Lithium iron manganese phosphate composite material, preparation method thereof, positive plate, lithium ion battery and electric device

Abstract

This disclosure relates to a lithium manganese iron phosphate composite material, its preparation method, a positive electrode, a lithium-ion battery, and an electrical device. The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core and a fast ion conductor layer and a conductive layer sequentially coating the lithium manganese iron phosphate core from the inside out. The lithium manganese iron phosphate composite material of this disclosure exhibits significantly improved electronic and ionic conductivity, structural stability, and excellent electrochemical performance.

Description

Technical Field

[0001] This disclosure relates to the field of lithium-ion battery technology, specifically to a lithium manganese iron phosphate composite material and its preparation method, a positive electrode, a lithium-ion battery, and an electrical device. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) cathode materials have advantages such as low cost, non-toxicity, safety, stable discharge platform, and good cycle stability, and are widely used in energy storage and power batteries. However, its relatively low operating voltage platform results in a low energy density, which cannot meet the market's demand for higher energy density. LiFeMnPO4 (LFMP) has a similar crystal structure to LiFePO4. It is a cathode material formed by replacing some of the Fe element with Mn element on the basis of LFP. The addition of manganese gives LFMP a higher voltage platform, thereby improving energy density, while also matching the operating voltage of existing electrolytes, showing great application potential.

[0003] However, LFMP cathode materials have low intrinsic lithium-ion conductivity and electronic conductivity, and Mn... 3+ The Jahn-Teltier effect causes lattice distortion and Mn dissolution. Furthermore, the significant volume change during the two-phase reaction stage of lithium insertion / extraction leads to stress concentration and particle breakage in the cathode material. These drawbacks typically result in longer charging times, poorer rate performance, increased DCR during discharge, and decreased cycle performance in lithium manganese iron phosphate (LFMP) batteries, posing a serious challenge to battery performance. Currently, common LFMP cathode material modification methods include nano-sizing, carbon coating, and metal ion doping, among others. Nano-sizing can shorten the transport paths of ions and electrons within the bulk material, but the higher specific surface area of ​​nano-sized LFMP increases side reactions with the electrolyte, reducing initial battery efficiency and deteriorating LFMP processing performance. Carbon coating cannot solve the problem of low LFMP ionic conductivity. While metal ion doping can improve the ionic and electronic conductivity of LFMP and enhance structural stability, the improvement is limited by the typically low doping concentration. Therefore, a method is needed that can effectively improve the ionic and electronic conductivity of lithium manganese iron phosphate cathode materials while maintaining their structural stability. Summary of the Invention

[0004] The purpose of this disclosure is to provide a lithium manganese iron phosphate composite material and its preparation method, positive electrode, lithium-ion battery and electrical device, so as to improve the electronic conductivity and ionic conductivity of LFMP material, while improving its structural stability.

[0005] To achieve the above objectives, the first aspect of this disclosure provides a lithium manganese iron phosphate composite material, the lithium manganese iron phosphate composite material comprising a lithium manganese iron phosphate core and a fast ion conductor layer and a conductive layer sequentially covering the lithium manganese iron phosphate core from the inside out; The powder resistivity of the lithium manganese iron phosphate composite material is 30~100 Ω·mm, and / or the ionic conductivity of the lithium manganese iron phosphate composite material is 1×10⁻⁶. -13 S / cm up to 3×10 -10 S / cm.

[0006] Optionally, based on the total weight of the lithium manganese iron phosphate composite material, the content of the fast ion conductor layer is 0.5 to 5% by weight, and the content of the conductive layer is 1.5 to 2% by weight.

[0007] Optionally, the fast ion conductor layer is made of lithium phosphate and / or lithium metaphosphate; and / or, The conductive layer is made of carbon.

[0008] Optionally, the chemical formula of the lithium manganese iron phosphate core is LiMn. x Fe 1-x PO4, where 0 <x<1。

[0009] Optionally, the thickness of the fast ion conductor layer is 1~20 nm; and / or, the thickness of the conductive layer is 1~20 nm.

[0010] A second aspect of this disclosure provides a method for preparing the lithium manganese iron phosphate composite material described in the first aspect of this disclosure, the method comprising: A mixture of raw materials containing lithium, iron, phosphorus and manganese sources is subjected to a first grinding and mixing process and a first sintering process to obtain lithium manganese iron phosphate material. The lithium manganese iron phosphate material is subjected to a second grinding and mixing process and a second sintering process with a fast ion conductor precursor and a conductive material precursor to obtain a lithium manganese iron phosphate composite material. The second sintering process includes a first stage and a second stage, wherein the sintering temperature of the first stage is lower than that of the second stage.

[0011] Optionally, the conditions for the first sintering include: a heating rate of 1~10℃ / min, a sintering temperature of 300~600℃, and a time of 0.5~5h.

[0012] Optionally, the conditions for the first stage include: a heating rate of 1~10℃ / min, a sintering temperature of 150~250℃, and a time of 2~5h; and / or, The conditions for the second stage include: a heating rate of 1~10℃ / min, a sintering temperature of 400~800℃, and a time of 5~8h.

[0013] Optionally, the fast-ion conductor precursor is lithium dihydrogen phosphate; and / or, The conductive material precursor is a carbon source.

[0014] Optionally, the molar ratio of the lithium source, the manganese source, the iron source, and the phosphorus source is (1~2):(0.1~0.9):(0.1~0.9):1; and / or, The weight ratio of the lithium manganese iron phosphate material, the fast ion conductor precursor, and the conductive material precursor is 100:(1~50):(1~50).

[0015] Optionally, the lithium source includes at least one of lithium hydroxide, lithium acetate, lithium nitrate, lithium carbonate, and lithium dihydrogen phosphate.

[0016] Optionally, the iron source includes at least one of ferric oxide, ferric oxide, ferrous phosphate, ferrous oxalate, ferrous nitrate, ferrous hydroxide, ferric phosphate, ferric oxalate, and ferric carbonate.

[0017] Optionally, the phosphorus source includes at least one of phosphoric acid, ferrous phosphate, ferric phosphate, manganese phosphate, lithium dihydrogen phosphate, lithium phosphate, and ammonium dihydrogen phosphate.

[0018] Optionally, the manganese source includes at least one of manganese tetroxide, manganese dioxide, manganese nitrate, manganese carbonate, manganese phosphate, manganese oxalate, and manganese sulfate.

[0019] Optionally, the carbon source includes at least one of oxalic acid, sucrose, cellulose, fructose, glucose, starch, maltose, lactose, citric acid, carbon black, and polyethylene glycol.

[0020] In a third aspect, this disclosure provides a positive electrode sheet comprising the lithium manganese iron phosphate composite material described in the first aspect of this disclosure.

[0021] In a fourth aspect, this disclosure provides a lithium-ion battery including the positive electrode sheet described in the third aspect of this disclosure.

[0022] In a fifth aspect, this disclosure provides an electrical device including the lithium-ion battery described in the fourth aspect of this disclosure.

[0023] Through the above technical solution, the lithium manganese iron phosphate composite material disclosed herein has a double coating layer consisting of a fast ion conductor layer and a conductive layer constructed sequentially from the inside to the outside on the surface of the lithium manganese iron phosphate core. The fast ion conductor layer can improve the lithium-ion conductivity of the lithium manganese iron phosphate composite material and establish a fast lithium-ion transport channel. The conductive layer can improve the electronic conductivity of the lithium manganese iron phosphate composite material and reduce the internal resistance of the battery. In addition, the double coating layer can more effectively prevent the positive electrode material from directly contacting the electrolyte, suppress side reactions and manganese dissolution, thereby effectively improving the structural stability of lithium manganese iron phosphate and significantly enhancing the electrochemical performance of the material.

[0024] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0025] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the structure of a lithium manganese iron phosphate composite material according to a specific embodiment; Figure 2 This is a scanning electron microscope image of the lithium manganese iron phosphate composite material from Example 1.

[0026] Explanation of reference numerals in the attached figures 1—Lithium manganese iron phosphate core, 2—Fast ion conductor layer, 3—Conductive layer. Detailed Implementation

[0027] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0028] In a first aspect, this disclosure provides a lithium manganese iron phosphate composite material, with reference to... Figure 1 As shown, the lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core 1 and a fast ion conductor layer 2 and a conductive layer 3 that sequentially cover the lithium manganese iron phosphate core 1 from the inside out.

[0029] The lithium iron manganese phosphate composite material of the present disclosure has a double coating layer composed of a fast ion conductor layer and a conductive layer constructed sequentially from the inside to the outside on the surface of the lithium iron manganese phosphate core. On the one hand, the fast ion conductor layer can improve the lithium ion conductivity of the lithium iron manganese phosphate composite material, establish a fast transmission channel for lithium ions, and promote the transmission of lithium ions. The conductive layer can improve the electronic conductivity of the lithium iron manganese phosphate composite material and reduce the internal resistance of the battery. On the other hand, the double coating layer can more effectively prevent the direct contact between the positive electrode material and the electrolyte, inhibit interfacial side reactions, and alleviate the occurrence of Mn / Fe dissolution, thereby effectively improving the structural stability of lithium iron manganese phosphate. Therefore, compared with traditional materials, the present disclosure can simultaneously improve the ionic and electronic conductivities of the LFMP material, improve the structural stability, and achieve the improvement of the electrochemical performance of the LFMP material.

[0030] According to the present disclosure, the material of the lithium iron manganese phosphate core 1 is a lithium iron manganese phosphate (LFMP) material. In one embodiment, the chemical formula of the lithium iron manganese phosphate core can be LiMn x Fe 1-x PO4, where 0 < x < 1. Specifically, the lithium iron manganese phosphate core may include at least one of LiMn 0.6 Fe 0.4 PO4, LiMn 0.5 Fe 0.5 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.8 Fe 0.2 PO4, preferably LiMn 0.6 Fe 0.4 PO4 and / or LiMn 0.7 Fe 0.3 PO4. Using the preferred lithium iron manganese phosphate material as the core is beneficial to further improving the electrochemical performance of the composite material.

[0031] The material of the fast ion conductor layer 2 is a fast ion conductor (FIC) material with high ionic conductivity. By constructing the fast ion conductor layer 2 on the surface of the lithium iron manganese phosphate core 1, not only can the side reaction between the electrode material and the electrolyte be prevented, but the charge transfer resistance at the interface can also be reduced, and the transmission of lithium ions at the interface can be accelerated. In a preferred embodiment, the material of the fast ion conductor layer is lithium phosphate (Li3PO4) and / or metaphosphate (LiPO3). The above materials are not only beneficial to improving the lithium ion conductivity of the lithium iron manganese phosphate LMFP composite material, but also have good compatibility with the lithium iron manganese phosphate core 1 in terms of material properties and preparation conditions, and can quickly achieve the coating of lithium phosphate and / or metaphosphate on the basis of the existing LMFP preparation process, with significant cost advantages.

[0032] The conductive layer 3 can be made of materials commonly used in the art that have a certain electronic conductivity. In one specific embodiment, the conductive layer can be made of carbon.

[0033] According to this disclosure, the ratio of the fast ion conductor layer 2 and the conductive layer 3 can be adjusted within a certain range. In one specific embodiment, based on the total weight of the lithium manganese iron phosphate composite material, the content of the fast ion conductor layer can be 0.1% to 10% by weight, and the content of the conductive layer can be 1% to 3% by weight. For example, based on the total weight of the lithium manganese iron phosphate composite material, the content of the fast ion conductor layer can be 0.1% by weight, 0.5% by weight, 1% by weight, 2% by weight, 3% by weight, 4% by weight, 5% by weight, 6% by weight, 7% by weight, 8% by weight, 9% by weight, 10% by weight, etc., and the content of the conductive layer can be 1% by weight, 1.5% by weight, 2% by weight, 3% by weight, etc.

[0034] The lithium manganese iron phosphate core 1 is formed into a spherical shape with a particle size of 5-500 nm. The fast ion conductor layer 2 and the conductive layer 3 are independent of each other and are formed into a layered structure that sequentially covers the lithium manganese iron phosphate core 1. The thickness of the fast ion conductor layer 2 can be 1-20 nm, for example, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, or 20 nm. The thickness of the conductive layer 3 can be 1-20 nm, for example, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, or 20 nm.

[0035] The lithium manganese iron phosphate composite material has suitable particle size and specific surface area, exhibiting excellent electrochemical performance; specifically, the primary particle size of the lithium manganese iron phosphate composite material can be 100~500 nm, and the specific surface area can be 10~19 m². 2 / g.

[0036] The lithium manganese iron phosphate composite material disclosed herein exhibits significantly improved electronic and ionic conductivity and structural stability. Specifically, the powder resistivity of the lithium manganese iron phosphate composite material can be 30~100 Ω·mm, and the ionic conductivity can be 1×10⁻⁶. -13 S / cm up to 3×10 -10 S / cm.

[0037] A second aspect of this disclosure provides a method for preparing the lithium manganese iron phosphate composite material described in the first aspect of this disclosure, the method comprising the following steps S1-S2: S1. A mixture of raw materials containing lithium source, iron source, phosphorus source and manganese source is subjected to a first grinding and mixing and a first sintering to obtain lithium manganese iron phosphate material; S2. The lithium manganese iron phosphate material is subjected to a second grinding and mixing process with a fast ion conductor precursor and a conductive material precursor, and then sintered to obtain a lithium manganese iron phosphate composite material; wherein, the second sintering includes a first stage and a second stage, and the sintering temperature of the first stage is lower than the sintering temperature of the second stage.

[0038] Existing cathode material coating processes are not fully compatible with cathode material production processes. Typically, coating is performed on the surface of the cathode material after its preparation, leading to a complex overall preparation process and increased production costs. The method disclosed herein achieves sequential coating of a fast-ion conductor layer and a conductive layer onto the surface of a lithium manganese iron phosphate core through two grinding and mixing processes and two sintering processes, with staged temperature control during the second sintering. This results in the lithium manganese iron phosphate composite material described in the first aspect of this disclosure.

[0039] In step S1, the amount of each component in the raw material mixture can be adjusted within a certain range. Specifically, the molar ratio of the lithium source, manganese source, iron source and phosphorus source can be (1~2):(0.1~0.9):(0.1~0.9):1, preferably (1~1.5):(0.5~0.8):(0.2~0.5):1.

[0040] The lithium source, iron source, phosphorus source, and manganese source can be common types in the art. Specifically, the lithium source can include at least one of lithium hydroxide, lithium acetate, lithium nitrate, lithium carbonate, and lithium dihydrogen phosphate; the iron source can include at least one of ferric oxide, ferric oxide, ferrous phosphate, ferrous oxalate, ferrous nitrate, ferrous hydroxide, ferric phosphate, ferric oxalate, and ferric carbonate; the phosphorus source can include at least one of phosphoric acid, ferrous phosphate, ferric phosphate, manganese phosphate, lithium dihydrogen phosphate, lithium phosphate, and ammonium dihydrogen phosphate; and the manganese source can include at least one of manganese tetroxide, manganese dioxide, manganese nitrate, manganese carbonate, manganese phosphate, manganese oxalate, and manganese sulfate.

[0041] The first grinding and mixing process can be wet ball milling, which can be carried out in the presence of a certain amount of liquid medium (such as water, ethanol, methanol, etc.). The amount of liquid medium can be adjusted as needed. For example, the ratio of the raw material mixture (by weight) to the liquid medium (by volume) can be 1:(1~10). After wet ball milling, drying can be performed to remove the liquid medium. The drying method can be common in the art, such as spray drying. Specifically, the conditions for the first grinding and mixing process can include: a material-to-ball ratio (referring to the weight ratio of the raw material mixture to the ball milling media) of 1~10, a rotation speed of 100~300 rpm, and a time of 0.5~20 h.

[0042] The first sintering can be carried out under an inert atmosphere. Specifically, the conditions for the first sintering may include: a heating rate of 1~10℃ / min, a sintering temperature of 300~600℃, and a time of 0.5~5h.

[0043] In step S2, the fast ion conductor precursor is used to form the fast ion conductor layer described in the first aspect of this disclosure, and the amount of the precursor is such that the prepared fast ion conductor layer has the content range described in the first aspect of this disclosure. Specifically, the weight ratio of the lithium manganese iron phosphate material to the fast ion conductor precursor can be 100:(1~50), preferably 100:(1~30).

[0044] In a specific embodiment where the fast ion conductor layer is made of lithium phosphate and / or lithium metaphosphate, the fast ion conductor precursor is lithium dihydrogen phosphate. Using lithium dihydrogen phosphate as the fast ion conductor precursor facilitates the rapid coating of lithium phosphate and / or lithium metaphosphate based on the preparation of lithium manganese iron phosphate materials, without requiring additional complex process steps.

[0045] Furthermore, the amount of lithium dihydrogen phosphate can be adjusted according to the amount of lithium source in step S1 to prepare the fast ion conductor layer of the desired material. Specifically, the lithium source in step S1 mainly reacts with iron, phosphorus, and manganese sources at high temperature to generate lithium manganese iron phosphate. When the lithium source in step S1 is in excess, after the high-temperature reaction to generate lithium manganese iron phosphate, the remaining lithium source will react with the fast ion conductor precursor lithium dihydrogen phosphate at high temperature in step S2 to produce a lithium phosphate fast ion conductor layer. When the lithium source in step S1 is not in excess and lithium manganese iron phosphate is completely generated, the lithium dihydrogen phosphate in step S2 will undergo high-temperature pyrolysis to generate a lithium metaphosphate fast ion conductor layer. By adjusting the amount of lithium dihydrogen phosphate in step S2 according to the amount of lithium source in step S1, a lithium phosphate fast ion conductor layer, a lithium metaphosphate fast ion conductor layer, and a lithium phosphate-lithium metaphosphate mixed fast ion conductor layer can be prepared respectively.

[0046] The conductive material precursor is used to form the conductive layer described in the first aspect of this disclosure, and its amount is such that the prepared conductive layer has the content range described in the first aspect of this disclosure. Specifically, the weight ratio of the lithium manganese iron phosphate material to the conductive material precursor can be 100:(1~50), preferably 100:(1~30). In a specific embodiment where the conductive layer is made of carbon, the conductive material precursor can be a carbon source. Further, the carbon source can be one of those commonly used in the art, such as at least one selected from oxalic acid, sucrose, cellulose, fructose, glucose, starch, maltose, lactose, citric acid, carbon black, and polyethylene glycol.

[0047] Similar to the first grinding and mixing, the second grinding and mixing can be wet ball milling. Specifically, the conditions for the second grinding and mixing can include: a ball-to-material ratio of 1 to 10, a rotation speed of 100 to 500 rpm, and a time of 0.5 to 10 h.

[0048] In the second sintering process, the sintering temperature of the first stage is lower than that of the second stage. Thus, in the first stage, the fast ion conductor precursor reacts at a lower temperature to obtain a fast ion conductor material and / or its intermediates coated on the surface of the lithium manganese iron phosphate material. At the same time, the conductive material precursor does not pyrolyze to form a conductive layer. In the second stage, the conductive material precursor pyrolyzes at a higher temperature to form a conductive layer coated on the outermost layer. Meanwhile, the fast ion conductor material intermediates can further react to obtain the fast ion conductor material. In addition, the high-temperature sintering in the second stage can also repair the crystal structure of the lithium manganese iron phosphate material, further improving the crystallinity of lithium manganese iron phosphate and enhancing product quality.

[0049] Both the first and second stages of the second sintering can be carried out under an inert atmosphere. In one specific embodiment, the sintering temperature of the first stage is lower than that of the second stage; specifically, the conditions of the first stage may include: a heating rate of 1~10℃ / min, a sintering temperature of 150~250℃, and a time of 2~5h; the conditions of the second stage may include: a heating rate of 1~10℃ / min, a sintering temperature of 400~800℃, and a time of 5~8h.

[0050] The preparation method disclosed herein is simple and can achieve a double-layer coating structure of lithium manganese iron phosphate composite material. It is highly compatible with the preparation process of lithium manganese iron phosphate, without the need for additional complex process steps, thus reducing preparation costs and process difficulty, and is suitable for large-scale production.

[0051] In a third aspect, this disclosure provides a positive electrode sheet comprising the lithium manganese iron phosphate composite material described in the first aspect of this disclosure. The positive electrode sheet prepared using the lithium manganese iron phosphate composite material of this disclosure exhibits excellent electrochemical performance, maintaining stable capacity output and low decay rate even under long-term cycling and high-rate charge-discharge conditions, providing a high-quality positive electrode material option for the development of high-energy-density batteries.

[0052] In a fourth aspect, this disclosure provides a lithium-ion battery, including the positive electrode sheet described in the third aspect of this disclosure. The specific structure of the lithium-ion battery is not particularly limited, and it may include structures common in the art, such as a negative electrode sheet and an electrolyte.

[0053] The lithium-ion battery disclosed herein has significantly improved cycle performance and is suitable not only for electric vehicles, portable devices and other fields that have high requirements for high energy density and fast charging and discharging, but also for large-scale energy storage systems, demonstrating broad application prospects and market potential.

[0054] In a fifth aspect, this disclosure provides an electrical device including the lithium-ion battery described in the fourth aspect of this disclosure. The electrical device may be, for example, a power battery module or an energy storage cabinet.

[0055] The present disclosure will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present disclosure and are not intended to limit the present disclosure.

[0056] All raw materials and reagents used in the examples and comparative examples are commercially available products.

[0057] In the embodiment, the specific surface area of ​​the material was detected by nitrogen (N2) adsorption-desorption test. The test conditions were: the test was conducted at the boiling point of liquid nitrogen at low temperature, and the relative pressure was controlled between 0.05 and 0.25.

[0058] The primary particle size of the material was determined using a scanning electron microscope (SEM). The test conditions were: 10 kV voltage and different magnifications for imaging.

[0059] The thickness of the fast ion conductor layer and the conductive layer was detected by transmission electron microscopy under the following conditions: 200 kV voltage at different magnifications.

[0060] The powder resistivity test method is as follows: The equipment is a Ruike Instruments automatic conductor powder resistivity tester. Select powder resistance as the measurement object; select cylinder as the sample state; automatic current; cross-sectional area 78.5; zeroing off; diameter 10; temperature compensation off; cylinder height not input; length (mm); input the stated sample mass; pressure selection (0~200kg, generally 180kg), select as needed; constant pressure time 10s; diameter 10mm. Click OK; zeroing and running for approximately 8 minutes; place the sample in the mold cavity, tighten the handle to zero, input the sample mass, click start, run for 8 minutes, and save the data.

[0061] The method for testing ionic conductivity is as follows: the AC impedance of the sample is measured using an electrochemical workstation, and the ionic conductivity of the sample is calculated.

[0062] The content of conductive carbon layer was tested using carbon-sulfur (CS) analysis. The equipment was a high-frequency carbon-sulfur infrared analyzer - Dekai HCS. The standard sample was rare earth magnesium spherical cast iron with C: 2.88% and S: 0.016%. The additive was a special solvent for carbon-sulfur analysis of ferrosilicon (1.5g). The sample amount was 0.1g. The content of fast ion conductor coating layer was calculated based on the chemical reaction formula.

[0063] Example 1 (1) Battery-grade raw materials lithium carbonate, manganese tetroxide, iron phosphate, and ammonium dihydrogen phosphate were weighed and mixed according to the molar ratio Li:Mn:Fe:P = 1.03:0.6:0.4:1. A certain amount of deionized water was added and the mixture was ball-milled. The solid-liquid ratio was 1g:1mL and the material-to-ball ratio was 8. The mixture was ball-milled at 250 rpm for 15 hours. The resulting yellow-green suspension was spray-dried to obtain lithium manganese iron phosphate precursor material. The precursor powder was heated to 500 ℃ at a heating rate of 5 ℃ / min under an inert atmosphere. After being kept at 500 ℃ for 3 hours, the temperature was cooled to room temperature. The resulting gray-black lithium manganese iron phosphate material was refined into powder with a median particle size D50 of 300 nm by sand milling.

[0064] (2) Lithium dihydrogen phosphate: lithium manganese iron phosphate: glucose = 3:100:10. Lithium dihydrogen phosphate, the prepared lithium manganese iron phosphate powder and glucose were added to ethanol, with a solid-liquid ratio of 1g:1mL and a material-to-ball ratio of 8. The mixture was ball-milled at 400 rpm for 6 hours and then spray-dried. The dried solid mixture was heated to 200℃ at a heating rate of 5℃ / min and held for 5 hours under an inert atmosphere. Then it was heated to 700℃ at a heating rate of 5℃ / min and held for 5 hours. After cooling to room temperature, it was pulverized through a 400-mesh sieve by an air jet mill to obtain the lithium manganese iron phosphate composite material, denoted as LiMn. 0.6 Fe 0.4The structure consists of PO4@LiPO3@C, where the content of the lithium metaphosphate fast ion conductor layer is 2.4% and the thickness is approximately 12 nm; the content of the carbon layer is 1.4% and the thickness is approximately 5 nm.

[0065] Electron micrographs of the lithium manganese iron phosphate composite material are as follows: Figure 2 As shown, the lithium manganese iron phosphate composite material consists of nanoparticles with uniform size distribution. The primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were measured, and the results are listed in Table 1.

[0066] Example 2 (1) Battery-grade raw materials lithium hydroxide, manganese carbonate, ferric oxide, and ammonium dihydrogen phosphate were weighed and mixed according to the molar ratio Li:Mn:Fe:P = 1.5:0.5:0.5:1. A certain amount of alcohol was added and ball-milled, with a solid-liquid ratio of 1g:1mL and a material-to-ball ratio of 8. After ball milling at 250 rpm for 15 h, the resulting suspension was spray-dried to obtain lithium manganese iron phosphate precursor material. The precursor powder was heated to 400 ℃ at a heating rate of 5℃ / min under an inert atmosphere, held at 400 ℃ for 5 h, and then cooled to room temperature. The resulting gray-black lithium manganese iron phosphate material was refined into powder with a median particle size D50 of 200 nm by sand milling.

[0067] (2) Lithium dihydrogen phosphate: lithium manganese iron phosphate: citric acid = 5:100:10. Lithium dihydrogen phosphate, the prepared lithium manganese iron phosphate powder and citric acid were added to ethanol, with a solid-liquid ratio of 1g:1mL and a material-to-ball ratio of 8. The mixture was ball-milled at 400 rpm for 8 hours. The material was then spray-dried. The dried solid mixture was heated to 180 ℃ at a heating rate of 5 ℃ / min and held for 5 hours under an inert atmosphere. Then it was heated to 600 ℃ at a heating rate of 5 ℃ / min and held for 8 hours. After cooling to room temperature, it was pulverized through a 400-mesh sieve using an air jet mill to obtain the lithium manganese iron phosphate composite material, denoted as LiMn. 0.5 Fe 0.5 The composition is PO4@Li3PO4@C, in which the content of the lithium metaphosphate fast ion conductor layer is 5% and the thickness is about 20 nm; the content of the carbon layer is 1.3% and the thickness is about 5 nm.

[0068] Electron micrographs of the lithium manganese iron phosphate composite material and Figure 2 Similarly, the primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were tested, and the results are listed in Table 1.

[0069] Example 3 (1) Battery-grade raw materials lithium dihydrogen phosphate, manganese oxalate, iron hydroxide, and lithium acetate were weighed and mixed in a molar ratio of 1.02:0.6:0.4:0.5, and then a certain amount of alcohol was added for ball milling. The solid-liquid ratio was 1g:1mL, and the material-to-ball ratio was 8. After ball milling at 250 rpm for 15 hours, the resulting suspension was spray-dried to obtain the core lithium manganese iron phosphate precursor material. The precursor powder was heated to 600 ℃ at a heating rate of 10 ℃ / min under an inert atmosphere, held at 600 ℃ for 2 hours, and then cooled to room temperature. The resulting gray-black lithium manganese iron phosphate material was refined into powder with a median particle size D50 of 500 nm by sand milling.

[0070] (2) The lithium dihydrogen phosphate, lithium manganese iron phosphate, citric acid, and polyethylene glycol were added to ethanol at a weight ratio of 5:100:10:10. The solid-liquid ratio was 1 g:1 mL and the ball-to-material ratio was 8. The mixture was ball-milled at 400 rpm for 8 h. The material was then spray-dried. The dried solid mixture was heated to 220 ℃ at a heating rate of 10 ℃ / min and held for 3 h under an inert atmosphere. Then it was heated to 700 ℃ at a heating rate of 10 ℃ / min and held for 5 h. After cooling to room temperature, it was pulverized through a 400-mesh sieve using an air jet mill to obtain the lithium manganese iron phosphate composite material, denoted as LiMn. 0.6 Fe 0.4 The structure is PO4@Li3PO4-LiPO3@C, in which the content of the lithium phosphate-lithium metaphosphate mixed fast ion conductor layer is 4.5% and the thickness is about 18 nm; the content of the carbon layer is 2.2% and the thickness is 10 nm.

[0071] Electron micrographs of the lithium manganese iron phosphate composite material and Figure 2 Similarly, the primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were tested, and the results are listed in Table 1.

[0072] Comparative Example 1 The method described in Example 1 is implemented, except that in step (2), lithium metaphosphate is used instead of lithium dihydrogen phosphate, and the weight ratio of lithium metaphosphate: lithium manganese iron phosphate: glucose = 0.027:1:0.1 is used. The second calcination is carried out without stages, and the conditions are: the temperature is raised to 700 ℃ at a heating rate of 5 ℃ / min and held for 10 h.

[0073] The lithium manganese iron phosphate composite material prepared in this comparative example is a C-LiPO3 layer-coated lithium manganese iron phosphate composite material, denoted as LiMn. 0.6 Fe 0.4 PO4@C-LiPO3, wherein the C-LiPO3 layer contains 3.8% and has a thickness of approximately 16 nm.

[0074] The primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were measured, and the results are listed in Table 1.

[0075] Comparative Example 2 The method described in Example 1 was implemented, except that in step (2), lithium metaphosphate and the prepared lithium manganese iron phosphate powder were added to ethanol at a mass ratio of lithium metaphosphate:lithium manganese iron phosphate = 4:100, with a solid-liquid ratio of 1g:1mL and a ball-to-material ratio of 8. After ball milling at 400 rpm for 6 hours, the material was spray-dried. The dried solid mixture was heated to 200℃ at a heating rate of 5℃ / min under an inert atmosphere and held at that temperature for 5 hours. After cooling to room temperature, LiMn was obtained. 0.6 Fe 0.4 PO4@LiPO3. Then, according to the mass ratio of LiMn... 0.6 Fe 0.4 The ratio of PO4@LiPO3:glucose = 100:10 was used to prepare LiMn 0.6 Fe 0.4 After mixing PO4@LiPO3 and glucose thoroughly, then heat at 5℃·min -1 The temperature was increased to 700 °C and held for 5 h. After cooling to room temperature, the mixture was pulverized through a 400-mesh sieve using an air jet mill to obtain a lithium manganese iron phosphate composite material, denoted as LiMn. 0.6 Fe 0.4 The structure is PO4@LiPO3@C, in which the content of the lithium metaphosphate fast ion conductor layer is 3.8% and the thickness is about 15nm; the content of the carbon layer is 1.4% and the thickness is 6nm.

[0076] The primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were measured, and the results are listed in Table 1.

[0077] Comparative Example 3 Ammonium dihydrogen phosphate, manganese acetate, ferrous acetate, magnesium acetate, titanium isopropoxide, and lithium carbonate were mixed in a molar ratio of Li:Mn:Fe:Co:Mg:Ti:P = 1.04:0.7:0.28:0.01:0.01:1.02. Polyethylene glycol-1000 was added at 3% of the sum of the mass of the phosphorus, iron, and manganese sources, and ethanol solvent was added at 130% of the sum of the mass of the phosphorus, iron, and manganese sources. The mixture was then ball-milled for 2 hours to achieve a solid content of 40%, forming a uniform slurry. The slurry was then transferred to a rotary evaporator for drying. After heating to 90°C in a water bath and rotary evaporating for 3 hours, the slurry became solid particles with a particle size of less than 5 mm. The particles were then pre-calcined at 400°C for 6 hours in a box furnace under nitrogen protection to obtain the pre-calcined product.

[0078] 1 kg of the pre-calcined material was mixed with 100 g of sucrose and 100 g of CNT aqueous slurry (5% by mass). The mixture was then coarsely ground in a stirred mill containing 1.8 kg of deionized water for 1 hour until D100 < 20 μm. The mixture was then transferred to a sand mill containing 0.2 mm zirconia balls and finely ground for 3 hours to obtain a uniform slurry with D50 = 197 nm. This slurry was then spray-dried, controlling the median diameter of the secondary agglomerates to approximately 20 μm. The secondary agglomerates were then sintered at 675 °C for 8 hours in a tunnel kiln under a nitrogen atmosphere to obtain a lithium manganese iron phosphate composite material.

[0079] The primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were measured, and the results are listed in Table 1.

[0080] Comparative Example 4 With a molar ratio of 1:0.5:0.5, weigh 3.0 g LiPO3, 1.56 g Mn(OH)2, and 1.57 g Fe(OH)2 into a ball mill jar and mill at 250 rad / min. -1 Ball milling was performed at a rotation speed of 3 h (ball-to-material ratio 5:1) to obtain a uniformly mixed LFMP precursor mixture. 0.06 g of lithium phosphate and 0.31 g of polydopamine were added to a ball mill jar at a mass ratio of 1:0.01:0.05, along with 30 mL of petroleum ether. The jar was then filled with N2, sealed, and ball milled at 400 rad / min. -1 After ball milling for 6 hours, the material was transferred to a single-necked flask, magnetically stirred, and heated at a stirring speed of 400 rad / min. -1 The heating temperature is 90℃. After the solvent has completely evaporated, the solid mixture is placed in a tube furnace and heated at 5℃ for 5 minutes under an Ar atmosphere. -1 The temperature was increased to 650℃ and sintered for 12 hours. After cooling to room temperature, C-Li3PO4-coated lithium iron manganese phosphate (LiFe) was obtained. 0.5 Mn 0.5 PO4@C-Li3PO4) cathode material.

[0081] The primary particle size, specific surface area, powder resistivity, and ionic conductivity of the lithium manganese iron phosphate composite material were measured, and the results are listed in Table 1.

[0082] Table 1

[0083] Test case The materials from the examples and comparative examples were assembled into lithium-ion coin cells and then subjected to cycle tests.

[0084] Assembly of lithium-ion button half-cells: The lithium manganese iron phosphate composite material, conductive carbon black and polyvinylidene fluoride prepared in the examples and comparative examples were mixed evenly in N-methylpyrrolidone solvent at a weight ratio of 100:2:3 to obtain a positive electrode slurry (solid content of 50% by weight). The slurry was then coated on aluminum foil, dried, and the aluminum foil was cut into round pieces with a diameter of 12 mm using a punching machine. The lithium-ion button half-cells were then assembled in a glove box.

[0085] Cyclic test conditions: Under a voltage range of 2.0~4.3V, constant current and constant voltage charge-discharge cycles were performed at a rate of 0.5C. The discharge specific capacity after 50 cycles was recorded and listed in Table 2. The capacity retention rate was calculated according to the following formula.

[0086] Capacity retention rate = Specific capacity at discharge cycle 50 / Specific capacity at discharge cycle 1 Table 2

[0087] As shown in Table 2, the lithium manganese iron phosphate composite material prepared in the examples exhibits higher discharge specific capacity and capacity retention rate in lithium-ion batteries, demonstrating excellent electrochemical performance.

[0088] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0089] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0090] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A lithium manganese iron phosphate composite material, characterized in that, The lithium manganese iron phosphate composite material includes a lithium manganese iron phosphate core and a fast ion conductor layer and a conductive layer that sequentially coat the lithium manganese iron phosphate core from the inside out. The powder resistivity of the lithium manganese iron phosphate composite material is 30~100 Ω·mm, and / or the ionic conductivity of the lithium manganese iron phosphate composite material is 1×10⁻⁶. -13 S / cm up to 3×10 -10 S / cm.

2. The lithium manganese iron phosphate composite material according to claim 1, wherein, Based on the total weight of the lithium manganese iron phosphate composite material, the content of the fast ion conductor layer is 0.1~10% by weight, and the content of the conductive layer is 1~3% by weight.

3. The lithium manganese iron phosphate composite material according to claim 1, wherein, The fast ion conductor layer is made of lithium phosphate and / or lithium metaphosphate; and / or, The conductive layer is made of carbon.

4. The lithium manganese iron phosphate composite material according to claim 1, wherein, The chemical formula of the lithium manganese iron phosphate core is LiMn. x Fe 1-x PO4, where 0 <x<1。 5. The lithium manganese iron phosphate composite material according to claim 1, wherein, The thickness of the fast ion conductor layer is 1~20 nm; and / or, the thickness of the conductive layer is 1~20 nm.

6. A method for preparing the lithium manganese iron phosphate composite material according to any one of claims 1 to 5, characterized in that, The method includes: A mixture of raw materials containing lithium, iron, phosphorus and manganese sources is subjected to a first grinding and mixing process and a first sintering process to obtain lithium manganese iron phosphate material. The lithium manganese iron phosphate material is subjected to a second grinding and mixing process and a second sintering process with a fast ion conductor precursor and a conductive material precursor to obtain a lithium manganese iron phosphate composite material. The second sintering process includes a first stage and a second stage, wherein the sintering temperature of the first stage is lower than that of the second stage.

7. The method according to claim 6, wherein, The conditions for the first sintering include: a heating rate of 1~10℃ / min, a sintering temperature of 300~600℃, and a time of 0.5~5h.

8. The method according to claim 6, wherein, The conditions for the first stage include: a heating rate of 1~10℃ / min, a sintering temperature of 150~250℃, and a time of 2~5h; and / or, The conditions for the second stage include: a heating rate of 1~10℃ / min, a sintering temperature of 400~800℃, and a time of 5~8h.

9. The method according to claim 6, wherein, The fast ion conductor precursor is lithium dihydrogen phosphate; and / or... The conductive material precursor is a carbon source.

10. The method according to claim 6, wherein, The molar ratio of the lithium source, the manganese source, the iron source, and the phosphorus source is (1~2):(0.1~0.9):(0.1~0.9):1; and / or, The weight ratio of the lithium manganese iron phosphate material, the fast ion conductor precursor, and the conductive material precursor is 100:(1~50):(1~50).

11. The method according to claim 6, wherein, The lithium source includes at least one selected from lithium hydroxide, lithium acetate, lithium nitrate, lithium carbonate, and lithium dihydrogen phosphate; and / or, The iron source includes at least one of ferric oxide, ferric oxide, ferrous phosphate, ferrous oxalate, ferrous nitrate, ferrous hydroxide, ferric phosphate, ferric oxalate, and ferric carbonate; and / or, The phosphorus source includes at least one selected from phosphoric acid, ferrous phosphate, ferric phosphate, manganese phosphate, lithium dihydrogen phosphate, lithium phosphate, and ammonium dihydrogen phosphate; and / or, The manganese source includes at least one selected from manganese tetroxide, manganese dioxide, manganese nitrate, manganese carbonate, manganese phosphate, manganese oxalate, and manganese sulfate; and / or, The carbon source includes at least one of oxalic acid, sucrose, cellulose, fructose, glucose, starch, maltose, lactose, citric acid, carbon black, and polyethylene glycol.

12. A positive electrode plate, characterized in that, The composite material of lithium manganese iron phosphate as described in any one of claims 1 to 5.

13. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 12.

14. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 13.

Images