Chemical pre-lithiation method to improve the first coulombic efficiency of carbon-based current collectors and its application in lithium metal batteries

Abstract

A chemical pre-lithiation method for improving the initial coulombic efficiency of carbon-based current collectors and its application in lithium metal batteries is disclosed. The method uses a carbonized cellulose film as the current collector, which is obtained by dropwise addition of an active lithium solution under an argon atmosphere until the solution reacts fully. This invention uses 4,4'-dimethylbiphenyllithium as the active lithium species and diphenyl ether and 2-methyltetrahydrofuran as a mixed solvent. By using a high-temperature carbonized cellulose film as the current collector, it is possible to achieve a better promoting effect on the active lithium species while increasing the initial coulombic efficiency to over 100%, greatly compensating for the irreversible capacity loss in the early cycling stages of carbon-based current collectors.

Description

Technical Field

[0001] This invention relates to a technology in the field of lithium batteries, specifically a chemical pre-lithiation method for improving the first coulombic efficiency of carbon-based current collectors and its application in lithium metal batteries. Background Technology

[0002] Existing chemical prelithiation systems are typically organic solutions of active lithium species. The active lithium species are generally aryl lithium, such as lithium naphthylene and lithium biphenyl; the solvents are usually ether compounds, such as ethylene glycol dimethyl ether and tetrahydrofuran. Currently proposed chemical prelithiation systems include lithium naphthylene in ethylene glycol dimethyl ether solution and lithium biphenyl in tetrahydrofuran. Due to the limited chemical activity of these systems, it is difficult to achieve good and sufficient prelithiation results, especially for carbon-based materials with low lithiation potentials. Summary of the Invention

[0003] This invention addresses the problem of excessively high redox potentials caused by the use of lithium naphthalene and lithium biphenyl in existing technologies. It proposes a chemical pre-lithiation method to improve the initial coulombic efficiency of carbon-based current collectors and its application in lithium metal batteries. Using 4,4'-dimethylbiphenyl lithium as the active lithium species and diphenyl ether and 2-methyltetrahydrofuran as a mixed solvent, and using a cellulose film carbonized at high temperature as the current collector, this method can achieve a better promoting effect on the active lithium species while increasing the initial coulombic efficiency to over 100%, greatly compensating for the irreversible capacity of carbon-based current collectors in the early cycle stage.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a method for preparing a pre-lithiated carbon-based current collector, which uses a carbonized cellulose film as the current collector and reacts it fully by adding an active lithium solution in an argon atmosphere.

[0006] The active lithium solution is obtained by fully dissolving lithium foil in a precursor solution containing 4,4'-dimethylbiphenyl.

[0007] The precursor solution uses 4,4'-dimethylbiphenyl as the solute and 2-methyltetrahydrofuran and diphenyl ether as a mixed solvent.

[0008] The mixed solvent is prepared by heating and holding diphenyl ether until it becomes liquid, then mixing it thoroughly with 2-methyltetrahydrofuran in a light-shielding blue-capped bottle until there is no obvious liquid separation, and then continuing to stir mechanically.

[0009] The precursor solution is obtained by adding 4,4'-dimethylbiphenyl to a mixed solvent at a concentration of 1 mol / L, shaking and mixing thoroughly until the 4,4'-dimethylbiphenyl dissolves, and then continuing mechanical stirring.

[0010] The active lithium solution is obtained by placing lithium foil in a precursor solution under an argon atmosphere and then mechanically stirring until the lithium foil is completely dissolved and the solution is completely opaque.

[0011] The carbonization refers to the process of heating a cellulose film to 800°C in a high-purity nitrogen gas atmosphere.

[0012] The complete reaction refers to the following: under an argon atmosphere, an active lithium solution is added dropwise to the current collector until it is completely submerged, and then allowed to stand to react until the solution changes from dark blue to dark green, and then gradually turns to reddish brown.

[0013] This invention relates to a pre-lithiated carbon-based current collector prepared by the above method, comprising: a pre-lithiated carbon-based current collector with a three-dimensional framework structure and lithium metal distributed therein.

[0014] This invention relates to the application of the aforementioned pre-lithiated carbon-based current collector, which is used as an electrode along with lithium foil, an electrolyte, and a separator to form a coin cell. Technical effect

[0015] This invention utilizes a mixed solvent system of aromatic and aliphatic ethers, employing 4,4'-dimethylbiphenyllithium, a lithium species with high lithiation activity, as the active species and diphenyl ether as the main solvent. Due to the conjugated system of the aromatic ether, the ether oxygen group of the diphenyl ether can achieve good coordination with lithium ions in the active lithium solution, thereby further enhancing the activity of the 4,4'-dimethylbiphenyl anion. 2-Methyltetrahydrofuran, as an auxiliary solvent, disperses the diphenyl ether to avoid side reactions of the aromatic ether and maintains the liquid phase system at room temperature through its miscibility with the diphenyl ether. The carbon-based current collector prepared by this invention exhibits a significantly improved initial coulombic efficiency exceeding 100%, a significantly reduced irreversible capacity, and improved cycle stability due to the partial pre-formation of the solid electrolyte interface, which is beneficial for realizing high-energy-density lithium metal batteries. Attached Figure Description

[0016] Figure 1 The differential charge diagram shows the chemical structure of the chemical pre-lithiation system prepared in this invention.

[0017] Figure 2 Scanning electron microscopy image of the unlithiated carbon-based current collector prepared in Example 1;

[0018] Figure 3 The image shows a scanning electron microscope image of the pre-lithiated carbon-based current collector prepared in Example 1.

[0019] Figure 4 A schematic diagram of the first constant current charge-discharge cycle curves of pre-lithiated (Example 1) and non-pre-lithiated (Comparative Example 1) carbon-based current collectors;

[0020] Figure 5 Coulombic efficiency diagrams for constant current charge-discharge cycles of pre-lithiated (Example 1) and un-lithiated (Comparative Example 1) carbon-based current collectors;

[0021] Figure 6 A schematic diagram of the first constant current charge-discharge cycle curves of pre-lithiation (Example 2) and non-pre-lithiation (Comparative Example 2) carbon-based current collectors;

[0022] Figure 7 Coulombic efficiency diagrams of carbon-based current collectors prepared under constant current charge-discharge cycles for pre-lithiation (Example 2) and non-pre-lithiation (Comparative Example 2). Detailed Implementation Example 1

[0023] This embodiment includes the following steps:

[0024] 1) Preparation of the chemical pre-lithiation system, specifically including:

[0025] 1.1) Measure 50 mL of 2-methyltetrahydrofuran and 50 g of diphenyl ether, place them separately in light-shielded blue-capped bottles, and heat them in a water bath to 30°C. Keep the temperature for 1 hour to allow the diphenyl ether to completely turn into a liquid. Measure 10 mL of the heated 2-methyltetrahydrofuran and 20 mL of diphenyl ether, and mix them thoroughly in the light-shielded blue-capped bottle until no obvious liquid separation is observed. Then, stir mechanically for 1 hour to obtain the prepared mixed solvent.

[0026] 1.2) Weigh 5.468 g of 4,4'-dimethylbiphenyl and add it to the mixed solvent obtained in step 1.1. Shake and mix thoroughly until completely dissolved, then mechanically stir for 2 hours to obtain the prepared precursor solution. Place the precursor solution in a glove box filled with argon (water and oxygen content are both below 0.1 ppm) and continue stirring for later use.

[0027] 1.3) In an argon-filled glove box, weigh 0.209 g of lithium foil, add it to the precursor solution, and mechanically stir for 10 hours until the lithium foil is completely dissolved and the solution is completely opaque, thus obtaining the prepared active lithium solution. Continue stirring the active lithium solution in an argon-filled glove box for later use.

[0028] 2) Preparation of carbon-based current collectors: A cellulose film with a thickness of approximately 30 μm was selected and cut into 50 × 50 mm squares. These squares were placed in a 55 × 55 × 50 mm alumina crucible, which was then pushed into a tube furnace. After three purgings with high-purity nitrogen, the quartz tube was kept in a high-purity nitrogen flow environment with a stable flow rate of 0.5 L / min. The tube furnace heating program was set as follows: heating to 800 °C at a rate of 5 °C / min, holding at that temperature for 2 hours, and then allowing to cool naturally. The tube furnace was then turned on, and after cooling, the crucible was removed, yielding a self-supporting carbon film. The self-supporting carbon film was cut into current collector discs with a diameter of 12 mm and placed in an argon-filled glove box for later use.

[0029] like Figure 2 The image shown is a scanning electron micrograph of an unlithiated carbon-based current collector. The sample exhibits a three-dimensional fibrous skeleton structure with numerous voids.

[0030] 4) Pre-lithiation of carbon-based current collector: In an argon-filled glove box, place the carbon-based current collector disc cut in step 2) into a 30mm diameter glass dish, and add the active lithium solution prepared in step 1) until completely submerged. Cover with a 35mm diameter glass dish and let stand for 15 minutes to allow the reaction to proceed. Carefully remove the reacted disc and wash it twice in a clean glass dish with 2-methyltetrahydrofuran and dimethyl carbonate, respectively. Allow it to air dry naturally in an argon-filled glove box to obtain the pre-lithiated carbon-based current collector.

[0031] like Figure 3 The image shown is a scanning electron microscope image of a pre-lithiated carbon-based current collector. The three-dimensional fiber skeleton structure of the sample is well preserved after pre-lithiation and exhibits a rich surface structure.

[0032] 5) Button cell assembly: using pre-lithiated carbon-based current collectors and lithium foil as electrodes, with 1 mol L... -1 Lithium hexafluorophosphate was dissolved in a solution of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate (volume ratio 1:1:1), and fluoroethylene carbonate (volume ratio 10%) as the electrolyte. A microporous polypropylene (Celgard 2325) membrane was used as the separator. The cells were assembled into 2025 coin cells in an argon-filled glove box (water and oxygen content were both below 0.1 ppm). After standing for 24 hours, constant current charge-discharge tests were performed.

[0033] like Figure 4 As shown, compared to the coin cell assembled with the un-lithiated carbon-based current collector, the open-circuit voltage of the pre-lithiated carbon-based current collector decreased from 2.5V to 0.1V at 1mA cm⁻¹. -2 1mAh cm -2 Under the test conditions, the coulombic efficiency increased from 65.6% to 102.3% for the first time, indicating a sufficient pre-lithiation effect.

[0034] The assembled button cells were subjected to constant current charge-discharge testing using a LAND CT-2001A battery testing system. At 1 mA cm⁻¹... -2 Current density, 1mAh cm -2 The above coin cells were subjected to constant current charge-discharge cycle tests under the conditions of specific area capacity and cutoff voltage of 1V.

[0035] like Figure 5 As shown, compared to the unlithiated carbon-based current collector, the prelithiated carbon-based current collector exhibits higher performance at 1 mA cm⁻¹. -2 1mAhcm -2 The long-cycle process is more stable. Example 2

[0036] The 2025 coin cell assembled in Example 1 was subjected to constant current charge-discharge testing using a LAND CT-2001A battery testing system. At 2mA cm⁻¹... -2 Current density, 4mAh cm -2 The above coin cells were subjected to constant current charge-discharge cycle tests under the conditions of specific area capacity and cutoff voltage of 1V.

[0037] like Figure 6 As shown, the pre-lithiated carbon-based current collector at 2 mA cm⁻¹ -2 4mAh cm -2 Under the test conditions, the coulombic efficiency increased from 87.5% to 100.5% for the first time, indicating a sufficient pre-lithiation effect.

[0038] like Figure 7 As shown, compared to the unlithiated carbon-based current collector, the prelithiated carbon-based current collector exhibits higher performance at 2 mA cm⁻¹. -2 4mAh cm -2 The long-cycle process is more stable. The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A method for preparing a pre-lithiated carbon-based current collector, characterized in that, Includes the following steps: A carbonized cellulose film was obtained by heating the cellulose film to 800°C in a high-purity nitrogen gas flow environment and then using it as a current collector. In an argon atmosphere, an active lithium solution is added dropwise to the current collector and allowed to react fully to obtain a pre-lithiated carbon-based current collector. The active lithium solution is obtained by fully dissolving lithium foil in a precursor solution; the precursor solution uses 4,4'-dimethylbiphenyl as a solute and 2-methyltetrahydrofuran and diphenyl ether as a mixed solvent, wherein the volume ratio of 2-methyltetrahydrofuran to diphenyl ether is 1:

2.

2. The method according to claim 1, characterized in that, The mixed solvent is prepared by heating and holding diphenyl ether until it becomes liquid, then mixing it thoroughly with 2-methyltetrahydrofuran in a light-shielding blue-capped bottle until there is no obvious liquid separation, and then continuing to stir mechanically.

3. The method according to claim 1, characterized in that, The precursor solution is obtained by adding 4,4'-dimethylbiphenyl to a mixed solvent at a concentration of 1 mol / L, shaking and mixing thoroughly until the 4,4'-dimethylbiphenyl dissolves, and then continuing mechanical stirring.

4. The method according to claim 1, characterized in that, The active lithium solution is obtained by placing lithium foil in a precursor solution under an argon atmosphere and then mechanically stirring until the lithium foil is completely dissolved and the solution is completely opaque.

5. The method according to claim 1, characterized in that, The complete reaction refers to the following: under an argon atmosphere, an active lithium solution is added dropwise to the current collector until it is completely submerged, and then allowed to stand to react until the solution changes from dark blue to dark green, and then gradually turns to reddish brown.

6. An application of a pre-lithiated carbon-based current collector prepared by any one of the methods described in claims 1-5, characterized in that, Using lithium foil as an electrode, and assembling it together with an electrolyte and a separator, a coin cell is obtained.

7. A lithium metal battery, characterized in that, Including pre-lithiated carbon-based current collectors prepared by any of the methods described in claims 1-5.

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