Lithium metal battery and modified negative electrode thereof

Abstract

The invention discloses a lithium metal battery and a modified cathode thereof. The preparation method of the negative electrode comprises the following steps: providing a clean metal lithium sheet; providing, as a treatment agent, an organic solution containing a compound of general formula R-A-X, where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, A is at least one metal selected from the group consisting of Mg, Al, Zn, K, Na, Ca, Sn, In and Fe, and X is at least one halogen selected from the group consisting of F, Cl, Br, I and At; and carrying out surface chemical treatment on the metal lithium sheet by using a treating agent under a protective atmosphere to prepare the negative electrode. The interface layer is formed on the surface of the lithium metal, so that on one hand, the lithium alloy contained in the interface layer can remarkably promote rapid transmission of lithium ions and effectively inhibit growth of lithium dendrites; on the other hand, the contained halide can be dissolved in the electrolyte in the cycle process of the battery to react with dead lithium generated by the lithium metal negative electrode to generate active lithium, so that the performance of the battery is remarkably improved.

Description

Technical Field

[0001] The present invention generally relates to lithium metal batteries. Background Art

[0002] Lithium metal faces many challenges in practical applications. In addition to the flammability and volatility of traditional organic electrolytes, lithium metal has high chemical activity and is prone to react with other materials, inducing uneven lithium deposition, thereby forming an unstable and non-dense solid electrolyte interface film. This may cause lithium dendrites to pierce the separator during cycling, forming dead lithium, reducing the Coulombic efficiency of the battery, causing battery short circuits, and seriously hindering the practical application process of lithium metal batteries. Summary of the Invention

[0003] An object of the present invention is to provide a lithium metal battery with stable performance.

[0004] According to one aspect of the present invention, there is provided a method for preparing a negative electrode for a liquid lithium metal battery, including:

[0005] Providing a clean metal lithium sheet;

[0006] Providing an organic solution containing a compound with the general formula R-A-X as a treatment agent, where R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, A is at least one metal selected from the group consisting of Mg, Al, Zn, K, Na, Ca, Sn, In, and Fe, and X is at least one halogen selected from the group consisting of F, Cl, Br, I, and At; and

[0007] Using the treatment agent to perform surface chemical treatment on the metal lithium sheet under a protective atmosphere to obtain the negative electrode.

[0008] According to the preparation method of the present invention, the concentration of the compound in the treatment agent is preferably 30-170 mM, more preferably 50-150 mM.

[0009] According to the preparation method of the present invention, it is also preferably included: directly performing a drying treatment under a protective atmosphere after the surface chemical treatment of the metal lithium sheet.

[0010] According to the preparation method of the present invention, the surface chemical treatment may include treatment methods such as infiltration, spraying, and spin coating to form an interface layer on the surface of the metal lithium negative electrode.

[0011] According to another aspect of the present invention, there is also provided a lithium metal battery, including an electrolyte and a negative electrode prepared according to the above method.

[0012] According to the lithium metal battery of the present invention, the thickness of the negative electrode interface layer is preferably 1-1.5 μm.

[0013] According to the lithium metal battery of the present invention, the electrolyte may include:

[0014] select at least one from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), fluorinated phosphate (FEBFP), tetrahydrofuran (THF), ethylene glycol dimethyl ether (DME), and dioxolane (DOL);

[0015] select at least one from the group consisting of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluoro(oxalato)borate (LiDFOB), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), and lithium trifluoromethanesulfonate (LiCF3SO3); and / or

[0016] at least one from the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), lithium nitrate (LiNO3), or propane sultone (PS).

[0017] After surface chemically treating lithium metal with the treatment agent of the present invention, a stable interfacial layer containing a lithium alloy and a halide can be constructed on its surface. Among them, the lithium alloy can induce uniform deposition of lithium ions, and the halide can in-situ eliminate the dead lithium generated during the cycling of the lithium metal negative electrode by regulating the voltage activation method. The specific chemical reaction during surface chemical treatment is:

[0018] R-A-X + 2Li → Li-R + A + Li-X

[0019] After the reaction, the metal element exists in the form of an alloy, while the halogen exists in the form of a lithium salt. During the cycling of the lithium metal battery, the lithium salt containing halogen will dissolve in the electrolyte and exist in the form of ion X - , and X - can remain stable during the cycling process without composition change.

[0020] During the charge and discharge cycling of the lithium metal battery, dead lithium will be generated on the lithium metal negative electrode. The main components of the dead lithium are Li or Li2O, etc. These dead lithium will accumulate and hinder the transmission of lithium ions, resulting in a rapid decline in the battery capacity. By CV testing, the voltage value of the X - → X3 - ion pair (the range can be, for example, 2.5V - 4V) during the battery charging process, so as to control the appropriate charging voltage, and the halogen redox ion pair can be in-situ activated to cause its chemical change of X - → X3 - . Subsequently, X3 - will react with the dead lithium generated on the lithium metal negative electrode to generate active lithium (Li +), the capacity of the lithium metal battery is restored:

[0021] 3Li2O+3X3 - =6Li + +XO3 - +8X -

[0022] 2Li+X3 - =2Li + +3X -

[0023] After the reaction, the halogen ion undergoes X3 - →X - The halogen ions are still in the form of X - exists in a stable form.

[0024] The present invention forms an interface layer on the surface of lithium metal. On the one hand, the lithium alloy contained therein can significantly promote the rapid transmission of lithium ions in the negative electrode, reduce the nucleation barrier of lithium, and effectively inhibit the growth of lithium dendrites; on the other hand, the lithium halide contained therein can react with the dead lithium generated by the lithium metal negative electrode during the battery cycle to generate active lithium, thereby significantly improving the cycle stability of the battery.

[0025] The present invention uses CV test to test the battery charging process. - →X3 - The voltage value of the ion pair can be controlled to control the appropriate charging voltage and realize the in-situ activation of dead lithium, thereby significantly improving the cycle performance of the battery.

[0026] The lithium metal battery negative electrode of the present invention has high ionic conductivity and low interface impedance, and exhibits good interface compatibility, long cycle stability and safety and reliability.

[0027] The method of the present invention is simple to operate and is compatible with a variety of lithium metal battery electrolyte systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 1 is a process flow chart according to an embodiment of the present invention.

[0029] Figure 2a , Figure 2b and Figure 2c The electronic images of lithium metal negative electrodes prepared according to different embodiments of the present invention are shown respectively.

[0030] Figure 3a and Figure 3b The following are scanning electron microscope images of lithium metal negative electrodes prepared according to different embodiments of the present invention.

[0031] Figure 4 : is the XRD spectrum of the lithium metal negative electrode prepared according to Example 1 of the present invention.

[0032] Figure 5a is a scanning electron microscope of the cross-sectional morphology of the lithium metal anode prepared according to Example 1 of the present invention; Figure 5b and Figure 5c are the EDS spectra of Mg and Br thereof, respectively.

[0033] Figure 6 is the charge-discharge curve of the symmetric battery according to Example 1 of the present invention.

[0034] Figure 7 is the charge-discharge cycle curve of the lithium iron phosphate full battery according to Example 1 of the present invention.

[0035] Figure 8a and Figure 8b are the CV curves of different scanning voltage ranges of the lithium iron phosphate full battery according to Example 1 of the present invention, respectively.

[0036] Figure 9 is the charge-discharge cycle curve of the in-situ dead lithium activation process of the lithium iron phosphate full battery according to Example 1 of the present invention. Detailed Embodiments

[0037] The present invention will be described in detail below with reference to examples and comparative examples. Those skilled in the art should understand that these examples and comparative examples are only used to explain rather than limit the present invention. Unless otherwise specified, the raw materials used can be obtained commercially.

[0038] Example 1

[0039] S1. In a glove box filled with Ar, the lithium metal anode was polished by a rolling method and cut into a 1.2 cm diameter original piece (thickness: 50 μm);

[0040] S2. Ethylmagnesium bromide (C2H5BrMg) was dissolved in tetrahydrofuran to form a treatment solution (where the concentration of ethylmagnesium bromide was 100 mM), and the lithium metal anode was immersed therein for 10 s and then placed in a clean glass dish for standby;

[0041] S3. The above-treated lithium metal was naturally dried in a glove box filled with Ar to obtain a lithium metal with a corresponding lithium-magnesium alloy and halide interface layer (thickness: about 1.25 μm) on the surface;

[0042] S4. The processed lithium metal anode, a separator (with a diameter of 1.9 cm), a lithium iron phosphate cathode (with a thickness of 50 μm), and an electrolyte with the following composition: the solvent is a 1:1 volume ratio of ethylene glycol dimethyl ether (DME) and dioxolane (DOL), the solute is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the additive is 65 μL of lithium nitrate with a mass ratio of 1% (1 M LiTFSI in DME:DOL = 1:1:Vol% with 1% LiNO3) are assembled into a coin cell. During the cycling process, the charge-discharge voltage range is controlled to be 2.4 V - 3.8 V.

[0043] Figure 1 The flowchart of the above anode treatment process is shown.

[0044] Example 2

[0045] Others are the same as in Example 1, except that in step S2, the treatment solution is evenly sprayed on the surface of the lithium metal anode by the spray method. The spray time is 10 s, the inner diameter of the nozzle is 0.5 mm, and the rotational speed of the peristaltic pump is 750 mL h -1 。

[0046] Example 3

[0047] Others are the same as in Example 1, except that in the treatment solution formed in step S2, the concentration of ethylmagnesium bromide is 50 mM.

[0048] Example 4

[0049] Others are the same as in Example 1, except that in the treatment solution formed in step S2, the concentration of ethylmagnesium bromide is 150 mM.

[0050] Example 5

[0051] Others are the same as in Example 1, except that in step S2, ethylaluminum bromide (C2H5BrAl) is dissolved in tetrahydrofuran to form a treatment solution (where the concentration of ethylaluminum bromide is 100 mM).

[0052] Example 6

[0053] Others are the same as in Example 1, except that in step S2, ethylmagnesium iodide (C2H5IMg) is dissolved in tetrahydrofuran to form a treatment solution (where the concentration of ethylmagnesium iodide is 100 mM).

[0054] Example 7

[0055] Others are the same as in Example 1, except that in step S2, diethylaluminum iodide (C4H 10AlI) was dissolved in tetrahydrofuran to form a treatment solution (where the concentration of diethylaluminum iodide was 100 mM). Additionally, the treatment solution was uniformly dropped onto the surface of the lithium metal anode by spin coating, with a dropping amount of 0.1 mL, a rotation speed of 800 r / min, and a duration of 10 s.

[0056] Example 8

[0057] Others were the same as in Example 7, except that in step S2, 4-ethylphenylzinc iodide (C8H9IZn) was dissolved in tetrahydrofuran to form a treatment solution (where the concentration of 4-ethylphenylzinc iodide was 100 mM).

[0058] Comparative Example 1

[0059] Others were the same as in Example 1, except that after the immersion treatment in step S2, the halides on the surface of the lithium metal anode were washed away with tetrahydrofuran.

[0060] Comparative Example 2

[0061] Others were the same as in Example 1, except that in step S2, only tetrahydrofuran was used as the immersion treatment solution (without adding ethylmagnesium bromide).

[0062] Morphology and Structure Performance Test

[0063] The electron micrographs of the surfaces of the lithium metal anodes treated in Example 1, Example 3, and Example 4 are respectively as Figure 2a , Figure 2b and Figure 2c shown.

[0064] The surface morphologies of the lithium metal anodes treated in Example 1 and Example 3 were observed using a scanning electron microscope (SEM), and the results are respectively shown in Figure 3a and Figure 3b . It can be observed that a uniform and dense interfacial layer was formed on the surface of the lithium metal anode.

[0065] The cross-sectional morphology of the lithium metal treated in Example 1 was observed using a scanning electron microscope (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS), and the cross-sectional morphology results are shown in Figure 5a . Figure 5b and 5c are the EDS energy spectra of the cross-section of the treated lithium metal, and obvious Mg and Br element distributions on the surface of the interfacial layer can be obtained.

[0066] The lithium metal anode treated in Example 1 was placed in an X-ray diffractometer for scanning tests, and the test results are as Figure 4 shown. Referring to Figure 4 , it can be known that after surface (dip coating) treatment of the lithium metal, Li3Mg7 alloy was formed, with large peak intensity, high crystallinity, and high characteristic peak intensity, proving that a complete crystal form had been formed.

[0067] Cyclic stability test

[0068] Assemble a symmetric battery according to the conventional method and perform charge-discharge cycle tests using a blue battery system.

[0069] At 25 °C and a charge-discharge current density of 1 mA cm -2 , the symmetric battery assembled with the lithium metal anode prepared in Example 1 can be cycled stably for more than 700 h with small and constant polarization; see Figure 6 .

[0070] At 25 °C and a charge-discharge rate of 1C, the lithium iron phosphate full battery assembled with the lithium metal anode prepared in Example 1 can be cycled stably for more than 250 cycles with a Coulombic efficiency of more than 99%; see Figure 7 .

[0071] CV test

[0072] Assemble a full battery with the lithium metal anode prepared in Example 1 according to the conventional method and perform cyclic voltammetry tests using an electrochemical workstation. The test results are shown in Figure 8:

[0073] Referring to Figure 8, at 25 °C and a scan rate of 0.1 mV / s, when the scanning voltage ranges are 2.4 V - 3.8 V ( Figure 8a ) and 2.4 V - 4.0 V ( Figure 8b ), it is measured that at about 3.95 V, an activation reaction of ion pair Br - →Br3 - occurs.

[0074] Dead lithium activation performance test

[0075] Assemble a full battery with the lithium metal anode prepared in Example 1 according to the conventional method and perform charge-discharge cycle tests using a blue battery system. The test results are as Figure 9 shown.

[0076] Figure 9 is the in-situ dead lithium activation process of the lithium iron phosphate full battery in Example 1 of the present invention. When a 50-μm-thick lithium is cycled 200 times at 6C and a voltage range of 2.4 V - 3.8 V, the capacity significantly decays. Subsequently, the voltage range is set to 2.4 V - 4.0 V. Within this voltage range, after cycling 20 times, it can be seen from the CV test results that at a charging voltage of 3.95 V, the conversion of Br - →Br3 - occurs. Subsequently, Br3 - reacts with the dead lithium or Li2O passivation layer generated on the lithium metal anode side to generate active lithium:

[0077] 3Li2O + 3Br3- = 6Li + + BrO3 - + 8Br -

[0078] 2Li + Br3 - = 2Li + + 3Br -

[0079] After successful activation, more active lithium is generated on the lithium metal anode side, and the capacity increases significantly.

[0080] Referring to Figure 8 and Figure 9 , a stable lithium-magnesium alloy + lithium halide functional layer will form on the surface of the lithium metal anode prepared according to the present invention. During cycling, the lithium halide can be activated by voltage regulation, thereby in-situ eliminating dead lithium and avoiding waste caused by secondary addition of halogen-containing salts (activating dead lithium). In summary, the lithium metal treatment method of the present invention is simple and easy to implement, has high cycle stability, and is convenient for commercial application.

Claims

1. A method for preparing a negative electrode for a liquid lithium metal battery, comprising: Provide clean lithium metal sheets; Providing an organic solution containing a compound of the general formula RAX as a treating agent, wherein R is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, A is at least one metal selected from the group consisting of Mg, Al, Zn, K, Na, Ca, Sn, In and Fe, and X is at least one halogen selected from the group consisting of F, Cl, Br, I and At; and The negative electrode is prepared by chemically treating the surface of a metal lithium sheet using a treating agent under a protective atmosphere.

2. The preparation method according to claim 1, wherein the concentration of the compound in the treating agent is 30 to 170 mM.

3. The preparation method according to claim 1, further comprising: After the metal lithium sheet is subjected to surface chemical treatment, it is directly dried under a protective atmosphere.

4. A lithium metal battery comprising an electrolyte and a negative electrode prepared by the method according to any one of claims 1 to 3. 5 . The lithium metal battery according to claim 4 , wherein the thickness of the interface layer formed on the lithium metal negative electrode by the treating agent is 1 to 1.5 μm.

6. The lithium metal battery according to claim 4, wherein the electrolyte comprises: At least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), fluorinated phosphate (FEBFP), tetrahydrofuran (THF), ethylene glycol dimethyl ether (DME) and dioxolane (DOL); At least one selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium bisfluorosulfonyl imide (LiFSI), lithium difluorooxalatoborate (LiDFOB), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium bisoxalatoborate (LiBOB) and lithium trifluoromethanesulfonate (LiCF3SO3); and / or At least one of the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), lithium nitrate (LiNO3) or propane sultone (PS).

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