Lithium-rich manganese-based positive electrode material, preparation method thereof and electrochemical device

Abstract

The invention provides a lithium-rich manganese-based positive electrode material, a preparation method thereof and an electrochemical device. The lithium-rich manganese-based positive electrode material comprises a base material and a coating layer, the coating layer comprises a lithium hexafluorozirconate coating layer located on the surface of the base material and a fluoride coating layer located on the side, away from the base material, of the lithium hexafluorozirconate coating layer, the chemical formula of the base material is Li (LixMnaNi1-a-bAlb) O2-delta, x is larger than or equal to 0.05 and smaller than or equal to 0.33, 0lt, y is larger than or equal to 0.05 and smaller than or equal to 0.33, 0lt, and y is larger than or equal to 0.05 and smaller than or equal to 0. A < lt >; 0.1 < = b < = 0.25, and 0 < = delta < = 0.2. According to the lithium-rich manganese-based positive electrode material, a unique lithium hexafluorozirconate-fluoride dual-coating structure is adopted, so that instability of a positive electrode / electrolyte interface and manganese dissolution of the lithium-rich manganese-based positive electrode material under high voltage are effectively relieved, and the lithium-rich manganese-based positive electrode material has high capacity, excellent rate capability and long cycle life at the same time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy, in particular to a lithium-rich manganese-based positive electrode material, a preparation method thereof and an electrochemical device. BACKGROUND

[0002] With the rapid growth of demand for electric vehicles and large-scale energy storage, developing lithium-ion battery positive electrode materials with high energy density and long cycle life has become a hot research topic. The specific capacity of traditional layered oxides (such as LiCoO2 and NCM) has approached the theoretical limit (about 200 mAh / g), which is difficult to meet the demand of breaking through 500 Wh / kg of battery energy density in the future. Under this background, lithium-rich manganese-based layered oxides (LRMOs) are considered to be one of the best choices for the next generation of high-energy lithium battery positive electrode materials due to their unique anion redox reaction mechanism, which can provide higher reversible specific capacity, while also having the advantages of low cost (low / no cobalt) and high operating voltage.

[0003] However, the practical application of lithium-rich manganese-based positive electrode materials still faces severe challenges. For example, during the cycling process, lithium-rich manganese-based positive electrode materials will undergo irreversible oxygen loss and layered-spinel phase transition, leading to voltage decay and capacity decay. At high voltage, lithium-rich manganese-based positive electrode materials will also undergo oxidation decomposition and other interfacial side reactions with the electrolyte interface (such as sulfide solid-state electrolyte), and the poor solid-solid interface contact between lithium-rich manganese-based positive electrode materials and the electrolyte interface leads to high lithium ion transport impedance, affecting the rate performance. The lattice expansion of lithium-rich manganese-based positive electrode materials when lithium is deintercalated will cause electrode cracking and destroy the interface stability. SUMMARY

[0004] In view of this, in order to solve at least one of the above technical problems, the present application provides a lithium-rich manganese-based positive electrode material.

[0005] In addition, the present application also provides a preparation method of the lithium-rich manganese-based positive electrode material and an electrochemical device using the lithium-rich manganese-based positive electrode material.

[0006] In a first aspect, the present application provides a lithium-rich manganese-based positive electrode material, which comprises a base material and a coating layer, including a lithium hexafluorozirconate coating layer located on the surface of the base material and a fluoride coating layer located on the side of the lithium hexafluorozirconate coating layer away from the base material, the chemical formula of the base material is Li(Li x Mn a Ni 1-a-b Al b )O 2-δ , wherein 0.05≤x≤0.33, 0

[0007] In some possible embodiments based on the first aspect, the fluoride coating layer comprises at least one of AlF3, TiF4 and ZrF4.

[0008] In some possible embodiments based on the first aspect, the mass percentage of the lithium hexafluorozirconate coating layer with respect to the base material is 1.0wt%-3.0wt%; and / or the mass percentage of the fluoride coating layer with respect to the base material is 0.05wt%-0.30wt%.

[0009] In the second aspect, the application provides a preparation method of a lithium-rich manganese-based positive electrode material, which comprises: mixing a manganese precursor, a lithium source and an aluminum source, and performing first sintering to obtain a base material; mixing the base material with a lithium hexafluorozirconate solution and performing drying treatment, and then performing second sintering to react and generate a lithium hexafluorozirconate coating layer on the surface of the base material, thereby obtaining an intermediate material; and mixing the intermediate material with a fluoride, and performing third sintering to form a fluoride coating layer on the surface of the intermediate material, thereby obtaining the lithium-rich manganese-based positive electrode material.

[0010] In some possible embodiments based on the second aspect, the concentration of the lithium hexafluorozirconate solution is 1.0mol / L-5.0mol / L; and / or the mass / volume ratio of the base material to the lithium hexafluorozirconate solution is 10kg:(0.1L-1.0L).

[0011] In some possible embodiments based on the second aspect, the fluoride comprises at least one of AlF3, TiF4 and ZrF4; and / or the mass percentage of the fluoride with respect to the base material is 0.05wt%-0.30wt%.

[0012] In some possible embodiments based on the second aspect, the manganese precursor comprises nickel-manganese hydroxide Ni x Mn y (OH)2, wherein x+y=1, 0.2≤x<0.5.

[0013] In some possible embodiments based on the second aspect, the mass percentage of the aluminum source in the total mass of the manganese precursor, the lithium source and the aluminum source is 0.05wt%-0.5wt%; and / or the aluminum source comprises at least one of aluminum hydroxide and aluminum oxide.

[0014] In some possible embodiments of the second aspect, the temperature of the drying treatment is 80-120 DEG C; the first sintering condition comprises sintering at 750-950 DEG C for 10-15 h with a temperature increasing rate of 2-3 DEG C / min; the second sintering condition comprises sintering at 300-600 DEG C for 2-5 h with a temperature increasing rate of 2-3 DEG C / min; and the third sintering condition comprises sintering at 550-750 DEG C for 7-12 h with a temperature increasing rate of 2-3 DEG C / min.

[0015] In a third aspect, the embodiments of the present application further provide an electrochemical device, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode material, and the positive electrode material is the lithium-rich manganese-based positive electrode material as described above.

[0016] Compared with the prior art, the lithium-rich manganese-based positive electrode material provided by the embodiments of the present application adopts an innovative double-layer coating structure design, the lithium hexafluorozirconate coating layer can act as a 'fast ion conductor layer', and the fluoride coating layer can act as an 'isolating protective layer', and the two layers work together to improve the rate performance of the lithium-rich manganese-based positive electrode material, effectively inhibit the dissolution of manganese ions in the lithium-rich manganese-based positive electrode material, reduce the interface side reaction between the lithium-rich manganese-based positive electrode material and the electrolyte or electrolyte, slow down the volume change of the lithium-rich manganese-based positive electrode material during the cycle process, and thus improve the cycle stability and structural stability of the lithium-rich manganese-based positive electrode material. The lithium-rich manganese-based positive electrode material provided by the present application adopts a unique double-coating structure, effectively alleviates the problems of positive electrode / electrolyte interface instability and manganese dissolution of the lithium-rich manganese-based positive electrode material at high voltage, and makes the lithium-rich manganese-based positive electrode material simultaneously have advantages of high capacity, excellent rate performance and long cycle life, and is suitable for next-generation high-energy-density lithium-ion battery systems. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A structural schematic diagram of the lithium-rich manganese-based positive electrode material provided by an embodiment of the present application.

[0018] Figure 2 A process flow chart of the preparation method of the lithium-rich manganese-based positive electrode material provided by an embodiment of the present application.

[0019] Figure 3 Scanning electron microscope images of the lithium-rich manganese-based positive electrode material in Example 1, Example 3 and Comparative Example 1 of the present application, wherein, Figure 3 Fig. (a-1) is a scanning electron microscope image of the lithium-rich manganese-based positive electrode material in Comparative Example 1 at a magnification of 10000 times, Figure 3 Fig. (a-2) is a scanning electron microscope image of the lithium-rich manganese-based positive electrode material in Comparative Example 1 at a magnification of 50000 times, Figure 3Fig. (b-1) is a scanning electron microscope (SEM) image of the Li-rich Mn-based positive electrode material in Example 1 at 10,000 times magnification, Figure 3 Fig. (b-2) is a SEM image of the Li-rich Mn-based positive electrode material in Example 1 at 50,000 times magnification, Figure 3 Fig. (c-1) is a SEM image of the Li-rich Mn-based positive electrode material in Example 3 at 10,000 times magnification, Figure 3 Fig. (c-2) is a SEM image of the Li-rich Mn-based positive electrode material in Example 3 at 50,000 times magnification.

[0020] Figure 4 Fig. is a graph of the first cycle discharge capacity and the first efficiency of the Li-rich Mn-based positive electrode material in Examples 1-6 and Comparative Examples 1-2.

[0021] Figure 5 Fig. is a graph of the high-temperature cycle performance of the Li-rich Mn-based positive electrode material in Examples 1-6 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0022] Embodiments of the present application are described in detail below. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present application and cannot be understood as a limitation of the present application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as understood by those skilled in the art in the technical field to which the present application belongs; the embodiments of the present application and the features in the embodiments can be combined with each other without conflict; many specific details are set forth in the following description in order to fully understand the present application, and the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0023] Referring to Figure 1 As shown in the drawings, the embodiments of the present application provide a new Li-rich Mn-based positive electrode material 100, which comprises a base material 10 and a coating layer 20, the coating layer 20 comprises a lithium hexafluorozirconate coating layer 21 located on the surface of the base material 10 and a fluoride coating layer 22 located on the side of the lithium hexafluorozirconate coating layer 21 away from the base material 10, and the chemical formula of the base material 10 is Li(Li x Mn a Ni 1-a-b Al b )O 2-δ , wherein 0.05≤x≤0.33, 0

[0024] The present application selects Li-rich Mn-based layered oxides (LRMOs) as the base material, and specifically, the chemical formula of the base material is Li(Lix Mn a Ni 1-a-b Al b )O 2-δ , wherein 0.05≤x≤0.33, 0 + a<0.6, 0.1≤b≤0.25, 0≤δ≤0.2. The host material has a high specific capacity (up to 250-300 mAh / g) while simultaneously having the advantages of low cost (low cobalt) and high operating voltage (3.0-4.8 V vs. Li 2- / Li). The anions and cations of the host material can synergistically undergo redox reactions: during lithium ion deintercalation, not only do the transition metals (such as Mn and Ni) contribute to electron transfer, but the oxygen ions (O 2- ) in the lattice also participate in reversible oxidation reactions (O - / O2 n- ), thereby effectively increasing the capacity. The manganese-rich characteristics of the host material can also reduce costs and improve the thermal stability of the host material. Furthermore, the host material is doped with Al elements, which can optimize the lattice structure of the host material, while improving the uniformity of the particle morphology of the host material, providing more channels for lithium ion diffusion and increasing the lithium ion diffusion rate. In addition, LRMOs can be applied in all-solid-state battery (ASSB) systems, and on the basis of the above advantages of LRMOs, the combination of LRMOs and solid-state electrolyte has broad prospects, wherein the wide electrochemical window (greater than 5 V) of the solid-state electrolyte can inhibit the decomposition of the solid-state electrolyte at high voltage, and the non-flammable nature of the solid-state electrolyte can alleviate the risk of thermal runaway caused by oxygen evolution in traditional liquid batteries.

[0025] To further improve the electrochemical performance and structural stability of the host material, a double coating layer is designed on the surface of the host material, and the double coating layer comprises a lithium hexafluorozirconate coating layer and a fluoride coating layer.

[0026] The lithium hexafluorozirconate (Li2ZrF6) coating layer is located on the side of the overall coating layer close to the host material, and the lithium hexafluorozirconate coating layer has excellent lithium ion conductivity (ionic conductivity greater than 10 -4 S / cm), and the lithium hexafluorozirconate coating layer can effectively promote the rapid transport of lithium ions on the surface of the lithium-rich manganese-based positive electrode material and at the lithium-rich manganese-based positive electrode material / electrolyte interface by providing a three-dimensional lithium ion channel, thereby improving the rate performance of the lithium-rich manganese-based positive electrode material. At the same time, the strong electronegativity of Zr 4+ in the lithium hexafluorozirconate coating layer is conducive to stabilizing the valence of the transition metals on the surface of the host material, thereby improving the surface chemical stability of the lithium-rich manganese-based positive electrode material.

[0027] In addition, compared with other lithium ion conductors, the lithium hexafluorozirconate coating layer has unique advantages. For example, compared with oxide lithium ion conductors (such as Li3PO4), Li2ZrF6, as a oxyfluoride salt, has better resistance to HF and better chemical compatibility with the fluoride coating layer. Li3PO4 can also transport lithium ions, but Li3PO4 has poor stability in HF acid environment and is easy to generate by-products such as LiF, metal phosphate or metal titanate, resulting in increased interface impedance of the positive electrode / electrolyte. Secondly, compared with oxide solid electrolytes (such as LLTO containing Ti 4+ and LCO containing Co 4+ ), the surface of LRMOs has strong oxidizing properties when charged to high voltage (>4.6V), and many oxide solid electrolytes will be oxidized and decomposed, resulting in a dramatic increase in interface impedance. F - in Li2ZrF6 has high electronegativity and is difficult to be further oxidized, so Li2ZrF6 has better high-voltage stability. Compared with sulfide electrolytes (such as LPS and LGPS), Li2ZrF6 has a wider electrochemical window and is difficult to burn, while the electrochemical window of sulfide electrolytes is narrow and will be severely oxidized and decomposed at high voltage. In addition, sulfide electrolytes are extremely sensitive to H2O and O2, and will react adversely when in contact with active oxygen generated on the surface of LRMOs, and cannot be adapted. Therefore, the use of lithium hexafluorozirconate in the inner layer has certain advantages.

[0028] The fluoride coating layer is located on the side of the overall coating layer away from the base material, which can effectively isolate the base material and the electrolyte / electrolyte. The fluoride coating layer has the following characteristics: (1) high chemical stability: the fluoride coating layer has high chemical stability, especially strong resistance to HF acid corrosion. Fluoride can preferentially react with trace HF in the electrolyte to form a stable interface to protect the base material, thereby reducing the dissolution of transition metals in the base material; (2) wide electrochemical window: fluoride is extremely stable at high voltage (higher than 4.5V) and is not easy to be oxidized and decomposed, which can provide long-term protection for lithium-rich manganese-based positive electrode materials; (3) physical barrier effect: fluoride can form a dense and uniform fluoride coating layer on the surface of the base material, which can effectively physically isolate the base material from direct contact with the electrolyte, thereby reducing side reactions and gas generation.

[0029] Specifically, the fluoride coating layer can include at least one of AlF3, TiF4, and ZrF4, all of which have high chemical stability, wide chemical window, and high structural stability. Further, the fluoride coating layer is preferably AlF3 because AlF3 has one of the widest electrochemical windows among all fluorides (stable voltage up to 5.5 V or more); AlF3 has a strong thermodynamic tendency to react with HF, which can durably maintain the stability of the positive electrode / electrolyte interface environment; AlF3 has high chemical stability (Al-F bond energy up to 664 kJ·mol -1 ) can effectively block direct contact between the base material and the electrolyte, inhibit the dissolution of transition metal ions (especially Mn 2+ ), and simultaneously inhibit the decomposition reaction of the electrolyte at high voltage; the AlF3 layer also has excellent mechanical strength, which can buffer the volume change of the base material during the cycle process; and, while providing optimal protection, the raw material cost of AlF3 is extremely low (Al and F elements are abundant), the synthesis process is simple, and the performance, cost, and scalable production requirements are perfectly balanced.

[0030] In addition, TiF4 and ZrF4 also have unique advantages: (1) TiF4 and ZrF4 can react with Li to generate lithium-containing fluoride fast ion conductors (fluoride coating layer), which makes the fluoride coating layer have both passivation and ion conduction functions; (2) compared with Al 3+ , the ionic radius of Ti 4+ and Zr 4+ is larger, the electronegativity is higher, and the binding energy with surface lattice oxygen (O 2- ) is stronger, which greatly inhibits oxygen loss; (3) the high valence state of Ti 4+ and Zr 4+ can better inhibit the reduction and migration of transition metals (especially Mn 3+ ) in the base material, slow down the transformation of the layered structure to spinel phase, and thus delay voltage decay.

[0031] In addition, the inner layer of lithium hexafluorozirconate needs to be protected by the outer layer of fluoride. Without the fluoride coating layer, the lithium hexafluorozirconate coating layer may be oxidized or corroded if it is exposed to a high-pressure or acidic electrolyte environment for a long time, resulting in functional degradation. Although the fluoride coating layer of the outer layer has high stability, the ionic conductivity of the fluoride coating layer is poor. If it is directly coated on the base material, it will seriously hinder the transmission of lithium ions, resulting in a decrease in the capacity and rate performance of the lithium-rich manganese-based positive electrode material. Therefore, the inner layer of lithium hexafluorozirconate and the outer layer of fluoride together form an "acid-resistant buffer zone", that is, the inner layer of the ion conductor serves as a "buffer" and "transition", effectively eliminating the corrosion effect of HF and reducing the transition metal dissolution of the base material, thereby improving the stability of the base material. The synergistic effect of the "high-lithium-conducting inner layer combined with the high-stability outer layer" design realizes "unobstructed and safe" ion transmission in the lithium-rich manganese-based positive electrode material.

[0032] In some embodiments, the mass percentage of the lithium hexafluorozirconate coating layer to the base material can be 1.0wt%-3.0wt%, which is beneficial to effectively improve the conductivity of the lithium-rich manganese-based positive electrode material. The mass percentage can exemplarily be 1.0wt%, 1.5wt%, 2.0wt%, 2.5wt%, 3.0wt%, or any value within the range formed by any two of the above values. The mass percentage can further be 1.5wt%-2.5wt%.

[0033] In some embodiments, the mass percentage of the fluoride coating layer to the base material can be 0.5wt%-3.0wt%, which is beneficial to balance the stability and capacity performance of the lithium-rich manganese-based positive electrode material. The mass percentage can exemplarily be 0.05wt%, 0.10wt%, 0.15wt%, 0.20wt%, 0.25wt%, 0.30wt%, or any value within the range formed by any two of the above values. The mass percentage can further be 0.05wt%-0.20wt%.

[0034] Compared with the prior art, the lithium-rich manganese-based positive electrode material of the present application adopts a unique lithium hexafluorozirconate-fluoride double-coating structure, effectively solving the key problems of interface instability and manganese dissolution of traditional lithium-rich manganese-based materials at high voltage, and making the lithium-rich manganese-based material have the advantages of high capacity, excellent rate performance, and long cycle life.

[0035] Referring to Figure 2 As shown in the accompanying drawings, the embodiment of the present application provides a preparation method of a lithium-rich manganese-based positive electrode material, which specifically comprises the following steps: Step S1, mixing a manganese precursor, a lithium source, and an aluminum source, and performing first sintering to obtain a base material.

[0036] Specifically, the manganese precursor, the lithium source and the aluminum source are proportioned by a high-speed mixer according to a specific Li / Me ratio, and after being uniformly mixed, dry mixture is obtained; then the dry mixture is charged into a crucible and subjected to a first sintering in an inert atmosphere or an air atmosphere, and after the material is cooled and collected, it is crushed to obtain a base material Li(Li x Mn a Ni 1-a-b Al b )O 2-δ , wherein 0.05≤x≤0.33, 0

[0037] In some embodiments, the manganese precursor can include a nickel-manganese hydroxide Ni x Mn y (OH)2, wherein x+y=1, 0.2≤x<0.5.

[0038] In some embodiments, the lithium source can include at least one of lithium hydroxide, lithium carbonate, and lithium nitrate.

[0039] In some embodiments, the ratio of the number of moles of lithium in the lithium source to the total number of moles of Me (including Ni and Mn) in the manganese precursor, Li:Me, can be (1.10-1.45):1, which is conducive to providing sufficient lithium source. Li:Me can exemplarily be 1.10:1, 1.15:1, 1.2:1, 1.25:1, 1.3:1, 1.35:1, 1.4:1, 1.45:1, or any value within the range formed by any two of the above values. Li:Me can further be (1.25-1.40):1.

[0040] In some embodiments, the aluminum source includes at least one of an aluminum hydroxide and an aluminum oxide.

[0041] In some embodiments, the mass percentage of the mass of the aluminum source in the total mass of the manganese precursor, the lithium source and the aluminum source (i.e., the dry mixture) can be 0.05wt%-0.5wt%, which is conducive to the aluminum element being formed in the base material while some aluminum elements remain on the surface to form an aluminum oxide inert coating layer, which cooperates with the fluoride coating layer on the outer layer to further reduce the direct contact between the base material and the electrolyte. The mass percentage can exemplarily be 0.05wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt%, 0.45wt%, 0.5wt%, or any value within the range formed by any two of the above values. The high mass percentage can further be 0.05wt%-0.15wt%.

[0042] In some embodiments, the conditions of the first sintering can include: sintering at a temperature of 750-950°C for 10-15h with a temperature increasing rate of 2-3°C / min. Slow temperature increasing is beneficial for mutual diffusion and reaction of raw materials at high temperature to generate a matrix material with perfect crystal structure. The temperature of the first sintering can exemplarily be 750°C, 800°C, 850°C, 900°C, 950°C or any value within a range defined by any two of the above values, and the time can exemplarily be 10h, 11h, 12h, 13h, 14h, 15h or any value within a range defined by any two of the above values.

[0043] During the first sintering, the loading amount of the crucible is controlled to be 1.5-3.0kg / crucible, the material inside the crucible is synchronously heated, and the entering and discharging efficiency of the sintering gas is improved, thereby improving the uniformity of the first sintering and being beneficial to normal capacity performance of the matrix material in the subsequent preparation of the lithium-rich manganese-based positive electrode material in the charging and discharging process.

[0044] In some embodiments, the median particle size Dv50 of the particles of the matrix material can be 5-10μm.

[0045] In step S2, after mixing the matrix material with the hexafluorozirconic acid solution and drying treatment, the second sintering is performed to generate a lithium hexafluorozirconate coating layer on the surface of the matrix material, thereby obtaining an intermediate material.

[0046] Specifically, the matrix material is mixed with the hexafluorozirconic acid solution, the residual lithium on the surface of the matrix material can react in situ with the hexafluorozirconic acid solution to generate lithium hexafluorozirconate, and then the mixture of the matrix material and the hexafluorozirconic acid solution is dried to preliminarily form a lithium hexafluorozirconate coating layer by using a wet coating process, and then the second sintering and crushing treatment are performed under an inert atmosphere to form a complete lithium hexafluorozirconate coating layer on the surface of the matrix material, thereby obtaining an intermediate material [Li(Li x Mn a Ni 1-a- b Al b )O 2-δ ]@Li2ZrF3, 0.05≤x≤0.33, 0

[0047] In some embodiments, the concentration of the hexafluorozirconic acid solution can be 1.0 mol / L to 5.0 mol / L, which is conducive to the uniform reaction of the hexafluorozirconic acid with the residual lithium on the surface of the base material to form a uniform and complete lithium hexafluorozirconate coating layer. The concentration of the hexafluorozirconic acid solution can exemplarily be 1.0 mol / L, 2.0 mol / L, 3.0 mol / L, 4.0 mol / L, 5.0 mol / L, or any value within the range consisting of any two of the above values. The concentration of the hexafluorozirconic acid solution can further be 2.0 mol / L to 4.0 mol / L.

[0048] Further, the mass / volume ratio of the base material to the hexafluorozirconic acid solution can be 10 kg: (0.1 L to 1.0 L), which is conducive to the sufficient contact of the base material with the hexafluorozirconic acid solution and the in-situ reaction of the hexafluorozirconic acid with the residual lithium on the surface of the base material to form a complete and uniform lithium hexafluorozirconate coating layer. The mass / volume ratio of the base material to the hexafluorozirconic acid solution can exemplarily be 10 kg: 0.25 L, 10 kg: 0.35 L, 10 kg: 0.45 L, 10 kg: 0.55 L, 10 kg: 0.65 L, 10 kg: 0.75 L, 10 kg: 0.85 L, 10 kg: 1.0 L, or any value within the range consisting of any two of the above values. The mass / volume ratio of the base material to the hexafluorozirconic acid solution can further be 10 kg: (0.35 L to 0.75 L).

[0049] In the drying process of the mixture of the base material and the hexafluorozirconic acid solution, the drying temperature can be 80°C to 120°C, which is conducive to the in-situ reaction of the hexafluorozirconic acid with the residual lithium on the surface of the base material to form lithium hexafluorozirconate and the fixation of the lithium hexafluorozirconate on the surface of the base material. The drying temperature can exemplarily be 80°C, 90°C, 100°C, 110°C, 120°C, or any value within the range consisting of any two of the above values. The drying temperature can further be 90°C to 110°C.

[0050] In some embodiments, the drying equipment can be an oven, which can perform constant temperature drying. It can be understood that the drying equipment includes but is not limited to an oven, and any equipment capable of drying can be used.

[0051] In some embodiments, the moisture content of the mixture of the base material and the hexafluorozirconic acid solution after drying is 0 to 1 wt%.

[0052] The second sintering condition can include: sintering at a temperature of 300-600°C for 2-5h with a temperature increasing rate of 2-3°C / min, and a lower sintering temperature is conducive to reducing the volatilization of fluorine in the lithium hexafluorozirconate coating layer and the phase transition of the substrate material, forming a more stable lithium hexafluorozirconate coating layer and improving the structural stability of the intermediate material. The temperature of the second sintering can exemplarily be 300°C, 400°C, 500°C, 600°C, or any value within a range defined by any two of the above values, and the time can exemplarily be 2h, 3h, 4h, 5h, or any value within a range defined by any two of the above values.

[0053] In the second sintering process, the charging amount of the crucible is controlled to be 1.5-3.0kg / crucible, the material inside the crucible is synchronously heated, and the entering and discharging efficiency of the sintering gas is improved, thereby improving the uniformity of the second sintering.

[0054] The process of drying treatment before wet coating and then low-temperature sintering is conducive to forming a uniform and stable lithium hexafluorozirconate coating layer, thereby improving the electronic conductivity and ionic conductivity of the substrate material and improving the rate performance of the lithium-rich manganese-based positive electrode material.

[0055] In some embodiments, the median particle size Dv50 of the particles of the intermediate material can be 7-9μm.

[0056] In step S3, the intermediate material and the fluoride are mixed and subjected to a third sintering to form a fluoride coating layer on the surface of the intermediate material, thereby obtaining the lithium-rich manganese-based positive electrode material.

[0057] Specifically, the intermediate material and the fluoride are dry mixed and coated by a high-speed mixer, and the third sintering is performed in an inert atmosphere to generate a fluoride coating layer on the surface of the intermediate material, thereby obtaining a double-coated lithium-rich manganese-based positive electrode material [Li(Li x Mn a Ni 1-a-b Al b )O 2-δ ]@Li2ZrF3@fluoride, 0.05≤x≤0.33, 0

[0058] In some embodiments, the fluoride can include at least one of AlF3, TiF4, and ZrF4, etc. The fluoride can further be AlF3.

[0059] Further, the mass percentage of fluoride to the base material can be 0.05wt%-0.30wt%, which is conducive to forming a uniform and dense fluoride coating layer, effectively protecting the base material, and taking into account the high capacity of the lithium-rich manganese-based positive electrode material. The mass percentage of fluoride to the base material can exemplarily be 0.05wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt% or any value within the range between any two of the above values. The mass percentage of fluoride to the base material can further be 0.05wt%-0.20wt%.

[0060] The third sintering conditions can include: sintering at a temperature of 550°C-750°C for 7-12h with a temperature rising rate of 2°C / min-3°C / min, which is conducive to improving the binding force of the fluoride coating layer and the intermediate material and improving the stability of the fluoride coating layer. The temperature of the third sintering can exemplarily be 300°C, 400°C, 500°C, 600°C or any value within the range between any two of the above values, and the time can exemplarily be 2h, 3h, 4h, 5h or any value within the range between any two of the above values.

[0061] During the third sintering process, the charging amount of the crucible is controlled to be 1.5-3.0kg / crucible, the material inside the crucible is synchronously heated, and the entering and discharging efficiency of the sintering gas is improved, thereby improving the uniformity of the third sintering.

[0062] In some embodiments, the median particle size Dv50 of the lithium-rich manganese-based positive electrode material particles can be 7μm-9μm.

[0063] Compared with the prior art, the preparation method of the lithium-rich manganese-based positive electrode material has the following beneficial effects: 1. By combining the wet coating of lithium hexafluorozirconate with the low-temperature sintering inner coating process, fast ion conductor lithium hexafluorozirconate capable of providing high ion conductivity is formed in situ on the surface of the base material, which can effectively improve the electronic conductivity and ion conductivity of the lithium-rich manganese-based positive electrode material, thereby improving the rate performance of the lithium-rich manganese-based positive electrode material.

[0064] 2. By forming a fluoride coating layer as an outer coating, since fluoride has good chemical stability and thermal stability, the fluoride coating layer can effectively prevent the electrolyte from directly contacting the base material, thereby reducing the occurrence of side reactions. Compared with traditional coating materials, the fluoride coating layer has better adhesion and is not easy to fall off from the surface of the material, thereby playing a long-term protection role.

[0065] 3. The double-layer coating process of the present application, compared with the traditional single coating method, realizes the performance improvement and balance of the lithium-rich manganese-based positive electrode material through the reasonable design of the double-layer coating structure, so that the lithium-rich manganese-based positive electrode material can still maintain good cycle stability under high-rate discharge conditions.

[0066] 4. The present application further optimizes the sintering temperature and time of the second and third sintering and other process conditions to further improve the coating effect, improve the cycle stability and service life of the lithium-rich manganese-based positive electrode material, and at the same time, improve the conductivity of the lithium-rich manganese-based positive electrode material, and further improve the comprehensive performance of the lithium-rich manganese-based positive electrode material.

[0067] The embodiment of the present application also provides an electrochemical device (for example, a secondary battery), which comprises a positive electrode sheet, and the positive electrode sheet comprises a positive electrode material, and the positive electrode material is the aforementioned lithium-rich manganese-based positive electrode material.

[0068] Compared with the prior art, the electrochemical device provided by the embodiment of the present application has high charge-discharge efficiency and rate performance, and also has high capacity and cycle life during the charge-discharge process, because the aforementioned lithium-rich manganese-based positive electrode material is applied.

[0069] The scheme of the present application will be explained below in combination with embodiments. Those skilled in the art will understand that the following examples are only used to explain the present application, and cannot be understood as a limitation of the present application. Unless otherwise indicated, the reagents, software and instruments involved in the following embodiments, which are not specifically indicated, are all conventional commercially available products or public.

[0070] Embodiment 1 Step S1, mixing Ni 0.3 Mn 0.7 (OH)2, lithium carbonate and Al(OH)3, the mixing method is as follows: first, put half of the mass of Ni 0.3 Mn 0.7 (OH)2 and lithium carbonate into a high-speed mixer, and then continue to mix the remaining half of the mass of Ni 0.3 Mn 0.7 (OH)2 and lithium carbonate after the Al(OH)3 is put in, through crucible feeding (the feeding amount of the crucible is controlled to be 1.5-3.0 kg / crucible), and the first sintering is performed at 900℃ under air atmosphere at a temperature rising rate of 2℃ / min-3℃ / min for 12h, and then the powder is crushed and sieved after cooling, to obtain a matrix material Li(Li 0.17 Mn 0.59 Ni 0.331 Al 0.1 )O2 with a median particle size Dv50 of 5-10μm. 0.3 Mn 0.7The ratio of Li:Me is 1.30:1, and Al(OH)3 accounts for Ni 0.3 Mn 0.7 The mass percentage of Al(OH)3 in the total mass of Li2CO3, LiOH·H2O and Al(OH)3 is 0.1wt%.

[0071] In step S2, the substrate material is mixed with a hexafluorozirconic acid solution with a concentration of 2mol / L, the ratio of the substrate material to the hexafluorozirconic acid solution is 10kg:0.45L, and after drying treatment at 120℃ using an oven (the moisture content of the dried material is less than or equal to 1wt%), the second sintering is performed at 450℃ for 3h under a nitrogen atmosphere at a temperature rising rate of 2℃ / min~3℃ / min through crucible feeding (the feeding amount of the crucible is controlled at 1.5~3.0kg / crucible), a lithium hexafluorozirconate coating layer is formed on the surface of the substrate material by reaction, and after cooling, the material is crushed and sieved to obtain an intermediate material [Li(Li x Mn a Ni 1-a-b Al b )O 2-δ ]@Li2ZrF3.

[0072] In step S3, the intermediate material is mixed with fluoride AlF3 through a high-speed mixer, wherein the mass percentage of AlF3 to the substrate material is 0.1wt%, and the second sintering is performed at 720℃ for 8h under an air atmosphere at a temperature rising rate of 2℃ / min~3℃ / min through crucible feeding (the feeding amount of the crucible is controlled at 1.5~3.0kg / crucible), a fluoride coating layer is formed on the surface of the intermediate material, and after cooling, the material is crushed and sieved to obtain a lithium-rich manganese-based positive electrode material [Li(Li x Mn a Ni 1-a-b Al b )O 2-δ ]@Li2ZrF3@AlF3.

[0073] Example 2: The difference from Example 1 is that the concentration of the hexafluorozirconic acid solution in step S2 is 5mol / L. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0074] Example 3: The difference from Example 1 is that the mass percentage of AlF3 to the substrate material in step S3 is 0.3wt%. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0075] Example 4: The difference from Example 1 is that the fluoride in step S3 is ZrF4, and the lithium-rich manganese-based positive electrode material formed is [Li(Li x Mn a Ni 1-a-b Al b )O 2-δ ]@Li2ZrF3@ZrF4. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0076] Example 5: The difference from Example 1 is that the fluoride in step S3 is TiF4, and the lithium-rich manganese-based positive electrode material formed is [Li(Li x Mn a Ni 1-a-b Al b )O 2-δ ]@Li2ZrF3@TiF4. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0077] Example 6: The difference from Example 1 is that the temperature of the second sintering in step S2 is 600°C. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0078] Comparative Example 1: The difference from Example 1 is that steps S2 and S3 are not performed, and the uncoated base material obtained in step S1 is the lithium-rich manganese-based positive electrode material. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0079] Comparative Example 2: The difference from Example 1 is that no aluminum source is added in step S1. The rest of the preparation method of the lithium-rich manganese-based positive electrode material is basically the same as that of Example 1.

[0080] The lithium-rich manganese-based positive electrode materials obtained in Examples 1-6 and Comparative Examples 1-2 are subjected to the following tests.

[0081] 1. Scanning electron microscope (SEM) test: A scanning electron microscope Axia ChemiSEM (model) with high-resolution imaging capability is used to observe the microstructure and structural characteristics of the material at a magnification of 10,000 times and 5,000 K.

[0082] 2. Mn dissolution: Referring to GB / T 37211-2018 Lithium Ion Battery Positive Electrode Material Metal Impurity Dissolution Test Method.

[0083] 3. Particle size test: A Malvern 3000 particle size tester is used to test the particle size distribution Dv50 after 5 minutes of ultrasonic dispersion.

[0084] 4. Electrochemical performance test: Preparation of battery: the lithium-rich manganese-based positive electrode material, polyvinylidene fluoride (PVDF), and conductive agent (such as acetylene black or conductive carbon black) were weighed in a certain proportion, then NMP was added to prepare a slurry. Then, the slurry was uniformly coated on an aluminum foil, and vacuum drying was performed at 115℃±5℃ / 8h, followed by compaction, cutting into a round piece, and using a lithium sheet as a negative electrode material, and finally transferring to a glove box to assemble a CR2025 specification button cell.

[0085] The test steps of the first charge-discharge efficiency include: under the condition of 25℃, the above-mentioned battery is charged to 4.55V at 0.1C (nominal capacity of 150mAh / g) current, then constant voltage charging is performed until the current decreases to 0.05C, the charging stops, and the first charge capacity is recorded; then it is discharged to the cut-off voltage of 2.3V at 0.1C, and the first discharge capacity is recorded, the 0.1C specific capacity of the positive electrode material is calculated according to the first discharge capacity, and the first charge-discharge efficiency of the battery is calculated according to the following formula, i.e. the first efficiency: first charge-discharge efficiency=(first discharge capacity / first charge capacity)*100%.

[0086] Charge-discharge capacity: the above-mentioned charging and discharging process is repeated once, and the 2nd charge capacity and the 2nd discharge capacity are recorded, and the 0.1C specific capacity of the lithium-rich manganese-based positive electrode material is calculated according to the 2nd discharge capacity.

[0087] High-temperature cycle retention rate test: the capacity retention rate of the test battery after 100 cycles at 45℃ with a charge-discharge current of 1C and a voltage of 2.3V~4.55V.

[0088] The test results of examples 1-6 and comparative examples 1-2 are shown in table 1 and Figures 3 to 5 .

[0089] Table 1 The above results show that: It can be seen from Figure 3 Fig. a-2 that the surface of the uncoated base material (comparative example 1) is relatively clean, and the secondary particles are spherical, indicating that the addition of aluminum source Al(OH)3 can optimize the crystal structure of the base material, so that the particle morphology of the base material has good uniformity. By comparing Figure 3 Fig. b-1, Figure 3 Fig. b-2, Figure 3 Fig. c-1 and Figure 3 Fig. c-2, it is found that compared with example 3 (AlF3 coating amount of 0.30wt%), the lithium-rich manganese-based positive electrode material in example 1 (AlF3 coating amount of 0.10wt%) has fewer small particles on the surface, the coating is more uniform, and the coating amount is more appropriate.

[0090] In combination with Table 1 and Figures 4 to 5 It can be seen that, compared with Comparative Example 1 and Comparative Example 2, the base material in Example 1 is doped with Al(OH)3, and the inner and outer layers are coated with Li2ZrF6 and AlF3 respectively, the initial capacity of the lithium-rich manganese-based positive electrode material prepared in Example 1 reaches 228.4 mAh / g at 0.1C, and the initial efficiency is 90.2%, which is significantly higher than that of Comparative Example 1 (220.1 mAh / g at 0.1C, and the initial efficiency is 88.4%) and Comparative Example 2 (218.5 mAh / g at 0.1C, and the initial efficiency is 86.7%). This is because the Al element in the base material can optimize the lattice structure of the base material, and at the same time improve the uniformity of the particle morphology of the base material, provide more channels for the diffusion of lithium ions, and improve the diffusion rate of lithium ions. In addition, the Li2ZrF6 coating layer in the inner layer mainly plays the role of fast ion conduction and structural stability, and the strong electronegativity of Zr 4+ can stabilize the Mn-O bond and inhibit the Jahn-Teller distortion of Mn 3+ . The dissolution rate of Mn 2+ is reduced. The outer AlF3 coating has the functions of chemical stability and mechanical protection of the positive electrode / electrolyte interface, and the high chemical stability of AlF3 (the bond energy of Al-F is 664 kJ / mol) can effectively block the contact between the base material and the electrolyte, reduce the erosion of HF to the base material and the dissolution of transition metals in the base material; and the strong Al-O bond can fix the surface lattice oxygen of the lithium-rich manganese-based positive electrode material, reducing the oxygen release amount of the lithium-rich manganese-based positive electrode material during the charging process. The synergistic effect of the double coating layers can promote the diffusion and migration of lithium ions in the electrode material, reduce the electrode polarization, improve the charge and discharge efficiency and rate performance of the battery; at the same time, as a physical barrier, it can effectively reduce the side reaction between the base material and the electrolyte, slow down the structure collapse and dissolution of active substances during the charging and discharging process of the base material, thereby improving the cycle life of the lithium-rich manganese-based positive electrode material.

[0091] On the basis of Example 3, the coating amount of AlF3 in Example 1 is moderately adjusted, and the lithium-rich manganese-based positive electrode material in Example 1 better balances the protection effect of the fluoride coating layer and the specific capacity and lithium ion transmission of the lithium-rich manganese-based positive electrode material, reduces the interface impedance and battery polarization, and the charge and discharge capacity, initial efficiency and high-temperature cycle stability of the lithium-rich manganese-based positive electrode material prepared in Example 1 are better.

[0092] It can be understood that the above embodiments are only exemplary embodiments adopted for illustrating the principles of the present application, and the present application is not limited thereto. Various modifications and improvements can be made by those skilled in the art without departing from the spirit and essence of the present application, and these modifications and improvements are also considered as the protection scope of the present application.

Claims

1. A lithium-rich manganese-based cathode material, characterized in that, The substrate material comprises a matrix material and a coating layer, the coating layer comprising a lithium hexafluorozirconate coating layer on the surface of the matrix material and a fluoride coating layer on the side of the lithium hexafluorozirconate coating layer away from the matrix material, wherein the matrix material has the chemical formula Li(Li) x Mn a Ni 1-a-b Al b )O 2-δ Where 0.05≤x≤0.33, 0 <a<0.6,0.1≤b≤0.25,0≤δ≤0.2。 2. The lithium-rich manganese-based cathode material as described in claim 1, characterized in that, The fluoride coating includes at least one of AlF3, TiF4, and ZrF4.

3. The lithium-rich manganese-based cathode material as described in claim 1, characterized in that, The lithium hexafluorozirconate coating has a mass percentage of 1.0 wt% to 3.0 wt% of the substrate material; and / or The mass percentage of the fluoride coating to the matrix material is 0.05wt% to 0.30wt%.

4. A method for preparing a lithium-rich manganese-based cathode material, characterized in that, include: The manganese-based precursor, lithium source, and aluminum source are mixed and subjected to a first sintering to obtain the matrix material; The matrix material is mixed with a hexafluorozirconic acid solution and dried, and then subjected to a second sintering to react on the surface of the matrix material to form a lithium hexafluorozirconate coating layer, thus obtaining an intermediate material. as well as The intermediate material and fluoride are mixed and sintered a third time to form a fluoride coating layer on the surface of the intermediate material, thereby obtaining the lithium-rich manganese-based cathode material.

5. The preparation method according to claim 4, characterized in that, The concentration of the hexafluorozirconic acid solution is 1.0 mol / L to 5.0 mol / L; and / or The mass / volume ratio of the matrix material to the hexafluorozirconic acid solution is 10 kg: (0.1 L~1.0 L).

6. The preparation method according to claim 4, characterized in that, The fluoride includes at least one of AlF3, TiF4, and ZrF4; and / or The mass percentage of the fluoride to the matrix material is 0.05wt% to 0.30wt%.

7. The preparation method according to claim 4, characterized in that, The manganese-based precursor includes nickel manganese hydroxide (Ni). x Mn y (OH)2, where x+y=1, 0.2≤x<0.

5.

8. The preparation method according to claim 4, characterized in that, The mass percentage of the aluminum source relative to the total mass of the manganese-based precursor, the lithium source, and the aluminum source is 0.05 wt% to 0.5 wt%; and / or The aluminum source includes at least one of aluminum hydroxide and aluminum oxide.

9. The preparation method according to claim 4, characterized in that, The drying temperature is 80℃~120℃; The conditions for the first sintering include: heating to 750℃~950℃ at a heating rate of 2℃ / min~3℃ / min and sintering for 10~15h; The conditions for the second sintering include: heating to 300℃~600℃ at a heating rate of 2℃ / min~3℃ / min and sintering for 2~5 hours; The conditions for the third sintering include: heating to 550℃~750℃ at a heating rate of 2℃ / min~3℃ / min and sintering for 7~12 hours.

10. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode sheet, the positive electrode sheet includes a positive electrode material, the positive electrode material being a lithium-rich manganese-based positive electrode material as described in any one of claims 1 to 3 or a lithium-rich manganese-based positive electrode material prepared by the method for preparing a lithium-rich manganese-based positive electrode material as described in any one of claims 4 to 9.

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