A modified separator for lithium metal batteries and a method of making the same

Abstract

The application provides a modified diaphragm for a lithium metal battery and a preparation method thereof, the modified diaphragm has the ability to have a conversion-alloying reaction with a lithium metal anode, thereby generating a multifunctional intermediate layer in situ, the functional layer has the effects of uniform lithium ion flux, improved lithium ion migration number, improved diaphragm wetting performance and mechanical strength. The method introduces air unstable functional components into the lithium metal battery through a simple diaphragm coating means, thereby realizing dendrite-free deposition of the anode, and greatly improving the cycle life and safety performance of the lithium metal battery.

Description

Technical Field

[0001] This invention belongs to the technical field of lithium metal batteries, specifically relating to a modified separator for lithium metal batteries, its preparation method, and its application. Background Technology

[0002] The growing market for portable electronics and electric vehicles has significantly influenced the technological revolution in lithium-ion batteries (LBs), driving them towards higher energy densities. Lithium metal, due to its low density (0.53 g / cm³), has become a key factor in this technological advancement. -3 High theoretical specific capacity (3860mAh g) -1 Lithium and its low redox potential (-3.040V compared to the standard hydrogen electrode) are considered ideal anodes for next-generation high-energy-density battery systems. However, problems arise during lithium deposition, such as anode volume expansion, solid electrolyte interface layer rupture, and dendrite growth due to uneven deposition. Furthermore, lithium stripping can lead to the formation of "dead lithium" and solid electrolyte interface layer repair, resulting in a relatively short cycle life for lithium metal batteries. Worse still, uncontrolled lithium dendrites can puncture the separator, causing short circuits and triggering a series of safety hazards. These are all key issues limiting the industrialization of lithium metal batteries.

[0003] Currently, most research on solving the application problems of lithium metal anodes focuses on electrolyte regulation, designing three-dimensional current collectors, membrane modification, and the use of solid electrolytes. Using additives in the electrolyte can form an artificial solid electrolyte interface layer, but the long-term sustainability of this strategy remains to be verified as the additives are consumed. Designing three-dimensional current collectors for lithium metal can reduce local current density, thereby suppressing lithium dendrite growth, but inevitably reduces the actual energy density of the battery. Solid electrolytes, due to their lower ionic conductivity and poorer interfacial contact, are still under continuous exploration and research.

[0004] As an indispensable component of batteries, the separator has a significant impact on battery performance. Especially in lithium metal batteries, commercially available separators often exhibit uneven pore structures, which inherently lead to uneven lithium flux distribution and consequently, lithium dendrite growth. Therefore, modifying the separator to homogenize lithium flux is a crucial measure to suppress lithium dendrite formation. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a modified separator for lithium metal batteries and a method for preparing the same, which can achieve uniform lithium deposition and solve the problem of lithium dendrites in lithium metal batteries.

[0006] The present invention provides a modified separator for lithium metal batteries, wherein a modified layer is disposed on the surface of the separator. The modified layer of the separator can spontaneously undergo an in-situ reaction with lithium metal through a simple physical contact or an electrochemical process. The in-situ reaction can be divided into two steps: a conversion reaction and an alloying reaction, and finally a multifunctional interlayer composed of transition metal oxides, lithium alloys and lithium oxide is formed. The interlayer has the important functions of regulating the uniform distribution of lithium ion flux, increasing the lithium ion transference number, and improving the wettability and mechanical strength of the separator, thereby greatly prolonging the cycle life of lithium metal batteries.

[0007] The modified layer is prepared from inorganic nano ceramic powder, and has a general chemical formula of A (+m) x B (+n) y O z (0 < x / y ≤ 4, z = 0.5*(mx+ny)), wherein: A is a transition metal element with a valence state of +m, specifically including at least one of La, Y, Fe and other elements; B is a main group metal element with a valence state of +n, specifically including at least one of In, Sn, Sb, Bi and other elements.

[0008] The present invention also provides a method for preparing the above modified separator, comprising the following steps:

[0009] Step 1, synthesize A x B y O z precursor powder;

[0010] Step 2, dispersing the precursor powder in a dispersant and performing sand milling treatment to obtain nano-sized finished powder;

[0011] Step 3, uniformly mixing the finished powder with a high molecular polymer solution, coating the mixture on the surface of a separator, and drying to obtain the modified separator.

[0012] Further, in step 2: the dispersant is at least one of water, ethanol and acetone, preferably ethanol; the mass fraction of the precursor powder dispersed in the dispersant is 5% to 25%, preferably 10%.

[0013] Further, in step 2, the rotation speed of the sand milling treatment is 2000 to 3000r min -1 , preferably 2400rmin -1 ; the sand milling time is 20 to 120min, preferably 40min.

[0014] Further, in step 3, the separator is at least one of a polyethylene membrane, a polypropylene membrane, a polyethylene / polypropylene / polyethylene three-layer membrane, a glass fiber separator, and a non-woven fabric membrane.

[0015] Furthermore, the thickness of the membrane modification layer is 1–20 μm.

[0016] The present invention does not impose any special restrictions on the synthesis scheme of the precursor powder. Solid-phase method, co-precipitation method and combustion method known to those skilled in the art are all acceptable, with combustion method being preferred.

[0017] The present invention does not impose any special restrictions on the coating method; any coating method known to those skilled in the art is acceptable, with scraping being preferred.

[0018] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0019] 1. This method provides a modified separator for lithium metal batteries. This separator is reactive with lithium metal and can generate a multifunctional intermediate layer composed of transition metal oxides, lithium alloys, and lithium oxide in situ through a conversion-alloying reaction. This intermediate layer plays an important role in regulating the uniform distribution of lithium-ion flux, increasing the lithium-ion transport number, and improving the wettability and mechanical strength of the separator, thereby significantly extending the cycle life of lithium metal batteries.

[0020] 2. This method introduces air-unstable functional components into lithium metal batteries through a simple separator coating process, achieving dendrite-free lithium deposition and thus significantly improving the cycle life and safety performance of lithium metal batteries. Furthermore, the modified separator has a simple preparation process, is lightweight, and has minimal impact on the actual energy density of the battery. Attached Figure Description

[0021] Figure 1 The images shown are scanning electron microscope (SEM) images (a, b) and X-ray diffraction patterns (c) of the precursor powder and the finished powder in Example 1.

[0022] Figure 2 The images are scanning electron microscope (SEM) images of the surface (a) and cross-section (b) of the modified diaphragm in Example 2.

[0023] Figure 3 These are optical photographs of the diaphragm before (a), after (b), and after (c, d) the reaction in Example 3.

[0024] Figure 4 The images show the in-situ X-ray diffraction pattern (a) and the non-in-situ X-ray photoelectron spectrum (b) of the reaction between the modified membrane and lithium metal in Example 4.

[0025] Figure 5 The deposition morphology of lithium metal in a lithium-copper half-cell using a commercial polypropylene separator (a) and a modified separator (b) in Example 5 is shown.

[0026] Figure 6 The lithium-ion transference number measurements for commercial polypropylene membranes (a) and modified membranes (b) used in Example 6 are shown in the inset, with electrochemical impedance spectroscopy before and after polarization.

[0027] Figure 7 The rate (a) and cycle performance (b) of the Li-LiFePO4 battery using commercial polypropylene membrane and modified membrane in Example 7 are shown. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, the drawings and embodiments do not constitute a limitation of this invention. This invention can be implemented in many different forms and is not limited to the embodiments described herein.

[0029] Example 1

[0030] Yttrium nitrate hexahydrate and bismuth nitrate pentahydrate were dissolved in dilute nitric acid at pH 2 at a molar ratio of 1:3. Citric acid monohydrate, in an equimolar amount to the metal ions, was added. The pH was adjusted to 7 using ammonia. The homogeneous solution was stirred and heated until a combustion reaction occurred. The product was then heated at 700°C for 3 hours in air to obtain the precursor powder. A 10% (w / w) ethanol dispersion of the precursor powder was then incubated at 2400 rpm. -1 The precursor powder was milled at a certain speed for 40 minutes to obtain the finished powder. Electron microscopy and X-ray diffraction were then performed on the precursor powder and the finished powder.

[0031] Figure 1 The electron microscope (SEM) images (a, b) and X-ray diffraction pattern (c) of the precursor powder and finished powder prepared in Example 1 of this invention show that nanoparticles of size A were successfully prepared. x B y O z (A: Y, B: Bi, x=0.5, y=1.5, z=3).

[0032] Example 2

[0033] The iridium-stabilized bismuth oxide powder prepared in Example 1, poly(vinylidene fluoride-co-hexafluoropropylene), and N-methylpyrrolidone were mixed at a mass ratio of 1:0.15:3 and ball-milled for 0.5 h to obtain a slurry. This slurry was then coated onto a commercial polypropylene membrane and dried in an oven at 80°C for 12 h to obtain a modified membrane, which was then punched into discs with a diameter of 16 mm for later use. The membrane was subjected to electron microscopy, and the results are shown in the figure below. Figure 2 As can be seen, the modified membrane layer is relatively dense and flat, with a thickness of about 7μm.

[0034] Example 3

[0035] This embodiment shows optical photographs before and after diaphragm modification and before and after the reaction.

[0036] Using lithium metal as the positive and negative electrodes, and 1M LiTFSIDME / DOL (volume ratio 1:1) with 2% LiNO3 as the electrolyte, a Li-Li symmetric cell was assembled using the modified separator from Example 2. 0.05 mA / cm² -2 After 50 constant current cycles at the current density, the battery was disassembled in an argon glove box, the separator was rinsed with DME, and optical photographs of both sides of the separator after the reaction were taken and compared with the initial commercial polypropylene separator and the modified separator in Example 2.

[0037] See results Figure 3 Commercial polypropylene separators are white discs (a), modified separators in Example 2 are pale yellow in color with the modified layer (b), and turn black after the reaction (c), while the unmodified side remains white (d) of the commercial polypropylene separator.

[0038] Example 4

[0039] This embodiment explores the products of the conversion-alloying reaction between the modification layer and lithium metal.

[0040] Using lithium metal as the negative electrode, a beryllium window as the positive electrode, and 1M LiTFSIDME / DOL (volume ratio 1:1) plus 2% LiNO3 as the electrolyte, a Li-Be half-cell was assembled in an argon glove box using the modified separator from Example 2. The phase changes of the modified separator layer during the first discharge-charge cycle were observed in situ (a). Furthermore, using lithium metal as both the positive and negative electrodes, and 1M LiTFSIDME / DOL (volume ratio 1:1) plus 2% LiNO3 as the electrolyte, a Li-Li symmetric cell was assembled using the modified separator from Example 2. 0.05 mA cm -2 After 50 cycles of constant current cycling at current density, the battery was disassembled in an argon glove box, the electrode plates were rinsed with DME, and after drying, the modified separator layer was subjected to X-ray photoelectron spectroscopy (b).

[0041] Figure 4 The in-situ X-ray diffraction pattern (a) characterizes the reaction products of the modified separator and lithium metal in Example 4. The pattern shows that as discharge proceeds, the YSB peak gradually disappears, while the Bi peak rapidly appears and then transforms into the corresponding Li3Bi peak, confirming the occurrence of the conversion-alloying reaction. X-ray photoelectron spectroscopy (b) detects signals from Li3Bi and Y2O3, further confirming that the reaction products are transition metal oxides (Y2O3 in this case), lithium alloys (Li3Bi in this case), and lithium oxide (Li2O).

[0042] Example 5

[0043] This embodiment compares the effects of modified membranes and commercial polypropylene membranes on lithium deposition morphology.

[0044] Li-Cu half-cells were assembled using lithium metal as the negative electrode, copper foil as the positive electrode, and 1M LiTFSIDME / DOL (volume ratio 1:1) with 2% LiNO3 as the electrolyte. The modified separator from Example 2 and a commercial polypropylene separator were used, respectively. The Li-Cu half-cells were subjected to constant current discharge at a current density of 1 mA / cm². -2 The discharge time was 3 hours, during which 3 mAh / cm³ of charge was deposited on the copper foil. -2 Lithium metal was used. The battery was disassembled in an argon glove box, the electrodes were rinsed with DME, dried, and then subjected to scanning electron microscopy. The results are shown in […]. Figure 5 .

[0045] Figure 5 The image shows the lithium deposition morphology in Example 5. In contrast, the lithium deposited in the battery using the modified separator exhibits a dense and smooth surface, indicating that the modified separator is beneficial for suppressing lithium dendrites.

[0046] Example 6

[0047] This embodiment tested lithium-ion transference numbers based on modified membranes and commercial polypropylene membranes.

[0048] Lithium metal was used as both the positive and negative electrodes. A 1M LiTFSIDME / DOL solution (volume ratio 1:1) with 2% LiNO3 was used as the electrolyte. Li-Li symmetric cells were assembled using the modified separator from Example 2 and a commercial polypropylene separator, respectively. Current-time response curves at a polarization voltage of 10 mV and electrochemical impedance spectroscopy before and after polarization were measured. The lithium-ion transference number was calculated using the following formula:

[0049]

[0050] Among them, I s For steady-state current, I o R is the initial current, ΔV is the DC polarization voltage pulse (10mV), and R is the voltage at rest. o and R s These are the initial and steady-state interface resistances, respectively.

[0051] Figure 6 These are the lithium-ion transference number test results from Example 6. Thanks to the adsorption of anions by the transition metal oxide (Y₂O₃ in this case), the lithium-ion transference number increased from 0.37 for the commercial polypropylene separator to 0.79 for the modified separator. Due to the increased transference number, dendrite nucleation and growth are effectively suppressed, which facilitates dendrite-free deposition, resulting in longer cycle life and higher safety performance.

[0052] Example 7

[0053] This embodiment tested the cycle and rate performance of Li-LiFePO4 batteries based on modified separators and commercial polypropylene separators.

[0054] Li-LiFePO4 batteries were assembled in an argon glove box using lithium metal as the negative electrode, LiFePO4 as the positive electrode, and 1M LiTFSIDME / DOL (volume ratio 1:1) plus 2% mass fraction of LiNO3 as the electrolyte. The modified separator from Example 2 and a commercial polypropylene separator were used respectively.

[0055] Preparation method of LiFePO4 positive electrode sheet: A slurry was prepared with lithium iron phosphate powder, carbon black, carbon nanofibers, polyvinylidene fluoride-hexafluoropropylene, polyvinylpyrrolidone, and N-methylpyrrolidone in a mass ratio of 37.43:4.68:2.34:4.94:1.23:49.38. The slurry was then planetarily ball-milled for 2 hours to obtain a uniform, viscous liquid slurry, which was then cast onto a PET carrier using a doctor blade. Next, it was rapidly transferred to deionized water and immersed for 2 hours to complete the phase inversion process. The electrode film was then removed. Finally, the electrode was dried in an 80°C oven.

[0056] The assembled Li-LiFePO4 battery was subjected to constant current charge-discharge testing, and the performance was obtained as follows: Figure 7 .

[0057] Figure 7 This is a graph showing the rate and cycle performance of the Li-LiFePO4 battery obtained in Example 7. The battery using the modified separator has a rate of 87 mAh / cm³ at 10C. -2 Capacity (1C = 160mAh g) -1 (While commercial polypropylene separators only have a capacity of 67mAh / cm³), -2 The capacity (a). Under high LiFePO4 loading, the battery using the improved separator retains approximately 80% of its capacity after 1700 cycles at 1C, and its cycle stability is significantly higher than that of the battery using a commercial polypropylene separator (b).

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A modified separator for lithium metal batteries, characterized in that: The modified separator is provided with a modified layer on the surface of the separator, and the general chemical formula of the inorganic nano-ceramic powder for preparing the modified layer is A (+m) x B (+n) y O z z, wherein: 0<x / y≤4, z=0.5×(mx+ny); A is at least one of transition metal elements La and Y, with an element valence of +m; B is at least one of main group metal elements In, Sn and Bi, with an element valence of +n; The modified layer can undergo a conversion reaction and alloying reaction with the lithium metal anode through an electrochemical process to form a multifunctional intermediate layer; the intermediate layer is composed of an oxide of transition metal element A, an alloy of lithium and main group metal element B, and lithium oxide Li2O.

2. The modified separator for lithium metal batteries according to claim 1, characterized in that: The thickness of the modified layer is 1~20 μm.

3. A method for preparing the modified diaphragm according to claim 1 or 2, characterized in that, Includes the following steps: Step 1, Synthesize A x B y O z Precursor powder; Step 2: Disperse the precursor powder in a dispersant and perform sand milling to obtain nano-sized finished powder; Step 3: After the finished powder is mixed evenly with the polymer solution, it is coated on the surface of the diaphragm and dried to obtain the modified diaphragm.

4. The preparation method according to claim 3, characterized in that: In step 2, the dispersant is at least one of water, ethanol and acetone, and the precursor powder is dispersed in the dispersant at a mass fraction of 5% to 25%.

5. The preparation method according to claim 3, characterized in that: In step 2, the grinding speed is 2000~3000 rpm and the grinding time is 20~120 min.

6. The preparation method according to claim 3, characterized in that: In step 3, the diaphragm is at least one of polyethylene film, polypropylene film, polyethylene / polypropylene / polyethylene three-layer film, glass cellulose diaphragm, and non-woven fabric film.

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