Low-temperature fast-charging lithium-supplementing positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, and particularly discloses a low-temperature fast-charging lithium-supplementing positive electrode material and a preparation method thereof. The low-temperature fast-charging lithium-supplementing positive electrode material specifically comprises composite particles formed by mixing lithium manganese iron phosphate and fluorinated lithium squarate and then carrying out high-energy ball milling, and inner and outer carbon coating layers coating the surfaces of the composite particles, wherein the lithium manganese iron phosphate is used as a main active material, the chemical formula of the lithium manganese iron phosphate is LiMnxFe1-xPO4, and x is more than or equal to 0.5 and less than or equal to 0.8; the fluorinated lithium squarate is used as a lithium supplementing agent, and the chemical formula is Li2C4O3F; the thickness of the inner carbon coating layer is 5-10 nm, and the thickness of the outer carbon coating layer is 10-20 nm. Through the synergistic effect of the lithium manganese iron phosphate and the fluorinated lithium squarate, the problems of poor low-temperature performance and fast charging bottleneck of the lithium manganese iron phosphate are solved, and the effects of low-temperature fast charging and simultaneous prolonging of the cycle life are achieved.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion battery cathode materials. More specifically, it relates to a low-temperature fast-charging lithium-supplementing cathode material and a preparation method thereof. Background Art

[0002] As a core energy storage device in the new energy field, the performance of the cathode material of a lithium-ion battery directly determines the energy density, cycle life, and environmental adaptability of the battery. Currently, commercially available cathode materials such as lithium cobalt oxide (LCO), ternary materials (NCM / NCA), and lithium iron phosphate (LFP) face significant technical bottlenecks in applications. First, at low temperatures (e.g., below -20 °C), the lithium-ion diffusion rate of traditional cathode materials drops sharply, resulting in a capacity attenuation rate exceeding 50% and a charging polarization voltage rising above 200 mV, severely limiting the applicability of electric vehicles in cold regions. Second, during high-rate charging (>3C), the mismatch between the lithium-ion transport rate and the electron conduction rate inside the electrode leads to safety hazards such as lithium metal precipitation and local overheating. In addition, a large amount of active lithium is consumed during the first charge and discharge process of the cathode material due to the formation of the solid electrolyte interface film (SEI film), resulting in irreversible capacity loss. The first Coulombic efficiency of lithium iron phosphate is usually lower than 90%, further restricting the improvement of the battery energy density.

[0003] To compensate for lithium source loss, existing technologies mostly adopt pre-lithiation strategies, but their limitations are significant. For example, the process of pre-lithiation with a lithium metal foil is complex and there is a risk of lithium dendrite growth; lithium-rich compounds (such as Li5FeO4, Li2NiO2) need to be activated under high voltage (>4.5 V), which exacerbates electrolyte decomposition and interfacial side reactions; although the silicon-based lithium-supplementing material has a high capacity, its volume expansion rate exceeds 300%, easily causing the collapse of the electrode structure. At the same time, lithium manganese iron phosphate (LiMn x Fe 1-x PO4, LMFP), which combines high safety and moderate energy density, has become a research hotspot. Its voltage platform (3.8 - 4.1 V) is significantly higher than that of LFP (3.4 V), and the theoretical energy density is increased by 15 - 20%. However, the manganese ion dissolution phenomenon of LMFP leads to a decrease in cycle stability (capacity retention rate after 1000 cycles < 85%), and its intrinsic electronic conductivity is low (<10 -9 S / cm), relying on carbon coating or nanosizing modification. The problem of insufficient ion diffusion coefficient (<10 -12 cm 2 / s) at low temperatures remains unresolved.

[0004] On the other hand, squaric acid lithium (Li2C4O4), as an organic lithium salt, has a high ionic conductivity (>10 -4(S / cm, -20 °C) and fast lithium-ion release characteristics have been explored for fast-charging systems, but their individual use has defects such as poor electrochemical stability (decomposition voltage < 4.0 V) and significant volume expansion. In the prior art, the structural stability of LMFP and the ion transport advantages of lithium squarate have not been effectively integrated, and there is a lack of material design for the synergistic enhancement of low-temperature fast charging and lithium supplementation.

[0005] In the existing publicly disclosed technologies, although there have been studies attempting to improve the performance of LMFP through composite modification. For example, Patent CN118173765A proposes carbon-coated LMFP to improve conductivity, but it does not involve the integration of lithium supplementation function; Patent CN119965251A discloses a method for preparing porous LMFP using metal-organic framework (MOF) precursors, but still does not solve the synergistic problem of low-temperature fast charging and lithium loss compensation. Therefore, developing an innovative cathode material based on the composite of LMFP and lithium squarate and achieving performance breakthroughs through structural design and process optimization has become the key technical path to promote the development of electric vehicles and high-power energy storage devices in alpine regions. Summary of the Invention

[0006] To solve the above technical problems, the present application provides a low-temperature fast-charging lithium-supplementing cathode material and its preparation method. Through the synergistic effect of lithium iron manganese phosphate and lithium fluorosquarate, the present application solves the problems of poor low-temperature performance and fast-charging bottleneck of lithium iron manganese phosphate, and achieves the effect of simultaneously improving low-temperature fast charging and cycle life.

[0007] In the first aspect, the present application provides a low-temperature fast-charging lithium-supplementing cathode material, adopting the following technical solution: A low-temperature fast-charging lithium-supplementing cathode material, comprising: Composite particles formed by high-energy ball milling after mixing lithium iron manganese phosphate and lithium fluorosquarate, and inner and outer carbon coating layers coated on the surface of the composite particles; Among them, The lithium iron manganese phosphate serves as the main active material, and its chemical formula is LiMn x Fe 1-x PO4, 0.5 ≤ x ≤ 0.8; The lithium fluorosquarate serves as a lithium supplementing agent, and its chemical formula is Li2C4O3F; The thickness of the inner carbon coating layer is 5 - 10 nm, and the thickness of the outer carbon coating layer is 10 - 20 nm.

[0008] In this application, lithium iron manganese phosphate is combined with lithium fluorosquarate. By utilizing the synergistic effect of the high-voltage platform (3.8 - 4.1 V) of lithium iron manganese phosphate and the fast lithium-ion release characteristics of lithium fluorosquarate, three-dimensional channels with high ion mobility can be constructed at low temperatures by filling the interstices of lithium iron manganese phosphate particles with lithium fluorosquarate. In addition, during charge and discharge, lithium fluorosquarate will partially decompose to generate LiF, which can cooperate with the components in the electrolyte to form a dense SEI film, further inhibiting manganese dissolution and structural collapse.

[0009] In this application, fluorine atoms are used to partially replace oxygen atoms in squaric acid lithium to obtain fluorine-doped squaric acid lithium. By using the high electronegativity of F - to stabilize the crystal structure and inhibit side reactions during charge and discharge.

[0010] Adopting a structure with two layers of carbon coating can not only improve the electronic conductivity but also prevent the manganese dissolution phenomenon caused by the expansion of composite particles.

[0011] Further preferably, after being mixed, the lithium iron manganese phosphate and lithium fluorosquarate are subjected to high-energy ball milling treatment to achieve nanoscale mixing.

[0012] Further preferably, the lithium iron manganese phosphate is impregnated with an aqueous phosphoric acid solution before being mixed with lithium fluorosquarate.

[0013] Further preferably, the lithium iron manganese phosphate is synthesized by a sol-gel method to obtain a lithium iron manganese phosphate precursor, and then obtained through high-temperature sintering.

[0014] Further preferably, the preparation steps of the lithium iron manganese phosphate are as follows: Dissolve lithium carbonate, manganese acetate, iron oxalate, and ammonium dihydrogen phosphate in deionized water, add citric acid and ethylene glycol, and heat and stir to form a sol; Vacuum dry the sol at 120 °C to obtain a porous gel precursor; The porous gel precursor is pre-fired and sintered successively under argon protection to obtain nanoscale lithium iron manganese phosphate.

[0015] Preferably, the pre-firing temperature is 300 - 320 °C, and the pre-firing time is 3.5 - 4 h to remove organic substances and initially form a Li-Mn-P-O framework.

[0016] Preferably, the specific sintering steps are as follows: Raise the temperature to 700 - 710 °C at a rate of 5 - 8 °C / min, keep it warm for 10 h, and then cool it naturally.

[0017] Preferably, the particle size of the lithium iron manganese phosphate is 80 - 150 nm.

[0018] Further preferably, the lithium fluorosquarate is prepared by reacting squaric acid and lithium fluoride as raw materials.

[0019] Further preferably, the preparation steps of the lithium fluorosquarate are as follows: Add squaric acid and lithium fluoride into ethylene glycol, react at 180 °C for 12 h, after centrifugal washing, dry in vacuum at 60 °C to obtain lithium fluorosquarate powder.

[0020] Preferably, the molar ratio of squaric acid to lithium fluoride is 1:(2 - 2.2).

[0021] Preferably, the particle size of the lithium fluorosquarate is less than 80 nm.

[0022] In a second aspect, the present application provides a preparation method of a low-temperature fast-charging lithium-supplemented cathode material, adopting the following technical solution: A preparation method of a low-temperature fast-charging lithium-supplemented cathode material, the specific preparation steps are as follows: Mix lithium iron phosphate manganese and lithium fluorosquarate and perform high-energy ball milling treatment to obtain composite particles, and successively coat inner and outer carbon coating layers on the surface of the composite particles. The inner carbon coating layer is coated by chemical vapor deposition method with a thickness of 5 - 10 nm, and the outer carbon coating layer is coated by carbothermal reduction method with a thickness of 10 - 20 nm.

[0023] By adopting the high-energy ball milling method, lithium fluorosquarate can be embedded in the gaps between lithium iron phosphate manganese particles in the form of nanoparticles to form a continuous ion conduction path. Lithium fluorosquarate releases lithium ions during the first charge, compensates for the consumption of SEI film formation, and improves the first Coulomb efficiency to more than 95%.

[0024] During charge and discharge, lithium iron phosphate manganese will have a volume change of about 10% due to the Jahn-Teller distortion of manganese ions and the insertion / extraction of lithium ions. Long-term cycling may lead to particle swelling and the appearance of microcracks, exacerbating the dissolution of manganese; in addition, as an organic lithium salt, lithium fluorosquarate may be accompanied by volume expansion when releasing lithium ions during the first charge. If the binding force between the carbon coating layer and the matrix is insufficient, it is easy to cause interface peeling. The present application adopts inner and outer carbon coating layers. The carbon coating layer coated by chemical vapor deposition method is an amorphous carbon flexible buffer layer, providing elastic buffer ability to adapt to the volume expansion of the composite particles. The carbon coating layer coated by carbothermal reduction method is a rigid conductive layer, improving electron conductivity and ensuring the structural stability of the composite particles.

[0025] Further preferably, the specific steps of the chemical vapor deposition method are as follows: Take the mixed gas of methane and argon as the carbon source, deposit at 500 °C for 2 h, and after cooling, it is obtained.

[0026] Further preferably, the specific steps of the carbothermal reduction method are as follows: adding the composite particles into an aqueous glucose solution, stirring and then performing ultrasonic treatment to obtain a mixed slurry, drying to obtain a glucose-coated precursor, placing the precursor in an argon atmosphere, heating to 300-320°C at a rate of 5-10°C / min, holding for 1-1.5 h, then continuing to heat to 800-810°C at a rate of 5-10°C / min, holding for 2.5-3 h, and grinding the product after natural cooling to obtain the product.

[0027] In summary, the present application has the following beneficial effects: (1) By using the high-energy ball milling method, lithium fluorosquarate can be embedded in the gaps between lithium iron phosphate manganese particles in the form of nanoparticles to form a continuous ion conduction path. Lithium fluorosquarate releases lithium ions during the first charge, compensates for the consumption of SEI film formation, and improves the first Coulomb efficiency to more than 95%; (2) The present application uses two layers of carbon, inner and outer, to coat the composite particles of lithium iron phosphate manganese and lithium fluorosquarate. The inner flexible carbon absorbs the volume change stress through elastic deformation and is in direct contact with the active material to reduce the interfacial impedance. The outer rigid carbon maintains the overall structural stability, avoids crack propagation, and improves the conductivity; (3) The positive electrode material prepared in the present application has a discharge capacity retention rate of >85% at -40°C at 0.5C, which is 40% higher than that of pure lithium iron phosphate manganese; it supports fast charging at 10C, and the capacity retention rate is >90% after 2000 cycles. Specific Embodiments

[0028] The following specific examples illustrate the implementation manners of the present application. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in this specification. The present application can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present application.

[0029] In addition, it should be understood that one or more method steps mentioned in the present application do not exclude the existence of other method steps before and after the combined steps or the insertion of other method steps between these clearly mentioned steps, unless otherwise stated. Moreover, unless otherwise stated, the numbers of the method steps are only convenient tools for identifying the method steps, rather than limiting the arrangement order of the method steps or the scope of implementation of the present application. The change or adjustment of their relative relationships, without substantial change in the technical content, should also be regarded as the scope of implementation of the present application.

[0030] If the specific experimental conditions are not specified in the examples, they are usually in accordance with the conventional conditions in the art or the conditions recommended by the reagent company; the materials, reagents, etc. used in the examples, unless otherwise specified, can all be obtained through commercial channels.

[0031] Preparation Example Preparation Example 1 Preparation of lithium iron manganese phosphate: 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate were dissolved in 200 mL of deionized water. 1.5 mol of citric acid and 1.8 mol of ethylene glycol were added, and the mixture was heated to 80 °C and stirred for 6 h to form a viscous sol. The sol was dried in vacuo at 120 °C for 12 h to obtain a porous gel precursor. The porous gel precursor was pre-calcined at 300 °C for 4 h under argon protection, then heated to 700 °C at a rate of 5 °C / min, held for 10 h, and naturally cooled to obtain nanoscale lithium iron manganese phosphate LiMn 0.5 Fe 0.5 PO4 with a particle size distribution in the range of 80 - 150 nm.

[0032] Preparation Example 2 Preparation of lithium iron manganese phosphate: 0.525 mol of lithium carbonate, 0.7 mol of manganese acetate, 0.3 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate were dissolved in 200 mL of deionized water. 1.5 mol of citric acid and 1.8 mol of ethylene glycol were added, and the mixture was heated to 80 °C and stirred for 6 h to form a viscous sol. The sol was dried in vacuo at 120 °C for 12 h to obtain a porous gel precursor. The porous gel precursor was pre-calcined at 300 °C for 4 h under argon protection, then heated to 700 °C at a rate of 5 °C / min, held for 10 h, and naturally cooled to obtain nanoscale lithium iron manganese phosphate LiMn 0.7 Fe 0.3 PO4 with a particle size distribution in the range of 80 - 150 nm.

[0033] Preparation Example 3 Preparation of lithium iron manganese phosphate: 0.525 mol of lithium carbonate, 0.8 mol of manganese acetate, 0.2 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate were dissolved in 200 mL of deionized water. 1.5 mol of citric acid and 1.8 mol of ethylene glycol were added, and the mixture was heated to 80 °C and stirred for 6 h to form a viscous sol. The sol was dried in vacuo at 120 °C for 12 h to obtain a porous gel precursor. The porous gel precursor was pre-calcined at 300 °C for 4 h under argon protection, then heated to 700 °C at a rate of 5 °C / min, held for 10 h, and naturally cooled to obtain nanoscale lithium iron manganese phosphate LiMn 0.8 Fe 0.2 PO4 with a particle size distribution in the range of 80 - 150 nm.

[0034] Preparation Example 4 Preparation of lithium fluorosquarate: Take 1 mol of squaric acid and 2.2 mol of lithium fluoride, add them to 200 ml of ethylene glycol, react at 180 °C for 12 h, after centrifugal washing, dry in vacuum at 60 °C to obtain lithium fluorosquarate powder Li2C4O3F with a particle size distribution less than 80 nm.

[0035] Example Example 1 Preparation of low-temperature fast-charging lithium-supplementing cathode material: Mix the lithium iron manganese phosphate obtained in Preparation Example 1 and the lithium fluorosquarate obtained in Preparation Example 4 according to a mass ratio of 85%:15%, add zirconia milling beads, control the ball-to-material ratio to 10:1, and process in a high-energy ball mill at a rotation speed of 400 rpm for 3 h to embed the lithium fluorosquarate particles into the gaps between the lithium iron manganese phosphate particles to obtain composite particles.

[0036] Place the composite particles in a tubular furnace, introduce a mixed gas of argon and methane with a volume ratio of 9:1, the flow rate of the mixed gas is 220 sccm, react at 500 °C for 2 h to form an amorphous inner carbon coating layer with a thickness of 5 nm on the surface of the composite particles.

[0037] Add the composite particles coated with the inner carbon coating layer to an aqueous glucose solution, control the mass ratio of the composite particles to glucose to 20:1, stir at 500 rpm for 1 h, then ultrasonically treat at 300 W and 40 kHz for 30 min to obtain a mixed slurry. After drying at 80 °C for 12 h, a glucose-coated precursor is obtained. Place the precursor in an argon atmosphere, heat it to 300 °C at a rate of 5 °C / min, hold for 1 h, then continue to heat to 800 °C at a rate of 5 °C / min, hold for 3 h, and grind the product after natural cooling to obtain a lithium-supplementing cathode material coated with an outer carbon coating layer with a thickness of 10 nm.

[0038] Example 2 Preparation of low-temperature fast-charging lithium-supplementing cathode material: The difference from Example 1 is that the lithium iron manganese phosphate is obtained from Preparation Example 2.

[0039] Example 3 Preparation of low-temperature fast-charging lithium-supplementing cathode material: The difference from Example 1 is that the lithium iron manganese phosphate is obtained from Preparation Example 3.

[0040] Example 4 Preparation of low-temperature fast-charging lithium-supplementing cathode material: The difference from Example 1 is that the mass ratio of lithium iron manganese phosphate to lithium fluorosquarate is controlled to 90%:10%.

[0041] Example 5 Preparation of low-temperature fast-charging lithium-supplementing cathode material: The difference from Example 1 is that the mass ratio of lithium iron manganese phosphate to lithium fluorosquarate is controlled to 95%:5%.

[0042] Example 6 Preparation of low-temperature fast-charging lithium-supplementing cathode material: Different from Example 1, lithium iron phosphate manganese is impregnated with phosphoric acid aqueous solution before being mixed with lithium fluorosquarate. The specific impregnation steps are as follows: Immerse lithium iron phosphate manganese into 3% phosphoric acid aqueous solution, treat it in a water bath at 60 °C for 1 h, wash it with water and then dry it to obtain lithium iron phosphate manganese with active -PO4H groups generated on the surface. This step of treatment can improve the interfacial binding force with lithium fluorosquarate and enhance the lithium ion transport efficiency.

[0043] Comparative Example Comparative Example 1 Preparation of lithium-supplementing cathode material: The preparation method is the same as that of Example 1, except that: Lithium squarate is used instead of lithium fluorosquarate.

[0044] Comparative Example 2 Preparation of lithium-supplementing cathode material: The raw material components are the same as those of Example 1, except that: Lithium iron phosphate manganese and lithium fluorosquarate are ball-milled separately and then mixed. The ball-milling conditions are the same as those of Example 1.

[0045] Comparative Example 3 Preparation of lithium-supplementing cathode material: The raw material components are the same as those of Example 1, except that: Only a carbon coating layer is coated on the surface of the composite particles by chemical vapor deposition method, and the thickness of the carbon coating layer is controlled to be 15 nm.

[0046] Comparative Example 4 Preparation of lithium-supplementing cathode material: The raw material components are the same as those of Example 1, except that: Only a carbon coating layer is coated on the surface of the composite particles by carbothermal reduction method, and the thickness of the carbon coating layer is controlled to be 15 nm.

[0047] Performance detection test Add the lithium-supplementing cathode materials prepared in each example and preparation example, conductive carbon black, and PVDF to N-methyl-2-pyrrolidone according to a mass ratio of 8:1:1, mix them evenly to make a slurry, coat the slurry on the aluminum foil, and dry it in an oven at 80 °C to make a positive electrode plate. Assemble the positive electrode plate into a button battery. The electrolyte is 1 mol / L LiPF6 (EC:DMC = 1:1), the negative electrode uses a lithium sheet, and the separator uses a Celgard 2400 polypropylene porous membrane.

[0048] 1. Initial coulombic efficiency: At 25 °C, charge and discharge the battery with a current of 0.1C, with a voltage of 2.5 - 4.2V, and calculate the initial coulombic efficiency as discharge capacity / charge capacity × 100%.

[0049] 2. High-rate cycle stability: At 25 °C, charge and discharge the battery with a current of 10C, and calculate the battery capacity retention rate after 3000 cycles.

[0050] 3. Low-temperature fast charging performance test: Charge the battery with a current of 0.5C at 25°C. After 5 minutes, discharge it with a current of 0.2C to 2.5V, record the capacity and denote it as Capacity 1. After recharging the battery with 0.5C, place the battery in an environment of -30°C and start discharging with a current of 0.2C, record the capacity and denote it as Capacity 2. The capacity retention rate of the battery at -30°C is Capacity 2 / Capacity 1×100%.

[0051] Charge the battery with a current of 5C in an environment of -30°C and record the time taken to charge to 80%.

[0052] The test results are recorded in Table 1 below.

[0053] Table 1 Test Results It can be seen from the test results in Table 1 that the positive electrode material prepared in this application has excellent low-temperature fast charging and long-life compatibility. It takes about 10 minutes to charge to 80% at -30°C with a current of 5C, and the retention rate after 3000 cycles is about 90%. The performance is significantly better than the prior art.

[0054] It can be seen from the test results in Table 1 that Example 6 is the optimal example. Comparing it with Example 1, it can be seen that when preparing the positive electrode material in Example 6, lithium iron manganese phosphate is first impregnated with phosphoric acid aqueous solution and then subjected to composite ball milling, and the performance of the obtained material is significantly improved, especially the improvement in high-rate long-cycle stability is obvious. It shows that the impregnation treatment with phosphoric acid aqueous solution can improve the binding force between lithium iron manganese phosphate and lithium fluorosquarate, ensure the stability of the ion transport channel, and thus improve the cycle stability of the material.

[0055] The above is only the preferred embodiment of the present invention, and it is not a limitation to any form and essence of the present invention. It should be pointed out that for those of ordinary skill in the art in this technical field, without departing from the method of the present invention, several improvements and supplements can still be made, and these improvements and supplements should also be regarded as the protection scope of the present invention. Those who are familiar with this professional technology, without departing from the spirit and scope of the present invention, when making some changes, modifications and evolutions of equivalent changes using the technical content disclosed above, are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A low-temperature fast-charging lithium-supplementing cathode material, characterized in that, Including: Composite particles formed by high-energy ball milling after mixing lithium iron manganese phosphate and lithium fluorosquarate, and inner and outer carbon coating layers coated on the surface of the composite particles; Wherein, The lithium iron manganese phosphate is used as the main active material, and the chemical formula is LiMn x Fe 1-x PO4, 0.5≤x≤0.8; The lithium fluorosquarate is used as a lithium supplement agent, and its chemical formula is Li2C4O3F; The thickness of the inner carbon coating layer is 5 - 10 nm, and the thickness of the outer carbon coating layer is 10 - 20 nm.

2. The low-temperature fast-charging lithium-supplementing cathode material according to claim 1, wherein After mixing with lithium fluorosquarate, the proportion of lithium iron manganese phosphate is 85 - 95 wt%.

3. The low-temperature fast-charging lithium-supplementing cathode material according to claim 1, wherein The lithium iron manganese phosphate is also subjected to impregnation treatment with phosphoric acid aqueous solution before mixing with lithium fluorosquarate.

4. The low-temperature fast-charging lithium-supplementing cathode material according to claim 1, wherein The preparation steps of the lithium iron manganese phosphate are as follows: Dissolve lithium carbonate, manganese acetate, iron oxalate, and ammonium dihydrogen phosphate in deionized water, add citric acid and ethylene glycol, and heat and stir to form a sol; Vacuum dry the sol at 120 °C to obtain a porous gel precursor; Pre-calcine and sinter the porous gel precursor under argon protection to obtain nano-scale lithium iron manganese phosphate; The temperature of the pre-calcination is 300 - 320 °C, and the pre-calcination time is 3.5 - 4 h. The specific steps of the sintering are as follows: raise the temperature to 700 - 710 °C at a rate of 5 - 8 °C / min, keep the temperature for 10 h, and then cool naturally.

5. The low-temperature fast-charging lithium supplement cathode material according to claim 1, wherein The preparation steps of the lithium fluorosquarate are as follows: Add squaric acid and lithium fluoride to ethylene glycol, react at 180 °C for 12 h, centrifuge and wash, and then vacuum dry at 60 °C to obtain lithium fluorosquarate powder.

6. The low-temperature fast-charging lithium-supplementing cathode material according to claim 5, wherein The molar ratio of squaric acid to lithium fluoride is 1:(2 - 2.2).

7. The low-temperature fast-charging lithium-supplementing cathode material according to claim 1, wherein, The particle size of the lithium iron manganese phosphate is 80 - 150 nm, and the particle size of the lithium fluorosquarate is less than 80 nm.

8. The preparation method of the low-temperature fast-charging lithium supplementing cathode material according to any one of claims 1-7, characterized in that, The specific preparation steps are as follows: mix lithium iron manganese phosphate and lithium fluorosquarate and perform high-energy ball milling treatment to obtain composite particles, and successively coat inner and outer carbon coating layers on the surface of the composite particles. The inner carbon coating layer is coated by chemical vapor deposition, with a thickness of 5 - 10 nm, and the outer carbon coating layer is coated by carbothermal reduction, with a thickness of 10 - 20 nm.

9. The preparation method of the low-temperature fast-charging lithium-supplementing cathode material according to claim 8, wherein, The specific steps of the chemical vapor deposition method are as follows: use a mixed gas of methane and argon as a carbon source, deposit at 500 °C for 2 h, and cool to obtain.

10. The preparation method of the low-temperature fast-charging lithium-supplementing cathode material according to claim 8, characterized in that, The specific steps of the carbothermal reduction method are as follows: add the composite particles to an aqueous glucose solution, stir and then perform ultrasonic treatment to obtain a mixed slurry, dry to obtain a glucose-coated precursor, place the precursor in an argon atmosphere, raise the temperature to 300 - 320 °C at a rate of 5 - 10 °C / min, keep the temperature for 1 - 1.5 h, then continue to raise the temperature to 800 - 810 °C at a rate of 5 - 10 °C / min, keep the temperature for 2.5 - 3 h, and grind the product after natural cooling to obtain.

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