Gradient doped single-layer coated lithium-rich lithium ferrite material as well as preparation method and application thereof

Abstract

The invention provides a gradient-doped single-layer coated lithium-rich lithium ferrite material as well as a preparation method and application thereof, and belongs to the technical field of lithium ion battery positive electrode lithium supplementing materials. According to the invention, a lithium source and an iron source are sintered at a low temperature to form an inner core, and then gradient doping and coating are realized through a ball milling process and dynamic CVD (Chemical Vapor Deposition). The material takes Li5FeO4 as an inner core, element distribution is optimized through a gradient doping technology, and a single-layer coating design is adopted, so that the air stability and the ionic conductivity of the material can be remarkably improved, and meanwhile, the loss of active lithium during first charge and discharge of a battery is compensated. When being used as a positive electrode lithium supplementing agent and matched with a silicon-carbon negative electrode, the lithium ion battery positive electrode lithium supplementing agent can improve the initial coulombic efficiency of a lithium ion battery to be greater than or equal to 88% and enhance the cycling stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode lithium replenishment materials, and in particular to a gradient-doped monolayer coated lithium iron ferrite material, its preparation method and application. Background Technology

[0002] Lithium-rich lithium iron ferrite (Li5FeO4) is an ideal cathode lithium replenisher due to its high theoretical capacity (864 mAh / g) and low cost. However, it is sensitive to water and CO2, and its easy decomposition in air leads to capacity decay. To address this issue, existing technologies employ double-layer coating (such as a carbon layer + polymer electrolyte layer) to improve its stability, but this suffers from complex processes and excessively thick coating layers (>15 nm), resulting in a capacity reduction of over 20%. Moreover, physical vapor deposition (PVD) is costly and difficult to achieve uniform doping. Furthermore, elemental doping of lithium-rich lithium iron ferrite can improve electrochemical performance, but traditional solid-state methods result in uneven doping, leading to a decrease in specific capacity. Summary of the Invention

[0003] The purpose of this invention is to provide a gradient-doped monolayer coated lithium iron ferrite material, its preparation method and application. Through the synergistic effect of gradient doping and monolayer coating, the stability problem of Li5FeO4's sensitivity to water / CO2 is solved, and the specific capacity reduction problem caused by traditional coating and doping is avoided. This provides key technical support for the industrial application of high-capacity, low-cost lithium-ion battery replenishing agents.

[0004] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a gradient-doped monolayer coated lithium iron ferrite material, which includes a Li5FeO4 core and a coating layer on the surface of the Li5FeO4 core. The coating layer is a Zr-Ti-OB gradient-doped amorphous silicon carbide layer.

[0005] Preferably, the mass ratio of the coating layer to the Li5FeO4 core is 2.5~6.5:100; The molar ratio of Zr, Ti and B in the coating layer is 0.3~1.2:0.2~0.8:0.05~0.3, and the molar ratio of Si and C is 0.16~0.25:1.

[0006] Preferably, the D of the Li5FeO4 core 90 The particle size is 2.8~4.5μm, and the specific surface area is 8~15m². 2 / g; The thickness of the coating layer is 3~12nm.

[0007] This invention provides a method for preparing the gradient-doped monolayer coated lithium iron ferrite material described above, comprising the following steps: A precursor is obtained by mixing a lithium source, an iron source, and water and carrying out a hydrothermal reaction. The precursor was ball-milled and mixed with zirconium source, titanium source and boron source, and then calcined to obtain a doped precursor; Silicon source gas and carbon source gas are introduced into the doped precursor, and chemical vapor deposition is performed to obtain a gradient-doped monolayer coated lithium iron ferrite material.

[0008] Preferably, the lithium source includes one or more of lithium hydroxide, lithium hydroxide monohydrate, lithium carbonate, lithium chloride, and lithium oxide; the iron source includes one or more of ferric chloride, ferric nitrate, ferric sulfate, and ferric oxide; and the molar ratio of the lithium source to the iron source is 5.1 to 5.4:1.

[0009] Preferably, the zirconium source includes one of zirconium dichloride, zirconium n-propoxide, zirconium butoxide, and zirconium acetylacetonate; The titanium source includes one of tetrabutyl titanate, isopropyl titanate, ethyl titanate, and titanium dioxide. The boron source includes one of trimethyl borate, triethyl borate, and boric acid. The molar ratio of the iron source, zirconium source, titanium source and boron source is 5~15:0.3~1.2:0.2~0.8:0.05~0.3.

[0010] Preferably, the hydrothermal reaction is carried out at a temperature of 180~220℃ for a time of 12~36h; The calcination temperature is 600~750℃, the time is 5~20h, and the atmosphere is argon or nitrogen.

[0011] Preferably, the silicon source gas includes one or more of silane, dichlorosilane, trichlorosilane and tetramethylsilane, and the carbon source gas includes one or more of methane, acetylene, ethylene and propane; the flow ratio of the silicon source gas to the carbon source gas is 1:3~4. The chemical vapor deposition temperature is 480~550℃, and the time is 30~90min.

[0012] Preferably, the process includes evaporating a boron source during or before the chemical vapor deposition.

[0013] This invention provides the application of the gradient-doped monolayer coated lithium-rich lithium ferrite material described in the above technical solution or the gradient-doped monolayer coated lithium-rich lithium ferrite material prepared by the preparation method described in the above technical solution as a positive electrode lithium replenishing agent in lithium-ion batteries.

[0014] This invention provides a gradient-doped monolayer coated lithium ferric oxide material. The material uses Li5FeO4 as its core, and optimizes the elemental distribution through gradient doping technology (e.g., high-valence elements stabilize the lattice, and low-valence elements end oxygen vacancies). A monolayer coating design is employed, with the doping elements transitioning from the Li5FeO4 core surface to an amorphous silicon carbide layer. This structure significantly improves the air stability and lithium-ion conductivity of the lithium ferric oxide material, while compensating for the loss of active lithium during the first charge-discharge cycle, thereby enhancing the battery's cycle stability.

[0015] This invention provides a method for preparing a gradient-doped monolayer coated lithium iron ferrite material. First, a lithium source and an iron source are sintered at low temperature to form a core. Then, gradient doping and coating are achieved through ball milling and dynamic CVD. Existing lithium iron ferrite materials suffer from high surface alkali residue when exposed to air due to the high reactivity of lithium ions and the instability of lattice oxygen, which affects the preparation of lithium-ion battery slurries. This invention, through gradient doping and monolayer coating, suppresses lattice corrosion of lithium iron ferrite, fills oxygen vacancies, and provides protection through a gradient energy barrier and a carbon layer, improving its air stability and thus enhancing lithium replenishment capacity. Furthermore, the amorphous silicon carbide layer in the coating layer improves both conductivity and air stability, thereby achieving a high specific capacity of the lithium iron ferrite material.

[0016] This invention uses a Zr-Ti-OB quaternary system as the doping element. Zr / Ti expands the lattice channels, B suppresses grain boundary corrosion, O fills oxygen vacancies, and Zr broadens the Li ion channels. Due to the different rates of high-temperature decomposition and element diffusion, a concentration gradient energy barrier is generated between the elements, resulting in a coating layer (such as...). Figure 2 As shown, from the inside out, the B concentration increases (0.05%→0.3%), the Ti concentration decreases (0.8%→0.2%), and the Zr concentration increases (0.32%-1.18%), but macroscopically, all elements exhibit a uniform distribution. Gradient-doped monolayer coating of lithium-rich lithium iron ferrite is employed. Zr / Ti metal doping optimizes the lattice channels, and the B / O gradient distribution constructs a dynamic barrier. The double-layer protection effect of the amorphous silicon carbide matrix improves the air stability and ion transport efficiency of lithium-rich lithium iron ferrite.

[0017] This invention achieves gradient doping by controlling the temperature levels and holding time of calcination. Ti relies on low-temperature diffusion, while B can be added before or during CVD. Gradient doping is achieved in two steps: solid-state diffusion in the calcination step initially establishes the gradient profile, and vapor deposition and high-temperature diffusion in the CVD stage further optimize the gradient distribution. Due to differences in molecular size and reactivity, B and Ti exhibit an increasing or decreasing concentration trend from the Li5FeO4 core interface to the amorphous silicon carbide layer.

[0018] This invention uses a gradient doped single-layer coating structure to control the coating thickness to within 12nm while ensuring air isolation, which is more than 40% thinner than a double-layer structure.

[0019] The thin monolayer structure achieves high capacity retention and long cycle life. This structure increases the proportion of active material, while optimized core particle size and surface area ensure complete coating coverage without hindering ion diffusion. When the gradient-doped monolayer coated lithium iron ferrite material described in this invention is used as a cathode lithium replenisher, adding 1.5% to 7.5% to the cathode active layer, it matches the silicon-carbon anode, improving the initial coulombic efficiency of lithium-ion batteries to ≥88% and enhancing cycle stability. This makes it suitable for high-energy-density battery development and significantly reduces the cost of lithium replenishment processes.

[0020] Traditional solid-phase doping requires excessive doping (e.g., V > 1.5%), which not only generates Li3VO4 impurities but also results in excessively high costs. In contrast, the doping amount of the doping element in this invention is as low as 0.8% relative to the lithium iron phosphate core, solving the problems of reduced specific capacity, impurity generation, and excessively high costs caused by traditional coating and doping methods. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the preparation and application process of the gradient-doped monolayer coated lithium iron ferrite material in this invention; Figure 2 This is a cross-sectional schematic diagram of the coating layer structure in the gradient-doped monolayer coated lithium iron ferrite material of the present invention; Figure 3 In the image, (a) is a TEM image of the gradient-doped monolayer coated lithium iron ferrite material prepared in Example 1, and (b) is a magnified view of a portion of (a). Figure 4 The image shows the EDS diagram of the gradient-doped monolayer coated lithium iron ferrite material prepared in Example 1. Detailed Implementation

[0022] In this invention, unless otherwise specified, the raw materials or reagents required for preparation are all commercially available products well known to those skilled in the art.

[0023] This invention provides a gradient-doped monolayer coated lithium iron ferrite material, comprising a Li5FeO4 core and a coating layer on the surface of the Li5FeO4 core, wherein the coating layer is a Zr-Ti-OB gradient-doped amorphous silicon carbide layer. A schematic diagram is shown below. Figure 2 .

[0024] In this invention, the mass ratio of the coating layer to the Li5FeO4 core is preferably 2.5~6.5:100, more preferably 2.5~4.5:100.

[0025] In this invention, the molar ratio of Zr, Ti and B in the coating layer is preferably 0.3~1.2:0.2~0.8:0.05~0.3, more preferably 0.55:0.2:0.15~0.19, and the molar ratio of Si and C is preferably 0.16~0.25:1, more preferably 0.20~0.25:1.

[0026] In this invention, the D of the Li5FeO4 core 90 The preferred particle size is 2.8~4.5μm, and the preferred specific surface area is 8~15m². 2 / g.

[0027] In this invention, the thickness of the coating layer is preferably 3~12nm, more preferably 8nm.

[0028] like Figure 1 As shown, the present invention provides a method for preparing the gradient-doped monolayer coated lithium iron ferrite material described in the above technical solution, comprising the following steps: A precursor is obtained by mixing a lithium source, an iron source, and water and carrying out a hydrothermal reaction. The precursor was ball-milled and mixed with zirconium source, titanium source and boron source, and then calcined to obtain a doped precursor; Silicon source gas and carbon source gas are introduced into the doped precursor, and chemical vapor deposition is performed to obtain a gradient-doped monolayer coated lithium iron ferrite material.

[0029] This invention involves mixing a lithium source, an iron source, and water to carry out a hydrothermal reaction to obtain a precursor.

[0030] In this invention, the lithium source preferably includes one or more of lithium hydroxide, lithium hydroxide monohydrate, lithium carbonate, lithium chloride, and lithium oxide; the iron source preferably includes one or more of ferric chloride, ferric nitrate, ferric sulfate, and ferric oxide; when the lithium source or iron source is two or more of the above, this invention does not have a special limitation on the ratio of different lithium sources or iron sources, and any ratio is acceptable.

[0031] In this invention, the molar ratio of the lithium source to the iron source is preferably 5.1 to 5.4:1, and more preferably 5.2 to 5.3:1.

[0032] In this invention, the iron source is preferably dissolved in water, and then a lithium source is added and stirred evenly to carry out a hydrothermal reaction. After filtration and drying, the precursor is obtained.

[0033] In this invention, the temperature of the hydrothermal reaction is preferably 180~220℃, more preferably 180~200℃, and the time is preferably 12~36h, more preferably 18~24h. This invention uses low-temperature hydrothermal synthesis of the precursor to avoid particle agglomeration in high-temperature solid-state methods, and limits the Li / Fe molar ratio to 5.1~5.4:1. Excess lithium can fill the lattice interstices and improve ionic conductivity.

[0034] After obtaining the precursor, the present invention ball-mills and mixes the precursor with zirconium source, titanium source and boron source, and then calcines it to obtain a doped precursor.

[0035] In this invention, the zirconium source preferably includes one of zirconium dichloride (ZrOCl2), zirconium n-propoxide, zirconium butoxide, and zirconium acetylacetonate.

[0036] In this invention, the titanium source preferably includes one of tetrabutyl titanate (Ti(OC4H9)4), isopropyl titanate, ethyl titanate, and titanium dioxide.

[0037] In this invention, the boron source preferably includes one of trimethyl borate, triethyl borate, and boric acid.

[0038] In this invention, the molar ratio of the iron source, zirconium source, titanium source and boron source is preferably 5~15:0.3~1.2:0.2~0.8:0.05~0.3, more preferably 6~10:0.55:0.2:0.15~0.19.

[0039] The present invention does not have any special limitations on the ball milling and mixing; the materials can be mixed evenly according to a process known in the art.

[0040] In this invention, the calcination temperature is preferably 600~750℃, more preferably 650~700℃, the calcination time is preferably 5~20h, more preferably 6~15h, and the atmosphere is argon or nitrogen. During high-temperature calcination, the titanium source partially decomposes into titanium dioxide, and during the CVD deposition stage, it completely decomposes into titanium dioxide, which is then embedded into Fe through surface diffusion. 3+ Lattice sites; the boron source will partially vaporize and escape during high-temperature calcination, and will be dynamically replenished at high temperature during the CVD deposition stage.

[0041] After the calcination is completed, the present invention preferably crushes the obtained product to obtain a doped precursor, places it in a rotary kiln, and introduces silicon source gas and carbon source gas into the doped precursor to perform chemical vapor deposition to obtain a gradient-doped monolayer coated lithium iron ferrite material.

[0042] In this invention, the silicon source gas preferably includes one or more of silane (SiH4), dichlorosilane, trichlorosilane, and tetramethylsilane, and the carbon source gas preferably includes one or more of methane (CH4), acetylene, ethylene, and propane; the flow ratio of the silicon source gas to the carbon source gas is preferably 1:3 to 4, more preferably 1:3. In this invention, the flow rate of the silicon source gas is preferably 0.5 L / min.

[0043] In this invention, the preferred temperature for chemical vapor deposition is 480-550°C, more preferably 500-520°C, and the preferred time is 30-90 min, more preferably 60-80 min. During chemical vapor deposition, the silicon source and carbon source mixture decomposes at 480-550°C to form SiC. X The matrix and dopant elements are embedded in the lattice in situ during the deposition process, improving the uniformity of distribution by 60%.

[0044] During or before chemical vapor deposition, the present invention preferably includes evaporating a boron source in the doped precursor to replenish B element; the present invention preferably uses a segmented evaporator to control the evaporation rate: 0.01 g / min for 0-20 min, 0.02 g / min for 20-40 min, and 0.03 g / min for 40-60 min, and is transported to the deposition section (500°C) with carrier gas H2 (flow rate 0.5-2 L / min) to form a gradient distribution of B concentration of 0.05~0.30 at% from the inside to the outside in the coating layer.

[0045] This invention provides the application of the gradient-doped monolayer coated lithium iron ferrite material described above as a positive electrode lithium replenishing agent in lithium-ion batteries.

[0046] The present invention uses the gradient-doped monolayer coated lithium iron ferrite material as a positive electrode lithium supplementer for the positive electrode sheet. The mass fraction of the gradient-doped monolayer coated lithium iron ferrite material in the active material layer of the positive electrode sheet is 1.5~7.5%, more preferably 2~5%.

[0047] The present invention does not have any special limitation on the specific composition of the active material layer; any active material layer for a positive electrode sheet well known in the art can be used.

[0048] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0049] Unless otherwise specified, the experimental methods described in the various embodiments of this invention are conventional methods; unless otherwise specified, the reagents and raw materials described below are all commercially available.

[0050] Example 1

[0051] (1) Weigh out 218.19 g of LiCl (5.2 mol) and 270.30 g of FeCl3 according to a Li:Fe molar ratio of 5.2:1. 6H2O (1 mol) to add FeCl3 6H2O was dissolved in 800g of water, and then LiCl was added and stirred evenly. The mixture was then transferred to a reaction vessel and heated at 180℃ for 24h. After filtration and drying, the precursor was obtained. (2) 100g (0.65mol) of precursor was mixed with 9.8g (0.055mol) ZrOCl2, 6.8g (0.02mol) Ti(OC4H9)4 and 1.2g (0.019mol) H3BO3 in a ball mill jar and ball milled. The mixture was sintered at 650℃ for 6h under argon protection in a tube furnace. After crushing, the doped precursor was obtained. (3) The doped precursor is transferred to a rotary kiln, and a mixture of SiH4 and CH4 is introduced. The flow rate of SiH4 is adjusted to 0.5 L / min and the flow rate of CH4 is 1.5 L / min. The mixture is deposited at 500℃ for 60 min to obtain a gradient-doped monolayer coated lithium iron ferrite material with a coating thickness of 8 nm. The mass ratio of the coating layer to the core is 4.5:100, the molar ratio of Zr, Ti and B in the coating layer is 0.55:0.2:0.19, and the molar ratio of Si and C is 0.25:1.

[0052] Examples 2-5

[0053] The only difference from Example 1 is that the mass ratio of the coating layer to the core in Examples 2 to 5 is 2.5:100, 6.5:100, 1.2:100 and 9.0:100 respectively.

[0054] Examples 6-9

[0055] The only difference from Example 1 is that the molar ratios of doping elements Zr, Ti and B in Examples 6 to 9 are 0.5:0.5:0.15, 0.8:0.3:0.2, 0.2:0.2:0.05 and 1.5:0.2:0.4, respectively.

[0056] Examples 10-13

[0057] The only difference from Example 1 is that the hydrothermal temperatures in Examples 10-13 are 200℃, 220℃, 170℃ and 240℃ respectively.

[0058] Examples 14-17

[0059] The only difference from Example 1 is that the lithium iron molar ratios in Examples 14-17 are 5.1:, 5.3:1, 4.8:1 and 5.6:1, respectively.

[0060] Examples 18-21

[0061] The only difference from Example 1 is that the vapor deposition temperatures in Examples 18-21 are 500°C, 530°C, 450°C and 580°C, respectively.

[0062] Comparative Example 1

[0063] The only difference from Example 1 is that step (3) is omitted.

[0064] Comparative Example 2

[0065] The only difference from Example 1 is that step (2) does not add zirconium source, titanium source and boron source.

[0066] Structural characterization and performance testing

[0067] Figure 3 In the image, (a) is a TEM image of the gradient-doped monolayer coated lithium iron ferrite material prepared in Example 1, and (b) is a magnified view of a portion of (a). Figure 3 It can be seen that the lithium iron phosphate is uniformly covered by the coating layer, which is a continuous thin layer on the surface of the particles, forming a clear interface with the lithium iron phosphate core. The coating layer thickness is 3~5nm.

[0068] Figure 4 The EDS image of the gradient-doped monolayer coated lithium iron ferrite material prepared in Example 1 proves that Zr, Ti, and B elements were successfully doped.

[0069] Test case

[0070] 1) Preparation of positive electrode sheet

[0071] 2g of polyvinylidene fluoride (PVDF) was dissolved in 64g of N-methylpyrrolidone (NMP), 0.5g of conductive carbon black (SP) was added, followed by 94.5g of active material NCM811 and 2g of lithium-rich lithium iron ferrite supplementary material prepared in the above different cases. Finally, 1.0g of carbon nanotubes (CNTs) was added, and the mixture was stirred in a vacuum mixer until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then coated onto aluminum foil by double-sided extrusion, dried in a drying oven, and then rolled and slit to obtain a positive electrode sheet.

[0072] 2) Preparation of negative electrode sheet

[0073] Dissolve 2.5g of carboxymethyl cellulose binder and 2.5g of styrene-butadiene rubber in 60g of deionized water, add 5g of SP conductive agent and sonicate, then add 90g of active material silicon-carbon composite material (Jiangxi Yijin New Energy Technology Co., Ltd., ASC series products), stir until homogeneous under vacuum mixer, coat on copper foil, dry, then roll and slit to obtain negative electrode sheet.

[0074] 3) Preparation of electrolyte

[0075] In a mixed organic solvent of ethylene carbonate, diethyl carbonate and dimethyl carbonate (volume ratio 1:1:1), lithium hexafluorophosphate (LiPF6, concentration 1 mol / L) is added, dissolved and mixed evenly to obtain an electrolyte.

[0076] 4) Battery assembly

[0077] Place the positive electrode shell into the positive electrode sheet, add an appropriate amount of the electrolyte, place the diaphragm in and add more electrolyte, place the negative electrode sheet in, add a gasket and a spring, and finally cover with the negative electrode shell for sealing.

[0078] Performance tests were conducted on each of the above embodiments and comparative examples, and the test results are shown in Table 1.

[0079] Table 1 Performance test results of different embodiments and comparative examples

[0080] As shown in Table 1, a comparison between Examples 1-21 and Comparative Examples 1-2 reveals that the lithium-rich lithium iron phosphate material prepared using the method of this invention, when applied to batteries, can not only improve the initial coulombic efficiency but also enhance the battery's cycle capacity retention. This demonstrates that the cathode lithium replenishment material provided by this invention can improve lithium replenishment performance and battery cycle performance.

[0081] Specifically, comparing Examples 1 to 5, Examples 1 to 3 have a more suitable coating layer to core ratio, resulting in better battery performance from the obtained positive electrode lithium replenishment material; Examples 4 and 5, one has incomplete coating, resulting in poor stability, and the other has excessive coating, leading to capacity decay, with excessive inactive material reducing effective capacity.

[0082] Examples 6-9 compare the proportions of doping elements. Examples 6 and 7 have appropriate proportions, which synergistically improve conductivity, and the batteries prepared with the prepared positive electrode lithium replenishment materials have better performance. Examples 8 and 9 show that the doping is insufficient, the grain boundary impedance is high, and when the boron (B) element is in excess, an insulating phase is formed.

[0083] Examples 10-13 compare hydrothermal temperatures. In Examples 10 and 11, the lithium iron phosphate particles were more uniform and had higher crystallinity within the temperature range of 180-220℃, resulting in better battery performance from the prepared positive electrode lithium replenishment material. In Examples 12 and 13, insufficient surface temperature led to crystallization defects, affecting the subsequent uneven coating. Excessive temperature caused particle breakage, which in turn accelerated surface side reactions due to the high specific surface area.

[0084] Examples 14-17 compare the Li / Fe molar ratios. Examples 14 and 15 show the preferred Li / Fe molar ratios, resulting in batteries with better performance from the prepared positive electrode lithium supplementation materials. Examples 16 and 17 illustrate that insufficient lithium leads to lattice vacancies, exacerbating oxidation side reactions, while excess lithium generates impurities such as lithium oxide.

[0085] Examples 18-21 compare deposition temperatures. The cathode lithium replenishment material prepared at the preferred temperatures in Examples 18 and 19 produces batteries with better performance. Examples 20 and 21 show that insufficient temperature leads to loose coating, while high temperature causes silicon carbide crystallization, increasing volumetric strain and causing the coating layer to crack.

[0086] As can be seen from the comparison between Example 1 and Comparative Examples 1 and 2, the material properties cannot be improved without silicon carbide coating or doping element layers.

[0087] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A gradient doped single-layer coated lithium-rich lithium-iron-phosphate material, characterized in that, The gradient-doped single-layer coated lithium-rich lithium-iron-oxide material comprises a Li5FeO4 core and a coating layer coated on the surface of the Li5FeO4 core, and the coating layer is a Zr-Ti-O-B gradient-doped amorphous silicon carbide layer.

2. The gradient doped monolayer coated lithium-rich lithium-iron-phosphate material of claim 1, wherein, The mass ratio of the coating layer to the Li5FeO4 core is 2.5-6.5:

100. The molar ratio of Zr, Ti and B in the coating layer is 0.3-1.2:0.2-0.8:0.05-0.3, and the molar ratio of Si and C is 0.16-0.25:

1. 3.The gradient-doped monolayer-coated lithium-rich lithium-iron-oxide material of claim 1 or 2, wherein, D of the Li5FeO4 core 90 particle size of 2.8 to 4.5 μm and a specific surface area of 8 to 15 m 2 / g; The thickness of the coating layer is 3-12 nm.

4. The method of producing a gradient-doped single-layer coated lithium-rich lithium-iron- phosphate material according to any one of claims 1 to 3, characterized in that The method comprises the following steps: Mixing a lithium source, an iron source and water to perform a hydrothermal reaction to obtain a precursor; Ball-milling mixing the precursor with a zirconium source, a titanium source and a boron source, and performing calcination to obtain a doped precursor; Introducing a silicon source gas and a carbon source gas to perform chemical vapor deposition to obtain a gradient-doped single-layer coated lithium-rich lithium-iron-oxide material.

5. The preparation method according to claim 4, characterized in that, The lithium source comprises one or more of lithium hydroxide, lithium hydroxide monohydrate, lithium carbonate, lithium chloride and lithium oxide; the iron source comprises one or more of iron chloride, iron nitrate, iron sulfate and diiron trioxide; and the molar ratio of the lithium source to the iron source is 5.1-5.4:

1.

6. The preparation method according to claim 4, characterized in that, The zirconium source comprises one of zirconium oxychloride, zirconium n-propylate, zirconium butylate and zirconium acetylacetonate; The titanium source comprises one of tetrabutyl titanate, isopropyl titanate, ethyl titanate and titanium dioxide; The boron source comprises one of trimethyl borate, triethyl borate and boric acid; The molar ratio of the iron source, the zirconium source, the titanium source and the boron source is 5-15:0.3-1.2:0.2-0.8:0.05-0.

3.

7. The preparation method according to claim 4, characterized in that, The temperature of the hydrothermal reaction is 180-220°C, and the time is 12-36h; The temperature of the calcination is 600-750°C, the time is 5-20h, and the atmosphere is argon or nitrogen.

8. The preparation method according to claim 4, characterized in that, The silicon source gas comprises one or more of monosilane, dichlorosilane, trichlorosilane and tetramethylsilane, and the carbon source gas comprises one or more of methane, acetylene, ethylene and propane; and the flow ratio of the silicon source gas to the carbon source gas is 1:3-4. The temperature of the chemical vapor deposition is 480-550°C, and the time is 30-90min.

9. The preparation method according to claim 4, characterized in that, The method further comprises evaporating a boron source when or before the chemical vapor deposition is performed.

10. Application of the gradient-doped single-layer coated lithium-rich lithium-iron-oxide material of any one of claims 1-3 or the gradient-doped single-layer coated lithium-rich lithium-iron-oxide material prepared by the method of any one of claims 4-9 as a positive electrode lithium supplement in a lithium ion battery.

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