Core-shell material, negative electrode material and preparation method thereof

Abstract

The invention relates to the technical field of batteries, in particular to a core-shell material, a negative electrode material and a preparation method thereof. The core-shell material provided by the invention comprises a core layer, a transition layer and a shell layer which are sequentially coated from inside to outside, wherein a bonding layer is arranged on the periphery of the shell layer and is bonded with the shell layer; the core-shell material is Al3Zr (at) Al3Li, the Al3Zr single crystal is a core layer of the core-shell material, the Al3Li single crystal is a shell layer of the core-shell material, the Al3 (Zr, Li) solid solution is an interface transition layer of the core-shell material, the bonding layer comprises Mg2Cu3 and Cu atoms, and the Mg2Cu3 and the Cu atoms are respectively bonded with the surface of the shell layer, or the Mg2Cu3 is bonded with the surface of the shell layer through an Mg-O bond, and the Cu atoms are bonded with the surface of the shell layer through a Cu-O-Al bond. The problem that when existing homogeneous alloy powder is applied to a negative electrode, the anti-expansion capacity is poor is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a core-shell material, a negative electrode material and a preparation method thereof. BACKGROUND

[0002] With the surge in demand for high energy density batteries, lithium metal negative electrode has become a research hotspot due to its ultra-high theoretical capacity, but the severe volume expansion and uncontrollable dendrite growth during the cycle process seriously restrict its practical application. The existing technology usually introduces a buffer structure to relieve the expansion, but there are still significant defects: for example, the existing technology uses a traditional homogeneous alloy electrode, although the homogeneous alloy powder has a certain anti-expansion ability, but under the huge volume change of long-term cycle of the battery, single alloy particles themselves crack and even pulverize. Therefore, it is urgent to develop a material with mechanical strengthening and volume buffering functions, which is of decisive significance for promoting the industrialization of lithium metal negative electrode. SUMMARY

[0003] In order to solve the problem of low eutectic efficiency of the preparation method of the traditional core-shell material, the present application provides a core-shell material, a negative electrode material and a preparation method thereof.

[0004] In order to solve the above technical problems, the present application provides the following technical solutions: a core-shell material, the core-shell material includes a core layer, a transition layer and a shell layer which are sequentially coated from inside to outside, and a bonding layer provided on the shell layer, the bonding layer is provided on the outer periphery of the shell layer and is bonded with the shell layer; and the core-shell material is Al3Zr@Al3Li, wherein the Al3Zr single crystal is the core layer of the core-shell material, the Al3Li single crystal is the shell layer of the core-shell material, the Al3(Zr, Li) solid solution is the interface transition layer of the core-shell material, the bonding layer includes Mg2Cu3 and Cu atoms, the Mg2Cu3 and the Cu atoms are respectively bonded with the surface of the shell layer, or the Mg2Cu3 is bonded with the surface of the shell layer through Mg-O bond, and the Cu atom is bonded with the surface of the shell layer through Cu-O-Al bond.

[0005] Preferably, the mass ratio of metal elements in the core-shell material is lithium: magnesium: copper: aluminum: zirconium = (70-80): (7.6-14.9): (3.8-7.4): (3.8-7.4): (0.3-1.0).

[0006] Preferably, the core-shell material is approximately spherical particles, the diameter of the core layer ranges from 20 to 50 nm, the thickness of the shell layer ranges from 5 to 10 nm, the thickness of the transition layer ranges from 2 to 5 nm, and the thickness of the bonding layer ranges from 2 to 5 nm.

[0007] The application provides another technical scheme to solve the above technical problems, and the technical scheme is as follows: a preparation method of a negative electrode material, the preparation method comprising the following steps: The modified carbon skeleton, the metal lithium material and the core-shell material are provided, the modified carbon skeleton, the core-shell material and the metal lithium material are stacked from bottom to top in an inert gas atmosphere, and then heat pressing treatment is performed, the metal lithium material is melted to infiltrate and densify the core-shell material and infiltrate into pores in the modified carbon skeleton, and then cooling treatment is performed; wherein the modified carbon skeleton infiltrated by the melted metal lithium material forms a base layer after cooling, the core-shell material infiltrated by the melted metal lithium material forms a buffer layer after cooling, and the metal lithium material which is melted and located on a side of the core-shell material away from the modified carbon skeleton forms a lithium-rich layer after cooling, so as to obtain the negative electrode material.

[0008] Preferably, the mass ratio of the modified carbon skeleton, the metal lithium material and the core-shell material is 25-35%:10-25%:50-55% of the modified carbon skeleton:the metal lithium material:the core-shell material.

[0009] Preferably, the step of providing the modified carbon skeleton comprises: providing a carbon fiber felt, a diamond lithium-avoiding layer, Ti3C2Tx x a lithium-philic layer, wherein Ti3C2Tx x is a general chemical formula of a MXene material, Ti3C2 is a fixed stoichiometric ratio structure of a MXene material parent body, T represents a -O, -OH, -F surface terminal group, and x represents a non-stoichiometric composition thereof; a diamond lithium-avoiding layer surface is deposited on the surface of each carbon fiber of the carbon fiber felt by using a chemical vapor deposition method to form a prefabricated carbon skeleton; and the Ti3C2Tx x lithium-philic layer on the surface of the prefabricated carbon skeleton to obtain the modified carbon skeleton.

[0010] Preferably, the heat pressing treatment, the melting of the metal lithium material to infiltrate and densify the core-shell material and infiltrate into pores in the modified carbon skeleton, and the cooling treatment comprise: setting the pressure of the inert gas atmosphere to 1-3 MPa, heating from room temperature to 230-270 DEG C; maintaining at 230-270 DEG C for 0.5-1 hour to complete the heat pressing treatment; and cooling to room temperature at a cooling rate of 5-15 DEG C / min to obtain the negative electrode material with the stacked base layer, the buffer layer and the lithium-rich layer; or setting the pressure of the inert gas atmosphere to 4-6 MPa, heating from room temperature to 230-270 DEG C; maintaining at 230-270 DEG C for 1-3 hours to complete the heat pressing treatment; and cooling to room temperature at a cooling rate of 5-15 DEG C / min to obtain the negative electrode material with the stacked base layer, the first interface transition layer, the buffer layer, the second interface transition layer and the lithium-rich layer.

[0011] The application provides another technical solution to solve the above technical problems, and is characterized in that the negative electrode material comprises a substrate layer, a buffer layer and a lithium-rich layer, the lithium-rich layer comprises a first lithium metal matrix, the buffer layer comprises a second lithium metal matrix, buffer pores distributed in the second lithium metal matrix and the core-shell material, and the substrate layer comprises a third lithium metal matrix and a modified carbon skeleton distributed in the third lithium metal matrix, and the first lithium metal matrix, the second lithium metal matrix and the third lithium metal matrix are sequentially stacked.

[0012] Preferably, the thickness of the substrate layer is 100-150 μm, the thickness of the buffer layer is 50-80 μm, and the thickness of the lithium-rich layer is 10-30 μm.

[0013] Preferably, the negative electrode material further comprises a first interface transition layer and a second interface transition layer, the first interface transition layer is arranged between the substrate layer and the buffer layer, and the second interface transition layer is arranged between the lithium-rich layer and the buffer layer; the first interface transition layer is a Cu-Al solid solution bonding layer, and the second interface transition layer is a Mg2Cu3 / Li2MgCu metallurgical bonding zone.

[0014] Compared with the prior art, the core-shell material, the negative electrode material and the preparation method thereof have the following beneficial effects: 1. The core-shell material provided by the embodiment of the application comprises a core layer, a transition layer and a shell layer which are sequentially coated from inside to outside, and a bonding layer arranged on the shell layer, wherein the bonding layer is arranged on the outer periphery of the shell layer and is bonded with the shell layer; the core-shell material is Al3Zr@Al3Li, wherein the Al3Zr single crystal is the core layer of the core-shell material, the Al3Li single crystal is the shell layer of the core-shell material, the Al3(Zr, Li) solid solution is the interface transition layer of the core-shell material, and the bonding layer comprises Mg2Cu3 and Cu atoms, wherein the Mg2Cu3 and the Cu atoms are respectively bonded with the surface of the shell layer, or the Mg2Cu3 is bonded with the surface of the shell layer through Mg-O bonds, and the Cu atoms are bonded with the surface of the shell layer through Cu-O-Al bonds. The core-shell material in the technical solution claimed in the application can be used as an important material of the buffer layer of the negative electrode material, wherein the core layer and the Mg2Cu3 form an elastic network, and plastic strain is absorbed. The Al3(Zr, Li) transition layer introduced in this embodiment alleviates the interface stress through gradient solid solution of Zr and Li, and solves the problem that the interface of the traditional core-shell material is prone to cracking after being applied to a battery and then being cycled.

[0015] 2. In the technical solution defined in the application, the mass ratio of metal elements in the core-shell material is lithium: magnesium: copper: aluminum: zirconium = (70-80): (7.6-14.9): (3.8-7.4): (3.8-7.4): (0.3-1.0). The element ratio directly affects the formation and distribution of the core-shell phase. If the lithium content is too high, such as more than 80%, it will cause free lithium proliferation and lead to dendrite and capacity attenuation. If the content is too low, it cannot provide sufficient active material. If the magnesium content is too high, such as more than 14.9%, it will generate excessive brittle phase, leading to electrode pulverization. If the content is too low, such as less than 7.6%, it will weaken the solid solution strengthening effect and reduce the anti-expansion ability. If the copper content is too high, such as more than 7.4%, it is easy to form inert "dead copper" phase, reducing the capacity. If the content is too low, such as less than 3.8%, it cannot form enough Mg2Cu3 elastic phase to buffer stress. If the aluminum content is too high, such as more than 7.4%, it will produce impurities, destroying the purity of the core-shell structure. If the content is too low, such as less than 3.8%, it will directly lead to the failure of the core layer and the shell layer to fully generate, losing the core function. If the zirconium content is too high, such as more than 1.0%, it will form harmful zirconium lithium compounds, blocking ion channels and increasing cost. If the content is too low, such as less than 0.3%, it cannot form enough Al. By limiting the element ratio in the core-shell material, the performance degradation problem caused by unbalanced composition of the core-shell material is solved.

[0016] 3. In the technical solution defined in the application, the diameter of the core layer is in the range of 20-50 nm. When the diameter of the core layer is greater than 50 nm, the stress distribution of the core layer is uneven, and cracks are easy to occur during the cycle process. When the diameter of the core layer is less than 20 nm, the specific surface area is too large, leading to an increase in surface energy and promoting the generation of interface reaction products. By limiting the diameter range of the core layer, the problem of stress concentration or insufficient modulus caused by improper size of the core layer is solved.

[0017] 4. In the technical solution defined in the application, the thickness of the shell layer is in the range of 5-10 nm. By controlling the thickness of the shell layer to be 5-10 nm, when the thickness of the shell layer is greater than 10 nm, the Li⁺ diffusion path is lengthened, leading to capacity loss at high rate. When the thickness of the shell layer is less than 5 nm, the lithium-philic site is insufficient, the nucleation overpotential is increased, and dendrite growth is easy to occur. By limiting the diameter range of the shell layer, the problem of ion transmission being hindered by too thick shell layer or the problem of insufficient deposition caused by too thin shell layer in traditional core-shell material is solved.

[0018] 5. In the technical solution defined in the application, the thickness of the transition layer is in the range of 2-5 nm. By limiting the thickness of the transition layer to be 2-5 nm, when the thickness of the transition layer is less than 2 nm, the buffering effect is insufficient, and stress concentration still exists at the interface. When the thickness of the transition layer is greater than 5 nm, the transition layer is brittle due to solid solution strengthening, and the elongation rate decreases. By limiting the diameter range of the transition layer, the technical difficulties of insufficient interface transition layer to relieve lattice mismatch or increased brittleness caused by too thick transition layer are solved.

[0019] 6. The embodiment of the present application further provides a preparation method of the negative electrode material, comprising the following steps: providing a modified carbon skeleton, a metal lithium material and the core-shell material as described above; stacking the modified carbon skeleton, the core-shell material and the metal lithium material from bottom to top in an inert gas atmosphere, and then performing a hot pressing treatment, so that the metal lithium material is melted and infiltrates and densifies the core-shell material and penetrates into pores in the modified carbon skeleton, and then performing a cooling treatment; wherein the modified carbon skeleton infiltrated by the melted metal lithium material forms a base layer after cooling, the core-shell material infiltrated by the melted metal lithium material forms a buffer layer after cooling, and the metal lithium material which is melted but located on a side of the core-shell material away from the modified carbon skeleton forms a lithium-rich layer after cooling, so as to obtain the negative electrode material. By arranging the stacking order of the modified carbon skeleton, the core-shell material and the metal lithium material, the metal lithium material is placed on the uppermost layer. During the hot pressing treatment, the metal lithium material and lithium in the core-shell material are melted to form a liquid phase, and the lithium metal matrix is formed after the metal lithium material is melted. The lithium metal matrix in the melted state gradually infiltrates and bonds the surface of the core-shell material to form a uniform buffer layer. The metal lithium material is melted in the heating stage to form a lithium in a molten state which penetrates into surface pores of the bottom modified carbon skeleton, reacts with the modified carbon skeleton to form a combination, and finally forms a firm whole similar to a sandwich structure. The prepared negative electrode material solves the problem that the interface of the core-shell material is prone to cracking after the traditional core-shell material is applied to a battery and then cycled.

[0020] 7. The embodiment of the present application further provides a negative electrode material, comprising a base layer, a buffer layer and a lithium-rich layer, the lithium-rich layer comprising a first lithium metal matrix, the buffer layer comprising a second lithium metal matrix, buffer pores distributed in the second lithium metal matrix and a core-shell material, and the base layer comprising a third lithium metal matrix and a modified carbon skeleton distributed in the third lithium metal matrix, and the first lithium metal matrix, the second lithium metal matrix and the third lithium metal matrix being sequentially stacked. The negative electrode material provided by the embodiment solves the technical contradiction that a single material cannot simultaneously have high capacity and low expansion. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative labor.

[0022] Figure 1 is a distribution diagram of the internal structure of the core-shell material provided by the first embodiment of the present application.

[0023] Figure 2is a sectional view of the core-shell material provided by the first embodiment of the present application along the diameter direction thereof.

[0024] Figure 3 is a flowchart of the preparation method of the negative electrode material provided by the second embodiment of the present application.

[0025] Figure 4 is a carbon fiber felt, a lithium sparse layer, a Ti3C2T x lithium sparse layer.

[0026] Figure 5 is a structure diagram of the negative electrode material provided by the third embodiment of the present application Figure 1 .

[0027] Figure 6 is a structure diagram of the negative electrode material provided by the third embodiment of the present application Figure 2 .

[0028] Explanation of the reference signs: 1, base layer; 2, buffer layer; 3, lithium-rich layer; 4, first interface transition layer; 5, second interface transition layer; 11, first lithium metal matrix; 21, second lithium metal matrix; 22, buffer pore; 23, core-shell material; 31, third lithium metal matrix; 32, modified carbon skeleton. DETAILED DESCRIPTION

[0029] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application.

[0030] In the embodiments provided by the present application, it should be understood that "B corresponding to A" means that B is associated with A, and B can be determined according to A. However, it should also be understood that the determination of B according to A does not mean that B is determined only according to A, but B can also be determined according to A and / or other information.

[0031] It should be understood that "one embodiment" or "an embodiment" mentioned throughout the specification means that the specific features, structures or characteristics related to the embodiment are included in at least one embodiment of the present application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. In addition, these specific features, structures or characteristics can be combined in any suitable manner in one or more embodiments. Those skilled in the art should also know that the embodiments described in the specification are all optional embodiments, and the actions and modules involved are not necessarily required by the present application.

[0032] In various embodiments of the present application, it should be understood that the size of the sequence number of the above-mentioned processes does not mean the inevitable sequence of execution, and the execution sequence of the processes should be determined according to its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0033] The flow diagrams and block diagrams in the drawings of the present application illustrate the architecture, functionality, and operation of possible implementations of various embodiments according to the present application. In this regard, each block in the flow diagrams or block diagrams can represent a portion of a step. It should also be noted that the functions noted in the blocks can occur in a different order than that noted in the figures in some alternative implementations. For example, two blocks noted in succession can actually be executed substantially in parallel, and they can also be executed in reverse order, depending on the functions involved.

[0034] With the surge in demand for high energy density batteries, lithium metal anode has become a research hotspot due to its ultra-high theoretical capacity, but the severe volume expansion and uncontrollable dendrite growth during the cycle process seriously restrict its practical application. The prior art usually introduces a buffer structure to alleviate the expansion, but there are still significant defects.

[0035] In the prior art, the homogeneous alloy electrode obtains a certain anti-expansion ability by introducing homogeneous alloy powder, however, when the battery is cycled, the entire alloy powder particles will uniformly expand and shrink as a whole. The whole particle "breathes" when lithium is inserted and extracted. That is, the entire particle expands when discharging, and a huge extrusion force is generated inside; the entire particle shrinks when charging, and a tensile stress is generated inside. This repeated "extrusion-tension" cycle is like repeatedly bending a metal wire, which eventually leads to cracking or even pulverization of the single alloy particle itself. Therefore, such a homogeneous alloy electrode still generates a large stress on the electrode structure, and may still cause particle pulverization or fall off from the current collector after long-term cycling. In addition, the high-temperature sintering process is often used in the preparation process of the traditional homogeneous alloy electrode, which is inconsistent with the low melting point characteristics of lithium metal. The smelting temperature is extremely high, the energy consumption is huge, and it is a great test for the equipment. At the same time, lithium element is extremely easy to volatilize and oxidize at high temperature, resulting in inaccurate composition control and poor batch stability. Therefore, it is urgent to develop a kind of mechanical strengthening and volume buffer, which is of decisive significance to promote the industrialization of lithium metal anode.

[0036] Please combine Figure 1 and Figure 2 , the first embodiment of the present application provides a core-shell material, the core-shell material comprises a core layer, a transition layer and a shell layer formed by being coated in turn from inside to outside, and a bonding layer provided on the shell layer, the bonding layer is provided at the outer periphery of the shell layer and is bonded with the shell layer; The core-shell material is Al3Zr@Al3Li, wherein the Al3Zr single crystal is the core layer of the core-shell material, the Al3Li single crystal is the shell layer of the core-shell material, the Al3(Zr, Li) solid solution is the interface transition layer of the core-shell material, the bonding layer includes Mg2Cu3 and Cu atoms, the Mg2Cu3 and the Cu atoms are respectively bonded to the surface of the shell layer, or the Mg2Cu3 is bonded to the surface of the shell layer through Mg-O bonds, and the Cu atoms are bonded to the surface of the shell layer through Cu-O-Al bonds.

[0037] It can be understood that the core-shell material of the embodiment is a four-level structure design of "core layer-transition layer-shell layer-bonding layer", wherein Al3Li is the shell layer: having excellent and uniform lithium affinity, capable of accelerating lithium ion conduction and greatly reducing lithium nucleation barrier, ensuring that lithium ions preferentially and uniformly nucleate and deposit on the entire particle surface rather than forming point-like nuclei. Al3Zr is the core layer: having extremely high mechanical strength. Even if there is a tendency for dendrite growth locally, the hard core layer can effectively resist volume expansion and physically block dendrite penetration. Mg2Cu3 exists in the core-shell material in an independent and stable solid state, so Mg2Cu3 is also called the Mg2Cu3 phase in the core-shell material. The core layer and the Mg2Cu3 phase can also form an elastic network to plastically absorb cyclic strain. It should be understood that the Al3Zr@Al3Li core-shell material of the embodiment first inhibits dendrites and secondarily resists expansion, and can also improve the cycle stability when used as a battery material. For inhibiting dendrites: the core layer can guide lithium deposition in a direction, eliminating random nucleation. The shell layer accelerates the lateral diffusion of lithium ions and neutralizes space charges. The bonding layer Mg2Cu3 phase improves the surface lithium hardness and blocks the longitudinal penetration of dendrites. For the poor anti-expansion ability of the homogeneous alloy powder in the prior art when applied to the negative electrode. The core-shell material in the embodiment can be used as a buffer layer for the negative material, wherein the core layer and the Mg2Cu3 phase form an elastic network to plastically absorb cyclic strain. The Al3(Zr, Li) transition layer introduced in the embodiment relieves the interface stress through the gradient solid solution of Zr and Li.

[0038] It should be noted that during the preparation of the core-shell material, a very small amount of oxygen may also be involved, forming trace amounts of oxides on the surface of the powder. Therefore, the Mg2Cu3 phase and the Cu atoms bonded to the shell layer may also exhibit the phenomenon that the Mg2Cu3 is bonded to the surface of the shell layer through Mg-O bonds, and the Cu atoms are bonded to the surface of the shell layer through Cu-O-Al bonds. The core-shell material formed in this way is only different in the bonding layer, and the effects that can be achieved are the same as those of the above-mentioned core-shell material, so no further description is given here.

[0039] Further, the mass ratio of the metal elements in the core-shell material is lithium: magnesium: copper: aluminum: zirconium = (70-80): (7.6-14.9): (3.8-7.4): (3.8-7.4): (0.3-1.0). It should be understood that lithium: magnesium: copper: aluminum: zirconium = (70-80): (7.6-14.9): (3.8-7.4): (3.8-7.4): (0.3-1.0) is a balanced formula for the performance of the core-shell material. Alternatively, the mass ratio of the metal elements in the core-shell material can also be lithium: magnesium: copper: aluminum: zirconium = (72-78): (12.0-16.0): (3.0-6.0): (3.0-6.0): (0.5-1.2), appropriately increasing the upper limit of the proportion of magnesium, while slightly tightening the lower limit of the proportion of copper and aluminum. As a lightweight element, magnesium can effectively improve the strength. The mass ratio of the metal elements in the core-shell material can also be lithium: magnesium: copper: aluminum: zirconium = (70-78): (5.0-10.0): (6.0-10.0): (6.0-10.0): (0.3-0.8), which focuses on the synergistic effect of Cu-Al. Or the mass ratio of the metal elements in the core-shell material can also be lithium: magnesium: copper: aluminum: zirconium = (78-85): (7.0-10.0): (3.0-5.0): (3.0-5.0): (0.3-0.6), which is a high-lithium-content formula. It can be understood that the present embodiment solves the performance degradation problem caused by the imbalance of the composition of the core-shell material by limiting the element ratio in the core-shell material. The element ratio directly affects the formation and distribution of the core-shell phase. Specifically, in the core-shell material, the content of Li element is greater than 70% to ensure that the ion channel can be provided when applied to the battery field; Zr content is too low to avoid agglomeration, and when Zr content is too high, hard and brittle phase is easily formed; wherein, the proportion of Mg, Cu and Al can be (40-50%: 20-25%: 20-25%), which can ensure the generation of Mg2Cu3 bonding phase. Alternatively, the mass ratio of Mg, Cu and Al can also be (48-58%): (18-23%): (18-23%), (35-45%): (25-30%): (25-30%) or (42-46%): (24-26%): (24-26%).

[0040] It should be understood that the present scheme ultimately forms a core-shell material by optimizing the ratio. In the Al3Zr@Al3Li core-shell material provided in the present embodiment, the role of Li is to provide a lithium source and construct an ion channel. The role of Mg is to inhibit dendrites and improve ductility; the role of Al is to form a core-shell phase skeleton; the role of Zr is to improve the hardness of the core phase and induce lithium deposition; and the role of Cu is to bond the modified carbon skeleton, and in the buffer layer, it can also form a Mg2Cu3 phase, which has the function of elastic buffer.

[0041] Further, the diameter of the core layer ranges from 20 nm to 50 nm. It can be understood that the present embodiment solves the problems of stress concentration or insufficient modulus caused by improper size of the core layer by limiting the diameter of the core layer. The size of the core layer plays a decisive role in the mechanical properties of the material. When the diameter of the core layer is greater than 50 nm, the stress distribution of the core layer is uneven, and cracks are easily generated in the cycle process. When the diameter of the core layer is less than 20 nm, the specific surface area is too large, which increases the surface energy and promotes the formation of interface reaction products. The present embodiment selects the diameter of the core layer to be 20-50 nm, so that the material has high modulus and good fatigue resistance. The cycle fracture problem caused by the too large core layer of the traditional core-shell material is avoided, and stable mechanical support is provided for the electrode. Alternatively, the diameter of the core layer can range from 20 nm to 30 nm, 25 nm to 45 nm, or 40 nm to 50 nm.

[0042] Further, the thickness of the shell layer ranges from 5 nm to 10 nm. It can be understood that the present embodiment solves the problems of hindering ion transmission or being too thin to guide deposition in the traditional core-shell material by controlling the thickness of the shell layer. The thickness of the shell layer directly affects the diffusion kinetics and deposition behavior of Li⁺. When the thickness of the shell layer is greater than 10 nm, the Li⁺ diffusion path is lengthened, resulting in capacity loss at high rate. When the thickness of the shell layer is less than 5 nm, there are not enough Li-philic sites, and the nucleation overpotential is increased, which easily causes dendrite growth. The optimized shell layer thickness of 5-10 nm in the present scheme controls the Li⁺ diffusion distance and ensures fast ion transmission; at the same time, it provides sufficient Li-philic sites. This thickness design makes the negative electrode material have no obvious dendrite protrusion on the electrode surface after being cycled for many times, solving the problems of poor rate performance or dendrite caused by improper size of the shell layer in the traditional core-shell material. The thickness of the shell layer ranges from 5 nm to 9 nm, 7 nm to 8 nm, or 6 nm to 10 nm.

[0043] Further, the thickness of the transition layer ranges from 2 nm to 5 nm. It can be understood that the present embodiment solves the technical problems of not being able to relieve lattice mismatch when the transition layer is too thin or increasing brittleness when the transition layer is too thick by limiting the thickness of the transition layer. The direct combination of the core layer and the shell layer will produce interface stress. The Al3(Zr, Li) transition layer in the present scheme forms a composition-continuous buffer zone through the gradient solid solution of Zr and Li, reducing the interface stress. When the thickness of the transition layer is less than 2 nm, the buffer effect is insufficient, and there is still stress concentration at the interface; when the thickness of the transition layer is greater than 5 nm, the transition layer is brittle due to solid solution strengthening, and the elongation decreases. The thickness design of 2-5 nm in the present embodiment not only ensures sufficient relief of lattice mismatch, but also avoids the brittleness of the transition layer, so that the fracture toughness of the core-shell structure is improved, and the long-term structural stability is maintained. The thickness of the transition layer can range from 4 nm to 5 nm, 2 nm to 3 nm, or 3 nm to 4 nm.

[0044] Further, the bonding layer has a thickness ranging from 2 nm to 5 nm. The bonding layer can enable the core-shell material and the lithium metal substrate to form a strong metallurgical bond or chemical bond. If the bonding layer is too thick, it will become a significant resistance layer, hindering the transport of electrons and lithium ions between the surface of the core-shell particles and the lithium substrate, ensuring efficient connection of the core-shell particles to the external circuit and ion channel, reducing the interface impedance of the electrode, and facilitating the improvement of the rate performance and coulombic efficiency of the battery. The thickness of the bonding layer can also range from 4 nm to 5 nm, 2 nm to 3 nm, or 3 nm to 4 nm.

[0045] Referring to Figure 3 The second embodiment of the present application also provides a preparation method of the negative electrode material. The preparation method comprises the following steps: S1, providing a modified carbon skeleton, a lithium metal material, and a core-shell material as in the first embodiment; and S2, stacking the modified carbon skeleton, the core-shell material, and the lithium metal material from bottom to top in an inert gas atmosphere, performing a hot pressing treatment, infiltrating and densifying the core-shell material after the lithium metal material is melted, and infiltrating the pores in the modified carbon skeleton, and then performing a cooling treatment; wherein the modified carbon skeleton infiltrated by the melted lithium metal material forms a base layer after cooling, the core-shell material infiltrated by the melted lithium metal material forms a buffer layer after cooling, and the lithium metal material that is melted but located on the side of the core-shell material away from the modified carbon skeleton forms a lithium-rich layer after cooling, to obtain the negative electrode material.

[0046] It can be understood that, in the embodiment, the order of the modified carbon skeleton, the core-shell material and the metal lithium material is set, so that the metal lithium material is placed in the uppermost layer. When the hot-pressing treatment is performed, the lithium in the metal lithium material and the core-shell material melts to form a liquid phase. After the metal lithium material melts, a lithium metal matrix is formed, which gradually infiltrates and bonds the surface of the core-shell material to form a uniform buffer layer. The metal lithium material retains certain pores when it infiltrates and bonds the surface of the core-shell material, and these pores are the buffer pores. Moreover, the molten lithium infiltrates the surface pores of the modified carbon skeleton in the bottom layer, reacts with the modified carbon skeleton, and finally forms a firm whole similar to a sandwich structure. The molten metal lithium material simultaneously infiltrates and penetrates the core-shell material, and the molten metal lithium material penetrates into the pores in the modified carbon skeleton. It should be noted that the metal lithium material is not the main body of the buffer layer, and the main body of the buffer layer is the core-shell material. The molten metal lithium material mainly plays a role similar to "pre-lithiation". Moreover, the role of the molten lithium is to bond, densify and form an interfacial bond, rather than simply coating the core-shell material. Specifically, the modified carbon skeleton, the core-shell alloy powder and the metal lithium material are sequentially stacked in an inert atmosphere and subjected to hot-pressing treatment at a specific temperature and pressure. During this process, the lithium foil and the lithium phase in the core-shell alloy powder melt to form a liquid phase. The liquid phase metal lithium simultaneously achieves the following functions: as a bonding medium, it infiltrates and densifies the core-shell alloy powder layer to form a uniform buffer layer; it infiltrates the surface pores of the modified carbon skeleton and forms a metallurgical bond with the lithiumophilic layer on the surface of the modified carbon skeleton to form a composite core skeleton layer; and it itself forms a continuous surface lithium-rich layer after solidification. Subsequently, after cooling, the three layers are combined to form a firm whole through metallurgical bonding, and a composite negative electrode material with a gradient structure is obtained. That is, at the hot-pressing temperature, the molten lithium metal infiltrates, corrodes and partially dissolves the outermost shell layer of the core-shell material. At the same time, the elements in the core-shell material also diffuse into the lithium matrix. Thus, at the interface between the particle and the matrix, a compositionally graded, atomically mixed diffusion layer, i.e., an interface layer, is formed. The shell layer and the lithium metal matrix are combined by metallurgical bonding, and this layer realizes high-strength metallurgical bonding from the particle to the matrix. It should be understood that the core layer and the shell layer are connected through a coherent interface, i.e., a transition layer, which is mainly combined by metal bonds and part of covalent bonds, has high bonding energy and good stability.

[0047] Specifically, the mass ratio of the modified carbon skeleton, the metal lithium material and the core-shell material is modified carbon skeleton:metal lithium material:core-shell material=25-35%:10-25%:50-55%, and the embodiment optimizes the mass ratio of the modified carbon skeleton, the core-shell material and the metal lithium material to solve the problem of excessive electrode expansion or insufficient capacity caused by unbalanced proportion of each layer. The gradient composite of the negative electrode material needs to balance the support, buffering and lithium storage functions: when the proportion of the modified carbon skeleton in the base layer is too low, the mechanical support is insufficient, and the electrode is easy to collapse in the cycle; if it is too high, the active material space is occupied, resulting in capacity decline. The scheme selects to limit the mass ratio of the modified carbon skeleton, the core-shell material and the metal lithium material, so that the base layer provides mechanical support through high porosity, the buffer layer core-shell material absorbs expansion stress through the "rigid core-flexible shell" structure, and the surface layer metal lithium material supplements active lithium. This ratio greatly improves the anti-expansion performance of the electrode, and balances the expansion rate and high capacity of the electrode. Compared with the expansion rate of the traditional homogeneous electrode, the expansion rate is significantly improved, and the contradiction between structure stability and high energy density of single material is solved.

[0048] Alternatively, the modified carbon skeleton:metal lithium material:core-shell material=(30-38%):(18-28%):(45-52%), this ratio scheme appropriately increases the upper limit of the proportion of the modified carbon skeleton and the metal lithium material, and slightly tightens the range of the core-shell material. It focuses on ensuring that the electrode has excellent mechanical support and higher theoretical capacity, and is suitable for scenarios with extremely high requirements for cycle life and absolute capacity. The modified carbon skeleton:metal lithium material:core-shell material=(25-32%):(8-18%):(55-62%), this ratio scheme significantly increases the upper limit of the proportion of the core-shell material, while correspondingly reducing the proportion of the lithium-rich layer. It focuses on maximizing the volume and stress absorption capacity of the buffer layer, and is suitable for application scenarios that need to deal with extreme volume expansion.

[0049] Specifically, in step S1, the step of providing the modified carbon skeleton further includes: S11, providing a carbon fiber felt, a diamond lithium-lean layer, Ti3C2T x a lithium-philic layer, wherein T is a surface terminal group, wherein Ti3C2T xis a general chemical formula of MXene materials, MXene materials are a kind of two-dimensional transition metal carbide, nitride or carbonitride with terminal functional groups (-O, -OH, -F) on the surface, which are obtained by selectively etching A atom layers in MAX phase materials. They have metal-level conductivity, hydrophilicity, adjustable surface chemistry and good mechanical properties, and are a new type of multifunctional two-dimensional material with great application prospects. MAX phase is a new type of processable ceramic material, which is composed of metal elements (M), main group elements (A) and carbon / nitrogen elements (X) with a hexagonal layered structure, combining the conductivity of metal and the high strength and high temperature resistance of ceramic. Ti3C2 is a fixed stoichiometric ratio structure of MXene material mother body, T represents surface terminal groups such as -O, -OH and -F, and x represents its non-stoichiometric composition. Non-stoichiometric composition refers to the proportion of various atoms in the compound is not a simple integer ratio, and can fluctuate within a certain range, such as Ti3C2O 1.3 (OH) 0.6 F 0.1 The value of x is not a single integer, but the sum of the number of all surface terminal groups. For example, x ≈ 2.0 (i.e. 1.3 + 0.6 + 0.1).

[0050] S12, using a chemical vapor deposition method to deposit a diamond lithium-averse layer on the surface of each carbon fiber of the carbon fiber felt to form a preformed carbon skeleton; and S13, spraying Ti3C2T x X on the surface of the preformed carbon skeleton to obtain a modified carbon skeleton.

[0051] It can be understood that the traditional modified carbon skeleton either causes excessive lithium penetration due to too strong lithium affinity, or causes too high interface impedance due to lithium aversion. In this embodiment, a chemical vapor deposition method (Chemical Vapor Deposition, CVD for short) is used to form a modified carbon skeleton similar to a cable structure, wherein the carbon fiber felt includes a large number of carbon fibers, and each carbon fiber is wrapped with a nanometer diamond lithium-averse layer and a Ti3C2T x X lithium-philic layer in sequence. Specifically, in the forming process, the nanometer diamond lithium-averse layer is first deposited on the surface of the carbon fiber and wraps the carbon fiber, and the Ti3C2T x X lithium-philic layer is deposited on the surface of the nanometer diamond lithium-averse layer and wraps the surface of the nanometer diamond lithium-averse layer, thereby forming a cable-like structure. Through the composite coating design of the diamond lithium-averse layer and the Ti3C2T x X lithium-philic layer, the problem of difficult regulation of the modified carbon skeleton and lithium wettability is solved. The lithium-averse layer can prevent molten lithium from excessively penetrating into the modified carbon skeleton pores during hot pressing, ensuring that ≥85% of the porosity is used as an expansion buffer pore. The sprayed Ti3C2T xLithium-philic layer, eliminating lithium deposition dead zone on the surface of carbon fiber, guiding lithium ion to deposit uniformly. The "lithium-phobic-lithium-philic" double-layer design reduces the interface impedance of the modified carbon skeleton, solving the problem of swelling out of control or capacity attenuation caused by improper wettability of traditional modified carbon skeleton.

[0052] Specifically, in step S2, the heat pressing and cooling treatment includes: setting the pressure of the inert gas atmosphere to 1-3 MPa, heating from room temperature to 230-270℃; continuing at a temperature of 230-270℃ for 0.5-1 hour to complete the heat pressing treatment; cooling to room temperature at a cooling rate of 5-15℃ / min after completing the heat pressing to obtain the negative electrode material with the substrate layer, the buffer layer and the lithium-rich layer stacked; or, setting the pressure of the inert gas atmosphere to 4-6 MPa, heating from room temperature to 230-270℃; continuing at a temperature of 230-270℃ for 1-3 hours to complete the heat pressing treatment; cooling to room temperature at a cooling rate of 5-15℃ / min after completing the heat pressing to obtain the negative electrode material with the substrate layer, the first interface transition layer, the buffer layer, the second interface transition layer and the lithium-rich layer stacked.

[0053] This embodiment solves the problem of weak interlayer bonding or cracking caused by thermal stress by optimizing the heat pressing process parameters. Heat pressing is the key to realizing the metallurgical bonding between the gradient composite electrode layers. In addition, different heat pressing process parameters will eventually form negative electrode materials with different structures. When the parameters of lower pressure and short holding time are used, the negative electrode material with the substrate layer, the buffer layer and the lithium-rich layer stacked will be formed. While specific high temperature and high pressure and long holding time will obtain the negative electrode material with the substrate layer, the first interface transition layer, the buffer layer, the second interface transition layer and the lithium-rich layer stacked.

[0054] It should be understood that when the heat pressing temperature is less than 230℃, the core-shell powder is not fully melted, the interlayer is only physically contacted, and the interface impedance is high; when the heat pressing temperature is greater than 270℃, the metal lithium material is excessively lost. In this embodiment, the heat pressing temperature is selected to be 230-270℃ and the heat pressing time is 1-3 hours, so that the Mg-Li eutectic phase (melting point ≈430℃) on the surface of the core-shell powder is partially melted to form a continuous metallurgical bonding zone; then cooling at a rate of 5-15℃ / min can avoid the concentration of thermal stress caused by rapid cooling, so that the interlayer interface shear strength can be improved. By limiting the heating and cooling process, this embodiment solves the problem of weak interlayer bonding or cracking caused by rapid cooling in the traditional cold pressing process, and ensures that the structure of the buffer layer is stable during the cycle process when the negative electrode material is used as an electrode.

[0055] Please refer to Figure 5The third embodiment of the present application also provides a negative electrode material, which comprises a substrate layer 1, a buffer layer 2 and a lithium-rich layer 3 arranged in a stack, the lithium-rich layer 3 comprises a first lithium metal matrix 11, the buffer layer 2 comprises a second lithium metal matrix 21 and buffer pores 22 and the above-mentioned core-shell material 23 distributed in the second lithium metal matrix, and the substrate layer 1 comprises a third lithium metal matrix 31 and a modified carbon skeleton 32 distributed in the third lithium metal matrix 31 connected to each other, and the first lithium metal matrix 11, the second lithium metal matrix 21 and the third lithium metal matrix 31 are arranged in a stack.

[0056] Specifically, as shown in Figure 5 , the arrangement of the first lithium metal matrix 11, the second lithium metal matrix 21 and the third lithium metal matrix 31 in a stack does not mean that the first lithium metal matrix 11, the second lithium metal matrix 21 and the third lithium metal matrix 31 will form a layered structure. The first lithium metal matrix 11, the second lithium metal matrix 21 and the third lithium metal matrix 31 in the embodiment are also called "lithium metal network" as Li-Mg-Cu matrix, which is a composite structure formed by continuous phase change, that is, the actual state of the first lithium metal matrix 11, the second lithium metal matrix 21 and the third lithium metal matrix 31 is connected together, only as a distinction between the substrate layer 1, the buffer layer 2 and the lithium-rich layer 3, Figure 5 , the layered structure is shown. The core-shell material 23 is embedded in the second lithium metal matrix 21, and the core-shell material 23 is dispersed, and the second lithium metal matrix 21 is continuous.

[0057] Specifically, please refer to Figure 4 , when the negative electrode material is prepared, the modified carbon skeleton 32, the core-shell material 23 and the metal lithium material are actually stacked in the order from bottom to top, and then heat-pressed, and in the heat-pressing process, the metal lithium material and the lithium in the core-shell material 23 are melted to form a liquid phase, and the liquid flow direction can refer to Figure 4The arrow direction, gradually infiltrate and bond the surface of the shell material, form a uniform buffer layer, specifically, the melted lithium material will retain a certain porosity when infiltrating and bonding the surface of the shell material, these porosities are the buffer porosities 22, the buffer porosities 22 provide crucial buffer space for the volume expansion of the negative electrode material. And the molten lithium infiltrates the surface layer porosities of the bottom modified carbon skeleton 32, reacts with the modified carbon skeleton 32 to form a combination, and finally forms a solid whole similar to a sandwich structure, and the interface layer is formed between the base layer 1 and the buffer layer 2, and between the buffer layer 2 and the lithium-rich layer 3. At the hot-pressing temperature, the molten lithium metal will infiltrate, corrode, and partially dissolve the outermost shell layer of the core-shell material. At the same time, the elements in the core-shell material 23 will also diffuse into the lithium matrix. Thus, at the interface between the core-shell material particles and the matrix, a compositionally graded, atomically mixed diffusion layer, i.e. an interface layer, is formed. This layer achieves high-strength metallurgical bonding from the core-shell material particles to the matrix. It should be understood that the core layer and the shell layer of the core-shell material are connected by a coherent interface, mainly bonded by metal bonds and part of covalent bonds, with high bonding energy and good stability. The shell layer is bonded to the lithium metal matrix by metallurgical bonding.

[0058] Specifically, the negative electrode material provided by the embodiment is a gradient design of "base layer 1-buffer layer 2-lithium-rich layer 3", which solves the technical contradiction that a single material cannot balance high capacity and low expansion. The structure realizes the synergistic integration of functions: the carbon fibers in the base layer 1 act as a mechanical skeleton to buffer the expansion of the lithium metal material; the core-shell material (A l3Zr@Al3Li) accommodates volume change through a "rigid core-soft shell" buffer; a surface layer 3, typically of lithium metal material, serves to supplement active lithium, similar to prelithiation, providing high capacity for the electrode material. The three form a functional gradient of "support-buffer-lithium reservoir". Specifically, the modified carbon skeleton maintains a porosity of greater than or equal to 85%, reserving deformation space for the middle layer expansion. The Mg solid solution in the surface layer 3 enhances ductility and fills micro voids through dynamic compensation of lithium. The metallurgical bonding of the gradient interface inhibits interlayer peeling and reduces overall expansion. The core layer is not directly connected to the Mg2Cu3 phase to form a network. They are mediated by the "lithium metal matrix" to synergistically work together to form an elastoplastic dissipation network. The anti-expansion function is not achieved by the direct connection of the core and Mg2Cu3, but by the following three-level dissipation mechanism: First level: the core layer has a very high modulus, which provides mechanical support for the surrounding soft lithium matrix, greatly improving the overall stiffness of the single composite particle, making it more difficult to be compressed and deformed. Second level: the Mg2Cu3 phase and the lithium metal matrix together form a "plastic deformation matrix" that wraps the core-shell material. The lithium matrix itself is relatively soft and will flow plastically, absorbing energy. The Mg2Cu3 phase, as a reinforcing phase, has a modulus much higher than lithium but has good elasticity. It will undergo reversible bending and deformation under cyclic stress, thereby absorbing and dissipating a large amount of strain energy. Third level: when the entire electrode expands, the stress is transmitted through the continuous lithium matrix. The stress first causes the soft lithium matrix to plastically deform. Then, the stress is transmitted to the Mg2Cu3 phase, causing it to elastically deform. Finally, the stress is transmitted to the hardest core layer, which successfully resists most of the remaining stress, thereby inhibiting the overall deformation amount.

[0059] The technical contradiction between high capacity and low expansion of a single material is solved by the gradient structure design of "substrate layer 1 carbon fiber-buffer layer 2 core-shell material-surface layer lithium-rich layer 3" in this embodiment. Among them, the substrate layer 1 carbon fiber, 100-150 μm thick, provides tensile strength as a mechanical skeleton and support; the buffer layer 2 core-shell material, 50-80 μm thick, buffers volume changes through the Al3Zr@Al3Li core-shell material; the surface layer lithium-rich layer 3, 10-30 μm thick, provides high theoretical capacity. This structure allows the electrode to absorb expansion stress through the plastic deformation of the buffer layer 2 core-shell material during the cycle process, and the remaining stress is dispersed by the substrate layer 1 carbon fiber to avoid the rupture of the surface layer metal lithium material. In the charge and discharge cycle, the gradient structure presents the following dynamic response: the modified carbon skeleton of the substrate layer 1 acts as a rigid anchor point, and the expansion stress is transmitted to the buffer layer 2 through the metallurgical bonding interface; the core-shell alloy powder and pores of the buffer layer 2 are vertically compressed and radially slightly expanded, absorbing the strain; the surface layer lithium-rich layer 3 freely expands / contracts without hard constraints and fills the micro voids through the dynamic compensation of lithium; the elastic potential energy of the buffer layer 2 is released during discharge, driving each layer to reset and reducing the overall expansion rate.

[0060] Please refer to Figure 6 Further, the negative electrode material further comprises a first interface transition layer 4 and a second interface transition layer 5, the first interface transition layer 4 is interposed between the substrate layer 1 and the buffer layer 2, and the second interface transition layer 5 is interposed between the lithium-rich layer 3 and the buffer layer 2; the first interface transition layer 4 is a Cu-Al solid solution bonding layer, and the second interface transition layer 5 is a Mg2Cu3 / Li2MgCu metallurgical bonding zone.

[0061] It should be understood that, during hot pressing, if the hot pressing time is continued to be extended, a metallurgical reaction between the molten lithium and the surface of the core-shell particles may also occur, and a compositionally graded interface transition layer realizes high-strength metallurgical bonding between the particles and the matrix. Illustratively, the Cu-Al solid solution bonding layer in this embodiment is a structure that may occur between the substrate layer 1 and the buffer layer 2, and the Mg2Cu3 / Li2MgCu metallurgical bonding zone is a structure that may occur between the lithium-rich layer 3 and the buffer layer 2. The Cu-Al solid solution bonding layer and the Mg2Cu3 / Li2MgCu metallurgical bonding zone that occur can solve the technical problems of high interlayer interface impedance and easy delamination of the negative electrode material. The interlayer interface of a traditional layered electrode is usually a physical contact, and gaps are easily generated during the cycle process, leading to an increase in impedance. In this scheme, a Cu-Al solid solution bonding layer is formed between the substrate layer 1 and the buffer layer 2, and metallurgical bonding is achieved through the interdiffusion of Cu and Al; a Mg2Cu3 / Li2MgCu metallurgical bonding zone is formed between the buffer layer 2 and the lithium-rich layer 3, and a continuous phase is formed through the eutectic reaction of Mg, Cu and Li. These two interface transition layers reduce the interlayer interface impedance.

[0062] In order to verify that the negative electrode material prepared in this embodiment has better performance, the following experimental tests are carried out: Example 1: Core-shell gradient composite electrode: Step 1: Pretreatment of raw materials: Select high-purity metal raw materials according to the mass ratio (Mg: 47.2%, Al: 21.9%, Cu: 21.9%), polish Mg with sandpaper, ultrasonic clean with 10% dilute acetic acid for 10 min, then rinse with deionized water, vacuum dry at 60°C for 2h. Al and Cu are soaked in 5% oxalic acid solution for 5 min, then washed with ethanol, vacuum dried at 80°C.

[0063] Step 2: Vacuum melting: Put the metal materials into the vacuum induction furnace for melting, the charging sequence is Cu at the bottom, Al in the middle, and Mg at the top, vacuumize to 0.05Mpa and then fill with high-purity argon. Heat from room temperature to 1050°C at a rate of 10°C / min and keep for 1h, realize atomic-level mixing of liquid metal by 25Hz electromagnetic stirring, and obtain alloy ingot.

[0064] Step 3: Gas atomization powder: The molten alloy is atomized through a nozzle with d=0.5mm and rapidly cooled under 5 MPa argon pressure (where the cooling rate is greater than or equal to 104 K / s), obtaining ternary alloy powder with structure (copper-aluminum-magnesium). The powder is sieved in a glove box (environment: O2 / H2O<0.1 ppm), and the 10~30μm particle size section is taken.

[0065] Step 4: Zr strengthening phase pre-dispersion: Mix the ternary alloy powder and Zr powder (mass ratio of alloy powder 96%: Zr 4%), uniformly disperse in argon atmosphere by ball milling at a speed of 50 rpm and a ball-to-material ratio of 8:1 for 3 hours, obtain quaternary alloy powder with structure (Mg-Al-Cu)-Zr.

[0066] Step 5: Low-temperature molten lithiumization: After preheating the quaternary alloy powder to 170°C, add Li powder (mass ratio of alloy powder 25%: Li 75%), heat to 200°C in argon atmosphere, and mechanically stir at a stirring speed of 60rpm for 20min using a closed propeller. Immediately stop stirring and cool to room temperature at a cooling rate of 15°C / min, obtain quinary alloy powder.

[0067] Step 5: Core-shell structure mechanical alloying: Put the quinary alloy powder into a tube furnace, heat from room temperature to 175°C at a rate of 5°C / min, keep at 175°C for 10 hours, cool at a rate of 15°C / min in argon atmosphere at 175°C for 12 hours, form Al3Zr@Al3Li core-shell strengthening phase.

[0068] Step 6: Preparation of modified carbon skeleton: carbon fiber felt with porosity greater than or equal to 90% is used to deposit a nano-diamond lithium-lean layer by chemical vapor deposition at 800°C for 30 min and spray 2 mg / mL Ti3C2T x Li-philic layer, where T is a surface termination group, x represents the type, number and proportion of terminal groups.

[0069] Step 7: Gradient composite: modified carbon skeleton, core-shell alloy powder metal lithium material stack (where the mass ratio is: modified carbon skeleton 30%; core-shell alloy powder 52%; metal lithium material 18%), heat pressed at 250°C / 5 MPa for 2 hours in a glove box, cooling rate 10°C / min, forming a gradient structure of the composite electrode of lithium-rich layer-buffer layer-substrate layer.

[0070] Comparative Example 1: Traditional homogeneous alloy electrode: Li, Mg, Al, Cu are directly melted at a temperature of 1500°C (where the mass ratio of Li, Mg, Al, Cu is: Li 75%, Mg 12.5%, Al 6.25%, Cu 6.25%) and then broken and powdered to prepare a traditional homogeneous alloy electrode. This scheme has no gradient / core-shell design. The expansion rate test is the same as step 8 and the cycle stability verification is the same as step 9.

[0071] Comparative Example 2: Single-element strengthened alloy electrode: Without core-shell composite steps 4-6, other conditions are the same, to prepare a single-element strengthened alloy electrode. The expansion rate test is the same as step 8 and the cycle stability verification is the same as step 9.

[0072] Corresponding test methods: In-situ expansion rate: an electrochemical in-situ expansion instrument is used to charge and discharge at 0.1C and record the thickness change of the electrodes in Experimental Example 1, Comparative Example 1 and Comparative Example 2 in real time.

[0073] Initial coulombic efficiency: Capacity retention after 500 cycles: electrodes in Experimental Example 1, Comparative Example 1 and Comparative Example 2 are directly hot-pressed and rolled into self-supporting lithium metal strips (thickness less than or equal to 50μm), assembled into CR2032 button half-cells, tested at a voltage of 0.005-1.5 V, cycled at a current density of 1C, and the capacity retention rate was recorded.

[0074]

[0075] As can be seen, compared with traditional homogeneous alloy electrodes and single-element strengthened alloy electrodes, the core-shell material gradient composite electrode has a lower in-situ expansion rate, mainly in two aspects.

[0076] The first aspect is the Al3Zr@Al3Li core-shell structure: a high modulus core layer with an elastic modulus of approximately 200 GPa can provide rigid support, a flexible shell layer buffers stress through plastic deformation, and a transition layer Al3(Zr, Li) solid solution alleviates the core-shell lattice mismatch. Specifically, in the Al3Zr@Al3Li core-shell material prepared by the method, the role of Li is to provide a lithium source and construct an ion channel. The role of Mg is to inhibit dendrites and improve ductility; the role of Al is to form a core-shell phase skeleton, in which Al3Zr as the core layer can guide the deposition orientation, and Al3Li as the shell layer can also accelerate ion conduction; the role of Zr is to improve the hardness of the core phase and induce lithium deposition; the role of Cu is to form a Mg2Cu3 phase and bond with the surface of the shell layer or directly bond with the surface of the shell layer. The Al3Zr@Al3Li core-shell material first inhibits dendrites and then can resist expansion, and can also improve the cycle stability when used as a battery material. For inhibiting dendrites: the core can guide directional deposition of lithium and eliminate random nucleation. It can accelerate the lateral diffusion of lithium ions and neutralize space charges. Specifically, during the process of low-temperature molten lithium, Mg begins to melt into molten Li to form an Mg-Li solid solution, and during the process of hot pressing for the preparation of the negative electrode material, the lithium metal in the buffer layer is melted again. This process further homogenizes the distribution of Mg in Li, ensuring the composition uniformity of the β-Li(Mg) solid solution matrix formed after the final solidification of the Mg element, which is locked in the lithium matrix of the middle buffer layer and the surface lithium-rich layer of the negative electrode material. During the charging and discharging cycle of the battery, lithium undergoes repeated deposition-dissolution, and the electrode volume changes accordingly. The Mg atoms melt into the Li lattice, causing lattice distortion, significantly improving the strength and hardness of the lithium metal. The harder lithium matrix is more resistant to dendrite penetration and plastic deformation. At the same time, the addition of Mg changes the deposition habit of lithium, making it more inclined to layer deposition rather than dendritic growth, thereby reducing uncontrolled volume changes that cause expansion from the root. Finally, the Mg2Cu3 phase in the buffer layer, the core-shell material, and the like work together to disperse and dissipate local stress. The Cu bonding layer forms an elastic buffer at the interface of the modified carbon skeleton, dispersing local stress. Li dynamic compensation drives dendrite backfilling through a gradient concentration difference.

[0077] The second aspect is the gradient composite layer: the lithium-avoiding nanodiamond layer of the modified carbon skeleton reserves expansion space to avoid structural collapse caused by excessive lithium infiltration. Further, the core-shell material gradient composite electrode has a relatively optimal cycle life. The base layer is a carbon fiber material, and the lithium-avoiding layer or the lithium-avoiding layer can be synergistically controlled: Ti3C2T xThe lithiumophilic layer reduces the lithium nucleation overpotential, and guides uniform deposition. The capacity retention of Example 1 is obviously higher than that of the comparative example after 500 cycles. In addition, the gradient composite electrode of core-shell material can inhibit dendrite growth, has a shell thickness of 5-10 nm, shortens the Li+ diffusion path, and avoids the dendrite nucleation caused by excessive local ion concentration. The present embodiment has the following beneficial effects through the synergistic design of "core-shell reinforced phase + gradient composite structure + interface layer": inhibiting electrode expansion: the high modulus core and the flexible shell cooperate to buffer the volume change, significantly reducing the structural stress during the cycle process; guiding uniform deposition: the lithiumophilic shell provides directional nucleation sites and eliminates the hidden danger of dendrite growth; improving cycle life: the unique interface design blocks the nanophase coarsening path and maintains long-term structural stability; enhancing safety: the mechanically reinforced network blocks the penetration of lithium dendrites and prevents battery short circuit. Finally, the stable and long cycle operation of high-energy-density lithium metal batteries is realized, and the problems of expansion failure and safety hazards are comprehensively solved.

[0078] The above discloses a core-shell material, a negative electrode material and a preparation method thereof. The principles and implementation modes of the present application are described by applying specific examples. The above examples are only used to help understand the method and core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range will be changed, and the above description should not be understood as a limitation of the present application. Any modification, equivalent replacement and improvement within the principles of the present application should be included in the protection scope of the present application.

Claims

1. A core-shell material, characterized in that: The core-shell material comprises a core layer, a transition layer and a shell layer formed in sequence from inside to outside, and a bonding layer arranged on the shell layer, the bonding layer being arranged at the outer periphery of the shell layer and bonded with the shell layer; and The core-shell material is Al3Zr@Al3Li, wherein the Al3Zr single crystal is the core layer of the core-shell material, the Al3Li single crystal is the shell layer of the core-shell material, the Al3(Zr, Li) solid solution is the interface transition layer of the core-shell material, and the bonding layer comprises Mg2Cu3 and Cu atoms, the Mg2Cu3 and the Cu atoms are respectively bonded with the surface of the shell layer, or the Mg2Cu3 is bonded with the surface of the shell layer through Mg-O bond, and the Cu atom is bonded with the surface of the shell layer through Cu-O-Al bond.

2. The core-shell material of claim 1, wherein: The mass ratio of metal elements in the core-shell material is lithium: magnesium: copper: aluminum: zirconium = (70-80): (7.6-14.9): (3.8-7.4): (3.8-7.4): (0.3-1.0).

3. The core-shell material of claim 1, wherein: The core-shell material is in the form of spherical particles, the diameter of the core layer ranges from 20 nm to 50 nm, the thickness of the shell layer ranges from 5 nm to 10 nm, the thickness of the transition layer ranges from 2 nm to 5 nm, and the thickness of the bonding layer ranges from 2 nm to 5 nm.

4. A method for producing a negative electrode material, characterized by: The preparation method comprises the following steps: providing a modified carbon skeleton, a metal lithium material and a core-shell material according to any one of claims 1-3; stacking the modified carbon skeleton, the core-shell material and the metal lithium material from bottom to top in an inert gas atmosphere to perform a hot-pressing treatment, the metal lithium material being melted to infiltrate and densify the core-shell material and to infiltrate into pores in the modified carbon skeleton, and then performing a cooling treatment; wherein the modified carbon skeleton infiltrated by the melted metal lithium material forms a base layer after cooling, the core-shell material infiltrated by the melted metal lithium material forms a buffer layer after cooling, and the metal lithium material which is melted but located on the side of the core-shell material away from the modified carbon skeleton forms a lithium-rich layer after cooling, to obtain the negative electrode material.

5. The method of claim 4, wherein the negative electrode material is prepared by the steps of: The mass ratio of the modified carbon skeleton, the metal lithium material and the core-shell material is modified carbon skeleton: metal lithium material: core-shell material = 25-35%: 10-25%: 50-55%. ​ 6. The method of claim 4, wherein the negative electrode material is prepared by the steps of: mixing the carbon material, the silicon material, and the binder to form a mixture; and compressing the mixture to form the negative electrode material. The provision of the modified carbon skeleton comprises: Providing carbon fiber felt, diamond lithium sparse layer, Ti3C2T x Lithiophilic layer, wherein Ti3C2T x Is the general chemical formula of MXene material, Ti3C2 is the fixed stoichiometric ratio structure of MXene material mother body, T represents -O, -OH, -F surface terminal group, x represents its non-stoichiometric composition; depositing a diamond lithium-lean layer on the surface of each carbon fiber of the carbon fiber felt to form a pre-prepared carbon skeleton by chemical vapor deposition; and Spraying Ti3C2T on the surface of pre-prepared carbon skeleton x The lithiumophilic layer obtains a modified carbon skeleton.

7. The method of claim 4, wherein the negative electrode material is prepared by the steps of: preparing a mixture of the active material, the binder, and the conductive agent; and mixing the mixture with the solvent. the hot-pressing treatment, the melting of the metal lithium material to infiltrate and densify the core-shell material and to infiltrate into pores in the modified carbon skeleton, and then the cooling treatment comprise: setting the pressure of the inert gas atmosphere to 1-3 MPa, and heating from room temperature to 230-270°C; maintaining at the temperature of 230-270°C for 0.5-1 hour to complete the hot-pressing treatment; cooling at a cooling rate of 5-15°C / min to room temperature after completing the hot-pressing to obtain the negative electrode material of the stacked base layer, buffer layer and lithium-rich layer; or, setting the pressure of the inert gas atmosphere to 4-6 MPa, and heating from room temperature to 230-270°C; maintaining at the temperature of 230-270°C for 1-3 hours to complete the hot-pressing treatment; After the hot pressing, cooling to room temperature at a cooling rate of 5-15 ℃ / min to obtain the negative electrode material of the substrate layer, the first interface transition layer, the buffer layer, the second interface transition layer and the lithium-rich layer arranged in layers.

8. A negative electrode material, characterized by: The negative electrode material comprises a substrate layer, a buffer layer and a lithium-rich layer, the lithium-rich layer comprises a first lithium metal matrix, the buffer layer comprises a second lithium metal matrix, buffer pores distributed inside the second lithium metal matrix and the core-shell material according to any one of claims 1-3, and the substrate layer comprises a third lithium metal matrix and a modified carbon skeleton distributed inside the third lithium metal matrix, and the first lithium metal matrix, the second lithium metal matrix and the third lithium metal matrix are arranged in layers in sequence.

9. The negative electrode material of claim 8, wherein: The thickness of the substrate layer is 100-150 μm, the thickness of the buffer layer is 50-80 μm, and the thickness of the lithium-rich layer is 10-30 μm.

10. The negative electrode material of claim 9, wherein: The negative electrode material further comprises a first interface transition layer and a second interface transition layer, the first interface transition layer is arranged between the substrate layer and the buffer layer, and the second interface transition layer is arranged between the lithium-rich layer and the buffer layer; the first interface transition layer is a Cu-Al solid solution bonding layer, and the second interface transition layer is a Mg2Cu3 / Li2MgCu metallurgical bonding zone.

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