Gradient artificial SEI layer and preparation method thereof, negative electrode containing SEI layer and solid-state battery

Abstract

The invention provides a gradient artificial SEI layer and a preparation method thereof, a negative electrode containing the SEI layer and a solid-state battery, the gradient artificial SEI layer comprises a surface layer, a middle layer and an inner layer which are sequentially stacked, the surface layer comprises lithium selenide and lithium fluoride, the middle layer comprises Li3PS4, and the inner layer comprises Li2S; in the gradient artificial SEI layer, the surface layer faces the solid electrolyte layer, and the inner layer faces the lithium negative electrode. The gradient artificial SEI layer can regulate and control the interface between the solid electrolyte layer and lithium metal and reduce the interface impedance, meanwhile, Li2S in the gradient artificial SEI layer is high in stability and not easy to decompose, the problems of chemical instability of the interface between the lithium metal negative electrode and the electrolyte and out-of-control interface impedance are solved, and the lithium metal negative electrode has good electrochemical performance. And finally, the cycle performance of the solid-state battery is improved and the interface impedance is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a gradient artificial SEI layer and its preparation method, as well as an anode containing an SEI layer and a solid-state battery. Background Technology

[0002] Because all-solid-state batteries use non-flammable solid electrolytes (such as sulfide Li6PS5Cl), their theoretical energy density has a significant advantage over liquid lithium-ion batteries, and the risk of thermal runaway is completely eliminated. However, the lithium metal anode interface of current sulfide solid-state batteries faces three major bottlenecks: (1) Chemical instability: The interface phase represented by Li2S undergoes an irreversible oxidation reaction (2Li2S→Li2S2+2Li) when the charging voltage is >3.6V. + +2e - This leads to the loss of active sulfur, as analyzed by XPS (Ar) deep profiling. + (Sputtering rate of 0.5 nm / s) confirms that the interface S in the traditional Li-PS system after 100 cycles 2- The content decreased sharply from the initial 78at% to 35at%, and the byproduct Li2S2 accumulated to 41at%, thus causing an increase in porosity; (2) Process damage: For example, plasma deposition requires the application of 50-200W of radio frequency power in a vacuum environment. Infrared thermal imaging shows that the local temperature of lithium foil reaches 185-210℃, inducing micro-melting areas (microhardness HV 0.012, normal area HV 0.038), and the thermal stress crack density is as high as 12.3 cracks / mm. 2 (3) Solvent residue defects: Although the solvents such as NMP and DMF used in the wet coating process are removed by vacuum drying at 80°C for 12 hours, TOF-SIMS will still detect residual molecular fragments, forming micropores of 50-200nm, which increases the risk of dendrite penetration by 45%.

[0003] Existing technologies have addressed the aforementioned problems by preparing LiF / Li3Sb composite layers and using solution dip-coating methods. However, current mainstream solutions all suffer from serious drawbacks: ① For example, Sb may form at the sulfide electrolyte interface of the LiF / Li3Sb composite layer. 3+ ① Migration reaction generates a high-resistivity Li3SbS3 phase; ② PVDF binder forms an ion-blocking layer at the LSPX interface, with EIS fitting showing an ionic conductivity of only 6.8 × 10⁻⁶. -5 S / cm is less than 1 / 30 of the bulk sulfide.

[0004] Based on the above research, there is a need to provide a gradient artificial SEI layer, which can significantly reduce the interface impedance of lithium metal anode and improve the chemical stability of the interface. Summary of the Invention

[0005] The purpose of this invention is to provide a gradient artificial SEI layer and its preparation method, an anode containing the SEI layer, and a solid-state battery. The gradient artificial SEI layer can regulate the interface between the solid electrolyte layer and lithium metal, reduce the interface impedance, and the Li2S in the gradient artificial SEI layer has high stability and is not easily decomposed, thus solving the problems of chemical instability and interface impedance runaway at the interface between the lithium metal anode and the electrolyte. Ultimately, this achieves improved cycle performance and a significant reduction in interface impedance of the solid-state battery.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a gradient artificial SEI layer, the gradient artificial SEI layer comprising a surface layer, an intermediate layer and an inner layer stacked sequentially, the surface layer comprising lithium selenide and lithium fluoride, the intermediate layer comprising Li3PS4, and the inner layer comprising Li2S.

[0008] In the gradient artificial SEI layer, the surface layer faces the solid electrolyte layer, and the inner layer faces the lithium anode.

[0009] This invention addresses the chemical instability of the lithium metal anode-electrolyte interface in solid-state batteries (Li2S oxidation and decomposition at >3.6V leading to sulfur loss exceeding 40%) and uncontrolled interface impedance (byproduct accumulation causing impedance >85Ω·cm2). It constructs a gradient artificial SEI layer, ultimately achieving improved full-cell cycle life and a significant reduction in interface impedance. Specifically, the artificial SEI layer of this invention has a three-layer gradient structure. The inner layer includes Li2S and faces the lithium anode, contacting it and significantly reducing the lithium anode interface impedance, thus improving chemical interface stability. The outer layer includes lithium selenide and lithium fluoride, facing the solid electrolyte layer and contacting it, significantly improving the interface stability between the SEI layer and the solid electrolyte, reducing interface side reactions and impedance growth. Simultaneously, Se doping suppresses Li2S decomposition at high voltage, further enhancing the electrochemical stability of the entire SEI layer. The middle layer includes Li3PS4, which provides an efficient lithium-ion transport path, reducing overall interface impedance, and also serves as a structural support layer, enhancing the mechanical integrity of the SEI layer and suppressing crack formation and dendrite penetration. Therefore, the gradient artificial SEI layer described in this invention modulates the interface between the solid electrolyte layer and lithium metal, ultimately achieving improved cycle performance and a significant reduction in interface impedance of the solid battery.

[0010] Preferably, the thickness of the inner layer is 50nm-100nm, for example, it can be 50nm, 60nm, 70nm, 80nm, 90nm or 100nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0011] Preferably, the content of Li2S in the inner layer is >60at%, for example, it can be 65at%, 70at%, 75at%, 85at%, or 90at%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0012] In the inner layer of the present invention, the content of Li2S is relatively high, which can effectively reduce the interface impedance. In addition, the inner layer may contain unreacted lithium, a small amount of Li3P, trace amounts of Li2Se or LiF diffused from the surface, and trace impurities (such as Li2O) that may be introduced during the process.

[0013] Preferably, the thickness of the intermediate layer is 100nm-300nm, for example, it can be 100nm, 150nm, 200nm, 250nm or 300nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0014] Preferably, the content of Li3PS4 in the intermediate layer is ≥90wt% (the remainder is unreacted Li2S, a small amount of Li3P, trace amounts of Li2Se or LiF diffused from the surface, and possible amorphous phases or impurities), for example, it can be 90wt%, 92wt%, 94wt%, 96wt%, 98wt% or 100wt%, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0015] Preferably, the thickness of the surface layer is 50nm-100nm, for example, it can be 50nm, 60nm, 70nm, 80nm, 90nm or 100nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Preferably, in the surface layer, the molar ratio of lithium selenide to lithium fluoride is 1:(0.5-2.0), for example, it can be 1:0.5, 1:1, 1:1.5 or 1:2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] Preferably, the thickness of the gradient artificial SEI layer is 200nm-500nm, for example, it can be 200nm, 300nm, 400nm or 500nm, and the ionic conductivity is >1.2×10⁻⁶. -4 S / cm, for example, could be 1.3 × 10 -4 S / cm, 1.5×10 -4 S / cm, 2.0×10 -4 S / cm, 2.5×10 -4 S / cm, 3.0×10 -4 S / cm, 3.5×10 -4S / cm or 4×10 -4 S / cm, but not limited to the listed values, other unlisted values ​​within the range also apply.

[0018] Secondly, the present invention provides a method for preparing a gradient artificial SEI layer as described in the first aspect, the method comprising the following steps:

[0019] (1) Phosphorus pentasulfide, selenium powder and lithium fluoride are mixed to obtain a mixed powder;

[0020] (2) After the mixed powder is placed on the lithium metal surface, the first rolling and the second rolling are performed. After the second rolling is completed, the pressure is released to obtain the gradient artificial SEI layer on the lithium metal surface.

[0021] The pressure of the second roller is greater than that of the first roller, and Raman online detection is performed in real time during step (2).

[0022] This invention addresses the problem of poor process compatibility in wet SEI layer preparation (wet solvent residue, such as >1200 pp). The invention employs a solvent-free in-situ rolling process (using DMF and plasma high temperature >180℃ to induce lithium melting) combined with online Raman real-time monitoring to precisely control and prepare the gradient artificial SEI layer. Specifically, in the first stage, the invention first places the mixed powder obtained by mixing raw materials on the surface of lithium metal. When the selenium powder comes into contact with the lithium metal, it triggers the primary selenization reaction: Se + 2Li → Li2Se, with a conversion rate >40% within 1 second, ultimately forming a 5-10nm thick Li2Se pre-coating layer (non-continuous island shape). In the second stage, the invention performs the first rolling under low pressure. At this time, the Se reaction deepens: the Li2Se layer becomes continuous (thickness → 50nm). Nano LiF particles are pressed into the gaps between the Li2Se layers. There is no chemical reaction, and a small amount of P2S5 reacts with lithium: P2S5 + 6Li → 2Li3P + 3Li2S (conversion rate <10%). At this time, the surface structure is fixed as a Li2Se-LiF composite layer with a density >90%, blocking the diffusion of subsequent reactants. In the third stage, a second rolling process is performed under high pressure. At this time, P2S5 undergoes a major reduction reaction: P2S5 + 6Li → 2Li3P + 3Li2S. The conversion rate is >95% within 5s. Li3P and Li2S diffuse in the solid state to form Li3PS4: Li3P + Li2S → Li3PS4, which generates the ionic conductor Li3PS4. The structure evolves as follows: the inner layer is unreacted Li2S, which is enriched on the lithium interface side; the middle layer is Li3PS4; and the surface layer is a Li2Se-LiF composite layer. In the fourth stage, due to the release of pressure, the Li3PS4 lattice relaxes and forms a crystalline phase (grain size 15±3nm) from the amorphous phase. The surface of the Li2Se layer is reconstructed, and Se atoms agglomerate to the surface layer. The three layers are locked in a gradient, resulting in a gradient artificial SEI layer.

[0023] In this invention, Raman online detection is performed in real time during step (2). The reaction conditions are adjusted according to the online feedback results, and it is determined whether to proceed to the next stage to prevent gradient structure disorder. For example, at t=2s (when step (2) is performed to the 2nd second): the surface Li2Se coverage (Raman 250cm) is detected. -1 Reduce roller speed when peak intensity <12000cps); at t=5s (when step (2) is performed up to the 5th second): detect I 375 / I 420 , if I 375 / I 420 If the value is greater than 1.5, automatic boosting will be triggered (e.g., boosting by 0.5 N / mm).

[0024] Preferably, in the mixed powder of step (1), the content of phosphorus pentasulfide is 85wt%-92wt%, for example, it can be 86wt%, 88wt%, 90wt% or 92wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] The amount of phosphorus pentasulfide added in this invention affects the content of the inner Li2S layer. Therefore, if the content of phosphorus pentasulfide is too low, sufficient Li2S inner layer cannot be generated, resulting in insufficient interface buffering capacity and increased interface impedance. If the content of phosphorus pentasulfide is too high, the surface Li2Se-LiF composite layer may be over-compressed or destroyed, and excessive P2S5 may react excessively with lithium to generate too much Li3P, affecting the purity and ionic conductivity of the intermediate Li3PS4 layer.

[0026] Preferably, in the mixed powder of step (1), the content of selenium powder is 5wt-10wt%, for example, it can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] The amount of selenium powder added in this invention affects the selenium content in the overall gradient artificial SEI layer. Since Se doping can inhibit Li2S decomposition, if the amount of selenium powder added is too small, the surface Li2Se layer will be too thin or discontinuous, resulting in insufficient inhibition of Li2S oxidation and increased sulfur loss under high voltage. If the amount of selenium powder added is too large, the surface layer will be too thick, which may hinder lithium ion transport, and excessive Se may diffuse inward, disrupting the crystallization and ion conduction performance of the intermediate Li3PS4 layer.

[0028] Preferably, the lithium fluoride content in the mixed powder in step (1) is 3wt%-8wt%, for example, it can be 3wt%, 4wt%, 5wt%, 6wt%, 7wt% or 8wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, the particle size D50 of the mixed powder in step (1) is 1μm-5μm, for example, it can be 1μm, 2μm, 3μm, 4μm or 5μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] Preferably, the mixing method in step (1) includes ball milling, which is carried out in an inert atmosphere. The ball milling speed is 280 rpm to 320 rpm, for example, 280 rpm, 290 rpm, 300 rpm, 310 rpm or 320 rpm. The ball-to-material ratio is (9-11):1, for example, 9:11, 10:1 or 11:1. The time is 110 min to 130 min, for example, 110 min, 115 min, 120 min, 125 min or 130 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, the lithium metal in step (2) includes a lithium foil with a thickness of 15μm-25μm, for example, it can be 15μm, 20μm or 25μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the method of setting the mixed powder on the lithium metal surface in step (2) includes spraying.

[0033] Preferably, in step (2), the loading of the mixed powder onto the lithium metal surface is 1.0 mg / cm³. 2 -2.0mg / cm 2 For example, it could be 1.0 mg / cm³. 2 1.2 mg / cm 2 1.4 mg / cm 2 1.6 mg / cm 2 1.8 mg / cm 2 Or 2.0 mg / cm 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0034] Preferably, in step (2), after the mixed powder is placed on the lithium metal surface and before the first rolling is performed, pre-compaction is also performed. The pre-compaction pressure is 0.3N / mm-0.5N / mm, for example, it can be 0.3N / mm, 0.35N / mm, 0.4N / mm, 0.45N / mm or 0.5N / mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, during step (2), spraying and pre-compaction are carried out within 0-1 seconds.

[0036] Preferably, in step (2), during the first rolling process, the rolling pressure increases linearly from N1 to N2, where N1 is 0.3N / mm-0.5N / mm, for example, it can be 0.3N / mm, 0.35N / mm, 0.4N / mm, 0.45N / mm or 0.5N / mm, and N2 is 0.8N / mm-1N / mm, for example, it can be 0.8N / mm, 0.85N / mm, 0.9N / mm, 0.95N / mm or 1N / mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] Preferably, in step (2), the temperature of the first roller is 25℃-40℃, for example, it can be 25℃, 30℃, 35℃ or 40℃, and the roller speed is 0.4m / min-0.6m / min, for example, it can be 0.4m / min, 0.45m / min, 0.5m / min, 0.55m / min or 0.6m / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] Preferably, during step (2), the first roller pressing is performed within the first 1-3 seconds.

[0039] Preferably, in step (2), during the second rolling process, the rolling pressure increases from N2 to N3 and then stops increasing. The N3 is 2.8 N / mm to 3.2 N / mm, for example, it can be 2.8 N / mm, 2.9 N / mm, 3 N / mm, 3.1 N / mm or 3.2 N / mm. The rate at which N2 increases to N3 is 0.3 N / (mm·s) to 0.5 N / (mm·s), for example, it can be 0.3 N / (mm·s), 0.35 N / (mm·s), 0.4 N / (mm·s), 0.45 N / (mm·s) or 0.5 N / (mm·s), but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] Preferably, the temperature of the second roller pressing in step (2) is 40℃-60℃, for example, it can be 40℃, 45℃, 50℃, 55℃ or 60℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] Preferably, during step (2), the second rolling is performed within the 3s-8s.

[0042] Preferably, during step (2), the pressure release is performed within 8-10 seconds.

[0043] Step (2) of the present invention is divided into four stages performed sequentially. The first stage is from 0 to 1 second, in which the mixed powder is sprayed and pre-compacted. The second stage is from 1 to 3 seconds (excluding 1 second), in which the first rolling is performed. The third stage is from 3 to 8 seconds (excluding 3 seconds), in which the second rolling is performed. The fourth stage is from 8 to 10 seconds (excluding 8 seconds), in which the pressure is released. Therefore, the preparation method of the present invention requires a short time, and the construction of the three-layer gradient structure can be completed within 5 seconds.

[0044] Preferably, the Raman online detection in step (2) includes detecting the coverage of lithium selenide on the lithium metal surface during the 2nd to 3rd second of step (2), when I 250 When the speed is less than 12000 cps, the roller speed is reduced. During step (2), I is detected within 5-6 seconds. 375 / I 420 , when I 375 / I 420 When the voltage is greater than 1.5, automatic boost is triggered.

[0045] Thirdly, the present invention provides a negative electrode containing an SEI layer, the negative electrode containing an SEI layer comprising a stacked lithium metal layer and a gradient artificial SEI layer as described in the first aspect, wherein the inner layer of the gradient artificial SEI layer is located on the side close to the lithium metal layer.

[0046] Fourthly, the present invention provides a solid-state battery, the solid-state battery comprising a gradient artificial SEI layer as described in the first aspect or a negative electrode containing an SEI layer as described in the third aspect.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] This invention addresses the chemical instability at the lithium metal anode-electrolyte interface in solid-state batteries (Li₂S oxidation and decomposition at >3.6V leading to sulfur loss exceeding 40%) and uncontrolled interface impedance (byproduct accumulation causing impedance >85 Ω·cm). 2To address the problem of poor process compatibility in wet SEI layer preparation (high solvent residue and lithium melting caused by plasma temperatures >180°C), this invention employs a solvent-free in-situ rolling process to trigger a stepwise reaction, combined with online Raman real-time monitoring, to precisely control and prepare the gradient artificial SEI layer. Attached Figure Description

[0049] Figure 1 This is a flowchart illustrating the generation timeline of the gradient artificial SEI layer as described in Embodiment 1 of the present invention.

[0050] Figure 2 This is a graph showing the pressure change over time during the generation of the gradient artificial SEI layer described in Embodiment 1 of the present invention. Detailed Implementation

[0051] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0052] Example 1

[0053] This embodiment provides a gradient artificial SEI layer, which includes a surface layer, a middle layer and an inner layer stacked sequentially. The surface layer includes Li2Se and LiF, the middle layer includes Li3PS4, and the inner layer includes Li2S.

[0054] The thickness of the gradient artificial SEI layer is 302 nm; the thickness of the inner layer is 80 nm, and the content of Li2S in the inner layer is 65.38 at%; the thickness of the middle layer is 150 nm, and the content of Li3PS4 in the middle layer is 95 at%; the thickness of the surface layer is 72 nm, and the molar ratio of Li2Se to LiF in the surface layer is 1:1.2.

[0055] The method for preparing the gradient artificial SEI layer includes the following steps:

[0056] (1) Preparation of precursor powder: Weigh P2S5 powder (90.00 g, D50=3.2 μm, 99.9% purity), Se powder (7.00 g, D50=1.1 μm), and LiF nanoparticles (3.00 g, D50=52 nm); put the weighed powder into a 250 mL zirconia ball mill jar, purge with argon gas three times (residual oxygen <0.5 ppm), and ball milling. The ball milling speed is 300 rpm, the ball-to-material ratio is 10:1, and the total time is 120 min (stop for 10 min every 30 min); after discharge, a mixed powder is obtained, and the D50 of the mixed powder is detected by a laser particle size analyzer (Malvern Mastersizer 3000) to be 2.8 ± 0.3 μm;

[0057] The mixed powder contains 90 wt% P2S5, 7 wt% Se powder, and 3 wt% lithium fluoride.

[0058] (2) Lithium foil pretreatment: 20 μm thick lithium foil (99.9%, thickness difference ±1.5 μm) was cut to the required specifications, ultrasonically cleaned 3 times with anhydrous ethanol (5 min each time), and treated with ultraviolet ozone for 10 min (wavelength 254 nm, intensity 30 mW / cm). 2 Finally, after purging with argon gas, it was transferred into a glove box (H2O < 0.1 ppm, O2 < 0.1 ppm).

[0059] (3) Roll forming and millisecond-level monitoring:

[0060] (3.1) Powder pre-compaction (0-1000ms): The powder spraying valve sprays the mixed powder at a pressure of 0.20MPa (load 1.50±0.05mg / cm³). 2 At t=500ms, the upper roller (with micro-bumps Ra=0.5μm) contacts the powder with an interference amount of -0.1mm and applies a linear pressure of 0.50N / mm; at t=800ms, 28 kHz ultrasonic vibration is started for 0.2 s to eliminate agglomerates >5 μm.

[0061] Raman feedback: t=1000 ms, focusing, Z=0 μm, acquisition at 250 cm. -1 When the peak strength is <12000 cps, the roller speed is reduced to 0.3 m / min and the pressure is increased by 0.1 N / mm;

[0062] (3.2) Surface construction (1000-3000 ms), the pressure increases linearly from 0.50 N / mm to 1.00 N / mm (the rate of increase is 0.25 N / (mm·s)), and the water-cooled lower roller maintains a temperature of ≤35℃;

[0063] Second scan at t=2000 ms, 250 cm-1 Peak: Intensity > 15000 cps and full width at half maximum (FWHM) < 18 cm -1 To pass, otherwise maintain the roller speed at 0.3 m / min;

[0064] (3.3) High-pressure reaction (3000-8000 ms), staged pressurization: the pressure increases from 1.0 N / mm to 2.6 N / mm within 3000-5000 ms (the rate of increase is 0.80 N / (mm·s)), and the pressure increases from 2.6 N / mm to 3.0 N / mm within 5000-8000 ms (the rate of increase is 0.13 N / (mm·s));

[0065] Layered monitoring, t=5000 ms, focusing Z=0 μm: 250 cm -1 Strength > 12000 cps (surface seal), t = 5000 ms focusing Z = 300 μm: calculated 375 cm -1 (Li₂S) and 420 cm -1 (Li3PS4) Peak intensity ratio R=I 375 / I 420 If R>1.5, the pressure increases instantaneously by 0.5 N / mm (lasting for 2000 ms).

[0066] (4) Relaxation lock-in (8000-10000 ms), pressure is released exponentially: P(t)=3.0×exp(-1.5×(t-8.0)), pressure ≈0.05 N / mm at t=10000 ms;

[0067] Monitoring 420 cm -1 Peak shift: If >422 cm -1 (Amorphization signs) triggered a 0.1 s micro-vibration (amplitude 2 μm).

[0068] In this embodiment, the upper roller of the roller pressing mechanism has micro-protrusions on its surface (Ra=0.5 μm) and a built-in pressure sensor (HBM U9C). The lower roller has a water-cooled channel (25℃) and an implanted K-type thermocouple. The conditions for online Raman detection are controlled as follows: laser probe: 785 nm, Z-axis adjustable (focusing depth 0-500 nm), focusing Z=0 μm: detection 250 cm. -1 (Li₂Se) intensity, focusing Z=300μm: detection 375 cm⁻¹ -1 (Li₂S) / 420 cm -1 (Li3PS4) Peak intensity ratio, control signal: when the intensity is <12000 cps, the roller speed is reduced to 0.3 m / min.

[0069] The generation timeline flowchart of the gradient artificial SEI layer described in this embodiment is as follows: Figure 1 As shown; the pressure change over time during the generation of the gradient artificial SEI layer is illustrated in the figure below. Figure 2 As shown.

[0070] Example 2

[0071] This embodiment provides a gradient artificial SEI layer, which includes a surface layer, a middle layer and an inner layer stacked sequentially. The surface layer includes Li2Se and LiF, the middle layer includes Li3PS4, and the inner layer includes Li2S.

[0072] The thickness of the gradient artificial SEI layer is 280 nm; the thickness of the inner layer is 70 nm, and the content of Li2S in the inner layer is 63.5 at%; the thickness of the middle layer is 130 nm, and the content of Li3PS4 in the middle layer is 92 at%; the thickness of the surface layer is 80 nm, and the molar ratio of Li2Se to LiF in the surface layer is 1:1.5.

[0073] The method for preparing the gradient artificial SEI layer includes the following steps:

[0074] (1) Preparation of precursor powder: Weigh P2S5 powder (85.00 g, D50=3.2μm, 99.9% purity), Se powder (10.00 g, D50=1.1μm), and LiF nanoparticles (5.00 g, D50=52nm); put the weighed powder into a 250mL zirconia ball mill jar, purge with argon gas three times (residual oxygen <0.5ppm), and ball milling. The ball milling speed is 320rpm, the ball-to-material ratio is 9:1, and the total time is 110min (stop for 10min every 30min); after discharge, a mixed powder is obtained, and the D50 of the mixed powder is detected by a laser particle size analyzer (Malvern Mastersizer 3000) to be 4μm;

[0075] The mixed powder contains 85 wt% P2S5, 10 wt% Se powder, and 5 wt% lithium fluoride.

[0076] (2) Lithium foil pretreatment: 20 μm thick lithium foil (99.9%, thickness difference ±1.5 μm) was cut to the required specifications, ultrasonically cleaned 3 times with anhydrous ethanol (5 min each time), and treated with ultraviolet ozone for 10 min (wavelength 254 nm, intensity 30 mW / cm). 2 Finally, after purging with argon gas, it was transferred into a glove box (H2O < 0.1 ppm, O2 < 0.1 ppm).

[0077] (3) Roll forming and millisecond-level monitoring:

[0078] (3.1) Powder pre-compaction (0-1000ms): The powder spraying valve sprays the mixed powder at a pressure of 0.20MPa (load 2mg / cm³). 2 At t=500ms, the upper roller (with micro-bumps Ra=0.5μm) contacts the powder with an interference amount of -0.1mm and applies a linear pressure of 0.50N / mm; at t=800ms, 28 kHz ultrasonic vibration is started for 0.2 s to eliminate agglomerates >5 μm.

[0079] Raman feedback: t=1000 ms, focusing, Z=0 μm, acquisition at 250 cm. -1 When the peak strength is <12000 cps, the roller speed is reduced to 0.3 m / min and the pressure is increased by 0.1 N / mm;

[0080] (3.2) Surface construction (1000-3000 ms), the pressure increases linearly from 0.50 N / mm to 1.00 N / mm (the rate of increase is 0.25 N / (mm·s)), and the water-cooled lower roller maintains a temperature of ≤35℃;

[0081] Second scan at t=2000 ms, 250 cm -1 Peak: Intensity > 15000 cps and full width at half maximum (FWHM) < 18 cm -1 To pass, otherwise maintain the roller speed at 0.3 m / min;

[0082] (3.3) High-pressure reaction (3000-8000 ms), staged pressurization: the pressure increases from 1.0 N / mm to 2.6 N / mm within 3000-5000 ms (the rate of increase is 0.80 N / (mm·s)), and the pressure increases from 2.6 N / mm to 3.0 N / mm within 5000-8000 ms (the rate of increase is 0.13 N / (mm·s));

[0083] Layered monitoring, t=5000 ms, focusing Z=0 μm: 250 cm -1 Strength > 12000 cps (surface seal), t = 5000 ms focusing Z = 300 μm: calculated 375 cm -1 (Li₂S) and 420 cm -1 (Li3PS4) Peak intensity ratio R=I 375 / I 420 If R>1.5, the pressure increases instantaneously by 0.5 N / mm (lasting for 2000 ms).

[0084] (4) Relaxation lock-in (8000-10000 ms), pressure is released exponentially: P(t)=3.0×exp(-1.5×(t-8.0)), pressure ≈0.05 N / mm at t=10000 ms;

[0085] Monitoring 420 cm -1 Peak shift: If >422 cm -1 (Amorphization signs) triggered a 0.1 s micro-vibration (amplitude 2 μm).

[0086] In this embodiment, the upper roller of the roller pressing mechanism has micro-protrusions on its surface (Ra=0.5 μm) and a built-in pressure sensor (HBM U9C). The lower roller has a water-cooled channel (25℃) and an implanted K-type thermocouple. The conditions for online Raman detection are controlled as follows: laser probe: 785 nm, Z-axis adjustable (focusing depth 0-500 nm), focusing Z=0 μm: detection 250 cm. -1 (Li₂Se) intensity, focusing Z=300μm: detection 375 cm⁻¹ -1 (Li₂S) / 420 cm -1 (Li3PS4) Peak intensity ratio, control signal: Intensity <12000 cps → Roller speed reduced to 0.3 m / min.

[0087] Example 3

[0088] This embodiment provides a gradient artificial SEI layer, which includes a surface layer, a middle layer and an inner layer stacked sequentially. The surface layer includes Li2Se and LiF, the middle layer includes Li3PS4, and the inner layer includes Li2S.

[0089] The thickness of the gradient artificial SEI layer is 320 nm; the thickness of the inner layer is 90 nm, and the content of Li2S in the inner layer is 68 at%; the thickness of the middle layer is 160 nm, and the content of Li3PS4 in the middle layer is 96 at%; the thickness of the surface layer is 70 nm, and the molar ratio of Li2Se to LiF in the surface layer is 1:0.8.

[0090] The method for preparing the gradient artificial SEI layer includes the following steps:

[0091] (1) Preparation of precursor powder: Weigh P2S5 powder (92.00 g, D50=3.2 μm, 99.9% purity), Se powder (5.00 g, D50=1.1 μm), and LiF nanoparticles (3.00 g, D50=52 nm); put the weighed powder into a 250 mL zirconia ball mill jar, purge with argon gas three times (residual oxygen <0.5 ppm), and ball milling. The ball milling speed is 280 rpm, the ball-to-material ratio is 11:1, and the total time is 130 min (stop for 10 min every 30 min); after discharge, a mixed powder is obtained, and the D50 of the mixed powder is detected by a laser particle size analyzer (MalvernMastersizer 3000) to be 2.0 μm;

[0092] The mixed powder contains 92 wt% P2S5, 5 wt% Se powder, and 3 wt% lithium fluoride.

[0093] (2) Lithium foil pretreatment: 25μm thick lithium foil (99.9%, thickness difference ±1.5μm) was cut to the required specifications, ultrasonically cleaned 3 times with anhydrous ethanol (5 min each time), and treated with ultraviolet ozone for 10 min (wavelength 254 nm, intensity 30 mW / cm). 2 Finally, after purging with argon gas, it was transferred into a glove box (H2O<0.1 ppm, O2<0.1ppm).

[0094] (3) Roll forming and millisecond-level monitoring:

[0095] (3.1) Powder pre-compaction (0-1000ms): The powder spraying valve sprays the mixed powder at a pressure of 0.20MPa (load 1.0mg / cm³). 2 At t=500ms, the upper roller (with micro-bumps Ra=0.5μm) contacts the powder with an interference amount of -0.1mm and applies a linear pressure of 0.50N / mm; at t=800ms, 28 kHz ultrasonic vibration is started for 0.2 s to eliminate agglomerates >5 μm.

[0096] Raman feedback: t=1000 ms, focusing, Z=0 μm, acquisition at 250 cm. -1 When the peak strength is <12000 cps, the roller speed is reduced to 0.3 m / min and the pressure is increased by 0.1 N / mm;

[0097] (3.2) Surface construction (1000-3000 ms), the pressure increases linearly from 0.50 N / mm to 1.00 N / mm (the rate of increase is 0.25 N / (mm·s)), and the water-cooled lower roller maintains a temperature of ≤35℃;

[0098] Second scan at t=2000 ms, 250 cm -1 Peak: Intensity > 15000 cps and full width at half maximum (FWHM) < 18 cm -1 To pass, otherwise maintain the roller speed at 0.3 m / min;

[0099] (3.3) High-pressure reaction (3000-8000 ms), staged pressurization: the pressure increases from 1.0 N / mm to 2.6 N / mm within 3000-5000 ms (the rate of increase is 0.80 N / (mm·s)), and the pressure increases from 2.6 N / mm to 3.0 N / mm within 5000-8000 ms (the rate of increase is 0.13 N / (mm·s));

[0100] Layered monitoring, t=5000 ms, focusing Z=0 μm: 250 cm -1 Strength > 12000 cps (surface seal), t = 5000 ms focusing Z = 300 μm: calculated 375 cm -1 (Li₂S) and 420 cm -1 (Li3PS4) Peak intensity ratio R=I 375 / I 420 If R>1.5, the pressure increases instantaneously by 0.5 N / mm (lasting for 2000 ms).

[0101] (4) Relaxation lock-in (8000-10000 ms), pressure is released exponentially: P(t)=3.0×exp(-1.5×(t-8.0)), pressure ≈0.05 N / mm at t=10000 ms;

[0102] Monitoring 420 cm -1 Peak shift: If >422 cm -1 (Amorphization signs) triggered a 0.1 s micro-vibration (amplitude 2 μm).

[0103] In this embodiment, the upper roller of the roller pressing mechanism has a micro-bump surface Ra=0.5μm and a built-in pressure sensor (HBMU9C). The lower roller has a water-cooled channel (25℃) and an implanted K-type thermocouple. The conditions for online Raman detection are controlled as follows: laser probe: 785 nm, Z-axis adjustable (focusing depth 0-500 nm), focusing Z=0 μm: detection 250 cm. -1 (Li₂Se) intensity, focusing Z=300μm: detection 375 cm⁻¹ -1 (Li₂S) / 420 cm -1 (Li3PS4) Peak intensity ratio, control signal: Intensity <12000 cps → Roller speed reduced to 0.3 m / min.

[0104] Example 4

[0105] This embodiment provides a gradient artificial SEI layer. Except for the preparation method, in which 80g of P2S5 powder, 15g of Se powder, and 5g of LiF nanoparticles are weighed to make the content of P2S5 in the mixed powder 80wt%, the content of Se powder 15wt%, and the content of lithium fluoride 5wt%, so that the resulting gradient artificial SEI layer adapts to changes, the rest is the same as in Example 1.

[0106] Example 5

[0107] This embodiment provides a gradient artificial SEI layer. Except for the preparation method, in which 95g of P2S5 powder, 2.5g of Se powder, and 2.5g of LiF nanoparticles are weighed to make the content of P2S5 in the mixed powder 95wt%, the content of Se powder 2.5wt%, and the content of lithium fluoride 2.5wt%, so that the resulting gradient artificial SEI layer adapts to changes, the rest is the same as in Example 1.

[0108] Comparative Example 1

[0109] This comparative example provides an artificial SEI layer. Except for step (3) of its preparation method, in which the rolling process is carried out with a constant pressure of 2.0 N / mm and a constant rolling speed of 0.5 m / min, and no Raman detection and feedback are performed, and each stage is controlled by a preset time, resulting in the mixing of various substances and the failure to obtain a gradient artificial SEI layer, the rest is the same as in Example 1.

[0110] Comparative Example 2

[0111] This comparative example provides an artificial SEI layer. Except for the preparation method, in which 97g of P2S5 powder and 3g of LiF nanoparticles are weighed to make the content of P2S5 in the mixed powder 97wt% and the content of lithium fluoride 3wt%, and no selenium powder is added, so that the adaptability of the artificial SEI layer is changed, the artificial SEI layer is otherwise the same as in Example 1.

[0112] Comparative Example 3

[0113] This comparative example provides an artificial SEI layer, the preparation method of which includes the following steps:

[0114] 97g of P2S5 powder and 3g of LiF nanoparticles were dispersed in a DMF solution (containing 10wt% PVDF) to obtain a mixed slurry. The mixed slurry was coated (the wet film thickness was 15μm), dried under vacuum at 80℃ for 12h, and then hot-pressed to form an SEI layer.

[0115] The thickness of the SEI layer obtained in Examples 1-5 and Comparative Examples 1-3, the Li2S content in the inner layer of Examples 1-5, and the Li2S content in the artificial SEI layer of Comparative Examples 1-2 are shown in Table 1. The SEI layers obtained in Examples 1-5 and Comparative Examples 1-3 are used to prepare solid-state batteries. The preparation process includes the following steps: (1) Preparation of positive electrode: The ternary positive electrode material (LiNi) is prepared. 0.8 Co 0.1 Mn 0.1O2), sulfide solid electrolyte (Li6PS5Cl) and vapor-grown carbon fiber (VGCF) conductive agent are mixed at a mass ratio of 70:28:2, and an appropriate amount of n-heptane is added as a dispersion medium. The mixture is stirred evenly in a planetary mixer to prepare a positive electrode slurry. The positive electrode slurry is coated on an aluminum foil current collector, dried under vacuum at 80°C for 12 hours, and then cold-pressed at a pressure of 300 MPa to obtain a positive electrode sheet. (2) Preparation of solid electrolyte layer: Li6PS5Cl powder is evenly spread in a mold and pressed under a pressure of 280 MPa to obtain a solid electrolyte sheet with a thickness of about 80 μm. (3) Assembly of negative electrode: The "lithium metal negative electrode with gradient artificial SEI layer" (i.e., the composite of pretreated lithium foil and artificial SEI layer) prepared in the examples or comparative examples is used as the negative electrode. (4) Full cell stacking and packaging: In an argon glove box (H2O, O2 < 0.1 ppm), the cells are stacked in the following order: "positive electrode shell - positive electrode sheet - solid electrolyte sheet - negative electrode with SEI layer - gasket - negative electrode shell". Using an insulating sealing ring, cold isostatic pressing is performed at 300 MPa pressure to produce a coin cell solid-state battery.

[0116] The interfacial impedance and 500-cycle retention rate of the solid-state battery were tested. The interfacial impedance was measured using electrochemical impedance spectroscopy (EIS). The test conditions were: after the battery was left to stand at open-circuit voltage for 2 hours, measurements were taken using an electrochemical workstation at a frequency range of 0.1 Hz to 1 MHz, an amplitude of 10 mV, and room temperature (25 ± 2℃). The interfacial impedance value (Rinterface) was obtained by fitting the diameter of the semicircle in the high-frequency region. The 500-cycle retention rate was tested using a Blue Battery testing system. The test conditions were: the battery was placed in a 25℃ constant temperature chamber at a voltage range of 2.7 V to 4.3 V (vs. Li). + Within the voltage range of / Li), constant current charge-discharge cycles were performed at a rate of 0.1C. The retention rate (%) after 500 cycles was calculated as: (discharge capacity of the 500th cycle / discharge capacity of the 3rd cycle) × 100%.

[0117] The test results are shown in Table 1 below:

[0118] Table 1

[0119]

[0120] As can be seen from Table 1 above:

[0121] As can be seen from Example 1 and Comparative Example 1, if a constant pressure is used for rolling, the gradient artificial SEI layer of the present invention cannot be obtained, which will lead to an imbalance in the conversion of Li2S and Li3PS4 (I 375 / I 420=2.37), insufficient Li2S residue in the inner layer causes cyclic decomposition, resulting in high interfacial impedance and decreased cycle performance; as shown in Example 1 and Comparative Example 2, when there is no Se doping, Li2S is oxidized to Li2S2 under high voltage (XPS detects a characteristic peak of 162.8 eV), and the ion channels are blocked; as shown in Example 1 and Comparative Example 3, conventional wet methods cannot prepare the gradient artificial SEI layer of the present invention, and there is DMF solvent residue, forming micropores (SEM shows pore size >100nm), which causes dendrites to penetrate prematurely, resulting in a decrease in solid-state battery performance; as shown in Example 1 and Examples 4-5, in the preparation process of the gradient artificial SEI layer of the present invention, the amount of raw materials added is preferably within a specific range, which is beneficial to improving the content and stability of Li2S in the inner layer.

[0122] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A gradient artificial SEI layer, characterized in that, The gradient artificial SEI layer comprises a surface layer, an intermediate layer and an inner layer arranged in sequence, the surface layer comprises lithium selenide and lithium fluoride, the intermediate layer comprises Li3PS4, and the inner layer comprises Li2S; In the gradient artificial SEI layer, the surface layer is towards the solid electrolyte layer, and the inner layer is towards the lithium negative electrode.

2. The gradient artificial SEI layer of claim 1, wherein, The thickness of the inner layer is 50 nm-100 nm; And / or, in the inner layer, the content of Li2S is >60 at%; And / or, the thickness of the intermediate layer is 100 nm-300 nm; And / or, in the intermediate layer, the content of Li3PS4 is ≥90 wt%; And / or, the thickness of the surface layer is 50 nm-100 nm; And / or, in the surface layer, the molar ratio of lithium selenide to lithium fluoride is 1:(0.5-2.0); and / or the gradient artificial SEI layer has a thickness of 200 nm to 500 nm and an ionic conductivity > 1.2 x 10 -4 S / cm.

3. A method for the preparation of a gradient artificial SEI layer as claimed in claim 1 or 2, characterized in that The preparation method comprises the following steps: (1) mixing phosphorus pentasulfide, selenium powder and lithium fluoride to obtain a mixed powder; (2) after arranging the mixed powder on the surface of lithium metal, performing first rolling and second rolling, and after the second rolling is completed, releasing the pressure to obtain the gradient artificial SEI layer on the surface of lithium metal; The pressure of the second rolling is greater than that of the first rolling, and real-time Raman online detection is performed during the process of step (2).

4. The production method according to claim 3, characterized by, In the mixed powder of step (1), the content of phosphorus pentasulfide is 85 wt%-92 wt%; And / or, in the mixed powder of step (1), the content of selenium powder is 5 wt-10 wt%; And / or, in the mixed powder of step (1), the content of lithium fluoride is 3 wt%-8 wt%; And / or, the particle size D50 of the mixed powder of step (1) is 1 μm-5 μm; And / or, the mixing method of step (1) comprises ball milling, and the ball milling is carried out in an inert atmosphere, the rotation speed of the ball milling is 280 rpm-320 rpm, the ball-to-material ratio is (9-11):1, and the time is 110 min-130 min.

5. The production method according to claim 3 or 4, characterized by, The method of arranging the mixed powder on the surface of lithium metal in step (2) comprises spraying; and / or, the loading amount of the mixed powder disposed on the surface of the lithium metal in step (2) is 1.0 mg / cm 2 -2.0 mg / cm 2 ; And / or, before the first rolling after arranging the mixed powder on the surface of lithium metal in step (2), pre-compaction is further performed, and the pressure of the pre-compaction is 0.3 N / mm-0.5 N / mm; And / or, during the process of step (2), spraying and pre-compaction are performed within 0-1 s.

6. The production method according to claim 3 or 4, characterized by, During the first rolling in step (2), the rolling pressure is increased linearly from N1 to N2, N1 is 0.3 N / mm-0.5 N / mm, and N2 is 0.8 N / mm-1 N / mm; And / or, the temperature of the first rolling in step (2) is 25℃-40℃, and the rolling speed is 0.4 m / min-0.6 m / min; And / or, during the process of step (2), the first rolling is performed within 1 s-3 s.

7. The production method according to claim 6, wherein During the second rolling in step (2), the rolling pressure is increased from N2 to N3 without further increasing the pressure, N3 is 2.8 N / mm-3.2 N / mm, and the rate of increasing from N2 to N3 is 0.3 N / (mm·s)-0.5 N / (mm·s). And / or, the second rolling in step (2) is performed at a temperature of 40-60°C. And / or, the second rolling in step (2) is performed within 3-8 seconds after the start of step (2).

8. The production method according to claim 3 or 4, characterized by, The pressure is released within 8-10 seconds after the start of step (2). And / or, the Raman online detection of step (2) includes detecting the coverage of lithium metal surface by lithium selenide when I 250 <12000 cps, then the roll speed is reduced, and when step (2) is performed to the 5th-6th second, I 375 / I 420 , when I 375 / I 420 >1.5, then automatic pressurization is triggered.

9. A negative electrode comprising an SEI layer, characterized by The SEI-containing negative electrode comprises a lithium metal layer and the gradient artificial SEI layer as claimed in claim 1 or 2, which are arranged in a stack, wherein the inner layer of the gradient artificial SEI layer is close to one side of the lithium metal layer.

10. A solid state battery, characterized by, The solid-state battery comprises the gradient artificial SEI layer as claimed in claim 1 or 2 or the SEI-containing negative electrode as claimed in claim 9.

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