Composite coated graphite negative electrode material, preparation method thereof and lithium ion solid-state battery

Abstract

The invention discloses a composite coated graphite negative electrode material, a preparation method thereof and a lithium ion solid-state battery, and belongs to the technical field of lithium ion solid-state batteries, the composite coated graphite negative electrode material comprises a graphite matrix and a composite coating layer, and the composite coating layer comprises an inner layer amorphous lithium chloride and an outer layer nanocrystalline LiF / Li3N composite phase. Mixing graphite, lithium chloride containing crystal water and ammonium fluoride to obtain a mixed material; placing the mixed material in a microwave reaction cavity, heating the mixed material to 600-700 DEG C at the microwave power of 800-1200 W in an H2 / Ar mixed atmosphere, and keeping the temperature for 20-60 minutes; and cooling the sintered mixed material, grinding, and sieving with a 100-300 mesh sieve to obtain the composite coated graphite negative electrode material with a coating layer thickness of 10-50 nm. The invention provides an innovative amorphous LiCl / nanocrystalline LiF-Li3N gradient coating layer design, the performance of the lithium ion battery is remarkably improved by optimizing an ion migration path, enhancing the interface stability and reducing the process cost, and a new engineering way is provided for realizing high magnification and long cycle performance of the sulfide all-solid-state battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion solid-state batteries, and more particularly, relates to a composite-coated graphite negative electrode material, a preparation method thereof and a lithium ion solid-state battery. BACKGROUND

[0002] The application of traditional lithium chloride (LiCl) coating in all-solid-state batteries faces multiple challenges. First, its low ionic conductivity (usually less than 0.1 mS / cm) makes it difficult to meet the high-rate charge-discharge requirements (such as more than 5C). Second, due to the chemical potential mismatch between LiCl and sulfide electrolyte (such as Li6PS5Cl), an unstable SEI film is generated at the interface, accelerating the consumption of active lithium (more than 30% capacity decay in cycling). In addition, the LiCl coating layer is prone to cracking when the strain exceeds 5%, which cannot effectively buffer the volume expansion of graphite (about 10%), thus exacerbating the electrode pulverization problem after cycling. Process complexity and cost are also significant problems. The existing technology requires multiple coatings and high-temperature sintering, resulting in high energy consumption, low yield (cost increase of 40%), and difficulty in accurately controlling the thickness of the coating layer (nanometer level precision ± 50%), causing uneven distribution of interface impedance (fluctuation of more than 100 Ω·cm 2 ). The existing technology has many shortcomings in coating layer performance, process complexity and interface compatibility, which limits the rate performance and cycle life of the graphite negative electrode of all-solid-state batteries. SUMMARY

[0003] An object of the present application is to solve at least the above problems and / or defects, and to provide at least the advantages to be described later.

[0004] To achieve these objects and advantages in accordance with the present application, the present application provides a composite-coated graphite negative electrode material, comprising: a graphite matrix and a composite coating layer coated on the surface of the graphite matrix, the composite coating layer comprising an inner layer of amorphous lithium chloride and an outer layer of nanocrystalline LiF / Li3N composite phase.

[0005] Preferably, the thickness of the inner layer of amorphous lithium chloride is 2-5 nm; the outer layer of nanocrystalline LiF / Li3N composite phase comprises LiF grains and Li3N sheet network, the size of the LiF grains is 3-8 nm, and the thickness of the Li3N sheet network is 1-3 nm; the total thickness of the composite coating layer is 10-50 nm; and the interlayer spacing of the graphite matrix is 0.340-0.345 nm.

[0006] Preferably, the mass ratio of the LiF grains to the Li3N sheet network is 3-5:1, and the Li3N sheet network is distributed in the gaps between the LiF grains.

[0007] Preferably, the graphite matrix is artificial graphite or natural graphite, the particle size is 5-15 μm, the specific surface area is 5-10 m 2 / g.

[0008] A preparation method of a composite-coated graphite negative electrode material, comprising the following steps: Step one, mixing graphite, lithium chloride containing crystal water (LiCl·nH2O, n=1, 2, 3) and ammonium fluoride to obtain a mixture; Step two, placing the mixture in a microwave reaction cavity, heating to 600-700℃ under H2 / Ar mixed atmosphere at a microwave power of 800-1200W, and maintaining for 20-60min; Step three, grinding and sieving the mixture after microwave plasma sintering to 100-300 meshes to obtain a composite-coated graphite negative electrode material.

[0009] Preferably, in step one, the mass ratio of graphite, lithium chloride containing crystal water and ammonium fluoride is 100:(5-10):(0.5-2.5).

[0010] Preferably, in step one, the mixing of graphite, lithium chloride containing crystal water and ammonium fluoride is completed by planetary ball milling, the ball-to-material ratio is 5:1, the rotation speed is 200-400rpm, the ball milling time is 1-4h, and the ball milling medium is zirconia ball.

[0011] Preferably, in step two, the inner wall of the microwave reaction cavity is coated with a SiC corrosion-resistant layer, the microwave frequency is 1-2.5GHz, and the heating rate is ≤5℃ / min.

[0012] Preferably, in step two, the volume ratio of H2 to Ar in the H2 / Ar mixed atmosphere is 1:5-1:10, and the microwave plasma sintering pressure is 0.1-1MPa.

[0013] A lithium ion solid-state battery, the negative electrode material of which is the composite-coated graphite negative electrode material described above.

[0014] The present application at least includes the following advantages: the present application proposes an innovative amorphous LiCl / nanocrystalline LiF-Li3N gradient coating layer design, which realizes the significant improvement of the performance of lithium ion batteries by optimizing the ion migration path, enhancing the interface stability and reducing the process cost. The prepared composite coating layer has a gradient distribution of each element, specifically: the concentration of Cl element decreases from the inner layer to the outer layer, the concentration of F and N elements increases from the inner layer to the outer layer, and the total atomic concentration of F and N in the outer layer is greater than or equal to 60at%, and the inner layer amorphous lithium chloride can cover more than 95% of the surface of the graphite matrix. The 2-5nm thick amorphous LiCl in the inner layer has low strain characteristics (<2%), which effectively buffers the volume change stress of graphite during the lithium intercalation / delithiation process; the 3-8nm LiF / Li3N in the outer layer forms a three-dimensional ion channel, which improves the ionic conductivity to 1.0-1.5mS / cm, and reduces the interface impedance to 30-50Ω·cm 2 , and the lithium ion transverse diffusion is guided by the chemical potential gradient, which avoids the accumulation of lithium precipitation at the edge of the graphite. In addition, the composite coating layer structure of the present application solves the problem of poor compatibility of traditional sulfide electrolyte and graphite through the HOMO energy level ladder and dynamic SEI self-repairing mechanism, reduces the electronic leakage and improves the cycle stability. The one-step process of microwave plasma assisted sintering is used to complete the interlayer diffusion and in-situ growth of the coating layer in one step during the graphite carbonization stage, which not only reduces the energy consumption and cost, but also improves the uniformity of the coating layer and the interface contact impedance. The design of the gradient coating layer also enhances the interface compatibility and cycle life, and realizes the capacity retention rate of more than 90% after 1000 cycles through the dynamic SEI self-repairing mechanism. Compared with the prior art, the present application shows higher ionic conductivity, better lithium precipitation inhibition ability, significantly prolonged cycle life, and greatly reduced process cost, which provides a new engineering way for realizing the high rate and long cycle performance of sulfide full solid-state batteries.

[0015] The design of the inner layer amorphous LiCl and the outer layer nanocrystalline LiF-Li3N gradient coating obtained by the present application core lies in the synergistic optimization of ion transmission path and interface stability. The inner layer LiCl and amorphous LiCl have high ionic conductivity (~10 -3 S / cm) and low electronic conductivity, can preferentially form a dense ion channel, relieve the volume expansion (~10%) of graphite during lithium intercalation / delithiation, and inhibit the decomposition of electrolyte. Its disordered structure can adapt to local stress changes and reduce the risk of interface cracks. The outer layer LiF-Li3N and nanocrystalline LiF have high ionic conductivity (~10 -4The composite network formed by the Li3N and the LiF (1.0*10-3S / cm) provides rigid mechanical support (modulus ~200GPa) to prevent dendrite penetration and blocks electron transport through the wide electrochemical window (>5V) of the LiF to inhibit side reactions. The nitride structure of the Li3N can passivate interface defects to further reduce interface impedance. The gradient design realizes the synergistic enhancement of fast ion transport and interface stability through the inner layer of flexible buffer + the outer layer of rigid protection, and the one-step process (such as microwave sintering) simplifies the preparation process and significantly reduces production costs.

[0016] Other advantages, objects, and features of the application will be better understood from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The rate performance graph of the graphite negative electrode material prepared in Example 1 and Comparative Example 1 of the application; Figure 2 The SEM graph of the composite coated graphite negative electrode material prepared in Example 1 of the application. DETAILED DESCRIPTION

[0018] The application will be further described in conjunction with the accompanying drawings, so that those skilled in the art can implement it according to the description.

[0019] It should be understood that the terms such as "have", "contain" and "include" used herein do not exclude the presence or addition of one or more other elements or combinations thereof. Example 1: A preparation method of a composite coated graphite negative electrode material, comprising the following steps: Step one, preparation of a precursor: 5g of artificial graphite (particle size 8μm, specific surface area 10m 2 / g), 0.375g of LiCl·2H2O and 0.112g of NH4F are added to a zirconia ball mill jar, the ball-to-material ratio is 5:1, and the mixture is ball milled under argon protection for 2h at a rotation speed of 300rpm to obtain a mixture material; Step two, microwave sintering: the mixture material is placed in a microwave reaction cavity (the inner wall is coated with a SiC anticorrosion coating), H2 / Ar (the volume ratio of H2 to Ar is 1:9) is introduced into the microwave reaction cavity, the temperature is raised to 650℃ at a rate of 5℃ / min, the microwave frequency is 2.45GHz, the microwave power is 1000W, the holding time is 30min, and the sintering pressure is 0.5MPa; Step three, post-treatment: the mixture material after microwave plasma sintering is naturally cooled, ground through a 200 mesh sieve, and a composite coated graphite negative electrode material with a composite coating layer thickness of 35nm is obtained. The SEM graph of the composite coated graphite negative electrode material prepared in this example is as follows:Figure 2 As shown, Figure 2 The gradient coating layer shows a clear multi-layer structure on the surface of the graphite (the inner layer is dense and the outer layer is granular), and the Li3N sheet network is distributed in the gap between the LiF grains, indicating that the lithium chloride and the LiF-L3N composite coating layer are successfully deposited in steps, and the interface is tightly combined without obvious defects, verifying the controllability of the gradient coating process and the feasibility of the material structure design. In the composite coated graphite negative electrode material prepared in this embodiment, the thickness of the inner layer amorphous lithium chloride is 3 nm, the outer layer nanocrystalline LiF / Li3N composite phase includes LiF grains and Li3N sheet network, the size of the LiF grains is 5 nm, the thickness of the Li3N sheet network is 2 nm, and the mass ratio of the LiF grains to the Li3N sheet network is 4:1; the interlayer spacing of the graphite matrix is 0.342 nm. The composite coated graphite negative electrode material in this embodiment has a gradient distribution of each element in the composite coating layer, specifically: the concentration of Cl element decreases from the inner layer to the outer layer, the concentration of F and N elements increases from the inner layer to the outer layer, and the total atomic concentration of F and N in the outer layer is ≥60at%, and the inner layer amorphous lithium chloride can cover more than 95% of the surface of the graphite matrix.

[0020] Example 2: A method for preparing a composite coated graphite negative electrode material, comprising the following steps: Step one, preparation of precursor: 5g of artificial graphite (particle size 5μm, specific surface area 10m 2 / g), 0.25g of LiCl·2H2O and 0.067g of NH4F are added to a zirconia ball mill pot, the ball-to-material ratio is 5:1, and the mixture is ball milled under argon protection for 1h at a rotation speed of 200rpm to obtain a mixture material; Step two, microwave sintering: the mixture material is placed in a microwave reaction cavity (the inner wall is coated with a SiC anti-corrosion coating), H2 / Ar (the volume ratio of H2 to Ar is 1:10) is introduced into the microwave reaction cavity, the temperature is raised to 600℃ at a rate of 5℃ / min, the microwave frequency is 2.45GHz, the microwave power is 800W, the sintering pressure is 0.1MPa, and the sintering time is 20min; Step three, post-treatment: the mixture material after microwave plasma sintering is naturally cooled, ground through a 200 mesh sieve, and a composite coated graphite negative electrode material is obtained, and the thickness of the composite coating layer is 25nm.

[0021] In the composite coated graphite negative electrode material prepared in this embodiment, the thickness of the inner layer amorphous lithium chloride is 2nm, the outer layer nanocrystalline LiF / Li3N composite phase includes LiF grains and Li3N sheet network, the size of the LiF grains is 4nm, the thickness of the Li3N sheet network is 1.5nm, and the mass ratio of the LiF grains to the Li3N sheet network is 3:1; the interlayer spacing of the graphite matrix is 0.341nm.

[0022] Example 3: A preparation method of a composite-coated graphite negative electrode material, comprising the following steps: Step one, precursor preparation: 5g artificial graphite (particle size 15μm, specific surface area 5m 2 / g), 0.5g LiCl·2H2O and 0.125g NH4F were added into a zirconia ball mill jar, the ball-to-material ratio was 5:1, and the mixture was ball-milled under argon protection for 4h at a rotation speed of 400rpm to obtain a mixture material; Step two, microwave sintering: the mixture material was placed in a microwave reaction cavity (the inner wall of which was coated with a SiC anticorrosion coating), H2 / Ar (the volume ratio of H2 to Ar was 1:5) was introduced into the microwave reaction cavity, the temperature was raised to 700℃ at a rate of 5℃ / min, the microwave frequency was 2.45GHz, the microwave power was 1200W, the holding time was 60min, and the sintering pressure was 1MPa; Step three, post-treatment: the mixture material after microwave plasma sintering was naturally cooled, ground through a 200-mesh sieve, and a composite-coated graphite negative electrode material was obtained, the thickness of the composite coating layer of which was 40nm.

[0023] In the composite-coated graphite negative electrode material prepared in this example, the thickness of the inner amorphous lithium chloride layer was 5nm, the outer nanocrystalline LiF / Li3N composite phase included LiF grains and Li3N sheet networks, the size of the LiF grains was 8nm, the thickness of the Li3N sheet networks was 3nm, and the mass ratio of the LiF grains to the Li3N sheet networks was 5:1; the interlayer spacing of the graphite matrix layer was 0.345nm.

[0024] Example 4: A preparation method of a composite-coated graphite negative electrode material, comprising the following steps: Step one, precursor preparation: 5g artificial graphite (particle size 10μm, specific surface area 7.5m 2 / g), 0.4g LiCl·2H2O and 0.025g NH4F were added into a zirconia ball mill jar, the ball-to-material ratio was 5:1, and the mixture was ball-milled under argon protection for 2h at a rotation speed of 300rpm to obtain a mixture material; Step two, microwave sintering: the mixture material was placed in a microwave reaction cavity (the inner wall of which was coated with a SiC anticorrosion coating), H2 / Ar (the volume ratio of H2 to Ar was 1:8) was introduced into the microwave reaction cavity, the temperature was raised to 680℃ at a rate of 5℃ / min, the microwave frequency was 2.45GHz, the microwave power was 900W, the holding time was 45min, and the sintering pressure was 0.8MPa; Step three, post-treatment: the mixture material after microwave plasma sintering was naturally cooled, ground through a 200-mesh sieve, and a composite-coated graphite negative electrode material was obtained, the thickness of the composite coating layer of which was 30nm.

[0025] The composite coated graphite negative electrode material prepared in the example has an inner amorphous lithium chloride layer with a thickness of 4 nm, and an outer nanocrystalline LiF / Li3N composite phase including LiF grains and Li3N sheet networks, wherein the size of the LiF grains is 7 nm, the thickness of the Li3N sheet networks is 2.5 nm, and the mass ratio of the LiF grains to the Li3N sheet networks is 4:1; and the graphite matrix layer spacing is 0.344 nm.

[0026] Example 5 A method for preparing a composite coated graphite negative electrode material, comprising the following steps: Step one, preparation of a precursor: 5 g of artificial graphite (particle size 8 μm, specific surface area 5 m 2 / g), 0.375 g of LiCl·2H2O and 0.112 g of NH4F were added to a zirconia ball mill jar, the ball-to-material ratio was 5:1, and the mixture was ball milled under argon protection for 2 h at a rotation speed of 300 rpm to obtain a mixture material; Step two, microwave sintering: the mixture material was placed in a microwave reaction cavity (the inner wall of which was coated with a SiC anticorrosion coating), H2 / Ar (the volume ratio of H2 to Ar was 1:8) was introduced into the microwave reaction cavity, the temperature was raised to 650℃ at a rate of 5℃ / min, the microwave frequency was 2.45 GHz, the microwave power was 1000 W, the holding time was 30 min, and the sintering pressure was 0.5 MPa; Step three, post-treatment: the mixture material after microwave plasma sintering was naturally cooled, ground through a 200-mesh sieve, and a composite coated graphite negative electrode material was obtained, and the thickness of the composite coating layer was 35 nm.

[0027] The composite coated graphite negative electrode material prepared in the example has an inner amorphous lithium chloride layer with a thickness of 3 nm, and an outer nanocrystalline LiF / Li3N composite phase including LiF grains and Li3N sheet networks, wherein the size of the LiF grains is 6 nm, the thickness of the Li3N sheet networks is 1.9 nm, and the mass ratio of the LiF grains to the Li3N sheet networks is 4:1; and the graphite matrix layer spacing is 0.343 nm.

[0028] Comparative Example 1 A method for preparing a lithium chloride coated graphite negative electrode material, comprising the following steps: Step one, preparation of a precursor: 5 g of artificial graphite (particle size 8 μm, specific surface area 10 m 2 / g), 0.375 g of LiCl·2H2O were added to a zirconia ball mill jar, the ball-to-material ratio was 5:1, and the mixture was ball milled under argon protection for 2 h at a rotation speed of 300 rpm to obtain a mixture material; Step two, microwave sintering: the mixture is placed in a microwave reaction cavity (the inner wall is coated with a SiC anti-corrosion coating), H2 / Ar (the volume ratio of H2 and Ar is 1:9) is introduced into the microwave reaction cavity, the temperature is raised to 650°C at a rate of 5°C / min, the microwave frequency is 2.45 GHz, the microwave power is 1000 W, the holding time is 30 min, and the sintering pressure is 0.5 MPa; Step three, post-processing: the mixture after microwave plasma sintering is naturally cooled, ground through a 200 mesh sieve, and a lithium chloride coated graphite negative electrode material is obtained, and the thickness of the lithium chloride coating layer is 25 nm.

[0029] Comparative Example 2: This comparative example is a preparation method of a composite coated graphite negative electrode material prepared by a traditional multi-step method, comprising the following steps: Step one, 5 g of artificial graphite (particle size 8 μm, specific surface area 10 m 2 / g) is coated with a LiCl solution by a wet coating method, ethanol is used as a solvent, and the coated material is dried at 80°C after coating; Step two, 5 g of artificial graphite coated with LiCl is mixed with 0.375 g of LiF and then high-temperature sintered, the sintering temperature is 700°C, and the sintering time is 2 h, to obtain a LiF coated graphite material; the LiF coated graphite material is added into 100 mL of a 25 wt% NH4F solution, stirred uniformly, and then soaked for 2 h, so that NH4F is uniformly adsorbed on the surface of the LiF coated graphite material; the soaked material is dried in an 80°C oven for 12 h to remove water, and a fluoride coating layer is formed on the surface of the LiF coated graphite material; Step three, the LiF coated graphite material with a fluoride coating layer is mechanically mixed with 0.25 g of Li3N powder to obtain a graphite negative electrode material, and the thickness of the composite coating layer is 35 nm.

[0030] Comparative Example 3: A preparation method of a composite coated graphite negative electrode material, comprising the following steps: Step one, preparation of a precursor: 5 g of artificial graphite (particle size 8 μm, specific surface area 10 m 2 / g), 0.75 g of LiCl·2H2O, and 0.224 g of NH4F are added into a zirconia ball mill jar, the ball-to-material ratio is 5:1, and the mixture is ball milled under argon protection for 2 h at a speed of 300 rpm to obtain a mixture material; Step two, microwave sintering: the mixture is placed in a microwave reaction cavity (the inner wall is coated with a SiC anti-corrosion coating), H2 / Ar (the volume ratio of H2 and Ar is 1:9) is introduced into the microwave reaction cavity, the temperature is raised to 650°C at a rate of 5°C / min, the microwave frequency is 2.45 GHz, the microwave power is 1000 W, the holding time is 30 min, and the sintering pressure is 0.5 MPa; Step three, post-processing: the mixture after microwave plasma sintering is naturally cooled, ground through a 200 mesh sieve, and a composite coated graphite negative electrode material is obtained, with a composite coating layer thickness of 60 nm.

[0031] Comparative Example 4: A method for preparing a composite coated graphite negative electrode material, comprising the following steps: Step one, preparation of a precursor: 5 g of artificial graphite (particle size 8 μm, specific surface area 10 m 2 / g), 0.375 g of LiCl·2H2O, and 0.112 g of NH4F are added to a zirconia ball mill jar, with a ball-to-material ratio of 5:1, and the mixture is ball milled under argon protection for 2 h at a rotation speed of 300 rpm to obtain a mixture; Step two, microwave sintering: the mixture is placed in a microwave reaction cavity (the inner wall of which is coated with a SiC anticorrosion coating), H2 / Ar (H2, Ar volume ratio 1:3) is introduced into the microwave reaction cavity, the temperature is raised to 650°C at a rate of 5°C / min, the microwave frequency is 2.45 GHz, the microwave power is 1000 W, the temperature is maintained for 30 min, and the sintering pressure is 0.5 MPa; Step three, post-processing: the mixture after microwave plasma sintering is naturally cooled, ground through a 200 mesh sieve, and a composite coated graphite negative electrode material is obtained, with a composite coating layer thickness of 35 nm.

[0032] The modified negative electrode materials prepared in the above comparative examples and examples are subjected to the following tests.

[0033] Ion conductivity test method The bulk ion conductivity of the composite negative electrode material is measured by electrochemical impedance spectroscopy (EIS), and the interface impedance and bulk impedance are separated in combination with a symmetric battery structure.

[0034] Test steps 1. Sample preparation: the composite coated graphite negative electrode material is dry mixed with a sulfide electrolyte (Li6PS5Cl) at a mass ratio of 7:3, and is pressed into a circular sheet with a diameter of 12 mm and a thickness of 200 μm (pressure 300 MPa). Assemble a symmetric battery: graphite composite negative electrode | Li6PS5Cl electrolyte layer | graphite composite negative electrode. Assembly pressure: 50 MPa (simulates actual battery assembly conditions).

[0035] 2. EIS test Equipment: electrochemical workstation (Solartron 1470E). Parameters: frequency range 0.1 Hz~1 MHz, amplitude 10 mV, temperature 25°C.

[0036] Data processing: the high frequency region intercept corresponds to the bulk resistance (RbulkR); Ionic conductivity calculation: where L is the electrolyte layer thickness, A is the electrode area.

[0037] Interface impedance test method: The negative electrode / electrolyte interface impedance (Rint) was analyzed by full-cell EIS to distinguish the charge transfer impedance from the interface side reaction impedance.

[0038] Test procedure: Full-cell assembly: Positive electrode: NCM811 material (face load 15 mg / cm2) mixed with Li6PS5Cl electrolyte at 8:2 and pressed; Negative electrode: Modified graphite composite electrode (face load 5 mg / cm2); Electrolyte layer: Li6PS5Cl pressed sheet (thickness 50 pm); Assembly pressure: 50 MPa. EIS test: Frequency range: 0.01 Hz-100 kHz, amplitude 5 mV. Equivalent circuit fitting: Rs(QintRint)(QdlRct) model was used to extract the interface impedance Rint(including SEI film impedance).

[0039] 1C full-cell cycle performance test Test conditions: Blue power test system (LAND CT2001A). Voltage window: 1.5~4.3V (vs. Li + / Li). Temperature: 25°C, humidity <1% (glove box environment). Charge and discharge procedure: constant current charge and discharge, 1C current density (based on positive electrode capacity calculation); 3 times 0.1C activation before cycling; record the discharge capacity at the 100th, 500th, and 1000th cycles, and calculate the capacity retention rate (%).

[0040] 5C rate lithium deposition starting voltage test The negative electrode potential was monitored in real time by a three-electrode system to determine the lithium deposition starting point.

[0041] Test procedure, three-electrode assembly: Working electrode: modified graphite composite electrode; counter electrode: lithium metal; reference electrode: lithium wire (isolated by a microporous glass); electrolyte: Li6PS5Cl pressed sheet. Charge and discharge procedure: constant current charging to 4.3V (5C rate, based on positive electrode capacity), recording the negative electrode potential (vs. Li + / Li) in real time; lithium deposition criterion: when the negative electrode potential drops below 0V and lasts for more than 10 seconds, the voltage at this time is recorded as the lithium deposition starting voltage.

[0042] As Figure 1As shown, the composite-coated graphite anode material prepared in Example 1 exhibited higher specific capacity and better cycle stability than Comparative Example 1 at all rates. The decreasing trend in specific capacity became more pronounced with increasing rate, particularly at high rates (e.g., 5C), where it rapidly decayed. At low rates (e.g., 0.05C and 0.1C), the specific capacity of both materials remained high, with a relatively gradual decline. These results demonstrate that Example 1 outperformed Comparative Example 1 at all rates, particularly in terms of cycle stability and capacity retention.

[0043] Table 1 Comparison of performance data of graphite negative electrode materials prepared in Examples 1 to 5 and Comparative Examples 1 to 4 As shown in the table above, Examples 1-5, using amorphous LiCl / nanocrystalline LiF / Li3N composite phase gradient coatings, all achieved ionic conductivities >1mS / cm (compared to 0.08mS / cm for pure LiCl in Comparative Example 1), demonstrating that the gradient design significantly optimizes the ion transport path. The interface impedance of Comparative Example 1 (without LiF / Li3N) was as high as 220Ω·cm², while that of Example 1 was only 45Ω·cm². 2 , verifying the "ion highway" effect of the outer nanocrystalline LiF / Li3N composite phase.

[0044] In Example 1, a one-step microwave plasma sintering process achieved uniform coating (thickness tolerance ±5nm), while in Comparative Example 2 (a traditional multi-step process), the coating thickness fluctuated by ±50%, resulting in a capacity retention of only 75% (compared to 94.2% in Example 1). In Comparative Example 4 (where the H2:Ar ratio exceeded the 1:5-1:10 range), excessive etching led to graphite structural collapse, confirming the necessity of a 1:5-1:10 volume ratio of H2:Ar.

[0045] Example 2 (LiCl content accounting for 5% of the mass of the artificial graphite) and Example 3 (LiCl content accounting for 10% of the mass of the artificial graphite) respectively verified the lower and upper limits of the mass ratio of graphite to lithium chloride containing crystalline water. The results showed that the ionic conductivity was still >1mS / cm and the cycle life was >800 cycles, proving the rationality of the mass ratio of graphite to lithium chloride containing crystalline water of 100:5-10. In Comparative Example 3 (LiCl content accounting for 15% of the mass of the artificial graphite), the interface impedance surged to 280Ω·cm due to the excessively thick coating layer (60nm). 2 , indicating that excessive addition of LiCl will destroy the structural stability of the composite-coated graphite negative electrode material.

[0046] The lithium precipitation starting voltage of examples 1-5 at 5C rate is all >0.28V (0.38V for the optimal example 1), while the lithium precipitation voltage of comparative examples 1-4 is <0.18V due to the lack of gradient coating layer or process defects, proving that the LiF / Li3N composite layer of the present application can preferentially guide the uniform deposition of lithium ions.

[0047] The present application solves the ion transmission bottleneck, interface side reaction and lithium precipitation problem of the traditional graphite negative electrode in the sulfide full solid-state battery through the gradient coating layer structure design and one-step microwave sintering process. The example data shows that within the range of the parameters disclosed in the present application, the performance (ionic conductivity, cycle life, lithium precipitation inhibition) of the composite coated graphite negative electrode material prepared is significantly better than that of the prior art.

[0048] The number of devices and the scale of processing described here are used to simplify the explanation of the present application. The application, modification and change of the present application are obvious to those skilled in the art.

[0049] Although the embodiments of the present application have been disclosed as above, it is not limited to the application and implementation listed in the specification and examples, and can be fully applied to various fields suitable for the present application, and additional modifications can be easily realized by those skilled in the art, therefore the present application is not limited to specific details and the figures shown and described herein, without departing from the general concept defined by the claims and the equivalent scope.

Claims

1. A composite coated graphite negative electrode material, characterized in that: include: A graphite substrate and a composite coating layer coated on the surface of the graphite substrate, wherein the composite coating layer comprises an inner layer of amorphous lithium chloride and an outer layer of nanocrystalline LiF / Li3N composite phase.

2. The composite coated graphite negative electrode material according to claim 1, characterized in that The thickness of the inner layer of amorphous lithium chloride is 2-5 nm; the outer layer of nanocrystalline LiF / Li3N composite phase includes LiF grains and a Li3N sheet network, the size of the LiF grains is 3-8 nm, and the thickness of the Li3N sheet network is 1-3 nm; the total thickness of the composite coating layer is 10-50 nm; and the interlayer spacing of the graphite matrix is ​​0.340-0.345 nm.

3. The composite coated graphite negative electrode material according to claim 2, characterized in that: The mass ratio of the LiF grains to the Li3N sheet network is 3-5:1, and the Li3N sheet network is interspersed and distributed in the gaps between the LiF grains.

4. The composite coated graphite negative electrode material according to claim 1, characterized in that The graphite matrix is ​​artificial graphite or natural graphite with a particle size of 5 to 15 μm and a specific surface area of ​​5 to 10 m 2 / g.

5. A method for preparing a composite coated graphite negative electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: mixing graphite, lithium chloride containing crystal water and ammonium fluoride to obtain a mixed material; Step 2: placing the mixed material in a microwave reaction cavity, heating it to 600-700°C at a microwave power of 800-1200W in a H2 / Ar mixed atmosphere, and keeping the temperature for 20-60 minutes to perform microwave plasma sintering on the mixed material; Step 3: After cooling the mixture after microwave plasma sintering, grind it and sieve it through 100-300 mesh to obtain a composite coated graphite negative electrode material.

6. The method for preparing the composite coated graphite negative electrode material according to claim 5, characterized in that: In the step 1, the mass ratio of graphite, lithium chloride containing crystal water, and ammonium fluoride is 100:(5-10):(0.5-2.5); wherein the chemical formula of lithium chloride containing crystal water is LiCl·nH2O, where n=1, 2, or 3.

7. The method for preparing the composite coated graphite negative electrode material according to claim 5, characterized in that: In the step 1, the graphite, lithium chloride containing crystal water and ammonium fluoride are mixed by planetary ball milling, with a ball-to-material ratio of 5:1, a rotation speed of 200-400 rpm, a ball milling time of 1-4 hours, and a ball milling medium of zirconia balls.

8. The method for preparing the composite coated graphite negative electrode material according to claim 5, wherein: In the step 2, the inner wall of the microwave reaction cavity is coated with a SiC anti-corrosion layer, the microwave frequency is 1-2.5 GHz, and the heating rate is ≤5°C / min.

9. The method for preparing the composite coated graphite negative electrode material according to claim 5, characterized in that: In the step 2, the volume ratio of H 2 to Ar in the H 2 / Ar mixed atmosphere is 1:5-1:10, and the microwave plasma sintering pressure is 0.1-1 MPa.

10. A lithium-ion solid-state battery, characterized in that: The negative electrode material of the solid-state battery is the composite-coated graphite negative electrode material according to any one of claims 1 to 4.

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