Lithium manganese iron phosphate composite positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention provides a lithium manganese iron phosphate composite positive electrode material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: preparing a lithium source, an iron source, a manganese source, a phosphorus source, a doping element source and a first organic carbon source into first slurry, and performing first calcination on the first slurry to obtain a first product; preparing second slurry from the first product and a one-dimensional carbon material, and drying the second slurry to obtain a second product; forming a SiO2 coating layer on the surface of the second product in an atomic layer deposition mode to obtain a third product; preparing third slurry from the third product and a second organic carbon source, and performing secondary calcination on the third slurry to obtain a fourth product; and putting the fourth product into an etching solution, and soaking to obtain the lithium iron manganese phosphate composite positive electrode material. According to the invention, through a mode of combining multiple times of carbon coating, arrangement of the middle coating layer and formation of the gap layer through etching treatment, microstructure design is carried out on the positive electrode material, so that the transmission rate of electrons and lithium ions in the structure is ensured.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a lithium iron manganese phosphate composite positive electrode material, a preparation method thereof and a lithium ion battery. Background Art

[0002] The energy density of lithium iron phosphate is close to the theoretical "ceiling". The introduction of Mn element on the basis of lithium iron phosphate can increase the discharge voltage to 4.1V, greatly improving the energy density of the positive electrode material, so it is expected to further improve the energy density of the corresponding battery cell. However, the corresponding lithium manganese iron phosphate material is not LiFePO 4 With LiMnPO 4 Simple physical mixing. Currently, by using Fe 2+ With Mn 2+ The ionic radius of LiFePO 4 With LiMnPO 4 The synergistic effect between them forms a stable and uniform solid solution, namely LiMn x Fe 1-x PO 4 , thus LiFePO 4 Stable electrochemical performance and LiMnPO 4 Therefore, lithium manganese iron phosphate has become an important direction for the development of positive electrode material technology. x Fe 1-x PO 4 On the basis of the material, if other elements are further introduced into it, its performance can be further improved.

[0003] In LiMn x Fe 1-x PO 4 In the material, x represents the manganese doping ratio, which can be any value between 0 and 1. Due to the high voltage platform of Mn and the good conductivity of Fe, different manganese-iron ratios make the performance of lithium manganese iron phosphate different. When the manganese content is too low, the voltage increase effect is limited and the energy density is not significantly better than that of lithium iron phosphate. When the manganese content is too high, the battery voltage and energy density are significantly improved, but the Jahn-Teller effect is prone to occur, resulting in Mn dissolution and rapid decay of cycle performance; in addition, there is also obvious voltage decay.

[0004] In particular, the existing lithium manganese iron phosphate cathode material has a significant manganese dissolution problem during the cycle. 3+ Dissolution will cause a disproportionation reaction on the surface of the positive electrode material to generate Mn 2+ , Mn 4+ , where Mn 2+ Will be through with HF (LiPF6 +H 2 O→POF 3 +HF+LiF) － Combined with Mn 2+ They dissolve into the electrolyte and migrate to the surface of the negative electrode where they are reduced and deposited, which in turn destroys the SEI film, causing the SEI film to continue to grow and thicken, consume active lithium, increase electrode impedance, and produce gas at the negative electrode. Ultimately, the battery capacity continues to decay and the voltage platform collapses, seriously affecting the battery's performance.

[0005] Based on this, how to provide a preparation method that can optimize the performance of lithium manganese iron phosphate positive electrode materials from the aspects of elemental composition and microstructure design so that the resulting positive electrode materials have better electrochemical performance is one of the important technical problems that need to be solved in this field. Summary of the invention

[0006] The main purpose of the present invention is to provide a lithium manganese iron phosphate composite positive electrode material, a preparation method thereof and a lithium ion battery, so as to solve the problem of poor electrochemical performance, especially poor cycle stability, of lithium ion battery positive electrode materials in the prior art.

[0007] In order to achieve the above-mentioned object, the first aspect of the present invention provides a method for preparing a manganese iron phosphate lithium composite positive electrode material, comprising: step S1, preparing a lithium source, an iron source, a manganese source, a phosphorus source, a doping element source and a first organic carbon source into a first slurry, and obtaining a first product after the first slurry is subjected to a first calcination; step S2, preparing the first product and a one-dimensional carbon material into a second slurry, and obtaining a second product after the second slurry is dried; step S3, forming SiO on the surface of the second product by atomic layer deposition. 2 coating layer, thereby obtaining a third product; step S4, preparing the third product and the second organic carbon source into a third slurry, and obtaining a fourth product after the third slurry is subjected to a second calcination; step S5, placing the fourth product in an etching solution, and obtaining a lithium manganese iron phosphate composite positive electrode material after soaking; wherein the etching solution is a strong acid solution or a strong alkaline solution.

[0008] The present invention realizes the formation of a barrier isolating the electrolyte on its surface by providing the above-mentioned preparation method, thereby alleviating the problem of Mn ion dissolution that may be generated in the application process of the obtained positive electrode material, so that it exhibits excellent electrochemical performance. The above-mentioned preparation method constructs a lithium manganese iron phosphate composite positive electrode material composed of a doped mixed phosphate core, a first carbon material skeleton, a first carbon layer, a nano-void layer and an outer second carbon layer through multi-step material microstructure design and processing. The unique microstructure designed and prepared effectively solves the common manganese ion dissolution and voltage decay problems in the cycle process of lithium-ion battery positive electrode materials. The finally prepared lithium manganese iron phosphate composite positive electrode material has higher energy density, longer cycle life and more excellent charge and discharge performance.

[0009] Further, in step S1, the molar ratio of the lithium source, the iron source, the manganese source, the phosphorus source and the doping element source is 1: (0.1-0.5): (0.5-0.9): 1: (0-0.08); and / or, based on the total weight of the lithium source, the iron source, the manganese source, the phosphorus source and the doping element source as 100%, the amount of the first organic carbon source is 8%-10%; and / or, the D50 of the first product is 3μm-6μm; and / or, the doping element source is selected from one or more of Mg, Sc, Ti, V, Co and Mo; and / or, the temperature of the first calcination is 680°C-730°C.

[0010] During the formation of the first product, by optimizing the above-mentioned preparation conditions, the first product with high capacity and high stability, i.e., a mixed phosphate core provided with a first carbon layer, can be more effectively prepared, thereby providing a material basis for forming a manganese iron phosphate lithium composite positive electrode material with excellent cycle performance.

[0011] Furthermore, in step S2, based on the total weight of the first product being 100%, the amount of the one-dimensional carbon material is 0.5% to 1.0%; and / or, the one-dimensional carbon material is selected from one or more of carbon nanotubes, carbon nanowires and carbon fibers, preferably carbon nanotubes.

[0012] By further optimizing the amount and type of one-dimensional carbon material added, especially carbon nanotubes, it is possible to construct a highly efficient and better-structured conductive network on the surface of the first product, thereby enhancing the mechanical and chemical stability of the resulting lithium manganese iron phosphate composite positive electrode material, and ultimately achieving a comprehensive improvement in its performance, including capacity retention and cycle performance.

[0013] Furthermore, in order to comprehensively improve the intrinsic electrochemical properties, structural stability and processing performance of the second product, so as to obtain a lithium manganese iron phosphate composite positive electrode material with higher cycle stability, the preferred step S2 includes: step S2-1, adding the first product and the one-dimensional carbon material to a solvent to obtain a dispersion; ultrasonically dispersing the dispersion at 10kHz to 50kHz for 0.5h to 2.0h to obtain a second slurry; step S2-2, air drying the second slurry at 100°C to 150°C to obtain a second product.

[0014] Further, in step S3, SiO 2 The thickness of the coating layer is 30nm to 50nm. 2 The coating layer can not only effectively form an optimized nanovoid structure, but also protect the mixed phosphate core and the first carbon layer on its surface from corrosion by hydrofluoric acid, thereby improving the overall electrochemical performance of the obtained lithium manganese iron phosphate composite positive electrode material.

[0015] Preferably, the atomic layer deposition comprises: placing the second product in an atomic layer deposition reaction chamber, 3 As a precursor, N 2 As a purge gas, surface deposition was performed at 200±20°C, and SiO was formed on the surface of the second product. 2 The atomic layer deposition conditions can make SiO 2 The deposition of the coating layer is more uniform and has fewer defects, thereby forming a lithium manganese iron phosphate composite positive electrode material with a more stable structure after etching.

[0016] Furthermore, in step S4, the amount of the second organic carbon source is 3.0% to 5.0% based on the total weight of the third product as 100%; and / or, the temperature of the second calcination is 600°C to 700°C, and the time is 2h to 4h. In the process of forming the second carbon layer, in order to achieve a denser coating, so that the etching solution can enter the interior more evenly in the subsequent etching process to achieve the SiO 2 The coating layer is etched to obtain a lithium manganese iron phosphate composite positive electrode material with a more stable structure and stronger conductivity.

[0017] Further, in step S5, the etching solution is a hydrofluoric acid solution or a sodium hydroxide solution, and the concentration of the etching solution is 0.05 mol / L to 0.5 mol / L; and / or, the soaking time is 1 min to 30 min. Through the above-mentioned more optimized etching solution concentration, type and etching soaking time, a more ideal nano-void structure can be formed, thereby more significantly reducing the contact between the obtained manganese iron phosphate lithium composite positive electrode material and the electrolyte during the application process, and further improving its electrochemical performance, especially the cycle stability.

[0018] In particular, the preferred etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L; and / or the immersion time is 5 min to 15 min. The hydrofluoric acid solution within the above concentration range, combined with the etching within the time range, can effectively remove SiO 2 While forming the coating layer, it more significantly inhibits excessive corrosion on the surface of the material and ultimately improves the long-cycle stability of the resulting lithium manganese iron phosphate composite positive electrode material.

[0019] Further, the lithium source, iron source, manganese source and doping element source are independently added in the form of one or more of nitrate, phosphate, sulfate, carbon source salt, acetate and oxide; and / or the phosphorus source is selected from one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid; and / or the first organic carbon source and the second organic carbon source are independently selected from one or more of glucose, quinoline solubles in coal tar, polyethylene glycol, polyvinyl alcohol, citric acid and dopamine. The manganese iron lithium phosphate composite positive electrode material prepared from the above raw materials has a more stable structure and better comprehensive electrochemical performance.

[0020] The second aspect of the present invention provides a lithium iron manganese phosphate composite positive electrode material, which is prepared by the above-mentioned lithium iron manganese phosphate composite positive electrode material preparation method, and the lithium iron manganese phosphate composite positive electrode material includes a mixed phosphate core, and the molecular formula of the mixed phosphate core is LiMn x Fe 1-x-y T y PO 4 , wherein x is 0.5 to 0.9, y is 0.001 to 0.08, and T is selected from one or more of Mg, Sc, Ti, V, Co and Mo. The obtained lithium manganese iron phosphate composite positive electrode material has three special coating layers and a skeleton interspersed therein, so when used, it can effectively avoid the possible dissolution of manganese, and finally make the lithium manganese iron phosphate composite positive electrode material show particularly excellent electrochemical properties, especially cycle stability.

[0021] The third aspect of the present invention provides a lithium ion battery, comprising a positive electrode sheet, a positive electrode sheet and an electrolyte, wherein the positive electrode sheet comprises the above-mentioned lithium iron manganese phosphate composite positive electrode material. Since the above-mentioned positive electrode material provided by the present invention has excellent stability and electrochemical properties, the corresponding lithium ion battery in which it is used also exhibits more superior performance.

[0022] By applying the technical solution of the present invention, a method for preparing a highly stable lithium iron manganese phosphate composite positive electrode material is provided. The microstructure of the prepared lithium iron manganese phosphate composite positive electrode material is designed by combining multiple carbon coatings, setting an intermediate coating layer, and etching to form a void layer, thereby ensuring the transmission channel and transmission rate of electrons and lithium ions in its structure. At the same time, the barrier that can effectively isolate the electrolyte formed by coating and etching during the preparation process can slow down the dissolution of Mn ions caused by the corrosion of the electrolyte to the positive electrode material, thereby achieving a highly stable cycle and suppressing the voltage decay during the cycle of the obtained positive electrode. The finally prepared lithium iron manganese phosphate composite positive electrode material has a higher energy density, a longer cycle life, and a more excellent charge and discharge performance. DETAILED DESCRIPTION

[0023] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0024] As described in the background technology, the positive electrode materials of lithium-ion batteries in the prior art have poor electrochemical performance, especially poor cycle stability. In order to solve the above technical problems, the first aspect of the present invention provides a method for preparing a lithium iron manganese phosphate composite positive electrode material, including: step S1, preparing a lithium source, an iron source, a manganese source, a phosphorus source, a doping element source and a first organic carbon source into a first slurry, and the first slurry is subjected to a first calcination to obtain a first product; step S2, preparing the first product and a one-dimensional carbon material into a second slurry, and the second slurry is dried to obtain a second product; step S3, forming SiO on the surface of the second product by atomic layer deposition 2 coating layer, thereby obtaining a third product; step S4, preparing the third product and the second organic carbon source into a third slurry, and obtaining a fourth product after the third slurry is subjected to a second calcination; step S5, placing the fourth product in an etching solution, and obtaining a lithium manganese iron phosphate composite positive electrode material after soaking; wherein the etching solution is a strong acid solution or a strong alkaline solution.

[0025] The present invention forms a barrier to isolate the electrolyte on the surface of the mixed phosphate core formed by lithium source, iron source, manganese source, phosphorus source and doping element source by multiple carbon coating, setting an intermediate coating layer and etching to form a void layer, thereby alleviating the Mn ion dissolution problem that may occur in the application process of the obtained positive electrode material, so that it exhibits excellent electrochemical performance. In the above preparation method, specifically:

[0026] In step S1, a doped mixed phosphate core is first formed to obtain a carbon-coated core material with good electrochemical properties and stable structure, i.e., the first product, which lays the foundation for the subsequent microstructure design. Then, through step S2, a one-dimensional carbon material is arranged on the surface of the outer carbon layer of the first product to construct a one-dimensional carbon material conductive network, and obtain a second product with a one-dimensional carbon material attached to the surface. The introduction of the one-dimensional carbon material can not only build a high-speed conductive bridge to enable more efficient contact between the above-mentioned phosphate core and the second carbon layer formed subsequently, so as to improve the overall conductivity of the material, but more importantly, it can form a stable, skeleton-like structure, thereby providing support for the void layer obtained by subsequent etching. In step S3, SiO is pre-deposited on the surface of the second product by atomic layer deposition technology. 2 The coating layer is the precursor of the insulating void layer formed subsequently.

[0027] And, further constructing a second carbon coating layer on its surface through step S4, which not only increases the conductivity of the final positive electrode material, but more importantly, acts as another barrier to further protect the positive electrode material from direct erosion by the electrolyte, and can also synergize with the one-dimensional carbon material introduced in step S2 to increase the transmission rate of lithium ions and electrons. At this time, the structure of the fourth product includes a doped mixed phosphate core, and a first carbon layer, SiO 2 The core step is step S6, in which the fourth product is immersed in an etching solution, and the etching solution is allowed to react with SiO 2 The coating layer produces etching and digestion, thereby forming a void layer between the first carbon layer and the second carbon layer. Of course, as mentioned above, the reason why this void layer can exist stably without collapse is that the one-dimensional carbon material can play a supporting role in it. The existence of this void layer can cooperate with the first carbon layer and the second carbon layer to significantly isolate the phosphate core of the obtained material from contact with the electrolyte during the application process, thereby reducing the dissolution of manganese ions and ultimately showing excellent electrochemical stability.

[0028] In particular, the present invention uses a strong acid solution or a strong alkaline solution as an etching solution to etch SiO 2 The coating layer is because strong acid and strong base solutions have high reactivity and can achieve more efficient and higher selectivity for SiO 2 The coating layer is etched, and then the target SiO is accurately etched without destroying the two carbon coating layers. 2 Of course, in addition to strong acid solutions or strong alkaline solutions, there are other types of compounds with etching effects in this field, such as CF 4 , SF 6However, such gases are highly toxic and easily decomposed, causing irritation and potential explosion risks in a closed environment, and therefore are not suitable for the above-mentioned preparation method provided by the present invention.

[0029] In summary, the above-mentioned preparation method provided by the present invention constructs a manganese iron phosphate lithium composite positive electrode material composed of a doped mixed phosphate core, a first carbon material skeleton, a first carbon layer, a nano-void layer and an outer second carbon layer through multi-step material microstructure design and processing. The unique microstructure designed and prepared effectively solves the common manganese ion dissolution and voltage decay problems in the cycle of lithium-ion battery positive electrode materials. The manganese iron phosphate lithium composite positive electrode material finally prepared has higher energy density, longer cycle life and better charge and discharge performance.

[0030] In some typical embodiments, in order to obtain a more ideal crystalline mixed phosphate core, and then to achieve the subsequent steps and obtain a positive electrode material with excellent electrochemical performance, it is preferred that in step S1, the molar ratio of the lithium source, iron source, manganese source, phosphorus source and doping element source is 1: (0.1-0.5): (0.5-0.9): 1: (0-0.08). Also, in order to form a more uniformly coated first carbon layer, and then to more effectively realize the composite of the obtained first product and the one-dimensional carbon material, and obtain a manganese iron phosphate lithium composite positive electrode material with a more stable structure and a longer service life, it is preferred that the total weight of the lithium source, iron source, manganese source, phosphorus source and doping element source is 100%, and the amount of the first organic carbon source is 8% to 10%.

[0031] Furthermore, in order to facilitate the subsequent attachment of one-dimensional carbon materials on its surface and also shorten the diffusion path of lithium ions, thereby comprehensively improving the electrochemical performance of the obtained lithium manganese iron phosphate composite positive electrode material, the D50 of the first product is preferably 3μm to 6μm. The preferred doping element source is selected from one or more of Mg, Sc, Ti, V, Co and Mo, which can improve the electronic structure and electrochemical properties of the obtained mixed phosphate core in a targeted manner, reduce structural changes during the cycle, inhibit the dissolution of Mn ions and voltage decay, and ultimately make the obtained lithium manganese iron phosphate composite positive electrode material show higher energy density and longer cycle life. And, in the formation process of the first product, in order to form a more perfect crystal form, while reducing the impurities introduced by the side reaction, and improving the purity and cycle life of the obtained lithium manganese iron phosphate composite positive electrode material, the first calcination temperature is preferably 680℃ to 730℃.

[0032] As for the selection of doping elements, in some more typical embodiments, in order to make the doping elements more compatible with the crystal structure of the mixed phosphate and thus achieve better electrochemical performance of the positive electrode material in which they are located, the inventors, after a large number of experiments, preferably use Mg and Ti as the doping element sources, and the molar ratio of Mg to Ti is (1.0-1.5):1; or use Mg and Co as the doping element sources, and the molar ratio of Mg to Co is (1.0-1.5):1.

[0033] In some typical embodiments, in order to form a more structurally stable conductive network, enhance the mechanical properties of the obtained second product, and reduce the low content of electrochemically active substances caused by excessive one-dimensional carbon materials, it is preferred that in step S2, the amount of one-dimensional carbon material is 0.5% to 1.0%, based on the total weight of the first product as 100%. The one-dimensional carbon material within this dosage range can be more evenly and efficiently attached to the surface of the first product, and provide a more stable support for the subsequently formed nanovoid layer, thereby facilitating the acquisition of a lithium manganese iron phosphate composite positive electrode material with higher cycle stability. Regarding the type of one-dimensional carbon material, it is preferably selected from one or more of carbon nanotubes, carbon nanowires and carbon fibers. In order to more effectively alleviate the stress caused by volume changes in the obtained lithium manganese iron phosphate composite positive electrode material during the application cycle, thereby improving its cycle life, it is more preferably a carbon nanotube.

[0034] Further, with respect to the process of one-dimensional carbon material adhering to the surface of the first product, the inventors have preferably selected step S2 after a large number of experiments, including: step S2-1, adding the first product and the one-dimensional carbon material to a solvent to obtain a dispersion; ultrasonically dispersing the dispersion at 10kHz to 50kHz for 0.5h to 2.0h to obtain a second slurry; step S2-2, air-drying the second slurry at 100°C to 150°C to obtain a second product. Preferably, the ultrasonic frequency of ultrasonic dispersion is 20±2kHz; and / or the air-drying temperature is 120±5°C.

[0035] The above preferred and more preferred ultrasonic frequency can more effectively break the agglomeration of the first product and the one-dimensional carbon material in the dispersion system, and promote the uniform distribution of the one-dimensional carbon material on the surface of the first product. Thus, a more stable conductive network with stronger electron transmission ability and more significant support for the subsequently formed nanovoid layer is formed, and ultimately the cycle stability of the lithium manganese iron phosphate composite positive electrode material in which it is located is improved. The above preferred and more preferred air drying temperature can effectively remove the residual solvent while improving the processing performance of the obtained second product, so that it can achieve the same as SiO in the subsequent atomic layer deposition process. 2 The good compounding of the coating layer reduces structural defects, thereby obtaining a lithium manganese iron phosphate composite positive electrode material with better electrochemical performance.

[0036] In some typical embodiments, in order to effectively form an optimized nanovoid structure while protecting the mixed phosphate core and the first carbon layer on its surface from corrosion by hydrofluoric acid, thereby improving the overall electrochemical performance of the obtained lithium manganese iron phosphate composite positive electrode material, in step S3, preferably, SiO 2 The thickness of the coating layer is 30nm to 50nm. In the process of atomic layer deposition, in order to deposit a more uniform structure and less defects in SiO 2 The coating layer is formed in a subsequent etching process to form a more stable nano-void layer, thereby improving the cycle stability of the lithium manganese iron phosphate composite positive electrode material. The preferred atomic layer deposition includes: placing the second product in an atomic layer deposition reaction chamber, and reacting tri(dimethylamino)silane and O 3 As a precursor, N 2 As a purge gas, surface deposition was performed at 200±20°C, and SiO was formed on the surface of the second product. 2 Coating layer.

[0037] In step S4, the second organic carbon source is first coated on the surface of the third product, and then a second conductive carbon layer is formed on its surface through a second calcination. In order to more effectively stabilize the structure while improving the conductivity of the material, and to form a nano-void layer in collaboration with the one-dimensional carbon material and the first carbon layer to improve the isolation of the electrolyte, it is preferred that the total weight of the third product is 100%, and the amount of the second organic carbon source is 3.0% to 5.0%. And, regarding the second calcination, the temperature is preferably 600°C to 700°C, and the time is 2h to 4h. This condition is conducive to promoting more complete carbonization of the second organic carbon source, forming a dense and uniform carbon coating layer, which facilitates the uniform occurrence of subsequent etching processes. At the same time, the setting and optimization of the above conditions reduce the material structure damage or excessive graphitization of the carbon layer caused by excessive temperature, and ultimately effectively improve the comprehensive performance of the obtained lithium manganese iron phosphate composite positive electrode material.

[0038] In the etching process of step S5, the etching solution is preferably a hydrofluoric acid solution or a sodium hydroxide solution, and the concentration of the etching solution is 0.05 mol / L to 0.5 mol / L, which can selectively remove most of the SiO 2 The concentration of the etching solution is preferably in the range of 0.05 mol / L to 0.5 mol / L, which can more accurately adjust the SiO 2The etching rate and degree of the coating layer are optimized to optimize the structure of the formed nano-void layer, so that it can more effectively isolate the electrolyte from the mixed phosphate core, and improve the long cycle performance of the manganese iron phosphate lithium composite positive electrode material. In addition, in order to improve the structural integrity and electrochemical performance of the obtained manganese iron phosphate lithium composite positive electrode material, the preferred immersion time is 1 min to 30 min.

[0039] As for the selection of etching solution and etching conditions, the inventors have conducted a lot of experiments and found that in some more typical embodiments, the preferred etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L; and / or the immersion time is 5 min to 15 min. The hydrofluoric acid solution within the above concentration range, combined with the etching within the time range, is more effective than other types of etching solutions such as sodium hydroxide (which will react with SiO 2 Sodium silicate is generated, which makes it difficult to remove by-products and ultimately fails to form a nano-void layer with a stable structure and no impurities). It can more effectively achieve SiO 2 The coating layer is etched to form a nano-void layer with stable structure and good insulation effect. More importantly, the above etching conditions, including the specific etching solution and etching time, can effectively remove SiO 2 While forming the coating layer, it more significantly inhibits excessive corrosion on the surface of the material, while also reducing the introduction of impurities, and ultimately improving the long-cycle stability of the resulting lithium manganese iron phosphate composite positive electrode material.

[0040] Further, the lithium source, iron source, manganese source and doping element source are each independently added in the form of one or more of nitrate, phosphate, sulfate, carbon source salt, acetate and oxide; and / or, the phosphorus source is selected from one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid; and / or, the first organic carbon source and the second organic carbon source are each independently selected from one or more of glucose, quinoline solubles in coal tar, polyethylene glycol, polyvinyl alcohol, citric acid and dopamine. Theoretically, the raw materials involved in the above preparation method can all be of the types commonly used in the art. However, the above specific types of raw materials selected by the inventor through a large number of experiments can be more effectively coordinated to obtain a lithium manganese iron phosphate composite positive electrode material with a more stable structure and superior electrochemical performance.

[0041] The second aspect of the present invention provides a lithium iron manganese phosphate composite positive electrode material, which is prepared by the above-mentioned lithium iron manganese phosphate composite positive electrode material preparation method, and the lithium iron manganese phosphate composite positive electrode material includes a mixed phosphate core, and the molecular formula of the mixed phosphate core is LiMn x Fe 1-x-y T y PO 4, wherein x is 0.5 to 0.9, y is 0.001 to 0.08, and T is selected from one or more of Mg, Sc, Ti, V, Co and Mo. And the structure of the obtained lithium manganese iron phosphate composite positive electrode material also includes a first carbon layer coated on the surface of the mixed phosphate core, SiO 2 The coating layer is etched to form a nano-void layer, the second carbon layer is formed by the second organic carbon source, and the one-dimensional carbon material is interspersed between the above three layers and plays a supporting role. When used, the three coating layers and the skeleton interspersed therein can significantly isolate the contact between the central mixed phosphate core and the electrolyte without affecting the electron transmission and mass transfer, thereby effectively avoiding the possible manganese dissolution phenomenon, and ultimately making the manganese iron phosphate lithium composite positive electrode material show particularly excellent electrochemical performance, especially cycle stability.

[0042] It should be noted that due to the particularity of the materials field and the limitations of existing testing and characterization methods, it is difficult to conduct a comprehensive quantitative characterization of the complex microstructure of the above-mentioned lithium manganese iron phosphate composite positive electrode material. However, the performance test results show that the lithium manganese iron phosphate composite positive electrode material obtained in this application has better electrochemical properties and can significantly improve the various performances of the lithium-ion battery in which it is used.

[0043] The third aspect of the present invention provides a lithium ion battery, comprising a positive electrode sheet, a positive electrode sheet and an electrolyte, wherein the positive electrode sheet comprises the above-mentioned lithium iron manganese phosphate composite positive electrode material. Since the above-mentioned positive electrode material provided by the present invention has excellent stability and electrochemical properties, when it is applied to a lithium ion battery, the corresponding lithium ion battery obtained also exhibits a higher specific capacity and more superior long-cycle stability.

[0044] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0045] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0046] Example 1

[0047] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0048] (1) Lithium carbonate (lithium source), iron phosphate (iron source), manganese carbonate (manganese source), lithium dihydrogen phosphate (phosphorus source), magnesium oxide and titanium dioxide (doping element source) are weighed according to the molar ratio of Li:Fe:Mn:P:Mg:Ti=1:0.4:0.55:1:0.03:0.02. Then, glucose is weighed as the first organic carbon according to 8% of the total weight of the above raw materials. After mixing the above materials, they are ground by a bead mill to control the slurry particle size D50=0.4μm. Then, the pellets are formed by spray drying, and then the first calcination is carried out in a nitrogen atmosphere at a temperature of 700°C and a time of 9h. Finally, a doped lithium manganese iron phosphate material with a D50 of 5μm is obtained, i.e., the first product.

[0049] (2) According to the weight ratio of the first product powder: carbon nanotube (CNT) = 100:1 (i.e., the total weight of the first product is 100%, and the amount of the one-dimensional carbon material is 1.0%), CNT is weighed as the one-dimensional carbon material. Using ethanol as a solvent, the first product powder and CNT are mixed to obtain a dispersion. The dispersion is ultrasonically dispersed at 20kHz for 1h to obtain a homogeneously dispersed second slurry. The dispersed second slurry is then dried in a blast oven at 120°C to obtain a second product.

[0050] (3) by atomic layer deposition technology, that is, taking the obtained second product and placing it in an atomic layer deposition reaction chamber, using tri(dimethylamino)silane and O 3 For the precursor, N 2 To purify the gas, SiO was deposited on the surface at 200°C. 2 , to deposit SiO with a thickness of 40 nm on the surface of the second product 2 The coating layer is then used to obtain a third product.

[0051] (4) The quinoline soluble matter in coal tar pitch was used as the second organic carbon source, and ethanol was used as the solvent. 2 The third product powder of the coating layer and the second organic carbon source are mixed and dispersed in ethanol according to the weight ratio of the third product: the second organic carbon source = 100:3 (that is, the total weight of the third product is 100%, and the amount of the second organic carbon source is 3.0%) to obtain a third slurry. The third slurry is heated until the solvent is completely volatilized, and the obtained powder is second calcined in a nitrogen atmosphere at 650° C. for 3 hours to obtain a fourth product.

[0052] (5) A hydrofluoric acid solution with a concentration of 0.1 mol / L was used as an etching solution, and the obtained fourth product was immersed in the solution for 15 min. During this process, the SiO 2The coating layer is etched by HF to become a nano-void layer, which is then centrifuged, washed, filtered and dried to obtain a manganese iron phosphate lithium composite positive electrode material.

[0053] The resulting material includes a mixed phosphate core, namely lithium iron manganese phosphate doped with Mg and Ti, with the molecular formula LiMn 0.55 Fe 0.4 Mg 0.03 Ti 0.02 PO 4 At the same time, the structure of the obtained lithium manganese iron phosphate composite positive electrode material also includes a first carbon layer coated on the surface of the mixed phosphate core, SiO 2 A nano-void layer formed by etching the coating layer, a second carbon layer formed by the second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers and playing a supporting role.

[0054] Example 2

[0055] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0056] (1) Lithium carbonate (lithium source), iron phosphate (iron source), manganese carbonate (manganese source), lithium dihydrogen phosphate (phosphorus source), magnesium oxide and titanium dioxide (doping element source) are weighed according to the molar ratio of Li:Fe:Mn:P:Mg:Ti=1:0.4:0.58:1:0.01:0.01. Then, glucose is weighed as the first organic carbon according to 9% of the total weight of the above raw materials. After mixing the above materials, they are ground by a bead mill to control the slurry particle size D50=0.3μm. Then, the pellets are formed by spray drying, and then the first calcination is carried out in a nitrogen atmosphere at a temperature of 680°C and a time of 10h. Finally, a doped lithium manganese iron phosphate material with a D50 of 3μm is obtained, i.e., the first product.

[0057] (2) According to the weight ratio of the first product powder: carbon nanotube (CNT) = 100:0.5 (i.e., the total weight of the first product is 100%, and the amount of the one-dimensional carbon material is 0.5%), CNT is weighed as the one-dimensional carbon material. Using ethanol as a solvent, the first product powder and CNT are mixed to obtain a dispersion. The dispersion is ultrasonically dispersed at 20kHz for 1h to obtain a homogeneously dispersed second slurry. The dispersed second slurry is then dried in a blast oven at 120°C to obtain a second product.

[0058] (3) by atomic layer deposition technology, that is, taking the obtained second product and placing it in an atomic layer deposition reaction chamber, using tri(dimethylamino)silane and O 3 For the precursor, N 2 To purify the gas, SiO was deposited on the surface at 200°C. 2, to deposit 50nm thick SiO on the surface of the second product 2 The coating layer is then used to obtain a third product.

[0059] (4) The quinoline soluble matter in coal tar pitch was used as the second organic carbon source, and ethanol was used as the solvent. 2 The third product powder of the coating layer and the second organic carbon source are mixed and dispersed in ethanol according to the weight ratio of the third product: the second organic carbon source = 100:3 (that is, the total weight of the third product is 100%, and the amount of the second organic carbon source is 3.0%) to obtain a third slurry. The third slurry is heated until the solvent is completely volatilized, and the obtained powder is second calcined in a nitrogen atmosphere at 700° C. for 2 hours to obtain a fourth product.

[0060] (5) A hydrofluoric acid solution with a concentration of 0.1 mol / L was used as an etching solution, and the obtained fourth product was immersed in the solution for 15 min. During this process, the SiO 2 The coating layer is etched by HF to become a nano-void layer, which is then centrifuged, washed, filtered and dried to obtain a manganese iron phosphate lithium composite positive electrode material.

[0061] The obtained lithium iron manganese phosphate composite positive electrode material includes a mixed phosphate core, that is, lithium iron manganese phosphate doped with Mg and Ti, and its molecular formula is LiMn 0.58 Fe 0.4 Mg 0.01 Ti 0.01 PO 4 At the same time, the structure of the obtained lithium manganese iron phosphate composite positive electrode material also includes a first carbon layer coated on the surface of the mixed phosphate core, SiO 2 A nano-void layer formed by etching the coating layer, a second carbon layer formed by the second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers and playing a supporting role.

[0062] Example 3

[0063] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0064] (1) Lithium carbonate (lithium source), iron phosphate (iron source), manganese carbonate (manganese source), lithium dihydrogen phosphate (phosphorus source), magnesium oxide and cobalt nitrate (doping element source) are weighed according to the molar ratio of Li:Fe:Mn:P:Mg:Co=1:0.4:0.55:1:0.03:0.02. Then, glucose is weighed as the first organic carbon according to 10% of the total weight of the above raw materials. After mixing the above materials, they are ground by a bead mill to control the slurry particle size D50=0.3μm. Then, the pellets are formed by spray drying, and then the first calcination is carried out in a nitrogen atmosphere at a temperature of 730°C and a time of 6h. Finally, a doped lithium manganese iron phosphate material with D50=6μm is obtained, i.e., the first product.

[0065] (2) According to the weight ratio of the first product powder: carbon nanotube (CNT) = 100:0.8 (i.e., the total weight of the first product is 100%, and the amount of the one-dimensional carbon material is 0.8%), CNT is weighed as the one-dimensional carbon material. Using ethanol as a solvent, the first product powder and CNT are mixed to obtain a dispersion. The dispersion is ultrasonically dispersed at 20kHz for 1h to obtain a homogeneously dispersed second slurry. The dispersed second slurry is then dried in a blast oven at 120°C to obtain a second product.

[0066] (3) by atomic layer deposition technology, that is, taking the obtained second product and placing it in an atomic layer deposition reaction chamber, using tri(dimethylamino)silane and O 3 For the precursor, N 2 To purify the gas, SiO was deposited on the surface at 200°C. 2 , to deposit 30nm thick SiO on the surface of the second product 2 The coating layer is then used to obtain a third product.

[0067] (4) The quinoline soluble matter in coal tar pitch was used as the second organic carbon source, and ethanol was used as the solvent. 2 The third product powder of the coating layer and the second organic carbon source are mixed and dispersed in ethanol according to the weight ratio of the third product: the second organic carbon source = 100:4 (that is, the total weight of the third product is 100%, and the amount of the second organic carbon source is 4.0%) to obtain a third slurry. The third slurry is heated until the solvent is completely volatilized, and the obtained powder is second calcined in a nitrogen atmosphere at 600° C. for 4 hours to obtain a fourth product.

[0068] (5) A hydrofluoric acid solution with a concentration of 0.2 mol / L is used as an etching solution, and the obtained fourth product is immersed in the solution for 5 min. During this process, the SiO 2The coating layer is etched by HF to become a nano-void layer, which is then centrifuged, washed, filtered and dried to obtain a manganese iron phosphate lithium composite positive electrode material.

[0069] The obtained lithium iron manganese phosphate composite positive electrode material includes a mixed phosphate core, that is, lithium iron manganese phosphate doped with Mg and Co, and its molecular formula is LiMn 0.55 Fe 0.4 Mg 0.03 Co 0.02 PO 4 At the same time, the structure of the obtained lithium manganese iron phosphate composite positive electrode material also includes a first carbon layer coated on the surface of the mixed phosphate core, SiO 2 A nano-void layer formed by etching the coating layer, a second carbon layer formed by the second organic carbon source, and a one-dimensional carbon material CNT interspersed between the above three layers and playing a supporting role.

[0070] Example 4

[0071] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0072] The only difference between this embodiment and embodiment 1 is that in step (4), quinoline solubles in coal tar pitch are added at a weight ratio of the third product to the second organic carbon source of 100:5. That is, based on the total weight of the third product being 100%, the amount of the second organic carbon source is changed to 5.0%.

[0073] Example 5

[0074] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0075] The only difference between this embodiment and embodiment 1 is that in step (5), the immersion time of the fourth product in the hydrofluoric acid solution is changed to 7 minutes.

[0076] Example 6

[0077] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0078] The only difference between this embodiment and embodiment 1 is that in step (1), the temperature of the first calcination is changed to 600° C., and the D50 of the obtained first product is 2 μm.

[0079] Example 7

[0080] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0081] The only difference between this embodiment and embodiment 1 is that in step (1), the temperature of the first calcination is changed to 750° C., and the D50 of the obtained first product is changed to 8 μm.

[0082] Example 8

[0083] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0084] The only difference between this embodiment and embodiment 1 is that in step (1), CNTs are weighed as one-dimensional carbon materials according to a weight ratio of the obtained first product powder: carbon nanotubes (CNTs) = 100:0.2.

[0085] That is, based on the total weight of the first product being 100%, the amount of the one-dimensional carbon material is changed to 0.2%.

[0086] Example 9

[0087] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0088] The only difference between this embodiment and embodiment 1 is that in step (1), CNTs are weighed as one-dimensional carbon materials according to a weight ratio of the obtained first product powder: carbon nanotubes (CNTs) = 100:1.5.

[0089] That is, based on the total weight of the first product being 100%, the amount of the one-dimensional carbon material is changed to 1.5%.

[0090] Example 10

[0091] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0092] The only difference between this embodiment and embodiment 1 is that the ultrasonic dispersion frequency in step (2) is changed to 10 kHz and the time is changed to 2 h.

[0093] Embodiment 11

[0094] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0095] The only difference between this embodiment and embodiment 1 is that the temperature of the forced air drying in step (2) is changed to 150°C.

[0096] Example 12

[0097] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0098] The difference between this embodiment and embodiment 1 is that in step (3), the SiO obtained by atomic layer deposition technology is changed to 2 The thickness of the coating layer is 20 nm.

[0099] Example 13

[0100] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0101] The difference between this embodiment and embodiment 1 is that in step (3), the SiO obtained by atomic layer deposition technology is changed to 2 The thickness of the coating layer is 60 nm.

[0102] Embodiment 14

[0103] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0104] The difference between this embodiment and embodiment 1 is only in step (4), which is described as follows.

[0105] (4) The quinoline soluble matter in coal tar pitch was used as the second organic carbon source, and ethanol was used as the solvent. 2 The third product powder of the coating layer and the second organic carbon source are mixed and dispersed in ethanol according to the weight ratio of the third product: the second organic carbon source = 100: 1 (that is, the total weight of the third product is 100%, and the amount of the second organic carbon source is 1.0%) to obtain a third slurry. The third slurry is heated until the solvent is completely volatilized, and the obtained powder is second calcined in a nitrogen atmosphere at 500° C. for 3 hours to obtain a fourth product.

[0106] Embodiment 15

[0107] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0108] The difference between this embodiment and embodiment 1 is only in step (4), which is described as follows.

[0109] (4) The quinoline soluble matter in coal tar pitch was used as the second organic carbon source, and ethanol was used as the solvent. 2 The third product powder of the coating layer and the second organic carbon source are mixed and dispersed in ethanol according to the weight ratio of the third product: the second organic carbon source = 100:10 (that is, the total weight of the third product is 100%, and the amount of the second organic carbon source is 10.0%) to obtain a third slurry. The third slurry is heated until the solvent is completely volatilized, and the obtained powder is second calcined in a nitrogen atmosphere at 800° C. for 3 hours to obtain a fourth product.

[0110] Example 16

[0111] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0112] The only difference between this embodiment and embodiment 1 is that the concentration of the hydrofluoric acid solution used in step (5) is changed to 0.05 mol / L, and the immersion time is changed to 30 min.

[0113] Embodiment 17

[0114] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0115] The only difference between this embodiment and embodiment 1 is that the concentration of the hydrofluoric acid solution used in step (5) is changed to 0.5 mol / L, and the immersion time is changed to 1 min.

[0116] Embodiment 18

[0117] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0118] The only difference between this embodiment and embodiment 1 is that a sodium hydroxide solution of equal concentration is used as the etching solution instead of the hydrofluoric acid solution.

[0119] Comparative Example 1

[0120] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0121] The only difference between this comparative example and Example 1 is that steps (2) to (5) are not performed, and the first product obtained in step (1) is directly used as the final manganese iron phosphate lithium composite positive electrode material.

[0122] Comparative Example 2

[0123] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0124] The only difference between this comparative example and Example 1 is that in step (2), an equal weight of citric acid is used to replace the one-dimensional carbon material CNT.

[0125] Comparative Example 3

[0126] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0127] The difference between this comparative example and Example 1 is that in step (3), a TiO layer of equal thickness is formed by atomic layer deposition technology. 2 Coating layer to replace SiO 2 Coating layer.

[0128] Comparative Example 4

[0129] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0130] The difference between this comparative example and Example 1 is that step (4) is not performed, but the third product obtained in step (3) is immersed in a hydrofluoric acid solution to make the outermost SiO 2 The cladding layer is etched.

[0131] Comparative Example 5

[0132] A method for preparing a lithium manganese iron phosphate composite positive electrode material:

[0133] The only difference between this comparative example and Example 1 is that step (5) is not performed, and the fourth product obtained in step (4) is directly used as the final manganese iron phosphate lithium composite positive electrode material.

[0134] That is, no etching is performed, SiO 2 The nano-void layer formed by etching the coating layer failed to form. The obtained composite material includes a mixed phosphate core, on the surface of which are sequentially coated with a first carbon layer formed by a first organic carbon source, a SiO 2 The coating layer, the second carbon layer formed by the second organic carbon source, and the one-dimensional carbon material CNT interspersed between the above three layers.

[0135] Battery sample preparation and test methods

[0136] Preparation of battery samples: For the positive electrode materials obtained in each embodiment and comparative example, according to the amount of active material (80wt.%), carbon black (Super-P, 10wt.%) and polyvinylidene fluoride (PDVF, 10wt.%), the three are mixed evenly in an agate mortar, and the resulting mixture is dispersed in N-methylpyrrolidone (NMP) to form a uniform slurry. Subsequently, the slurry is evenly coated on an aluminum foil on a coater, and the coated aluminum foil is dried in a vacuum oven at 120°C for 12 hours. The dried electrode sheet is cut into small discs with a diameter of 14mm, namely the positive electrode sheet. After that, the battery is assembled and sealed in the order of negative electrode shell, lithium sheet, electrolyte, diaphragm, electrolyte, positive electrode sheet, and positive electrode shell.

[0137] Battery performance test: At 25°C, the battery samples obtained above were subjected to charge and discharge performance tests with a voltage range of 2.0V to 4.5V. The following results were obtained: the first discharge specific capacity at 0.2C; the first discharge specific capacity at 1C; the average discharge voltage of the first discharge at 1C (obtained by the Blue Electric test system, representing the average voltage of a battery sample during the entire discharge process); the discharge specific capacity after 200 cycles at 1C, the average discharge voltage after 200 cycles at 1C, and the capacity retention rate after 200 cycles at 1C. The average voltage decay rate is calculated as follows: (1-V 循环200圈后 / V 首圈 )×100%, the lower the value is, the better the cycle stability of the battery sample is.

[0138] The results of the above tests are shown in Table 1.

[0139] Table 1

[0140]

[0141]

[0142] From the above description, it can be seen that the above embodiments of the present invention achieve the preparation of a composite material with excellent electrochemical properties. When the obtained composite material is used as a positive electrode material and applied to a lithium-ion battery, the corresponding lithium-ion battery exhibits excellent electrical properties, especially higher discharge specific capacity and cycle stability. Specifically:

[0143] By comparing Examples 1 to 5 with Examples 6 and 7, it can be seen that the preferred first calcination temperature and the D50 of the first product formed can form a more complete crystal form and reduce impurities introduced by side reactions. It also facilitates the subsequent attachment of one-dimensional carbon materials on its surface, shortens the lithium ion diffusion path, and ultimately further improves the purity and cycle life of the obtained lithium manganese iron phosphate composite positive electrode material.

[0144] By comparing Examples 1 to 5 with Examples 8 and 9, it can be seen that the preferred amount of one-dimensional carbon material is 0.5% to 1.0%, which can make it more evenly and efficiently adhere to the surface of the first product, and provide a more stable support for the subsequently formed nanovoid layer, thereby obtaining a lithium manganese iron phosphate composite positive electrode material with higher cycle stability.

[0145] By comparing Examples 1 to 5 with Examples 10 and 11, it can be seen that by optimizing the conditions for the attachment of the one-dimensional carbon material to the surface of the first product, the one-dimensional carbon material can be evenly distributed on the surface of the first product, while reducing structural defects, forming a conductive network with better performance, and further improving the cycle stability of subsequent corresponding battery samples.

[0146] By comparing Examples 1 to 5 with Examples 12 and 13, it can be seen that for the SiO formed in the intermediate step 2 The coating layer, preferably with a thickness of 30 nm to 50 nm, can effectively form an optimized nanovoid structure while better protecting the mixed phosphate core and the first carbon layer on its surface from corrosion by hydrofluoric acid, thereby improving the overall electrochemical performance of the obtained lithium manganese iron phosphate composite positive electrode material.

[0147] By comparing Examples 1 to 5 with Examples 14 and 15, it can be seen that by optimizing the amount of the second organic carbon source and the temperature conditions of the second calcination, the second organic carbon source can be more fully carbonized to form a dense and uniform carbon coating layer, which facilitates the uniform occurrence of the subsequent etching process and ultimately effectively improves the electrochemical properties of the obtained lithium manganese iron phosphate composite positive electrode material.

[0148] By comparing Examples 1 to 5 with Examples 16 and 17, it can be seen that, for the etching process, when the etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L and the immersion time is 5 min to 15 min, the SiO 2 While forming the coating layer, it more significantly inhibits excessive corrosion of the material surface and reduces the introduction of impurities, thereby more effectively improving the electrochemical performance of the obtained lithium manganese iron phosphate composite positive electrode material. Example 18 shows that compared with other types of etching solutions such as sodium hydroxide, a hydrofluoric acid solution with a suitable concentration can more effectively achieve SiO on the basis of protecting the core material. 2 The coating layer is etched to form a nano-void layer with a stable structure and good isolation effect, and ultimately significantly improves the long-cycle stability of the obtained lithium manganese iron phosphate composite positive electrode material.

[0149] On this basis, comparative examples 1 to 5 show that the above-mentioned preparation method provided by the present invention, through multi-step material microstructure design and processing, constructs a lithium manganese iron phosphate composite positive electrode material composed of a doped mixed phosphate core, a first carbon material skeleton, a first carbon layer, a nanovoid layer and an outer second carbon layer, which can effectively solve the common manganese ion dissolution and voltage decay problems in the cycle of lithium-ion battery positive electrode materials. The finally prepared lithium manganese iron phosphate composite positive electrode material has higher energy density, longer cycle life and better charge and discharge performance, especially after 200 cycles, the voltage decay situation is significantly improved.

[0150] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those described herein, for example.

[0151] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate composite positive electrode material, characterized in that: The preparation method of the lithium manganese iron phosphate composite positive electrode material comprises: Step S1, preparing a first slurry from a lithium source, an iron source, a manganese source, a phosphorus source, a doping element source and a first organic carbon source, and subjecting the first slurry to a first calcination to obtain a first product; Step S2, preparing the first product and the one-dimensional carbon material into a second slurry, and drying the second slurry to obtain a second product; Step S3, forming a SiO2 coating layer on the surface of the second product by atomic layer deposition, thereby obtaining a third product; Step S4, preparing the third product and the second organic carbon source into a third slurry, and subjecting the third slurry to a second calcination to obtain a fourth product; Step S5, placing the fourth product in an etching solution, and obtaining the lithium manganese iron phosphate composite positive electrode material after soaking; Wherein, the etching solution is a strong acid solution or a strong alkaline solution.

2. The method for preparing the lithium iron manganese phosphate composite positive electrode material according to claim 1, characterized in that: In step S1, the molar ratio of the lithium source, the iron source, the manganese source, the phosphorus source and the doping element source is 1: (0.1-0.5): (0.5~0.9):1:(0~0.08); and / or, Based on the total weight of the lithium source, the iron source, the manganese source, the phosphorus source and the doping element source as 100%, the amount of the first organic carbon source is 8% to 10%; and / or, The D50 of the first product is 3 μm to 6 μm; and / or, The doping element source is selected from one or more of Mg, Sc, Ti, V, Co and Mo; and / or the temperature of the first calcination is 680°C to 730°C.

3. The method for preparing the lithium iron manganese phosphate composite positive electrode material according to claim 1 or 2, characterized in that: In step S2, Based on the total weight of the first product being 100%, the amount of the one-dimensional carbon material is 0.5% to 1.0%; and / or, The one-dimensional carbon material is selected from one or more of carbon nanotubes, carbon nanowires and carbon fibers, and is preferably carbon nanotubes.

4. The method for preparing a lithium manganese iron phosphate composite positive electrode material according to any one of claims 1 to 3, characterized in that: Step S2 includes: Step S2-1, adding the first product and the one-dimensional carbon material into a solvent to obtain a dispersion; ultrasonically dispersing the dispersion at 10kHz to 50kHz for 0.5h to 2.0h to obtain the second slurry; Step S2-2, drying the second slurry by forced air at 100° C. to 150° C. to obtain the second product.

5. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to any one of claims 1 to 4, characterized in that: In step S3, the thickness of the SiO2 coating layer is 30nm to 50nm; Preferably, the atomic layer deposition includes: placing the second product in an atomic layer deposition reaction chamber, using tri(dimethylamino)silane and O3 as precursors, N2 as a purge gas, performing surface deposition at 200±20°C, and forming the SiO2 coating layer on the surface of the second product.

6. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to any one of claims 1 to 5, characterized in that: In step S4, Based on the total weight of the third product being 100%, the amount of the second organic carbon source is 3.0% to 5.0%; And / or, the temperature of the second calcination is 600° C. to 700° C., and the time is 2 h to 4 h.

7. The method for preparing a lithium iron manganese phosphate composite positive electrode material according to any one of claims 1 to 6, characterized in that: In step S5, the etching solution is a hydrofluoric acid solution or a sodium hydroxide solution, and the concentration of the etching solution is 0.05 mol / L to 0.5 mol / L; and / or the immersion time is 1 min to 30 min; Preferably, the etching solution is a hydrofluoric acid solution with a concentration of 0.1 mol / L to 0.2 mol / L; and / or the immersion time is 5 min to 15 min.

8. The method for preparing a lithium manganese iron phosphate composite positive electrode material according to any one of claims 1 to 7, characterized in that: The lithium source, the iron source, the manganese source and the doping element source are each independently added in the form of one or more of nitrate, phosphate, sulfate, carbon source salt, acetate and oxide; And / or, the phosphorus source is selected from one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid; And / or, the first organic carbon source and the second organic carbon source are independently selected from one or more of glucose, quinoline soluble matter in coal tar pitch, polyethylene glycol, polyvinyl alcohol, citric acid and dopamine.

9. A lithium manganese iron phosphate composite positive electrode material, characterized in that: The lithium iron manganese phosphate composite positive electrode material is prepared by the preparation method of the lithium iron manganese phosphate composite positive electrode material according to any one of claims 1 to 8, and the lithium iron manganese phosphate composite positive electrode material includes a mixed phosphate core, and the molecular formula of the mixed phosphate core is LiMn x Fe 1-x-y T y PO4, wherein x is 0.5 to 0.9, y is 0.001 to 0.08, and T is selected from one or more of Mg, Sc, Ti, V, Co and Mo.

10. A lithium ion battery, comprising a positive electrode sheet, a positive electrode sheet and an electrolyte, characterized in that: The positive electrode plate includes the lithium manganese iron phosphate composite positive electrode material as described in claim 9.

Images