Background
Currently, with the rapid development of society, lithium ion batteries with high energy density, low cost and long life are becoming increasingly demanded. Among the cathode materials of many lithium ion batteries, a high-voltage high-nickel low-cobalt ternary cathode material (LiNi)xCoyMnzO2X is more than or equal to 0.6, y is less than or equal to 0.1, and x + y + z is 1) fully shows the great advantages of high specific energy and low cost due to high voltage, high nickel content and low content of expensive metal cobalt, and meets the requirement of the current social development. However, the layered structure of the ternary material is unstable due to deep lithium deintercalation caused by its high voltage, and severe Li is caused by the high nickel content and the low cobalt content+/Ni2+The output specific capacity of the ternary material in the circulation process is rapidly reduced due to mixed discharging and surface phase transition, and the harm seriously limits the practical application of the high-voltage high-nickel low-cobalt ternary cathode material. The doping modification is to dope elements with larger ionic radius, more multi-core external electron quantity and higher oxygen binding energy into the crystal lattice of the ternary anode material, so that the properties of the ternary layered anode material can be fundamentally improved, the output specific capacity and the charge-discharge cycle stability of the ternary layered anode material are improved, and the ternary layered anode material is beneficial to better popularization and application.
The doping modification comprises lithium-site doping, transition metal-site doping and oxygen anion-site doping, wherein the lithium-site doping elements are generally alkali metal elements such as Na, K, Rb, Mg and the like, the atomic radii of the alkali metal elements are larger than that of Li, and when the lithium-site doping elements are doped into the ternary layered positive electrodeWhen the lithium layer of the cathode material is formed, the lithium layer can play a role of a pillar, the collapse of a layered structure during deep lithium removal (high voltage) is inhibited, and the structural stability of the cathode material is well maintained. Meanwhile, due to the larger ionic radius, the lithium intercalation and deintercalation of lithium can be facilitated, the diffusion coefficient of lithium is improved, and the rate capability of the cathode material is improved. Finally, the bulky electron clouds formed by the more extra-nuclear electrons they possess can effectively inhibit transition metal migration through electrostatic repulsion and magnetic interactions, thereby attenuating Li+/Ni2+Mixed arrangement and phase transition, and the cycling stability of the ternary layered cathode material is improved. These excellent modifications can greatly improve the cycle stability of the high-voltage high-nickel low-cobalt ternary cathode material, thereby achieving the goal of commercial application thereof. Currently, lithium site doping modification is widely studied, but they are often not easy to be applied in industrial production, for example: the patent CN101540400A adopts a ball milling method to prepare the lithium-sodium-doped lithium iron phosphate cathode material, and the ball milling method has high energy consumption in industrial production and difficult large-scale production, thereby limiting the practical application of the ball milling method. In patent CN111653775A, a sodium silicate coating layer is formed on the surface of the lithium-manganese-rich base material in advance by using an organic liquid phase method, and the surface sodium-doped lithium-manganese-rich base anode material is obtained after roasting, which limits the large-scale industrial production application thereof. The patent CN111463428A combines the solvothermal method and the twice calcining method to prepare the sodium-doped ternary cathode material with a hollow structure, and the method is not easy to be applied to large-scale industrial practical production. Therefore, the method for modifying the ternary cathode material by in-situ lithium-position doping on the premise of not changing the production process and equipment of the conventional ternary cathode material has important application significance, is convenient for large-scale production, greatly improves the electrochemical lithium storage performance of the high-voltage high-nickel low-cobalt ternary cathode material, and well meets the requirements of current social development.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a lithium-doped and modified high-nickel low-cobalt ternary cathode material for a lithium ion battery and a preparation method thereof, and the obtained high-nickel low-cobalt ternary cathode material has excellent electrochemical lithium storage performance.
In order to achieve the technical purpose and achieve the technical effect, the invention is realized by the following technical scheme:
the invention firstly provides a high-nickel low-cobalt ternary cathode material for a lithium-site-doped modified lithium ion battery, which is obtained by mixing and grinding sodium salt or potassium salt powder, lithium salt and a precursor of the high-nickel low-cobalt ternary cathode material.
The invention also provides a preparation method of the lithium-site-doped modified high-nickel low-cobalt ternary cathode material for the lithium ion battery, which comprises the following steps of:
the method comprises the following steps: grinding the sodium salt or potassium salt solid powder in a mortar to obtain sodium salt or potassium salt powder;
step two: uniformly mixing the high-nickel low-cobalt ternary positive electrode material precursor, lithium salt and the sodium salt or potassium salt powder obtained in the step one to obtain a mixture;
step three: and D, roasting and grinding the mixture obtained in the step II to obtain the lithium-doped modified high-nickel low-cobalt ternary cathode material for the lithium ion battery.
Preferably, in the first step, the sodium salt is sodium carbonate, sodium acetate, sodium oxalate, sodium nitrate or sodium sulfate.
Preferably, in the first step, the potassium salt is potassium carbonate, potassium acetate, potassium oxalate, potassium nitrate or potassium sulfate.
Preferably, the precursor of the high-nickel and low-cobalt ternary cathode material in the second step is NixCoyMnz(OH)2Wherein x is more than or equal to 0.6 and less than or equal to 0.9, y is more than or equal to 0 and less than or equal to 0.1, z is more than or equal to 0 and less than or equal to 0.4, and x + y + z is equal to 1.
Preferably, the lithium salt in the second step is LiOH & H2O or Li2CO3。
Preferably, the molar ratio of the high-nickel and low-cobalt ternary positive electrode material precursor to the mixed powder of lithium salt and sodium salt or potassium salt in the second step is 1: 1.05, wherein the molar ratio of the lithium salt to the sodium salt or the potassium salt is (1-a): a, wherein a is 0.005-0.3.
Preferably, the atmosphere of the roasting process in the third step is air or oxygen.
Preferably, the roasting process in the third step is to firstly preserve heat at 500 ℃ for 3h and then preserve heat at 700-900 ℃ for 8-15 h.
The invention has the advantages of
The invention provides a high-nickel low-cobalt ternary cathode material for a lithium-site-doped modified lithium ion battery and a preparation method thereof. Meanwhile, Na or K element with larger ionic radius is doped at the lithium position, which can play a role of a pillar, and can prevent the structure from collapsing even when lithium is deeply de-intercalated (high voltage), thereby stabilizing the layered structure of the ternary cathode material; furthermore, Li can be inhibited by doping Li sites with multi-electron Na or K (electron cloud with larger electron outside atomic nucleus)+/Ni2+Mixed arrangement and transition metal migration, thereby effectively inhibiting the phase transition in the circulation process, and well maintaining the stability of a laminated structure; both greatly improve the cycling stability of the anode material, and further prolong the service life of the lithium ion battery.
The experimental results show that: the lithium-doped modified high-nickel low-cobalt ternary cathode material obtained by the method keeps good appearance, and elements are uniformly doped into crystal lattices. Example 1 the 2% mol Na doped NCM60535 ternary material prepared by the above method has a first discharge specific capacity of 184.5mAh g at a voltage range of 3-4.5V and a current density of 0.1C-1. The specific capacity of 152.5mAh g is output after the circulation for 100 times under the current density of 0.5C-1The capacity retention rate can reach 93.3 percent. Example 2 the 1% mol K-doped NCM60535 ternary material prepared by the above method has a first discharge specific capacity of 181.4mAh g under a voltage range of 3-4.5V and a current density of 0.1C-1. The specific capacity of 152.8mAh g is output after the circulation for 100 times under the current density of 0.5C-1The capacity retention rate can reach 96.5%.
Therefore, the lithium-doped modified high-nickel low-cobalt ternary cathode material prepared by the method has excellent charge-discharge cycle stability, can be widely applied to the cathode material of the lithium ion battery, and is suitable for popularization and application.
Detailed Description
The invention firstly provides a high-nickel low-cobalt ternary cathode material for a lithium-site-doped modified lithium ion battery, which is obtained by mixing and grinding sodium salt or potassium salt powder, lithium salt and a precursor of the high-nickel low-cobalt ternary cathode material.
The invention also provides a preparation method of the lithium-site-doped modified high-nickel low-cobalt ternary cathode material for the lithium ion battery, which comprises the following steps of:
the method comprises the following steps: grinding the sodium salt or potassium salt solid powder in a mortar to obtain sodium salt or potassium salt powder; the sodium salt is preferably sodium carbonate, sodium acetate, sodium oxalate, sodium nitrate or sodium sulfate; the potassium salt is preferably potassium carbonate, potassium acetate, potassium oxalate, potassium nitrate or potassium sulfate, and the sources of the sodium salt or the potassium salt are all commercially available;
step two: uniformly mixing the high-nickel low-cobalt ternary positive electrode material precursor, lithium salt and the sodium salt or potassium salt powder obtained in the step one to obtain a mixture;
the precursor of the high-nickel low-cobalt ternary cathode material is NixCoyMnz(OH)2Wherein x is more than or equal to 0.6 and less than or equal to 0.9, y is more than or equal to 0 and less than or equal to 0.1, z is more than or equal to 0 and less than or equal to 0.4, and x + y + z is equal to 1.
The lithium salt is preferably LiOH H2O or Li2CO3When the molar fraction of Ni is 0.7 or more (i.e., Ni)xCoyMnz(OH)2When x is not less than 0.7), only LiOH. H can be used2O。
In the second step, a precursor (Ni) of the ternary cathode material with high nickel content and low cobalt content is preparedxCoyMnz(OH)2) Lithium salt (Li)+) With sodium salt (Na)+) Or potassium salt (K)+) The molar ratio of the two mixed powders is 1: 1.05, lithium salt (Li)+) With sodium salt (Na)+) Or potassium salt (K)+) The molar ratio of the two is (1-a): a, wherein a is 0.005-0.3. Namely, the lithium salt and the sodium salt or the potassium salt mixture are excessive by 5 mol percent relative to the precursor of the high-nickel and low-cobalt ternary cathode material to make up for the loss caused by high-temperature volatilization, and the total amount is Na+Or K+The mol fraction of doping is between 0.5% and 3%.
Step three: and D, roasting and grinding the mixture obtained in the step II to obtain the lithium-doped modified high-nickel low-cobalt ternary cathode material for the lithium ion battery.
The atmosphere of the roasting process in the third step is as follows: ni for high-nickel low-cobalt ternary cathode materialxCoyMnz(OH)2When x is more than or equal to 0.6 and less than or equal to 0.7, the roasting atmosphere is air, and when x is more than 0.7 and less than or equal to 0.9, the roasting atmosphere is oxygen.
And in the third step, the roasting process comprises the steps of firstly preserving heat at 500 ℃ for 3 hours, then preserving heat at 700-900 ℃ for 8-15 hours, then cooling, grinding and sieving with a 300-mesh sieve to obtain the sodium or potassium doped modified high-nickel low-cobalt ternary cathode material.
Other aspects, features and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention. But this example does not limit the invention.
Example 1
1) The purchased sodium carbonate was ground well to a powder in a mortar.
2) Mixing Ni0.6Co0.05Mn0.35(OH)2High-nickel low-cobalt ternary positive electrode material precursor and LiOH & H2O powder and the above-mentioned well-milled Na2CO3The powder was prepared according to the following formula 1: 1.0395: 0.0105 molar ratio, weighing, grinding and mixing uniformly.
3) And (2) preserving the uniformly mixed raw materials in an air atmosphere at 500 ℃ for 3h, then preserving the heat at 800 ℃ for 5h, finally preserving the heat at 870 ℃ for 10h, then cooling, grinding and sieving by a 300-mesh sieve to obtain LiNi with 2% mol of Na doped at the lithium position0.6Co0.05Mn0.35O2(noted as NCM-Na) high nickel low cobalt ternary positive electrode material.
The SEM test result of the NCM-Na doped modified material obtained in example 1 is shown in FIG. 1b, which is a dense secondary sphere having a diameter of about 10 μm and formed by stacking a large number of primary particles. The XRD pattern of the NCM-Na doped modified material is shown in figure 2, which is an impurity-free single-phase material with hexagonal alpha-NaFeO
2Mold structure
And the pattern shows a clear split between the (006)/(102) and (108)/(110) peaks, indicating a better layered structure of the material.
Example 2
1) The purchased potassium carbonate was ground well to a powder in a mortar.
2) Mixing Ni0.6Co0.05Mn0.35(OH)2High-nickel low-cobalt ternary positive electrode material precursor and LiOH & H2O powder and the above-mentioned well-ground K2CO3The powder was prepared according to the following formula 1: 1.04475: 0.00525 are weighed, ground and mixed evenly.
3) Keeping the uniformly mixed raw materials at 500 ℃ for 3h, keeping the temperature at 800 ℃ for 5h, keeping the temperature at 870 ℃ for 10h, cooling, and grindingSieving with a 300-mesh sieve to obtain LiNi doped with 1% mol K at lithium position0.6Co0.05Mn0.35O2(noted as NCM-K) high nickel low cobalt ternary positive electrode material.
The SEM test results of the NCM-K doped modified material obtained in example 2 are shown in FIG. 1c, which is a compact secondary sphere with a diameter of about 10 μm and formed by stacking a large number of primary particles. The XRD pattern of the NCM-K doped modified material is shown in figure 2, which is an impurity-free single-phase material with hexagonal alpha-NaFeO
2Mold structure
And the pattern shows a clear split between the (006)/(102) and (108)/(110) peaks, indicating a better layered structure of the material.
Comparative example 1
1) Only mixing Ni0.6Co0.05Mn0.35(OH)2High-nickel low-cobalt ternary positive electrode material precursor and LiOH & H2O powder is prepared according to the following ratio of 1: the molar ratio of 1.05 is weighed, ground and mixed uniformly.
2) And (2) preserving the uniformly mixed raw materials at 500 ℃ for 3h in an air atmosphere, preserving the heat at 800 ℃ for 5h, preserving the heat at 870 ℃ for 10h, cooling, grinding and sieving by a 300-mesh sieve to obtain the modified LiNi without Na or K doping0.6Co0.05Mn0.35O2(noted as NCM) high nickel low cobalt ternary positive electrode material.
The results of SEM test of the NCM material obtained in comparative example 1 are shown in FIG. 1a, which is a dense secondary sphere having a diameter of about 10 μm and formed by stacking a large number of primary particles. The XRD pattern of the NCM material is shown in FIG. 2, which is an impurity-free single-phase material having hexagonal alpha-NaFeO
2Mold structure
And the pattern shows a clear split between the (006)/(102) and (108)/(110) peaks, indicating a good layered structure of the material.
Application example 1
The high-nickel low-cobalt ternary cathode materials prepared in example 1, example 2 and comparative example 1 are subjected to electrochemical lithium storage performance tests. The method comprises the following specific steps:
mixing the positive electrode active material, C45, KS-6 and PVDF in a mass ratio of 95:2:1.5:1.5 in an N-methylpyrrolidone (NMP) solvent, setting the solid content of the slurry to be 55%, uniformly mixing the slurry by using a homogenizer, coating the slurry on an aluminum foil, drying the aluminum foil in an oven at 100 ℃ for 1h, rolling and cutting the aluminum foil, and standing the aluminum foil in a vacuum oven overnight. The loading capacity of the obtained pole piece active material is about 5.5mg cm-2. The cathode adopts a metal lithium sheet, the diaphragm is a polypropylene porous membrane, and the electrolyte adopts 1mol L-1LiPF of6The lithium salt is dissolved in a solvent system with the volume ratio of EC/EMC being 3/7, a 2025 type button cell is adopted as the cell, and the lithium storage performance test is carried out in a voltage interval of 3-4.5V.
The first charge-discharge curve of the cells prepared from the ternary materials of NCM-Na, NCM-K and NCM high-nickel and low-cobalt obtained in example 1, example 2 and comparative example 1 at the current density of 0.1C and the charge-discharge cycle performance of the cells prepared from the ternary materials of NCM-Na, NCM-K and NCM high-nickel and low-cobalt at the current density of 0.5C in the voltage interval of 3-4.5V are shown in FIGS. 3 and 4, and it can be seen that: the specific capacity difference of the first discharge of the NCM-Na, the NCM-K and the NCM is smaller, and the specific capacity difference is 184.5mAh g-1、181.4mAh g-1And 187.6mAh g-1However, the cycle stability of the NCM-Na and NCM-K doped modified material is obviously superior to that of the undoped NCM material, which fully proves that the Na or K doped modification at the lithium position well plays a role in stabilizing the stability of the layered structure of the high-nickel low-cobalt ternary cathode material, thereby improving the charge-discharge cycle stability of the high-nickel low-cobalt ternary cathode material under high pressure, and therefore, the invention has more commercial popularization superiority.
The present invention includes, but is not limited to, the above embodiments, and any equivalent substitutions or partial modifications made under the principle of the spirit of the present invention are considered to be within the scope of the present invention.