A surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor and a preparation method thereof

Abstract

A surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor and its preparation method are disclosed, belonging to the field of lithium-ion battery materials. The general chemical formula is Ni. x Mn y M k (OH)2 or Ni x Mn y M k The CO3 precursor exhibits a continuous gradient distribution of elements within its interior. The Mn concentration decreases gradually from the particle center to the surface; the Ni concentration increases gradually from the particle center to the surface; and the concentration of the highly doped element M increases in the opposite direction from the particle center to the surface. M is selected from at least one of Al, Mg, Ti, Nb, Ta, Mo, W, and Zr. Through a "dual-tank gradient co-precipitation" process combined with "variable pH control," a gradient of transition metal concentration is achieved while simultaneously achieving an "opposite gradient" distribution of dopant element concentration increasing from the core to the surface. The surface-enriched dopant elements construct a robust "protective shield," suppressing surface oxygen evolution and side reactions while maintaining high capacity activity in the core region.

Description

Technical Field

[0001] This invention belongs to the field of key material preparation technology for lithium-ion batteries, specifically relating to a lithium-rich manganese-based cathode material precursor with an in-situ doped structure with an anti-gradient gradient, the cathode material prepared therefrom, and related preparation methods. Background Technology

[0002] With the rapid development of new energy vehicles and large-scale energy storage systems, higher requirements have been placed on the energy density of lithium-ion batteries. Lithium-rich manganese-based cathode materials (LLOs), generally written as xLi2MnO3\(1-x)LiTMO2, are considered one of the most promising candidate cathode materials for next-generation high-energy-density power batteries due to their high discharge specific capacity (over 250 mAh / g) and high operating voltage, as well as the abundance and low cost of manganese resources.

[0003] However, lithium-rich manganese-based materials still face many challenges in practical applications, mainly in the following aspects:

[0004] Poor structural stability: During high-voltage (>4.5V) charging, the precipitation of lattice oxygen (Oxygen Evolution) causes the migration of transition metal ions, inducing an irreversible transformation of the layered structure to the spinel phase, resulting in severe voltage decay and capacity loss.

[0005] Interfacial side reactions and secondary particle cracking: The high operating voltage of lithium-rich materials can easily exacerbate the oxidative decomposition of the electrolyte on the material surface, forming a thick and unstable positive electrode electrolyte interphase (CEI) film. At the same time, during long-term cycling, the volume expansion / contraction (anisotropic) caused by repeated lithium ion insertion / extraction can easily lead to microcracks inside the particles, exacerbating side reactions.

[0006] To address these issues, researchers typically employ elemental doping (such as Al, Mg, Ti, Zr, etc.) or surface coating techniques. Elemental doping can enhance the binding energy of metal-oxygen bonds and stabilize the crystal structure. However, traditional bulk homogeneous doping often fails to meet the different needs of the material's interior and surface: the interior requires high capacity utilization, and excessive inactive dopants can reduce specific capacity; the material surface, in direct contact with the electrolyte, is a major area for structural degradation and side reactions, requiring high concentrations of dopants to stabilize the surface structure and suppress phase transitions.

[0007] Furthermore, current mainstream concentration gradient materials mainly focus on the gradient distribution of transition metals (Ni, Mn), with limited research on the spatial distribution control of doped elements. Existing fabrication processes are often quite complex, making it difficult to precisely control the primary particle morphology of the precursor while achieving dual concentration gradients.

[0008] Therefore, developing a lithium-rich manganese-based precursor and cathode material that can simultaneously achieve the transition metal concentration gradient and the dopant element "anti-gradient" (i.e., surface enrichment) distribution, and has a special microstructure, is of great significance for improving the overall electrochemical performance of lithium-ion batteries. Summary of the Invention

[0009] The purpose of this invention is to overcome the defects of existing lithium-rich manganese-based cathode materials, such as poor cycle stability, rapid voltage decay, and unstable surface interface, and to provide a reverse gradient doped lithium-rich manganese-based cathode material precursor and its preparation method.

[0010] This invention utilizes a unique "dual-tank gradient co-precipitation" combined with a "variable pH control" process to achieve a Mn-Ni transition metal concentration gradient (high Mn internally, high Ni externally) while simultaneously constructing an "anti-gradient" distribution of dopant elements (such as Al and Mg) with increasing concentrations from the core to the surface. This design aims to utilize the surface-enriched dopant elements to build a robust "protective shield," suppressing surface oxygen evolution and side reactions, while maintaining high capacity activity in the core region.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] In a first aspect, the present invention provides a lithium-rich manganese-based cathode material precursor with reverse gradient doping. The precursor is a spherical or near-spherical particle with the general chemical formula Ni. x Mn y M k (OH)2 or Ni x Mn y M k CO3, of which 0 <x<1,0<y<1,0<k≤0.1,x+y+k=1。

[0013] The elements inside the precursor exhibit a continuous gradient distribution, with the following specific characteristics:

[0014] Transition metal gradient: Mn concentration decreases gradually from the particle center to the surface; Ni concentration increases gradually from the particle center to the surface. This design utilizes Mn-rich cores to provide high capacity and structural support, and Ni-rich surfaces to enhance conductivity and electrochemical activity.

[0015] Anti-gradient dopant: The concentration of dopant element M increases in an anti-gradient manner from the particle center to the surface. That is, the average molar concentration of dopant element M in the core region (radius 0-30%) of the precursor is lower than the average molar concentration of dopant element M in the surface region (radius 70-100%).

[0016] Type of doping element: The doping element M is selected from at least one of Al, Mg, Ti, Nb, Ta, Mo, W, Zr, etc.

[0017] Preferably, the molar content C of the doping element M in the core region core and the molar content C in the surface layer region shell have a ratio that satisfies 1.5 ≤ C shell / C core ≤ 10; this significant concentration difference ensures that the surface layer region has extremely strong interfacial stability (inhibiting the dissolution of transition metals and electrolyte corrosion), while the central region, due to the lower doping amount, retains the capacity contribution of the active lithium manganese oxide to the greatest extent.

[0018] The precursor particles do not form an obvious core-shell interface along the radial direction, and the element concentration change curve is a continuous and smooth curve, which helps to relieve the stress concentration during charge and discharge and prevent particle cracking.

[0019] Microscopic morphological characteristics: The precursor is aggregated by primary particles, and the primary particles exhibit a radially growing morphology from the center outward. This special radial structure can effectively buffer the volume expansion of the material during cycling and provide a radial channel conducive to lithium ion transport.

[0020] Physical indexes: The median particle size D50 of the precursor is 3 - 15 μm, the particle size distribution coefficient (D90 - D10) / D50 ≤ 1.2, the tapped density ≥ 1.8 g / cm 3 , and the specific surface area is 5 - 15 m 2 / g.

[0021] Second, the present invention provides a method for preparing the above-mentioned precursor of the lithium-rich manganese-based cathode material with inverse gradient doping.

[0022] This method includes the following steps:

[0023] (1) Prepare a metal salt solution:

[0024] Solution A (manganese-rich core solution): contains manganese salt and nickel salt, where the molar ratio of Mn to Ni is a:b, a > b, and does not contain the doping element M or the content of M is significantly lower than that of Solution B; this solution serves as the source of the "bottom liquid" of the reaction and determines the high-Mn and low-doping characteristics of the particle core.

[0025] Solution B (nickel-rich shell solution): contains manganese salt, nickel salt and the salt of the doping element M, where the molar ratio of Mn to Ni is e:f, e < f, a > e, b < f, and contains a high concentration of the doping element M (here, the high concentration is relative to the content concentration of M in Solution A); this solution is used to construct the high-Ni and high-doping characteristics of the particle surface layer.

[0026] Salt selection: The salt of the dopant element M is preferably at least one of aluminum sulfate, aluminum nitrate, magnesium sulfate, and magnesium nitrate. The total concentration of metal ions in both solution A and solution B is 1.5~2.5 mol / L.

[0027] (2) Dual-tank gradient co-precipitation, using a dual-feeding tank and reactor linkage mode:

[0028] Add the base solution (a mixture of pure water, ammonia, and alkali) to the reactor. While stirring, pump solution B into a container containing solution A at a constant flow rate v1 for mixing (at this time, the concentration of metal ions and the amount of doping in container A change over time). Simultaneously, pump the mixed solution AB from container A into the reactor at a flow rate v2.

[0029] Preferably, the flow rates v1 and v2 are equal. This operation ensures that the concentration of metal ions entering the reactor changes continuously over time, thereby forming a gradient structure without interfaces.

[0030] (3) Reaction control (key step):

[0031] Basic parameters: Under the protection of inert gas (such as nitrogen or argon), the temperature inside the reactor is controlled at 40~60℃, the ammonia concentration is 0.3~0.8mol / L, the stirring speed is 500~1500rpm, and the reaction time is 5~30h.

[0032] Variable pH control strategy: During the reaction, the pH of the reaction system is not constant, but increases linearly or stepwise with the progress of the reaction time, with a total increase of 0.2~0.5 (pH range from 8.0~11.5 to 8.2-12.0). The lower pH value in the early stage of the reaction is conducive to the explosive growth of crystal nuclei and the formation of dense cores; as the salt solution enters the reactor for co-precipitation reaction, appropriately increasing the pH value can better precipitate dopants and regulate the growth rate and stacking mode of primary particles, which is beneficial to the formation of specific surface morphologies and the integration of high concentrations of dopants.

[0033] Aging: After the reaction is completed, the resulting slurry is aged in the original solution at 50~70℃ for 2~10h to eliminate internal stress and improve the crystal structure.

[0034] Thirdly, the present invention provides a cathode material prepared from the above-mentioned precursor.

[0035] The obtained precursor material is mixed with lithium carbonate at a molar ratio of Li:(Ni+Mn+M)=(1.15~1.45):1, and the final product is obtained by high-temperature solid-state sintering (e.g., 800-950℃). This cathode material inherits the gradient structure characteristics of the precursor.

[0036] 3. Beneficial effects

[0037] Compared with existing technologies, the present invention has the following significant advantages:

[0038] Significantly enhanced surface / interface stability (anti-gradient advantage): By enriching the particle surface region with dopant elements (such as Al and Mg), a robust chemical protective layer is constructed. The high concentration of dopant elements on the surface effectively stabilizes lattice oxygen, inhibits the decomposition of the electrolyte and the dissolution of transition metals under high voltage, thereby blocking the initiation point of the layered structure to spinel phase transformation and significantly reducing voltage decay.

[0039] Balancing high capacity and structural stability: The low doping level in the core region of the particles avoids the dilution of capacity by inactive elements, ensuring the high specific capacity of lithium-rich materials. This "high internal capacity, high external stability" design is more advantageous than traditional uniform doping.

[0040] Optimized lithium-ion transport: The high Ni content on the particle surface, combined with the radially arranged primary particle structure, significantly reduces the interfacial charge transfer impedance and shortens the lithium-ion diffusion path, thereby improving the rate performance of the material.

[0041] Enhanced particle mechanical strength: The continuous concentration gradient distribution eliminates the interfacial stress common in core-shell structures, while the radial arrangement of primary particles can effectively buffer the anisotropic volume expansion during charging and discharging, suppress the pulverization and cracking of secondary particles, and extend the battery cycle life.

[0042] High process controllability: Through the unique "variable pH value (increase pH)" co-precipitation process, the microstructure of the precursor can be precisely controlled, and the prepared material has good sphericity and high tap density, which is conducive to commercial processing. Attached Figure Description

[0043] Figure 1 The image shows a scanning electron microscope (SEM) image of the reverse gradient doped lithium-rich manganese-based cathode material prepared in Example 1 of this invention; the secondary particles are spherical.

[0044] Figure 2 The image shows an energy dispersive X-ray spectroscopy (EDS) line scan of the precursor along a radial cross section in Embodiment 1 of the present invention; it shows that Mn decreases from the center to the surface, Ni increases from the center to the surface, while the dopant Al shows an inverse gradient increasing trend from the center to the surface. Detailed Implementation

[0045] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0046] Example 1: Preparation of reverse gradient Al-doped lithium-rich cathode material precursor and material

[0047] Solution preparation: Solution A (manganese-rich core solution + no / low doping): Prepare a 2 mol / L metal salt solution, with nickel sulfate and manganese sulfate as solutes. The Ni:Mn molar ratio is 0.25:0.75. This solution contains no aluminum source (or only trace amounts). This solution is placed in container A containing a stirrer.

[0048] Solution B (Nickel-rich shell solution + high doping): Prepare a 2 mol / L metal salt solution with nickel sulfate, manganese sulfate, and aluminum sulfate as solutes. The Ni:Mn molar ratio is 0.45:0.55. Add aluminum sulfate to make the Al:(Ni+Mn) molar ratio 1:1000 (i.e., 0.1 mol%); this solution is used to construct a high-alumina surface layer.

[0049] Base solution and precipitant: Prepare a 2 mol / L sodium carbonate solution as a precipitant; prepare a 0.8 mol / L ammonia solution as a complexing agent; add appropriate amounts of pure water and ammonia solution to the reactor beforehand.

[0050] Dual-tank gradient co-precipitation:

[0051] Turn on the agitator (1000 rpm) and heating system of the reactor, introduce nitrogen for protection, and control the temperature inside the reactor to 55℃. Turn on the pump for solution B and pump solution B into solution A at a flow rate (v1) of 10 mL / min. Keep solution A vigorously stirred to ensure instantaneous and uniform mixing.

[0052] Simultaneously turn on the pump for solution A and pump the mixed solution AB into the reactor at a flow rate of 10 mL / min (v2); at the same time, pump in sodium carbonate solution and ammonia water.

[0053] Variable pH reaction control (pH-increasing mode):

[0054] At the beginning of the reaction, the salt solution, complexing agent, and precipitated gold were simultaneously pumped into the reactor (core growth stage), and the system pH was controlled at 7.8 to facilitate the formation of dense, manganese-rich cores. As the reaction progressed, the pH was gradually increased linearly by fine-tuning the amount of sodium carbonate added. After 20 hours of reaction (at the end), the feed was stopped, and the system pH reached 8.0 (a total increase of 0.2). The ammonia concentration was maintained at approximately 0.5 mol / L. This pH-raising strategy, combined with a high doping concentration, facilitated the complete precipitation of dopant ions. After the reaction was completed, stirring was stopped, and the slurry was aged in the original solution at 50°C for 2 hours.

[0055] (4) Post-treatment and sintering:

[0056] The aged precursor was filtered, washed, and dried to obtain an anti-gradient Al-doped precursor powder material. Tests showed that its D50 was 10.5 μm and its tap density was 2.0 g / cm³.

[0057] The obtained powder material was mixed with lithium carbonate at a molar ratio of Li:(Ni+Mn+Al)=1.45:1. Under an inert gas atmosphere, it was first pre-calcined at 500℃ for 5 hours, then heated to 850℃ for sintering for 12 hours, and then naturally cooled to obtain the final cathode material.

[0058] Example 2: Preparation of reverse gradient Mg-doped lithium-rich cathode material precursor and material

[0059] The steps are basically the same as in Example 1, except that:

[0060] Doping elements: Magnesium nitrate is added to solution B instead of aluminum sulfate, and the Mg content is set to 1.0 mol% of the total metal (high surface concentration); solution A does not contain magnesium.

[0061] pH control: The initial pH of the reaction was 7.8, and the final pH was 8.1 (increasing linearly).

[0062] Example 3: Preparation of reverse gradient Co-doped lithium-rich cathode material precursor and material

[0063] The steps are basically the same as in Example 1, except that:

[0064] Doping elements: Cobalt sulfate is added to solution B instead of aluminum sulfate, and the Co content is set to 1.0 mol% of the total metal (high surface concentration); solution A does not contain cobalt.

[0065] pH control: The initial pH of the reaction was 7.8, and the final pH was 8.1 (increasing linearly).

[0066] Comparative Example 1: Uniformly Doped Material

[0067] A traditional single-tank co-precipitation method was employed. A single metal salt solution was prepared with a Ni:Mn ratio of 0.3:0.7 (the average value from Example 1), and 0.1 mol% aluminum sulfate was added. The pH was maintained at a constant 8.0 throughout the reaction. The resulting precursor exhibited a uniform elemental distribution and no gradient structure.

[0068] Comparative Example 2: Traditional transition metal gradient (undoped element gradient)

[0069] The solution was prepared using a two-tank method. The Ni and Mn ratios in solutions A and B were the same as in Example 1, but no doping elements were added to either solution A or solution B. That is, there was only a Ni-Mn gradient, without any doping elements.

[0070] Comparative Example 3: Traditional transition metal gradient (undoped element gradient)

[0071] The solution was prepared using a two-tank method. The Ni and Mn ratios in solutions A and B were the same as in Example 1, but the same molar ratio of dopant elements was added to both solutions A and B. That is, there was only a Ni-Mn gradient, and no dopant element concentration gradient.

[0072] Performance Testing and Result Analysis

[0073] The cathode materials prepared in Examples 1-3 and Comparative Examples 1-3 were assembled into CR2032 coin cells for electrochemical performance testing. The test voltage range was 2.0-4.8V, and the test temperature was 25℃. Their electrochemical performance is shown in Table 1. The first-cycle discharge specific capacity was 208.28 mAh / g. Cyclic tests were conducted at 25℃ and a 1C rate. Comparative Example 1 had no transition metal salt concentration gradient and only added Al dopant; Comparative Example 2 had only a transition metal salt concentration gradient without elemental doping; and Comparative Example 3 had a transition metal salt concentration gradient and uniform elemental doping without a concentration gradient of doped elements.

[0074] Table 1 Comparison of battery performance of coin cells assembled from different modified samples

[0075] sample Sample composition 1C discharge specific capacity (mAh / g) First-lap coulomb efficiency (%) Capacity retention rate after 300 laps (%) Example 1 G-LLOs-Al-inverse gradient 215.2 93.0 93.2 Example 2 G-LLOs-Mg-inverse gradient 215.3 93.2 92.3 Example 3 G-LLOs-Co-inverse gradient 218.3 94.2 91.3 Comparative Example 1 LLOs-Al 208.3 90.3 60.2 Comparative Example 2 G-LLOs 207.8 89.6% 65.3 Comparative Example 3 G-LLOs-Al-uniform 206.4 89.1% 85.2

[0076] .

Claims

1. A surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor, characterized in that, The precursor is a spherical or near-spherical particle with the general chemical formula Ni. x Mn y M k (OH)2 or Ni x Mn y M k CO3, of which 0 <x<1,0<y<1,0<k≤0.1,x+y+k=1; Each element inside the precursor shows a continuous gradient distribution, and the specific characteristics are as follows: Transition metal gradient: The concentration of Mn decreases in a gradient from the particle center to the surface; the concentration of Ni increases in a gradient from the particle center to the surface; Doping element inverse gradient: The concentration of the doping element M increases in an inverse gradient (GradientIncrease) from the particle center to the surface; that is, the average molar concentration of the doping element M in the core region of the precursor is lower than the average molar concentration of the doping element M in the surface layer region; Types of doping elements: The doping element M is selected from at least one of Al, Mg, Ti, Nb, Ta, Mo, W, Zr.

2. The surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor according to claim 1, characterized in that, The molar content C of the dopant element M in the core region core With molar content C in the surface region shell The ratio satisfies 1.5 ≤ C shell / C core ≤10.

3. A surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor according to claim 1, characterized in that, There is no obvious core-shell interface formed along the radial direction of the precursor particles, and the element concentration change curve is a continuous and smooth curve.

4. A surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor according to claim 1, characterized in that, Microscopic morphology characteristics: The precursor is aggregated by primary particles, and the primary particles show a radially growing morphology from the center outwards.

5. A surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor according to claim 1, characterized in that, Physical properties: The median particle size D50 of the precursor is 3~15µm, the particle size distribution coefficient (D90-D10) / D50 ≤ 1.2, and the tap density ≥ 1.8g / cm³. 3 Specific surface area is 5~15m² 2 / g.

6. A method for preparing a surface-enriched gradient in-situ doped gradient lithium-rich cathode material precursor according to any one of claims 1-5, characterized in that, This method includes the following steps: (1) Prepare a metal salt solution: Solution A, rich-manganese core solution: containing manganese salt and nickel salt, where the molar ratio of Mn to Ni is a:b, a > b, and without the doping element M or the content of M is significantly lower than that in Solution B; Solution B, rich-nickel shell solution: containing manganese salt, nickel salt and the salt of the doping element M, where the molar ratio of Mn to Ni is e:f, e < f, a > e, b < f, and containing a high concentration of the doping element M, here the high concentration is relative to the content concentration of M in Solution A; (2) Dual-tank gradient co-precipitation, adopting a mode of linkage between a dual-feeding tank and a reaction kettle: Add into the reaction kettle. Under stirring, pump Solution B into the container containing Solution A at a constant flow rate v1 for mixing; simultaneously pump the mixed solution AB in the Solution A container into the reaction kettle at a flow rate v2; the bottom liquid is a mixed solution of pure water, ammonia water and alkali; (3) Reaction control: Basic parameters: Under the protection of an inert gas (such as nitrogen, argon), control the temperature in the reaction kettle to be 40 - 60 °C, the ammonia water concentration to be 0.3 - 0.8 mol / L, the stirring speed to be 500 - 1500 rpm, and the reaction time to be 5 - 30 h; Variable pH value control strategy: During the reaction process, the pH value of the reaction system is not constant, but increases linearly or stepwise as the reaction time progresses, and the total increase in the pH value is 0.2 - 0.5, and the pH range increases from pH 8.0 - 11.5 to 8.2 - 12.0; Aging: After the reaction is completed, the obtained slurry is aged in the original solution at 50 - 70 °C for 2 - 10 h to eliminate internal stress and perfect the crystal structure.

7. The method according to claim 6, characterized in that, Selection of salts: The salt of the doping element M is preferably at least one of aluminum sulfate, aluminum nitrate, magnesium sulfate, magnesium nitrate; the total metal ion concentration of Solution A and Solution B is 1.5 - 2.5 mol / L.

8. The method according to claim 6, characterized in that, The flow rates v1 and v2 are equal.

9. A positive electrode material, characterized in that, Mix a surface enrichment type gradient in-situ doped gradient rich-lithium cathode material precursor according to any one of claims 1 - 5 with lithium carbonate in a molar ratio of Li:(Ni + Mn + M) = (1.15 - 1.45):1, and the final product can be obtained through high-temperature solid-state sintering.

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