Active material for lithium ion battery and preparation method and product thereof

Abstract

The invention discloses an active material for a lithium ion battery as well as a preparation method and a product thereof, and belongs to the field of lithium ion batteries. The active component comprises lithium manganese iron phosphate; the active component exists in the form of particulate matter, and the particulate matter comprises a core structure and a carbon coating layer coating the surface of the core structure; the core structure has a manganese element whose concentration gradually decreases continuously from inside to outside, and an iron element whose concentration gradually increases continuously from inside to outside. According to the scheme, the problem that manganese is dissolved in the circulation process can be solved, and the circulation performance is improved.

Description

Technical Field

[0001] This application belongs to the field of ion batteries, especially the field of lithium-ion batteries. In particular, this application relates to an active material for lithium-ion batteries, its manufacturing method and products. Background Art

[0002] Due to advantages such as high energy density, long cycle life, and low self-discharge rate, lithium-ion batteries are widely used in fields such as electric vehicles, portable electronic devices, smart grid energy storage, and aerospace.

[0003] In the complex system of lithium-ion batteries, the cathode material is a key component. The cathode material undertakes the storage and release of lithium ions, and also determines the core indicators such as the energy density, charge and discharge performance, cycle stability, and safety of lithium-ion batteries, thus playing a decisive role in the overall performance of lithium-ion batteries.

[0004] As the cathode material of lithium-ion batteries, lithium iron manganese phosphate has advantages such as rich resources, simple synthesis process, low cost, and high energy density. However, in practical applications, for example, when the temperature exceeds 55 °C, the manganese element therein will intensify the internal chemical reaction due to high temperature, resulting in the manganese ions being released from the lattice and dissolved into the electrolyte, thereby gradually destroying the crystal structure of the material, accelerating the attenuation of battery capacity, shortening the cycle life, possibly increasing internal side reactions, and reducing the safety and stability of the battery. Summary of the Invention

[0005] The purpose of this application is to provide an active material for lithium-ion batteries, its manufacturing method and products, to overcome the problem of poor stability of lithium-ion batteries in the prior art.

[0006] The solution of this application is implemented through the following content.

[0007] In the first aspect, the active component for manufacturing the cathode material of lithium-ion batteries disclosed in this application includes lithium iron manganese phosphate; the active component exists in the form of particulate matter, and the particulate matter includes a core structure and a carbon coating layer coated on its surface; the core structure has a manganese element with a continuously decreasing concentration from the inside to the outside, and an iron element with a continuously increasing concentration from the inside to the outside.

[0008] In the second aspect, this application discloses a cathode material for lithium-ion batteries; it includes the above active component.

[0009] In the third aspect, the cathode structure disclosed in this application includes: A cathode current collector; and The cathode slurry made from the aforementioned active component is attached, In the fourth aspect, this application discloses a lithium-ion battery including the above active component.

[0010] In a fifth aspect, a method for manufacturing an active ingredient in a cathode material for a lithium-ion battery disclosed in the present application includes: manufacturing a full-concentration gradient manganese iron phosphate, and after mechanically mixing the full-concentration gradient manganese iron phosphate with a carbon source and a lithium source in a solid state, performing calcination to form a full-concentration gradient lithium manganese iron phosphate.

[0011] Beneficial effects: The active ingredient for manufacturing a cathode material for a lithium-ion battery disclosed in the present application is a full-concentration gradient lithium manganese iron phosphate. By constructing the compositional gradient distribution characteristics of rich manganese inside and rich iron on the surface, the effective inhibition of the dissolution and precipitation of manganese can be achieved. At the same time, the carbon coating layer on the surface can also provide good structural stability.

[0012] The above structure with rich manganese inside and rich iron on the surface can not only utilize manganese to improve the discharge capacity, but also rely on iron to enhance the surface corrosion resistance and improve the electron / ion conductivity. At the same time, the dissolution of manganese in the electrolyte is reduced, the chemical stability of the material is improved, and the electrochemical performance of the battery is comprehensively enhanced.

[0013] Therefore, through the concentration gradient structure design, the stability of lithium manganese iron phosphate at high temperatures (such as 55 °C) can be improved, laying a foundation for its large-scale commercial application.

[0014] Overall, the process of the present application has the advantages of being green, environmentally friendly, and simple to operate. Moreover, for the first time, the co-precipitation method is used to prepare the cathode material lithium manganese iron phosphate with a full-concentration gradient structure for a lithium-ion battery, and the prepared product has a small charge transfer and ion diffusion resistance, and exhibits excellent electrochemical performance. Description of the Drawings

[0015] The following will briefly introduce the drawings required for the description.

[0016] Figure 1 SEM image of the full-concentration gradient structure lithium manganese iron phosphate of the cathode material in Example 1; Figure 2 TEM image of the full-concentration gradient structure lithium manganese iron phosphate of the cathode material in Example 1; Figure 3 EDS image of the full-concentration gradient structure lithium manganese iron phosphate of the cathode material in Example 1; Figure 4 XRD pattern of the full-concentration gradient structure lithium manganese iron phosphate of the cathode material in Example 1; Figure 5 Mapping image of the full-concentration gradient structure lithium manganese iron phosphate of the cathode material in Example 1; Figure 6Cycling performance graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Example 1 at high temperature; Figure 7 XRD graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Example 1 under different calcination conditions; Figure 8 SEM graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Comparative Example 1; Figure 9 TEM graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Comparative Example 1; Figure 10 EDS graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Comparative Example 1; Figure 11 XRD graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Comparative Example 1; Figure 12 Mapping graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Comparative Example 1; Figure 13 Cycling performance graph of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Comparative Example 1 at high temperature; Figure 14 Charge-discharge curves of the lithium iron manganese phosphate with a full concentration gradient structure as the cathode material in Example 1 and Comparative Example 1 after the 70th and 150th cycles at high temperature; Figure 15 Schematic flow chart of the manufacturing processes in Example 1 and Comparative Example 1 of this application; Figure 16 For Example 2 of this application, directly heated to 650 o SEM graph of the formed g-Mn after calcination at C; Figure 17 Shows the CV curve graph of g-Fe and g-Mn in Example 2 of this application at 0.2 mV s -1 under; Figure 18 Shows the charge-discharge plateau curve graph of g-Fe in Comparative Example 1 at different temperatures; Figure 19 Charge-discharge plateau curve graph of g-Mn in Example 1 of this application at different temperatures; Figure 20 Charge-discharge curve graph of g-Mn in Example 1 of this application at different rates at room temperature; Figure 21 Charge-discharge curve graph of g-Fe in Comparative Example 1 of this application at different rates at room temperature; Figure 22 Rate performance comparison graph of g-Mn and g-Fe in Example 1 and Comparative Example 1 of this application at room temperature; Figure 23 Cycling performance graphs of g-Mn and g-Fe in Example 1 and Comparative Example 1 of this application at 0.5C; Figure 24 and Figure 25 XPS graphs of g-Mn in Example 1 and g-Fe in the Comparative Example at different etching depths; Figure 26 Schematic diagram of the cross-section of the argon ion-cut particles and the atomic ratio of Mn / Fe at different cutting depths. Detailed implementation manners

[0017] Lithium-ion batteries have advantages such as high working voltage, high energy density, long cycle life, light weight, small volume, low self-discharge rate, no memory effect, and environmental protection. For power batteries, the cathode material of lithium-ion batteries must have a high specific capacity. Currently, the cathode materials used in lithium-ion batteries mainly include lithium iron phosphate and ternary materials. Lithium iron phosphate has characteristics such as high safety and long cycle life, but poor conductivity and low actual specific capacity (such as 140 mAh / g), so it cannot fully meet the high requirements for energy density of power batteries.

[0018] Therefore, in some improvement attempts, lithium-rich manganese-based cathode materials have been proposed. However, there are still some problems with such materials, such as: low initial Coulomb efficiency (≤85%), rapid capacity / voltage decay, and poor rate performance. For this, it is possible to try to perform surface coating treatment on the lithium-rich manganese-based cathode material to protect the material surface from the erosion of the electrolyte, the dissolution of transition metal ions, and the phase change of the surface structure, and can improve the electron / ion conduction rate on the material surface. In addition, doping and modification of the cathode material with heteroatoms in the bulk / surface can also play a role in stabilizing the crystal structure of the material, improving the initial Coulomb efficiency, rate performance, and cycling performance of the lithium-rich manganese-based cathode material.

[0019] Among manganese-based cathode materials, lithium iron manganese phosphate has advantages such as high energy density and high safety. Lithium iron manganese phosphate uses relatively inexpensive metal elements Fe and Mn, and has a stable olivine structure, occupying advantages in material cost and safety. However, under high-temperature (such as greater than 55 °C) usage conditions, it is prone to manganese-based dissolution and precipitation, thus affecting the cycle life of the corresponding battery.

[0020] In view of this, in this application, the applicant has proposed an improvement scheme to optimize the lithium iron manganese phosphate material.

[0021] Primarily, in this application, the applicant attempts to achieve the above purpose by controlling the distribution of iron and manganese elements. Specifically, it is achieved by designing a specific concentration gradient structure.

[0022] The so-called concentration gradient structure refers to a gradual change in the composition of the material in at least one direction, and more specifically, a continuous gradient characteristic. Its core is to achieve the optimization and improvement of material properties by constructing a spatially non-uniform microstructure to meet diverse application requirements and performance standards.

[0023] Based on this consideration, the material components can be precisely placed in ideal positions to enhance the thermal stability and structural stability of the material. For lithium manganese iron phosphate, by forming a specific concentration gradient—in this application, a structure such as internal manganese-rich and surface iron-rich—manganese can be used to increase the discharge capacity, and iron can be used to enhance the surface corrosion resistance and improve the electronic / ionic conductivity. At the same time, such a solution can also reduce the dissolution of manganese in the electrolyte, thereby improving the chemical stability of the material and comprehensively improving the electrochemical performance of the battery.

[0024] It should be pointed out that the full concentration gradient described in the present application refers to a complete continuous concentration change of the Mn element content and the Fe element content, and the concentration of the Mn element increases continuously and gradually from the center of the particle to the surface of the particle, rather than a continuous step-like change.

[0025] Therefore, in an example, an active component for manufacturing a positive electrode material for a lithium-ion battery is disclosed, wherein the active component comprises lithium iron manganese phosphate in the form of particles, and the particles comprise a core structure and a carbon coating layer coated on the surface thereof.

[0026] The core structure has a manganese element whose concentration decreases continuously from the inside to the outside, and an iron element whose concentration increases continuously from the inside to the outside. That is, the core structure has a full concentration gradient for manganese and iron.

[0027] For existing lithium manganese iron phosphate, functional stability at 50°C can be considered a higher problem, and manganese dissolution is relatively serious at such high temperatures (manganese dissolution also occurs at temperatures below 50°C, although relatively slower). This is because high temperatures make Mn 3+ More likely to undergo disproportionation reaction to produce Mn 2+ and Mn 4+ , and the Mn 2+ Easy to dissolve. In addition, high temperature makes it easier for the electrolyte to undergo a side reaction to generate HF, which will destroy the Mn-O bond and cause Mn to dissolve.

[0028] For the active ingredients in this application, the main component of the surface of lithium manganese iron phosphate with a concentration gradient structure is an iron-rich phase, which can effectively resist the erosion of HF to the internal manganese-rich area during the cycle. The iron-rich phase reduces the dissolution of manganese elements, thereby stabilizing the olivine structure and avoiding structural collapse to ensure stable performance.

[0029] In particular, the use of a carbon coating layer to wrap a core structure with full concentration gradient characteristics can reduce the resistance of the electrode material. For example, during the charge and discharge process, electrons can be transmitted more smoothly, reducing the polarization of the electrode and improving the charge and discharge performance of the battery.

[0030] The carbon coating can also improve the structural stability of the positive electrode material; because the carbon layer can buffer the stress caused by structural changes during the charge and discharge process, thereby inhibiting the agglomeration and growth of particles, maintaining the integrity of the material structure, and thus improving the cycle stability of the battery.

[0031] Furthermore, the presence of the carbon coating can optimize the diffusion path of lithium ions on the surface and in the bulk of the material, shorten the diffusion distance of lithium ions, and make lithium ions embed and extract more quickly, thereby enhancing the lithium ion diffusion capacity. Based on this, the rate performance of the battery can be improved, making the performance better during high current charging and discharging.

[0032] In addition, the carbon coating layer acts as a physical barrier, which can reduce the direct contact between the material and the electrolyte, prevent the material from reacting with the electrolyte, inhibit the occurrence of side reactions, and improve the chemical stability and service life of the battery.

[0033] The carbon coating can inhibit the growth of lithium manganese iron phosphate grains, thereby increasing the specific surface area. It also enhances the conductivity of particles and surface electrons, reducing the occurrence of battery polarization. The carbon coating can also act as a reducing agent to avoid the formation of iron, and can also act as a nucleating agent to reduce the particle size of the product, adsorb and maintain the stability of the electrolyte.

[0034] In the example, lithium manganese iron phosphate in the form of particles is crystalline, which enables it to have better structural stability at high temperatures and gives the material higher cycle stability, while also improving the theoretical specific capacity.

[0035] Furthermore, the particles can be made porous, and in the example, these pores are mesopores or micropores with a pore size of less than 50 nm. The porous structure can increase the specific surface area of ​​the material, facilitate the exchange of lithium ions at the interface between the active material and the electrolyte, improve the electrode reaction kinetics, and thus improve the rate performance of the battery.

[0036] In addition, the porous structure provides abundant and shorter lithium ion transmission channels, shortening the diffusion distance of lithium ions inside the material, allowing lithium ions to be embedded and extracted more efficiently.

[0037] The porous structure can also serve as a buffer area to alleviate the stress caused by volume changes during the cycle, thereby preventing the material from breaking due to repeated volume changes, maintaining structural stability, and extending the cycle life of the battery.

[0038] Based on the above active ingredients, an example also discloses a cathode material for a lithium-ion battery including the active ingredient.

[0039] In specific use, the cathode material is configured in the form of a slurry for use. The slurry can be formed by mixing the active ingredient, a conductive agent, a binder, and a solvent.

[0040] Furthermore, based on the above cathode material, it is used by fabricating the cathode structure of a lithium-ion battery. The cathode structure includes a cathode current collector; and a cathode material layer in which the above cathode slurry adheres to and is dried and solidified on the current collector.

[0041] To enhance the adhesion strength of the cathode material layer on the surface of the cathode current collector, in an example, the current collector is selected to have a rough surface (which can be defined and delimited by roughness). The rough surface of the current collector has depressions, so that it can match the small particle size of the active ingredient, form minute air gaps, and produce a structure similar to a rivet, thereby enabling the active material layer to firmly adhere to the current collector. Thus, the structural stability of the cathode structure can be improved.

[0042] In addition, the current collector with a rough surface can modify the surface morphology of the cathode material layer, enabling the cathode structure to have a larger activation area, which helps to increase the reaction rate and thus improve the rate performance. At the same time, such a structure can also improve the uniformity of the current density distribution on the cathode surface and enhance the capacity performance of the cathode under high current.

[0043] In addition, the rough surface of the cathode active material layer is modified by adapting to the current collector, which can also prevent potential puncture of the lithium-ion battery separator by the cathode material layer, resulting in internal short circuit of the battery, and ensuring that the lithium-ion battery has a high safety performance.

[0044] In addition, an example also discloses a lithium-ion battery based on the aforementioned active material. The lithium-ion battery further includes a cathode structure, an anode structure, a separator, and an electrolyte.

[0045] To facilitate those skilled in the art to implement the above solutions, in an example, a method for manufacturing the active ingredient in the cathode material of a lithium-ion battery is also disclosed.

[0046] Basically, the method includes: manufacturing a full-concentration gradient manganese iron phosphate, and after mechanically mixing the full-concentration gradient manganese iron phosphate with a carbon source and a lithium source in a solid state, performing calcination to form a full-concentration gradient lithium manganese iron phosphate.

[0047] Among them, in some examples, the calcination can be carried out in segments based on a gradient temperature, and the calcination process causes the carbon source to carbonize (forming a carbon coating layer). In addition, the method for manufacturing the full-concentration-gradient lithium iron manganese phosphate may include: forming a gradient-concentration mixed solution of a phosphorus source, a manganese source, and an iron source, and performing centrifugation, washing, and drying.

[0048] Based on the above method, considering from the material structure level, due to the continuous change structure characteristic of the active ingredient in the present application being a full-concentration gradient, there is no interface mutation phenomenon, thus being beneficial to the migration of Li + This structure in the present application can effectively alleviate the problem of the increase in the Li + diffusion energy barrier caused by the interface mutation, providing more favorable conditions for the migration of Li⁺.

[0049] Particularly, in the application, by controlling the mixing method of the reaction raw materials, the formation of the full-concentration gradient is achieved.

[0050] Basically, in the present application, a phosphoric acid solution is first prepared, and then a manganese source and an iron source are added in different sequences. After the three co-precipitation reactions for a period of time, the obtained product is rinsed several times with deionized water and ethanol to obtain a full-concentration-gradient lithium iron manganese phosphate precursor. Then, the precursor and a lithium source (such as lithium carbonate) are mixed and treated by a method of staged calcination to obtain an active ingredient with better crystallinity.

[0051] More specifically, the preparation method of the full-concentration-gradient lithium iron manganese phosphate cathode material for a lithium-ion battery includes the following steps: 1) Preparation of full-concentration-gradient lithium iron manganese phosphate.

[0052] First, a certain amount of phosphoric acid is added to a certain volume of ethylene glycol aqueous solution to form solution a. Subsequently, a certain amount of manganese source is dissolved in a certain volume of ethylene glycol aqueous solution to form solution b. A certain amount of iron source is dissolved in a certain volume of ethylene glycol aqueous solution to form solution c.

[0053] Stir at a certain temperature for a certain time, and use a constant-current pump to drip solution c into solution b at a certain flow rate (v). At the same time, drip solution b into solution a at twice the flow rate (2v).

[0054] After centrifugation, washing, and drying, full-concentration-gradient lithium iron manganese phosphate is obtained.

[0055] 2) Preparation of full-concentration-gradient lithium iron manganese phosphate lithium.

[0056] The above full-concentration gradient manganese iron phosphate, carbon source, and lithium carbonate are placed in a ball mill in a certain proportion and mixed and ball milled for a certain time at a certain rotation speed to obtain a mixture. The mixture is calcined in stages. First, it is calcined at a certain temperature for a certain time, then the temperature is raised to a certain temperature and calcined for a certain time, and finally, the full-concentration gradient lithium manganese iron phosphate coated with a carbon layer is successfully prepared.

[0057] In the step of adding a certain content of phosphoric acid to a certain volume of ethylene glycol aqueous solution to form solution a, it is 6-10 mmol of phosphoric acid and 20-30 mL of ethylene glycol aqueous solution (where the volume ratio of alcohol to water is 1-10) respectively.

[0058] In the steps of dissolving a certain amount of manganese source in a certain volume of ethylene glycol aqueous solution to form solution b and dissolving a certain amount of iron source in a certain volume of ethylene glycol aqueous solution to form solution c, the amount of manganese source is 3-5 mmol, the manganese source is one of manganese acetate and manganese oxalate, the amount of iron source is 3-5 mmol, and the iron source is one of iron acetate and ferrous oxalate. The volume of the ethylene glycol aqueous solution is 20-30 mL (where the volume ratio of alcohol to water is 1-10).

[0059] Use a constant flow pump to drip solution c into solution b at a certain flow rate (v). At the same time, drip solution b into solution a at twice the flow rate (2v). The specific implementation can be to drip c into solution b at a flow rate of 1-3 mL·min −1 and drip solution b into solution a at a flow rate of 2-6 mL·min −1 .

[0060] That is, dissolve phosphoric acid, manganese source, and iron source in the same volume of a solvent respectively, and the molar ratio of phosphoric acid, manganese, and iron is 2:1:1 to form three solutions a, b, and c.

[0061] There are containers I, II, and III. Among them, solution c is stored in container III, solution b is stored in container II, and solution a is stored in container I.

[0062] Input solution c from container III into container II at a flow rate of v to mix with the solution b therein to form a first mixture. And at the same time, input the first mixture from container II into container I at a flow rate of 2v to mix with the solution a therein to form a second mixture. When the solutions in containers II and III are delivered completely, the product is formed in container I.

[0063] It should be particularly noted that in the above process, the principle of different combinations of flow rates is: to ensure that the titrations of solution b and solution c are completed simultaneously to ensure the continuous change of Mn element and Fe element. More importantly, it is necessary to ensure a low flow rate first to ensure the slow growth of the precursor.

[0064] In terms of the reaction process of the process of this application, solution a can be regarded as the reaction vessel in the coprecipitation reaction process, so b is added to a.

[0065] From the very beginning of the titration, when b is added to a, the pure manganese source reacts with phosphoric acid to form manganese phosphate. From the titration process, manganese in b gradually flows into a (iron also flows into a during this process, but relatively less), and iron in c flows into b, and the manganese-iron ratio in b gradually decreases. Therefore, when the coprecipitation reaction occurs in a, it can be regarded that as the particles grow, the manganese phosphate center at the beginning gradually grows into a manganese-iron phosphate surface layer with a gradually decreasing manganese-iron ratio, and the manganese-iron ratio changes continuously. In the above reaction process, since both manganese phosphate and manganese-iron phosphate are insoluble in water and can be directly precipitated, there is no need to separately prepare a coprecipitation reagent; that is, the solution of this application can avoid using the coprecipitation reagent as adopted in some existing technologies.

[0066] In the aforementioned practice, since solutions b and c have the same volume, to ensure that the titrations of solutions b and c are completed simultaneously, the flow rate relationship of v and 2v will be maintained. Subsequently, Examples 1 and 2 disclosed in this application altogether adopt two mixing methods (two different dropping sequences) to prepare two gradient structures with completely opposite trends. Through characterization and tests, it is shown that the concentration gradient structure with rich Fe on the surface can effectively inhibit the dissolution problem of Mn at high temperatures.

[0067] In the above mixing method, the proportions of manganese and iron in the mixed solution (coprecipitation reaction solution) change dynamically and gradually with the progress of the reaction, and are continuously changing throughout the process to ensure the formation of a continuously changing concentration gradient product.

[0068] On the contrary, in the prior art, when mixing reaction solutions, only several fixed ratios are selected, or gradient ratios are selected at some local stages or several stages during the entire reaction process. Such a method will result in the formation of obvious composition mutation interfaces, that is, the full concentration gradient material described in this application cannot be achieved. Due to the existence of the above mutations, it is extremely unfavorable for the migration of Li + and thus adversely affects the battery performance.

[0069] Preferably, the manufacturing method of this application only needs to simply form a reaction solution through the mixing method of reaction raw materials (i.e., dropping sequence and speed), and the concentrations of iron and manganese components in the reaction solution change continuously in a gradient manner for the reaction, without the need for complex control of the reaction process (including reaction steps, reaction conditions, reaction cycle, etc.) and with high control difficulty.

[0070] In addition, this application does not premix multiple raw materials (iron source, manganese source) according to a specific concentration ratio. That is, this application does not directly pump the aforementioned premix and other reaction raw materials into the reaction vessel, that is, the raw materials are not gradually mixed and reacted in the reaction vessel.

[0071] In step 1, after centrifugally washing with deionized water 3 times and ethanol 2 times, it is dried at 60 °C for 12 h to obtain iron manganese phosphate with a full concentration gradient.

[0072] In step 2, the carbon source is one of sucrose, glucose, dopamine hydrochloride, ascorbic acid, and starch. The carbon source and iron manganese phosphate with a full concentration gradient are placed in a ball mill according to a mass ratio of 1:6 to 1:4, and iron manganese phosphate and lithium carbonate are placed in a ball mill according to a molar ratio of 1:1.1, and mixed and ball milled at 500 rpm for 10 h.

[0073] Furthermore, the mixture is subjected to staged calcination treatment. For example, the mixture is first calcined in an argon-hydrogen environment at 300 - 350 °C for 2 - 5 h, and then heated to 500 - 650 °C and calcined for another 3 - 5 h. The heating rate of the heat treatment is 3 °C·min −1 .

[0074] Among them, staged calcination can improve crystallinity to improve the cycle stability during use.

[0075] Staged calcination can also remove impurities; because in the initial stage of calcination, the calcination process can remove volatile impurities, chemically bound and physically adsorbed water, gases, organic substances, etc. in the raw materials, thereby improving the purity of the raw materials. For example, as shown in the subsequent Figure 7 XRD pattern.

[0076] Staged calcination can also improve the pore structure. Specifically, by controlling the calcination temperature and time, the pore structure of the material can be improved, forming more micropores and mesopores, increasing the specific surface area of the material, which is conducive to the contact of reactants and the improvement of the reaction rate.

[0077] Staged calcination can also control the crystal form and microcrystalline grain size. For example, during the calcination process, through processes such as solid-state reaction, mutual dissolution, and recrystallization, a certain crystal form, microcrystalline grain size, pore diameter, and specific surface area can be obtained, further affecting the material properties.

[0078] Staged calcination also improves the thermal stability of the material. By staged calcination, the thermal stability of the material can be increased, enabling it to better withstand the reaction under high-temperature environments.

[0079] Multiple specific examples will be given below Example 1 From the particle center to the particle surface, the Mn content gradually decreases, and the Fe element gradually increases; rich in manganese inside and rich in iron on the surface.

[0080] The process of the manufacturing process is as Figure 15 shown.

[0081] Preparation of full-concentration-gradient manganese iron phosphate. First, 8 mmol of phosphoric acid was added to 30 mL of ethylene glycol aqueous solution (where the alcohol-to-water ratio was 5:1) to form solution a. Subsequently, 4 mmol of manganese acetate was dissolved in 30 mL of ethylene glycol aqueous solution (where the alcohol-to-water ratio was 5:1) to form solution b. 4 mmol of iron acetate was dissolved in 500 mL of ethylene glycol aqueous solution (where the alcohol-to-water ratio was 5:1) to form solution c.

[0082] After stirring at 60 °C for 30 min, solution c was added dropwise to solution b at a flow rate (v) of 2 mL min−1 using a constant flow pump. At the same time, solution b was added dropwise to solution a at a rate (2v) of 4 mL min−1. After centrifugally washing 3 times with deionized water and 2 times with ethanol, full-concentration-gradient manganese iron phosphate was obtained after drying at 60 °C for 12 h.

[0083] Preparation of full-concentration-gradient lithium manganese iron phosphate.

[0084] 1000 mg of the above full-concentration-gradient manganese iron phosphate, 200 mg of sucrose, and 270 mg of lithium carbonate were placed in a ball mill, and an appropriate amount of alcohol was added as a dispersant. They were mixed and ball-milled at 500 rpm for 10 h, and the obtained mixture was calcined in stages. First, it was calcined at 350 °C for 3 h, and then the temperature was raised to 650 °C and calcined for another 5 h (where the heating rate was 3 °C min −1 ), and finally, full-concentration-gradient lithium manganese iron phosphate (g-Mn) was successfully prepared. The core component of this full-concentration-gradient lithium manganese iron phosphate is LiMnPO4 / C, and the surface component is LiFePO4 / C. The content of Mn gradually decreases from the core to the surface, and the content of Fe gradually increases. For comparison, the same mixture was also calcined at 650 °C for 5 h (as Figure 7 ).

[0085] Characterization was carried out by SEM, TEM, EDS, XRD, and Mapping respectively, and the results are as Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 shown. The cycling performance at high temperature is as Figure 6 . The XRD patterns under different calcination conditions are as Figure 7 shown.

[0086] Comparative Example 1 From the particle center to the particle surface, the Mn element gradually increases, while the Fe element gradually decreases; it is rich in Fe inside and rich in Mn on the surface.

[0087] Preparation of full-concentration-gradient manganese iron phosphate. First, 8 mmol of phosphoric acid was added to 30 mL of an ethylene glycol aqueous solution (where the alcohol-to-water ratio was 5:1) to form solution a. Subsequently, 4 mmol of manganese acetate was added to 30 mL of an ethylene glycol aqueous solution (where the alcohol-to-water ratio was 4:1) to form solution b. 4 mmol of iron acetate was dissolved in 30 mL of an ethylene glycol aqueous solution (where the alcohol-to-water ratio was 4:1) to form solution c.

[0088] After stirring at 60 °C for 30 min, solution b was added dropwise to solution c at a flow rate (v) of 2 mL min −1 using a constant-flow pump. At the same time, solution c was added dropwise to solution a at a flow rate (2v) of 4 mL min −1 . After centrifugal washing with ionized water 3 times and ethanol 2 times, full-concentration-gradient manganese iron phosphate was obtained after drying at 60 °C for 12 h.

[0089] Preparation of full-concentration-gradient lithium manganese iron phosphate. 1000 mg of the above full-concentration-gradient manganese iron phosphate, 200 mg of glucose, and 270 mg of lithium carbonate were placed in a ball mill, and an appropriate amount of alcohol was added as a dispersant. The mixture was mixed and ball-milled at 500 rpm for 10 h, and the obtained mixture was calcined in stages. First, it was calcined at 350 °C for 3 h, then heated to 600 °C and calcined for 5 h, and finally full-concentration-gradient lithium manganese iron phosphate was successfully prepared. The core component of this full-concentration-gradient lithium manganese iron phosphate is LiFePO4 / C, and the surface component is LiMnPO4 / C. From the core to the surface, the content of Mn gradually increases and the content of Fe gradually decreases.

[0090] Characterization was carried out by SEM, TEM, EDS, XRD, and Mapping respectively, and the results are as shown in Figure 8 、 Figure 9 、 Figure 10 、 Figure 11 、 Figure 12 . The cycling performance at high temperature is as shown in Figure 13 .

[0091] According to Figure 13 the disclosed content, the material of Comparative Example 1 is a full-concentration-gradient-structured lithium manganese iron phosphate (g-Fe) that is rich in Fe inside and rich in Mn on the surface. During the cycling process shown in Figure 13 , due to the continuous dissolution of Mn on the material surface, its cycling stability becomes poor, especially at higher temperatures (55 o °C), and this phenomenon of performance deterioration is particularly prominent.

[0092] Using the lithium iron manganese phosphate at full concentration gradients in Example 1 and Comparative Example 1 as the cathode material, batteries with the same structure were assembled - the only difference being the cathode material. Cyclic charge and discharge were carried out at a high temperature (55 °C), and the charge and discharge curves after the 70th and 150th cycles were compared, as Figure 14 shown.

[0093] As Figure 14 can be seen, with the increase in the number of cycles, the charge and discharge plateau corresponding to the Mn redox reaction in g-Fe of Comparative Example 1 significantly shortened. In sharp contrast, only a slight reduction occurred in the charge and discharge plateau of Mn in g-Mn of Example 1. The applicant is convinced that this difference stems from the unique structural characteristics of g-Mn. The Fe-rich phase on its surface can not only effectively resist the erosion of the electrolyte, but also provide stress buffering for the crystal structure when Jahn-Teller distortion occurs inside the particles, thereby suppressing the crystal structure damage caused by this distortion, maintaining the stability of the material structure, and ensuring the relative stability of the charge and discharge plateau.

[0094] The two manufacturing processes in Example 1 and Comparative Example 1 are as Figure 15 follows; by setting different solution addition sequences, two materials with opposite full concentration gradient distributions can be obtained, namely, g-Mn in Example 1 and g-Fe in Comparative Example 1.

[0095] Example 2 The main difference from Example 1 is that the calcination process is g-Mn formed after calcination only at 650 °C (abbreviated as one-time calcination for convenience). And through the SEM characterization shown in Figure 16 it and comparison with the scheme of Example 1, it can be seen that compared with the staged calcination in Example 1, the secondary particle agglomeration of the material in Example 2 is serious and the surface is rough, which will significantly disadvantage the lithium ion transport.

[0096] Test Example 1 The CV curves of the materials of g-Mn in Example 1 and g-Fe in Comparative Example 1 were tested at 0.2 mV s-1 respectively, as Figure 17 shown. The results show that the spacing between the oxidation and reduction peaks of g-Mn is smaller, indicating that it has higher electrochemical activity.

[0097] The charge and discharge tests of the materials of g-Mn in Example 1 and g-Fe in Comparative Example 1 were carried out at different temperatures respectively, as Figure 18 (g-Fe) and Figure 19 (g-Mn) shown in the charge and discharge plateau curves at different temperatures.

[0098] Test Example 2 The g-Mn of Example 1 and the g-Fe material of Comparative Example 1 were tested respectively to determine the rate platform, rate test and cycling performance of the materials at room temperature. The results are as follows: Figure 20 The charge-discharge curves of g-Mn at different rates at room temperature shown, Figure 21 The charge-discharge curves of g-Fe at different rates at room temperature shown, Figure 22 The rate performance of g-Mn and g-Fe at room temperature shown, Figure 23 The cycling performance diagrams of g-Mn and g-Fe at 0.5C shown.

[0099] Test Example 3 XPS tests were carried out on the g-Mn of Example 1 and the g-Fe material of Comparative Example 1 at different etching depths, and the XPS results are respectively as Figure 24 、 Figure 25 shown. The cross-section of the argon ion cut particles and the atomic ratio of Mn / Fe at the cutting depth are as Figure 26 shown.

[0100] The results show that for the g-Fe material, the Mn / Fe ratios on the outermost surface, and at the etching depths of 50 and 100 nm are approximately 1.91, 1.12 and 0.55 respectively. This data indicates that with the increase of depth, the content of Fe element increases significantly, while the content of Mn element decreases significantly. The chemical formula of its surface composition is approximately LiMn 2 / 3 Fe 1 / 3 PO4, and the chemical formula of the internal central composition is approximately LiMn 1 / 3 Fe 2 / 3 PO4.

[0101] Different from the g-Fe material, for the g-Mn material, with the increase of etching depth, the Mn / Fe ratios are approximately 0.63, 1.05 and 2.04 respectively, showing a trend opposite to that of the g-Fe material. That is, the content of Fe element decreases significantly with the increase of depth, while the content of Mn element increases significantly. The chemical formulas of the surface composition and the internal central composition of the g-Mn material are approximately LiMn 1 / 3 Fe 2 / 3 PO4 and LiMn 2 / 3 Fe 1 / 3 PO4.

[0102] Test Example 4 The materials of Example 1 and Comparative Example 1 were tested respectively, and the results shown in Table 1 can be obtained.

[0103] Table 1 Comparison of the electrochemical performance of g-Mn and g-Fe

[0104] The structure, features and effects of the present application have been described in detail based on the embodiments shown in the drawings. The above is only the preferred embodiment of the present application. However, the present application is not limited to the scope of implementation shown in the drawings. Any changes made according to the concept of the present application, or equivalent embodiments modified into equivalent changes, should still be within the protection scope of the present application as long as they do not exceed the spirit covered by the description and the drawings.

Claims

1. An active ingredient for manufacturing anode materials for lithium-ion batteries, characterized in that: The active ingredient includes lithium iron manganese phosphate; The active ingredient exists in the form of particles, and the particles include a core structure and a carbon coating layer coated on the surface thereof; The core structure has a manganese element whose concentration gradually decreases continuously from the inside to the outside, and an iron element whose concentration gradually increases continuously from the inside to the outside.

2. The active ingredient according to claim 1, characterized in that The particles are crystalline.

3. The active ingredient according to claim 1, characterized in that The particles are porous, and the pores are mesopores or micropores with a pore size below 50 nm.

4. A positive electrode material for a lithium ion battery, characterized in that: The active ingredient comprises any one of claims 1 to 3.

5. The lithium-ion battery positive electrode material according to claim 4, characterized in that: The positive electrode material exists in the form of slurry, and further includes a conductive agent, a binder and a solvent.

6. A positive electrode structure, characterized in that: include: Positive electrode current collector; as well as A positive electrode material layer is formed by attaching a positive electrode slurry made from the active ingredient according to any one of claims 1 to 3 and solidifying the positive electrode slurry on the current collector after drying.

7. The active ingredient for producing a positive electrode material according to claim 5, characterized in that: The current collector surface is rough.

8. A lithium ion battery, characterized in that: The active ingredient comprises any one of claims 1 to 3.

9. A method for producing an active ingredient in a positive electrode material for a lithium ion battery, characterized in that: The method comprises: Making a full concentration gradient of ferromanganese phosphate, and After the full concentration gradient manganese iron phosphate is mechanically mixed with a carbon source and a lithium source in a solid state, calcination is performed to form a full concentration gradient manganese iron lithium phosphate.

10. The method according to claim 9, characterized in that The calcination is performed in stages based on a gradient temperature, and the calcination process causes carbonization of the carbon source; And / or, the method for preparing full concentration gradient manganese ferrophosphate includes: forming a gradient concentration mixed solution of a phosphorus source, a manganese source, and an iron source, and centrifuging, washing, and drying.

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