A preparation method of a lithium metal high-voltage battery based on the anchoring effect of hydrophilic PTFE diaphragm and DMC

Abstract

The application provides a preparation method of a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE diaphragm and DMC, and belongs to the technical field of lithium batteries. A lithium salt is dissolved in a polymer monomer to obtain solution a, the lithium salt is dissolved in dimethyl carbonate to obtain solution b, after solution a and solution b are mixed, an initiator is added to obtain a precursor solution; after a hydrophilic PTFE diaphragm is assembled with an electrode, the precursor solution is injected, the hydrophilic PTFE diaphragm is anchored with dimethyl carbonate to carry out in-situ polymerization, and a lithium metal high-voltage battery based on the anchoring effect of the hydrophilic PTFE diaphragm and DMC is obtained. In the application, the hydrophilic PTFE diaphragm is used as a support framework, is anchored with DMC to carry out in-situ polymerization, decomposition at the positive electrode is inhibited, a thin and dense CEI is formed, and the cycle stability of the lithium metal battery under a high cut-off voltage is improved.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE diaphragm and DMC, and belongs to the technical field of lithium batteries. BACKGROUND

[0002] Lithium metal batteries are considered as the core of the next generation of energy storage technology due to their high energy density, but the application of high-voltage cathode materials (such as high-nickel layered oxides and lithium-rich manganese-based materials) exacerbates the complexity of the battery system. Under high-voltage conditions, traditional liquid electrolytes are prone to decomposition, which intensifies the side reactions at the cathode interface, forms an excessively thick cathode electrolyte interface phase (CEI), and causes active material particle rupture and transition metal dissolution, which seriously damages the cycle life. Under high-voltage conditions (>4.3 V), the cathode material is prone to lattice oxygen release and structural phase transition, which causes active particle rupture and transition metal (Ni, Mn) dissolution, and the dissolved metal ions migrate to the negative electrode surface to catalyze lithium dendrite growth, forming a vicious cycle. At the same time, the traditional liquid electrolyte (such as EC / DMC) is oxidized and decomposed under high voltage, generating impedance byproducts, which promotes the excessive thickening of the cathode electrolyte interface phase (CEI), and greatly reduces the lithium ion transmission efficiency. In addition, lithium dendrite growth, poor heat resistance of the diaphragm, and insufficient interface compatibility further limit the high-voltage performance of lithium metal batteries.

[0003] To balance the ionic conductivity and mechanical stability, gel electrolytes (GPE) have been widely studied, but their traditional preparation process is complex, and the organic solvent (such as DMC) is prone to oxidative decomposition on the surface of the high-voltage cathode, resulting in a complex CEI composition and high impedance. Existing in-situ polymerized gel electrolytes mostly rely on ether solvents (such as DME), but their oxidation resistance is weak (decomposition potential <4.0 V), making it difficult to adapt to high-voltage systems; while DMC has high oxidation stability (decomposition potential >4.5 V), but it has side reactions with lithium metal, and its solubility is limited in high-concentration lithium salt (>3 mol / L), which increases the risk of interface failure. On the other hand, commercial polyolefin diaphragms (PE / PP) have low melting points, uneven porosity, and poor electrolyte wettability, making it difficult to inhibit lithium dendrite penetration and thermal runaway, further limiting the safety of high-voltage batteries.

[0004] In view of the above problems, it is urgent to develop a battery system that has high ionic conductivity, excellent interface stability, and high-voltage compatibility. SUMMARY

[0005] Therefore, the application provides a preparation method of a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE diaphragm and DMC, which combines a hydrophilic polytetrafluoroethylene (PTFE) diaphragm with a specific in-situ polymerized gel electrolyte to inhibit the decomposition of the cathode electrolyte interface and improve the cycle stability of the battery, and the obtained battery can adapt to a high-voltage use environment of 4.3 V or higher.

[0006] Specifically, the application is realized by the following scheme:

[0007] A preparation method of a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE diaphragm and DMC, the steps being as follows:

[0008] Step one, dissolve a lithium salt in a polymer monomer to obtain solution a;

[0009] Step two, dissolve a lithium salt in dimethyl carbonate (DMC) to obtain solution b;

[0010] Step three, after mixing solution a and solution b, add an initiator to obtain a precursor solution;

[0011] Step four, after assembling a hydrophilic PTFE diaphragm and an electrode, inject the precursor solution, and the hydrophilic PTFE diaphragm and dimethyl carbonate are anchored to carry out in-situ polymerization reaction to obtain a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE diaphragm and DMC and in-situ polymerization gel electrolyte.

[0012] The above scheme uses a hydrophilic PTFE diaphragm to anchor in-situ polymerization gel electrolyte to prepare a lithium metal high-voltage battery, uses a hydrophilic polytetrafluoroethylene (PTFE) film as a support skeleton, and uses azobisisobutyronitrile (AIBN) to initiate in-situ polymerization of a precursor solution to obtain a gel polymer electrolyte. The polytetrafluoroethylene hydrophilic film can form C-H···F weak hydrogen bond interaction and n → π* interaction with DMC molecules in the in-situ polymerization gel electrolyte, thereby anchoring the DMC molecules and inhibiting their decomposition at the positive electrode, thereby forming a thin and dense CEI and improving the cycle stability of the lithium metal battery at a high cutoff voltage.

[0013] Further, as preferred:

[0014] In step one,

[0015] The polymer monomer is 2-methoxyethyl acrylate (2-MTA).

[0016] In solution a, the concentration of the lithium salt is 1-4 M.

[0017] In solution b, the concentration of the lithium salt is 4-12 M.

[0018] The lithium salt is lithium bisfluorosulfonylimide (LiFSI).

[0019] In step three,

[0020] The mixing volume ratio of solution a to solution b is 3:1-10, and is preferably 3:4-7.

[0021] The initiator is azobisisobutyronitrile (AIBN), and the addition amount is 0.5-4 wt% of the mass of the polymer monomer.

[0022] In step four,

[0023] The surface of the hydrophilic PTFE diaphragm is modified by plasma treatment or chemical grafting, and the contact angle is <40°, the porosity is ≥60%, and the pore size is 0.2-0.5 μm.

[0024] The hydrophilic PTFE diaphragm is obtained by modifying the surface of a PTFE diaphragm by plasma treatment or chemical grafting, and more preferably, the plasma treatment power is 100 W, the treatment time is 5 minutes, and the oxygen flow rate is 5 L / min.

[0025] The thickness of the hydrophilic PTFE diaphragm is 20-80 μm.

[0026] The temperature of the anchoring in-situ polymerization reaction is 60°C, and the reaction time is 10-24 h (preferably 10-12 h).

[0027] In this application, the hydrophilic PTFE diaphragm is synergistically designed with the DMC anchoring in-situ polymerization gel electrolyte:

[0028] The hydrophilic PTFE diaphragm is modified by plasma treatment or chemical grafting, and the contact angle is reduced to <40°, and the porosity is increased to ≥60%. The high porosity and uniform pore size distribution (0.2-0.5 μm) optimize the uniformity of lithium ion flow, and the weak hydrogen bond (C-H···F) and n→π* interaction of fluorine atoms anchor the DMC molecules and inhibit their oxidative decomposition, providing a reliable path for the commercialization of high-pressure lithium metal batteries. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0030] Figure 1 The long cycle performance of lithium lithium symmetrical batteries of Example 2 and Comparative Example 1 under test conditions of current density of 0.1 mA cm -2 and capacity density of 0.2 mA cm -2 is shown in the following figure:

[0031] Figure 2 The long cycle performance of lithium lithium symmetrical batteries of Example 2 and Comparative Example 1 under test conditions of current density of 0.2 mA cm -2 and capacity density of 0.2 mA cm -2The long cycle performance of lithium lithium symmetric battery under the test conditions of Example 2 and Comparative Example 1 is shown in the figure below.

[0032] Figure 3 The cycle performance of lithium / electrolyte / NCM811 full battery under constant current density in the voltage range of 2.8V-4.4V for Example 2 and Comparative Example 1 is shown in the figure below.

[0033] Figure 4 The cycle performance of lithium / electrolyte / NCM811 full battery under constant current density in the voltage range of 2.8V-4.7V for Example 2 and Comparative Example 1 is shown in the figure below. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and beneficial effects to be solved in the present application more clear and explicit, the technical solutions in the embodiments of the present application will be further described in detail below with reference to the drawings in the embodiments of the present application. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the technical solutions of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0035] Example 1

[0036] In this embodiment, the preparation and performance verification of hydrophilic PTFE separator are carried out.

[0037] 1. Material selection:

[0038] Commercially available PTFE separator (initial contact angle 110°, thickness 50 μm, pore size 0.3 μm, porosity 50%).

[0039] 2. Hydrophilic modification:

[0040] Oxygen plasma treatment (power 100 W, treatment time <5 minutes, oxygen flow rate 5 L / min) is used.

[0041] After modification, the contact angle of the separator is reduced to 25°, and the porosity is increased to 65%.

[0042] 3. Performance test:

[0043] The infiltration time of the electrolyte (DMC+LiFSI) is shortened from >60 seconds to <5 seconds.

[0044] XPS analysis shows that the content of oxygen-containing polar groups (-OH, -COOH) on the surface of the modified separator increases by 20%.

[0045] Example 2

[0046] In this embodiment, the preparation of gel electrolyte is carried out, and the steps are as follows:

[0047] Step 1, solution a: lithium bisfluorosulfonylimide (LiFSI) was dissolved in 2-methacryloxyethyl methacrylate (2-MTA) with a concentration of 2 M.

[0048] Step 2, solution b: LiFSI was dissolved in dimethyl carbonate (DMC) with a concentration of 8 M.

[0049] Step 3, precursor solution: solution a and solution b were mixed in a volume ratio of 3:7, and 1 wt% of azobisisobutyronitrile (AIBN) was added, and stirred uniformly.

[0050] Step 4, in-situ polymerization anchoring: the precursor solution was injected into the assembled full cell (NCM811 positive electrode, lithium metal negative electrode, hydrophilic PTFE separator), and placed in a 60°C oven for 12 hours. The hydrophilic PTFE separator was in-situ polymerized with DMC to form a gel electrolyte.

[0051] The loading of the NCM811 positive electrode was 3 mg / cm 2 ; the thickness of the lithium metal negative electrode was 20 μm, and the hydrophilic PTFE separator was prepared by the method of Example 1 (thickness 50 μm, pore size 0.3 μm).

[0052] Test results:

[0053] 1) Gel electrolyte: the ionic conductivity reached 1.5×10 -3 S / cm (25°C). Li + The transference number was improved to 0.58 (0.45 for the traditional PE separator system).

[0054] 2) Full cell cycle performance:

[0055] (1) Test conditions: voltage range: 3.0~4.5 V; cycle rate: 1C charge and discharge; temperature: 25°C.

[0056] (2) Results: after 300 cycles, the capacity retention rate was 87.3%, and the CEI layer thickness was 8.2 nm; the discharge capacity at 5C rate was >120 mAh / g.

[0057] Comparative Example 1

[0058] This comparative example was the same as Example 2, except that the separator was replaced with a commercial PE separator (contact angle >90°, melting point 120°C).

[0059] The results show that: the electrolyte wettability is poor, the interface contact resistance increases, the lithium ion transmission path is blocked, the polarization voltage rises, the separator shrinks at high temperature (80°C), the capacity retention rate is 55% after 100 cycles, the ion transmission efficiency is low, and the high-rate discharge capacity decreases. Moreover, the PE separator cannot anchor the DMC molecules through the C-H···F interaction, and the DMC is oxidized and decomposed on the surface of the high-voltage positive electrode (such as NCM811), forming a thick and porous CEI layer (the CEI layer thickness is as high as 24.5 nm).

[0060] By comparing Example 2 with Comparative Example 1, the synergistic advantages of the hydrophilic PTFE separator and the DMC anchoring effect are verified: the hydrophilic PTFE separator used in the application has significant advantages in wettability, high temperature resistance (327°C) and the ability to anchor DMC molecules. In the in-situ polymerization process, the hydrophilic PTFE separator not only anchors DMC and inhibits DMC decomposition, but also forms a dense CEI layer (thickness <10 nm) rich in LiF with 2-MTA / DMC mixed solvent (volume ratio 3:7) and high concentration LiFSI (8 M), which is only 1 / 3 of that of Comparative Example 1, significantly reducing the interface impedance. It also significantly reduces the risk of thermal runaway of the full battery at a 4.5 V cutoff voltage, and the in-situ gel electrolyte has no leakage, and the battery has no swelling or short circuit in a 150°C environment. Under the same conditions, the Li+ migration number is increased from 0.45 in the PE system to 0.58 in the application, the lithium deposition overpotential is reduced by 40%, the discharge capacity at 5C rate is >120 mAh / g, and the capacity retention rate is >85% after 300 cycles.

[0061] Figure 1 It is shown that: in the 800h cycle, the PTFE curve voltage fluctuation is minimal, and is close to or even better than that of the PE separator. This indicates that the PTFE separator can stably maintain the lithium metal deposition / dissolution process and is not prone to interface deterioration caused by cycling, which reflects its electrochemical compatibility and stability, which is related to the strong chemical inertness of PTFE, ensuring the voltage consistency of the battery during long-term cycling.

[0062] Figure 2 When the PE separator is used in the middle, the voltage fluctuation range expands rapidly and the strip widens as the cycle progresses, especially after 400h, indicating that the lithium deposition / dissolution process is out of control, the interface polarization intensifies, and the dendrite growth / separator damage is severe, which may eventually fail prematurely due to short circuit and other problems. The test current density is higher than before Figure 1 The PTFE separator can still control the voltage, while the PE is obviously not suitable. This also proves that the PTFE separator used in the application has significant advantages at high current.

[0063] Figure 3 and Figure 4In the full battery cycle test with NCM811 as the positive electrode and metal lithium as the negative electrode, the PTFE separator exhibits a significant advantage, regardless of whether the voltage interval is 2.8 V-4.4 V (long cycle scenario) or is increased to 2.8 V-4.7 V (high-voltage harsh scenario). Compared with the PE separator, the PTFE separator is suitable for high-nickel positive electrodes and high-voltage environments, has a gentle capacity attenuation during long cycles, and has a stable high coulombic efficiency, which can effectively inhibit lithium dendrite growth, side reactions and electrolyte degradation, and protect uniform lithium ion transmission and stable electrode interfaces.

[0064] Comparative Example 2

[0065] The present comparative example has the same settings as Example 2, except that the separator is replaced with an unmodified PTFE separator (contact angle 110°, porosity 50%).

[0066] The infiltration time of the electrolyte obtained in Step Four is longer than that of Example 2, and after 200 cycles, the capacity retention rate is only 65%; the CEI layer thickness is 22 nm, and the interface impedance increases by 45%.

[0067] Comparative Example 3

[0068] The present comparative example has the same settings as Example 2, except that in Step Three, the volume ratio of 2-MTA to DMC in the precursor solution is adjusted from 3:7 to 3:1, 3:4 and 3:10, respectively.

[0069] The results show that when the DMC ratio is too high (such as 3:10), the DMC anchoring is saturated, the PTFE separator has limited anchoring capacity for DMC, and the excess DMC is oxidized and decomposed on the positive electrode surface, forming a thick and porous CEI layer (thickness > 15 nm) with a significantly increased interface impedance; at the same time, the low 2-MTA concentration leads to a decrease in polymerization degree and a decrease in gel mechanical strength, which is prone to rupture during cycling, increasing the risk of lithium dendrite penetration. When the DMC ratio is too low (such as 3:1), DMC is almost replaced by 2-MTA and cannot be anchored by C-H···F interaction, and the DMC on the positive electrode surface is severely decomposed, forming a CEI layer rich in organic matter (thickness > 20 nm) with a significantly increased impedance. At this time, the solubility of LiFSI in high-concentration 2-MTA decreases, the actual concentration of the electrolyte decreases, and the ionic conductivity decreases to 5 x 10 -4 S / cm. The excess 2-MTA increases the rigidity of the gel, and the electrolyte cracks during battery charging and discharging, which deteriorates the contact between the active material and the electrode.

[0070] Comparative Example 4

[0071] The present comparative example has the same settings as Example 2, except that the oven standing time is adjusted from 12 hours to 6 hours and 24 hours, respectively.

[0072] SEM test shows: the oven standing time is shortened to 6 hours, the obtained gel network is loose, and cracks or holes appear in local areas, the insufficient crosslinking of monomer (2-MTA) due to insufficient reaction time, the existence of un-polymerized liquid residues in the gel electrolyte, and the ion conductivity is also reduced to 8.5 x 10 -4 S / cm, the capacity attenuation is 30% after 50 cycles, the probability of lithium dendrite penetrating the separator increases due to the insufficient mechanical strength of the gel, and the battery expansion rate increases in high-temperature tests. The oven standing time is increased to 24 hours, although the cycle stability is close to that of Example 2, the discharge capacity at 5C rate is reduced to 110 mAh / g due to the increased brittleness of the gel.

[0073] The above-described examples only express several feasible implementation manners of the present application, the description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the patent of the present application, and the examples are not used to limit the protection scope in the claims of the present application. For ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, and any equivalent implementation or change made without departing from the present application should be included in the present application.

Claims

1. A method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC, characterized in that, The steps are as follows: Step 1: Dissolve the lithium salt in the polymer monomer to obtain solution a; Step 2: Dissolve the lithium salt in dimethyl carbonate to obtain solution b; Step 3: After mixing solution a and solution b, add an initiator to obtain the precursor solution; Step four: After assembling the hydrophilic PTFE membrane and the electrode, a precursor solution is injected. The hydrophilic PTFE membrane undergoes an in-situ polymerization reaction with dimethyl carbonate (DMC) to obtain a lithium metal high-voltage battery based on the anchoring effect of the hydrophilic PTFE membrane and DMC.

2. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: In step one, the polymer monomer is 2-methoxyethyl 2-acrylate.

3. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: In step one, the lithium salt concentration is 1~4 M.

4. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: In step two, the lithium salt concentration is 4~12 M.

5. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: The lithium salt is lithium difluorosulfonylimide.

6. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: In step three, the volume ratio of solution a to solution b is 3:1~10.

7. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: In step three, the initiator is azobisisobutyronitrile (AIBN), and the amount added is 0.5 to 4 wt% of the polymer monomer mass.

8. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: In step four, the hydrophilic PTFE membrane has a contact angle of <40°, a porosity of ≥60%, and a pore size of 0.2~0.5μm.

9. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: The hydrophilic PTFE membrane is obtained by surface plasma treatment or chemical grafting modification of PTFE membrane.

10. The method for preparing a lithium metal high-voltage battery based on the anchoring effect of a hydrophilic PTFE membrane and DMC according to claim 1, characterized in that: The temperature for the anchoring in-situ polymerization reaction is 60℃, and the reaction time is 10~24 h.

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