Preparation and application of strontium lanthanum manganate coated lithium ferric manganese phosphate composite material

Abstract

The invention belongs to the technical field of battery materials, and particularly discloses preparation and application of a strontium lanthanum manganate-coated lithium ferric manganese phosphate composite material, and the method comprises the following steps: firstly, preparing strontium lanthanum manganate by adopting a lanthanum source, a strontium source and a manganese source as raw materials; respectively adding a lithium source, a phase transfer agent, a phosphorus source, L-ascorbic acid, an iron source and a manganese source into a solvent, and preparing a lithium ferric manganese phosphate precursor by using a solvothermal method; the lithium ferric manganese phosphate material is coated twice through strontium lanthanum manganate and carbon. According to the invention, strontium lanthanum manganate is used as a coating material, and the addition of lanthanide elements improves the crystal stability of lithium ferric manganese phosphate, keeps the integrity of an olivine-type structure, and avoids capacity fading caused by structure collapse, thereby improving the cycle stability of the lithium ion battery.

Description

Technical Field

[0001] The invention belongs to the technical field of electrode materials, and specifically relates to the preparation and application of a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material. Background Art

[0002] As an indispensable part of lithium-ion batteries, cathode materials directly determine important performances of batteries, such as capacity, life, and safety. With the development of electric vehicles in recent years, it is necessary to develop battery systems with high charge and discharge efficiency, high specific capacity, and excellent low-temperature performance to meet the rapidly developing market demand.

[0003] Olivine structure lithium iron phosphate (LiFePO 4 , LFP) as the positive electrode material of lithium-ion batteries, with a theoretical specific capacity of 170 mAh g -1 , and has the advantages of abundant raw materials, low price, environmental friendliness, pollution-free, good thermal stability, good cycle performance (can be charged and discharged more than 2000 times under 100% DD conditions). In recent years, LFP has been rapidly growing at home and abroad due to its high safety stability, long cycle life and lower cost advantages. It is mainly used in electric vehicles, commercial vehicles and energy storage. However, as people put forward higher requirements for the cruising range of electric vehicles, the existing LiFePO 4 The energy density of Li / Li is no longer sufficient because of its operating voltage (3.4V vs Li / Li + ) and theoretical capacity (170mAhg -1 ) are not satisfactory.

[0004] Lithium iron manganese phosphate (LMFP) is a new type of phosphate lithium battery positive electrode material formed by doping a certain proportion of manganese on the basis of lithium iron phosphate (LFP). The structure of lithium iron manganese phosphate is similar to that of lithium iron phosphate, and the crystals are all olivine structures, but the energy density is higher than that of lithium iron phosphate. At the same time, lithium iron manganese phosphate has advantages in energy density, safety, low temperature performance and cost. On the one hand, manganese has a higher voltage platform. LMFP can increase the voltage platform from 3.4V to 4.1V by doping manganese. The high voltage brings higher energy density, which can increase the energy density of the battery by 10% to 20%, thereby increasing the cruising range. On the other hand, at -20°C, the Mn platform capacity of LMFP can be maintained at 95% of that at room temperature. In contrast, the capacity retention rate of LFP is about 50%, showing its performance advantage under low temperature conditions.

[0005] The disadvantages of lithium iron manganese phosphate are its low conductivity and poor cycle life. Lithium iron manganese phosphate has a hexagonal stacking structure, with discontinuous FeO 4 (MnO4 ) edge-sharing octahedral networks and the PO 4 The tetrahedron affects electron transfer and Li + embedding and de-embedding; In addition, Li + The diffusion path of Li is easily blocked by Fe-Li antisite defects, which leads to + The diffusion coefficient is much lower than the theoretical value, which in turn affects the performance of lithium iron manganese phosphate batteries.

[0006] During the charge and discharge process of lithium iron manganese phosphate, manganese ions are prone to Jahn-Teller effect, resulting in lattice distortion and manganese dissolution. The dissolved manganese ions will migrate to the negative electrode, destroy the SEI film, consume active lithium and reduce the battery capacity and cycle life. The literature (ACS Nano 2021, 15, 1358-1369) discloses the use of gradient Al 2 O 3 The coating technology effectively suppresses the Jahn-Teller distortion of manganese ions through surface passivation. However, the coating layer produces microcracks due to side reactions with the electrolyte during the cycle, resulting in continuous dissolution of manganese ions along the grain boundaries. Although patent CN113851654A proposes a polydopamine-carbon composite coating process to improve interface stability, the stratification phenomenon of the organic-inorganic interface under high voltage (>4.2V) still causes local SEI film reconstruction and accelerates the loss of active lithium. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a strontium lanthanum manganate coated lithium iron manganese phosphate composite material, preparation and application. The lithium iron manganese phosphate material is coated twice with strontium lanthanum manganate conductive metal oxide and a carbon source to form a complete and continuous conductive network on the surface of the lithium iron manganese phosphate, thereby enhancing the electronic conductivity of the material and improving the performance of the battery system.

[0008] According to the first aspect of the present invention, the present invention first provides a method for preparing strontium lanthanum manganate coated lithium iron manganese phosphate, comprising the following steps:

[0009] (1) Preparing strontium lanthanum manganate: adding a lanthanum source, a strontium source, and a manganese source to a solvent respectively, stirring them sufficiently, washing and drying the precipitate by centrifugation, and then subjecting the precipitate to a thermal reaction to obtain a strontium lanthanum manganate solid;

[0010] (2) Synthesis of lithium iron manganese phosphate: adding a lithium source, a phase transfer agent, phosphoric acid, L-ascorbic acid, an iron source, and a manganese source to a solvent, respectively, stirring the mixture evenly, and subjecting the mixed solution to a solvothermal reaction at room temperature to obtain a lithium iron manganese phosphate precursor solution; washing and centrifuging the mixture with anhydrous ethanol and deionized water for multiple times, drying and grinding the mixture to obtain a lithium iron manganese phosphate precursor powder; subjecting the lithium iron manganese phosphate precursor powder to a carbon coating treatment, and calcining the mixture to obtain a lithium iron manganese phosphate active material;

[0011] (3) Preparation of composite materials: The active material is surface coated with strontium lanthanum manganate and subjected to heat treatment to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0012] Further, in step (1), the lanthanum source is one of lanthanum nitrate, lanthanum chloride, and lanthanum oxide; the strontium source is one of strontium nitrate, strontium carbonate, and strontium chloride; the manganese source is one of manganese nitrate, manganese acetate, and manganese sulfate; and the solvent is deionized water or ethylene glycol;

[0013] Furthermore, in step (1), the reaction stirring time is 1 to 7 hours, and the stirring rate during the reaction is 300 to 1500 rpm;

[0014] Furthermore, in step (1), the temperature of the thermal reaction is 400 to 1200° C., the time of the thermal reaction is 3 to 10 hours, and the heating rate is 1 to 20° C. / min;

[0015] Furthermore, in step (2), the lithium source is one or more of lithium hydroxide, lithium carbonate, and lithium sulfate; the phase transfer agent is one of benzyltriethylammonium chloride, hexadecyltrimethylammonium chloride, and hexadecyltrimethylammonium bromide;

[0016] Furthermore, in step (2), the mass concentration of phosphoric acid used is 85wt%; the iron source is one or more of ferric oxide, ferrous chloride, ferrous oxalate, ferrous sulfate, ferric phosphate, and ferric chloride; the manganese source is one or more of manganese sulfate, manganese nitrate, manganese carbonate, manganese acetate, and manganese oxide; wherein the molar ratio of the lithium source, the manganese source, and the iron source is (1-5):(0.1-1):(0.1-1);

[0017] Furthermore, in step (2), the solvent is one or more of deionized water, methanol, ethanol, propanol, isopropanol, and ethylene glycol; the temperature of the solvent thermal reaction is 150 to 260° C., and the reaction time is 10 to 20 hours;

[0018] Furthermore, in step (2), the carbon source used in the carbon coating treatment is at least one of glucose, fructose, sucrose, citric acid, and PVP;

[0019] Furthermore, in step (2), the calcination atmosphere is Ar or 5% H2 / Ar, the calcination temperature is 400-800°C, the calcination time is 1-24h, and the heating rate is 1-20°C / min;

[0020] Furthermore, in step (3), the amount of strontium lanthanum manganate added is 1 to 30 wt %; the heat treatment temperature is 200 to 700° C., the heat treatment time is 1 to 24 h, and the heating rate is 1 to 20° C. / min.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. The strontium lanthanum manganate coating in the strontium lanthanum manganate-coated lithium iron manganese phosphate composite material provided by the present invention is similar to the effect of carbon coating. Strontium lanthanum manganate can form a physical barrier on the surface of lithium iron manganese phosphate through its own perovskite structural characteristics (such as: high oxygen vacancy concentration), reduce the direct contact between manganese ions and the electrolyte, inhibit manganese dissolution, thereby slowing down the destruction of the SEI film and significantly improving its cycle stability. The addition of lanthanide elements improves the crystal stability of lithium iron manganese phosphate and inhibits the lattice distortion of lithium iron manganese phosphate. In addition, strontium lanthanum manganate has high thermal stability. After coating, it can reduce the volume expansion and structural stress of lithium iron manganese phosphate during charging and discharging, inhibit particle rupture, and thus extend the life of the material.

[0023] 2. The preparation method of the present invention uses lanthanum strontium manganate and carbon coating to coat the lithium iron manganese phosphate material twice, so that the overall coating layer forms a complete and continuous conductive network on the surface of the lithium iron manganese phosphate particles, enhances the electronic conductivity of the material, improves the structural stability of the lithium iron manganese phosphate, and improves the cycle performance of the lithium ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the crystal structure diagram of lanthanum strontium manganate (LSM) coated lithium iron manganese phosphate (LFMP).

[0025] Figure 2 The first cycle charge and discharge curves of the button-type lithium batteries provided in Examples 1, 2, 3 and Comparative Example 1 of the present invention at room temperature and 0.1C.

[0026] Figure 3 The first cycle charge and discharge curves of the button-type lithium batteries provided in Example 3 and Comparative Example 2 of the present invention at room temperature and 0.1C.

[0027] Figure 4 The cycle performance diagram provided for Example 3 of the present invention and Comparative Example 2.

[0028] Figure 5 X-ray diffraction patterns provided for Example 3 and Comparative Example 1 of the present invention.

[0029] Figure 6 This is a scanning electron microscope image of the strontium lanthanum manganate coated lithium iron manganese phosphate material in Example 3 of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0031] Example 1

[0032] In this embodiment, the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate is prepared according to the following steps:

[0033] (1) adding lanthanum nitrate, strontium nitrate and manganese nitrate to ethylene glycol solvent in sequence, placing the mixed solution on a magnetic stirrer and stirring at a rate of 500 rpm for 3 hours, centrifuging and washing the precipitate in the solution, drying and grinding it into a powder state, placing it in a muffle furnace for heat treatment at 600° C. for 5 hours, and obtaining lanthanum strontium manganate and its composite oxide;

[0034] (2) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was centrifuged and washed three times with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0035] (3) Carrying out carbon coating treatment, selecting sucrose as a carbon source, weighing a certain amount of lithium iron manganese phosphate precursor and dissolving it in anhydrous ethanol, placing it in an ultrasonic cleaner for ultrasonic dispersion, and then weighing 15wt% of sucrose and dissolving it in deionized water. After dissolution, the sucrose is added to the anhydrous ethanol dispersion of lithium iron manganese phosphate. After ultrasonic treatment, the mixture is placed in an oven at 60°C for drying. After the solid is ground, a carbon-containing lithium iron manganese phosphate precursor is obtained. The carbon-containing lithium iron manganese phosphate powder is placed in a tubular furnace and heated at H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, a carbon-coated lithium iron manganese phosphate active material was obtained.

[0036] (4) Preparation of composite materials: LMFP / C was dispersed in anhydrous ethanol, 5 wt% lanthanum strontium manganate powder was added, and after ultrasonic treatment, it was dried and placed in a tube furnace at H 2 / Ar atmosphere, and the temperature is increased to 400°C at a heating rate of 5°C / min and kept at that temperature for 4h, and then cooled to room temperature to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0037] (5) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a slurry with a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0038] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6 The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0039] Example 2

[0040] In this embodiment, the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate is prepared according to the following steps:

[0041] (1) adding lanthanum nitrate, strontium nitrate and manganese nitrate to ethylene glycol solvent in sequence, placing the mixed solution on a magnetic stirrer and stirring at a rate of 700 rpm for 3 hours, centrifuging and washing the precipitate in the solution, drying and grinding it into a powder state, placing it in a muffle furnace and heat treating it at 650° C. for 5 hours to obtain lanthanum strontium manganate and its composite oxide;

[0042] (2) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was centrifuged and washed three times with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0043] (3) Carrying out carbon coating treatment, selecting glucose as a carbon source, weighing a certain amount of lithium iron manganese phosphate precursor and dissolving it in anhydrous ethanol, placing it in an ultrasonic cleaner for ultrasonic dispersion, and then weighing 15wt% of glucose and dissolving it in deionized water. After dissolution, adding it to the anhydrous ethanol dispersion of lithium iron manganese phosphate, after ultrasonic treatment, placing it in a 60°C oven for drying, grinding the solid to obtain a carbon-containing lithium iron manganese phosphate precursor, placing the carbon-containing lithium iron manganese phosphate powder in a tube furnace, and heating it under H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, a carbon-coated lithium iron manganese phosphate active material was obtained.

[0044] (4) Preparation of composite materials: LMFP / C was dispersed in anhydrous ethanol, 3 wt% lanthanum strontium manganate powder was added, and after ultrasonic treatment, it was dried and placed in a tube furnace at H 2 / Ar atmosphere, and the temperature is increased to 400°C at a heating rate of 5°C / min and kept at that temperature for 4h, and then cooled to room temperature to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0045] (5) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a slurry with a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0046] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6 The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0047] Example 3

[0048] This embodiment provides a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material, and a lithium ion battery is assembled based on the composite material, as follows:

[0049] The preparation of the positive electrode material includes the following steps:

[0050] In this embodiment, the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate is prepared according to the following steps:

[0051] (1) adding lanthanum nitrate, strontium nitrate and manganese nitrate to ethylene glycol solvent in sequence, placing the mixed solution on a magnetic stirrer and stirring at a rate of 420 rpm for 4 hours, centrifuging and washing the precipitate in the solution, drying and grinding it into a powder state, placing it in a muffle furnace and heat treating it at 700° C. for 5 hours to obtain lanthanum strontium manganate;

[0052] (2) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was centrifuged and washed three times with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0053] (3) Carrying out carbon coating treatment, selecting sucrose as a carbon source, weighing a certain amount of lithium iron manganese phosphate precursor and dissolving it in anhydrous ethanol, placing it in an ultrasonic cleaner for ultrasonic dispersion, and then weighing 15wt% of sucrose and dissolving it in deionized water. After dissolution, it is added to the anhydrous ethanol dispersion of lithium iron manganese phosphate. After ultrasonic treatment, it is placed in an oven at 60°C for drying. After the solid is ground, a carbon-containing lithium iron manganese phosphate precursor is obtained. The carbon-containing lithium iron manganese phosphate powder is placed in a tubular furnace and heated at H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, a carbon-coated lithium iron manganese phosphate active material was obtained.

[0054] (4) Preparation of composite materials: LMFP / C was dispersed in anhydrous ethanol, 1 wt% lanthanum strontium manganate powder was added, and after ultrasonic treatment, it was dried and placed in a tube furnace at H 2 / Ar atmosphere, and heat to 380°C at a heating rate of 5°C / min and keep warm for 4h, and then cool to room temperature to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0055] (5) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a slurry with a mass loading of 2 to 4 mg cm -2 The positive electrode.

[0056] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6 The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0057] Example 4

[0058] In this embodiment, the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate is prepared according to the following steps:

[0059] (1) adding lanthanum nitrate, strontium nitrate and manganese nitrate to ethylene glycol solvent in sequence, placing the mixed solution on a magnetic stirrer and stirring at a rate of 420 rpm for 4 hours, centrifuging and washing the precipitate in the solution, drying and grinding it into a powder state, placing it in a muffle furnace for heat treatment at 700° C. for 5 hours, and obtaining lanthanum strontium manganate and its composite oxide;

[0060] (2) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was centrifuged and washed three times with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0061] (3) Carrying out carbon coating treatment, selecting glucose as a carbon source, weighing a certain amount of lithium iron manganese phosphate precursor and dissolving it in anhydrous ethanol, placing it in an ultrasonic cleaner for ultrasonic dispersion, weighing 10wt% of glucose and dissolving it in deionized water, adding it to the anhydrous ethanol dispersion of lithium iron manganese phosphate after dissolution, placing it in an oven at 60°C for drying after ultrasonic treatment, grinding the solid to obtain a carbon-containing lithium iron manganese phosphate precursor, placing the carbon-containing lithium iron manganese phosphate powder in a tube furnace, and heating it under H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, a carbon-coated lithium iron manganese phosphate active material was obtained.

[0062] (4) Preparation of composite materials: LMFP / C was dispersed in anhydrous ethanol, 17 wt% of lanthanum strontium manganate powder was added, and after ultrasonic treatment, it was dried and placed in a tube furnace at H 2 / Ar atmosphere, and the temperature is increased to 400°C at a heating rate of 5°C / min and kept at that temperature for 4h, and then cooled to room temperature to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0063] (5) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a slurry with a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0064] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6 The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0065] Example 5

[0066] In this embodiment, the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate is prepared according to the following steps:

[0067] (1) adding lanthanum nitrate, strontium nitrate and manganese nitrate to ethylene glycol solvent in sequence, placing the mixed solution on a magnetic stirrer and stirring at a rate of 500 rpm for 3 hours, centrifuging and washing the precipitate in the solution, drying and grinding it into a powder state, placing it in a muffle furnace for heat treatment at 800° C. for 5 hours, and obtaining lanthanum strontium manganate and its composite oxide;

[0068] (2) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 180° C. for 15 h. After the reaction was completed and the temperature was lowered to room temperature, the mixture was centrifuged and washed three times with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0069] (3) Carrying out carbon coating treatment, selecting sucrose as a carbon source, weighing a certain amount of lithium iron manganese phosphate precursor and dissolving it in anhydrous ethanol, placing it in an ultrasonic cleaner for ultrasonic dispersion, and then weighing 10wt% of sucrose and dissolving it in deionized water. After dissolution, it is added to the anhydrous ethanol dispersion of lithium iron manganese phosphate, and after ultrasonic treatment, it is placed in an oven at 60°C for drying. After the solid is ground, a carbon-containing lithium iron manganese phosphate precursor is obtained; the carbon-containing lithium iron manganese phosphate powder is placed in a tubular furnace and heated at H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, a carbon-coated lithium iron manganese phosphate active material was obtained.

[0070] (4) Preparation of composite materials: LMFP / C was dispersed in anhydrous ethanol, 25 wt% of lanthanum strontium manganate powder was added, and after ultrasonic treatment, it was dried and placed in a tube furnace at H 2 / Ar atmosphere, and the temperature is increased to 400°C at a heating rate of 5°C / min and kept at that temperature for 4h, and then cooled to room temperature to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0071] (5) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a slurry with a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0072] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0073] Example 6

[0074] In this embodiment, the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate is prepared according to the following steps:

[0075] (1) adding lanthanum nitrate, strontium nitrate and manganese nitrate to ethylene glycol solvent in sequence, placing the mixed solution on a magnetic stirrer and stirring at a rate of 500 rpm for 3 hours, centrifuging and washing the precipitate in the solution, drying and grinding it into a powder state, placing it in a muffle furnace for heat treatment at 600° C. for 5 hours, and obtaining lanthanum strontium manganate and its composite oxide;

[0076] (2) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was centrifuged and washed three times with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0077] (3) Carrying out carbon coating treatment, selecting sucrose as a carbon source, weighing a certain amount of lithium iron manganese phosphate precursor and dissolving it in anhydrous ethanol, placing it in an ultrasonic cleaner for ultrasonic dispersion, and then weighing 15wt% of sucrose and dissolving it in deionized water. After dissolution, it is added to the anhydrous ethanol dispersion of lithium iron manganese phosphate. After ultrasonic treatment, it is placed in an oven at 60°C for drying. After the solid is ground, a carbon-containing lithium iron manganese phosphate precursor is obtained; the carbon-containing lithium iron manganese phosphate powder is placed in a tubular furnace and heated at H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, a carbon-coated lithium iron manganese phosphate active material was obtained.

[0078] (4) Preparation of composite materials: LMFP / C was dispersed in anhydrous ethanol, 5 wt% lanthanum strontium manganate powder was added, and after ultrasonic treatment, it was dried and placed in a tube furnace at H 2 / Ar atmosphere, and the temperature is increased to 400°C at a heating rate of 5°C / min and kept at that temperature for 4h, and then cooled to room temperature to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

[0079] (5) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0080] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6 The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0081] Comparative Example 1

[0082] This comparative example uses lithium iron manganese phosphate prepared without a carbon source and lanthanum strontium manganate and assembles a lithium ion battery based on it. The specific steps are as follows:

[0083] Preparation of positive electrode materials:

[0084] (1) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, respectively, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was washed three times by centrifugation with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0085] (2) Place the lithium iron manganese phosphate precursor powder in a tube furnace under H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, the lithium iron manganese phosphate active material was obtained.

[0086] (3) The prepared positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a slurry with a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0087] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0088] Comparative Example 2

[0089] This comparative example uses lithium iron manganese phosphate prepared from only a carbon source and no strontium lanthanum manganate and assembles a lithium ion battery based on the phosphate. The specific steps are as follows:

[0090] (1) Lithium hydroxide, hexadecyltrimethylammonium bromide, phosphoric acid, L-ascorbic acid, ferrous sulfate, and manganese sulfate were dissolved in a mixed solvent of deionized water and ethylene glycol in a volume ratio of 1:1, respectively, and stirred on a magnetic stirrer for 3 h. The mixed liquid was transferred to a stainless steel reactor and subjected to a solvothermal reaction at 200° C. for 15 h. After the reaction was completed and the mixture was cooled to room temperature, it was washed three times by centrifugation with anhydrous ethanol and deionized water respectively, and dried in an oven at 60° C. The solid was taken out and ground to obtain a lithium iron manganese phosphate precursor.

[0091] (2) Carry out carbon coating treatment, select sucrose as the carbon source, weigh a certain amount of lithium iron manganese phosphate precursor and dissolve it in anhydrous ethanol, place it in an ultrasonic cleaner for ultrasonic dispersion, then weigh 10wt% of sucrose and dissolve it in deionized water, add it to the anhydrous ethanol dispersion of lithium iron manganese phosphate after dissolution, place it at 60°C for drying after ultrasonic treatment, and grind it to obtain a carbon-containing lithium iron manganese phosphate precursor.

[0092] (3) Placing the carbon-containing lithium iron manganese phosphate precursor powder in a tube furnace under H 2 / Ar atmosphere, the temperature was raised to 700°C at a heating rate of 5°C / min and then kept at that temperature for 5h. After cooling to room temperature, carbon-coated lithium iron manganese phosphate powder was obtained.

[0093] (4) The positive electrode material, conductive carbon black (Ketjen black), and dispersant (PVDF) prepared above were mixed with N-methylpyrrolidone (NMP) solvent in a mass ratio of 8:1:1 to form a slurry. The slurry was then coated on a 200 μm thick aluminum foil, dried in a vacuum oven at 80°C for 12 h, and then rolled and punched to obtain a mass loading of 2 to 3.5 mg cm -2 The positive electrode.

[0094] Battery assembly: The battery was assembled in an argon-protected glove box. The positive electrode used the lithium iron manganese phosphate positive electrode sheet prepared in this example, the negative electrode material used a lithium sheet, and the electrolyte used was 1M LiPF 6 The electrolyte of DMC:EC:EMC=1:1:1Vol% was used, and the separator was 2325PP.

[0095] 1. The batteries assembled in the above embodiments and comparative examples were subjected to the following performance tests:

[0096] Full battery room temperature test: The test conditions are constant current charge and discharge, and the voltage range is 2.5V-4.5V. Examples 1-6 and Comparative Examples 1-2 are charged to 4.5V, 4.5V, 4.5V, 4.5V, 4.5V, 4.5V, 4.5V, 4.5V, 4.5V, respectively at room temperature (25°C) at a rate of 0.1C, and then discharged to 2.5V, 2.5V, 2.5V, 2.5V, 2.5V, 2.5V, 2.5V, 2.5V at room temperature at a rate of 0.1C, and the discharge performance at room temperature is measured. The results are shown in the table.

[0097]

[0098]

[0099] It can be seen from the table that the battery assembled from the iron manganese phosphate lithium composite material prepared in Example 3 has a first cycle discharge capacity of 123.96 mAh g -1 , the capacity retention rate is 85.8%, which is significantly better than that of the batteries in Examples 1, 2, 4, 5, and 6. Moreover, compared with Comparative Example 2, after adding strontium lanthanum manganate, Example 3 has a significantly increased first cycle discharge specific capacity and an increased capacity retention rate (200 cycles), indicating that the addition of strontium lanthanum manganate can effectively improve the cycle stability of the battery.

[0100] 2. Figure 1 This is the crystal structure diagram of strontium lanthanum manganate coated lithium iron manganese phosphate.

[0101] 3. Figure 2 The first cycle charge and discharge curves of the button-type lithium batteries provided in Examples 1, 2, 3 of the present invention and Comparative Example 1 at room temperature and 0.1C are consistent with the results in Table 1 through curve comparison. The battery assembled with the lithium iron manganese phosphate composite material of Example 3 has the highest initial discharge specific capacity.

[0102] 4. Figure 3 The first cycle charge and discharge curves of the button-type lithium batteries provided in Example 3 and Comparative Example 2 of the present invention at room temperature and 0.1C are shown. By comparing the battery assembled with lithium iron manganese phosphate material with added lanthanum strontium manganate with the battery without adding lanthanum strontium manganate, its initial discharge capacity is significantly improved.

[0103] 5. Figure 4 The charge and discharge cycle performance comparison curve of the lithium battery provided in Example 3 of the present invention and Comparative Example 2 at room temperature and 2C shows that the capacity of the battery assembled with the lithium iron manganese phosphate composite material coated with strontium lanthanum manganate of Example 3 decays slightly, and the capacity retention rate is 80.2% after 400 cycles, which has the best cycle stability and electrochemical performance compared with the battery assembled with the lithium iron manganese phosphate composite material without adding strontium lanthanum manganate in Comparative Example 2.

[0104] 6. Figure 5 These are the X-ray diffraction diagrams provided by Example 3 of the present invention and Comparative Example 1. Through comparative analysis, the XRD diagram after adding strontium lanthanum manganate has higher crystallinity and smaller particles than that without adding strontium lanthanum manganate.

[0105] 7. Figure 6 This is the scanning electron microscope (SEM) characterization result of the strontium lanthanum manganate coated lithium iron manganese phosphate material in Example 3 of the present invention. The image shows that the particles of lithium iron manganese phosphate have a polyhedral morphology, an average particle size range of 200-500nm, clear crystal surfaces, and no significant sintering and adhesion phenomenon.

[0106] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a composite material of strontium lanthanum manganate coated with lithium iron manganese phosphate, characterized in that: The following steps are involved: (1) preparing strontium lanthanum manganate: adding a lanthanum source, a strontium source and a manganese source to a solvent respectively, stirring them sufficiently, washing and drying the precipitate by centrifugation, and then performing a thermal reaction to obtain a strontium lanthanum manganate solid; (2) Synthesis of lithium iron manganese phosphate: adding a lithium source, a phase transfer agent, phosphoric acid, L-ascorbic acid, an iron source, and a manganese source to a solvent, respectively, stirring the mixture to be uniform, and then subjecting the mixture to a solvothermal reaction at room temperature to obtain a lithium iron manganese phosphate precursor solution; Washing and centrifuging with anhydrous ethanol and deionized water for multiple times, drying and grinding to obtain lithium iron manganese phosphate precursor powder; carbonizing the lithium iron manganese phosphate precursor powder, and calcining to obtain carbon-coated lithium iron manganese phosphate active material; (3) Preparation of composite materials: The active material is surface coated with strontium lanthanum manganate and subjected to heat treatment to obtain a strontium lanthanum manganate-coated lithium iron manganese phosphate composite material.

2. The preparation method according to claim 1, characterized in that: In step (1), the lanthanum source is one or more of lanthanum nitrate, lanthanum chloride, and lanthanum oxide; the strontium source is one or more of strontium nitrate, strontium carbonate, and strontium chloride; the manganese source is one or more of manganese nitrate, manganese acetate, and manganese sulfate; and the solvent is a mixed solution of one or more of deionized water, methanol, ethanol, isopropanol, and ethylene glycol.

3. The preparation method according to claim 1, characterized in that: In step (1), the reaction stirring time is 1 to 7 hours, the stirring rate during the reaction is 300 to 1500 rpm, the thermal reaction temperature is 400 to 1200° C., the thermal reaction time is 3 to 10 hours, and the heating rate is 1 to 20° C. / min.

4. The preparation method according to claim 1, characterized in that: In step (2), the lithium source is one or more of lithium hydroxide, lithium carbonate, and lithium sulfate; the phase transfer agent is one of benzyltriethylammonium chloride, hexadecyltrimethylammonium chloride, and hexadecyltrimethylammonium bromide; in step (3), the carbon source used in the carbon coating treatment is at least one of glucose, fructose, sucrose, citric acid, and PVP.

5. The preparation method according to claim 1, characterized in that: In step (2), the mass concentration of phosphoric acid used is 85wt%; the iron source is one or more of ferric oxide, ferrous chloride, ferrous oxalate, and ferrous sulfate; the manganese source is one or more of manganese sulfate, manganese nitrate, manganese carbonate, manganese acetate, and manganese oxide, wherein the molar ratio of the lithium source, the manganese source, and the iron source is (1-5):(0.1-1):(0.1-1); the solvent is one or more of deionized water, methanol, ethanol, isopropanol, and ethylene glycol.

6. The preparation method according to claim 1, characterized in that: The temperature of the solvent thermal reaction in step (2) is 150-260° C., and the reaction time is 10-72 hours.

7. The preparation method according to claim 1, characterized in that: In step (3), the calcination atmosphere is Ar or 5% H2 / Ar, the calcination temperature is 400-800°C, the calcination time is 1-24h, and the heating rate is 1-20°C / min.

8. The preparation method according to claim 1, characterized in that: In step (3), the amount of lanthanum strontium manganate added is 1 to 30 wt %; the heat treatment temperature is 200 to 700° C., the heat treatment time is 1 to 24 h, and the heating rate is 1 to 20° C. / min.

9. A lithium iron manganese phosphate composite material having a surface coating of strontium lanthanum manganate obtained by the preparation method as claimed in any one of claims 1 to 8.

10. Use of the lithium iron manganese phosphate material coated with lanthanum strontium manganate as claimed in claim 9 in a lithium ion battery.

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