallic lithium increased to 89.3%. Figure 6 The first-cycle charge-discharge curves of the full cells of the Example, Comparative Example 1 and Comparative Example 2 are shown. It can be seen that after pre-lithiation, the discharge specific capacity, i.e., the reversible specific capacity, of the full cell increased significantly. The discharge specific capacity of Comparative Example 1 increased by 10.8 mAh / g, while the discharge specific capacity of the Example increased to 19.6 mAh / g. The greater increase in the reversible capacity of the Example compared to Comparative Example 1 stems from the higher utilization rate of metallic lithium of this pre-lithiation method (Table 1). The presence of the interface layer enables metallic lithium to react fully with the anode, thus avoiding the residue of metallic lithium on the anode surface after pre-lithiation and the side reaction between the electrolyte and the residual metallic lithium during the formation process. From Figure 7 it can be seen that after 100 cycles of the full cell, the Example can still maintain a higher reversible specific capacity than Comparative Example 1 and Comparative Example 2, proving that this method will not have a negative impact on the cycle performance of the battery and can effectively improve the capacity of the battery. By comparing the Example and the Comparative Examples, it can be clearly seen that the spontaneously formed lithium oxide, lithium nitride, lithium carbonate anode interface layer prepared by the present invention and the pre-lithiated anode with this interface layer can significantly improve the charge-discharge efficiency and cycle life of the battery.

[0053] Those of ordinary skill in the art should understand that: the discussion of any above embodiment is only exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features between the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A pre-lithiated negative electrode having a spontaneously formed lithium-containing compound interface layer, characterized in that: It includes a negative electrode, a lithium compound interface layer located on the negative electrode, and a metal lithium layer for pre-lithiation; The negative electrode comprises a current collector and an active material layer located on the current collector; the active material layer comprises an active material, a conductive agent and a binder; The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer is prepared by the following steps: (1) drying and heating the negative electrode and placing it in a dry gas environment to allow the surface of the electrode to adsorb gas; (2) Covering the surface of the negative electrode adsorbing the gas with a layer of metallic lithium and leaving it for 0.5-1h, the gas adsorbed on the surface of the negative electrode and the metallic lithium spontaneously react to form an interface layer containing a lithium compound; (3) Placing the negative electrode containing the lithium compound interface layer in an inert gas or vacuum environment for a period of time to complete the negative electrode pre-lithiation process, thereby obtaining a pre-lithiation negative electrode.

2. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, characterized in that: The current collector is one or more of copper foil, copper-plated Al foil, and stainless steel foil.

3. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, characterized in that: The active material includes one or more of natural graphite, artificial graphite, silicon, silicon oxide, porous silicon-carbon composite, tin, and germanium.

4. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, wherein the conductive agent comprises: One or more of conductive carbon black, Ketjen black, carbon fiber, carbon nanotube, and graphene; The adhesive includes: one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyamide, polyimide, polymethyl methacrylate, carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, and polyvinyl alcohol.

5. According to claim 1, the pre-lithiation negative electrode with a spontaneously formed lithium-containing compound interface layer, the mass proportion of active material in the active material layer is 70%-97%, the mass proportion of conductive agent is 1.5%-15%, and the mass proportion of binder is 1.5%-15%; the thickness of the active material layer is 10μm-300μm.

6. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, characterized in that: The drying method of step (1) is vacuum drying or forced air drying, and the drying temperature is 30° C.-110° C. The drying gas environment of step (1) is composed of one or more of nitrogen, oxygen, carbon dioxide, and dry air.

7. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, characterized in that: The method for covering the surface of the negative electrode with a layer of metallic lithium in step (2) comprises a combination of one or more of vacuum resistance heating evaporation, vacuum electron beam evaporation, vacuum laser heating evaporation, magnetron sputtering and vacuum coating, and the thickness of the metallic lithium is 0.5 μm-20 μm.

8. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, characterized in that: In step (2), the lithium-containing compound interface layer is composed of one or more of Li3N, Li2O, and Li2CO3, and the coverage of the lithium-containing compound interface layer on the negative electrode surface is between 20% and 80%. The interface layer does not completely cover the negative electrode surface.

9. The pre-lithiation negative electrode having a spontaneously formed lithium-containing compound interface layer according to claim 1, characterized in that: The inert gas in step (3) includes one or more of argon, carbon dioxide, nitrogen and helium. The temperature of the negative electrode pre-lithiation process is 10°C-80°C.

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