Modified lithium iron oxide material, its manufacturing method and uses

Abstract

This application provides a modified lithium iron oxide material, a method for producing the same, and its applications. The modified lithium iron oxide material comprises a nickel-doped lithium iron oxide substrate and a thin layer of lithium iron oxide and a carbon coating layer sequentially laminated on the surface of the nickel-doped lithium iron oxide substrate. The thin layer of lithium iron oxide is formed in situ on the surface of the nickel-doped lithium iron oxide substrate. In the modified lithium iron oxide material, the molar ratio of lithium element to iron element is 5:(0.9~1.1), the molar ratio of nickel element to lithium element is (0.01~0.05):5, and the amount of carbon coating is 0.5~5 wt%. The modified lithium iron oxide material described in this application contains a unique thin layer of lithium iron oxide, significantly improves the stability of the lithium iron oxide material, can reduce or suppress the generation of residual alkali, and has excellent lithium replenishment performance.

Description

Technical Field

[0001] This application belongs to the field of lithium-ion battery materials, and relates to a modified lithium iron phosphate material, a method for manufacturing the same, and uses thereof.

Background Art

[0002] In a lithium-ion battery, a lithium-containing interface layer (SEI) is formed on the surface of the negative electrode during the first charge-discharge process. This interface has the properties of a solid electrolyte and is very important for the performance of the lithium-ion battery. The SEI film is insoluble in organic solvents and can exist stably in an organic electrolyte, effectively preventing the co-insertion of solvent molecules, thereby improving the cycle performance and service life of the electrode. However, the formation of SEI consumes active lithium in the battery, resulting in a loss of the reversible capacity of the battery. Currently, the initial efficiency of commercially available graphite negative electrodes is usually about 90%, but the initial efficiency of silicon-based negative electrode materials with higher specific capacity is lower, usually about 85% or less.

[0003] To compensate for the active lithium lost during the first charge-discharge process, the current mainstream technologies include two types: a positive electrode lithium replenishment process and a negative electrode lithium replenishment process. From the perspective of technology maturity, currently, there are relatively many manufacturers engaged in the production of materials and equipment related to lithium replenishment with metallic Li powder and Li foil, as well as the research on the lithium replenishment process, and the industrial maturity is relatively high. However, negative electrode lithium replenishment has relatively high requirements for equipment and the environment, and it is also difficult to ensure safety. In comparison, the positive electrode lithium replenishment process has good safety and does not affect the conventional production process. Currently, there are relatively few manufacturers and scientific research institutions engaged in the research on positive electrode lithium replenishment materials, and there is still room for research on the manufacture, modification, and improvement of related lithium replenishment materials and the lithium replenishment process.

[0004] Lithium iron oxide is an ideal lithium-rich material for cathode lithium replenishment, as it has a theoretical specific capacity of 867 mAh / g, a suitable operating voltage, and the inert material generated after the initial charge and discharge does not participate in subsequent electrochemical processes.

[0005] CN108878849A discloses a synthesis process for lithium-rich oxide and a lithium-ion battery containing lithium-rich oxide, the specific synthesis process for lithium-rich oxide being as follows: Appropriate amounts of lithium source and iron source in molar ratio are weighed, dispersed in deionized water, and completely dissolved. Then, an appropriate amount of organic carbon source is weighed and added to the solution. The solution is stirred at 80°C until a sol is formed, and the sol is spray-dried to obtain a spherical precursor powder. Next, the precursor powder is calcined in an inert gas atmosphere for a certain period of time, and as the furnace cools, a core-shell structured lithium-rich oxide Li5FeO4 / C is obtained.

[0006] CN110459748A discloses a method for producing a carbon-coated lithium iron oxide material, the method comprising: (1) a step of mixing an iron source and a lithium source, sintering them to obtain lithium iron oxide, wherein the molar ratio of the lithium source to the iron source is (5-25):1; (2) a step of pulverizing the lithium iron oxide obtained in step (1); and (3) a step of vapor-coating the lithium iron oxide pulverized in step (2) with a carbon source to obtain a carbon-coated lithium iron oxide material.

[0007] The lithium iron oxide materials described in the above proposal have problems such as high residual alkali content or poor stability. Therefore, there is a great need to develop lithium iron oxide materials that have low residual alkali content and good stability. [Overview of the project]

[0008] The following is an overview of the topics described in detail herein. This overview is not intended to limit the scope of the claims.

[0009] This application provides a modified lithium iron oxide material, a method for producing the same, and its applications. In this application, a lithium iron oxide powder material can be doped with a nickel source, and a unique thin layer of nickel-iron-lithium oxide can be formed on the surface of the lithium iron oxide material by mechanical polishing. This significantly improves the stability of the lithium iron oxide material, reduces or suppresses the generation of residual alkali, and has excellent lithium replenishment performance.

[0010] In a first embodiment, the present application provides a modified lithium iron oxide material comprising a nickel-doped lithium iron oxide substrate and a thin layer of lithium iron oxide and a carbon coating layer sequentially laminated on the surface of the nickel-doped lithium iron oxide substrate, wherein the thin layer of lithium iron oxide is formed in situ on the surface of the nickel-doped lithium iron oxide substrate, and the modified lithium iron oxide material comprises lithium and iron elements. The molar ratio is 5:(0.9~1.1) (e.g., 5:0.9, 5:0.95, 5:1, 5:1.05 or 5:1.1), the molar ratio of nickel to lithium is (0.01~0.05):5 (e.g., 0.01:5, 0.02:5, 0.03:5, 0.04:5 or 0.05:5), and the carbon coating amount is 0.5~5wt%, for example, 0.5wt%, 1wt%, 2wt%, 3wt%, or 5wt%.

[0011] In the modified lithium iron oxide material described in this application, a unique thin layer of nickel-iron-lithium oxide is formed on the surface of the lithium iron oxide material, significantly improving the stability of the lithium iron oxide material, reducing or suppressing the generation of residual alkali, and having excellent lithium replenishment performance.

[0012] The amount of nickel added affects the performance of the modified lithium iron oxide material produced. Controlling the molar ratio of nickel in the nickel source to lithium in the lithium source to (0.01-0.05):5 results in a relatively good performance of the modified lithium iron oxide material. If the amount of nickel added is too small, the reaction is not uniform during the ball milling process, and the improvement effect is limited. If the amount of nickel added is too large, it affects the lithium iron oxide structure of the main phase, reducing its capacity.

[0013] In the modified lithium iron oxide material provided by this application, the electrical performance of the modified lithium iron oxide material can be improved by controlling the amount of carbon coating to 0.5 to 5 wt%. Preferably, the amount of carbon coating is 0.8 to 3 wt%, more preferably 1 to 3 wt%, and even more preferably 1 to 2.5 wt%, thereby obtaining even better electrical performance.

[0014] According to the embodiments provided herein, the molar ratio of nickel to lithium includes, but is not limited to, 0.01:5, 0.02:5, 0.03:5, 0.04:5, or 0.05:5.

[0015] Preferably, the nickel doping method is polishing, and more preferably, ball milling. The nickel doping process does not include high-temperature sintering and grinding operations.

[0016] Preferably, the average thickness of the carbon coating layer is 10 to 500 nm, more preferably 80 to 400 nm, and even more preferably 100 to 300 nm.

[0017] In this application, the method for measuring the average thickness of the carbon coating layer involves performing powder cross-sectional SEM measurement on the obtained modified lithium iron oxide material, then selecting at least 10 spots from the SEM cross-sectional view, calculating their thickness values, removing the maximum and minimum values, and calculating the average value to obtain the average thickness.

[0018] The coating described in this application is a complete coating; that is, the carbon material achieves 100% coating of the nickel-doped lithium iron oxide substrate.

[0019] In the modified lithium iron oxide material provided by this application, the carbon coating layer is a homogeneous carbon coating layer, and "homogeneous" means that, according to the SEM measurement results of the modified lithium iron oxide material, there is no obvious deposition or aggregation phenomenon in the carbon material outside the substrate over an area of ​​at least 50%, thereby obtaining superior overall performance.

[0020] Preferably, the carbon source forming the carbon coating layer is a gaseous carbon source and includes, but is not limited to, one or at least two of ethylene, acetylene, methane, ethane, propylene, propane, or butene. Other gaseous carbon sources of the same type that can achieve the same or similar effects can also be used in this application.

[0021] According to some embodiments provided herein, the residual alkali content of the modified lithium iron oxide material is <5%, according to some other embodiments provided herein, the residual alkali content of the modified lithium iron oxide material is <4.5%, and according to some embodiments provided herein, the residual alkali content of the modified lithium iron oxide material is <3.5%.

[0022] The above residual alkali includes LiOH and Li2CO3.

[0023] In a second embodiment, the present application provides a method for producing a modified lithium iron oxide material, the above production method is: (1) A step of mixing a lithium source and an iron source and obtaining a lithium iron oxide substrate by sintering, (2) The step of polishing the lithium iron oxide substrate with a nickel source to obtain a nickel-doped lithium iron oxide substrate, (3) The step of applying a gas-phase carbon coating to the nickel-doped lithium iron oxide substrate obtained in step (2) using a carbon source to obtain the modified lithium iron oxide material.

[0024] By subjecting the lithium iron phosphate powder material to a doping reaction with a nickel source, strictly controlling the molar ratios of lithium, iron, and nickel elements, and combining with appropriate process conditions, a unique nickel iron lithium oxide thin layer can be formed on the surface of the lithium iron phosphate material, significantly improving the stability of the lithium iron phosphate material, reducing or suppressing the generation of residual alkali, and having excellent lithium replenishment performance.

[0025] Preferably, the lithium source described in step (1) includes any one or at least a combination of two of lithium oxide, lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrite, lithium nitrate, lithium oxalate, lithium acetate, or lithium phosphate.

[0026] Preferably, the iron source includes any one or at least a combination of two of iron chloride, triiron tetraoxide, diiron trioxide, iron hydroxide, iron oxyhydroxide, iron nitrate, iron acetate, iron citrate, iron phosphate, ferrous oxalate, iron formate, or iron acetate.

[0027] Preferably, the molar ratio of lithium element in the lithium source to iron element in the iron source is 5:(0.9 - 1.1), for example, 5:0.9, 5:0.92, 5:0.95, 5:1, or 5:1.1, etc.

[0028] Preferably, the sintering treatment described in step (1) includes one - stage sintering and two - stage sintering.

[0029] Preferably, the temperature of the two - stage sintering is higher than that of the one - stage sintering.

[0030] Preferably, the one - stage sintering is carried out at 100 - 250 °C for 0.5 - 4 h.

[0031] Preferably, the two - stage sintering is carried out at 600 - 900 °C for 5 - 20 h.

[0032] Preferably, the nickel source described in step (2) includes one or at least two of the following: nickel monooxide, nickel trioxide, nickel hydroxide, nickel hydroxyoxide, nickel nitrate, nickel nitrite, nickel carbonate, nickel oxalate, or nickel acetate.

[0033] Preferably, the molar ratio of nickel element in the nickel source to lithium element in the lithium source is (0.01~0.05):5, for example, 0.01:5, 0.02:5, 0.03:5, 0.04:5, or 0.05:5.

[0034] Preferably, the lithium iron oxide substrate is crushed and sieved before the polishing treatment described in step (2).

[0035] Preferably, the mesh count of the sieve in the above sieving process is 400 to 500 mesh, for example, 400 mesh, 420 mesh, 450 mesh, 480 mesh, or 500 mesh.

[0036] Preferably, the polishing treatment includes mechanochemical ball milling.

[0037] Preferably, the ball material ratio in the above mechanochemical ball milling is (10-45):1, for example, 10:1, 15:1, 20:1, 30:1, 40:1, or 45:1.

[0038] This invention utilizes mechanochemical ball milling to react a nickel source with excess lithium on the surface of lithium iron oxide nanoparticles to produce lithium nickelate, thereby creating a thin layer of iron-nickel-lithium composite material on the surface of the lithium iron oxide nanoparticles and forming an excellent core-shell structure. Since lithium nickelate is more stable than lithium iron oxide, the amount of residual alkali produced can be effectively reduced, improving the overall stability of the modified lithium iron oxide material, and the lithium nickelate itself also possesses a certain lithium replenishment capacity.

[0039] Compared to the mixed sintering method, the mechanochemical ball milling method allows for control of the reaction to the surface layer of the powder and avoids processes such as grinding and sieving after sintering, as opposed to high-temperature sintering.

[0040] Preferably, the carbon source described in step (3) includes one or at least two of the following: ethylene, acetylene, methane, ethane, propylene, propane, or butene.

[0041] This invention sets the temperature of the gas phase carbon coating relative to the decomposition temperature of different carbon sources, and the temperature of the gas phase carbon coating is close to or higher than the decomposition temperature (if it is slightly lower, decomposition is possible, but the reaction is insufficient), for example, 700 to 1000°C.

[0042] Preferably, the gas-phase carbon coating described in step (3) is carried out in an inert gas atmosphere.

[0043] Preferably, the inert gas atmosphere includes nitrogen gas and / or argon gas.

[0044] This invention allows for sufficient and complete coating of a modified lithium iron oxide material by sintering a carbon source gas at high temperature and depositing it on the surface in the form of carbon nanovapors, thereby improving the material's stability, reducing the amount of residual alkali, and enhancing its conductivity. Compared to conventional carbon powder mixture coatings and polymer coatings, carbon nanoparticles have a smaller particle size, resulting in better surface modification and coating effects on the material.

[0045] Furthermore, a better coating effect can be obtained by rotating the cavity of the chemical vapor deposition furnace at a rotation speed of 0.3 to 15.0 rpm, especially 0.5 to 10.0 rpm. If the rotation speed is too low, the modified lithium iron oxide material in the bottom layer does not easily come into contact with the carbon source gas, and if the rotation speed is too high, the powder material may be swept up by centrifugal force or pressed into clumps, which is unfavorable for carbon coating.

[0046] Preferably, the duration of the gas-phase carbon coating is 1 to 3 hours.

[0047] In a third embodiment, the present application provides a positive electrode piece comprising the modified lithium iron oxide material described in the first embodiment, or a modified lithium iron oxide material manufactured by the method described in the second embodiment.

[0048] In a fourth embodiment, the present application provides a lithium-ion battery comprising the positive electrode piece described in the third embodiment.

[0049] Compared to related technologies, this invention has the following beneficial effects.

[0050] (1) The modified lithium iron oxide material provided by this application is obtained by doping lithium iron oxide powder material with a nickel source, controlling the molar ratio of lithium, iron, and nickel elements, and combining it with appropriate process conditions, thereby generating a unique nickel-iron-lithium oxide thin layer on the surface of the lithium iron oxide material, significantly improving the stability of the lithium iron oxide material, reducing or suppressing the generation of residual alkali, and having excellent lithium replenishment performance.

[0051] (2) By employing mechanochemical ball milling, the present invention achieves the effect of a chemical reaction by mechanical force, avoiding the need to employ high-temperature sintering and eliminating the need for grinding after sintering, thereby saving process and energy consumption. Furthermore, by employing mechanochemical ball milling, the nickel source can be reacted mainly with excess lithium elements on the surface or surface layer of lithium iron oxide fine particles. The resulting iron-nickel-lithium composite oxide is advantageous in that it coats the lithium iron oxide fine particles, blocking contact between lithium iron oxide and moisture and oxygen in the external environment, reducing side reactions, improving the stability of the lithium iron oxide powder material, providing a protective effect, and suppressing or reducing the amount of residual alkali generated.

[0052] (3) The modified lithium iron oxide material provided herein can form a homogeneous carbon coating layer using a gaseous carbon source, has excellent conductivity, and can improve electrochemical performance. Furthermore, such a homogeneous carbon coating layer can better block air, further improving environmental stability.

[0053] (4) The residual alkali content of the modified lithium iron oxide described in this application can be 1.56% or less, the initial Coulomb efficiency of the manufactured battery can be 85% or more, the 3C discharge capacity retention rate can be 88% or more, and the capacity retention rate at 500 cycles at 25°C can be 99% or more.

[0054] Other aspects will become clear after reading and understanding the drawings and detailed descriptions. [Brief explanation of the drawing]

[0055] [Figure 1] This is a cross-sectional SEM diagram of the modified lithium iron oxide material described in Example 1 of the present application. [Figure 2] This is an SEM diagram of the modified lithium iron oxide material described in Comparative Example 2 of the present application. [Modes for carrying out the invention]

[0056] The technical solutions of this application will be further explained below with reference to specific embodiments. Those skilled in the art should understand that the embodiments described are merely intended to aid in understanding this application and do not specifically limit it.

[0057] Example 1 This embodiment provides a modified lithium iron oxide material, and the method for producing the above modified lithium iron oxide material includes the following steps: (1) Mixing 298.81 g of lithium oxide (10 mol, equivalent to 20 mol of lithium element) and 312.99 g of ferric oxide (1.96 mol, equivalent to 3.92 mol of iron element) in a mixer to obtain a mixed powder. In an argon gas atmosphere, the mixed powder is heated to 100°C and maintained for 4.0 hours, then heated to 600°C and maintained for 20.0 hours to perform high-temperature sintering, and then allowed to cool naturally to 25°C to obtain lithium iron oxide. (2) In an argon gas atmosphere, lithium iron oxide is dispersed and pulverized using a pulverizer, sieved through a 500-mesh sieve, the residue from sieving is pulverized again, sieved again, 2.99 g of nickel tertian oxide (0.04 mol, equivalent to 0.04 mol of nickel element) is added, and the mixture is mixed using a mixer to obtain a mixed powder of lithium iron oxide and nickel tertian oxide. Next, in an argon gas atmosphere, mechanochemical ball milling is performed on the above mixed powder using a ball mill for a ball milling time of 5.0 h and a ball material ratio of 30:1 to obtain a nickel-doped lithium iron oxide substrate. (3) In an argon gas atmosphere, 100 g of the nickel-doped lithium iron oxide substrate obtained in step (2) is left in a chemical vapor deposition furnace, the temperature is raised to 700°C, then acetylene is added, the cavity of the chemical vapor deposition furnace is rotated at a rotation speed of 0.5 rpm, the temperature is maintained and the rotation is continued for 3.0 hours, then the filling of the carbon source gas is stopped, the temperature is lowered to 25°C, and the above modified lithium iron oxide material is obtained, the carbon coating amount of which is 1.4 wt%.

[0058] A cross-sectional SEM diagram of the modified lithium iron oxide material described above is shown in Figure 1. As can be seen from Figure 1, a single coating layer exists on the powder surface of each particle of the modified lithium iron oxide manufactured in this application, and the average thickness of the coating layer is 200 nm.

[0059] Example 2 This embodiment provides a modified lithium iron oxide material, and the method for producing the above modified lithium iron oxide material includes the following steps: (1) Mixing 298.81 g of lithium oxide (10 mol, equivalent to 20 mol of lithium element) and 319.38 g of ferric oxide (2 mol, equivalent to 4 mol of iron element) in a mixer to obtain a mixed powder. In an argon gas atmosphere, the mixed powder is heated to 250°C, maintained for 0.5 hours, then heated to 900°C, maintained for 5.0 hours, and after high-temperature sintering, it is allowed to cool naturally to 25°C to obtain lithium iron oxide. (2) In an argon gas atmosphere, lithium iron oxide is dispersed and pulverized using a pulverizer, sieved through a 500-mesh sieve, the residue from sieving is pulverized again, 14.94 g (0.2 mol, equivalent to 0.2 mol of nickel element) of nickel steric oxide is added, and the mixture is mixed using a mixer to obtain a mixed powder of lithium iron oxide and nickel steric oxide. Next, in an argon gas atmosphere, mechanochemical ball milling is performed on the above mixed powder using a ball mill for a ball milling time of 0.5 hours and a ball material ratio of 10:1 to obtain a nickel-doped lithium iron oxide substrate. (3) In an argon gas atmosphere, 100 g of the nickel-doped lithium iron oxide substrate obtained in step (2) is left in a chemical vapor deposition furnace, the temperature is raised to 1000°C, then ethane is added, the cavity of the chemical vapor deposition furnace is rotated at a rotation speed of 10.0 rpm, the temperature is maintained and the rotation is continued for 1.0 hour, then the filling of the carbon source gas is stopped, the temperature is lowered to 25°C, and the above-mentioned modified lithium iron oxide material is obtained.

[0060] Of this amount, the carbon coating content was 2.01 wt%, and the average thickness of the carbon coating layer was 300 nm.

[0061] Example 3 The only difference between this example and Example 1 was that the lithium source was changed to 20 mol of lithium hydroxide, the iron source to 4.4 mol of iron chloride, and the nickel source to 0.2 mol of nickel nitrate; all other conditions and parameters were exactly the same as in Example 1. Of this amount, the carbon coating content was 1.4 wt%, and the average thickness of the carbon coating layer was 200 nm.

[0062] Example 4 The only difference between this example and Example 1 was that the lithium source was changed to 10 mol of lithium oxalate, the iron source to 3.6 mol of iron chloride, and the nickel source to 0.2 mol of nickel hydroxide; all other conditions and parameters were exactly the same as in Example 1. Of this amount, the carbon coating content was 1.4 wt%, and the average thickness of the carbon coating layer was 200 nm.

[0063] Comparative Example 1 The only difference between this comparative example and Example 1 was the absence of a nickel source; all other conditions and parameters were exactly the same as in Example 1. Of this amount, the carbon coating content was 1.4 wt%, and the average thickness of the carbon coating layer was 200 nm.

[0064] Comparative Example 2 The only difference between this comparative example and Example 1 is that a solid-phase coating process was adopted in step (3), the coating carbon source was changed to sucrose, and solid-phase coating was performed; all other conditions and parameters were exactly the same as in Example 1. Of this, the carbon coating content was 0.87 wt%, and the SEM image of the modified lithium iron oxide material is shown in Figure 2.

[0065] Comparative Example 3 The only difference between this comparative example and Example 1 is that the nickel source was directly mixed with the iron and lithium sources and sintered, and mechanochemical ball milling was not performed. All other conditions and parameters were exactly the same as in Example 1. Of this amount, the carbon coating content was 1.4 wt%, and the average thickness of the carbon coating layer was 200 nm.

[0066] Comparative Example 4 The only difference between this comparative example and Example 1 was that the mass of nickel(1) oxide described in step (2) was 2.4 g (0.032 mol, equivalent to 0.032 mol of nickel element), while all other conditions and parameters were exactly the same as in Example 1. Of this amount, the carbon coating content was 1.4 wt%, and the average thickness of the carbon coating layer was 200 nm.

[0067] Comparative Example 5 The only difference between this example and Example 1 is that the mass of nickel(1) oxide described in step (2) is 19.74 g (0.24 mol, equivalent to 0.24 mol of nickel element), while all other conditions and parameters were exactly the same as in Example 1. Of this amount, the carbon coating content was 1.4 wt%, and the average thickness of the carbon coating layer was 200 nm.

[0068] Performance testing: 0.3 g of lithium iron oxide samples prepared in the examples and comparative examples were added to 100 mL of methanol solvent, stirred at 800 rpm for 5 min using a 3 cm magnetic stirrer, filtered by suction, and the filtrate was collected and measured using a Mettler GS20 automatic titrator to obtain the total values ​​of residual alkali LiOH and Li2CO3.

[0069] (1) The modified lithium iron oxide material obtained in LFP, Examples 1-4 and Comparative Examples 1-5, the conductive agent SuperP, carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) were uniformly mixed with N-methylpyrrolidone (NMP) in a mass ratio of 94:2:1.5:0.5:2 to produce a positive electrode paste (solid content 60%), which was applied to the aluminum foil current collector at a surface density of 340 g / cm2 on both sides, dried at 70°C, then cold-pressurized at 4 MPa at room temperature, followed by edge trimming, cutting, and slitting, and finally the tabs were welded to produce a positive electrode piece. (2) In an inert protective atmosphere, the artificial graphite anode material, the conductive agent SuperP, and the binder PVDF are uniformly mixed with NMP in a mass ratio of 97.5:1.0:1.5 to produce anode paste (solid content 50%), which is applied to the copper foil of the current collector to a thickness of 100 μm, dried at 100°C, then cold-pressurized at 4 MPa at room temperature, followed by edge trimming, cutting, and slitting, and then tabs are welded to produce anode pieces. (3) A porous PE polymer film is used as the separator film. The manufactured positive electrode piece, separator film, and negative electrode piece are sequentially laminated, with the separator film positioned between the positive and negative electrode pieces. The cell is then wound up to obtain a bare cell. The bare cell is placed in an aluminum laminate pack and subjected to a relative vacuum pressure of -0.95 × 10⁻⁶. 5 The cells were dried at Pa at 100°C until the moisture content was 100 ppm or less. The electrolyte was injected into the dried bare cells, and the electrolyte consisted of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) (EC:EMC:DEC volume ratio = 1:1:1), and LiPF6 (concentration 1.0 M). The cells were then sealed, allowed to stand, chemically converted (constant current charging at 0.02C for 2 hours, then constant current charging at 0.1C for 2 hours), shaped, and capacity measured (capacity division) to manufacture laminate-type lithium-ion batteries.

[0070] 1. Initial Coulomb efficiency test: Using a battery test cabinet, the batteries manufactured as described above were subjected to chemical conversion and capacity splitting. In the chemical conversion process, the batteries were charged at a constant current of 0.05C for 2.0 hours, then at a constant current of 0.15C for 2.5 hours, and further charged at a constant current of 0.33C up to 4.2V, after which they were charged at a constant voltage of 4.2V down to a cutoff current of 0.02C. In capacity splitting, the batteries were discharged at 0.33C up to 2.5V. The sum of the above charging capacities and the discharge capacity were recorded as the initial charging capacity and initial discharge capacity, and the initial charge ratio capacity, initial discharge ratio capacity, and initial Coulomb efficiency were calculated.

[0071] 2. Room temperature cycling performance test: At 25°C, a battery that had undergone chemical conversion and capacity splitting was charged at 0.5C with constant current and constant voltage up to 3.7V, with a cutoff current of 0.02C, and left for 5 minutes. Then, it was discharged at 1C with constant current up to 2.5V, and left for 5 minutes. After 500 charge / discharge cycles in this manner, the capacity retention rate at the 500th cycle was calculated using the following formula:

[0072] Capacity retention rate at 500 cycles (%) = (Discharge capacity at 500 cycles / Discharge capacity at 1 cycle) × 100%.

[0073] The test results are shown in Table 1:

[0074] [Table 1]

[0075] As can be seen from Table 1 and Examples 1-4, the residual alkali content of the modified lithium iron oxide described in this application can be reduced to 1.56% or less, the initial Coulomb efficiency of the manufactured battery can be reduced to 85% or more, the 3C discharge capacity retention rate can be reduced to 88% or more, and the capacity retention rate after 500 cycles at 25°C can be reduced to 99% or more.

[0076] As can be seen from the comparison between Example 1 and Comparative Example 1, the present invention makes it possible to generate a unique nickel-iron-lithium oxide thin layer on the surface of the lithium iron oxide material by doping lithium iron oxide powder material with a nickel source, controlling the molar ratio of lithium, iron, and nickel elements, and combining it with appropriate process conditions. This significantly improves the stability of the lithium iron oxide material, reduces or suppresses the generation of residual alkali, and has excellent lithium replenishment performance.

[0077] As can be seen from the comparison between Example 1 and Comparative Example 2, the present invention can sufficiently and completely coat the surface of a modified lithium iron oxide material by sintering a carbon source gas at high temperature and depositing it on the surface in the form of carbon nanovapors, thereby improving the stability of the material, reducing the amount of residual alkali, and improving conductivity. Regarding solid-phase coating, as can be seen from Figure 2, the surface of the solid-phase coating is non-uniform, some particles are exposed, there is no carbon layer coating, and it is difficult to form a homogeneous carbon coating layer in the coated areas.

[0078] As can be seen from the comparison between Example 1 and Comparative Example 3, the present invention utilizes mechanochemical ball milling to react a nickel source with excess lithium on the surface of lithium iron oxide nanoparticles to produce lithium nickelate, thereby creating a thin layer of iron-nickel-lithium composite material on the surface of the lithium iron oxide nanoparticles and forming an excellent core-shell structure. Since lithium nickelate is more stable than lithium iron oxide, the amount of residual alkali produced can be effectively reduced, improving the overall stability of the modified lithium iron oxide material, and lithium nickelate itself also has a certain lithium replenishment capacity.

[0079] As can be seen from the comparison between Example 1 and Comparative Examples 4-5, in the modified lithium iron oxide material described in this application, the amount of nickel added affects the performance of the modified lithium iron oxide material produced. When the molar ratio of nickel element in the nickel source to lithium element in the lithium source is controlled to (0.01-0.05):5, the effect of the modified lithium iron oxide material produced is relatively good. If the amount of nickel added is too small, the reaction does not proceed uniformly during the ball milling process and the improvement effect is limited. If the amount of nickel added is too large, it affects the lithium iron oxide structure of the main phase and reduces the volume.

[0080] The applicant declares that the above description is merely a specific embodiment of the present application, and that the scope of protection of the present application is not limited thereto. A person skilled in the art should understand that any changes or substitutions that a person skilled in the art could easily conceive of within the scope of the art disclosed herein are included within the claims and disclosures of the present application.

Claims

1. A modified lithium iron oxide material comprising a nickel-doped lithium iron oxide substrate and a thin layer of iron-nickel-lithium oxide and a carbon coating layer sequentially laminated on the surface of the nickel-doped lithium iron oxide substrate, wherein the iron-nickel-lithium oxide thin layer is formed in situ on the surface of the nickel-doped lithium iron oxide substrate, and in the modified lithium iron oxide material, the molar ratio of lithium element to iron element is 5:(0.9-1.1), the molar ratio of nickel element to lithium element is (0.01-0.05):5, and the amount of carbon coating is 0.5-5 wt%. Modified lithium iron oxide material.

2. The average thickness of the carbon coating layer is 10 to 500 nm. The modified lithium iron oxide material according to claim 1.

3. The carbon source forming the carbon coating layer is a gaseous carbon source and includes one or at least two of the following: ethylene, acetylene, methane, ethane, propylene, propane, or butene. The modified lithium iron oxide material according to claim 1.

4. The residual alkali content of the modified lithium iron oxide material is <5%. The modified lithium iron oxide material according to claim 1.

5. A method for producing a modified lithium iron oxide material, (1) A step of mixing a lithium source and an iron source and obtaining a lithium iron oxide substrate by sintering, (2) The step of polishing the lithium iron oxide substrate with a nickel source to obtain a nickel-doped lithium iron oxide substrate, (3) The process includes the step of applying a gas-phase carbon coating to the nickel-doped lithium iron oxide substrate obtained in step (2) using a carbon source to obtain the modified lithium iron oxide material, Manufacturing method.

6. The lithium source described in step (1) includes one or at least two of the following: lithium oxide, lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrite, lithium nitrate, lithium oxalate, lithium acetate, or lithium phosphate. Preferably, the iron source includes one or at least two of the following: iron chloride, triiron tetroxide, diiron trioxide, iron hydroxide, iron oxyhydroxide, iron nitrate, iron acetate, iron citrate, iron phosphate, ferrous oxalate, iron formate, or iron acetate. Preferably, the molar ratio of lithium element in the lithium source to iron element in the iron source is 5:(0.9 to 1.1). The manufacturing method according to claim 5.

7. The sintering process includes one-stage sintering and two-stage sintering, wherein the temperature of the two-stage sintering is higher than the temperature of the one-stage sintering. The manufacturing method according to claim 5.

8. The nickel source described in step (2) includes one or at least two of the following: nickel monooxide, nickel trioxide, nickel hydroxide, nickel hydroxyoxide, nickel nitrate, nickel nitrite, nickel carbonate, nickel oxalate, or nickel acetate. Preferably, the molar ratio of nickel element in the nickel source to lithium element in the lithium source is (0.01 to 0.05):

5. The manufacturing method according to claim 5.

9. Before the polishing treatment described in step (2), the lithium iron oxide substrate is crushed and sieved. Preferably, the polishing process includes mechanochemical ball milling. Preferably, the ball material ratio in the mechanochemical ball milling is (10 to 45):

1. The manufacturing method according to claim 5.

10. The carbon source described in step (3) includes one or at least two of the following: ethylene, acetylene, methane, ethane, propylene, propane, or butene. Preferably, the gas phase carbon coating is carried out in an inert gas atmosphere. The manufacturing method according to claim 5.

11. A modified lithium iron oxide material according to claims 1 to 4, or a modified lithium iron oxide material produced by the method according to any one of claims 5 to 10, Positive electrode piece.

12. Including the positive electrode piece described in claim 11, Lithium-ion battery.

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