Method for synthesizing lithium manganese iron phosphate material coated with discontinuous vapor deposition carbon

Abstract

The present invention belongs to the technical field of lithium battery preparation. Disclosed is a method for synthesizing a lithium manganese iron phosphate material coated with discontinuous vapor deposition carbon. The method for synthesizing the discontinuous vapor deposition carbon-coated lithium manganese iron phosphate material comprises the following steps: (1) preparing a precursor; (2) carrying out pre-sintering organic carbon coating; (3) carrying out rotary kiln vapor deposition carbon coating; and (4) carrying out crushing and sieving. In the method for synthesizing a lithium manganese iron phosphate material coated with discontinuous vapor deposition carbon, raw materials are selected to prepare a precursor, the precursor is then subjected to two-stage sintering of pre-sintering organic carbon coating and rotary kiln vapor deposition carbon coating, and same is then crushed and sieved to obtain a lithium manganese iron phosphate material coated with discontinuous vapor deposition carbon. The lithium manganese iron phosphate material coated with discontinuous vapor deposition carbon prepared by means of the present synthesis method has a compact and uniform coated carbon layer and has relatively good electronic conductivity.

Description

A method for synthesizing discontinuous vapor deposition carbon-coated lithium manganese iron phosphate material Technical Field

[0001] The present disclosure relates to the field of lithium battery preparation, and in particular to a method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material. Background Art

[0002] Olivine-type phosphates, LiMPO4 (M = Fe, Mn), are considered the most promising cathode materials for lithium-ion power batteries. Among them, lithium iron phosphate (LiFePO4) (LFP) has been widely used in lithium batteries due to its excellent cycle performance, structural stability, safety, and low cost. However, the energy density of LiFePO4 is approaching the theoretical limit for mass production (175-185 Wh / kg).

[0003] Lithium iron manganese phosphate (LMP) combines the safety of LMP with the high energy density of ternary materials, making it a promising candidate for cathode material upgrades. LMP has a higher voltage platform than LMP, at 4.1V, 0.7V higher than LMP. Consequently, its theoretical energy density is approximately 15% higher than LMP.

[0004] Lithium manganese iron phosphate material LiMn with olivine structure 1～x Fe x PO4 has inherent defects, one of which is that the lithium manganese iron phosphate material LiMn 1～x Fe x The strong PO bond in PO4 limits the free movement of electrons, resulting in low electronic conductivity; another reason is that the lithium manganese iron phosphate material LiMn 1～x Fe x The olivine structure of PO4 causes its Li + The diffusion channel is one-dimensional, Li + The transmission rate is low. Therefore, it is necessary to use carbon coating, nano-sizing, metal ion doping and other means to improve the performance of lithium manganese iron phosphate material LiMn 1～x Fe x PO4 material is modified.

[0005] Existing coating methods typically involve high-temperature carbonization of organic carbon sources or carbon coating by vapor deposition (CVD). High-temperature carbonization of organic carbon sources has a low carbonization temperature, and the resulting carbon layer can inhibit particle growth, but the coating is loose and uneven. While CVD carbon coating produces a uniform and dense coating, it has disadvantages such as high reaction temperatures, harsh reaction conditions, and a low deposition rate.

[0006] Chinese patent application CN116230932A discloses a method for preparing a carbon- and lithium phosphate-coated lithium iron manganese phosphate material. The method uses an organic carbon source and an inorganic carbon source as a primary carbon-coating material, and sintering, crushing, and screening with an iron source, a manganese source, and a lithium source in an inert gas atmosphere to obtain a primary carbon-coated lithium iron manganese phosphate; then, a vapor deposition coating method is used to carbon-coat the lithium iron manganese phosphate material after the primary carbon source coating; and then, lithium phosphate coating is performed to obtain a carbon- and lithium phosphate-coated lithium iron manganese phosphate material. However, when synthesizing the primary carbon-coated lithium iron phosphate material, the lithium iron manganese phosphate crystals have already grown. When the secondary carbon coating and lithium phosphate coating are subsequently performed, it is easy to cause the conductive network to be discontinuous and the coating layers to be loose.

[0007] Summary of the Invention

[0008] Based on this, the purpose of the present disclosure is to provide a method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material, which improves the material properties of lithium manganese iron phosphate by obtaining a dense and uniform carbon coating layer through a two-stage sintering method of low-temperature and short pre-sintering organic carbon coating and rotary kiln CVD vapor deposition coating.

[0009] To achieve the above objectives, the present disclosure adopts the following technical solutions:

[0010] A method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material comprises the following steps:

[0011] (1) Preparing a precursor: adding a lithium source, a manganese source, an iron source, a phosphorus source, and an organic carbon source into a sander, then adding water, and grinding to obtain a precursor slurry; the ratio of the amount of each element in the precursor slurry is Li:Mn:Fe:P=1.05:x:1~x:1, wherein x=0.3~0.7; the amount of the organic carbon source added is 4~10wt% of the theoretical yield of lithium manganese iron phosphate; spray drying the precursor slurry to obtain a precursor;

[0012] (2) Pre-sintering organic carbon coating: placing the precursor in a kiln with a protective atmosphere, heating it to 530-570°C, and keeping it warm for 1-3 hours; after the end of the insulation, stopping heating and cooling to obtain a pre-sintered material;

[0013] (3) Rotary kiln vapor deposition (CVD) carbon coating: the pre-sintered material is placed in a rotary kiln with a protective atmosphere, heated to 680-780°C, and kept warm for 5-7 hours. After keeping warm for 4-5 hours, a reducing gas is introduced. The reducing gas is one or a combination of at least two of acetylene, methane, ethanol vapor, and benzene vapor. The ventilation time of the reducing gas is 1-2 hours. After the end of the insulation, the heating is stopped and the material is cooled to obtain a sintered material.

[0014] (4) Crushing and screening: The sintered material is crushed and screened to obtain a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material.

[0015] The method for synthesizing a non-continuous vapor-deposited carbon-coated lithium manganese iron phosphate material disclosed herein comprises selecting raw materials to prepare a precursor, then subjecting the precursor to a two-stage sintering process involving pre-sintering with organic carbon coating and rotary kiln vapor deposition (CVD) carbon coating, followed by crushing and sieving to obtain the non-continuous vapor-deposited carbon-coated lithium manganese iron phosphate material. The non-continuous vapor-deposited carbon-coated lithium manganese iron phosphate material prepared by this synthesis method exhibits a dense and uniform carbon coating layer and good electronic conductivity.

[0016] Wherein, in step (2), the precursor is pretreated by pre-sintering organic carbon coating. By performing a low-temperature short-term pre-sintering treatment on the precursor, that is, keeping the temperature at 530-570°C for 1-3 hours, the organic carbon source is carbonized to form a porous carbon network and the volatile substances in the precursor are removed, and the generation and growth of lithium manganese iron phosphate (LMFP) crystal nuclei are avoided (that is, lithium manganese iron phosphate is not generated in this process), effectively avoiding the problem of discontinuous conductive network and loose coating layer caused by the simultaneous carbonization of organic carbon source and the generation of lithium manganese iron phosphate. The porous carbon network formed by the carbonization of the organic carbon source uses amorphous carbon as a skeleton to confine the precursor particles to a nanometer-sized pore size. The porous carbon network has the functions of limiting crystal size and conductivity; and pre-sintering can remove the volatile components in the precursor, preventing the subsequent loose and porous coating layer due to the volatilization of the volatile components.

[0017] In step (3), the pre-sintered material is further sintered in a rotary kiln, and is kept warm for 5 to 7 hours after reaching the nucleation temperature of 680 to 780°C; wherein, during the process of reaching the nucleation temperature of 680 to 780°C and keeping warm for 4 to 5 hours, lithium manganese iron phosphate (LMFP) crystals nucleate and grow in the nanoscale pores of the porous carbon network, and after 4 to 5 hours, the growth of the lithium manganese iron phosphate (LMFP) crystals is nearly completed, and the lithium manganese iron phosphate (LMFP) crystals are tightly attached to the porous carbon network without destroying the conductivity of the porous carbon network; then, a reducing gas is introduced at 680 to 780°C for 1 to 2 hours, and the reducing gas can be adsorbed on the surface of the material and directly carbonized, filling the gaps in the porous carbon network, generating a complete and dense high-graphitized carbon coating layer, and greatly improving the electronic conductivity of the material. In this step, the use of a rotary kiln can cause the material to be continuously turned over, thereby increasing the contact area between the material and the reducing gas, improving production efficiency and making the carbon coating layer tighter, thereby generating a lithium manganese iron phosphate-carbon composite material.

[0018] In one embodiment, in step (1), the lithium source is one or a combination of at least two of lithium carbonate, lithium hydroxide, lithium nitrate, lithium sulfate, lithium oxalate, and lithium acetate. The above lithium sources are suitable for conversion into lithium manganese iron phosphate.

[0019] In one embodiment, in step (1), the manganese source is one or a combination of at least two of manganese iron phosphate, manganese nitrate, manganese carbonate, and manganese oxalate. The above manganese sources are suitable for conversion into lithium manganese iron phosphate.

[0020] In one embodiment, in step (1), the iron source is one or a combination of at least two of ferromanganese phosphate, ferric hydroxide, ferric phosphate, ferric nitrate, and ferrous oxalate. The above iron sources are suitable for conversion into lithium ferromanganese phosphate.

[0021] In one embodiment, in step (1), the phosphorus source is one or a combination of at least two of ferric phosphate, phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate. The above phosphorus sources are suitable for conversion into lithium manganese iron phosphate.

[0022] By selecting the above-mentioned lithium source, manganese source, iron source and phosphorus source, the nucleation and growth of lithium manganese iron phosphate (LMFP) crystals are avoided during the low-temperature and short pre-sintering treatment process; while during the high-temperature 680-780°C rotary kiln process, lithium manganese iron phosphate crystals can be nucleated and grown.

[0023] In one embodiment, in step (1), the organic carbon source is one or a combination of at least two of glucose, sucrose, citric acid, and polyethylene glycol. Using the above organic carbon source, the organic carbon source can be carbonized to form a porous carbon network during a low-temperature and short pre-sintering process, i.e., a temperature of 530-570° C. for 1-3 hours.

[0024] In one embodiment, in step (1), the solid content of the mixture obtained after adding water is 20-35 wt %. By controlling the solid content of the mixture after adding water, the raw materials can be better mixed during grinding in the grinder and the particle size can be better controlled, thereby controlling the particle size of the final product.

[0025] In one embodiment, in step (1), the particle size D50 of the precursor slurry is 0.4 to 0.45 μm. After grinding, the precursor slurry with a particle size D50 of 0.4 to 0.45 μm is obtained. If the particle size after grinding is too small, the grinding efficiency is low and the production capacity is affected; if the particle size after grinding is too large, there will be residual particles, making the solid phase reaction difficult in the subsequent reaction process, affecting the performance of the finished product.

[0026] In one embodiment, in step (2), the protective atmosphere is one of nitrogen and argon; in step (3), the protective atmosphere is one of nitrogen and argon. It should be understood that the protective atmospheres in step (2) and step (3) can be the same or different.

[0027] In one embodiment, the kiln is a tube furnace, a muffle furnace, a rotary kiln, or a roller kiln. The above kilns can facilitate a low-temperature, short-term pre-sintering treatment of the precursor.

[0028] In one embodiment, in step (2), the temperature is raised to 530-570°C at a heating rate of 2-5°C / min; and in step (3), the temperature is raised to 680-780°C at a heating rate of 2-5°C / min. The heating rate is selected so that the material can be heated uniformly to a specific temperature. In other embodiments, other heating rates may be selected for heating. It should be understood that the heating rates in step (2) and step (3) may be the same or different.

[0029] In one embodiment, in step (2), the pre-sintered material is naturally cooled to below 80°C before being removed; and in step (3), the sintered material is naturally cooled to below 80°C before being removed. In other embodiments, other cooling techniques may be selected to cool to other temperatures so that the processed material can be removed.

[0030] In one embodiment, in step (3), the flow rate of the reducing gas is 0.02 to 0.2 L / min. The introduction of a suitable flow rate of reducing gas allows the reducing gas to be adsorbed on the surface of the material and directly carbonized, filling the voids in the porous carbon network to form a complete and dense highly graphitized carbon coating.

[0031] In one embodiment, in step (4), the sintered material is crushed by air flow milling or mechanical grinding. In other embodiments, other crushing methods can also be selected to crush the sintered material.

[0032] In one embodiment, in step (4), a 200-mesh screen is used for screening, and the material above the screen is crushed and re-screened, and the material below the screen is the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material. In other embodiments, other screens can also be selected for screening according to actual needs.

[0033] The present disclosure also provides a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material, which is characterized in that it is obtained by using any of the above-mentioned synthesis methods for the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material.

[0034] The present disclosure discloses a lithium iron manganese phosphate material prepared by discontinuous vapor deposition carbon coating, namely, the discontinuous vapor deposition carbon-coated lithium iron manganese phosphate material, which has a continuous conductive network coating, primary particles concentrated in the range of 150 to 200 nm, and a spherical material structure, which reduces the specific surface area of ​​the material and improves the actual processing performance of the lithium iron manganese phosphate. The discontinuous vapor deposition carbon-coated lithium iron manganese phosphate material disclosed in the present disclosure has a dense and uniform carbon coating layer, and the material has good electronic conductivity; the size of the primary particles of the material is less than 200 nm. The reduced material size shortens the lithium ion migration path, resulting in improved electrochemical performance; the low free carbon content in the material facilitates close contact between particles, thereby increasing the compaction density.

[0035] For better understanding and implementation, the present disclosure is described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is a charge-discharge curve diagram of the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material of Example 1;

[0037] FIG2 is a charge and discharge curve diagram of the lithium iron manganese phosphate material of Comparative Example 1;

[0038] FIG3 is an SEM image of the lithium manganese iron phosphate materials of Example 1, Comparative Example 1, Comparative Example 3, and Comparative Example 6. DETAILED DESCRIPTION

[0039] The present disclosure is further illustrated below with reference to the following examples. These examples are intended to illustrate the present disclosure only and are not intended to limit the scope of the present disclosure. Experimental methods in the following examples, where specific conditions are not specified, generally follow conventional conditions in the art or conditions recommended by the manufacturer; raw materials and reagents used, unless otherwise specified, are all commercially available from conventional markets.

[0040] Example 1

[0041] This embodiment provides a method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material, comprising the following specific steps:

[0042] (1) preparing a precursor;

[0043] ①Ingredients:

[0044] Lithium carbonate, manganese carbonate, ferrous oxalate, ammonium dihydrogen phosphate and glucose were weighed according to the stoichiometric ratio, wherein the molar ratio of each element was Li:Mn:Fe:P=1.05:0.6:0.4:1, and the amount of glucose added was 6% of the mass of the theoretically generated pure-phase lithium manganese iron phosphate.

[0045] ② Sand milling: slowly add the lithium source, manganese source, iron source, phosphorus source and glucose weighed in the "ingredients" step into the sand mill in turn, add an appropriate amount of deionized water as a dispersion medium to make the solid content 25wt%, and after grinding, obtain a precursor slurry with a particle size D50 of 0.4μm.

[0046] ③ Spray drying: The precursor slurry is spray dried. The spray tower inlet temperature is set to 180°C, the air outlet temperature is set to 80°C, and the compressed air pressure is 0.4 MPa. After spray drying, the precursor is obtained.

[0047] (2) Pre-sintered organic carbon coating:

[0048] The precursor was placed in a tubular furnace with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 570°C at a heating rate of 5°C / min and kept at this temperature for 2 hours; after the insulation was completed, the heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a pre-sintered material; in this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0049] (3) Rotary kiln vapor deposition (CVD) carbon coating:

[0050] The pre-sintered material was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was increased to 720°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours. After keeping warm for 5 hours, acetylene gas was introduced with a ventilation flow rate of 0.05L / min and a ventilation time of 2 hours. After the insulation was completed, heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a sintered material. In this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0051] (4) Crushing and screening:

[0052] The sintered material is crushed by mechanical grinding, and the crushed material is screened through a 200-mesh sieve. The material above the sieve is crushed again, and the material below the sieve is the lithium manganese iron phosphate material, that is, the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material.

[0053] Example 2

[0054] This embodiment provides a method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material, comprising the following specific steps:

[0055] (1) preparing a precursor;

[0056] ① Ingredients: lithium carbonate, manganese carbonate, ferrous oxalate, ammonium dihydrogen phosphate and glucose were weighed according to the stoichiometric ratio, wherein the molar ratio of each element was Li:Mn:Fe:P=1.05:0.6:0.4:1, and the amount of glucose added was 4% of the mass of the theoretically generated pure-phase lithium manganese iron phosphate.

[0057] ② Sanding: Same as Example 1.

[0058] ③ Spray drying: same as in Example 1.

[0059] (2) Pre-sintered organic carbon coating:

[0060] The precursor was placed in a tubular furnace with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 530°C at a heating rate of 5°C / min and kept at this temperature for 3 hours; after the insulation was completed, the heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a pre-sintered material; in this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0061] (3) Rotary kiln vapor deposition (CVD) carbon coating:

[0062] The pre-sintered material was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was increased to 780°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours. After keeping warm for 5 hours, methane gas was introduced with a ventilation flow rate of 0.2L / min and a ventilation time of 2 hours. After the insulation was completed, heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a sintered material. In this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0063] (4) Crushing and screening:

[0064] Same as Example 1.

[0065] Example 3

[0066] This embodiment provides a method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material, comprising the following specific steps:

[0067] (1) preparing a precursor;

[0068] 1. Ingredients: same as in Example 1.

[0069] ② Sanding: Same as Example 1.

[0070] ③ Spray drying: same as in Example 1.

[0071] (2) Pre-sintered organic carbon coating:

[0072] The precursor was placed in a tubular furnace with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 550°C at a heating rate of 5°C / min and kept at this temperature for 1 hour; after the insulation was completed, the heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a pre-sintered material; during this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0073] (3) Rotary kiln vapor deposition (CVD) carbon coating:

[0074] The pre-sintered material was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was increased to 680°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours. After keeping warm for 5 hours, acetylene gas was introduced with a ventilation flow rate of 0.05L / min and a ventilation time of 2 hours. After the insulation was completed, heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a sintered material. In this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0075] (4) Crushing and screening:

[0076] Same as Example 1.

[0077] Comparative Example 1

[0078] This comparative example provides a method for synthesizing lithium manganese iron phosphate material, comprising the following specific steps:

[0079] (1) preparing a precursor;

[0080] 1. Ingredients: same as in Example 1.

[0081] ② Sanding: Same as Example 1.

[0082] ③ Spray drying: same as in Example 1.

[0083] (2) Rotary kiln vapor deposition (CVD) carbon coating:

[0084] The precursor was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 720°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours. After keeping warm for 5 hours, acetylene gas was introduced with a ventilation flow rate of 0.05L / min and a ventilation time of 2 hours. After the insulation was completed, heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a sintered material. Protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0085] (3) Crushing and screening:

[0086] The sintered material is crushed by mechanical grinding, and the crushed material is screened through a 200-mesh sieve. The material above the sieve is crushed again, and the material below the sieve is the lithium manganese iron phosphate material.

[0087] Comparative Example 2

[0088] This comparative example provides a method for synthesizing lithium manganese iron phosphate material, comprising the following specific steps:

[0089] (1) preparing a precursor;

[0090] 1. Ingredients: same as in Example 1.

[0091] ② Sanding: Same as Example 1.

[0092] ③ Spray drying: same as in Example 1.

[0093] (2) Pre-sintered organic carbon coating:

[0094] The precursor was placed in a tubular furnace with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 570°C at a heating rate of 5°C / min and kept at this temperature for 2 hours; after the insulation was completed, the heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a pre-sintered material; in this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0095] (3) Crushing and screening:

[0096] The pre-sintered material is crushed by mechanical grinding, and the crushed material is screened through a 200-mesh sieve. The material above the sieve is crushed again, and the material below the sieve is the lithium manganese iron phosphate material.

[0097] Comparative Example 3

[0098] This comparative example provides a method for synthesizing lithium manganese iron phosphate material, comprising the following specific steps:

[0099] (1) Preparation of precursor:

[0100] ① Ingredients: lithium carbonate, manganese carbonate, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio, wherein the molar ratio of each element was Li:Mn:Fe:P=1.05:0.6:0.4:1.

[0101] ② Sand milling: slowly add the lithium source, manganese source, iron source and phosphorus source weighed in the "batch" step into the sand mill in turn, add an appropriate amount of deionized water as a dispersion medium to make the solid content 25wt%, and after grinding, obtain a precursor slurry with a particle size D50 of 0.4μm.

[0102] ③ Spray drying: same as in Example 1.

[0103] (2) Pre-sintered organic carbon coating:

[0104] Same as Example 1.

[0105] (3) Rotary kiln vapor deposition (CVD) carbon coating:

[0106] The pre-sintered material was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was increased to 720°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours. After keeping warm for 5 hours, acetylene gas was introduced with a ventilation flow rate of 0.05L / min and a ventilation time of 2 hours. After the insulation was completed, heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a sintered material. In this process, protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0107] (4) Crushing and screening:

[0108] Same as Example 1.

[0109] Comparative Example 4

[0110] This comparative example provides a method for synthesizing lithium manganese iron phosphate material, comprising the following specific steps:

[0111] (1) Preparation of precursor:

[0112] 1. Ingredients: same as in Example 1.

[0113] ② Sanding: Same as Example 1.

[0114] ③ Spray drying: same as in Example 1.

[0115] (2) Pre-sintered organic carbon coating:

[0116] Same as Example 1.

[0117] (3) Rotary kiln vapor deposition (CVD) carbon coating:

[0118] The pre-sintered material was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 720°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours; after the holding period, heating was stopped, and the material was naturally cooled to below 80°C and then taken out; protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0119] (4) Crushing and screening:

[0120] Same as Example 1.

[0121] Comparative Example 5

[0122] This comparative example provides a method for synthesizing lithium manganese iron phosphate material, comprising the following specific steps:

[0123] (1) Preparation of precursor:

[0124] 1. Ingredients: same as in Example 1.

[0125] ② Sanding: Same as Example 1.

[0126] ③ Spray drying: same as in Example 1.

[0127] (2) Pre-sintered organic carbon coating:

[0128] Same as Example 1.

[0129] (3) Tubular furnace vapor deposition (CVD) carbon coating:

[0130] The pre-sintered material was placed in a tubular furnace with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 720°C at a heating rate of 5°C / min, and kept at this temperature for 7 hours. After keeping warm for 5 hours, acetylene gas was introduced with a ventilation flow rate of 0.05L / min and a ventilation time of 2 hours. After the insulation was completed, heating was stopped, and the material was naturally cooled to below 80°C and then taken out. Protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0131] (4) Crushing and screening:

[0132] Same as Example 1.

[0133] Comparative Example 6

[0134] This comparative example provides a method for synthesizing lithium manganese iron phosphate material, comprising the following specific steps:

[0135] (1) Preparation of precursor:

[0136] 1. Ingredients: same as in Example 1.

[0137] ② Sanding: Same as Example 1.

[0138] ③ Spray drying: same as in Example 1.

[0139] (2) Pre-sintered organic carbon coating:

[0140] The precursor was placed in a rotary kiln with nitrogen as a protective atmosphere for sintering, and the temperature was raised to 600°C at a heating rate of 5°C / min and kept at this temperature for 2 hours; after the insulation was completed, the heating was stopped, and the material was naturally cooled to below 80°C and taken out to obtain a pre-sintered material; protective gas nitrogen was passed throughout the entire process from the start of heating to the end of cooling.

[0141] (3) Rotary kiln vapor deposition (CVD) carbon coating:

[0142] Same as Example 1.

[0143] (4) Crushing and screening:

[0144] Same as Example 1.

[0145] Performance Testing

[0146] The following tests were performed on the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate materials prepared in Examples 1 to 3 and the lithium manganese iron phosphate materials prepared in Comparative Examples 1 to 6:

[0147] Carbon content test: carried out in accordance with the provisions of GB / T 223.69.

[0148] Volume resistivity test: carried out in accordance with Appendix D of GB / T 33822-2017.

[0149] Tap density test: carried out in accordance with the provisions of GB / T 5162.

[0150] Initial discharge specific capacity and coulombic efficiency tests: Conducted in accordance with Appendix G of GB / T 33822-2017, the battery's charge and discharge voltage range is 2.0V to 4.3V.

[0151] Please refer to Table 1 for the test results:

[0152] Table 1 Test results of lithium manganese iron phosphate material

[0153] In Comparative Example 1, relative to Example 1, the low-temperature and short pre-sintering organic carbon coating step was skipped; in Comparative Example 2, relative to Example 1, the rotary kiln vapor deposition (CVD) carbon coating was skipped; in Comparative Example 3, relative to Example 1, no organic carbon source was added when preparing the precursor; in Comparative Example 4, relative to Example 1, no reducing gas was introduced during the rotary kiln vapor deposition (CVD) carbon coating; in Comparative Example 5, relative to Example 1, a tubular furnace was used for vapor deposition (CVD) carbon coating; in Comparative Example 6, relative to Example 1, the temperature was raised to 600°C during the pre-sintering organic carbon coating.

[0154] It can be seen from Examples 1 to 3 and Comparative Examples 1 and 2 that the performance of the lithium manganese iron phosphate material obtained by single sintering is inferior to that of the discontinuously vapor-deposited carbon-coated lithium manganese iron phosphate material obtained by organic carbon coating and rotary kiln vapor deposition (CVD) carbon coating in the present disclosure. This is because the present disclosure uses low-temperature, short-term pre-sintering organic carbon coating to carbonize the organic carbon source to form a porous carbon network and avoid the generation and growth of lithium manganese iron phosphate (LMFP) crystal nuclei, thereby effectively limiting particle growth; the present disclosure then performs rotary kiln vapor deposition (CVD) carbon coating on the pre-sintered material, so that after reaching the nucleation temperature, the lithium manganese iron phosphate (LMFP) crystals nucleate and grow within the nanoscale pores of the porous carbon network. The lithium manganese iron phosphate (LMFP) crystals are tightly attached to the carbon layer without destroying the conductive network. The vapor deposition (CVD) carbon coating fills the gaps in the porous carbon network, forming a complete and dense highly graphitized carbon coating layer, which greatly improves the electronic conductivity of the material.

[0155] From Examples 1 to 3 and Comparative Examples 3 and 4, it can be seen that only the addition of a carbon source and a reducing atmosphere within the appropriate range of the present disclosure will improve the material properties. Using only an organic carbon source or a reducing gas for carbon coating will cause an increase in the material resistivity and a decrease in the electrochemical performance. Comparative Example 3 does not add an organic carbon source and only uses the reducing gas acetylene in the rotary kiln vapor deposition (CVD) carbon coating for carbon coating, which causes an increase in the material resistivity and a decrease in the electrochemical performance. Comparative Example 4 only uses an organic carbon source for carbon coating and does not use the reducing gas acetylene for carbon coating in the rotary kiln vapor deposition (CVD) carbon coating, which also causes an increase in the material resistivity and a decrease in the electrochemical performance. Comparative Example 3 using only rotary kiln vapor deposition (CVD) carbon coating has the worst effect. This is because the addition of the organic carbon source not only reduces the particle size but also inhibits the oxidation of divalent iron during the ball milling process.

[0156] As can be seen from Examples 1-3 and Comparative Example 5, Examples 1-3 utilize a rotary kiln, which allows for continuous rotation during the CVD process to increase the contact area between the material and the reducing gas, resulting in more uniform carbon layer deposition and improved electrochemical performance. Comparative Example 5 utilizes a tubular furnace for CVD carbon coating, resulting in high volume resistivity and relatively poor initial discharge specific capacity and coulombic efficiency.

[0157] By comparing Examples 1 to 3 with Comparative Example 6, it can be seen that adjusting the sintering system within a reasonable range has little effect on the performance of the finished product. In Comparative Example 6, during the pre-sintering organic carbon coating process, the temperature is 600°C, which exceeds the temperature range of the present disclosure. In Comparative Example 6, lithium manganese iron phosphate is directly synthesized in the pre-sintering organic carbon coating step. Since the growth of lithium manganese iron phosphate (LMFP) and the formation of the carbon coating layer occur simultaneously, a continuous coating layer cannot be obtained, and the loose carbon coating layer formed on the surface of the material has a weaker effect on limiting the particle size than in Examples 1 to 3. The subsequent rotary kiln vapor deposition (CVD) carbon coating only serves to improve the conductivity.

[0158] This disclosure also plots charge-discharge curves for the lithium manganese iron phosphate materials of Example 1 and Comparative Example 1. Referring to Figure 1 , the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material of Example 1 exhibits an initial discharge capacity of 153.2 mAh / g and a coulombic efficiency of 97.2%. Referring to Comparative Example 1, the lithium manganese iron phosphate material of Comparative Example 1 exhibits an initial discharge capacity of only 122.2 mAh / g and a relatively low coulombic efficiency of 91.3%.

[0159] The present disclosure also performs SEM detection on the lithium iron manganese phosphate materials prepared in Example 1, Comparative Example 1, Comparative Example 3 and Comparative Example 6, please refer to Figure 3. From the perspective of morphology and structure, the morphology and structure of the lithium iron manganese phosphate material coated with discontinuous vapor-deposited carbon in Example 1 is a spherical lithium iron manganese phosphate material with a continuous conductive network coating and primary particles concentrated in the range of 150 to 200 nm. The morphology and structure of the lithium iron manganese phosphate materials in Comparative Examples 1, Comparative Example 3 and Comparative Example 6 do not have an obvious conductive network; from the perspective of material particle size, Example 1 < Comparative Example 6 < Comparative Example 1 < Comparative Example 3. The reduction in particle size of the discontinuous vapor-deposited carbon-coated lithium iron manganese phosphate material in Example 1 is beneficial to improving ionic conductivity, which is also consistent with the charge and discharge test results.

[0160] Compared with the prior art, the synthesis method of the non-continuous vapor deposition carbon-coated lithium manganese iron phosphate material disclosed in the present invention improves the material properties of lithium manganese iron phosphate by obtaining a dense and uniform carbon coating layer through a two-stage sintering method of low-temperature and short-term pre-sintering organic carbon coating and rotary kiln CVD vapor deposition coating. The lithium manganese iron phosphate material prepared by non-continuous vapor deposition carbon coating disclosed in the present invention, that is, the non-continuous vapor deposition carbon-coated lithium manganese iron phosphate material, has a continuous conductive network coating, primary particles concentrated in the range of 150 to 200 nm, and the material structure is spherical, which reduces the specific surface area of ​​the material and improves the actual processing performance of lithium manganese iron phosphate. The non-continuous vapor deposition carbon-coated lithium manganese iron phosphate material disclosed in the present invention has a dense and uniform carbon coating layer, and the material has good electronic conductivity; the size of the primary particles of the material is less than 200 nm, and the reduction in the size of the material shortens the lithium ion migration path, resulting in improved electrochemical performance; the low free carbon in the material is conducive to close contact between particles, thereby improving the compaction density.

[0161] The above-described embodiments merely represent several implementation methods of the present disclosure. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous modifications and improvements without departing from the spirit of the present disclosure, and the present disclosure is intended to encompass such modifications and variations.

Claims

1. A method for synthesizing a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material, characterized in that: The following steps are involved: (1) Preparing a precursor: adding a lithium source, a manganese source, an iron source, a phosphorus source, and an organic carbon source into a sander, and then adding water, and grinding to obtain a precursor slurry; the ratio of the amount of each element in the precursor slurry is Li:Mn:Fe:P=1.05:x:1~x:1, wherein x=0.3~0.7; the amount of the organic carbon source added is 4~10wt% of the theoretical output of lithium manganese iron phosphate; spray drying the precursor slurry to obtain a precursor; (2) Pre-sintering organic carbon coating: placing the precursor in a kiln with a protective atmosphere, heating it to 530-570° C., and keeping it warm for 1-3 hours; after the end of the insulation, stopping heating and cooling to obtain a pre-sintered material; (3) Rotary kiln vapor deposition carbon coating: placing the pre-sintered material in a rotary kiln with a protective atmosphere, heating it to 680-780° C., and keeping it warm for 5-7 hours, wherein a reducing gas is introduced after keeping it warm for 4-5 hours, wherein the reducing gas is one of acetylene, methane, ethanol vapor, and benzene vapor, or a combination of at least two of them, and the ventilation time of the reducing gas is 1-2 hours; after the insulation is completed, the heating is stopped, and the material is cooled to obtain a sintered material; (4) Crushing and screening: The sintered material is crushed and screened to obtain a discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material.

2. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: The lithium source is one or a combination of at least two of lithium carbonate, lithium hydroxide, lithium nitrate, lithium sulfate, lithium oxalate, and lithium acetate; 3. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (1), the manganese source is one or a combination of at least two of ferromanganese phosphate, manganese nitrate, manganese carbonate, and manganese oxalate.

4. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (1), the iron source is one or a combination of at least two of ferromanganese phosphate, ferric hydroxide, ferric phosphate, ferric nitrate, and ferrous oxalate; 5. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (1), the phosphorus source is one or a combination of at least two of ferric phosphate, phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate; 6. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (1), the organic carbon source is one of glucose, sucrose, citric acid, polyethylene glycol, or a combination of at least two of them.

7. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (1), the solid content of the mixture obtained after adding water is 20 to 35 wt%.

8. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (1), the particle size D50 of the precursor slurry is 0.4 to 0.45 μm.

9. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (2), the protective atmosphere is one of nitrogen and argon; in step (3), the protective atmosphere is one of nitrogen and argon.

10. The method for synthesizing the discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (2), the kiln is one of a tubular furnace, a muffle furnace, a rotary kiln, and a roller kiln.

11. The method for synthesizing the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (2), the temperature is increased to 530-570° C. at a heating rate of 2-5° C. / min; in step (3), the temperature is increased to 680-780° C. at a heating rate of 2-5° C. / min.

12. The method for synthesizing the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (2), the pre-sintered material is taken out after being naturally cooled to below 80° C.; in step (3), the sintered material is taken out after being naturally cooled to below 80° C.

13. The method for synthesizing the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (3), the ventilation flow rate of the reducing gas is 0.02 to 0.2 L / min.

14. The method for synthesizing the discontinuous vapor-deposited carbon-coated lithium manganese iron phosphate material according to claim 1, characterized in that: In step (4), the sintered material is crushed by air flow crushing or mechanical grinding. In step (4), a 200-mesh sieve is used for screening, and the material on the sieve is crushed and sieved again, and the material under the sieve is the non-continuous vapor deposited carbon-coated lithium manganese iron phosphate material.

15. A discontinuous vapor deposited carbon-coated lithium manganese iron phosphate material, characterized in that: The material is obtained by the synthesis method of the non-continuous vapor deposition carbon-coated lithium manganese iron phosphate material as claimed in any one of claims 1 to 14.

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