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

Abstract

The application discloses a kind of composite coated graphite negative material and its preparation method and lithium ion solid-state battery, belong to lithium ion solid-state battery technical field, include: graphite matrix and composite coating, the composite coating includes inner layer amorphous lithium chloride and outer layer nanocrystalline LiF / Li3N composite phase.Mixture is obtained by graphite, lithium chloride containing crystallization water and ammonium fluoride;Mixed material is placed in microwave reaction cavity, under H2 / Ar mixed atmosphere, with 800~1200 W microwave power heating to 600~700 DEG C, and keeps warm for 20~60 min;After sintering, the mixture is cooled, ground and sieved 100~300 mesh, to obtain the composite coated graphite negative material of coating layer thickness 10~50 nm.The application proposes an innovative amorphous LiCl / nanocrystalline LiF-Li3N gradient coating design, by optimizing ion migration path, enhancing interface stability and reducing process cost, realize the significant improvement of lithium ion battery performance, provide a new engineering approach for realizing the high rate and long cycle performance of sulfide full 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, and 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:

[0009] Step one, mixing graphite, lithium chloride containing crystal water (LiCl nH2O, n = 1, 2, 3) and ammonium fluoride to obtain a mixture;

[0010] 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;

[0011] Step three, grinding and sieving the mixture after microwave plasma sintering to 100-300 mesh to obtain a composite-coated graphite negative electrode material.

[0012] 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).

[0013] 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.

[0014] 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.

[0015] 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.

[0016] A lithium ion solid-state battery, wherein the negative electrode material is the composite-coated graphite negative electrode material described above.

[0017] 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.

[0018] The design of the inner layer amorphous LiCl and the outer layer nanocrystalline LiF-Li3N gradient coating obtained by the present application is to synergistically optimize the 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 (ionic conductivity of 10-3 S / cm) provides rigid mechanical support (modulus of 200 GPa) to prevent the penetration of dendrites and blocks the electron transmission through the wide electrochemical window (>5 V) of the LiF to inhibit the side reactions. The nitride structure of the Li3N can passivate the interface defects to further reduce the interface impedance. The gradient design realizes the synergistic enhancement of the fast ion transmission and the interface stability through the inner layer of the flexible buffer + the outer layer of the rigid protection, and the one-step process (such as microwave sintering) simplifies the preparation process and significantly reduces the production cost.

[0019] Other advantages, objects, and features of the present application will be better understood from the following description taken in conjunction with the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The rate performance graph of the graphite negative electrode material prepared in Example 1 and Comparative Example 1 of the present application;

[0021] Figure 2 The SEM graph of the composite coated graphite negative electrode material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0022] The present application will be further described in conjunction with the drawings to enable those skilled in the art to carry out the present application with reference to the description and drawings.

[0023] 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.

[0024] Example 1:

[0025] A preparation method of a composite coated graphite negative electrode material, comprising the following steps:

[0026] Step one, preparation of a precursor: 5 g of artificial graphite (particle size of 8 μm, specific surface area of 10 m 2 / g), 0.375 g of LiCl·2H2O, and 0.112 g 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 2 h at a rotation speed of 300 rpm to obtain a mixture material;

[0027] 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 (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 2.45 GHz, the microwave power is 1000 W, the holding time is 30 min, and the sintering pressure is 0.5 MPa;

[0028] 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. The SEM image of the composite coated graphite negative electrode material prepared in this embodiment is shown in FIG. 1. Figure 2 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 amorphous lithium chloride in the inner layer 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 elements in the composite coating layer of the composite coated graphite negative electrode material of this embodiment show a gradient distribution, 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 amorphous lithium chloride in the inner layer can cover more than 95% of the surface of the graphite matrix.

[0029] Embodiment 2

[0030] A method for preparing a composite coated graphite negative electrode material, comprising the following steps:

[0031] Step one, preparation of a precursor: 5 g of artificial graphite (particle size 5 μm, specific surface area 10 m 2 / g), 0.25 g of LiCl·2H2O, and 0.067 g 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 1 h at a rotation speed of 200 rpm to obtain a mixture material;

[0032] 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°C at a rate of 5°C / min, the microwave frequency is 2.45 GHz, the microwave power is 800 W, the sintering pressure is 0.1 MPa, and the sintering time is 20 min;

[0033] 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. The SEM image of the composite coated graphite negative electrode material prepared in this embodiment is shown in FIG. 1.

[0034] ​The composite coated graphite negative electrode material prepared in the embodiment has an inner amorphous lithium chloride layer with a thickness of 2 nm, and an outer nanocrystalline LiF / Li3N composite phase including LiF grains and Li3N sheet networks, wherein the size of the LiF grains is 4 nm, the thickness of the Li3N sheet networks is 1.5 nm, and the mass ratio of the LiF grains to the Li3N sheet networks is 3:1; and the graphite matrix layer spacing is 0.341 nm.

[0035] Example 3

[0036] A method for preparing a composite coated graphite negative electrode material, comprising the following steps:

[0037] Step one, preparation of a precursor: 5 g of artificial graphite (particle size 15 μm, specific surface area 5 m 2 / g), 0.5 g of LiCl·2H2O and 0.125 g of NH4F are added to a zirconium oxide ball mill jar, the ball-to-material ratio is 5:1, and the mixture is ball milled under argon protection for 4 h at a rotation speed of 400 rpm to obtain a mixture material;

[0038] 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 1:5) is introduced into the microwave reaction cavity, the temperature is raised to 700℃ at a rate of 5℃ / min, the microwave frequency is 2.45 GHz, the microwave power is 1200 W, the holding time is 60 min, and the sintering pressure is 1 MPa;

[0039] 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, wherein the thickness of the composite coating layer is 40 nm.

[0040] The composite coated graphite negative electrode material prepared in the embodiment has an inner amorphous lithium chloride layer with a thickness of 5 nm, and an outer nanocrystalline LiF / Li3N composite phase including LiF grains and Li3N sheet networks, wherein the size of the LiF grains is 8 nm, the thickness of the Li3N sheet networks is 3 nm, and the mass ratio of the LiF grains to the Li3N sheet networks is 5:1; and the graphite matrix layer spacing is 0.345 nm.

[0041] Example 4

[0042] A method for preparing a composite coated graphite negative electrode material, comprising the following steps:

[0043] Step one, preparation of a precursor: 5 g of artificial graphite (particle size 15 μm, specific surface area 5 m 2 / g), 0.4 g LiCl·2H2O and 0.025 g NH4F were added into a zirconium oxide 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;

[0044] 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°C at a rate of 5°C / min, the microwave frequency was 2.45 GHz, the microwave power was 900 W, the temperature was maintained for 45 min, and the sintering pressure was 0.8 MPa;

[0045] 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 30 nm.

[0046] In the composite-coated graphite negative electrode material prepared in this example, the thickness of the inner amorphous lithium chloride layer was 4 nm, the outer nanocrystalline LiF / Li3N composite phase included LiF grains and Li3N sheet networks, the size of the LiF grains was 7 nm, the thickness of the Li3N sheet networks was 2.5 nm, and the mass ratio of the LiF grains to the Li3N sheet networks was 4:1; the interlayer spacing of the graphite matrix layer was 0.344 nm.

[0047] Example 5:

[0048] A method for preparing a composite-coated graphite negative electrode material, comprising the following steps:

[0049] 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 LiCl·2H2O and 0.112 g NH4F were added into a zirconium oxide 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;

[0050] 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°C at a rate of 5°C / min, the microwave frequency was 2.45 GHz, the microwave power was 1000 W, the temperature was maintained for 30 min, and the sintering pressure was 0.5 MPa;

[0051] 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 35 nm.

[0052] The composite coated graphite negative electrode material prepared in the embodiment 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 network, wherein the size of the LiF grains is 6 nm, the thickness of the Li3N sheet network is 1.9 nm, and the mass ratio of the LiF grains to the Li3N sheet network is 4:1; and the graphite matrix layer spacing is 0.343 nm.

[0053] Comparative Example 1

[0054] A preparation method of a lithium chloride coated graphite negative electrode material, comprising the following steps:

[0055] Step one, preparation of a precursor: 5 g of artificial graphite (particle size 8 μm, specific surface area 10 m 2 / g) and 0.375 g of LiCl·2H2O were added to a zirconium oxide 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;

[0056] 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 1:9) 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 sintering pressure was 0.5 MPa, and the material was kept at the temperature for 30 min;

[0057] Step three, post-treatment: the mixture material after microwave plasma sintering was naturally cooled, ground through a 200-mesh sieve, and a lithium chloride coated graphite negative electrode material was obtained, wherein the thickness of the lithium chloride coating layer was 25 nm.

[0058] Comparative Example 2

[0059] The present 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:

[0060] Step one, wet coating of a LiCl solution on the surface of 5 g of artificial graphite (particle size 8 μm, specific surface area 10 m 2 / g), using ethanol as a solvent, and drying at 80 ℃ after coating;

[0061] Step two, after mixing 5g artificial graphite coated with LiCl with 0.375g LiF, high-temperature sintering, sintering temperature 700℃, sintering time 2h, to obtain LiF coated graphite material; after adding LiF coated graphite material into 100mL NH4F solution with concentration of 25wt%, stirring uniformly, then soaking for 2h, making NH4F uniformly adsorbed on the surface of LiF coated graphite material; after placing the impregnated material in 80℃ oven for drying for 12h, removing water, forming fluoride coating layer on the surface of LiF coated graphite material;

[0062] Step three, mechanically mixing LiF coated graphite material with fluoride coating layer with 0.25g Li3N powder, to obtain graphite negative electrode material, the thickness of the composite coating layer is 35nm.

[0063] Comparative example 3:

[0064] A preparation method of a composite coated graphite negative electrode material, comprising the following steps:

[0065] Step one, precursor preparation: adding 5g artificial graphite (particle size 8μm, specific surface area 10m 2 / g), 0.75g LiCl·2H2O and 0.224g NH4F into a zirconia ball mill jar, ball-to-material ratio 5:1, ball milling mixing under argon protection for 2h, rotation speed 300rpm, to obtain mixture material;

[0066] Step two, microwave sintering: placing the mixture material into a microwave reaction cavity (inner wall coated with SiC anticorrosion coating), introducing H2 / Ar (volume ratio of H2, Ar 1:9) into the microwave reaction cavity, heating to 650℃ at 5℃ / min, microwave frequency 2.45GHz, microwave power 1000W, holding for 30min, sintering pressure 0.5MPa;

[0067] Step three, post-treatment: naturally cooling the mixture material after microwave plasma sintering, grinding through 200 mesh sieve, to obtain composite coated graphite negative electrode material, the thickness of the composite coating layer is 60nm.

[0068] Comparative example 4:

[0069] A preparation method of a composite coated graphite negative electrode material, comprising the following steps:

[0070] Step one, precursor preparation: adding 5g artificial graphite (particle size 8μm, specific surface area 10m 2 / g), 0.375g LiCl·2H2O and 0.112g NH4F into a zirconia ball mill jar, ball-to-material ratio 5:1, ball milling mixing under argon protection for 2h, rotation speed 300rpm, to obtain mixture material;

[0071] Step two, microwave sintering: the mixture is placed in a microwave reaction cavity (the inner wall is coated with SiC anticorrosion coating), H2 / Ar (the volume ratio of H2 and Ar is 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 2.45GHz, the microwave power is 1000W, the sintering pressure is 0.5MPa, and the sintering time is 30min;

[0072] 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 with a composite coating layer thickness of 35nm is obtained.

[0073] The modified negative electrode materials prepared in the above examples and comparative examples are subjected to the following tests.

[0074] Ion conductivity test method

[0075] 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 the symmetric battery structure.

[0076] Test steps

[0077] 1. Sample preparation: the composite coated graphite negative electrode material is dry mixed with sulfide electrolyte (Li6PS5Cl) at a mass ratio of 7:3, and is pressed into a circular sheet with a diameter of 12mm and a thickness of 200μm (pressure 300MPa). Assemble the symmetric battery: graphite composite negative electrode | Li6PS5Cl electrolyte layer | graphite composite negative electrode. Assembly pressure: 50MPa (simulating the actual battery assembly conditions).

[0078] 2. EIS test

[0079] Equipment: electrochemical workstation (Solartron 1470E). Parameters: frequency range 0.1Hz~1 MHz, amplitude 10mV, temperature 25°C.

[0080] Data processing: the high-frequency region intercept corresponds to the bulk resistance (RbulkR);

[0081] Ion conductivity calculation: where L is the electrolyte layer thickness, and A is the electrode area.

[0082] Interface impedance test method: the negative electrode / electrolyte interface impedance (Rint) is analyzed by full battery EIS to distinguish the charge transfer impedance and the interface side reaction impedance.

[0083] Test steps: full battery assembly: the positive electrode: NCM811 material (surface load 15mg / cm²) is mixed with Li6PS5Cl electrolyte at a ratio of 8:2 and pressed;

[0084] Negative electrode: modified graphite composite electrode sheet (surface loading 5 mg / cm2); electrolyte layer: Li6PS5Cl pressed sheet (thickness 50 μm); 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).

[0085] 1C full battery cycle performance test

[0086] Test conditions: blue electricity test system (LAND CT2001A). Voltage window: 1.5~4.3V (vs. Li + / Li). Temperature: 25℃, humidity <1% (glove box environment). Charge and discharge steps: 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 of the 100th, 500th, and 1000th cycle, and calculate the capacity retention rate (%).

[0087] 5C rate lithium deposition starting voltage test

[0088] The negative electrode potential was monitored in real time by a three-electrode system to determine the lithium deposition starting point.

[0089] Test steps, three-electrode assembly: working electrode: modified graphite composite electrode sheet; counter electrode: lithium metal; reference electrode: lithium wire (isolated by a microporous glass); electrolyte: Li6PS5Cl pressed sheet. Charge and discharge steps: constant current charging to 4.3V (5C rate, based on positive electrode capacity), real-time recording of negative electrode potential (vs. Li + / Li); 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.

[0090] As Figure 1 shown, the composite coated graphite negative electrode material prepared in Example 1 exhibited higher specific capacity and better cycle stability than Comparative Example 1 at all rates. As the rate increased, the trend of specific capacity decline became more obvious, especially at high rates (such as 5C), the specific capacity rapidly decayed. At low rates (such as 0.05C and 0.1C), the specific capacity of both materials could be maintained at a relatively high level, and the decline trend was relatively gentle. These results show that the performance of Example 1 is superior to that of Comparative Example 1 at different rates, especially in terms of cycle stability and capacity retention.

[0091] Table 1 Performance data comparison of graphite negative electrode materials prepared in Example 1-Example 5, Comparative Example 1-Comparative Example 4

[0092]

[0093] As shown in the above table, examples 1-5 use amorphous LiCl / nanocrystalline LiF / Li3N composite phase gradient coating layer, and the ion conductivity is all >1 mS / cm (comparative example 1 pure LiCl is only 0.08 mS / cm), which proves that the gradient design significantly optimizes the ion transmission path. The interface impedance of comparative example 1 (without LiF / Li3N) is as high as 220 Ω·cm², while example 1 is only 45 Ω·cm 2 , verifying the “ion highway” effect of the outer nanocrystalline LiF / Li3N composite phase.

[0094] Example 1 realizes the uniformity of the coating layer (thickness error ±5 nm) by a one-step method of microwave plasma sintering, while comparative example 2 (traditional multi-step process) has a coating layer thickness fluctuation of ±50%, resulting in a capacity retention rate of only 75% (example 1 is 94.2%). Comparative example 4 (H2, Ar ratio exceeds 1:5~1:10 range) causes the collapse of graphite structure due to excessive etching, which proves the necessity of H2, Ar volume ratio of 1:5~1:10.

[0095] Example 2 (LiCl dosage accounts for 5% of the mass of artificial graphite) and example 3 (LiCl dosage accounts for 10% of the mass of artificial graphite) respectively verify the lower limit and upper limit of the mass ratio of graphite and lithium chloride containing crystal water, and the results show that the ion conductivity is still >1 mS / cm, and the cycle life is >800 times, proving the rationality of the mass ratio of graphite and lithium chloride containing crystal water in the range of 100:5~10. Comparative example 3 (LiCl dosage accounts for 15% of the mass of artificial graphite) causes the interface impedance to surge to 280 Ω·cm 2 , indicating that excessive addition of LiCl will destroy the structural stability of the composite coated graphite negative electrode material.

[0096] Examples 1-5 have a lithium precipitation starting voltage of >0.28V at 5C rate (the optimal example 1 reaches 0.38V), while comparative examples 1-4 have a lithium precipitation voltage of <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.

[0097] The present application solves the ion transmission bottleneck, interface side reaction and lithium precipitation problem of traditional graphite negative electrode in sulfide full solid-state battery through gradient coating layer structure design and one-step microwave sintering process. The example data shows that within the parameter range disclosed in the present application, the performance (ion conductivity, cycle life, lithium precipitation inhibition) of the composite coated graphite negative electrode material prepared is significantly better than that of the prior art.

[0098] The number of devices and processing stages described herein are used to simplify the description of the application. Applications, modifications and variations of the application will be apparent to those skilled in the art without departing from the general concept of the application.

[0099] While the embodiments of the application have been disclosed in connection with the specification and examples herein, it should be understood that they are not limited to the particular details described but rather can be practiced with modifications and changes apparent to one of ordinary skill in the art. It is therefore contemplated to cover any and all modifications, variations or equivalents that fall within the scope of the present application as claimed.

Claims

1. A method for preparing a composite-coated graphite negative electrode material, characterized by, The method comprises the following steps: Step one, mixing graphite, lithium chloride containing crystal water and ammonium fluoride to obtain a mixture, the mass ratio of graphite, lithium chloride containing crystal water and ammonium fluoride being 100: (5-10): (0.5-2.5); Step two, placing the mixture in a microwave reaction cavity, heating to 600-700 DEG C under H2 / Ar mixed atmosphere with microwave power of 800-1200 W, and keeping for 20-60 min to perform microwave plasma sintering on the mixture; the volume ratio of H2 and Ar in the H2 / Ar mixed atmosphere being 1:5-1:10; Step three, grinding and sieving the mixture after microwave plasma sintering to obtain a composite-coated graphite negative electrode material.

2. The method for preparing the composite-coated graphite anode material according to claim 1, characterized in that, In step one, the chemical formula of lithium chloride containing crystal water is LiCl·nH2O, and n=1, 2 or 3.

3. The method for preparing the composite-coated graphite anode material according to claim 1, characterized in that, 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 being 5:1, the rotation speed being 200-400 rpm, the ball milling time being 1-4 h, and the ball milling medium being zirconia balls.

4. The method for preparing the composite-coated graphite anode material according to claim 1, characterized in that, In step two, the inner wall of the microwave reaction cavity is coated with a SiC corrosion-resistant layer, the microwave frequency is 1-2.5 GHz, and the temperature rising rate is ≤5 DEG C / min.

5. The method for preparing the composite-coated graphite anode material according to claim 1, characterized in that, In step two, the microwave plasma sintering pressure is 0.1-1 MPa.

6. A composite-coated graphite negative material prepared by the method of any one of claims 1-5, characterized in that, The composite-coated graphite negative electrode material comprises a graphite matrix and a composite coating layer coated on the surface of the graphite matrix, and the composite coating layer comprises inner amorphous lithium chloride and outer nanocrystalline LiF / Li3N composite phase.

7. The composite-coated graphite anode material of claim 6, wherein, The thickness of the inner amorphous lithium chloride is 2-5 nm; the outer nanocrystalline LiF / Li3N composite phase comprises LiF grains and Li3N sheet-shaped networks, the size of the LiF grains is 3-8 nm, and the thickness of the Li3N sheet-shaped networks 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.

8. The composite-coated graphite anode material of claim 7, wherein, The mass ratio of the LiF grains to the Li3N sheet-shaped networks is 3-5:1, and the Li3N sheet-shaped networks are distributed in the gaps between the LiF grains.

9. The composite-coated graphite anode material of claim 6, wherein, The graphite matrix is artificial graphite or natural graphite, with a particle size of 5-15 μm and a specific surface area of 5-10 m 2 / g.

10. A lithium-ion solid-state battery, characterized by, The negative electrode material of the solid-state battery is the composite-coated graphite negative electrode material according to any one of claims 6-9.

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