Preparation method of cobalt-free single crystal lithium-rich positive electrode material

Abstract

The invention relates to a preparation method of a cobalt-free single crystal lithium-rich positive electrode material. The preparation method comprises the following steps: preparing a nickel-manganese oxygen-containing compound precursor from manganese salt and nickel salt by using a solid-phase method or a coprecipitation method; mixing the prepared nickel-manganese oxygen-containing compound precursor with a low-melting-point lithium source, pre-sintering and melting the mixture, performing high-energy ball milling, adding a conventional lithium source once or multiple times during ball milling, and calcining the mixture after ball milling to obtain an initial product of the cobalt-free single crystal lithium-rich positive electrode material; and carrying out acid treatment and secondary calcination on the initial product to obtain the cobalt-free single crystal lithium-rich positive electrode material product. The single crystal lithium-rich material with excellent performance can be prepared with high efficiency and low cost, and has excellent electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the field of lithium batteries, and in particular to the cathode material technology in lithium-ion batteries, and aims to provide a cobalt-free single crystal lithium-rich cathode material. Background Art

[0002] Currently, lithium-ion batteries are widely used in portable electronics and electric vehicles. However, due to the limited capacity and energy density of current cathode materials, the long-distance driving requirements of electric vehicles with commercial lithium-ion batteries are still unattainable. In addition, layered oxide cathode materials such as lithium-rich layered oxides and high-nickel ternary layered oxides are considered promising candidates due to their high energy density. Compared with high-nickel ternary layered oxides, xLi 2 MnO 3 ·(1-x)LiMO 2 The capacity of the lithium-rich layered oxide positive electrode material (M=Mn, Co, Ni) is greater than 250 mAh / g, and the energy density can reach 1000 Wh / kg.

[0003] However, the electrochemical performance of lithium-rich layered oxide cathode materials is still unsatisfactory, including large initial irreversible capacity, unstable cycling, and poor rate performance, which may be caused by irreversible structural rearrangement during the first charge, oxygen loss in the surface area, and destruction of the surface structure of the material. In addition, due to the long-term charge and discharge cycle of polycrystalline lithium-rich layered oxides, the aggregated spherical polycrystalline lithium-rich cathode promotes crack generation due to the accumulation of internal strain, which further aggravates the penetration and volume expansion of the electrolyte, resulting in overall instability from the whole to the interface. Single-crystalline lithium-rich materials are believed to improve cycle stability by delaying particle cracking. The most attractive advantage of single-crystalline lithium-rich is the elimination of intergranular cracks, thanks to its intrinsic integrity and superior mechanical strength.

[0004] The methods reported so far for preparing single-crystal lithium-rich materials usually involve extensive molten salt-assisted sintering, and these molten salts need to be removed through subsequent complex washing steps, which leads to expensive production costs for large-scale preparation of single-crystal lithium-rich materials. Therefore, it is of great significance to develop a simple method for preparing high-performance single-crystal lithium-rich materials. Summary of the invention

[0005] In view of the above-mentioned problems in the prior art, the present invention provides a method for preparing a cobalt-free single crystal lithium-rich positive electrode material, which surpasses the traditional method for preparing single crystal lithium-rich materials and can prepare single crystal lithium-rich materials with excellent performance with high efficiency and low cost. The material can be used in lithium-ion batteries with high energy density and long cycle life, and has broad market prospects.

[0006] The specific scheme of the present invention is as follows:

[0007] A method for preparing a cobalt-free single crystal lithium-rich positive electrode material comprises the following steps:

[0008] S1: preparing a nickel-manganese oxygen-containing compound precursor by a solid phase method or a coprecipitation method using a manganese salt and a nickel salt;

[0009] S2: mixing the prepared nickel-manganese oxygen-containing compound precursor with a low-melting-point lithium source, pre-calcining and melting the mixture, and then subjecting the mixture to high-energy ball milling, adding a conventional lithium source once or multiple times during the ball milling, and calcining the mixture after ball milling to obtain a primary product of a cobalt-free single crystal lithium-rich positive electrode material;

[0010] S3: The primary product is subjected to acid treatment and secondary calcination to obtain a cobalt-free single crystal lithium-rich positive electrode material product.

[0011] The above method of the present invention firstly uses only two transition metal sources, manganese salt and nickel salt, to prepare the precursor. The obtained nickel-manganese oxygen-containing compound is conducive to forming relatively loose flaky morphology particles. In subsequent operations, on the one hand, it is convenient to fully refine in ball milling, and on the other hand, it is convenient to form a low-melting point eutectic with a low-melting point lithium source, so that a viscous mixture that is conducive to the growth of small-particle single crystals can be formed during the ball milling process of step S2, and then a good single-crystal morphology of cobalt-free single-crystal lithium-rich positive electrode material is formed by calcination. Finally, the surface of the initial product is modified by acid treatment and secondary calcination, so that the single crystal surface of the layered structure is transformed into a stable spinel structure, which promotes the stability of small-particle single crystal particles. The present invention creatively regulates the precursor and the low-melting point lithium source to make them high-energy ball milled in a molten state, which provides a favorable environment for the uniform growth of small-particle single crystals during the calcination process, and provides a new preparation idea for the effective preparation of high-performance single-crystal lithium-rich materials.

[0012] Optimized preparation conditions are conducive to ensuring the efficient formation of the primary product. In some schemes, the precursor of the nickel-manganese oxygen-containing compound is nickel-manganese hydroxide, which is prepared by coprecipitation, which is conducive to obtaining more refined and loose fine particles with fish scale-like longitudinal stacking and a particle size of 2-5μm. In some schemes, such precursors are combined with lithium salts with a melting point below 300°C to more easily obtain a mixture that maintains a stable molten state during ball milling. In some schemes, such mixtures can be fully broken and refined during ball milling with a ball-to-material ratio greater than 6:1, preferably 7:1, and a rotation speed greater than 300rpm, forming an ideal state that promotes single crystal growth.

[0013] In some optimized schemes, the prepared cobalt-free single crystal lithium-rich cathode material has the following general formula: Li 1+ x Ni y Mn 0.8-y O 2, where 0 < x < 0.25 and 0 < y < 0.8, preferably 0.15 < x < 0.25 and 0.1 < y < 0.3; correspondingly, the molar ratio of nickel to manganese in the nickel-manganese oxide precursor satisfies y: 0.8 - y, 0 < y < 0.8, preferably 0.1 < y < 0.3; the optimized chemical ratio is beneficial to improving the formation efficiency of the precursor of flaky morphology particles and the low-melting eutectic while ensuring the formation of a layered single-crystal structure, and increasing the lithium content to ensure the energy density of the product.

[0014] In some optimized embodiments, the manganese salt is one or more of manganese carbonate, manganese nitrate, manganese acetate, and manganese sulfate; the nickel salt is one or more of nickel carbonate, nickel nitrate, nickel acetate, and nickel sulfate.

[0015] In some optimized embodiments, the low-melting lithium source is selected from one or more of organic acid lithium salts, inorganic lithium salts, or halogen-containing lithium salts with a melting point below 300°C. Exemplarily, the organic acid lithium salts include lithium acetate, lithium propionate, lithium butyrate, lithium valerate, lithium hexanoate, lithium heptanoate, and lithium octanoate; the inorganic lithium salts include lithium nitrate; the halogen-containing lithium salts include lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide, and lithium thiocyanate. Further preferably, it is lithium nitrate, which is beneficial to providing an acidic environment for the synthesis.

[0016] In some exemplary embodiments, the solid-phase method generally directly mechanically mixes the manganese salt and the nickel salt evenly to obtain the nickel-manganese oxide precursor. The mechanical mixing method can be mechanical stirring, and the mechanical method is simple and convenient to operate.

[0017] In some exemplary embodiments, the co-precipitation method generally dissolves the manganese salt and the nickel salt in a solvent to prepare solution A. Under the protection of a protective gas, solution A, an alkali solution, and ammonia water are mixed for a co-precipitation reaction to obtain a suspension, which is then filtered, washed, and dried to obtain the nickel-manganese oxide precursor. The co-precipitation method has high condition controllability and is more conducive to obtaining a precursor structure with refined and loose fish-scale-like longitudinal stacking of particles compared to the solid-phase method. Among them, the solvent is generally deionized water, and the alkali solution can be selected from sodium hydroxide solution or sodium carbonate solution. As a more optimized reaction condition, the total concentration of manganese ions and nickel ions in solution A is 2 - 5 mol / L, the concentration of the alkali solution is 2 - 7 mol / L, the concentration of ammonia water is 1 - 4 mol / L, and the volume ratio of solution A, the alkali solution, and ammonia water is 1:1:1 - 2; the pH value of the suspension is 9.5 - 12.0, the co-precipitation reaction temperature is 30 - 80°C, and the reaction time is 1 - 4 hours.

[0018] In some optimized schemes, the conventional lithium source is selected from lithium sources with a melting point higher than 600° C., including but not limited to one or more of lithium carbonate, lithium sulfate, lithium phosphate, lithium borate, lithium sulfide, lithium fluoride, etc. The conventional lithium source is distributed and added to the system during the ball milling process, which is conducive to stabilizing the low eutectic mixture and preventing the single crystal grains from precipitating prematurely during the calcination process, resulting in abnormal grain enlargement.

[0019] In some optimized schemes, the total amount of low-melting-point lithium source and conventional lithium source is fed according to the theoretical stoichiometric ratio of the target product, and the molar amount of lithium contained in each of the low-melting-point lithium source and the conventional lithium source is calculated in a ratio of (2-2.5):(0.5-0.75).

[0020] In some optimized schemes, conventional lithium source is added multiple times during ball milling according to the total amount of material to be added, with the amount added each time decreasing, preferably 2-3 times.

[0021] In some optimized schemes, high-energy ball milling uses zirconium beads as ball milling media with a diameter of 5-12 mm and a total ball milling time of 2-5 h, which is beneficial to improving ball milling efficiency.

[0022] In some optimized schemes, the pre-firing temperature is generally not limited, and it is best to obtain a molten mixture. The optimized pre-firing temperature is 200-300°C, taking into account both the sufficiency of melting and the stability of the melt.

[0023] In some optimized schemes, the mixture is screened after ball milling, and the mesh size of the screen is preferably 5000-7000, which is beneficial to promoting the uniformity of the ball milled mixture.

[0024] In some optimized schemes, the calcination temperature is preferably 850-900°C, which is conducive to the formation of single crystals in a low eutectic environment. Furthermore, in order to ensure the integrity of the single crystal particles and reduce defects, a more optimized stage heating system can be adopted, with stage insulation at 300-500°C, 700-800°C, and 850-900°C, and the heating rate before heating to the temperature of each insulation stage is preferably 3-8°C / min.

[0025] In some optimized schemes, the acid used for acid treatment includes but is not limited to one or more of citric acid, acetic acid, phosphoric acid, boric acid, and hydrofluoric acid, and the acid concentration is preferably 1-5 mol / L. In order to obtain a more stable spinel surface and further reduce the stabilizing effect on the internal layered structure of the product, a mixed acid solution of citric acid and acetic acid, or citric acid and phosphoric acid can be optimized. In the mixed acid solution, the volume ratio of citric acid to other acids is preferably (1-2):1.

[0026] In some optimized schemes, secondary sintering promotes the stabilization of the surface structure of the product after acid treatment, and it is better to control the secondary sintering temperature at 300-500℃ and 1-3h.

[0027] By the above method of the present invention, a cobalt-free single crystal lithium-rich positive electrode material can be obtained, and the positive electrode material includes at least one of the following characteristics:

[0028] (1) The particle size is between 100nm and 300nm;

[0029] (2) The interior is layered and the surface is spinel phase;

[0030] (3) The reversible discharge capacity at 0.1C is greater than 260 mAh / g, and the coulombic efficiency is greater than 93%;

[0031] (4) The capacity retention rate after 200 cycles at 1C is greater than 80%, and the discharge capacity is greater than 180 mAh / g;

[0032] (5) Reversible discharge capacity at 10C is greater than 98 mAh / g;

[0033] (6) XRD shows that I(003) / I(104) is greater than 2.22.

[0034] Compared with the prior art, the present invention has at least the following beneficial effects:

[0035] 1. The method of the present invention can successfully prepare a cobalt-free single crystal lithium-rich positive electrode material with a small particle single crystal morphology, and the particle size is between 100nm and 300nm, which can effectively improve the transmission rate of lithium ions, improve the reversible specific capacity and rate performance of the material, and the single crystal cycle stability is better than that of polycrystalline lithium-rich materials.

[0036] 2. The present invention optimizes and modifies the cobalt-free single crystal lithium-rich positive electrode material through acid treatment, which can significantly improve the first-week coulomb efficiency of the lithium-rich material. It not only consumes the lithium carbonate and other products on the surface of the primary product material, but also produces a spinel phase on the surface, which can inhibit surface side reactions and reduce irreversible charging capacity.

[0037] 3. The present invention obtains a cobalt-free single crystal lithium-rich positive electrode material product with excellent discharge specific capacity, coulombic efficiency, cycle and rate performance through optimized process conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0039] Figure 1 The fish scale flake Mn prepared in the embodiment of the present invention 0.75 Ni 0.25 (OH) 2 Precursor SEM image;

[0040] Figure 2 XRD patterns of the products prepared in Examples and Comparative Examples;

[0041] Figure 3 The SEM images of the products prepared in the examples and comparative examples are as follows;

[0042] Figure 4 The first week charge and discharge curves of the products prepared in the examples and comparative examples are shown;

[0043] Figure 5 The cyclic charge-discharge curves of the products prepared in the examples and comparative examples are shown;

[0044] Figure 6 The curve diagram of the discharge medium voltage variation during the cycle of the products prepared in the examples and comparative examples;

[0045] Figure 7 The figure is the rate performance diagram of the products prepared in the examples and comparative examples;

[0046] Figure 8 The EIS curves of the products prepared in the examples and comparative examples before activation are shown;

[0047] Fig. 9 The EIS curves of the products prepared in the examples and comparative examples after activation for 3 cycles;

[0048] Fig.10 The first week charge and discharge curves of the initial product and the product prepared in the example. DETAILED DESCRIPTION

[0049] Detailed preparation methods and technical steps are provided in the embodiments of the present invention, which are intended to fully illustrate the specific implementation process of the present invention. However, these detailed technical details only represent a specific way to realize the present invention, and are not intended to limit the core content of the present invention or its protection scope. It should be understood by those skilled in the art that these embodiments are only for illustrative purposes and should not be regarded as limiting the only or exclusive embodiment of the present invention. In addition, unless otherwise stated, in the present invention, the expression range of "greater than" and "less than" includes this number.

[0050] Example 1

[0051] Precursor selection: Mn 0.75 Ni 0.25 (OH) 2The precursor is prepared by coprecipitation reaction. The specific steps are as follows: nickel sulfate hexahydrate and manganese sulfate monohydrate are dissolved in deionized water in a stoichiometric ratio to prepare solution A with a total concentration of manganese ions and nickel ions of 2 mol / L; and sodium hydroxide is prepared into an aqueous solution with a concentration of 3 mol / L and ammonia water with a concentration of 2 mol / L. Solution A, alkali solution and ammonia water are added to N 2 In the reactor under protection, the pH value was adjusted to 12, the reaction temperature was 60°C, the reaction time was 4h, and the coprecipitated product was washed with water and dried at 50°C in vacuum to obtain Mn 0.75 Ni 0.25 (OH) 2 Precursor.

[0052] The obtained Mn 0.75 Ni 0.25 (OH) 2 The precursor powder is mixed with low-melting-point lithium nitrate in a molar ratio of 1:2.2, and the mixed material is placed in a crucible; then the crucible is placed in a muffle furnace, and the temperature is increased to 220°C at a heating rate of 5°C / min, and the mixture is kept warm for 1 hour. When the temperature displayed by the furnace reaches 180°C, the mixture is taken out and placed in a ball mill, and zirconium beads are added. The material and zirconium beads are mixed in a ball-to-material ratio of 7:1, and the ball mill is performed at a speed of 500 rpm for 1 hour, and then lithium carbonate with a molar ratio of 1:0.2 to the precursor powder is added. After an interval of 10 minutes, the ball mill is continued at a speed of 500 rpm for 1 hour, and the addition is stopped again. The molar ratio of lithium carbonate to precursor powder is 1:0.1. After an interval of 10 minutes, continue to ball mill at 500rpm for 1 hour. After the ball milling is completed, take out the material and put it into an ultrasonic 6000 mesh screen for sieving. Repeat twice. The final uniform granular material is placed in a crucible. The crucible is placed in a muffle furnace. The low temperature section is heated at a heating rate of 5℃ / min and kept at 350℃ for 5 hours; the medium temperature section is heated to 800℃ at a heating rate of 3℃ / min and kept for 5 hours; the high temperature section is heated to 850℃ at a heating rate of 3℃ / min, kept for 10 hours, and then cooled with the furnace. Li 1.2 Mn 0.6 Ni 0.2 O 2 The initial product of cobalt-free single crystal lithium-rich positive electrode material.

[0053] Prepare a 1 mol / L mixed acid solution, in which the volume ratio of citric acid and acetic acid is 1:1, Li 1.2 Mn 0.6 Ni 0.2 O 2The initial product of the cobalt-free single crystal lithium-rich cathode material was immersed in the mixed acid solution for 15 minutes, filtered twice with anhydrous ethanol, and the obtained material was placed in a vacuum oven at 50°C for 3 hours, and then placed in a crucible; then the crucible was placed in a muffle furnace, heated at a heating rate of 5°C / min, kept at 400°C for 3 hours, and then cooled with the furnace to obtain the modified Li 1.2 Mn 0.6 Ni 0.2 O 2 Cobalt-free single crystal lithium-rich positive electrode material products.

[0054] Example 2

[0055] The same method as in Example 1 is used to prepare the initial product of cobalt-free single crystal lithium-rich positive electrode material and the product of cobalt-free single crystal lithium-rich positive electrode material, the difference being that after the precursor powder and low-melting-point lithium nitrate are heated in the crucible and taken out, the ball-to-material ratio in the ball mill is changed to 6:1.

[0056] Example 3

[0057] The same method as in Example 1 is used to prepare the initial product of cobalt-free single crystal lithium-rich positive electrode material and the product of cobalt-free single crystal lithium-rich positive electrode material, the difference being that after the precursor powder and low-melting-point lithium nitrate are heated in the crucible and taken out, the ball-to-material ratio in the ball mill is changed to 8:1.

[0058] Example 4

[0059] The same method as in Example 1 is used to prepare the initial product of cobalt-free single crystal lithium-rich positive electrode material and the product of cobalt-free single crystal lithium-rich positive electrode material, the difference being that after the precursor powder and low-melting-point lithium nitrate are heated in the crucible and taken out, the ball-to-material ratio in the ball mill is changed to 10:1.

[0060] Example 5

[0061] The same method as in Example 1 was used to prepare a cobalt-free single crystal lithium-rich cathode material preliminary product and a cobalt-free single crystal lithium-rich cathode material product, the difference being that the low melting point lithium salt was selected to be lithium acetate.

[0062] Comparative Example 1

[0063] The same method as in Example 1 was used to prepare Mn 0.75 Ni 0.25 (OH) 2 Precursor, the obtained Mn 0.75 Ni 0.25 (OH) 2Precursor powder and lithium carbonate were mixed at a molar ratio of 1:1.4, and the materials were placed in a crucible; then the crucible was placed in a muffle furnace, the low temperature section was heated at a heating rate of 5°C / min, and kept at 350°C for 5 hours; the medium temperature section was heated to 800°C at a heating rate of 3°C / min, and kept at this temperature for 5 hours; the high temperature section was heated to 850°C at a heating rate of 3°C / min, and kept at this temperature for 10 hours, and then cooled with the furnace. 1.2 Mn 0.6 Ni 0.2 O 2 The cobalt-free lithium-rich positive electrode material initial product is then prepared by continuing to use the same method as in Example 1 to prepare the cobalt-free lithium-rich positive electrode material product.

[0064] Comparative Example 2

[0065] The same method as in Example 1 was used to prepare Mn 0.75 Ni 0.25 (OH) 2 Precursor, the obtained Mn 0.75 Ni 0.25 (OH) 2 The precursor powder and lithium carbonate are mixed in a molar ratio of 1:1.4, put into a ball mill, zirconium beads are added, and the material and zirconium beads are mixed in a ball-to-material ratio of 7:1. The method of Example 1 is continued to prepare a cobalt-free lithium-rich positive electrode material preliminary product and a cobalt-free lithium-rich positive electrode material product.

[0066] Comparative Example 3

[0067] The same method as in Example 1 is used to prepare the initial product of cobalt-free single crystal lithium-rich positive electrode material and the product of cobalt-free single crystal lithium-rich positive electrode material, the difference being that after the precursor powder and low-melting-point lithium nitrate are heated in the crucible and taken out, the ball-to-material ratio in the ball mill is changed to 3:1.

[0068] Comparative Example 4

[0069] The same method as in Example 1 is used to prepare the initial product of cobalt-free single crystal lithium-rich positive electrode material and the product of cobalt-free single crystal lithium-rich positive electrode material, the difference being that after the precursor powder and low-melting-point lithium nitrate are heated in the crucible and taken out, the ball-to-material ratio in the ball mill is changed to 5:1.

[0070] The initial products obtained in Examples 1-5 and Comparative Examples 1-4 were tested for power-off index (2.0-4.8V), and the results are recorded in Table 1.

[0071] Table 1

[0072]

[0073] Figure 1 Mn prepared in the embodiment of the present invention 0.75 Ni 0.25(OH) 2 The SEM image of the precursor shows that its size is between 2-5μm and its surface morphology is quite special. The primary particles are longitudinally stacked like fish scales. The surface is easily broken by ball milling and has a high degree of uniformity after breaking. The conventional morphology precursor has particles of different sizes after ball milling. Therefore, this special morphology provides a basis for the subsequent ball milling and breaking to prepare single crystals rich in lithium.

[0074] The phase analysis of the initial products prepared in Example 1 and Comparative Examples 1-4 of the present invention was carried out by X-ray powder diffraction (XRD). Figure 2 As shown in the figure, all samples can be indexed by structures with hexagonal R-3m and monoclinic C2 / m space groups, indicating that all samples have ordered layered structures. However, the diffraction intensity ratios of I(003) / (104) peaks are different. It can be found that the product obtained by ball milling and re-calcining in Example 1 has lower Li + / Ni 2+ Mixed arrangement. Relative to Comparative Example 2 in which no low-melting point lithium salt was added for preheating before ball milling, the addition of molten lithium nitrate in the early stage of ball milling in Example 1 helps to increase the crystallinity of the product during calcination. This is because the molten state is more conducive to the uniform dispersion of lithium salt on the ball-milled precursor material, allowing more uniform growth between grains during calcination, and ultimately obtaining a product with high crystallinity. Relative to Comparative Examples 3-4 with low ball-to-material ratio, Example 1 uses a high ball-to-material ratio, which helps to break up the precursor and obtain more uniform and smaller particles, allowing the lithium salt to more uniformly adhere to the precursor material. For the products prepared in Example 1 and Comparative Examples 1-4, after adding low-melting point lithium salt, as the ball-to-material ratio increases, the diffraction peaks of (006) / (012) and (018) / (110) are clearly separated, and the (003) diffraction peak shifts to a low angle, indicating that the interlayer spacing increases, which is conducive to lithium ion transmission. Therefore, adding molten lithium nitrate before ball milling and a high ball-to-material ratio are the best methods to obtain high crystallinity and lower Li + / Ni 2+ Mixed lithium-rich materials are a necessary condition. The synergistic effect of the two makes the lithium salt and the precursor material evenly dispersed, which is beneficial to improving the crystallinity and uniformity of particle size of the lithium-rich.

[0075] Figure 3 The SEM images of the products prepared in the examples and comparative examples are shown in FIG. Figure 3 a Corresponding to Example 1, it can be seen that the product morphology is a typical polycrystalline morphology, and the secondary particles are formed by the accumulation of primary particles. Figure 3 b Corresponding to proportional 2, it can be seen that the material size has decreased and the primary particles on the surface have grown. This is the result of the precursor being broken up and calcined due to the high ball-to-material ratio. The figure shows that the material size uniformity is poor and both large and small particles exist. Figure 3c Corresponding to Example 3, it can be seen that the secondary particle morphology and size of the product prepared in Example 3 are not much different from those in Example 1, but the primary particles on the surface are spread out and aggregated. This is because the ball-to-material ratio of 3:1 can only open the primary particles on the surface but cannot break the material. Figure 3 d Corresponding to Example 4, it can be seen that there is a big difference in the secondary particle morphology and size between the product prepared in Example 4 and that in Example 1, and its size is reduced from the original 5μm to 2μm, indicating that the ball-to-material ratio of 5:1 can break up the precursor material, but the obtained material is not in a single crystal morphology. Figure 3 e corresponds to Example 1. It can be seen that the product prepared in Example 1 has a good single crystal morphology. Its particles are small nano-sized particles grown by calcining after the polycrystalline secondary particles are broken. Its ion transmission path is reduced, which is beneficial to lithium ion transmission and is conducive to lithium-rich Li 2 MnO 3 Phase activation improves the reversible capacity of the material, indicating that a ball-to-material ratio of 7:1 can break up the precursor material and obtain nano-scale cobalt-free single crystal products through a sintering process.

[0076] Figure 4 The first week charge and discharge curves of the initial products prepared in Example 1 and Comparative Example 1 show that the small particle single crystal product of Example 1 has a higher discharge capacity, which can reach 251.0 mAh / g. This is attributed to the lithium ion transmission performance brought by the optimized transmission path, which makes Li 2 MnO 3 The phase is more fully activated and more capacity is released.

[0077] Figure 5 The cyclic charge and discharge curves of the initial products prepared in Example 1 and Comparative Example 1 are shown. From the test results, it can be seen that the cobalt-free single crystal product prepared in Example 1 has excellent cycle stability. After 200 cycles at a rate of 1C, the capacity retention rate is as high as 85.6%.

[0078] Figure 6 The curve diagram of the discharge medium voltage variation during the cycle of the initial products prepared in Example 1 and Comparative Example 1 shows that the cobalt-free single crystal product prepared in Example 1 can slow down the voltage decay during the cycle, with an average decay of 3.16 mV per cycle.

[0079] Figure 7 The figure is a rate performance diagram of the initial products prepared in Example 1 and Comparative Example 1. From the test results, it can be seen that the cobalt-free single crystal product prepared in Example 1 has excellent rate performance, and the discharge capacity is as high as 98.3 mAh / g at a rate of 10C.

[0080] Figure 8The EIS curves of the initial products prepared in Example 1 and Comparative Example 1 before activation show that the cobalt-free single crystal product prepared in Example 1 has a lower impedance before activation. Fig. 9 The EIS curves of the initial products prepared in Example 1 and Comparative Example 1 after 3 cycles of activation are shown. From the test results, it can be seen that the impedance of the cobalt-free lithium-rich single crystal prepared in Example 1 is still low even after activation, indicating that the cobalt-free lithium-rich single crystal prepared in Example 1 has fewer surface side reactions during activation, better film formation quality, and is beneficial to lithium ion transport.

[0081] Fig.10 The first week charge and discharge curves of the initial product and the product prepared in Example 1. From the test results, it can be seen that the product of Example 1 has a higher discharge specific capacity than the initial product, which can reach 251.0 mAh / g, and the coulombic efficiency is as high as 93.3%. This is because the ball milling process takes a long time and there is no special atmosphere protection. The surface of the prepared lithium-rich material is prone to residue, which seriously affects the lithium-rich charge and discharge efficiency in the first week. After being treated with a mixed solution of citric acid and acetic acid, products such as lithium carbonate on the surface of the lithium-rich material are consumed, and at the same time, a spinel phase is generated near the surface of the lithium-rich material, which can further inhibit side reactions on the surface of the material, thereby improving the coulombic efficiency and stability.

[0082] The technical features of the above-described embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above-described embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. Although the embodiments of the present invention have been shown and described above, it can be understood that the above-described embodiments are exemplary and cannot be understood as limitations of the present invention. Those of ordinary skill in the art can change, modify, replace and deform the above-described embodiments within the scope of the present invention. In addition, those of ordinary skill in the art can combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples without contradiction.

Claims

1. A preparation method of a cobalt-free single-crystal lithium-rich cathode material, comprising the following steps: S1: Prepare a nickel-manganese oxygen-containing compound precursor by a solid-phase method or a co-precipitation method using a manganese salt and a nickel salt; S2: Mix the prepared nickel-manganese oxygen-containing compound precursor with a low-melting-point lithium source. After the mixture is pre-calcined and melted, high-energy ball milling is carried out. During the ball milling, a conventional lithium source is added one or more times. After ball milling, the mixture is calcined to obtain a primary product of the cobalt-free single-crystal lithium-rich cathode material; S3: The primary product is subjected to acid treatment and secondary calcination to obtain a cobalt-free single-crystal lithium-rich cathode material product.

2. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The nickel-manganese oxygen-containing compound precursor is a hydroxide of nickel and manganese, prepared by a co-precipitation method, having a fish-scale-like longitudinal stacking morphology and a particle size of 2-5 μm; The low-melting-point lithium source uses a lithium salt with a melting point below 300 °C; The ball-to-material ratio of the mixture in high-energy ball milling is greater than 6:1, preferably 7:1, and the rotation speed is greater than 300 rpm.

3. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The cobalt-free single-crystal lithium-rich cathode material has the following general formula: Li 1+x Ni y Mn 0.8-y O2, where 0 < x < 0.25, 0 < y < 0.8, preferably 0.15 < x < 0.25, 0.1 < y < 0.3; The molar ratio of nickel to manganese in the nickel-manganese oxygen-containing compound precursor satisfies y:0.8 - y, 0 < y < 0.8, preferably 0.1 < y < 0.

3.

4. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The manganese salt uses one or more of manganese carbonate, manganese nitrate, manganese acetate, and manganese sulfate; The nickel salt uses one or more of nickel carbonate, nickel nitrate, nickel acetate, and nickel sulfate; The low-melting-point lithium source is selected from one or more of organic acid lithium salts, inorganic lithium salts, or halogen-containing lithium salts with a melting point below 300 °C. The organic acid lithium salts include lithium acetate, lithium propionate, lithium butyrate, lithium valerate, lithium hexanoate, lithium heptanoate, and lithium octanoate; the inorganic lithium salts include lithium nitrate; the halogen-containing lithium salts include lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide, and lithium thiocyanate.

5. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The solid-phase method directly mechanically mixes the manganese salt and the nickel salt evenly to obtain a nickel-manganese oxygen-containing compound precursor. The mechanical mixing method is preferably mechanical stirring; The co-precipitation method dissolves the manganese salt and the nickel salt in a solvent to obtain solution A. Under the protection of a protective gas, solution A, an alkali solution, and ammonia water are mixed for a co-precipitation reaction to obtain a suspension, which is then filtered, washed, and dried to obtain a nickel-manganese oxygen-containing compound precursor; Preferably, in the co-precipitation method, the solvent is deionized water; the alkali solution is selected from sodium hydroxide solution or sodium carbonate solution; Preferably, in the co-precipitation method, the total concentration of manganese ions and nickel ions in solution A is 2-5 mol / L, the concentration of the alkali solution is 2-7 mo1 / L, the concentration of ammonia water is 1-4 mo1 / L, and the volume ratio of solution A, the alkali solution, and ammonia water is 1:1:1 - 2; the pH value of the suspension is 9.5 - 12.0, the co-precipitation reaction temperature is 30 - 80 °C, and the reaction time is 1 - 4 hours.

6. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The conventional lithium source is selected from lithium sources with a melting point above 600 °C, including one or more of lithium carbonate, lithium sulfate, lithium phosphate, lithium borate, lithium sulfide, and lithium fluoride.

7. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The total amount of the low-melting-point lithium source and the conventional lithium source is fed according to the theoretical stoichiometric ratio of the target product, and the ratio of the two, calculated by the molar amount of lithium contained in each, is (2 - 2.5):(0.5 - 0.75); Preferably, the conventional lithium source is added multiple times during the ball milling in a manner of decreasing the input amount each time according to the total feeding amount, preferably 2 - 3 times.

8. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: High-energy ball milling uses zirconium beads as the ball milling medium, with a diameter of 5-12 mm and a total ball milling time of 2-5 h; Preferably, the pre-firing temperature is 200-300°C; Preferably, after ball milling, the mixture is screened using a sieve having a mesh size of 5000-7000; Preferably, the calcination temperature is 850-900°C; further preferably, the calcination adopts a stage heating system, with stage insulation at 300-500°C, 700-800°C, and 850-900°C respectively, and the heating rate before heating to the temperature of each insulation stage is 3-8°C / min.

9. The method for preparing a cobalt-free single crystal lithium-rich cathode material according to claim 1, characterized in that: The acid used in the acid treatment includes one or more of citric acid, acetic acid, phosphoric acid, boric acid, and hydrofluoric acid, and the acid concentration is 1-5 mol / L; Preferably, a mixed acid solution of citric acid and acetic acid, or citric acid and phosphoric acid is selected, and in the mixed acid solution, the volume ratio of citric acid to the other acid is (1-2):

1. Preferably, the secondary sintering temperature is 300-500° C. and the time is 1-3 h.

10. The cobalt-free single crystal lithium-rich positive electrode material obtained by the preparation method according to any one of claims 1 to 9 comprises at least one of the following characteristics: (1) The particle size is between 100nm and 300nm; (2) The interior is layered and the surface is spinel phase; (3) The reversible discharge capacity at 0.1C is greater than 260 mAh / g, and the coulombic efficiency is greater than 93%; (4) The capacity retention rate after 200 cycles at 1C is greater than 80%, and the discharge capacity is greater than 180 mAh / g; (5) Reversible discharge capacity at 10C is greater than 98 mAh / g; (6) XRD shows that I(003) / I(104) is greater than 2.22.

Images