Modified positive electrode material and preparation method and application thereof

Abstract

The invention provides a modified positive electrode material and a preparation method and application thereof. The modified positive electrode material comprises a lithium manganese iron phosphate inner core and a coating layer coating the surface of the lithium manganese iron phosphate inner core, and the material of the coating layer comprises oxyfluoride; ti and S are co-doped in the lithium manganese iron phosphate core. Ti and S are co-doped in a lithium manganese iron phosphate core, Ti doping can introduce additional electron holes to enhance the intrinsic electron conductivity of the material, S doping can expand a lithium ion channel and improve the Li < + > diffusion rate, and the Ti doping and the S doping are synergistically optimized through two channels of electron conduction-ion migration, so that the rate capability and the cycling stability of the positive electrode material are improved; the oxyfluoride is adopted as a coating layer material, so that HF corrosion and dissolution of transition metal can be reduced, an interface side reaction is inhibited, meanwhile, the excellent thermal stability and wide electrochemical window of the oxyfluoride can inhibit decomposition and gas production of an electrolyte in a high-temperature environment, and the thermal stability and electrochemical stability of the modified positive electrode material are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a modified positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries have outstanding advantages such as large discharge specific capacity, high voltage platform, safety, long service life, environmental friendliness, and are currently more and more widely used in various fields. In lithium ion batteries, the positive electrode material is the core technology of the update and replacement of lithium ion batteries, and therefore, the in-depth research on the positive electrode material of lithium ion batteries has important practical significance.

[0003] As one of the important candidates for lithium ion battery positive electrode materials, lithium manganese iron phosphate (LMFP) has a broad application prospect in the fields of power batteries and energy storage batteries due to its high theoretical specific capacity, stable charge / discharge platform, excellent structural stability and low cost advantage, and has become a research hotspot in recent years. However, LMFP still faces the following key technical problems in practical application, which seriously restricts its commercialization process: 1) interface side reaction and HF corrosion problem, since the electrolyte is easy to decompose to produce HF and other acidic substances during the charge / discharge process (especially at high voltage), these products will continuously corrode the lattice structure of the surface of the LMFP material, leading to the dissolution of Mn 2+ , Fe 2+ and other metal ions, at the same time, HF and Li + on the surface of the LMFP material combine to form LiF, which not only hinders the transmission of electrons and ions, but also increases the interface impedance, leading to rapid capacity decay and cycle performance deterioration of the battery; 2) poor high-temperature performance, high temperature will accelerate the decomposition of the electrolyte and the generation of HF, exacerbate the interface corrosion, and the dissolution of Mn elements in the LMFP material will increase, leading to lattice distortion and structure collapse, in addition, high temperature will also cause rapid capacity decay and thermal runaway risk, which is difficult to meet the use requirements of power batteries under high temperature working conditions; 3) unstable CEI film, during the charge / discharge cycle process, the unstable CEI film will repeatedly break and re-form, consuming the electrolyte and active lithium, and at the same time, leading to the continuous increase of the interface impedance, ultimately affecting the cycle life and rate performance of the battery; 4) low electronic conductivity, low electronic conductivity leads to the inability of the electric charge to be quickly transmitted between the material particles, especially during high-rate charge / discharge, the electrode is prone to polarization, which limits the rate performance of the battery; 5) slow ion diffusion rate, in the olivine structure of LMFP, the diffusion channel of Li + is narrow, leading to low Li+diffusion coefficient, and the slow ion diffusion rate makes it difficult for Li + to quickly intercalate / deintercalate the lattice during the charge / discharge process, which exacerbates the electrode polarization, leading to the reduction of the capacity utilization rate of the material.

[0004] Therefore, how to effectively inhibit the interface side reaction, reduce the HF corrosion and the dissolution of transition metals, and improve the thermal stability, electronic conductivity and ion diffusion rate of the positive electrode material, so as to improve the cycle performance and rate performance of the positive electrode material, is a technical problem to be solved. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the present application is to provide a modified positive electrode material, a preparation method and application thereof. Ti and S are co-doped in the lithium manganese iron phosphate core, Ti doping can introduce additional electron holes, enhance the intrinsic electronic conductivity of the material, S doping can expand the lithium ion channel, improve the Li + diffusion rate, and the two are optimized through the "electron conduction-ion migration" double channel, which improves the rate performance and cycle stability of the positive electrode material. The fluorine oxide as the coating layer material can reduce the HF corrosion and the dissolution of transition metals, inhibit the interface side reaction, and the excellent thermal stability and wide electrochemical window of the fluorine oxide can inhibit the electrolyte decomposition gas at high temperature environment, improve the thermal stability and electrochemical stability of the modified positive electrode material.

[0006] To achieve this purpose, the following technical solutions are adopted in the present application:

[0007] In a first aspect, the present application provides a modified positive electrode material, which comprises a lithium manganese iron phosphate core and a coating layer coated on the surface of the lithium manganese iron phosphate core, the material of the coating layer comprises fluorine oxide; and Ti and S are co-doped in the lithium manganese iron phosphate core.

[0008] In one aspect, Ti and S are co-doped in the lithium manganese iron phosphate core, wherein Ti doping can introduce additional electron holes, enhance the intrinsic electronic conductivity of the material, accelerate the charge transmission between particles, and S doping can expand the lithium ion channel, reduce the space hindrance of PO4 3- tetrahedron to Li + migration, and improve the Li + diffusion rate, and the two are optimized through the "electron conduction-ion migration" double channel, which fundamentally improves the rate performance and cycle stability of the positive electrode material. On the other hand, fluorine oxide is used as the coating layer material, which can block the direct contact of the electrolyte with the lithium manganese iron phosphate core, reduce the HF corrosion and the dissolution of transition metals, thereby effectively inhibiting the interface side reaction. In addition, the excellent thermal stability and wide electrochemical window of the fluorine oxide can inhibit the electrolyte decomposition gas at high temperature environment, significantly reduce the risk of side reaction under high temperature and high pressure conditions, and improve the thermal stability and electrochemical stability of the modified positive electrode material. Therefore, the above modification strategy comprehensively improves the rate performance, cycle life and thermal stability of the positive electrode material, and lays a foundation for the commercial application of the positive electrode material in the field of secondary batteries.

[0009] Preferably, the oxyfluoride is a metal oxyfluoride.

[0010] Preferably, the metal oxyfluoride comprises AlOF.

[0011] In the present application, due to the fact that the ionic radius of Al 3+ in the crystal structure of AlOF is close to that of Fe 2+ and Mn 2+ in the crystal lattice of lithium manganese iron phosphate, and the bond length of Al-O bond is close to that of Fe-O bond and Mn-O bond in lithium manganese iron phosphate, such structural similarity enables the AlOF coating layer to form a coherent or semi-coherent interface with the inner core of lithium manganese iron phosphate, thereby reducing the interface stress and defects, so that the AlOF coating layer is less likely to peel off due to volume change during charging and discharging, and can continuously block the electrolyte erosion in long-term cycling. Moreover, the empty orbital of Al 3+ can form an "ion transport channel", which not only allows Li + to diffuse quickly, but also improves the interface electron conduction through the electronic delocalization of Al-O bond.

[0012] Preferably, the thickness of the coating layer is 5-50 nm, for example, it can be 5 nm, 10 nm, 20 nm, 30 nm, 40 nm or 50 nm, etc.

[0013] In the present application, the appropriate thickness is conducive to forming a complete and dense protective barrier on the surface of the lithium manganese iron phosphate inner core, which not only effectively blocks the direct contact between the electrolyte and the inner core, reduces HF corrosion and transition metal ion dissolution, and suppresses the interface side reaction, but also avoids the increase of ion / electron transport path and impedance caused by excessive thickness, thereby ensuring the efficient migration of Li + and electrons between the coating layer and the inner core; at the same time, the appropriate thickness can match the volume change of lithium manganese iron phosphate during charging and discharging, thereby reducing the risk of cracking of the coating layer due to stress concentration and ensuring the stability of the coating layer in long-term cycling.

[0014] Preferably, there is also a carbon layer between the lithium manganese iron phosphate inner core and the coating layer.

[0015] Preferably, the thickness of the carbon layer is 1-10 nm, for example, it can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm, etc.

[0016] Preferably, the chemical formula of the lithium manganese iron phosphate inner core is LiMn 1-x-y Fe x Ti y PO 4-z S zwherein 0.2≤x≤0.8, such as 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8, etc., 0

[0017] Preferably, the doping molar ratio of Ti and S is 1:(0.04-0.4), such as 1:0.04, 1:0.05, 1:0.1, 1:0.12, 1:0.2, 1:0.25, 1:0.3, 1:0.35 or 1:0.4, etc.

[0018] In the present application, the appropriate doping molar ratio is conducive to the full play of the synergistic effect of the two, not only can form a match between the electron conduction ability and the ion diffusion rate, avoid the transmission effect to be worse due to the excessive doping of one of them (such as Ti too much may cause lattice distortion or S too much may destroy the stability of the crystal structure), but also can ensure the uniform distribution of the doping elements in the core, reduce the local defects, improve the material rate performance while maintaining the integrity of the crystal structure, reduce the risk of capacity attenuation caused by structure collapse in the cycle process, and finally realize the double improvement of electron-ion transmission synergistic optimization and structure stability.

[0019] Preferably, the particle size D50 of the lithium iron manganese phosphate core is 100-800nm, such as 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm or 800nm, etc.

[0020] In a second aspect, the present application provides a preparation method of the modified positive electrode material as described in the first aspect, the preparation method comprising the following steps:

[0021] Mixing a lithium source, a manganese source, an iron source, a phosphorus source, a sulfur source, a titanium source and a carbon source to perform sintering treatment to obtain a sintering product.

[0022] Depositing a coating layer on the surface of the sintering product to obtain the modified positive electrode material; wherein the material of the coating layer comprises oxyfluoride.

[0023] In the present application, the carbon source plays an important role in the sintering process, including reduction effect (such as inducing the third-order iron to be reduced to divalent iron, etc.), limited growth effect and inhibiting segregation of doping elements, etc.

[0024] Preferably, the sulfur source comprises ammonium sulfide and / or thiourea.

[0025] Preferably, the titanium source comprises titanium dioxide and / or tetrabutyl titanate.

[0026] Preferably, the lithium source comprises any one or a combination of at least two of lithium hydroxide monohydrate, lithium carbonate, lithium nitrate or lithium chloride.

[0027] Preferably, the manganese source comprises any one or a combination of at least two of manganese carbonate, manganese acetate, manganese dioxide, trimanganese tetroxide or manganese nitrate.

[0028] Preferably, the iron source comprises any one or a combination of at least two of ferrous oxalate dihydrate, ferric oxide, magnetite, ferrous chloride or ferrous nitrate.

[0029] Preferably, the phosphorus source comprises any one or a combination of at least two of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate or phosphoric acid.

[0030] Preferably, the carbon source comprises any one or a combination of at least two of starch, sucrose, glucose, citric acid, phenolic resin, acetylene black, carbon black or carbon nanotube.

[0031] Preferably, the sintering process comprises pre-sintering and main sintering in sequence.

[0032] The present application adopts a step-by-step sintering method, which helps to fully decompose the carbon source and build lithium iron manganese phosphate crystal nucleus.

[0033] Preferably, the pre-sintering temperature is 300-500℃, for example, it can be 300℃, 400℃ or 500℃, etc., and the time is 2-6h, for example, it can be 2h, 3h, 4h, 5h or 6h, etc.

[0034] Preferably, the pre-sintering atmosphere is an inert atmosphere. For example, it can be nitrogen or argon, etc.

[0035] Preferably, the main sintering temperature is 650-800℃, for example, it can be 650℃, 700℃, 750℃ or 800℃, etc., and the time is 5-15h, for example, it can be 5h, 10h or 15h, etc.

[0036] Preferably, the main sintering atmosphere is an inert atmosphere. For example, it can be nitrogen or argon, etc.

[0037] Preferably, the deposition method of the coating layer comprises co-precipitation or sol-gel method.

[0038] Preferably, the deposition method of the coating layer is co-precipitation, and the specific steps comprise:

[0039] (a) mixing the metal fluoride mixed solution and the sintering product to obtain a suspension.

[0040] (b) adjusting the pH of the suspension, performing a co-precipitation reaction, so that the surface of the sintered product is attached with metal hydroxyl fluoride, and then performing a calcination treatment, so that the metal hydroxyl fluoride is converted into metal oxyfluoride.

[0041] Preferably, in the metal fluoride mixed solution of step (a), the total concentration of metal ions and fluoride ions is 300-5000 mg / L, for example, it can be 300 mg / L, 500 mg / L, 1000 mg / L, 2000 mg / L, 3000 mg / L, 4000 mg / L or 5000 mg / L, etc.

[0042] Preferably, in the co-precipitation reaction of step (b), the temperature is 70-90℃, for example, it can be 70℃, 80℃ or 90℃, etc., and the time is 1-5h, for example, it can be 1h, 2h, 3h, 4h or 5h, etc.

[0043] Preferably, in the calcination treatment of step (b), the temperature is 400-600℃, for example, it can be 400℃, 500℃ or 600℃, etc., and the time is 2-6h, for example, it can be 2h, 3h, 4h, 5h or 6h, etc.

[0044] Preferably, the preparation method comprises the following steps:

[0045] (1) mixing a lithium source, a manganese source, an iron source, a phosphorus source, a sulfur source, a titanium source and a carbon source, adding an alcohol solvent, performing wet ball milling, then performing pre-sintering in an inert atmosphere at 300-500℃ for 2-6h, performing main sintering in an inert atmosphere at 650-800℃ for 5-15h, and then performing grinding and sieving to obtain a sintered product with a particle size D50 of 100-800nm (for example, it can be 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm or 800nm, etc.).

[0046] (2) mixing an aluminum source (for example, it can be aluminum nitrate, aluminum chloride, aluminum sulfate or aluminum isopropoxide, etc.), a fluorine source (for example, it can be ammonium fluoride, hydrofluoric acid, ammonium bifluoride or sodium fluoride, etc.) and a solvent to obtain an aluminum fluoride mixed solution; wherein the total concentration of aluminum ions and fluoride ions in the aluminum fluoride mixed solution is 300-5000 mg / L.

[0047] Disperse the sintered product in the aluminum fluoride mixed solution, and after ultrasonic treatment for 20-40min (for example, it can be 20min, 30min or 40min, etc.), a suspension is obtained.

[0048] The pH of the suspension is adjusted to 2-4 (for example, 2, 2.5, 3, 3.5 or 4), and a coprecipitation reaction is carried out at 70-90° C. for 1-5 hours to allow the hydroxyaluminum fluoride precursor to adhere to the surface of the sintered product. The product is then centrifuged, washed and dried to obtain an intermediate.

[0049] The intermediate is calcined under an inert atmosphere to convert the aluminum hydroxyfluoride precursor into AlOF to obtain a modified positive electrode material; wherein the calcination temperature is 400-600° C. and the time is 2-6 hours.

[0050] In a third aspect, the present invention provides an application of the modified positive electrode material as described in the first aspect in the field of secondary batteries.

[0051] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] On the one hand, the present invention co-dopes Ti and S in the lithium manganese iron phosphate core, wherein Ti doping can introduce additional electron holes, enhance the intrinsic electronic conductivity of the material, and accelerate the transfer of charge between particles, while S doping can expand the lithium ion channel and reduce PO4 3- Tetrahedron Li + Space barriers to migration, increase Li + Diffusion rate, the two are synergistically optimized through the dual pathways of "electron conduction-ion migration", fundamentally improving the rate performance and cycle stability of the positive electrode material; on the other hand, the use of fluoride oxide as the coating material can block the direct contact between the electrolyte and the lithium manganese iron phosphate core, reduce HF corrosion and the dissolution of transition metals, thereby effectively inhibiting interfacial side reactions. At the same time, the excellent thermal stability and wide electrochemical window of fluoride oxide can inhibit the decomposition and gas production of the electrolyte in a high-temperature environment, significantly reducing the risk of side reactions under high temperature and high-pressure conditions, and improving the thermal stability and electrochemical stability of the modified positive electrode material. Therefore, the above-mentioned modification strategy comprehensively improves the rate performance, cycle life and thermal stability of the positive electrode material, laying the foundation for the commercial application of positive electrode materials in the field of secondary batteries. DETAILED DESCRIPTION

[0054] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0055] Example 1

[0056] The embodiment provides a modified positive electrode material, which comprises a lithium iron manganese phosphate core and a coating layer coated on the surface of the lithium iron manganese phosphate core, wherein the material of the coating layer is AlOF; Ti and S are co-doped in the lithium iron manganese phosphate core.

[0057] The thickness of the coating layer is 27 nm; the lithium iron manganese phosphate core and the coating layer further have a carbon layer therebetween, and the thickness of the carbon layer is 5 nm; the chemical general formula of the lithium iron manganese phosphate core is LiMn 1-x-y Fe x Ti y PO 4-z S z x=0.5, y=0.025, and z=0.0055, the molar ratio of the Ti and S is 1:0.22; and the particle size D50 of the lithium iron manganese phosphate core is 450 nm.

[0058] The embodiment further provides a preparation method of the modified positive electrode material, and the preparation method comprises the following steps:

[0059] (1) mixing lithium hydroxide monohydrate, manganese carbonate, ferrous oxalate dihydrate, diammonium hydrogen phosphate, ammonium sulfide, titanium dioxide and sucrose, adding ethanol, performing wet ball milling, then performing pre-sintering at 400 DEG C in a nitrogen atmosphere for 4 h, performing main sintering at 750 DEG C in an inert atmosphere for 10 h, grinding and sieving after the end of the main sintering, and obtaining a sintered product with a particle size D50 of 450 nm.

[0060] (2) stirring and mixing aluminum isopropoxide and hydrofluoric acid in deionized water to obtain an aluminum-fluorine mixed solution; wherein the total concentration of aluminum ions and fluorine ions in the aluminum-fluorine mixed solution is 2600 mg / L.

[0061] dispersing the sintered product in the aluminum-fluorine mixed solution, and performing ultrasonic treatment for 30 min to obtain a suspension.

[0062] adjusting the pH of the suspension to 3, performing a co-precipitation reaction at 80 DEG C for 3 h to make the surface of the sintered product adhere to a hydroxyl aluminum fluoride precursor, then performing centrifugal separation, washing and drying to obtain an intermediate.

[0063] performing calcination treatment on the intermediate under a nitrogen atmosphere to convert the hydroxyl aluminum fluoride precursor into AlOF, and obtaining a modified positive electrode material; wherein the temperature of the calcination treatment is 500 DEG C, and the time is 4 h.

[0064] Embodiment 2

[0065] The embodiment provides a modified positive electrode material, which comprises a lithium iron manganese phosphate core and a coating layer coated on the surface of the lithium iron manganese phosphate core, wherein the material of the coating layer is AlOF; and Ti and S are co-doped in the lithium iron manganese phosphate core.

[0066] The thickness of the coating layer is 5 nm; the lithium iron manganese phosphate core and the coating layer further have a carbon layer therebetween, and the thickness of the carbon layer is 3 nm; the chemical general formula of the lithium iron manganese phosphate core is LiMn 1-x-y Fe x Ti y PO 4-z S z x=0.2, y=0.03, z=0.0012, the molar ratio of the Ti and S is 1:0.04; and the particle size D50 of the lithium iron manganese phosphate core is 100 nm.

[0067] The embodiment further provides a preparation method of the modified positive electrode material, and the preparation method comprises the following steps:

[0068] (1) mixing lithium hydroxide monohydrate, manganese carbonate, ferrous oxalate dihydrate, diammonium hydrogen phosphate, ammonium sulfide, titanium dioxide and sucrose, adding ethanol, performing wet ball milling, then performing pre-sintering at 400 DEG C in a nitrogen atmosphere for 6 h, performing main sintering at 650 DEG C in a nitrogen atmosphere for 15 h, grinding and sieving after the end, and obtaining a sintered product with a particle size D50 of 100 nm.

[0069] (2) stirring and mixing aluminum isopropoxide and hydrofluoric acid in deionized water to obtain an aluminum-fluorine mixed solution; wherein the total concentration of aluminum ions and fluorine ions in the aluminum-fluorine mixed solution is 300 mg / L.

[0070] dispersing the sintered product in the aluminum-fluorine mixed solution, and performing ultrasonic treatment for 20 min to obtain a suspension.

[0071] adjusting the pH of the suspension to 2, performing a co-precipitation reaction at 70 DEG C for 5 h to make the surface of the sintered product adhere to a hydroxyl aluminum fluoride precursor, then performing centrifugal separation, washing and drying to obtain an intermediate.

[0072] performing calcination treatment on the intermediate under a nitrogen atmosphere to convert the hydroxyl aluminum fluoride precursor into AlOF, and obtaining a modified positive electrode material; wherein the temperature of the calcination treatment is 400 DEG C, and the time is 6 h.

[0073] Embodiment 3

[0074] The embodiment provides a modified positive electrode material, which comprises a lithium iron manganese phosphate core and a coating layer coated on the surface of the lithium iron manganese phosphate core, the material of the coating layer is AlOF, and Ti and S are co-doped in the lithium iron manganese phosphate core.

[0075] The thickness of the coating layer is 50 nm; the lithium iron manganese phosphate core and the coating layer further have a carbon layer therebetween, the thickness of the carbon layer is 10 nm; the chemical general formula of the lithium iron manganese phosphate core is LiMn 1-x-y Fe x Ti y PO 4-z S z x=0.8, y=0.02, z=0.008, the molar ratio of the Ti and S is 1:0.4; the particle size D50 of the lithium iron manganese phosphate core is 800 nm.

[0076] The embodiment further provides a preparation method of the modified positive electrode material, and the preparation method comprises the following steps:

[0077] (1) lithium hydroxide monohydrate, manganese carbonate, ferrous oxalate dihydrate, diammonium hydrogen phosphate, ammonium sulfide, titanium dioxide and sucrose are mixed and then added into ethanol, wet ball milling is carried out, pre-sintering is carried out at 500 DEG C in a nitrogen atmosphere for 2h, main sintering is carried out at 800 DEG C in an argon atmosphere for 5h, grinding and sieving are carried out after the end, and a sintered product with a particle size D50 of 800 nm is obtained.

[0078] (2) aluminum isopropoxide and hydrofluoric acid are added into deionized water and stirred to obtain an aluminum-fluorine mixed solution; wherein the total concentration of aluminum ions and fluorine ions in the aluminum-fluorine mixed solution is 5000 mg / L.

[0079] The sintered product is dispersed in the aluminum-fluorine mixed solution, and after ultrasonic treatment for 40 min, a suspension is obtained.

[0080] The pH of the suspension is adjusted to 4, a co-precipitation reaction is carried out at 90 DEG C for 1h, so that the surface of the sintered product is attached with a hydroxy aluminum fluoride precursor, and then centrifugal separation, washing and drying are carried out, so that an intermediate is obtained.

[0081] The intermediate is subjected to calcination treatment under an argon atmosphere, so that the hydroxy aluminum fluoride precursor is converted into AlOF, and a modified positive electrode material is obtained; wherein the temperature of the calcination treatment is 600 DEG C, and the time is 2h.

[0082] Embodiment 4

[0083] The difference between the embodiment and embodiment 1 is that aluminum isopropoxide in step (2) is replaced by iron nitrate, so that the material of the coating layer is FeOF.

[0084] The rest of the preparation method and parameters are consistent with Example 1.

[0085] Example 5

[0086] The difference between this example and Example 1 is that the thickness of the coating layer is 2 nm.

[0087] The rest of the preparation method and parameters are consistent with Example 1.

[0088] Example 6

[0089] The difference between this example and Example 1 is that the thickness of the coating layer is 100 nm.

[0090] The rest of the preparation method and parameters are consistent with Example 1.

[0091] Example 7

[0092] The difference between this example and Example 1 is that the doping molar ratio of Ti and S is 1:0.01.

[0093] The rest of the preparation method and parameters are consistent with Example 1.

[0094] Example 8

[0095] The difference between this example and Example 1 is that the doping molar ratio of Ti and S is 1:1.

[0096] The rest of the preparation method and parameters are consistent with Example 1.

[0097] Example 9

[0098] The difference between this example and Example 1 is that pre-burning is not performed in step (1).

[0099] The rest of the preparation method and parameters are consistent with Example 1.

[0100] Comparative Example 1

[0101] The difference between this comparative example and Example 1 is that the ammonium sulfide in step (1) is replaced with ammonium chloride, so that Ti and Cl are co-doped in the lithium iron manganese phosphate core.

[0102] The rest of the preparation method and parameters are consistent with Example 1.

[0103] Comparative Example 2

[0104] The difference between this comparative example and Example 1 is that the coating layer is an aluminum oxide layer, that is, hydrofluoric acid in step (2) is replaced with an equal mass of aluminum isopropyl alcohol.

[0105] The rest of the preparation method and parameters are consistent with Example 1.

[0106] Performance test

[0107] Preparation of lithium ion battery based on modified positive electrode material provided in the above examples and comparative examples: the modified positive electrode material, conductive carbon black and polyvinylidene fluoride prepared above are weighed according to the mass ratio of 8:1:1, added into N-methyl pyrrolidone for mixing and pulping, then the slurry is coated on aluminum foil, dried, sliced to obtain positive electrode sheet; with lithium metal sheet as counter electrode, polyethylene microporous membrane as separator, electrolyte with solute concentration of 1 mol / L (wherein, the solvent is a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1, and the solute is LiPF6); assemble into CR2025 button cell in an argon glove box.

[0108] The CR2025 button cell is subjected to cycle performance test and rate performance test.

[0109] Test conditions of cycle performance: after the assembled battery is rested for 2h, the cycle test is carried out for 300 cycles at 25℃, voltage range 2.5-4.35V and 2C rate.

[0110] Test conditions of rate performance: after the assembled battery is rested for 2h, the cycle is carried out for 5 cycles at 25℃, voltage range 2.5-4.35V and 0.2C, 0.5C, 1C, 2C and 5C respectively.

[0111] Test method of thermal stability: the material is soaked in electrolyte (1 mol / L electrolyte, wherein the solvent is a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1, and the solute is LiPF6) at 80℃ for 24h, and the dissolved Mn 2+ concentration is detected by ICP-MS.

[0112] The test results are shown in Table 1.

[0113] Table 1

[0114]

[0115]

[0116] Analysis

[0117] As shown in Table 1, in one aspect of the present application, Ti and S are co-doped in the manganese iron lithium phosphate core, wherein Ti doping can introduce additional electron holes, enhance the intrinsic electronic conductivity of the material, and accelerate the transmission of charges between particles, and S doping can expand the lithium ion channel, reduce the spatial hindrance of PO4 3- tetrahedron to Li + migration, and improve the Li +The diffusion rate is optimized by the synergistic effect of the "electron conduction-ion migration" dual channel, which fundamentally improves the rate performance and cycle stability of the positive electrode material. On the other hand, fluorine oxide is used as a coating material, which can block the direct contact between the electrolyte and the inner core of lithium manganese iron phosphate, reduce HF corrosion and the dissolution of transition metals, thereby effectively inhibiting the interface side reaction. At the same time, the excellent thermal stability and wide electrochemical window of fluorine oxide can inhibit the decomposition of electrolyte gas at high temperature, significantly reduce the risk of side reactions under high temperature and high pressure conditions, and improve the thermal stability and electrochemical stability of the modified positive electrode material.

[0118] As can be seen from the comparison of Example 1 and Example 4, if the material of the coating layer is FeOF, due to the lower Fe-O bond energy than Al-O bond, it is easy to be corroded by HF in the electrolyte at high temperature or high pressure, which may lead to Fe 2+ dissolution and catalyze electrolyte decomposition, while Fe 2+ is easy to be oxidized to Fe 3+ , causing lattice distortion of FeOF, cracking of the coating layer, and losing the protection of the LMFP core.

[0119] As can be seen from the comparison of Example 1 and Example 5-6, if the thickness of the coating layer is too small, the thin coating layer is easy to be damaged due to stress in the cycle process, and cannot maintain the protection effect for a long time, thereby affecting the cycle stability of the material. If the thickness of the coating layer is too large, the over-thick coating layer may produce interface stress due to the decrease of lattice matching with the core, and is easy to peel off in the charge-discharge volume change, which increases the risk of interface failure.

[0120] As can be seen from the comparison of Example 1 and Example 7-8, if the molar ratio of Ti and S doping is too small, the intrinsic electronic conductivity of lithium manganese iron phosphate is limited, even if the S doping expands the lithium ion channel, it is difficult to play the advantage of ion diffusion due to the low efficient conduction of electrons, which limits the improvement of the rate performance of the material. Excessive S may damage the lattice integrity, increase the structural defects, and aggravate the capacity decay in the cycle process. If the molar ratio of Ti and S doping is too large, the diffusion rate of lithium ions is limited, and lithium ions cannot migrate quickly, and excessive Ti may cause lattice distortion, damage the structural stability of lithium manganese iron phosphate, and reduce the cycle life of the material.

[0121] As can be seen from the comparison of Example 1 and Example 9, if the pre-burning in step (1) is not performed, the carbon source reaction is not sufficient, and the raw material particles directly subjected to main burning lack effective constraints, which is easy to cause excessive sintering and grain growth, resulting in poor rate performance of the material, and uneven particle size distribution further reduces the cycle stability. The uneven distribution of doping elements leads to the difficulty in effectively improving the performance of the material.

[0122] From the comparison between Example 1 and Comparative Example 1, it can be seen that if Ti and Cl are co-doped into the lithium manganese iron phosphate core, the ionic radius of chloride ions is larger than that of PO4 3- O in 2- , after doping, it is easy to embed into the lattice gap or replace O 2- sites, resulting in aggravated lattice distortion, undermining structural stability and prone to structural collapse during cycling; at the same time, chloride ions easily combine with lithium ions in the electrolyte to form LiCl at high temperatures, which not only consumes active lithium but also may form a loose LiCl layer at the interface to hinder lithium ion migration; in addition, the electron holes introduced by Ti may be neutralized due to the electron affinity of chlorine, weakening its effect of improving electronic conductivity.

[0123] Comparing Example 1 with Comparative Example 2 shows that if the coating layer is aluminum oxide, its insulating oxide significantly increases interfacial impedance, hindering electron transport and leading to a decrease in the material's rate performance. Furthermore, aluminum oxide has a low lithium ion diffusion coefficient, which prolongs the lithium ion migration path. Furthermore, the poor lattice matching between aluminum oxide and lithium manganese iron phosphate makes it prone to cracking due to stress concentration during the volume change during the charge-discharge cycle. This makes it unable to provide long-term protection against HF corrosion in the electrolyte, ultimately leading to increased transition metal ion dissolution and poor cycling stability.

[0124] It should be noted that the present invention uses the above-described embodiments to illustrate the technical solutions of the present invention, but the present invention is not limited to the above-described embodiments, that is, it does not mean that the present invention must rely on the above-described embodiments to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent replacements of various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., all fall within the scope of protection and disclosure of the present invention.

Claims

1. A modified positive electrode material, characterized in that The modified positive electrode material includes a lithium iron manganese phosphate core and a coating layer coated on the surface of the lithium iron manganese phosphate core, wherein the material of the coating layer includes fluoride oxide; Ti and S are co-doped in the lithium iron manganese phosphate core.

2. The modified positive electrode material according to claim 1, characterized in that The oxyfluoride is a metal oxyfluoride; Preferably, the metal oxyfluoride comprises AlOF; Preferably, the coating layer has a thickness of 5-50 nm; Preferably, a carbon layer is further provided between the lithium manganese iron phosphate core and the coating layer; Preferably, the carbon layer has a thickness of 1-10 nm.

3. The modified positive electrode material according to claim 1 or 2, characterized in that The general chemical formula of the lithium manganese iron phosphate core is LiMn 1-x-y Fe x Ti y PO 4-z S z , where 0.2≤x≤0.8, 0<y<0.05, 0<z<0.02; Preferably, the doping molar ratio of Ti to S is 1:(0.04-0.4); Preferably, the particle size D50 of the lithium manganese iron phosphate core is 100-800 nm.

4. A method for preparing a modified positive electrode material according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: mixing a lithium source, a manganese source, an iron source, a phosphorus source, a sulfur source, a titanium source and a carbon source, and performing a sintering process to obtain a sintered product; A coating layer is deposited on the surface of the sintered product to obtain the modified positive electrode material; wherein the material of the coating layer includes fluoride oxide.

5. The preparation method according to claim 4, characterized in that The sulfur source includes ammonium sulfide and / or thiourea; Preferably, the titanium source comprises titanium dioxide and / or tetrabutyl titanate; Preferably, the lithium source comprises any one or a combination of at least two of lithium hydroxide monohydrate, lithium carbonate, lithium nitrate or lithium chloride; Preferably, the manganese source comprises any one of manganese carbonate, manganese acetate, manganese dioxide, trimanganese tetraoxide or manganese nitrate, or a combination of at least two thereof; Preferably, the iron source comprises any one of ferrous oxalate dihydrate, ferrous oxide, ferrosoferric oxide, ferrous chloride or ferrous nitrate, or a combination of at least two thereof; Preferably, the phosphorus source includes any one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate or phosphoric acid, or a combination of at least two thereof.

6. The preparation method according to claim 4 or 5, characterized in that The sintering process includes pre-firing and main firing performed in sequence; Preferably, the pre-calcination temperature is 300-500°C and the time is 2-6 hours; Preferably, the pre-firing atmosphere is an inert atmosphere; Preferably, the main firing temperature is 650-800°C and the time is 5-15h; Preferably, the atmosphere of the main firing is an inert atmosphere.

7. The preparation method according to any one of claims 4 to 6, characterized in that The deposition method of the coating layer includes a co-precipitation method or a sol-gel method; Preferably, the coating layer is deposited by a co-precipitation method, and the specific steps include: (a) mixing a metal-fluorine mixed solution and a sintered product to obtain a suspension; (b) adjusting the pH of the suspension, performing a coprecipitation reaction, so that the metal hydroxyfluoride is attached to the surface of the sintered product, and then performing a calcination treatment to convert the metal hydroxyfluoride into a metal oxyfluoride.

8. The preparation method according to claim 7, characterized in that In the metal-fluorine mixed solution of step (a), the total concentration of metal ions and fluoride ions is 300-5000 mg / L; Preferably, the coprecipitation reaction in step (b) is carried out at a temperature of 70-90° C. and for a time of 1-5 h; Preferably, the calcination temperature in step (b) is 400-600° C. and the calcination time is 2-6 hours.

9. The preparation method according to any one of claims 4 to 8, characterized in that The preparation method comprises the following steps: (1) A lithium source, a manganese source, an iron source, a phosphorus source, a sulfur source, a titanium source, and a carbon source are mixed, an alcohol solvent is added, and wet ball milling is performed, followed by pre-calcination in an inert atmosphere at 300-500° C. for 2-6 hours, and main calcination in an inert atmosphere at 650-800° C. for 5-15 hours. After the calcination, the mixture is ground and sieved to obtain a sintered product with a particle size D50 of 100-800 nm; (2) mixing an aluminum source, a fluorine source, and a solvent to obtain an aluminum-fluorine mixed solution; wherein the total concentration of aluminum ions and fluoride ions in the aluminum-fluorine mixed solution is 300-5000 mg / L; Dispersing the sintered product in the aluminum-fluorine mixed solution, and ultrasonically treating for 20-40 minutes to obtain a suspension; Adjusting the pH of the suspension to 2-4, performing a coprecipitation reaction at 70-90° C. for 1-5 hours to allow the hydroxyaluminum fluoride precursor to adhere to the surface of the sintered product, and then centrifuging, washing and drying to obtain an intermediate; The intermediate is calcined under an inert atmosphere to convert the aluminum hydroxyfluoride precursor into AlOF to obtain a modified positive electrode material; wherein the calcination temperature is 400-600° C. and the time is 2-6 hours.

10. Use of the modified positive electrode material according to any one of claims 1 to 3 in the field of secondary batteries.

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