A high-rate long-cycle lithium manganese iron phosphate cathode material and a preparation method thereof

Abstract

The application relates to the field of electrochemical energy storage, and discloses a high-rate long-cycle lithium manganese iron phosphate positive electrode material and a preparation method thereof, which comprises a lithium manganese iron phosphate base body and a modified layer in-situ coated on the surface of the base body; the modified layer has neodymium ions, strontium ions and nickel ions with a concentration gradient distribution; the outermost layer has a perovskite-like structure phase; the inner part has ion permeation channels composed of non-stoichiometric oxygen defects and forms an interface lattice phase locking structure; the application reduces interface charge transfer impedance by establishing a high-conductivity network, restrains lattice volume deformation by using an interlocking network formed by hetero-ions, suppresses manganese element dissolution and widens ion migration paths, breaks through the restriction of the cycle stability and the rate performance of the lithium manganese iron phosphate, and improves the structural stability of the active substance in the charge and discharge cycle.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, and particularly relates to a high-rate, long-cycle lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] Currently, lithium manganese iron phosphate (LFP) has become a key positive electrode active material due to its high safety performance and voltage platform. However, with the continuous improvement of energy density and charge / discharge rate requirements of battery systems, LFP materials, due to the asymmetry of Mn-O bond lengths within their crystal lattice, are prone to severe Gaindale structure distortion in the deep delithiation state, leading to irreversible physical collapse of the active center. Furthermore, due to the extremely low intrinsic conductivity of LFP materials, typically around 10... -10 S / cm to 10 -14 The extremely low S / cm range leads to its significantly slow kinetic characteristics. Existing technologies commonly employ surface carbon coating combined with nano-processing to address the low intrinsic conductivity of lithium manganese iron phosphate and the kinetic barriers induced by manganese. However, this can only alleviate surface ohmic polarization and cannot fundamentally curb the accumulation of lattice stress and the dissolution of manganese in high-manganese systems. The solution is to shorten the migration path of lithium ions by constructing a conductive network or to anchor lattice sites by using heterogeneous ion doping to reduce the dissolution rate of manganese ions in the electrolyte.

[0003] While the aforementioned techniques can alleviate the initial polarization phenomenon of materials, existing technologies face irreconcilable physical constraints under the dual demands of high energy density and long cycle life. Conventional carbon layer modification can only improve the surface conductivity of materials and cannot reduce the charge transfer energy barrier inside the bulk phase. Furthermore, the introduction of carbon components reduces the compaction density of the active material. Traditional ion doping often results in the discrete distribution of dopant elements at the lattice edges, preventing the formation of continuous electron transport channels during charge-discharge dynamics. Under extreme conditions, the Jan Taylor distortion of the manganese-oxygen octahedron can still induce lattice framework collapse, leading to irreversible capacity decay. For example, Chinese invention patent CN116682962B discloses a method for preparing high-capacity lithium manganese iron phosphate cathode material. It improves the material's conductivity and inhibits grain growth by combining nickel ion doping with modified organic carbon sources. However, in actual working conditions, the combination of nickel and carbon sources is based on the pyrolysis path of organic matter, resulting in nickel oxides that are discretely distributed on the particle surface. This does not form atomic-level chemical continuity with the lithium manganese iron phosphate matrix, making it difficult to generate sufficient lattice constraint to counteract the manganese oxygen octahedral Jan Taylor distortion. Single-dimensional doping lacks dynamic synergy between improving bulk diffusion and reducing interfacial energy barriers. During high-current-rate cycling, the interfacial impedance deteriorates with the formation of microcracks.

[0004] Therefore, how to reconstruct a modified architecture with both high conductivity and lattice anchoring functions in situ on the surface of a manganese source, and break through the impedance barrier of the material in the energy conversion process, has become the technical problem to be solved by this invention. Summary of the Invention

[0005] The present invention aims to solve the problems of lattice distortion and increased interfacial charge transfer impedance in lithium manganese iron phosphate materials during energy conversion.

[0006] In this technical solution, a high-rate, long-cycle lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate matrix and a modified layer in situ coated on the surface of the lithium manganese iron phosphate matrix. The modified layer has neodymium and strontium ions distributed in decreasing concentration from the surface of the lithium manganese iron phosphate substrate, and nickel ions distributed in increasing concentration from the surface of the lithium manganese iron phosphate substrate. The outermost layer of the modified layer has a perovskite-like structure phase composed of neodymium ions, strontium ions and nickel ions, and the thickness of the perovskite-like structure phase is 2 nm to 10 nm. The modified layer contains non-stoichiometric oxygen defects that serve as diffusion channels for guiding neodymium ions, strontium ions, and nickel ions to penetrate deep into the bulk phase of the lithium manganese iron phosphate matrix. These non-stoichiometric oxygen defects are used to balance the local potential generated by heterogeneous ion doping within the modified layer. The interface between the perovskite-like phase and the lithium manganese iron phosphate matrix has an atomic-level lattice phase-locked structure achieved through non-stoichiometric oxygen defects. Inside the modified layer, neodymium ions, strontium ions, and nickel ions precisely occupy specific sites around the manganese oxygen octahedra in the lithium manganese iron phosphate matrix lattice to form a three-dimensional interlocking anchoring network. The neodymium ion content is from 0.05 wt% to 0.3 wt%. The strontium ion content is 0.1 wt% to 0.5 wt%; The nickel ion content is 0.2 wt% to 0.8 wt%; The balance of the nickel-based modified lithium manganese iron phosphate formulation is the lithium manganese iron phosphate matrix.

[0007] Preferably, the neodymium ions, strontium ions and nickel ions in the modified layer constrain the volume deformation of the manganese oxygen octahedra in the lithium manganese iron phosphate matrix under the charge-discharge delithiation state through the lattice anchoring effect; the width of the lithium ion diffusion channel in the modified layer is 15% to 25% wider than that of the unmodified lithium manganese iron phosphate matrix.

[0008] Preferably, the molecular formula of the lithium manganese iron phosphate matrix is: ,in, The lithium manganese iron phosphate matrix consists of hollow microspheres or near-spherical particles; the average particle size of the lithium manganese iron phosphate matrix is ​​0.5 μm to 5 μm.

[0009] Preferably, the bulk density of non-stoichiometric oxygen defects within the modified layer is: to The slopes of the concentration gradients of neodymium, strontium, and nickel ions within the modified layer are monotonically positively correlated with the radial concentration distribution of non-stoichiometric oxygen defects.

[0010] Preferably, the perovskite-like structure phase has a neodymium-strontium-nickel composite oxide crystal structure; the interfacial charge transfer impedance of the perovskite-like structure phase in the frequency range of 0.1 Hz to 100 kHz is 30% to 50% lower than that of the unmodified lithium manganese iron phosphate matrix.

[0011] Preferably, in the three-dimensional interlocking anchoring network, neodymium ions occupy the vertices, strontium ions occupy the edges, and nickel ions occupy the faces; the lattice volume shrinkage rate of the three-dimensional interlocking anchoring network in the state of complete lithium ion depletion is less than 2%.

[0012] Preferably, the internal lattice of the lithium manganese iron phosphate matrix remains intact and there is no manganese element segregation; the perovskite-like structure phase is an atomically dense layer continuously coated on the surface of the lithium manganese iron phosphate matrix; and the residual lithium content on the surface of the manganese source particles in the nickel-based modified lithium manganese iron phosphate formulation is less than 500 ppm.

[0013] Preferably, the compacted density of the nickel-based modified lithium manganese iron phosphate formulation is not less than 2.4 g / L. The nickel-based modified lithium manganese iron phosphate formulation retains no less than 85% of its discharge capacity at 10C rate and at 0.1C rate.

[0014] A method for preparing a high-rate, long-cycle lithium manganese iron phosphate cathode material includes the following steps: a nickel-based modified lithium manganese iron phosphate formulation is obtained by atomic layer deposition combined with an asymmetric pressure switching thermal quenching process; wherein, the asymmetric pressure switching thermal quenching process includes an oxygen-deficient diffusion stage with an oxygen partial pressure of 0.01 MPa to 0.05 MPa and an oxygen-enriched shaping stage with an oxygen partial pressure of 0.1 MPa to 0.3 MPa.

[0015] Compared to existing technologies, this invention significantly improves the intrinsic kinetic properties of lithium manganese iron phosphate through precise lattice repair: 1. It greatly improves the capacity performance of lithium manganese iron phosphate under high-rate conditions. The discharge capacity retention rate at 10C rate is increased from less than 70% of conventional materials to more than 85%, and it maintains extremely high voltage plateau stability during 1000 cycles.

[0016] 2. By constructing a neodymium-strontium-nickel superconductor precursor coating with strong electronic correlation effect around the active center of lithium manganese iron phosphate, this structure forms chemical continuity with the LMFP matrix, reducing the interfacial charge transfer impedance of lithium manganese iron phosphate by 30% to 50%, and solving the polarization problem of high manganese formulation in low temperature environment.

[0017] 3. To address the persistent problem of manganese dissolution during the cycling process of lithium manganese iron phosphate (LMFP), neodymium ions, strontium ions, and nickel ions are used to precisely anchor manganese oxygen octahedral sites, reducing the volume shrinkage rate of LMFP during charge-discharge cycles to below 2%, thereby physically curbing capacity decay caused by lattice strain. Attached Figure Description

[0018] Figure 1 This is a diagram showing the ion gradient distribution and atomic-level phase-locked structure of the modified lithium manganese iron phosphate of this invention; Figure 2 This is a flowchart of the asymmetric pressure-switching thermal shock and crystal phase reconstruction process of the nickel-based modified material of this invention. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0020] It should be noted that all directional and positional terms used in this invention, such as: up, down, left, right, front, back, vertical, horizontal, inner, outer, top, bottom, transverse, longitudinal, center, etc., are only used to explain the relative positional relationship and connection between components in a specific state (as shown in the accompanying drawings). They are only for the convenience of describing this invention and do not require that this invention be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention. In addition, the descriptions of "first," "second," etc., in this invention are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated.

[0021] In the description of this invention, unless otherwise explicitly specified and limited, the terms installation, connection, and linking should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections; they can refer to direct connections or indirect connections through an intermediate medium; they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances.

[0022] In the description of this specification, references to the terms "an embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example, and the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0023] A high-rate, long-cycle lithium manganese iron phosphate cathode material includes a lithium manganese iron phosphate matrix and a modified layer in situ coated on the surface of the lithium manganese iron phosphate matrix. The modified layer has neodymium and strontium ions distributed in decreasing concentration from the surface of the lithium manganese iron phosphate substrate, and nickel ions distributed in increasing concentration from the surface of the lithium manganese iron phosphate substrate. The outermost layer of the modified layer has a perovskite-like structure phase composed of neodymium ions, strontium ions and nickel ions, and the thickness of the perovskite-like structure phase is 2 nm to 10 nm. The modified layer contains non-stoichiometric oxygen defects that serve as diffusion channels for guiding neodymium ions, strontium ions, and nickel ions to penetrate deep into the bulk phase of the lithium manganese iron phosphate matrix. These non-stoichiometric oxygen defects are used to balance the local potential generated by heterogeneous ion doping within the modified layer. The interface between the perovskite-like phase and the lithium manganese iron phosphate matrix has an atomic-level lattice phase-locked structure achieved through non-stoichiometric oxygen defects. Inside the modified layer, neodymium ions, strontium ions, and nickel ions precisely occupy specific sites around the manganese oxygen octahedra in the lithium manganese iron phosphate matrix lattice to form a three-dimensional interlocking anchoring network. The neodymium ion content is from 0.05 wt% to 0.3 wt%. The strontium ion content is 0.1 wt% to 0.5 wt%; The nickel ion content is 0.2 wt% to 0.8 wt%; The balance of the nickel-based modified lithium manganese iron phosphate formulation is the lithium manganese iron phosphate matrix.

[0024] Preferably, the neodymium ions, strontium ions and nickel ions in the modified layer constrain the volume deformation of the manganese oxygen octahedra in the lithium manganese iron phosphate matrix under the charge-discharge delithiation state through the lattice anchoring effect; the width of the lithium ion diffusion channel in the modified layer is 15% to 25% wider than that of the unmodified lithium manganese iron phosphate matrix.

[0025] Preferably, the molecular formula of the lithium manganese iron phosphate matrix is: ,in, The lithium manganese iron phosphate matrix consists of hollow microspheres or near-spherical particles; the average particle size of the lithium manganese iron phosphate matrix is ​​0.5 μm to 5 μm.

[0026] Preferably, the bulk density of non-stoichiometric oxygen defects within the modified layer is: to The slopes of the concentration gradients of neodymium, strontium, and nickel ions within the modified layer are monotonically positively correlated with the radial concentration distribution of non-stoichiometric oxygen defects.

[0027] Preferably, the perovskite-like structure phase has a neodymium-strontium-nickel composite oxide crystal structure; the interfacial charge transfer impedance of the perovskite-like structure phase in the frequency range of 0.1 Hz to 100 kHz is 30% to 50% lower than that of the unmodified lithium manganese iron phosphate matrix.

[0028] Preferably, in the three-dimensional interlocking anchoring network, neodymium ions occupy the vertices, strontium ions occupy the edges, and nickel ions occupy the faces; the lattice volume shrinkage rate of the three-dimensional interlocking anchoring network in the state of complete lithium ion depletion is less than 2%.

[0029] Preferably, the internal lattice of the lithium manganese iron phosphate matrix remains intact and there is no manganese element segregation; the perovskite-like structure phase is an atomically dense layer continuously coated on the surface of the lithium manganese iron phosphate matrix; and the residual lithium content on the surface of the manganese source particles in the nickel-based modified lithium manganese iron phosphate formulation is less than 500 ppm.

[0030] Preferably, the compacted density of the nickel-based modified lithium manganese iron phosphate formulation is not less than 2.4 g / L. The nickel-based modified lithium manganese iron phosphate formulation retains no less than 85% of its discharge capacity at 10C rate and at 0.1C rate.

[0031] A method for preparing a high-rate, long-cycle lithium manganese iron phosphate cathode material includes the following steps: a nickel-based modified lithium manganese iron phosphate formulation is obtained by atomic layer deposition combined with an asymmetric pressure switching thermal quenching process; wherein, the asymmetric pressure switching thermal quenching process includes an oxygen-deficient diffusion stage with an oxygen partial pressure of 0.01 MPa to 0.05 MPa and an oxygen-enriched shaping stage with an oxygen partial pressure of 0.1 MPa to 0.3 MPa.

[0032] Example 1: In the application scenario of this invention for preparing positive electrode materials for power batteries, surface atomic-level modification of the manganese source precursor is performed to reduce charge transfer impedance and suppress manganese ion dissolution. An atomic layer deposition process is used, in which gaseous neodymium source precursor, nickel source precursor, and strontium source precursor are alternately fed into a fluidized bed reactor. Specifically, 100g of manganese source powder, after acid washing and drying, is first weighed and placed in the fluidized bed reaction chamber. The vacuum pump is started to evacuate the reaction chamber to a pressure of 13.3 Pa to 1333.2 Pa, and the temperature is heated to a reaction temperature of 150°C to 250°C. This temperature is maintained for 30 to 60 minutes to completely remove free moisture and impurities adsorbed on the surface of the manganese source. After the temperature and pressure of the reaction chamber stabilize, the following self-confined atomic layer deposition cycles are sequentially executed, with a total number of cycles set to 10 to 150. 00 cycles: First, a precursor A pulse is executed, introducing the gaseous neodymium source precursor into the reaction chamber with a pulse duration controlled between 0.1 and 2 seconds, allowing it to chemically adsorb onto the surface of the manganese source particles and reach saturation. Then, a first purging is performed, introducing high-purity inert gas for 5 to 60 seconds to thoroughly remove unreacted precursor A and volatile byproducts. Next, a precursor B pulse is executed, simultaneously introducing the gaseous strontium and nickel source precursors into the reaction chamber at a stoichiometric molar ratio with a pulse duration of 0.1 to 2 seconds, allowing them to fully react with the precursor A adsorbed on the particle surface, generating the target infinite-layer structure coating layer in situ. Finally, a second purging is performed, again introducing high-purity inert gas for 5 to 60 seconds to remove the remaining precursor B and byproducts, ensuring a conformal distribution of the manganese source powder surface at a reaction temperature of 150°C to 250°C. Infinite-layer precursor coating; in which The doping ratio coefficient ranges from 0.002 to 0.06. The coated manganese source powder is then placed in a reaction apparatus with dynamic pressure regulation. The initial ambient oxygen partial pressure is controlled to be 0.02 MPa to 0.05 MPa, while the pure oxygen flow rate is maintained at a low diffusion range of 0.1 L / min to 0.4 L / min. The temperature is increased to 300°C to 350°C at a heating rate of 1°C / min. In this low oxygen partial pressure environment, non-stoichiometric oxygen defects are generated on the surface of the manganese source, serving as diffusion channels to guide large-radius neodymium and strontium ions in the precursor layer to in-situ gradient penetration into the lattice sites surrounding the manganese-oxygen octahedron. Within 5 minutes, the ambient oxygen partial pressure is increased to 0.15 MPa to 0.2 MPa. The pure oxygen flow rate was dynamically adjusted to a high flow rate of 0.5 L / min to 0.7 L / min for stabilization, while the heating rate was switched to 10 °C / min to 15 °C / min to bring it into the 500 °C to 550 °C excitation temperature range. Utilizing the high oxygen partial pressure environment, the surface nickel ions underwent phase reconstruction to form a perovskite-like structure phase with a thickness of 2 nm to 10 nm. Thermodynamic time difference was used to restrict the diffusion of oxygen atoms into the internal bulk phase. Based on Fick's second law, which reveals the solid-phase concentration gradient mass transfer law, under the dual physical constraints of a specific temperature window of 500 °C to 550 °C and an excitation time not exceeding 5 minutes, the dynamic diffusion length of free oxygen atoms was physically limited to the 10 nm scale, forming a kinetic barrier that prevents external oxygen elements from penetrating deep into the particles. The matrix maintains the integrity of the crystal lattice structure; in which The value of manganese at the metal site ranges from 0.2 to 0.8. Under the synergistic effect of this specific temperature window and oxygen-rich state, the in-situ reconstruction process is completely governed by the principle of minimum thermodynamic energy, thus breaking the original random solid solution distribution. Driven by the local crystallization heat released by the phase transformation, heterogeneous ions undergo short-range ordered rearrangement. Neodymium ions, with the largest ionic radius, spontaneously accumulate at the vertices with the most abundant void space inside the polyhedron to minimize lattice distortion stress. Strontium ions, based on the local charge compensation effect, migrate directionally under thermal drive and are stably anchored at the edge center site. At the same time, nickel ions, with the smallest ionic radius and electron shell orbital preference, are precisely embedded at the face center site to maximize their orbital hybridization overlap with the surrounding oxygen atoms, driven by the physical drive of minimizing the crystal field stabilization energy. Thus, the absolute and precise spatial positioning of multi-component heterogeneous ions is achieved without external mechanical intervention.

[0033] Based on this, the device pressure was adjusted to 0.1 MPa and held at that temperature for 3 to 5 hours to eliminate interfacial lattice distortion through stress annealing. Finally, the modified manganese source was weighed and mixed with lithium, iron, phosphorus, and carbon sources according to stoichiometric ratios. In the specific weighing and mixing process, the gas-phase neodymium source precursor was tris(2,2,6,6-tetramethyl-3,5-heptadecanoic acid)neodymium, the gas-phase strontium source precursor was bis(2,2,6,6-tetramethyl-3,5-heptadecanoic acid)strontium, and the gas-phase nickel source precursor was bis(cyclopentadiene)nickel, i.e., nickel dicerocene. The manganese source used as the substrate was in sheet form. The carbon source can be any one or a combination of glucose, sucrose, lactose, maltose, and sorbitol. During preparation, strictly adhere to the stoichiometric ratio, weighing 0.07 mol of the pre-modified manganese source powder and adding 0.102 mol of lithium hydroxide monohydrate accordingly. 0.03 mol of ferric phosphate 0.01 mol of phosphoric acid The carbon source, comprising 6 wt% of the total raw and auxiliary materials, is added together with a polyvinylpyrrolidone dispersant comprising 0.01% to 0.05% of the total material weight. The mixture is vacuum dried at 100°C for 4 hours, and sintered at 730°C for 3 hours under a high-purity nitrogen protective atmosphere, with the global heating rate controlled at 2°C / min during the solid-phase reaction process, to obtain a modified lithium manganese iron phosphate cathode material. Testing showed that the interfacial charge transfer impedance of this material in the frequency range of 0.1Hz to 100kHz is 30% to 50% of the impedance of the unmodified matrix, and the discharge capacity retention rate at a 10C charge-discharge rate is not less than 85%. In the process condition test, when the above... The process conditions for the pure oxygen thermal shock reaction section were selected as sintering for 5 to 6 hours at a temperature range of 350℃ to 500℃, and then performing a corresponding cooling annealing treatment for 3 to 5 hours at a rate of 1℃ / min to 5℃ / min. The final sample was assembled into a lithium-ion coin cell and tested twice per cycle at a 1C rate. After 200 charge-discharge cycles, the initial discharge capacity was 153.7 mAh / g, the powder resistivity was 121.9 Ω·cm, the electrochemical capacity retention rate was 82% after 200 cycles, and the amount of manganese dissolved in the electrolyte was controlled at 68.59 ppm.

[0034] Example 2: In the experimental verification of the high-rate charge-discharge cycle stability of power batteries, an electrochemical testing system integrating a high and low temperature environmental chamber was used to evaluate the electrochemical response characteristics of modified lithium manganese iron phosphate materials. The data acquisition accuracy of the system was set to 0.1mV, the current response resolution was 10nA, and the polarization evolution under high-frequency pulse current was captured by a sampling frequency of 1kHz. To simulate electromagnetic interference under real working conditions, Gaussian white noise with a signal-to-noise ratio of 20dB and power frequency interference harmonics with a frequency of 50Hz were superimposed on the sampled signal. The sampling period was set according to the spectral bandwidth of the measured signal, that is, under the guidance of the Nyquist sampling theorem, the sampling period was set between 0.5s and 2s to balance data fidelity and storage load. The setting of the excitation temperature was controlled by two limiting factors: the surface nickel ion crystallization rate and the bulk manganese oxidation depth. When the temperature was in the 500℃ to 550℃ window, a perovskite-like structural phase could be induced within 5 minutes. Atomic-level lattice locking is achieved while thermodynamic hysteresis is used to block oxygen atoms from penetrating deep into the bulk phase. When the excitation temperature exceeds 550℃, the oxygen diffusion depth exceeds 15nm and causes matrix lattice distortion, resulting in a decrease in capacity retention to 78.4%. It should be noted that the atomic-level lattice phase locking achieved by non-stoichiometric oxygen defects is not based on mechanical obstruction in spatial structure, but on the basis of cross-interface electronic orbital correlation and recombination. Oxygen defects at the interface inevitably generate unpaired d-shell localized electrons around their adjacent transition metal atoms. These unpaired electrons spontaneously cross the phase boundary and undergo strong dpd orbital hybridization between the surface nickel atoms of the perovskite-like phase and the surface manganese and iron atoms of the lithium manganese iron phosphate matrix, thereby constructing a high-density cross-interface localized covalent bond network in situ. This enhanced underlying chemical bonding force is used to electronically offset and lock the overall surface volume expansion generated during lithium insertion / extraction cycles.

[0035] The test sample group includes the sample group of the present invention, which is prepared using the above-described process. And the strontium source doping ratio The value is 0.032. In specific process adaptation scenarios, the gas-phase neodymium source precursor used in preparing the sample group of this invention is switched to tris(isopropylcyclopentadiene)ne, the gas-phase nickel source precursor is switched to bis(ethylcyclopentadiene)nickel, i.e., 1,1'-diethyldicenocene, the gas-phase strontium source precursor is bis(pentamethylcyclopentadiene)strontium, and the manganese source as the substrate is high-purity manganese dioxide. The carbon source for in-situ carbonization is a homogeneous mixture of starch and cellulose. The slurry dispersant used is carboxymethyl cellulose (CMC), and the fluid solvent is N-methylpyrrolidone. The subsequent solid-phase reaction is completed under a pure argon atmosphere. When the reaction temperature of the pure oxygen thermal shock section is switched and adjusted to sintering at 450°C to 600°C for 5 to 6 hours, combined with an annealing stress relief process of 3 to 5 hours at a cooling rate of 1°C / min to 5°C / min, the resulting sample material of this invention is assembled into a lithium-ion coin cell. At 1C rate, it is tested twice per cycle. After 200 cycles of charge-discharge long-cycle testing, it exhibits an initial capacity of 157.2 mAh / g, a bulk resistivity of 88.09 Ω·cm, and a corresponding 200-cycle electrochemical capacity retention of 88%. Furthermore, the tested manganese dissolution is further reduced to 30.08 ppm. Control group A, which uses atomic layer deposition but lacks the asymmetric transformer reconstruction step, and control group B, which... The value is 0.1, which exceeds the preset range of 0.002 to 0.06. This is the doping ratio factor. The value is 0.6, representing the proportion of manganese at the metal sites. In the aforementioned Example 1, the in-situ coated infinite-layer precursor coating initially had a uniform composition distribution. The reason it transformed into a bidirectional, opposite concentration gradient after heat treatment was driven by the dynamic oxidation potential gradient applied by the asymmetric pressure-switching process. In the low-pressure, oxygen-deficient stage (0.01 MPa to 0.05 MPa), the outermost layer of the precursor first exposed a high density of oxygen vacancies. The concentration difference formed by these vacancies generated an inward penetrating force, preferentially driving the neodymium and strontium ions with larger ionic radii to diffuse in reverse into the matrix, forming a decreasing gradient. Distribution; When the ambient oxygen partial pressure abruptly jumps to the oxygen-rich shaping stage of 0.1 MPa to 0.3 MPa, the extremely high external oxygen potential field reverses and draws in nickel ions with stronger oxygen affinity from the coating interior, causing them to accumulate and crystallize on the outermost surface. Thus, by purely utilizing the physical lever of the step-like ambient oxygen pressure combined with the thermodynamic differences in oxygen affinity of various elements, in-situ decoupling and gradient recombination of heterogeneous ions within the uniform coating are achieved. Original input data confirms that the unmodified lithium manganese iron phosphate matrix reaches an initial polarization potential of 0.45 V at a 10C rate. According to the key intermediate data monitored, the interfacial charge transfer impedance of the sample group after thermal excitation is... The Ω decreased from the initial 125.4Ω to 42.6Ω, of which As the interfacial charge transfer impedance, in contrast, control group A, due to the lack of crystal phase reconstruction induced by a high oxygen partial pressure environment, still maintains an amorphous distribution of the surface precursor, leading to... The impedance value remained at 98.2Ω, making it impossible to establish a high-conductivity electron transport network. In control group B, under excessively high concentration doping, the excessive strontium ions caused interfacial lattice stress accumulation, resulting in microcrack evolution in the perovskite-like coating after 50 cycles at 10C, and the capacity retention rate deteriorated from the initial 88.2% to 71.5%.

[0036] The experimental results show that after 1000 cycles of 10C charge-discharge, the discharge capacity retention rate of the sample group of this invention is 87.6%, and the average median voltage decreases by 0.045V. This confirms that the present invention suppresses the Genyleryl distortion of manganese ions and solves the problem of the interface impedance continuously increasing with the cycle time through the atomic-level lattice locking mechanism of the perovskite-like phase. In the electrode preparation scenario to verify the high-rate charge-discharge characteristics of the modified lithium manganese iron phosphate cathode material, the modified lithium manganese iron phosphate cathode material, acetylene black as the conductive component, and polyvinylidene fluoride as the binder component are mixed in a mass ratio of 90:5:5. For example, N-methylpyrrolidone solvent was added to the mixture and placed in a dual planetary mixer. The solid content of the positive electrode slurry was maintained in the range of 45% to 55% by controlling the stirring time at 2000 rpm. The positive electrode slurry was coated on the surface of an aluminum foil with a thickness of 12 μm using an automatic coating machine. After vacuum drying at 120°C for 12 hours, the compaction density of the electrode was controlled in the range of 2.4 g / cm³ to 2.6 g / cm³ using a precision roller press. The electrode sample formed in this way was used to verify the inhibitory effect of the perovskite-like structure on manganese ion dissolution at a rate of 10C.

[0037] Example 3: When the specific surface area of ​​the manganese source precursor particles is 15... Up to 25 In continuous production conditions with fluctuations within a certain range, to eliminate the impact of uneven interfacial stress distribution on the stability of the perovskite-like structural phase, this scheme adopts a parameter calibration method based on reaction heat flow feedback. The heat flow changes during the asymmetric pressure transformation stage are collected in real time within the reaction unit. The sampling frequency of the heat flow sensor is set to 10Hz, with a resolution of 0.01J / s. By monitoring the heat of crystallization released instantaneously during the switching of the high-pressure oxygen environment, the cumulative heat flow value during the interfacial lattice reconstruction process is calculated. The duration of stress annealing is determined based on this physical quantity. .

[0038] Specifically, the duration of stress annealing treatment The following relationship must be satisfied: ,in, The duration of stress annealing. This represents the cumulative heat flux during the crystal phase reconstruction process. is the specific surface area of ​​the manganese source precursor. The stress compensation coefficient, k, is determined based on the principle of solid-state lattice relaxation kinetics. In the solid-state lattice relaxation kinetics system, the physical essence of this stress compensation coefficient k is the area-normalized time constant required for the precursor surface to release a unit crystallization enthalpy and complete lattice interface rearrangement. Its dimensionless unit is defined as h·m² / (g·J). Introducing this coefficient as a core adjustment parameter enables the processing algorithm of this system to directly convert the extremely complex surface lattice distortion stress relaxation process into an equivalent linear process constrained only by the combined constraints of the powder's absolute specific surface area and the overall exothermic heat release. The attenuation algebraic equation was used to establish the underlying dimensional consistency of its physical mapping derivation. At least three standard test samples of manganese source precursors with different nominal specific surface areas were selected, and the cumulative heat flux values ​​of the samples during the thermal reconstruction stage were recorded. Using in-situ monitoring with X-ray diffraction, the shortest annealing time required for the interface lattice mismatch to decrease to the critical threshold of 0.8% under constant pressure annealing was obtained. A scatter plot of the shortest annealing time versus the corresponding cumulative heat flux value and specific surface area quotient was established. Linear fitting was performed, and the slope of the fitted straight line was extracted as the stress compensation coefficient under production conditions. The baseline input value ranges from 0.12 to 0.15; in a production batch, when the manganese source precursor... It is 18.5 And the measured When the value is 125.8J, select It is 0.132, determined by calculation. For 0.9 hours, a constant-pressure annealing process was performed at 0.1 MPa, reducing the lattice mismatch at the interface between the perovskite-like phase and the lithium manganese iron phosphate matrix from 5.4% to below 0.8%, and stabilizing the non-stoichiometric oxygen defect volume fraction within the modified layer within the range of 2.5% to 3.8%. Experimental data confirmed that the modified lithium manganese iron phosphate material prepared using the above procedure exhibited consistent electrochemical performance across different production batches, with a capacity retention fluctuation of less than 0.5% over 1000 cycles, and an interfacial charge transfer impedance deviation controlled within 2.0 Hz in the 0.1 Hz to 100 kHz frequency range. Within this framework, the compensation mechanism eliminates the interference of material property fluctuations on internal structure locking, achieving stable construction in industrial environments. In the selected high-performance, high-consistency industrial construction batches in this group, the gas-phase neodymium source precursor is tris(N,N'-diisopropyl-2-methylamidinyl)neodymium, the gas-phase nickel source precursor is bis(methylcyclopentadiene)nickel, the gas-phase strontium source precursor is bis(triisopropylcyclopentadiene)strontium, and the manganese source for the matrix polyhedral construction is manganese sulfate. The added external carbon source is selected from at least one of citric acid, tartaric acid, and ascorbic acid. The dispersant used is hexadecyltrimethylammonium bromide (CTBA), and the solvent used for the mixed dissolving fluid is butanol. When this batch of material powder undergoes high-temperature solid-state crystal growth in an atmosphere furnace, the pure oxygen thermally stimulated crystal growth temperature range in the front stage is correspondingly optimized to 550°C to 700°C, with a holding time of 5 to 6 hours. Under this reconstructed state, the annealing time ta is controlled to 3 to 5 hours, and the subsequent solid-state sintering... The process was completed under a uniform inert gas atmosphere composed of high-purity nitrogen and pure argon in a volume ratio of 1:1. After testing, the modified cathode material prepared in this stable batch, when assembled into a coin cell, exhibited an excellent initial discharge capacity of 160.6 mAh / g and an extremely low intrinsic resistivity of 57.38 Ω·cm under two cycles at 1C. After 200 cycles, its capacity retention rate reached 97%, and the amount of manganese dissolved after cycling was reduced to 12.65 ppm.

[0039] Example 4: To clarify the microstructural boundaries and technical parameters of the modified layer, the concentration gradient distribution, atomic-level lattice phase-locked structure, three-dimensional interlocking anchoring network, and production parameter calibration mechanism all possess clear physical evolution characteristics and technical basis. The concentration gradient distribution of Nd:YAG, Sr:Nd ... In-situ decoupling and gradient recombination of multi-component heterogeneous ions; the physical essence of atomic-level lattice phase-locked structures is based on the cross-interface electronic orbital correlation and recombination at the phase interface; driven by asymmetric pressure-switching thermal irradiation, due to the presence of non-stoichiometric oxygen defects, unpaired electrons are inevitably generated around the transition metal atoms at the phase interface; these unpaired electrons spontaneously cross the phase boundary, and strong orbital hybridization occurs between the surface nickel atoms of the outer perovskite-like phase and the surface manganese and iron atoms of the inner lithium manganese iron phosphate matrix, thereby constructing a high-density cross-interface local covalent bond network in situ, realizing the chemical iso-lattice continuity locking of the two phase interfaces at the electronic structure level, effectively offsetting and locking the overall surface volume expansion generated during lithium insertion / extraction cycles.

[0040] The construction of a three-dimensional interlocking anchoring network refers to the short-range ordered rearrangement of heterogeneous ions under the dual physical drive of localized crystallization heat released during phase transformation and the environmental oxidation potential gradient, completely governed by the thermodynamic minimum energy law. Neodymium ions, with the largest ionic radius, spontaneously accumulate at the vertices with the most abundant void space within the crystal lattice polyhedrons. Strontium ions migrate directionally based on local charge compensation effects and are stably anchored at the edge centers. Nickel ions, with the smallest ionic radius and specific electron shell orbital preferences, are precisely embedded at the face centers. This absolutely precise spatial positioning of multi-component heterogeneous ions effectively constrains the crystal framework under the condition of complete lithium ion depletion, keeping the overall lattice volume shrinkage rate below 2%. Under continuous production conditions, to eliminate the influence of uneven interfacial stress distribution on structural stability, the duration of stress annealing is... The duration of stress annealing is determined by parameter calibration based on reaction heat flow feedback. It satisfies a specific linear positive correlation algebraic relationship, that is, the annealing duration is equal to the total crystallization heat flow value collected and accumulated by the heat flow sensor during the crystal phase reconstruction process, divided by the absolute specific surface area of ​​the manganese source precursor, and then multiplied by the stress compensation coefficient determined by solid phase lattice relaxation kinetics calibration. The physical essence of the stress compensation coefficient is the area normalized time constant required for the precursor surface to release unit crystallization enthalpy and complete lattice interface rearrangement, and its value ranges from 0.12 to 0.15. This mechanism converts the complex surface lattice distortion stress relaxation process into a linear algebraic equation constrained by the absolute specific surface area of ​​the powder and the total exothermic heat release, ensuring the stability and reliability of the modified structure and the batch stability of electrochemical performance under industrial mass production conditions.

[0041] Example 5: In determining the oxygen partial pressure setting parameters, the mass change of the manganese source precursor was measured in a thermogravimetric analyzer with a resolution of 1 μg. Under isothermal conditions of 300℃ to 350℃, the proportion of ambient oxygen was adjusted at intervals of 0.005 MPa to obtain the equilibrium curve of sample weight change with ambient oxygen partial pressure. Based on the law of conservation of mass, a quantitative mapping mechanism between weight loss and surface defect volume fraction was constructed to eliminate the interference of early free moisture volatilization. It was determined that the mass change of the manganese source precursor during the isothermal plateau period of 300℃ to 350℃ was solely attributed to the release of lattice oxygen atoms. The absolute mass decrease value of the thermogravimetric analyzer was collected, and the relative proportion with the initial total lattice oxygen mass of the precursor sample was calculated. Combining the proportion with the geometric conversion relationship of the theoretical unit cell volume of the precursor, the non-stoichiometric oxygen defect volume fraction under a specific oxygen partial pressure steady state was calculated. The oxygen vacancy concentration at different pressure points is calculated and stored in a preset mapping table of the control system, thereby adjusting the ambient oxygen partial pressure. Compared with non-stoichiometric oxygen defect volume fraction The numerical correlation between them is used to set the pressure during the hypoxia-induced diffusion stage in asymmetric pressure swing thermal shock, where... The partial pressure of oxygen in the environment, To determine the non-stoichiometric oxygen defect volume fraction, and to obtain the surface volume density of non-stoichiometric oxygen defects within the aforementioned 2nm to 10nm perovskite-like modified layer, the control system, based on a spatial geometric modeling algorithm, multiplies the nominal specific surface area of ​​the powder by the set coating thickness to obtain the absolute physical volume of the precursor shell participating in the modification reaction. Using Avogadro's constant, the total number of moles of overall defects calculated based on thermogravimetric data is converted into the absolute number of missing oxygen atoms on the surface. Finally, the extracted absolute number of oxygen atoms is directly divided by the aforementioned derived shell physical volume. Through this deterministic mapping path combining pure overall thermodynamic parameters with spatial geometric conversion, a precise output of 10 is obtained without the need for destructive characterization instruments. 18cm-3 Up to 10 20cm-3 The absolute quantitative volume density value.

[0042] When adapting process parameters between fluidized bed reactors of different specifications, pressure sensors monitor the real-time pressure change curve of the reaction chamber from 0.05 MPa to 0.2 MPa, based on the volume of the reaction chamber. The replenishment rate of the high-pressure oxygen source was determined, and the response time constant of the controller was adjusted to keep the linearity deviation of the pressure ramp-up curve within 5%. This ensured that the ambient oxygen partial pressure jumped from 0.05 MPa to 0.2 MPa within 5 minutes. During the thermal shock process, the driving force generated by the pressure gradient guided surface nickel ions to undergo crystal phase reorganization towards the perovskite-like structure phase. In this specific parameter control scenario, the gaseous neodymium source precursor used was tris(N,N'-diisopropyl-2-dimethylaminoguanidinyl)neodymium, the gaseous nickel source precursor was bis(acetylacetone)nickel, and the gaseous strontium source precursor was bis(2,2,6,6-tetramethyl-3,5-heptadecyl)strontium. The manganese source for the matrix crystal growth reaction was selected from either manganese oxalate monohydrate MnC2O4·2H2O or manganese phosphate Mn3(PO4)2. The composite introduced carbon source was lauric acid or alginic acid, combined with... The particle barrier dispersant used was polyethylene glycol (PEG), and the solvent used for mixing the reaction fluid was an equal volume ratio of anhydrous ethanol and butanol. High-purity nitrogen was introduced into the tube furnace as an inert protective atmosphere for subsequent heat treatment and sintering steps. When the process reaction temperature during the pure oxygen thermal shock stage was stabilized at 550℃ to 700℃ and the sintering time was 5 to 6 hours, but the annealing time was extended to 8 to 10 hours (cooling rate controlled at 1℃ / min to 5℃ / min), the modified cathode material obtained by crystal growth, when assembled into a coin cell and tested at 1C, exhibited an initial capacity of 158.7 mAh / g, a resistivity of 63.27 Ω·cm, and a capacity retention rate of 87% after 200 charge-discharge cycles. Furthermore, due to long-term thermal stress relaxation of the crystal lattice, the manganese dissolution was extremely suppressed to a trace level of 3.52 ppm. Example 6: In an atomic layer deposition process adapted to fluctuations in the saturated vapor pressure of a gas-phase precursor, precursor pulse timing is established. To maintain the consistency of the modified layer thickness, a reference calibration relationship is used. Specifically, real-time saturated vapor pressure data of gaseous neodymium source, gaseous nickel source and gaseous strontium source are collected in the feeding tank. The evolution curve of precursor concentration over time is monitored using a residual gas analyzer installed at the outlet of the deposition chamber. When the ion concentration at the monitoring point reaches 95% of the saturation concentration, the pulse duration at this time is recorded. This time parameter is written into the register of the fluidized bed controller as a control parameter. The resulting infinite-layer precursor coating maintains a single-layer thickness in the range of 0.5nm to 1.5nm.

[0043] When faced with the flow response delay during the high partial pressure switching phase of a large-capacity atmosphere furnace, a method based on the volumetric response time constant is adopted. The flow compensation method involves starting a pressure boost experiment from 0.05 MPa to 0.2 MPa in an empty furnace to obtain the system's pressure response envelope. A proportional-integral-derivative (PID) control algorithm is used to map the adjustment step size of the oxygen supply valve. The intake flow is adjusted in real-time based on the difference between the ambient oxygen partial pressure and the preset gradient, ensuring that the partial pressure jump during the excitation phase completes the transition from 0.05 MPa to 0.2 MPa within 5 minutes. Under thermodynamic driving force, surface nickel ions recombine in situ with neodymium and strontium ions that have penetrated into the crystal lattice to form a perovskite-like structural phase with a thickness of 2 nm to 10 nm. The resulting material maintains an impedance modulus of 50 at a frequency of 0.1 Hz as measured in an electrochemical workstation. The following describes the in-situ gradient modified layer structure of this invention, which maintains high selectivity compatibility even under the industrial atomic layer deposition (ALD) conditions of drastic fluctuations in the saturated vapor pressure of a fluidized bed reactor. Specifically, the gas-phase neodymium source precursor is tris(2-methoxyethoxy)neodymium, the gas-phase nickel source precursor is nickel acetylacetone hydrate, the gas-phase strontium source precursor is bis(pentamethylcyclopentadiene)strontium, and the manganese source for the synthetic matrix is ​​selected from manganese chloride. or manganese acetate One of the following: The carbon source introduced in the subsequent slurry mixing step includes at least one of the following: phenolic resin (phenol-formaldehyde polymer), furfural resin (furan-formaldehyde polymer), urea-formaldehyde resin (urea-formaldehyde polymer), water-soluble epoxy resin (epoxychloropropane-bisphenol A polymer), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylamide (PAM), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), chitosan, and melamine. The appropriate dispersant is polyvinyl ester. When this batch of powder undergoes the final solid-state reaction sintering in the later stage, the protective atmosphere is either pure helium or an argon-helium mixed inert protective gas atmosphere with a volume ratio of 1:2. Tests show that the above-mentioned outstanding ultra-low charge transfer impedance and volume distortion suppression effect can still be achieved.

[0044] The embodiments of this application have been described above with reference to the accompanying drawings. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. This application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit of this application and the scope of protection of this invention, and all of these forms are within the protection scope of this application.

Claims

1. A high-rate, long-cycle lithium manganese iron phosphate cathode material, characterized in that, Includes a lithium manganese iron phosphate matrix and a modified layer in situ coated on the surface of the lithium manganese iron phosphate matrix: The modified layer has neodymium and strontium ions distributed in decreasing concentration from the surface of the lithium manganese iron phosphate substrate, and nickel ions distributed in increasing concentration from the surface of the lithium manganese iron phosphate substrate. The outermost layer of the modified layer has a perovskite-like structure phase composed of neodymium ions, strontium ions and nickel ions, and the thickness of the perovskite-like structure phase is 2 nm to 10 nm. The modified layer contains non-stoichiometric oxygen defects that serve as diffusion channels for guiding neodymium ions, strontium ions, and nickel ions to penetrate deep into the bulk phase of the lithium manganese iron phosphate matrix. These non-stoichiometric oxygen defects are used to balance the local potential generated by heterogeneous ion doping within the modified layer. The interface between the perovskite-like phase and the lithium manganese iron phosphate matrix has an atomic-level lattice phase-locked structure achieved through non-stoichiometric oxygen defects. Inside the modified layer, neodymium ions, strontium ions, and nickel ions precisely occupy specific sites around the manganese oxygen octahedra in the lithium manganese iron phosphate matrix lattice to form a three-dimensional interlocking anchoring network. The neodymium ion content is from 0.05 wt% to 0.3 wt%. The strontium ion content is 0.1 wt% to 0.5 wt%; The nickel ion content is 0.2 wt% to 0.8 wt%; The balance of the nickel-based modified lithium manganese iron phosphate formulation is the lithium manganese iron phosphate matrix.

2. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, Neodymium, strontium, and nickel ions in the modified layer constrain the volume deformation of manganese-oxygen octahedra in the lithium manganese iron phosphate matrix under charge-discharge delithiation state through lattice anchoring effect; the width of lithium ion diffusion channels in the modified layer is 15% to 25% wider than that of the unmodified lithium manganese iron phosphate matrix.

3. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The molecular formula of lithium manganese iron phosphate matrix is ,in, The lithium manganese iron phosphate matrix consists of hollow microspheres or near-spherical particles; the average particle size of the lithium manganese iron phosphate matrix is ​​0.5 μm to 5 μm.

4. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The bulk density of non-stoichiometric oxygen defects within the modified layer is to The slopes of the concentration gradients of neodymium, strontium, and nickel ions within the modified layer are monotonically positively correlated with the radial concentration distribution of non-stoichiometric oxygen defects.

5. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The perovskite-like structure phase has a neodymium-strontium-nickel composite oxide crystal structure; the interfacial charge transfer impedance of the perovskite-like structure phase in the frequency range of 0.1 Hz to 100 kHz is 30% to 50% lower than that of the unmodified lithium manganese iron phosphate matrix.

6. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In the three-dimensional interlocked anchoring network, neodymium ions occupy the vertex corner positions, strontium ions occupy the edge center positions, and nickel ions occupy the face center positions; the lattice volume shrinkage rate of the three-dimensional interlocked anchoring network in the state of complete lithium ion depletion is less than 2%.

7. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The internal lattice of the lithium manganese iron phosphate matrix remains intact and there is no manganese element segregation; the perovskite-like structure phase is an atomically dense layer continuously coated on the surface of the lithium manganese iron phosphate matrix; the residual lithium content on the surface of manganese source particles in the nickel-based modified lithium manganese iron phosphate formulation is less than 500 ppm.

8. The high-rate, long-cycle lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The compacted density of the nickel-based modified manganese iron phosphate formulation is not less than 2.4 g / L. The nickel-based modified lithium manganese iron phosphate formulation retains no less than 85% of its discharge capacity at 10C rate and at 0.1C rate.

9. A method for preparing a high-rate, long-cycle lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: The nickel-based modified lithium manganese iron phosphate formulation was obtained by atomic layer deposition combined with an asymmetric pressure swing thermal quenching process. The asymmetric pressure swing thermal quenching process includes an oxygen-deficient diffusion stage with an oxygen partial pressure of 0.01 MPa to 0.05 MPa and an oxygen-enriched shaping stage with an oxygen partial pressure of 0.1 MPa to 0.3 MPa.

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