CN121123200A

CN121123200A - A core-shell structured cathode material and its preparation method, and a lithium-ion battery.

Info

Publication number: CN121123200A
Application number: CN202511051886.7A
Authority: CN
Inventors: 唐俊; 胡宏辉; 杨智伟; 侯任重; 谭淑慧; 林卫楷
Original assignee: Shenzhen Technology University
Current assignee: Shenzhen Technology University
Priority date: 2025-07-29
Filing date: 2025-07-29
Publication date: 2025-12-12

Abstract

The invention relates to the technical field of battery materials, and discloses a positive electrode material with a core-shell structure, a preparation method thereof and a lithium ion battery, wherein the positive electrode material consists of a core layer and a shell layer, the core layer is an LNCM material doped with aluminum, zirconium and niobium, the chemical general formula of the LNCM material is LiNi _xCo_yMn_zO₂, x+y+z=1, 0< x, y and z <1, and the shell layer is a lithium-aluminum-zirconium-niobium composite oxide. The cathode material provided by the invention has the dual functions of lattice distortion strengthening and interface stabilization, the cycle stability and the multiplying power performance of the cathode material are obviously improved while the high specific capacity is maintained, and the assembled lithium ion battery can realize stable and rapid charge and discharge with high specific capacity under high multiplying power (such as 1C-5C). Most importantly, an innovative path is provided for low-cost and large-scale modification of the high-nickel ternary cathode material, and the high-nickel ternary cathode material has the capacity of large-scale production.

Description

Cathode material with core-shell structure, preparation method of cathode material and lithium ion battery

Technical Field

The invention relates to the technical field of battery materials, in particular to a positive electrode material with a core-shell structure, a preparation method of the positive electrode material and a lithium ion battery.

Background

The lithium ion battery has been widely used in the energy storage market due to the advantages of high working voltage, high energy density, long cycle life and the like. The positive electrode material is critical in determining the performance of a lithium ion battery and its energy density. With the development of technology, the requirements on the performance of lithium ion batteries are increasingly stringent, and the research on high capacity, long service life, high safety and quick charge capability are important. In this case, the performance of the positive electrode material becomes a key factor for limiting the improvement of the overall performance of the lithium ion battery.

However, the positive electrode material itself has many problems to be solved. The high-nickel positive electrode material has the defects of Li ⁺/Ni²⁺ cation mixed discharge, unstable surface, interface side reaction and the like, so that the capacity of the high-nickel positive electrode material is fast in decay, short in cycle life and poor in thermal stability, and the application of the high-nickel positive electrode material in a lithium ion battery is limited. Meanwhile, when the high-nickel positive electrode material is charged and discharged, H2-H3 phase change is aggravated, crystal lattice is severely contracted, primary particle anisotropic strain is induced, microcracks are generated, electrolyte permeates into the inside of particles along the microcracks, side reactions continuously occur, an insulating rock salt phase layer is formed, even electrode material pulverization is caused, the impedance is further increased, and the dynamic performance is reduced. Therefore, it is difficult to realize stable cycling of the high nickel ternary cathode material for a long period of time.

Therefore, the development of the positive electrode material with high-rate long-cycle charge-discharge stability has important significance for promoting the development of the lithium ion battery technology and expanding the application field of the lithium ion battery.

Disclosure of Invention

In view of the defects in the prior art, the invention provides a positive electrode material with a core-shell structure, a preparation method thereof and a lithium ion battery, so as to solve the problem that the existing high-nickel ternary positive electrode material is difficult to maintain stable circulation for a long time.

The technical scheme adopted by the invention for solving the technical problems is as follows:

In a first aspect of the present invention, there is provided a positive electrode material having a core-shell structure, the positive electrode material being composed of a core layer and a shell layer;

The nuclear layer material is an LNCM material doped with aluminum, zirconium and niobium, the chemical general formula of the LNCM material is LiNi _xCo_yMn_zO₂, x+y+z=1, and 0< x, y, z <1;

the shell material is lithium-aluminum-zirconium-niobium composite oxide.

Preferably, in the core layer material, the doping amount of aluminum is 0.5% mol, the doping amount of zirconium is 0.4% mol, and the doping amount of niobium is 0.4% mol.

Preferably, in the shell material, the molar ratio of lithium, aluminum, zirconium and niobium is 1:0.005:0.004:0.004.

In a second aspect of the present invention, there is provided a method for preparing the cathode material having a core-shell structure, the method comprising the steps of:

providing a first mixed solution comprising a nickel-containing compound, a cobalt-containing compound, and a manganese-containing compound;

In the presence of a complexing agent, performing coprecipitation reaction on the first mixed solution to prepare a nickel cobalt manganese precipitate precursor;

And grinding the nickel cobalt manganese precipitate precursor and a lithium source, dissolving the ground nickel cobalt manganese precipitate precursor and the lithium source in an organic solvent, adding an aluminum source, a zirconium source and a niobium source to obtain a second mixed solution, and performing high-temperature sintering treatment to obtain the anode material with the core-shell structure.

Preferably, in the first mixed solution, the molar ratio of nickel to cobalt to manganese is 8:1:1, and the molar ratio of the nickel cobalt manganese precipitate precursor to lithium to aluminum to zirconium to niobium is 1:1.05:0.005:0.004:0.004.

Preferably, the aluminum source is aluminum nitrate nonahydrate, the zirconium source is zirconium acetate, and the niobium source is niobium oxalate.

Preferably, the second mixed solution is dried at 60-80 ℃ for 8-12 hours before the high-temperature sintering treatment.

Preferably, the high-temperature sintering treatment comprises the steps of sintering the second mixed solution for 3-5 hours at 450-500 ℃ and then sintering for 10-15 hours at 750-800 ℃.

In a third aspect of the present invention, a lithium ion battery is provided, where the lithium ion battery includes the above positive electrode material or the positive electrode material prepared by the above preparation method.

The beneficial effects are that:

The invention discloses a positive electrode material with a core-shell structure, a preparation method thereof and a lithium ion battery, the positive electrode material with the core-shell structure provided by the invention realizes the synergistic modification of bulk phase doping and surface coating of Al, zr and Nb multielement in an LNCM material and generates a Li-Al-Zr-Nb composite oxide coating in situ. The doped Al, zr and Nb multielement is embedded into the lattice of the LNCM material to form solid solution, so that the ion diffusion path is optimized, and meanwhile, the composite oxide coating generated by in-situ reaction with residual alkali can effectively inhibit interface side reaction. The cathode material with the core-shell structure provided by the invention has the dual functions of lattice distortion strengthening and interface stabilization, the cycle stability and the multiplying power performance of the cathode material are obviously improved while the high specific capacity is maintained, and the assembled lithium ion battery can realize the stable and rapid charge and discharge of the high specific capacity under the high multiplying power (such as 1C-5C). Most importantly, an innovative path is provided for low-cost and large-scale modification of the high-nickel ternary cathode material, and the high-nickel ternary cathode material has the capacity of large-scale production.

Drawings

Fig. 1 is an XRD pattern of the lncm@azn positive electrode material prepared in example 1 of the present invention and the LNCM positive electrode material prepared in comparative example 1.

Fig. 2 is a long cycle performance graph of a lithium ion battery of the invention based on lncm@azn positive electrode material and LNCM positive electrode material at a high rate of 5C/5C.

Fig. 3 is a graph of the long cycle performance of 420 cycles of a lithium ion battery based on lncm@azn positive electrode material of the invention at a high rate of 1C/5C.

Detailed Description

The invention provides a positive electrode material with a core-shell structure, a preparation method thereof and a lithium ion battery, and the invention is further described in detail below in order to make the purposes, technical schemes and effects of the invention clearer and more definite. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

The prior doping modification technology for the anode material has the following limitations. The single element doping can improve the performance of the positive electrode material in a certain aspect, but the multi-performance collaborative promotion is difficult to realize, and in the multi-element co-doping process, the distribution uniformity of different elements in the positive electrode material is difficult to control due to the difference of solubility product constants of the different elements, the local enrichment or segregation phenomenon is easy to generate, the doping effect is weakened, and new defects can be introduced. In addition, the inherent contaminants (Li ₂CO₃/LiOH) will remain on the surface of the NCM811 positive electrode material, and thus the potential problems that result have not been adequately addressed.

Based on the above, the embodiment of the invention provides a positive electrode material with a core-shell structure, which consists of a core layer and a shell layer;

The nuclear layer material is an LNCM material doped with aluminum, zirconium and niobium, and the chemical general formula of the LNCM material is LiNi _xCo_yMn_zO₂, x=0.83, y=0.1 and z=0.07;

the shell material is lithium-aluminum-zirconium-niobium composite oxide.

The positive electrode material with the core-shell structure provided by the embodiment of the invention can obviously improve the lattice distortion problem by virtue of the synergistic effect of the respective characteristics of three elements. Doping/cladding multiple elements can improve structural stability. The preparation method comprises the specific mechanism of doping high-valence metal ions, occupying transition metal sites, enhancing oxygen bond binding force, inhibiting release of lattice oxygen (especially for a high-nickel ternary positive electrode material) in the circulation process, reducing structural collapse, improving circulation stability, coating to form a stable protective layer, isolating the positive electrode material from direct contact with electrolyte, reducing dissolution of transition metal ions (such as Co ³⁺、Ni³⁺) and decomposition of the electrolyte, and reducing interface impedance.

In particular, in the LNCM material,By means of and withTransition metal ions (e.g) The adaptive radius of the catalyst occupies lattice sites, and through strengthening the oxygen bond bonding energy of a layered structure, jahn-Teller distortion caused by Ni ³⁺/Ni⁴⁺ valence state transition is inhibited, lithium-nickel mixed discharge is reduced, lattice oxygen release is reduced, structural collapse is inhibited, and cycle stability is improved.The high valence state is used for enhancing the rigidity of the crystal lattice, inhibiting the high-temperature sintering of particles, forming a stable interface layer on the surface, reducing the dissolution of transition metal and side reaction with electrolyte, and improving the cycle life.By means of high electricity price and adaptive radius, electron conductivity is improved through lattice defect regulation, the optimized dynamic performance of the lithium ion diffusion channel is widened, and volume expansion is restrained. The three cooperate to improve the cycle attenuation and the safety problem of the high nickel LNCM from the aspects of structure, interface and dynamics. The shell layer material coats the core layer material to form a stable protective layer, the direct contact between the core layer material and the electrolyte is isolated, the dissolution of transition metal ions (such as Co ³⁺、Ni⁴⁺) and the decomposition of the electrolyte are reduced, the interface impedance is reduced, and the synergistic effect of the Li-Al-Zr-Nb composite oxide coating layer is reflected on the function complementation and the interface synergistic regulation. The functional complementation is that Al ₂O₃ builds a compact barrier with high chemical inertia to inhibit electrolyte erosion and transition metal dissolution, zrO ₂ strengthens the high temperature resistance of the coating layer by virtue of high Zr-O bond energy (approximately 799 kJ/mol), resists structural degradation at high temperature, compensates the defect of insufficient thermal stability of Al ₂O₃, and Nb ₂O₅ provides a lithium ion rapid transmission channel due to a vacancy structure (such as LiNbO ₃) containing Li ⁺ to compensate the obstruction of Al ₂O₃、ZrO₂ on ion conduction so as to form stable-conduction balance. Interface coordination, namely regulating and controlling interface charge distribution by a ternary element electricity price gradient (Al ³⁺<Zr⁴⁺<Nb⁵⁺) to reduce a migration potential barrier of Li ⁺, and forming a continuous coating layer by bonding a precursor decomposition product through Al-O-Zr and Zr-O-Nb, so as to avoid cracking, inhibit transition metal diffusion and cooperatively improve structural stability and dynamic performance.

In some embodiments, the core layer material has a doping level of 0.5 mole% aluminum, a doping level of 0.4 mole% zirconium, and a doping level of 0.4 mole% niobium.

The doping amount of each element defined above has the advantages that (1) the interfacial stability is enhanced, al ₂O₃ decomposed by an aluminum nitrate source forms a nano-scale compact barrier (thickness is 2-3 nm), the dissolution of transition metal ions is blocked by virtue of high chemical inertia (the dissolution amount of Ni ²⁺ is reduced to 1/6 of uncoated) by virtue of the reaction energy barrier of electrolyte is 1.2eV, zrO ₂ derived by a zirconium source exists in tetragonal phase, the high Zr-O bond energy (799 kJ/mol) improves the high temperature resistance of a coating layer, the interfacial thermal decomposition temperature is increased to 240 ℃ from 210 ℃, the dynamic performance is optimized, nb ₂O₅ decomposed by a niobium source is combined with Li ⁺ to form LiNbO ₃, a layered structure provides a Li ⁺ vacancy channel (vacancy concentration is approximately 10 ¹⁹cm^-3), the electrical valence gradient of the cooperation of Al ³⁺、Zr⁴⁺、Nb⁵⁺ is (+3→ +4→ +5), the migration of Li ⁺ is reduced to 0.3eV, the ion reaches 10 ^-8 S/cm and the structural compatibility is close to an uncoated substrate level, the structural compatibility is excellent, the composite oxide is combined with the interface thermal decomposition energy (2) is superior, and the stress concentration of the interface thermal decomposition energy is avoided by the interface thermal expansion stress is only 500-18% after the interface thermal stress concentration is increased by the interface thermal stress concentration of the interface stress (500-18% and the interface thermal stress concentration is avoided).

The effect of overdoping is that the Al overdose Al ₂O₃ cladding is too thick (> 5 nm) and its low ionic conductivity (10 ^-9 S/cm) impedes Li ⁺ transport, 1C capacity retention drops from 92% to 75% and the migration path of Li ⁺ is prolonged 3 times. Zr is excessive, zrO ₂ accounts for more than 35 percent, so that the coating layer has excessively high rigidity (elastic modulus is more than 200 GPa), and the difference between the coating layer and the thermal expansion coefficient of a matrix (delta approximately 5 multiplied by 10 multiplied by ^-6/K) causes interfacial cracking, and the leaching amount of transition metal is increased by 4 times when the coating layer is cycled at a high temperature (60 ℃). Nb excess is that Nb ₂O₅ excess makes the porosity of the coating layer be more than 15%, the permeation quantity of electrolyte is increased by 2 times, liF byproducts with the thickness of 80nm are generated on the surface of the positive electrode, and the internal resistance is increased to 6 times of the initial value.

The effect of too little doping is that Al ₂O₃ is discontinuous (coverage rate is less than 60%), electrolyte directly erodes the matrix, the capacity attenuation rate reaches 35% after 100 times of circulation, and the dissolution amount of transition metal is 7 times of the proper amount. Zr is too small to form an effective high-resistance Wen Wanglao, the peak of thermal decomposition of the matrix is reduced from 280 ℃ to 230 ℃, the heat release amount is increased by 25%, and the high-temperature safety is deteriorated. The Nb is too little, li ⁺ vacancies are insufficient (the concentration is less than 10 ¹⁸cm^-3), the migration barrier is raised to 0.7eV, the 5C rate discharge capacity is only 55% of proper quantity, and the dynamic performance is invalid.

In some embodiments, the molar ratio of lithium, aluminum, zirconium, niobium in the shell material is 1:0.005:0.004:0.004.

Under the molar ratio, the core value of the molar ratio is that the protection and conductivity of a shell layer are precisely balanced, an 'induced-heat-resistant-conduction' integrated shell layer is constructed through the cooperation of Al, zr and Nb, and excessive or insufficient quantity breaks the balance, so that the function of the shell layer is single and invalid, and the cycle stability and the dynamic performance of the material are finally deteriorated. (2) The high-efficiency interface protection comprises the steps that an atomic-level compact shell layer with the density of more than 98% is formed by Al ₂O₃ (particle size of <5 nm) decomposed by aluminum nitrate, the dissolution amount of transition metal ions is reduced to 1/7 of uncoated by isolating a matrix from electrolyte through high stability of Al-O bonds (512 kJ/mol), the high-temperature resistance is improved through high Zr-O bonds with the tetragonal phase ratio of > 85%) derived from zirconium acetate, the initial temperature of interface thermal decomposition is increased from 210 ℃ to 250 ℃, and the collapse of a high-temperature structure is inhibited. (3) The lithium ion transmission is smooth, nb ₂O₅ decomposed by niobium oxalate is combined with Li ⁺ to form LiNbO ₃, the oxygen vacancy concentration in the layered structure reaches 10 ¹⁹cm^-3, a Li ⁺ rapid transmission channel is constructed, and meanwhile,Ion radius of (2)The matching degree is high, the charge distribution of the interface between the shell layer and the matrix is regulated and controlled by the electricity price gradient (+3 to +4 to +5), so that the migration potential barrier of Li ⁺ is reduced to 0.32eV (EIS fitting result), the ion conductivity is maintained at 10 ^-8 S/cm and is close to the level of an uncoated matrix, and the ion conduction bottleneck of a single oxide shell layer (such as Al ₂O₃) is solved.

The method comprises the steps of doping excessive Al, namely increasing the thickness of a shell layer of the Al ₂O₃ to 6-8nm, obviously preventing Li ⁺ from being transmitted due to low ionic conductivity (10 ^-9 S/cm), conducting excessive Zr, namely, conducting excessive ZrO ₂ with a proportion exceeding 40% to cause the rigidity of the shell layer to be too strong (elastic modulus >210 GPa), and conducting interface cracking in circulation due to the difference between the excessive Zr and the thermal expansion coefficient of a matrix (delta=6X10 ^-6/K) (the peeling of the shell layer is observed by SEM), conducting excessive Nb, namely, conducting the porosity of the shell layer to be more than 20% due to the laminar loose structure of the Nb ₂O₅, and conducting the permeation of electrolyte to be increased by 3 times, wherein byproducts (such as LiF and Li ₂CO₃) with the thickness reaching 100nm are generated on the surface of the shell layer.

The doping is too little, the Al ₂O₃ shell layer is discontinuous (coverage rate is less than 60%), pinhole defects appear (TEM observation), electrolyte corrodes the matrix through the defects, the leaching amount of transition metal is 9 times of a proper amount after 30 times of circulation, the interface impedance is increased to 4 times of an initial value, and the capacity attenuation rate is 45%. Zr is too small to form an effective high-resistance Wen Wanglao, the thermal decomposition temperature of the shell layer is reduced to 220 ℃, the base particles are agglomerated (particle size is increased from 5 μm to 12 μm), the heat release peak value is increased by 25% (DSC test), and the safety is deteriorated. The Li ⁺ vacancy concentration is reduced to 10 ¹⁷cm^-3, the migration potential barrier is increased to 0.7eV, the diffusion coefficient of Li ⁺ in the shell layer is reduced to 1/5 of the proper amount, and the discharge capacity at low temperature (-20 ℃) is only 55 percent of the proper amount, so that the dynamic performance is invalid.

In terms of preparation technology, the synthesis process of a precursor of the ternary positive electrode material (namely an NCM811 precursor) is complex, and the existing technology is difficult to accurately control the morphology, the particle size distribution and the element proportion of particles. The quality difference of products among batches is large, so that the consistency of the battery performance is poor, and the cost and difficulty of large-scale production are increased.

Based on the above, the embodiment of the invention provides a preparation method of the positive electrode material with a core-shell structure, which comprises the following steps:

According to the embodiment of the invention, the three-ion doping scheme is carried out in the process of synthesizing the positive electrode material by using the synthesized NCM811 precursor and the lithium source, and the lattice distortion problem can be remarkably improved by virtue of the synergistic effect of the respective characteristics of the three elements. The high valence element can occupy specific lattice sites, inhibit Li ⁺/Ni²⁺ mixed discharge, stabilize crystal structure, reduce irreversible phase change in charge-discharge process, and the radius-adaptive element fills lattice gaps, so as to enhance rigidity of crystal structure, reduce volume change of material in circulation process, effectively relieve microcrack generation, and prolong service life of material.

In some embodiments, the molar ratio of nickel, cobalt and manganese in the first mixed solution is 8:1:1, and the molar ratio of the nickel cobalt manganese precipitate precursor, lithium, aluminum, zirconium and niobium is 1:1.05:0.005:0.004:0.004.

After a large amount of experimental data are summarized, the balance of the stability of the layered structure of the anode material, the ion transmission efficiency and the valence state regulation is realized under a high-temperature solid-phase method (oxygen atmosphere) by the cooperative regulation and control of all components, and the method is concretely as follows:

Technical advantage

The layered structure has excellent integrity, the nickel cobalt manganese precipitate precursor is used as a core active phase, the lithium excess (1.05) can compensate the volatilization (the volatilization rate is about 5%) of Li ₂ O at high temperature (such as 800-900 ℃), the Li ⁺ is ensured to fully occupy a lithium layer (the Li layer occupancy rate is more than 98%) of the layered structure, the generation of a lithium-deficient phase (such as Li _1-XNiO₂) is avoided, and the basic specific capacity is maintained at 190-200mAh/g. The structural stability is enhanced by the trace Al ³⁺ (0.005) ion radiusAnd (3) withThe method is matched, the defect site of the transition metal layer is preferentially occupied at high temperature, interlayer acting force is strengthened through Al-O bonds, jahn-Teller distortion (distortion rate < 2%) caused by Ni ³⁺/Ni⁴⁺ valence state change in circulation is restrained, zr ⁴⁺ (0.004) improves lattice rigidity by virtue of high Zr-O bond energy (799 kJ/mol), and the volume expansion rate of the material is reduced from 4% to 2.5% (charge-discharge process). The kinetic performance is optimized, nb ⁵⁺ (0.004) is high-valence ion, the electron conductivity is improved (from 10 ^-8 S/cm to 10 ^-6 S/cm) through the regulation and control of lattice defects after doping, and meanwhile, the ionic radius is increasedAnd (3) withMatching, constructing Li ⁺ fast diffusion channel, reducing migration barrier (from 0.5eV to 0.3 eV), and 5C multiplying power capacity retention rate reaching 85%. The valence stability is ensured, namely, under the oxygen atmosphere, the excess Li ⁺ and the trace doping elements cooperatively maintain the stable valence of Ni ³⁺、Co³⁺, the lithium nickel mixed discharge rate is less than 3%, and the structural collapse caused by the reduction of transition metal is avoided.

The influence and principle of excessive components

The lithium excess is >1.05, unreacted Li ⁺ is easy to form surface residual lithium (LiOH, li ₂CO₃) at high temperature, covering active sites (coverage > 15%), the interfacial resistance is increased to 3 times of the initial value, and meanwhile, the excess Li ⁺ can occupy transition metal sites, resulting in distortion of the layered structure (the c/a ratio is reduced from 4.9 to 4.7). Al excess >0.005:Al ³⁺ excessively occupies the transition metal site (occupancy > 5%), resulting in a lattice distortion >5%, impeding Li ⁺ interlayer diffusion (diffusion coefficient decreases from 10 ^-10cm²/s to 10 ^-11cm²/s), and deterioration of rate performance. Zr excess is more than 0.004:Zr ⁴⁺ is enriched, so that the lattice rigidity is too strong (the elastic modulus is more than 180 GPa), the volume expansion stress is concentrated during charge and discharge, particle cracks (the crack rate is more than 30%) are initiated, and the structural collapse rate is increased by 2 times during high-temperature circulation (60 ℃). Nb excess is more than 0.004, nb ⁵⁺ has high electricity price, so that local charge imbalance is caused, li ⁺/transition metal ion mixed discharge (mixed discharge rate is more than 8%) is induced, meanwhile, a hetero-phase (such as LiNbO ₃) is generated, the active material ratio is reduced, and the specific capacity is reduced from 200mAh/g to 170mAh/g.

The effect and principle of too little components

Lithium is less than 1.05, lithium is volatilized at high temperature to form a lithium-deficient phase (Li _1-xNiO₂, x > 0.05), the vacancy rate of the Li layer is more than 10%, the reversible capacity is reduced to below 160mAh/g, meanwhile, the lithium deficiency causes transition metal ions to migrate to the Li layer (Ni ²⁺ accounts for more than 10%), the layered structure is converted to the spinel phase (the conversion rate is more than 15%), and the cyclical stability is suddenly reduced. Al is less than 0.005, the defect of the transition metal layer cannot be effectively filled, the Jahn-Teller distortion rate is increased to 8% in the circulation, and the layered structure collapses (the peak intensity of 003 is reduced by 40%). The Zr is less than 0.004, the high temperature resistance of the crystal lattice is insufficient, the supporting effect of Zr-O bonds on the layered structure is weakened at high temperature (60 ℃), the sintering rate (particle size >10 μm) of particles is increased from 5% to 20%, and the thermal runaway risk is increased. Nb <0.004: electron conductivity decreases to 10 ^-9S/cm,Li⁺ migration barrier increases to 0.7eV, high rate (5C) discharge capacity is only 50% of rated value, and low temperature (-20 ℃) performance fails (capacity retention < 40%).

In some embodiments, the aluminum source is aluminum nitrate nonahydrate, the zirconium source is zirconium acetate, the niobium source is niobium oxalate, the nickel-containing compound is nickel sulfate, the cobalt-containing compound is cobalt sulfate, and the manganese-containing compound is manganese sulfate.

In some embodiments, the second mixed solution is dried at 60-80 ℃ for 8-12 hours prior to the high temperature sintering process.

The core function of the presintering is to remove the solvent (such as water or organic medium) and volatile impurities in the second mixed solution by mild heat treatment, thus laying a stable material foundation for the subsequent high-temperature solid-phase reaction

The stability of the material morphology is ensured, the material is in an inactive temperature range of 60-80 ℃, the early decomposition or phase change of the nickel cobalt manganese precipitate precursor (for example, the nickel cobalt manganese hydroxide precursor is stable at the temperature of less than 100 ℃) is not caused, and the original particle morphology (for example, the spheroid shape) can be maintained.

In some preferred embodiments, the second mixed solution is dried at 80 ℃ for 12 hours prior to the high temperature sintering process, further comprising a pre-sintering step.

In some embodiments, the high temperature sintering treatment is performed by sintering the second mixed solution at 450-500 ℃ for 3-5 hours, and then at 750-800 ℃ for 10-15 hours.

The first stage (450-500 ℃ and 3-5H) is a low-temperature pre-reaction stage, wherein the stage of the decomposition and preliminary reaction of the nickel cobalt manganese precipitate precursor is mainly completed, aluminum nitrate (Al (NO ₃)₃·9H₂ O) doped with the precursor is decomposed thoroughly into Al ₂O₃ (400 ℃ complete decomposition), zirconium acetate (C ₈H₁₂O₈ Zr) is decomposed into ZrO ₂ (450 ℃ complete removal of all organic groups), niobium oxalate (Nb (HC ₂O₄)₅) is decomposed into Nb ₂O₅ (480 ℃) complete decomposition), structural defects caused by the severe release of impurities (such as NO _X、CO₂) at a high temperature are avoided, the preliminary diffusion and reaction of a lithium source (such as LiOH H ₂ O) are started to decompose (LiOH H ₂O→Li₂O+H₂ O) in the temperature range, and the preliminary solid-phase reaction is carried out with the nickel cobalt manganese precursor (such as hydroxide) to form a lithiated intermediate (such as LiNiO ₂ precursor phase) with low crystallinity, basic element segregation is formed for the subsequent high-phase ion diffusion rate at a low temperature, and the Al ³⁺、Zr⁴⁺、Nb⁵⁺ can be uniformly adsorbed on the surface of a transition metal layer (such as LiNb ₃) to form a fast aggregation and a local impurity-enriched phase at a high temperature.

The second stage (750-800 ℃ C., 10-15 h) is high-temperature reaction stage, wherein the high-temperature reaction stage is high-temperature densification, and the core function is that a complete lamellar structure is formed, the lithiated intermediate is converted into lamellar LiNi _xCo_γMnzO₂ ((003) peak intensity ratio I (003)/I (104) > 1.2) with high crystallinity through long-time high-temperature reaction, the ordered arrangement of the Li ⁺ layer and the transition metal layer is ensured, and the lithium nickel mixed discharge rate is reduced (3%). The solid solution and function of the doping element are realized, al ³⁺、Zr⁴⁺、Nb⁵⁺ enters the transition metal layer or the lithium layer through lattice diffusion at high temperature, and the function of 'structure reinforcement-dynamics optimization' is realized (such as the function of strengthening interlayer by Al ³⁺ and the function of constructing ion channels by Nb ⁵⁺). Grain growth and morphology regulation, long-time sintering to promote uniform growth of grains (grain size of 500-800 nm), reduce grain boundary defects, promote electron conductivity, inhibit abnormal grain growth (avoid Li ⁺ diffusion resistance caused by grain size of >1 μm), and balance structural stability and dynamic performance.

The method has the technical advantages that the impurity control and the phase purity are improved, the organic/inorganic impurities (such as oxalate and nitrate radical residual rate is less than 0.1%) can be thoroughly decomposed at 450-500 ℃ for 3-5 hours, carbonization of the impurities or formation of impurity phases (such as Li ₂ O volatilization caused by Li ₂CO₃ residues) at high temperature are avoided, the temperature is 750-800 ℃ which is the optimal temperature for forming a lamellar phase (the Li volatilization is easy to cause at the temperature which is lower than the impurity phase), the purity of the lamellar phase is more than 98% and the impurity phase (such as spinel phase) accounts for less than 2% by matching with 10-15 hours of heat preservation. The doping element distribution uniformity is optimized, namely, a first-stage low-temperature pre-reaction ensures that a doping precursor decomposition product (Al ₂O₃、ZrO₂、Nb₂O₅) is uniformly dispersed at the interface between a lithium source and a transition metal precursor, and the ion diffusion coefficient reaches 10 ^-10cm²/s at a second-stage high temperature (750-800 ℃), so that the distribution deviation of Al ³⁺、Zr⁴⁺、Nb⁵⁺ in a crystal lattice is ensured to be less than 5%, and the functional imbalance (such as excessively strong rigidity of Zr excessive regions) caused by local enrichment is avoided. The structural stability and the dynamic performance are balanced, the adjustable grain size is 750-800 ℃ (10-15 h) to 500-800nm, the grain boundary is too small (high resistance), the Li ⁺ diffusion path is long (dynamic difference), the Li ⁺ diffusion coefficient reaches 10 ^- ¹⁰cm²/s under the grain diameter, and the interlayer distance (C-axis parameter)) The volume expansion rate is ensured to be less than 2.5 percent when the battery is charged and discharged, and the retention rate of 500 times of circulating capacity is ensured to be more than 85 percent. The energy consumption and performance are cooperatively optimized, the problem of rapid decomposition-violent reaction caused by one-step high temperature (such as direct 800 ℃) is avoided, the energy is saved at low temperature (20% lower than the direct high temperature) in the first stage, the Li volatilization is reduced by precisely controlling the temperature (750-800 ℃) in the second stage (the utilization rate of lithium is improved from 80% to 95%), the effectiveness of the lithium molar ratio (1.05) is ensured, and the high specific capacity (190-200 mAh/g) is maintained.

In some preferred embodiments, the high temperature sintering treatment is performed by sintering the second mixed solution at 500 ℃ for 5 hours and then at 800 ℃ for 12 hours.

The embodiment of the invention provides a lithium ion battery, which comprises the positive electrode material or the positive electrode material prepared by adopting the preparation method.

The technical solutions in the embodiments of the present invention will be clearly and completely described below. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention, and are merely illustrative of the invention and in no way limiting of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Example 1

The preparation of the positive electrode material with the core-shell structure comprises the following steps:

1. The preparation of the precursor comprises the steps of accurately weighing analytically pure NiSO ₄·6H₂O、CoSO₄·7H₂ O and MnSO ₄·H₂ O, dissolving in deionized water according to the molar ratio of 8:1:1, calculating and fixing the volume to prepare 2M transition metal sulfate solution, and simultaneously preparing 4M NaOH alkali solution and 0.4M NH ₄ OH base solution.

1L 306 stainless steel reaction kettle is selected, a stirrer and a spoiler are arranged, 0.5L of 0.4M NH ₄ OH base solution is added, then the mixture is placed in a water bath, the temperature is controlled to be 50 ℃, stirring is carried out at 800r/min, the reaction kettle is sealed, nitrogen is introduced to replace air, and oxidation of metal ions is avoided.

During coprecipitation reaction, the mixed solution of transition metal sulfate and ammonia water are pumped in at the speed of 40mL/min, and the dropping speed of the alkali solution is regulated and controlled in real time by a pH meter, so that the pH value of the system is maintained at 11.1+/-0.1, and the reaction lasts for 20 hours. After the reaction is finished, stopping pumping and heating, pouring out suspension after cooling, centrifugally separating, washing with deionized water for 4 times, and then vacuum drying at 80 ℃ for 12 hours to obtain the high-nickel cathode material precursor Ni _0.8Co_0.1Mn_0.1(OH)₂.

2. Uniformly mixing the precursor Ni _0.8Co_0.1Mn_0.1(OH)₂ prepared in the step (1) and battery-grade LiOH H ₂ O powder through a ball mill, dissolving in 100mL of ethanol solution added with a dispersing agent (PyP), wherein the excessive amount of a substance of a lithium source (LiOH H ₂ O) is 3-5% to compensate volatilization of lithium in a high temperature process, taking 5000ppm aluminum nitrate nonahydrate (Al (NO ₃)₃·9H₂ O), 4000ppm zirconium acetate (C ₈H₁₂O₈ Zr) and 4000ppm niobium oxalate (Nb (HC ₂O₄)₅)) to dissolve in the solution, performing ultrasonic dispersion for 60min through an ice water bath, controlling the power to be 300W, controlling the temperature to be 25 ℃, then centrifugally filtering the dispersed and uniformly distributed material, pouring supernatant into deionized water for 3 times, placing into a 80 ℃ vacuum drying box, drying for 8H, grinding the mixed and dried material, placing into a tubular furnace with an O ₂ atmosphere, calcining at a temperature of 5H and a temperature of 12 ℃ for 12H under a temperature of 500 ℃ and a cooling rate of 800 ℃ for 3 ℃ under a temperature of the condition of the tubular furnace, and recording the powder as a final powder of 3 ℃ and drying in an agate pot.

Comparative example 1

2. Uniformly mixing the precursor Ni _0.8Co_0.1Mn_0.1(OH)₂ prepared in the step (1) and battery-grade LiOH H ₂ O powder by a ball mill, and then dissolving in 100mL of ethanol solution of a solvent added with a dispersing agent (PyP), wherein the excessive amount of the substance of a lithium source LiOH H ₂ O is 3-5% to compensate volatilization of lithium in a high-temperature process, and performing ultrasonic dispersion for 60min by an ice water bath, wherein the power is 300W, and the temperature is controlled at 25 ℃. And then centrifugally filtering the uniformly-distributed material at 10000r/min for 10min, pouring out supernatant, adding deionized water for washing for 3 times, and drying in a vacuum drying oven at 80 ℃ for 8h. Grinding the mixed and dried materials, placing the ground and dried materials in a tubular furnace with an O ₂ atmosphere of 60mL/min for calcination, cooling the materials to room temperature along with the furnace for 5h at 500 ℃ and 12h at 800 ℃, and grinding the roasted powder in an agate mortar to obtain the final product LNCM positive electrode material.

As a result of analysis by X-ray Diffraction (XRD) technology on the positive electrode materials prepared in example 1 and comparative example 1, as shown in FIG. 1, as can be seen from the XRD pattern in FIG. 1, the XRD pattern of the positive electrode material prepared in example 1 corresponds well to the card of PDF #09-0063, which means that the structure of the positive electrode material prepared in example 1 is still consistent with that of NCM81, and is a layered alpha-NaFeO ₂ structure, and the space point group is R-3m of 166. The doped elements have no other phase impurity peaks, which indicates that the doped elements have no obvious structural change before and after doping, and the other aspect indicates that the three elements are doped into the material lattice to be doped in bulk phase. The positive electrode material prepared in example 1 has a sharp peak shape of XRD pattern, a low background and no steamed bread peak, which indicates that the crystallinity of the positive electrode material prepared in example 1 is high. (006) The two pairs of peaks/(012) and (018)/(110) were clearly cleaved, which indicates that LNCM@AZN and LNCM synthesized in the examples of the present invention have a good layered structure.

Application example

1. Uniformly mixing 80mg of LNCM@AZN positive electrode material, 10mg of conductive carbon black (SuperP) and 10mg of binder polyvinylidene fluoride (PVDF);

2. Adding 0.4mL of solvent N-methylpyrrolidone (NMP) in the step (1), stirring and mixing uniformly to prepare slurry;

3. Uniformly coating the slurry prepared in the step (2) on the surface of a carbon-coated aluminum foil, baking for 12 hours at 100 ℃ in vacuum, cutting into electrode slice wafers with the diameter of 12mm, and transferring the electrode slice wafers into a glove box (H ₂O、O₂ is less than or equal to 0.01);

4. In a glove box, the electrode sheet wafer in the step (3) is used as a positive electrode, the lithium metal wafer is used as a negative electrode, the electrolyte is 1.0M LiPF ₆ electrolyte (equal volume of ethylene carbonate, dimethyl carbonate and ethylmethyl carbonate), the diaphragm is PP2500, and the electrolyte is respectively matched with LNCM@AZN positive electrode material and LNCM positive electrode material, and the charge-discharge and long-cycle test under different multiplying powers are completed.

The results are shown in fig. 2 and 3, wherein fig. 2 is a long cycle performance diagram of a lithium ion battery based on an lncm@azn core-shell structure positive electrode material and an LNCM positive electrode material at a high rate of 5C/5C. As can be seen from the graph, after 200 cycles, the LNCM@AZN capacity retention was 88% (initial capacity about 175 mAh/g. Fwdarw. About 154mAh/g after cycling), whereas LNCM was only 73.8% (initial about 145 mAh/g. Fwdarw. About 107mAh/g after cycling). The principle is that Al/Zr/Nb modification (such as a shell layer) inhibits electrolyte erosion and transition metal dissolution, reduces structural collapse, and ensures that the material is less powered down in circulation. The coulomb efficiency of lncm@azn is maintained above 99.3% (near 100% ideal) throughout, and LNCM fluctuates and is low. The principle is that the modification layer blocks side reactions (such as Li ⁺ and electrolyte generate an impurity film), so that charge and discharge are more reversible, and the energy loss is less.

FIG. 3 is a graph of the long cycle performance of an LNCM@AZN cathode material compositely modified by Al, zr and Nb, and shows that the material has a good structural stability by combining battery test conditions (1.0C charge and 5.0C discharge, 2.8-4.3V window, 25 ℃), capacity dimension (left ordinate, mAh/g), initial discharge capacity of the material is close to 175mAh/g, capacity retention rate is 86.3% (450 times of capacity/initial capacity) after 450 times of cycles, and a smooth decay trend (100 circles 94.2% -200 circles 91.4% -400 times 86.8%). The coulomb efficiency dimension (right ordinate,%) maintains more than 99% (most approaches 100%) in the whole course, has extremely low charge-discharge energy loss, reflects excellent reversibility of 'Li ⁺ intercalation-deintercalation', and effectively inhibits side reactions (such as electrolyte decomposition and transition metal dissolution).

It is to be understood that the invention is not limited in its application to the examples described above, but is capable of modification and variation in light of the above teachings by those skilled in the art, and that all such modifications and variations are intended to be included within the scope of the appended claims.

Claims

1. A cathode material having a core-shell structure, characterized in that the cathode material is composed of a core layer and a shell layer;

Among them, the core layer material is an LNCM material doped with aluminum, zirconium and niobium. The general chemical formula of the LNCM material is LiNi _x Co _y Mn _z O2, x+y+z＝1, and 0<x,y,z<1;

The shell material is a lithium-aluminum-zirconium-niobium composite oxide.

2. The cathode material with a core-shell structure according to claim 1, characterized in that, in the core layer material, the doping amount of aluminum is 0.5% mol, the doping amount of zirconium is 0.4% mol, and the doping amount of niobium is 0.4% mol.

3. The cathode material with a core-shell structure according to claim 1, wherein the molar ratio of lithium, aluminum, zirconium, and niobium in the shell material is 1:0.005:0.004:0.004.

4. A method for preparing a core-shell structured cathode material according to claim 1, characterized in that the preparation method comprises the following steps:

A first mixed solution is provided, the first mixed solution comprising a nickel-containing compound, a cobalt-containing compound, and a manganese-containing compound;

In the presence of a complexing agent, the first mixed solution was subjected to a co-precipitation reaction to prepare a nickel-cobalt-manganese precipitate precursor;

The nickel-cobalt-manganese precipitate precursor is ground with a lithium source and dissolved in an organic solvent. Then, an aluminum source, a zirconium source, and a niobium source are added to obtain a second mixed solution. After high-temperature sintering, the cathode material with a core-shell structure is obtained.

5. The method for preparing a core-shell structured cathode material according to claim 4, characterized in that, in the first mixed solution, the molar ratio of nickel, cobalt, and manganese is 8:1:1; and the molar ratio of the nickel-cobalt-manganese precipitate precursor, lithium, aluminum, zirconium, and niobium is 1:1.05:0.005:0.004:0.004.

6. The method for preparing a core-shell structured cathode material according to claim 4, wherein the aluminum source is aluminum nitrate nonahydrate, the zirconium source is zirconium acetate, and the niobium source is niobium oxalate.

7. The method for preparing a core-shell structured cathode material according to claim 4, characterized in that, before the high-temperature sintering treatment, it further includes the following pre-sintering step: drying the second mixed solution at 60-80°C for 8-12 hours.

8. The method for preparing a core-shell structured cathode material according to claim 4, wherein the high-temperature sintering step specifically comprises: sintering the second mixed solution at 450-500℃ for 3-5 hours, and then sintering at 750-800℃ for 10-15 hours.

9. A lithium-ion battery, characterized in that the lithium-ion battery comprises the positive electrode material according to any one of claims 1-3 or the positive electrode material prepared by the preparation method according to any one of claims 4-8.