A lithium iron manganese phosphate composite material and a preparation method thereof

Abstract

This invention provides a lithium manganese iron phosphate composite material and its preparation method, relating to the field of lithium battery technology. The preparation method includes: combining a manganese iron oxide precursor with graphene oxide via electrostatic self-assembly to obtain a first composite product; treating the composite product with a ferric ion solution to obtain a second composite product; and mixing the second composite product with at least one of a lithium source, a phosphorus source, and a carbon source to obtain a precursor mixture. This invention utilizes electrostatic self-assembly and iron ion crosslinking to construct a stable conductive network and form an iron-rich outer shell in situ. This solves the problem of poor conductivity caused by graphene stacking and utilizes the iron-rich layer to isolate and protect the internal manganese elements, effectively inhibiting manganese dissolution, thereby significantly improving the cycle life and conductivity of the material.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and more specifically, to a lithium manganese iron phosphate composite material and its preparation method. Background Technology

[0002] In today's global energy structure transformation and the urgent need for sustainable development, advanced energy storage technologies, especially rechargeable battery technologies, play a crucial role. Lithium-ion batteries (LIBs), with their advantages of high energy density, long cycle life, and low self-discharge rate, have dominated the portable electronic device market and are rapidly becoming the preferred technology for electric vehicles (EVs) and large-scale grid energy storage systems. Among the four key components of lithium-ion batteries (cathode, anode, electrolyte, and separator), the performance of the cathode material largely determines the battery's overall energy density, operating voltage, rate performance, safety, and cost. Therefore, developing novel, efficient, low-cost, and environmentally friendly cathode materials has always been a core research topic in the field of battery engineering.

[0003] Among numerous cathode material systems, phosphate materials with an olivine structure, such as lithium iron phosphate (LiFePO4, LFP), have achieved tremendous commercial success due to their excellent thermal stability, long cycle life, high safety, and abundant and inexpensive raw material (iron). However, LFP materials have a voltage rating of approximately 3.4V (vs. Li / Li). + The relatively low operating voltage plateau of LFPs limits their potential in applications requiring higher energy density. To overcome this limitation, researchers compared LFPs with those having a higher voltage plateau (approximately 4.1V vs. Li / Li). + By solid-solution of lithium manganese phosphate (LiMnPO4, LMP), lithium manganese iron phosphate (LiMn) was developed. 1-x Fe x PO4 (LMFP) materials. LMFP materials combine the structural stability of LFP with the high voltage characteristics of manganese (Mn), and therefore have attracted much attention due to their high operating voltage and low cost, and are considered one of the most promising next-generation high-energy-density cathode materials.

[0004] However, despite the significant theoretical advantages of LMFP materials, their commercial application still faces two inherent challenges. First, LMFP materials inherit the common drawback of phosphate materials: extremely low intrinsic electronic conductivity, which severely hinders electron transport within the active material. Second, this material suffers from severe manganese leaching during cycling. Under the corrosive effects of the electrolyte, especially at high temperatures or under high charge conditions, Mn... 2+Ions dissolve from the LMFP lattice and migrate to the negative electrode surface, causing damage to the solid electrolyte interphase (SEI) film, continuous decomposition of the electrolyte, and loss of active materials, which in turn leads to rapid capacity decay and shortened cycle life.

[0005] To address the aforementioned issues, particularly the low electronic conductivity, the most common modification strategy is to coat LMFP particles with conductive carbon. However, traditional carbon coating methods, such as high-temperature pyrolysis of organic materials (e.g., sucrose, glucose), struggle to form a uniform and dense coating layer on the particle surface. This uneven coating results in a discontinuous conductive network, with some active materials remaining electronically insulating and unable to fully realize their electrochemical activity. To construct a more efficient conductive network, graphene, due to its excellent conductivity and large specific surface area, has also been introduced into composite materials. However, the strong van der Waals forces between graphene nanosheets make them prone to recombination and aggregation. This stacking phenomenon not only negates the advantage of high specific surface area but also hinders the formation of three-dimensional conductive pathways. Therefore, both the inhomogeneity of traditional carbon coating and the easy stacking of graphene lead to low ion and electron transport efficiency within the composite material, preventing LMFP materials from meeting the rate performance and cycle stability requirements of high-performance lithium-ion batteries.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a lithium manganese iron phosphate composite material and its preparation method. This method constructs a three-dimensional conductive network and an iron-rich core-shell structure through electrostatic self-assembly and iron ion cross-linking, simultaneously solving the problems of low conductivity and manganese leaching, and significantly improving the battery cycle performance.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a method for preparing the lithium manganese iron phosphate composite material as described in the foregoing embodiments, comprising: The first composite product was obtained by combining the manganese iron oxide precursor with graphene oxide through electrostatic self-assembly. The composite product is treated with a ferric ion solution to coordinate and crosslink the ferric ions with the graphene oxide and enrich them on the surface of the manganese iron oxide precursor, thereby obtaining a second composite product. The second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source to obtain a precursor mixture; wherein the molar ratio of lithium, phosphorus, and manganese iron in the precursor mixture satisfies the stoichiometric ratio.

[0009] In an optional embodiment, the phosphorus source includes at least one selected from the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate; and / or, The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride; and / or, the carbon source includes at least one of glucose, polypyrrole, polythiophene, Ketjen black, conductive fiber, acetylene black, carbon nanotubes, and graphene.

[0010] In an optional embodiment, the first composite product obtained by combining the manganese iron oxide precursor and graphene oxide through electrostatic self-assembly comprises: The manganese iron oxide precursor is positively charged modified; The positively charged manganese iron oxide precursor was dispersed in a graphene oxide solution that served as a negatively charged carbon source, and then subjected to electrostatic self-assembly by mechanical stirring.

[0011] In an optional implementation, the method of positive charge modification includes: The manganese iron oxide precursor particles and APTES were added to a toluene solution and mixed evenly. The mixture was then mechanically stirred and refluxed under water bath heating conditions.

[0012] In an optional embodiment, the treatment of the composite product with a ferric ion solution causes the ferric ions to coordinate and crosslink with the graphene oxide and accumulate on the surface of the manganese iron oxide precursor, yielding a second composite product, comprising: The first composite product was immersed in a solution of ferric ions; After removing and washing to remove excess ferric ions, the product is vacuum dried to obtain the second composite product.

[0013] In an optional embodiment, the second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source, and then subjected to high-temperature treatment to obtain the lithium manganese iron phosphate composite material, comprising: The second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source, and then kept at a temperature of 650°C to 850°C for 120 to 720 minutes under an inert or reducing atmosphere. After cooling, the lithium manganese iron phosphate composite material is obtained.

[0014] Secondly, the present invention provides a lithium manganese iron phosphate composite material, which is prepared by the preparation method described in the foregoing embodiments.

[0015] Thirdly, the present invention provides a positive electrode comprising the lithium manganese iron phosphate composite material as described in the foregoing embodiments. Alternatively, the lithium manganese iron phosphate composite material prepared by the preparation method described in the foregoing embodiments.

[0016] Fourthly, the present invention provides a battery comprising a positive electrode as described in the foregoing embodiments.

[0017] Fifthly, the present invention provides an electrical device including a battery as described in the foregoing embodiments.

[0018] This invention provides a lithium manganese iron phosphate composite material and its preparation method. The preparation method utilizes electrostatic self-assembly, employing the interaction between charges to uniformly disperse manganese iron oxide precursors between graphene oxide sheets. This microstructure control, on the one hand, uses the precursor particles as physical spacers, effectively preventing the recombination and agglomeration of graphene oxide sheets in subsequent processing; on the other hand, it ensures the uniformity of carbon coating, overcoming the defects of discontinuous and uneven conductive network construction in traditional mechanical mixing or simple carbon coating processes.

[0019] Building upon this foundation, a ferric ion solution was further introduced to treat the composite product. Ferric ions act as a coordinating crosslinking agent, strongly interacting with the oxygen-containing groups on the surface of graphene oxide. This chemical crosslinking significantly enhances the bonding force between graphene oxide sheets and the rigidity of the network structure, enabling it to maintain a stable three-dimensional porous framework without collapse during high-temperature heat treatment. This results in the construction of a highly efficient and continuous three-dimensional conductive network, greatly improving the material's electronic conductivity.

[0020] More importantly, the ferric ions introduced in this step not only act as a cross-linking network fixation agent but also accumulate on the surface of the manganese iron oxide precursor. During the subsequent high-temperature reaction, this enriched iron source promotes the in-situ formation of a unique core-shell structure in the final product: a "manganese-rich inner layer and an iron-rich outer layer." The iron-rich outer layer acts as a protective barrier, effectively isolating the highly reactive manganese elements from direct contact with the electrolyte, fundamentally suppressing the manganese ion dissolution effect, and significantly improving the material's structural stability and cycle life. This synergistically solves the dual technical challenges of poor conductivity and severe manganese dissolution in lithium manganese iron phosphate materials. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a schematic flowchart of the preparation method of lithium manganese iron phosphate composite material in the embodiments of this application; Figure 2 is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material prepared in Example 1 of the present invention; Figure 3 This is an EDS surface scan and layered distribution diagram of iron (Fe) element in the lithium manganese iron phosphate composite material in Example 1 of the present invention, used to show the distribution state of iron element inside and on the surface of the particles. Detailed Implementation

[0023] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0024] refer to Figure 1 This application provides a method for preparing the lithium manganese iron phosphate composite material as described in the foregoing embodiments, comprising: Step S1: The manganese iron oxide precursor and graphene oxide are combined by electrostatic self-assembly to obtain the first composite product.

[0025] The above steps utilize the surface charge between the materials to spontaneously combine the two precursors together through electrostatic attraction (positive and negative attract).

[0026] This process requires the two materials to carry opposite charges.

[0027] Among them, manganese iron oxide precursor (Mn a Fe b O n Since a is greater than 0, b is greater than 0, and n is greater than 0, it does not carry the required charge and therefore needs to be "positively charged modified".

[0028] One specific implementation method is as follows: 0.5g of nano-manganese iron oxide particles and 0.5mL of APTES ((3-aminopropyl)triethoxysilane) are added to 50mL of toluene solution. The mixture is mechanically stirred and refluxed in an 80℃ water bath for 24 hours. After separation, positively charged APTES-(Mn) is obtained. a Fe b ) m O n .

[0029] The graphene oxide (GO) mentioned above has carboxyl and hydroxyl groups on its surface in solution, which gives it a natural negative charge and makes it a negative carbon source.

[0030] The above electrostatic self-assembly refers to the process of assembling dried, positively charged APTES-(Mn) a Fe b ) m O n The particles were dispersed in a GO solution (e.g., 1.0 mg / mL) and mechanically stirred at room temperature for 24 hours.

[0031] Step S2: The composite product is treated with a ferric ion solution to coordinate and crosslink the ferric ions with the graphene oxide and enrich them on the surface of the manganese iron oxide precursor, thereby obtaining the second composite product.

[0032] This step involves chemically treating the first composite product, utilizing the chemical properties of ferric ions to achieve two objectives simultaneously: (1) "Glue" the GO sheets together; (2) Iron is "enriched" on the surface of oxide particles. Process and implementation: Coordination crosslinking: Fe 3+ Cations can coordinate with oxygen-containing groups such as carboxyl, hydroxyl, and epoxy groups on the GO surface. Fe 3+ It acts as a "bridge," connecting (crosslinking) different GO sheets together. Enrichment: Fe 3+ Ions can also be adsorbed and enriched in (Mn) a Fe b ) m O n / GO particle surface. Specific implementation: The first composite product ((Mn) is applied to the surface of the particles. a Fe b ) m O n / GO particles) were placed in 1.0 mol / L Fe 3+ Soak in the solution for 2 hours. After soaking, remove and wash repeatedly (to remove excess Fe). 3+ Then, it is vacuum dried at 60°C to obtain the "second composite product", namely, manganese iron oxide @GO, which is rich in Fe on the outside and manganese on the inside.

[0033] This step enhances network stability, Fe 3+ The coordination and cross-linking effect of Fe greatly improved the adhesion strength between GO nanosheets and enhanced the stability of the GO film (i.e., the network precursor); 3+ The cross-linking effect will (Mn a Fe b ) m O n Nanoparticles are confined between GO sheets, further improving the stability of the three-dimensional network structure; Fe 3+The enrichment process creates a precursor structure that is "Fe-rich on the outside and manganese-rich on the inside," which is key to ultimately achieving particles that are "manganese-rich on the inside and iron-rich on the outside." This design allows the manganese-rich portion of the final material to be kept away from the electrolyte, improving the material's stability and cycle performance.

[0034] Step S3: The second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source to obtain a precursor mixture; wherein the molar ratio of lithium, phosphorus, and manganese iron in the precursor mixture satisfies the stoichiometric ratio; the precursor mixture is subjected to high-temperature treatment to obtain the lithium manganese iron phosphate composite material.

[0035] It should be noted that the phrase "mixed with at least one of a lithium source, a phosphorus source, and a carbon source" aims to ensure that the mixed system simultaneously contains lithium, phosphorus, and manganese-iron from the second composite product. In practice, a compound that serves as both a lithium and phosphorus source, such as lithium dihydrogen phosphate, can be added; alternatively, a lithium source (such as lithium carbonate) and a phosphorus source (such as ammonium dihydrogen phosphate) can be added separately. If enhanced conductivity or reducing power is required, a carbon source (such as glucose) can be added. Regardless of the combination used, the molar ratio of each element in the mixture must meet stoichiometric requirements.

[0036] It should be noted that, prior to mixing, the metal element content of the second composite product needs to be determined (e.g., using ICP-OES inductively coupled plasma atomic emission spectrometry) to ascertain the total molar amount (n) of manganese and iron. Total_Metal =n Mn +n Fe Then proceed to step S3. Based on the measured total molar amount, calculate the required mass of lithium and phosphorus sources according to a lithium:phosphorus:metal (Mn+Fe) molar ratio of 1:1:1 (or a slight excess of lithium source, such as 1.02~1.05:1:1). (If a compound containing both lithium and phosphorus is used, calculate the mass of that compound.) Mix the second composite product with the calculated and weighed lithium, phosphorus, and carbon sources (ensuring the mixture contains both lithium and phosphorus).

[0037] This step is the final synthesis step. Through a high-temperature solid-state reaction, the precursor containing manganese, iron, lithium, and phosphorus is transformed into the target crystalline phase (LMFP), while the GO network is transformed into a graphene network.

[0038] Specific processing procedures and implementation methods may include, for example: Mixing: The second composite product (Fe-rich manganese iron oxide @GO) is mixed with lithium source and phosphorus source (such as Li2CO3, NH4H2PO4, etc.).

[0039] High-temperature treatment: The mixture is placed in a tube furnace and kept at high temperature under an inert atmosphere (such as nitrogen). After cooling, the final "lithium manganese iron phosphate composite material" is obtained.

[0040] High temperature treatment makes (Mn) a Fe b ) m O n enriched Fe 3+ In-situ reaction of lithium and / or phosphorus and / or carbon sources generates LMFP particles with an "iron-rich outer layer and manganese-rich inner layer" structure. A conductive network is formed: simultaneously, high temperature causes Fe... 3+ The cross-linked GO network undergoes carbonization (thermal reduction) and transforms into a graphene network with excellent conductivity. This is because the GO network was already affected by Fe in the second step. 3+ Cross-linking reinforcement prevents it from collapsing during high-temperature processing, thus ensuring the stable three-dimensional structure of the final LMFP particles embedded and constrained by the graphene network. The carbon source here can further play a role in reduction and conductivity.

[0041] In summary, this method utilizes electrostatic self-assembly and ferric ion coordination crosslinking technology to construct a robust three-dimensional conductive graphene network, while simultaneously enriching the iron source on the particle surface. This process design not only effectively suppresses the stacking of graphene sheets and significantly improves the conductivity of the material, but also forms a core-shell structure with a manganese-rich inner layer and an iron-rich outer layer through in-situ reaction. The iron-rich layer blocks electrolyte erosion, thereby effectively inhibiting manganese dissolution and significantly enhancing the electrochemical activity and cycle stability of the material.

[0042] In some embodiments, the phosphorus source includes at least one selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate.

[0043] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride.

[0044] In some embodiments, the carbon source may include at least one of glucose, polypyrrole, polythiophene, Ketjen black, conductive fiber, acetylene black, carbon nanotubes, and graphene. In some embodiments, step S1 involves combining the manganese iron oxide precursor with graphene oxide via electrostatic self-assembly to obtain a first composite product, comprising: Step S11: The manganese iron oxide precursor is positively charged modified.

[0045] This step involves chemically surface-treating the "manganese iron oxide precursor" used as raw material to impart a positive charge to its surface. This is a prerequisite for subsequent "electrostatic self-assembly".

[0046] Specific implementation methods, for example, could involve using nano-manganese iron oxide (Mn) a Fe b ) m O n The particles and APTES ((3-aminopropyl)triethoxysilane) were added to a toluene solution. The reaction was carried out under specific conditions (such as heating in a water bath at 80°C, mechanical stirring, and reflux for 24 hours).

[0047] The aforementioned APTES is a coupling agent. One end of it can bind to the surface of an oxide, while the exposed amino group (-NH2) at the other end can be protonated to (-NH3) under specific conditions. + This causes the particle surface to carry a positive charge.

[0048] The above steps can yield a positively charged manganese iron oxide precursor (which can be APTES-(Mn)). a Fe b ) m O n ).

[0049] Step S12: The positively charged modified manganese iron oxide precursor is dispersed in a graphene oxide solution that serves as a negatively charged carbon source, and then mechanically stirred to perform electrostatic self-assembly.

[0050] This step utilizes the physical principle of "mutual attraction between positive and negative charges" (i.e., electrostatic self-assembly) to combine the positively charged particles from the first step with the negatively charged graphene oxide (GO) sheets.

[0051] Among these, the positively charged component can be the dried, positively charged APTES-(Mn) obtained in the aforementioned steps. a Fe b ) m O n Particles. Negatively charged particles can be used in solutions of graphene oxide (GO) (e.g., 1.0 mg / mL). The surface of GO sheets contains a large number of carboxyl and hydroxyl groups, giving them a natural negative charge, thus serving as a "negatively charged carbon source".

[0052] Then, the positively charged particles were dispersed into a negatively charged GO solution and mechanically stirred at room temperature (e.g., for 24 hours).

[0053] Due to the strong attraction between positive and negative charges, the positively charged (Mn) a Fe b ) m On The particles spontaneously adsorb onto the negatively charged GO sheet surface, forming a uniform composite, namely the "first composite product" ((Mn) a Fe b ) m O n / GO particles).

[0054] Through the electrostatic self-assembly in this step, GO pairs (Mn) can be achieved. a Fe b ) m O n Uniform "pre-coating" of particles. This solves the problem of uneven coating in traditional carbon coating methods. In this composite structure, (Mn a Fe b ) m O n The nanoparticles, located between the GO sheets, act as "spacers," physically inhibiting the re-stacking of the GO sheets. This solves the problem of graphene's tendency to stack, which leads to a decrease in ion / electron transport efficiency.

[0055] In some embodiments, the positive charge modification method in step S11 includes: Step S111: The manganese iron oxide precursor particles and APTES are added to a toluene solution and mixed evenly. The mixture is then mechanically stirred and refluxed under water bath heating conditions.

[0056] In some embodiments, step S2 involves treating the composite product with a ferric ion solution to coordinate and crosslink the ferric ions with the graphene oxide and enrich them on the surface of the manganese iron oxide precursor, thereby obtaining a second composite product, comprising: Step S21: Immerse the first composite product in a ferric ion solution.

[0057] This step involves a chemical reaction to achieve the goals of "coordination crosslinking" and "enrichment".

[0058] Specifically, the "first composite product" obtained in the previous step (i.e., (Mn) can be used as a reference. a Fe b ) m O n / GO particles) are immersed in a solution containing ferric ions.

[0059] For example, the particles can be placed in a 1.0 mol / L Fe solution. 3+ Soak in the solution for 2 hours.

[0060] During this soaking process, Fe 3+ Ions play two key roles simultaneously: Coordination crosslinking: Fe 3+ The cations coordinate with the carboxyl, hydroxyl, and epoxy groups on the surface of graphene oxide (GO) in the "first composite product". This interaction acts like an "adhesive", greatly improving the bonding strength between GO nanosheets and enhancing the stability of the GO film (i.e., the network precursor).

[0061] Enrichment effect: Fe 3+ Ions will also accumulate in (Mn) a Fe b ) m O n The surface of the particles is a precursor step in forming the "iron-rich outer layer" structure of the final product.

[0062] Step S22: After removing and washing to remove excess ferric ions, vacuum drying is performed to obtain the second composite product.

[0063] The first step described above is purification and separation. The treatment method can be: removing the soaked product from Fe... 3+ Remove it from the solution and wash it repeatedly.

[0064] The purpose of this step is to remove free "excess Fe" that has not undergone coordination cross-linking or has not accumulated on the particle surface. 3+ This ensures the accuracy of the reactant ratio during subsequent high-temperature processing and removes impurities.

[0065] The vacuum drying described above is the final step in removing solvents (such as moisture) and obtaining a stable intermediate. Specific processing methods may include: Vacuum drying at 60°C yields the "second composite product".

[0066] The product is a "Fe-rich external and manganese-rich internal manganese-rich manganese iron oxide @GO". This is a stable intermediate that already possesses the precursor structure of "externally iron-rich" and "Fe-rich internal". 3+ The stable network structure of "crosslinking" immediately laid the foundation for the final high-temperature synthesis step.

[0067] In some embodiments, step S3 involves mixing the second composite product with at least one of a lithium source, a phosphorus source, and a carbon source to obtain a precursor mixture; wherein the molar ratio of lithium, phosphorus, and manganese / iron in the precursor mixture satisfies a stoichiometric ratio, including: Step S31: The second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source, and then kept at a temperature of 650°C to 850°C (e.g., 650°C, 700°C, 750°C, 800°C, 850°C, etc.) for 120 minutes to 720 minutes (e.g., 120 minutes, 180 minutes, 240 minutes, 300 minutes, 360 minutes, 420 minutes, 480 minutes, 540 minutes, 600 minutes, 660 minutes, 720 minutes, etc.) under an inert or reducing atmosphere. After cooling, the lithium manganese iron phosphate composite material is obtained.

[0068] This application also provides a lithium manganese iron phosphate composite material, which is prepared by the preparation method provided in the above embodiments.

[0069] Thanks to the unique electrostatic self-assembly process and the coordination crosslinking and in-situ reaction mechanism of ferric ions in the above preparation process, the prepared lithium manganese iron phosphate composite material exhibits unique structural features in its microstructure. Specifically, the lithium manganese iron phosphate composite material includes lithium manganese iron phosphate particles with a manganese-rich inner layer and an iron-rich outer layer, and a graphene three-dimensional conductive network; wherein the lithium manganese iron phosphate particles are embedded and constrained in the graphene three-dimensional conductive network.

[0070] The formation of this structure lies in the fact that the graphene three-dimensional conductive network is formed by the coordination crosslinking of graphene oxide with ferric ions and high-temperature treatment, and the outer iron-rich structure of the lithium manganese iron phosphate particles originates from the ferric ions used in the coordination crosslinking.

[0071] Based on this unique structure, the above material mainly consists of the following two parts (components): (1) Lithium manganese iron phosphate particles (LiMn) 1-x Fe x PO4): This is the main substance that plays an electrochemical role in the material.

[0072] (2) Graphene three-dimensional conductive network: This is a conductive framework made of graphene material that is interconnected in three-dimensional space.

[0073] The aforementioned lithium manganese iron phosphate particles "have a manganese-rich inner layer and an iron-rich outer layer structure," providing information on the elemental distribution within a single lithium manganese iron phosphate particle. It is not a homogeneous solid solution, but rather has a core-shell or gradient distribution structure. The core region (inner layer) has a higher concentration of manganese (Mn), while the region near the particle surface (outer layer) has a higher concentration of iron (Fe).

[0074] While manganese (Mn) in lithium manganese iron phosphate (LMFP) provides high voltage, its chemical properties are relatively unstable, making it prone to dissolution upon contact with the electrolyte (a phenomenon known as "manganese leaching"). By designing the manganese-rich portion as an inner layer, away from the electrolyte, and then coating it with a more chemically stable iron-rich outer layer, this structural design effectively inhibits manganese dissolution, thereby improving the material's structural stability and cycle performance.

[0075] The aforementioned three-dimensional conductive graphene network was formed by cross-linking graphene oxide with trivalent iron ions and then subjected to high-temperature treatment.

[0076] Its raw material is graphene oxide (GO), which is a precursor of graphene. Its sheet surface has a large number of oxygen-containing groups (such as carboxyl groups, hydroxyl groups, etc.).

[0077] "trivalent iron ions (Fe 3+ "Coordination crosslinking", that is, Fe 3+ Cations are introduced between the GO layers and coordinate with these oxygen-containing groups on the GO surface. Fe 3+ Ions act like "bridges" or "adhesives," connecting (crosslinking) the originally separate GO sheets together.

[0078] The aforementioned "high-temperature treatment" refers to the reduction (carbonization) of the cross-linked GO network at high temperatures, which removes most of the oxygen-containing groups and transforms it into graphene (carbon source) with excellent conductivity.

[0079] In traditional material preparation, van der Waals forces exist between graphene (or GO) sheets, making them prone to recombination and resulting in the loss of their conductivity. However, through Fe... 3+ The coordination and cross-linking of ions greatly improves the adhesion strength between GO nanosheets and enhances the stability of the GO network (precursor). This prevents the network from collapsing or severely stacking during subsequent high-temperature processing, thus ensuring that the final graphene three-dimensional conductive network is uniform, stable, and efficient.

[0080] Regarding the relationship between the particles and the network, in this embodiment, they are "embedded and constrained within the graphene three-dimensional conductive network." The lithium manganese iron phosphate particles (or their precursors) are not simply attached to the surface of the network, but are "embedded" inside the network and "constrained" (i.e., fixed) between the graphene sheets. This structure offers synergistic advantages.

[0081] (1) On the one hand, graphene networks provide efficient electron transport channels for particles; (2) On the other hand, these embedded particles act as "spacers," physically expanding the graphene sheets and further inhibiting the recombination of graphene. This mutually constraining and supporting structure ensures the overall structural stability of the composite material and its high ion / electron transport efficiency.

[0082] The aforementioned "outer iron-rich structure originates from the ferric ions used in the coordination crosslinking," which links the particle structure with network formation. The aforementioned F used for crosslinking GO... 3+ Ions are also the source of iron (Fe) that forms the "iron-rich outer layer" structure.

[0083] Fe was introduced during the preparation process. 3+ While coordinating and crosslinking with GO, the ions also accumulate on the surface of the particles (precursors) and form an iron-rich outer layer of lithium manganese iron phosphate particles after high-temperature treatment.

[0084] This "killing two birds with one stone" design allows Fe 3+ Ions play two roles simultaneously: (1) It was used as a "crosslinking agent" to construct a stable GO network; (2) As a "reactant", it forms an iron-rich protective layer for the particles.

[0085] This design utilizes Fe 3+ It forms a strong interaction with the abundant oxygen-containing groups in the GO lamellars, which can improve the stability of the composite material structure.

[0086] This application also provides a positive electrode, comprising the lithium manganese iron phosphate composite material as described in the foregoing embodiments.

[0087] The aforementioned positive electrode is one of the core components of a lithium-ion battery. This positive electrode includes (or is made from) the aforementioned lithium manganese iron phosphate composite material as its active material. By employing this specific composite material, the positive electrode directly inherits all the beneficial effects brought about by the material's unique structure. These effects include the high stability and inhibition of manganese dissolution brought about by the "manganese-rich inner layer and iron-rich outer layer" particle structure, as well as the high stability and inhibition of manganese dissolution brought about by the Fe... 3+ The excellent conductivity provided by the three-dimensional conductive network of graphene formed after cross-linking makes this a high-performance cathode designed to achieve high discharge specific capacity (not less than 140 mAh / g) and high conductivity.

[0088] In this embodiment, a battery is provided, including a positive electrode as described in the foregoing embodiments.

[0089] The aforementioned battery uses the cathode described above. By integrating this high-performance cathode, the battery achieves all the beneficial effects of the lithium manganese iron phosphate composite material. These effects stem from the unique structure of the composite material: the "manganese-rich inner layer and iron-rich outer layer" design of the particles effectively suppresses manganese dissolution, thereby improving the battery's cycle stability and chemical stability; simultaneously, the Fe... 3+ The cross-linked graphene three-dimensional conductive network ensures that the battery has a high specific capacity (not less than 140 mAh / g) and excellent conductivity. As a complete electrochemical energy storage device, in addition to this high-performance positive electrode, the battery may also include, but is not limited to, a negative electrode that works with it, an electrolyte for transporting ions, and a separator for preventing direct contact between the positive and negative electrodes.

[0090] In this application embodiment, an electrical device is provided, including a battery as described in the foregoing embodiments.

[0091] The aforementioned electrical equipment includes the aforementioned battery as its power source or energy storage unit. Because this battery integrates the aforementioned high-performance positive electrode, which has an "inner layer rich in manganese, outer layer rich in iron" structure and Fe... 3+ The device is made of a composite material with a cross-linked graphene network, thus directly benefiting from all the advantages of this composite material. This means the device is powered by a high-performance lithium-ion battery with high specific capacity (not less than 140 mAh / g), excellent conductivity, and high cycle stability (due to suppression of manganese leaching). Examples of such devices include, but are not limited to, portable electronic devices (such as smartphones and laptops), electric vehicles, power tools, and grid energy storage systems.

[0092] The present invention will be further illustrated below with specific embodiments. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0093] Example 1 This embodiment provides a lithium manganese iron phosphate composite material.

[0094] Experimental methods: (1) Positive charge modification: 0.5g of nano-manganese iron oxide Mn 1.8 Fe 1.2 O4 particles and 0.5 mL of APTES were added to 50 mL of toluene solution and mixed thoroughly. The mixture was then heated in a water bath at 80 °C with mechanical stirring and refluxed for 24 h. After magnetic separation, APTES-Mn was obtained. 1.8 Fe 1.2 O4; (2) Self-assembly: The dried APTES-Mn 1.8 Fe 1.2O4 particles were dispersed in a GO (negatively charged carbon source) solution (1.0 mg / mL). After mechanical stirring at room temperature for 24 h, the mixture was vacuum filtered through a microporous membrane (0.45 μm pore size) and dried to obtain the first composite product (Mn). 1.8 Fe 1.2 O4 / GO particles); (3) Coordination and crosslinking: The first composite product particles were soaked in 1.0 mol / L ferric chloride (FeCl3) solution for 2 h. After being taken out, they were repeatedly washed with deionized water and ethanol to remove excess iron ions and chloride ions. After vacuum drying at 60 °C, the second composite product (Fe-rich on the outside and manganese-rich on the inside manganese oxide @GO) was obtained. (4) The second composite product obtained in step (3) was analyzed by ICP elemental content determination. The molar amount of Mn in the 48.75g second composite product was 0.3535 mol, the molar amount of Fe was 0.2892 mol, that is, the molar amount of total metal elements (Mn+Fe) was 0.6427 mol. According to the molar ratio of Li:(Mn+Fe):P = 1.02:1:1, the required molar amount of phosphorus (P) should be 0.6427 mol.

[0095] Since the molar ratio of Li to P in the lithium dihydrogen phosphate (LiH2PO4, molecular weight approximately 103.93) used is 1:1, in order to meet the ratio requirements, 66.8g of lithium dihydrogen phosphate (approximately 0.6427 mol) was weighed and mixed with the second composite product.

[0096] The mixture was placed in a tube furnace at 700℃ in a mixed atmosphere of N2 and H2 (hydrogen volume ratio of 10%~20%) and kept at that temperature for 300 min. After cooling, a lithium manganese iron phosphate composite material (a lithium manganese iron phosphate@graphene composite material with an iron-rich outer layer and a manganese-rich inner layer) was obtained.

[0097] Because the precursor manganese-iron oxide has a high metal valence state, high-temperature treatment with a nitrogen-hydrogen mixed gas helps to fully reduce Mn and Fe to the +2 valence state through the synergistic effect of the reducing gas, ensuring the formation of a pure-phase olivine-structured LiMn. 1-x Fe x PO4 is used to prevent the formation of impurity phases.

[0098] Combination Figure 2 Scanning electron microscope (SEM) images show that graphene forms a three-dimensional conductive network after high-temperature carbonization and is uniformly distributed between the positive electrode particles, which can significantly improve the overall conductivity of the material.

[0099] Meanwhile, to further verify the microstructure and elemental distribution gradient of the material prepared in this embodiment, EDS elemental surface scanning and layer analysis were performed on it. Combined with, for example... Figure 3 As shown in the EDS surface scan and layered distribution diagram of iron (Fe), iron (Fe) is highly enriched in the surface region of the composite material particles, gradually decreasing from the surface to the interior, exhibiting a clear gradient distribution characteristic of iron-rich outer layer and manganese-rich inner layer. This further provides direct evidence that the preparation method of this invention successfully constructs a continuous and dense iron-rich outer shell structure on the particle surface through coordination crosslinking and in-situ enrichment of ferric ions. This iron-rich outer layer can effectively serve as a dual physical and chemical protective barrier, preventing direct contact between the electrolyte and the internal manganese-rich region, inhibiting manganese dissolution from the source, significantly improving the material's structural stability and cycle life, and fully demonstrating the technical mechanism of this invention.

[0100] Comparative Example 1 This comparative example provides a lithium manganese iron phosphate material.

[0101] The experimental method is basically the same as in Example 1, except that: nano-manganese iron oxide (Mn) is not used. 1.8 Fe 1.2 O4 particles are modified with positive charge; The experimental method is as follows: (1) Take 0.5g Mn 1.8 Fe 1.2 O4 particles were dispersed in a GO (negatively charged carbon source) solution (1.0 mg / mL). After mechanical stirring at room temperature for 24 h, the mixture was vacuum filtered through a microporous membrane (0.45 μm pore size) and dried to obtain the first composite product (Mn). 1.8 Fe 1.2 (a mixture of O4 and GO); (2) Coordination crosslinking: The first composite product particles were placed in 1.0 mol / L Fe... 3+ Soak in the solution for 2 hours, then remove and wash repeatedly to remove excess Fe. 3+ The second composite product can be obtained by vacuum drying at 60℃. (3) 48.75g of the second composite product obtained in step (2) was weighed and 66.8g of lithium dihydrogen phosphate (as lithium source and phosphorus source) was weighed according to the molar ratio of Li:(Mn+Fe):P=1.02:1:1. The mixture was mixed with the second composite product and placed in a tube furnace at 700℃ in a mixed atmosphere of N2 and H2 (hydrogen volume ratio of 10%~20%). The mixture was kept at the atmosphere for 300min and cooled to obtain lithium manganese iron phosphate composite material.

[0102] Comparative Example 2 This comparative example provides a lithium manganese iron phosphate material.

[0103] The experimental method is basically the same as in Example 1, except that Fe is not used. 3+ Coordination crosslinking.

[0104] Experimental methods: (1) Positive charge modification: 0.5g of nano-manganese iron oxide Mn 1.8 Fe 1.2 O4 particles and 0.5 mL of APTES were added to 50 mL of toluene solution and mixed thoroughly. The mixture was then heated in a water bath at 80 °C with mechanical stirring and refluxed for 24 h. After magnetic separation, APTES-Mn was obtained. 1.8 Fe 1.2 O4; (2) Self-assembly: The dried APTES-Mn 1.8 Fe 1.2 O4 particles were dispersed in a GO (negatively charged carbon source) solution (1.0 mg / mL). After mechanical stirring at room temperature for 24 h, the mixture was vacuum filtered through a microporous membrane (0.45 μm pore size) and dried to obtain the first composite product (Mn). 1.8 Fe 1.2 O4 / GO particles); (3) 48.75g of the first composite product obtained in step (2) was weighed and 66.8g of lithium dihydrogen phosphate (as lithium source and phosphorus source) was weighed according to the molar ratio of Li:(Mn+Fe):P=1.02:1:1. The mixture was mixed with the first composite product and placed in a tube furnace at 700℃ in a mixed atmosphere of N2 and H2 (hydrogen volume ratio of 10%~20%). The mixture was kept at the atmosphere for 300min and cooled to obtain lithium manganese iron phosphate composite material.

[0105] Test Experiment 1. Testing method: (1) Test the intrinsic properties of the material, such as particle size, specific surface area, and tap density.

[0106] (2) The lithium manganese iron phosphate prepared in Examples 1 and Comparative Examples 1-2 was mixed with a conductive agent and a binder to prepare a positive electrode slurry. The proportion of solid matter in the slurry was as follows: active material accounted for 97.2%, conductive agent (conductive carbon black) accounted for 1.7%, and binder (polyvinylidene fluoride) accounted for 1.1%. The content of the solvent N-methylpyrrolidone was adjusted to make the solid content of the slurry about 60%. The slurry was coated on the surface of the current collector aluminum foil after stirring, dried, and then rolled and sliced ​​to obtain the positive electrode sheet. The square full cell assembled using the above positive electrode sheet was subjected to a 1C charge-1C discharge cycle test for 1000 cycles. The square battery had a capacity of 20Ah, a thickness of 15mm, a width of 119mm, and a height of 208mm.

[0107] 2. Test Results: Table 1. Test Results (Morphology and Physicochemical Properties)

[0108] Table 2. Test Results (Electrochemical Performance)

[0109] 3. Analysis: Based on Tables 1 and 2, it can be seen that, after adopting the preparation method of this embodiment, the conductivity, compaction performance, and capacity retention rate of the lithium manganese iron phosphate material in Example 1 are greatly improved compared with Comparative Examples 1 and 2.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate composite material, characterized in that, include: The first composite product was obtained by combining the manganese iron oxide precursor with graphene oxide through electrostatic self-assembly. The composite product is treated with a ferric ion solution to coordinate and crosslink the ferric ions with the graphene oxide and enrich them on the surface of the manganese iron oxide precursor, thereby obtaining a second composite product. The second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source to obtain a precursor mixture; wherein the molar ratio of lithium, phosphorus, and manganese iron in the precursor mixture satisfies the stoichiometric ratio.

2. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, The phosphorus source includes at least one selected from the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate; and / or, The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride.

3. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, The method of combining the manganese iron oxide precursor with graphene oxide through electrostatic self-assembly to obtain the first composite product includes: positively charging the manganese iron oxide precursor; dispersing the positively charged manganese iron oxide precursor in a graphene oxide solution that serves as a negatively charged carbon source, and performing electrostatic self-assembly by mechanical stirring.

4. The preparation method of the lithium manganese iron phosphate composite material as described in claim 3, characterized in that, The method for positive charge modification includes: The manganese iron oxide precursor particles and APTES were added to a toluene solution and mixed evenly. The mixture was then mechanically stirred and refluxed under water bath heating conditions.

5. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, The composite product is treated with a ferric ion solution to coordinate and crosslink the ferric ions with the graphene oxide and enrich them on the surface of the manganese iron oxide precursor, resulting in a second composite product comprising: The first composite product was immersed in a solution of ferric ions; After removing and washing to remove excess ferric ions, the product is vacuum dried to obtain the second composite product.

6. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, The second composite product is mixed with at least one of a lithium source, a phosphorus source, and a carbon source, and then subjected to high-temperature treatment to obtain the lithium manganese iron phosphate composite material, comprising: The second composite product is mixed with at least one of a lithium source, a phosphorus source and a carbon source, and then kept at a temperature of 650°C to 850°C for 120 minutes to 720 minutes under an inert atmosphere and / or a reducing atmosphere. After cooling, the lithium manganese iron phosphate composite material is obtained.

7. A lithium manganese iron phosphate composite material, characterized in that, It is prepared by the preparation method according to any one of claims 1-6.

8. A positive electrode, characterized in that, This includes the lithium manganese iron phosphate composite material as described in claim 7, or the lithium manganese iron phosphate composite material prepared by the preparation method described in any one of claims 1-6.

9. A battery, characterized in that, Includes the positive electrode as described in claim 8.

10. An electrical-related device, characterized in that, Includes the battery as described in claim 9.

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