CN114512661A - Modified positive electrode active material and preparation method, positive electrode, lithium ion secondary battery - Google Patents

Abstract

The present invention relates to a modified positive electrode active material comprising: the primary particles comprise a spinel phase and a rock-salt-like phase, the primary particles have a core-shell-like structure, the spinel phase is a core, and the rock-salt-like phase is distributed on the surface of the spinel phase to form an outer shell; the spinel phase is formed from a lithium-containing compound having a spinel crystal structure; the halite-like phase comprises at least one occupying element of Al, Nb, B, Si, F and S, and the occupying element occupies 16c or 8a vacancy of spinel octahedron or the position of oxygen ion in spinel octahedron; the primary particles are also doped with phosphorus elements, and the phosphorus elements are distributed in a gradient manner from outside to inside. The invention further relates to a preparation method of the modified positive electrode active material, a positive electrode of a lithium ion secondary battery containing the positive electrode active material and the lithium ion secondary battery.

Description

Modified positive electrode active material and preparation method, positive electrode, lithium ion secondary battery

technical field

The invention relates to the field of lithium ion batteries, in particular to a modified positive electrode active material and a preparation method, a positive electrode, and a lithium ion secondary battery.

Background technique

Compared with other rechargeable battery systems, lithium-ion secondary batteries have the advantages of high operating voltage, light weight, small size, no memory effect, low self-discharge rate, long cycle life, and high energy density. Mobile terminal products such as mobile phones, notebook computers, and tablet computers. In recent years, due to the consideration of environmental protection, electric vehicles have been rapidly developed under the impetus of governments and automakers, and lithium-ion secondary batteries have become an ideal power source for a new generation of electric vehicles due to their excellent performance. . At present, the positive active materials of lithium ion secondary batteries that people pay attention to can be roughly divided into three categories: layered materials represented by lithium cobalt oxide (LiCoO ₂ ), and olivine represented by lithium iron phosphate (LiFePO ₄ ). type materials and spinel structure materials represented by lithium manganate (LiMn ₂ O ₄ ).

High-voltage materials with spinel structure, as an advanced cathode active material, are considered to be the most likely cathode active materials for next-generation high-performance lithium batteries. For the high-pressure spinel positive active material, during the cycle, due to the interaction between the traditional carbonate electrolyte and the positive active material, the surface of the positive active material loses oxygen and dissolves the surface of the material, which eventually leads to the reduction of active substances. . In order to solve this technical problem, it is proposed to use element doping to modify the positive electrode active material. Doping elements can form new chemical bonds inside and on the surface of the material to stabilize the bulk and surface lattice oxygen, but bulk element doping Excessive amount will lead to a decrease in the capacity of the cathode active material and affect the electrochemical performance of the cathode active material.

SUMMARY OF THE INVENTION

Based on this, it is necessary to provide a modified positive electrode active material and a preparation method, a positive electrode, and a lithium ion secondary battery, which can improve the structural stability of the positive electrode active material without sacrificing the electrochemical activity of the positive electrode active material.

The present invention provides a modified positive electrode active material, the modified positive electrode active material comprises:

Primary particles comprising a spinel phase and a rock-salt-like phase, the primary particles have a spinel octahedral structure, the spinel phase is an inner core, and the rock-salt-like phase is distributed on the surface of the spinel phase to form shell;

the spinel phase is formed from a lithium-containing compound having a spinel crystal structure;

The rock-salt-like phase contains at least one placeholder element among Al, Nb, B, Si, F, and S, and the placeholder element occupies the 16c or 8a vacancy of the spinel octahedron or in the spinel octahedron. the position of the oxygen ion;

The primary particles are doped with phosphorus elements, and the phosphorus elements are distributed in a gradient from the outside to the inside.

In one embodiment, the lithium-containing compound is a lithium manganese phosphate compound, and the chemical formula of the lithium-containing compound is Li _1+x Ni _0.5-y Mn _1.5-z O _u , wherein 0.2≤x≤0.2, - 0.2≤y≤0.2, -0.2≤z≤0.2, 3.8≤u≤4.2.

In one embodiment, the spinel phase has a thickness of 0.1 μm˜30 μm.

In one embodiment, the thickness of the rock-salt-like phase is 0.5 nm to 50 nm.

In one embodiment, the structure in which the phosphorus element in the primary particles is distributed in a gradient is a phosphorus gradient doped layer, and the thickness of the phosphorus gradient doped layer is 0.5 nm to 40 nm.

In one embodiment, the primary particles further contain one or more elements selected from magnesium, calcium, iron, copper, zinc, titanium, zirconium, cobalt, cadmium and vanadium.

In one of the embodiments, the doping amount of the phosphorus element in the primary particles is gradually decreased from the outside to the inside.

The present invention also provides a method for preparing the modified positive electrode active material, comprising the following steps:

provide lithium-containing compounds;

mixing a phosphorus source, a rock salt-like phase inducer, and the lithium-containing compound to obtain a mixture; and

The mixture is sintered at 600°C to 1200°C for 0.5 to 10 hours.

In one embodiment, the phosphorus source includes nickel phosphate, cobalt phosphate, manganese phosphate, magnesium phosphate, calcium phosphate, iron phosphate, copper phosphate, zinc phosphate, titanium phosphate, zirconium phosphate, lithium phosphate, cobalt pyrophosphate, pyrophosphate Nickel Phosphate, Manganese Pyrophosphate, Magnesium Pyrophosphate, Calcium Pyrophosphate, Iron Pyrophosphate, Copper Pyrophosphate, Zinc Pyrophosphate, Titanium Pyrophosphate, Zirconium Pyrophosphate, Ammonium Phosphate, Ammonium Dihydrogen Phosphate, Diammonium Phosphate, Diammonium Phosphate One or more of lithium hydrogen, dilithium hydrogen phosphate, lithium pyrophosphate, pyrophosphoric acid, phosphoric acid and phosphorus pentoxide.

In one embodiment, the rock-salt-like phase inducer is one or more of oxides, elements, salts and compounds of the place-occupying element.

In one embodiment, the rock salt-like phase inducer further comprises one or more of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, formic acid, oxalic acid and citric acid.

In one embodiment, the mass ratio of the phosphorus source, the rock-salt-like phase inducer and the lithium-containing compound is (1-30):(1-30):2000.

The present invention further provides a positive electrode of a lithium ion secondary battery, comprising a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, the positive electrode active material layer comprising the modified positive electrode active material.

The present invention also provides a lithium ion secondary battery, comprising:

A positive electrode as described above;

a negative electrode comprising a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector;

Diaphragm and electrolyte.

The modified positive electrode active material provided by the present invention has a primary particle with a spinel-phase core and a shell-like core structure of a rock-salt-like shell, the rock-salt-like shell contains place-occupying elements, and the primary particles also contain a gradient distribution of phosphorus elements . The structure is to build a rock-salt-like shell containing a placeholder element on the surface of the original electrode material. Due to the introduction of the placeholder element, the placeholder element induces a phase change in the crystal structure of the surface of the original electrode material, which changes the surface of the original electrode material. The lattice constant can be reduced, and the barrier of phosphorus element doping into the electrode material structure is reduced, so that phosphorus element can be doped into the positive electrode active material in a gradient distribution manner. The gradient distribution of phosphorus element relieves the structural stress generated during the deintercalation process of lithium ions and reduces the reactivity between the cathode active material and the electrolyte. At the same time, occupying elements can further improve the electronic conductivity and interface stability of cathode active materials. The gradient-doped phosphorus element and the placeholder elements in the rock-like salt phase can synergistically improve the stability of the surface structure of the cathode active material, thereby improving the capacity retention rate and charge-discharge coulombic efficiency of the battery. At the same time, each of these elements has its own unique function. For aluminum, it can stabilize the lattice constant and reduce the ginger-Taylor effect of the spinel structure, which is very important for the long cycle of lithium nickel manganate. The benefits of niobium can not only improve the cycle, but also change the morphology of the synthesized lithium manganate, effectively controlling the morphology, particle size and vibration of the final synthesized lithium manganate. For non-metallic elements B, Si, F, and S, they can generate interfacial films containing B, Si, F, and S when they encounter the electrolyte at high voltage. The interfacial films containing these elements will have higher Rate performance and better stability.

Description of drawings

Fig. 1 is the STEM image of the modified positive electrode active material obtained in Example 1;

Fig. 2 is the STEM line scan of the modified positive electrode active material obtained in Example 1;

3 is an STEM image of the surface of the modified positive electrode active material prepared in Example 2;

Fig. 4 is the relative content change of the surface phosphorus element that the modified positive electrode active material obtained in Example 2 is characterized by XPS under different etching depths;

5 is an STEM image of the surface of the modified positive active material prepared in Example 3;

FIG. 6 is the XPS diagram of the F element on the surface of the modified positive electrode active material prepared in Example 4. FIG.

Detailed ways

In order to facilitate understanding of the present invention, the present invention will be described more fully hereinafter with reference to the related drawings. Preferred embodiments of the invention are shown in the accompanying drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that a thorough and complete understanding of the present disclosure is provided.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terms used herein in the description of the present invention are for the purpose of describing specific embodiments only, and are not intended to limit the present invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Except as shown in the working examples or otherwise indicated, all numbers used in the specification and claims indicating amounts, physicochemical properties, etc. of ingredients are understood to be adjusted in all cases by the term "about". For example, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that those skilled in the art can seek to obtain the desired properties using the teachings disclosed herein, as appropriate Change these approximations. The use of numerical ranges by endpoints includes all numbers within that range and any range within that range, eg, 1 to 5 includes 1, 1.1, 1.3, 1.5, 2, 2.75, 3, 3.80, 4, and 5, etc. .

The generally defined core-shell structure is an ordered assembly formed by one material wrapping another material through chemical bonds or other forces. The core-shell structure "core" and "shell" as defined in the present invention are actually integral. The modified cathode active material structure of the present invention includes two phases, resulting in a different microstructure of the surface layer from that of the interior of the material. In the present invention, the interior of the material thus formed is called "core", and the surface layer is called "shell" ”, and define materials with such structures as core-shell-like materials.

The "rock-salt-like phase" in the present invention is formed when the 16c or 8a positions of the spinel octahedron are occupied by elements.

The embodiment of the present invention provides a modified positive electrode active material, and the modified positive electrode active material comprises:

Primary particles comprising a spinel phase and a rock-salt-like phase, the primary particles have a core-shell structure, the spinel phase is an inner core, and the rock-salt-like phase is distributed on the surface of the spinel phase to form an outer shell;

the spinel phase is formed from a lithium-containing compound having a spinel crystal structure including lithium, nickel and manganese,

The rock-salt-like phase contains at least one placeholder element among Al, Nb, B, Si, F, and S, and the placeholder element occupies the 16c or 8a vacancy of the spinel octahedron or in the spinel octahedron. the position of the oxygen ion;

The primary particles are doped with phosphorus elements, and the phosphorus elements are distributed in a gradient from the outside to the inside.

In the modified positive active material provided by the embodiment of the present invention, the primary particles have a shell-like core structure of a spinel-phase core and a rock-salt-like shell, the rock-salt-like shell contains space-occupying elements, and the primary particles also contain gradient-distributed Phosphorus element. The structure is to build a rock-salt-like shell containing a placeholder element on the surface of the original electrode material. Due to the introduction of the placeholder element, the placeholder element induces a phase change in the crystal structure of the surface of the original electrode material, which changes the surface of the original electrode material. The lattice constant can be reduced, and the barrier of phosphorus element doping into the electrode material structure is reduced, so that phosphorus element can be doped into the positive electrode active material in a gradient distribution manner. The gradient distribution of phosphorus element relieves the structural stress generated during the deintercalation process of lithium ions and reduces the reactivity between the cathode active material and the electrolyte. At the same time, occupying elements can further improve the electronic conductivity and interface stability of cathode active materials. The gradient-doped phosphorus element and the placeholder elements in the rock-like salt phase can synergistically improve the stability of the surface structure of the cathode active material, thereby improving the capacity retention rate and charge-discharge coulombic efficiency of the battery.

The primary particle refers to the smallest unit constituting the positive electrode active material, and specifically refers to the smallest unit that can be determined based on the geometric configuration of the appearance. Aggregates of primary particles are secondary particles. The primary particles have a core-like structure in which a spinel-like inner core and a rock-salt-like shell are integrated, and there are no crystal grains at the boundary between the spinel-like and rock-salt-like phases boundary, and the spinel phase and the rock salt-like phase cannot be separated from each other by oxygen bonding. The positive electrode active material having the above-described configuration has higher structural stability.

Preferably, the lithium-containing compound is a lithium manganese phosphate compound. More preferably, the chemical formula of the lithium-containing compound may be Li _1+x Ni _0.5-y Mn _1.5-z O _u , wherein 0.2≤x≤0.2, -0.2≤y≤0.2, -0.2≤z≤0.2, 3.8≤u≤4.2. The values of x, y, and z may vary depending on the ratio between the elements, but are all set within a range such that the compound represented by the formula can exhibit a spinel structure.

The place occupant element is preferably Al, and the Al element is more conducive to improving the structural stability of the positive electrode active material and reducing the barrier for phosphorus element to be doped into the spinel structure.

The thickness of the spinel phase can be any value between 0.1 μm and 30 μm, for example, it can also include 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm , 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm.

The thickness of the rock-salt-like phase can be any value between 0.5nm and 50nm, for example, it can also include 0.5nm, 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, 25nm, 26nm, 27nm, 28nm, 29nm, 30nm, 31nm, 32nm, 33nm, 34nm, 35nm, 36nm, 37nm, 38nm, 39nm, 40nm, 41nm, 42nm, 43nm, 44nm, 45nm, 46nm, 47nm, 48nm, 49nm, 50nm.

The modified positive electrode active material provided by the present invention is doped with phosphorus element, but is different from the phosphate-coated positive electrode active material in the prior art. Phosphate-coated positive active material refers to a material formed by a phosphate crystal structure or an amorphous phosphate covered on the surface of a spinel positive electrode material, and a coating layer can be seen on the surface of the material through transmission electron microscopy. In the modified positive electrode active material provided by the present invention, phosphorus element is doped in the primary particles, and phosphorus element is doped into the spinel structure in a gradient from the surface of the primary particle to the interior.

Both the spinel phase and the rock-salt-like phase of the primary particles are doped with phosphorus element, but phosphorus element is preferentially doped in the rock-salt-like phase. The doping amount of the phosphorus element in the primary particles is gradually decreased from the outside to the inside.

The structure in which the phosphorus element in the primary particles is distributed in a gradient can be defined as a phosphorus gradient doped layer, and the thickness of the phosphorus gradient doped layer can be any value between 0.5 nm and 40 nm, for example, it can also include 0.5 nm and 1 nm. , 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, 25nm, 26nm , 27nm, 28nm, 29nm, 30nm, 32nm, 35nm, 38nm.

The primary particles also contain one or more elements selected from magnesium, calcium, iron, copper, zinc, titanium, zirconium, cobalt, cadmium and vanadium.

The positive electrode material provided by the present invention, the rock-salt-like surface layer and the phosphorus gradient doped layer can be characterized by common characterization methods in the art, such as scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) Characterization, in which the rock-salt-like phase distribution on the surface due to part of the site-occupying elements occupying the spinel octahedron 16c or 8a can be accurately seen by STEM, and the STEM line scan can also prove the gradient distribution of phosphorus elements. At the same time, the etching analysis of X-ray photoelectron spectroscopy can also prove the gradient distribution of phosphorus element in the phosphorus gradient doped layer. For specific characterization methods, please refer to M.Lin, L.Ben, Y.Sun, H.Wang, Z.Yang, L.Gu, X.Yu, X.-Q.Yang, H.Zhao, R.Yu, M. Armand, X. Huang, Insight into the Atomic Structure of High-VoltageSpinelLiNi _0.5 Mn _1.5 O ₄ Cathode Material in the First Cycle. Chemistry of Materials 27, 292-303 (2015), Y. Wu, L. Ben, H. Yu, W. Qi, Y. Zhan, W. Zhao, X. Huang, Understanding the Effect of Atomic-Scale Surface Migration of Bridging Ions in Binding Li3PO4 to the Surface of Spinel Cathode Materials. Acs Applied Materials & Interfaces 11, 6937-6947 (2019).

The present invention also provides a preparation method of the above-mentioned modified positive electrode active material, comprising the following steps:

S10, providing lithium-containing compounds;

S20, mixing the phosphorus source, the rock-salt-like phase inducer and the lithium-containing compound to obtain a mixture; and

S30, the mixture is sintered at 600° C.˜1200° C. for 0.5˜10 hours.

The lithium-containing compound can be prepared by methods known to those skilled in the art. For example, it can be prepared by a low temperature solid phase method. Specifically, the precursor can be prepared by mixing nickel salt, manganese salt, lithium hydroxide and oxalic acid by ball milling, and then the precursor can be calcined at high temperature to obtain the lithium-containing compound.

The phosphorus source may include nickel phosphate, cobalt phosphate, manganese phosphate, magnesium phosphate, calcium phosphate, iron phosphate, copper phosphate, zinc phosphate, titanium phosphate, zirconium phosphate, lithium phosphate, cobalt pyrophosphate, nickel pyrophosphate, manganese pyrophosphate , magnesium pyrophosphate, calcium pyrophosphate, iron pyrophosphate, copper pyrophosphate, zinc pyrophosphate, titanium pyrophosphate, zirconium pyrophosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, dihydrogen phosphate One or more of lithium, lithium pyrophosphate, pyrophosphoric acid, phosphoric acid and phosphorus pentoxide. Preferably, the phosphorus source is one or more of titanium phosphate, copper phosphate, cobalt phosphate and phosphoric acid.

The rock-salt-like phase inducer may be one or more of oxides, elements and salts of the place-occupying elements. For example, Al ₂ O ₃ , Nb ₂ O, Nb ₂ O ₅ , B ₂ O ₃ , SiO ₂ , Al(OH) ₃ , H3BO3 , NaAlO ₂ , Na ₂ SiO ₃ , NH ₄ F, S, and the like.

The rock-salt-like phase inducer may also include one or more of organic acids or inorganic acids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, formic acid, oxalic acid, citric acid, and the like. The organic or inorganic acid can further promote the generation of a rock salt-like phase.

The mass ratio of the phosphorus source, the rock-salt-like phase inducer and the lithium-containing compound can be any ratio between (1-30):(1-30):2000, for example, it can also be 1:1:50, 1:1 1:80, 1:1:100, 1:1:150, 1:1:200, 1:1:250, 1:1:300, 1:1:350, 1:1:400, 1:1: 500, 1:1:600, 1:1:700, 1:1:800, 1:1:900, 1:1:1000, 1:1:1200, 1:1:1500, 1:1:1800.

In step S20, the phosphorus source, the rock salt-like phase inducer, and the lithium-containing compound may be mixed by methods known to those skilled in the art, such as mechanical mixing, ultrasound, ball milling, and the like.

The sintering in step S30 may be performed in oxygen, air, or an inert atmosphere (eg, nitrogen or argon) and an atmosphere containing oxygen. Preferably, the specific operation of the sintering process is as follows: raising the temperature to 600°C-1200°C at a heating rate of 0.5-10°C/min, then sintering for 0.5-10 hours, and then decreasing the temperature at a cooling rate of 0.5-10°C/min. to room temperature.

The present invention also provides a positive electrode of a lithium ion secondary battery, comprising a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, the positive electrode active material layer comprising the above-mentioned modified positive electrode active material.

As the positive electrode current collector, a conductive member formed of a highly conductive metal as used in the positive electrode of a lithium ion secondary battery of the related art is preferable. For example, aluminum or an alloy including aluminum as a main component may be used. The shape of the positive electrode current collector is not particularly limited because it may vary depending on the shape and the like of the lithium ion secondary battery. For example, the positive electrode current collector may have various shapes such as a rod shape, a plate shape, a sheet shape, and a foil shape.

The positive electrode active material layer further includes a conductive additive and a binder.

The conductive additive may be a conventional conductive additive in the art, which is not particularly limited in the present invention. For example, in some embodiments, the conductive additive is carbon black (eg, acetylene black or Ketjen black).

The adhesive can be a conventional adhesive in the art, which is not particularly limited in the present invention, and can be composed of polyvinylidene fluoride (PVDF), or can be composed of carboxymethyl cellulose (CMC) and butylbenzene Made of rubber (SBR). In some embodiments, the binder is polyvinylidene fluoride (PVDF).

The present invention also provides a lithium ion secondary battery, comprising:

A positive electrode as described above;

a negative electrode comprising a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector;

Diaphragm and electrolyte.

As a negative current collector,

The negative electrode, separator and electrolyte can use conventional negative electrode current collector, separator and electrolyte materials in the art, which are not particularly limited in the present invention.

The negative electrode current collector may be copper, and the shape of the negative electrode current collector is also not particularly limited, and may be rod-shaped, plate-shaped, sheet-shaped, and foil-shaped, and may vary depending on the shape of the lithium ion secondary battery and the like. The negative electrode active material layer includes a negative electrode active material, a conductive additive and a binder. Negative active materials, conductive additives and binders are also conventional materials in the art. In some embodiments, the negative active material is metallic lithium. The conductive additives and binders are as described above and will not be repeated here.

The separator can be selected from those commonly used in lithium ion secondary batteries, including microporous films made of polyethylene and polypropylene; porous polyethylene films and polypropylene multi-layer films; Nonwoven fabrics formed of aramid fibers, glass fibers, etc.; and base films formed by adhering ceramic particles such as silica, alumina, and titania to their surfaces, and the like. In some embodiments, the separator is a triple layer film of PP/PE/PP coated on both sides with alumina.

The electrolytic solution may include an electrolyte and a non-aqueous organic solvent. The electrolyte is preferably LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ . The non-aqueous organic solvent can be carbonate, ester and ether. Among them, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) can be preferably used.

The following are specific examples, which are intended to further describe the present invention in detail to help those skilled in the art and researchers to further understand the present invention, and the relevant technical conditions and the like do not constitute any limitation to the present invention. Any modifications made within the scope of the claims of the present invention are within the protection scope of the claims of the present invention.

In the following examples, STEM was performed with a spherical aberration-corrected scanning transmission microscope model JEM ARM200F (JEOL, Tokyo, Japan); X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB 250 model produced by Thermo Fisher Company. The X-ray radiation source is Mg Kα to study the types of elements and chemical environment on the surface of powder samples.

Example 1

9 g of LiNi _0.5 Mn _1.5 O ₄ material, 0.05 g of B ₂ O ₃ and 0.1 g of (NH ₄ ) ₂ HPO ₄ were uniformly mixed, and the resulting mixture was calcined in oxygen at 600 °C for 5 h, the heating rate was 3 °C/min, and the temperature was lowered. The rate was 5°C/min, and a modified positive electrode active material was obtained.

FIG. 1 shows the STEM image of the modified cathode active material prepared in Example 1. It can be seen from the STEM image of the modified cathode active material in Figure 1 that there is a rock-salt-like phase generated by the 16c atomic occupancy of the spinel octahedron on the surface of the material, and the thickness of the rock-salt-like phase is about 10 nm.

Figure 2 is a STEM line scan of the phosphorus element content on the surface of the modified positive electrode active material prepared in Example 1. It can be seen from Figure 2 that there is no coating layer on the surface of lithium nickel manganate after doping, and it can be seen in conjunction with Figure 1 Phosphorus is distributed in the rock salt-like phase, and the content of phosphorus gradually decreases from the surface to the interior.

Example 2

9 g of LiNi _0.4 Mn _1.6 O ₄ material, 0.2 g of H ₃ PO ₄ and 0.267 g of Nb ₂ O ₅ were uniformly mixed, and the resulting mixture was calcined in oxygen at 800 °C for 5 h, with a heating rate of 3 °C/min and a cooling rate of 5 °C/min to obtain a modified positive electrode active material.

FIG. 3 shows the STEM image of the modified cathode active material prepared in Example 2. It can be seen from the figure that there is a rock-salt-like phase occupied by spinel octahedron 8a atoms on the surface of the material, and the thickness of the rock-salt-like phase is about 5 nm.

Figure 4 shows the change of the relative content of phosphorus elements on the surface of the modified cathode active material prepared in Example 2 by XPS characterization at different etching depths. We can see that the phosphorus elements change from the surface to the interior with the etching depth. Increasing content keeps decreasing.

Example 3

9g of LiNi _0.4 Mn _1.6 O ₄ material, 0.1g SiO ₂ and 0.133g (NH ₄ ) ₂ HPO ₄ and 20ml of deionized water were added to the beaker and mixed evenly. The beaker was placed in an oil bath at 120°C and heated for 5h with stirring. A dry mixture is obtained. The obtained mixture was calcined in oxygen at 850 °C for 5 h, with a heating rate of 3 °C/min and a cooling rate of 5 °C/min to obtain a modified positive electrode active material.

FIG. 5 shows the STEM image of the modified cathode active material prepared in Example 3. FIG. It can be seen from the figure that the surface of the material has a rock-salt-like phase occupied by 2-3 nm spinel octahedra 8a and 16c atoms, and the thickness of the surface rock-salt-like phase is about 2 nm.

Example 4

9 g of LiNi _0.4 Mn _1.6 O ₄ material, 0.2 g of H ₃ PO ₄ and 0.267 g of NH ₄ F were uniformly mixed, and the resulting mixture was calcined in oxygen at 800 °C for 5 h with a heating rate of 3 °C/min and a cooling rate of 5 °C /min to obtain a modified positive electrode active material.

Figure 6 shows the XPS spectrum of the modified cathode active material prepared in Example 4, and the results show that the surface of the material contains F element.

Example 5

9 g of LiNi _0.5 Mn _1.5 O ₄ material, 0.03 g of Al ₂ O ₃ and 0.125 g of (NH ₄ ) ₂ HPO ₄ were uniformly mixed, and the resulting mixture was calcined in air at 800 °C for 5 h, the heating rate was 3 °C/min, and the temperature was lowered. The rate was 5°C/min, and a modified positive electrode active material was obtained.

Example 6

9 g of LiNi _0.5 Mn _1.5 O ₄ material, 0.015 g of S and 0.15 g of (NH ₄ ) ₂ HPO ₄ were uniformly mixed, and the resulting mixture was calcined at 825 °C for 5 h in a closed space with a heating rate of 3 °C/min and a cooling rate of 5°C/min to obtain a modified positive electrode active material.

Comparative Example 1

The preparation method is basically the same as that in Example 1, except that the phosphorus source (NH ₄ ) ₂ HPO ₄ is not added.

Comparative Example 2

The preparation method is basically the same as that in Example 1, except that the rock-salt-like phase inducer B ₂ O ₃ is replaced with hydrochloric acid.

Performance Testing

The positive electrode active materials prepared in Examples 1 to 4 and Comparative Examples 1 to 2 were assembled into coin cells according to the following steps.

(1) Preparation of positive electrode pieces

的圆片，然后置于真空烘箱中于120℃下烘6小时，自然冷却后，取出置于手套箱中用作正极极片。The positive active material prepared in the example, carbon black as a conductive additive and polyvinylidene fluoride (PVDF) as a binder, were dispersed in N-methylpyrrolidone (NMP) according to a weight ratio of 80:10:10, and mixed. uniform, and prepared into a uniform positive electrode slurry. The uniform positive electrode slurry was evenly coated on the aluminum foil current collector with a thickness of 15 μm, and dried at 55 ° C to form a pole piece with a thickness of 100 μm, and the pole piece was placed under a roller press for rolling (pressure about 1MPa). ×1.5cm ² ), cut to a diameter of

The discs were then placed in a vacuum oven at 120 °C for 6 hours, and after natural cooling, they were taken out and placed in a glove box for use as a positive pole piece.

(2) Assembly of lithium ion secondary battery

In a glove box filled with an inert atmosphere, metal lithium is used as the negative electrode of the battery, and a triple-layer film of PP/PE/PP coated with alumina on both sides is placed between the positive electrode and the negative electrode as a separator, and commonly used carbonates are added dropwise. The electrolyte solution is assembled into a button battery with a model of CR2032, using the positive electrode plate prepared in step (1) as the positive electrode.

High temperature cycle test:

After standing the prepared button battery at room temperature (25°C) for 10 hours, the button battery was activated by charging and discharging, and then the button battery prepared above was charged and discharged using a blue battery charge and discharge tester. Loop test. First, cycle at room temperature (25°C) at a rate of 0.1C for 1 week, and then continue to cycle at a rate of 0.2C for 4 weeks, wherein the charge-discharge voltage range of the control battery is 3.5V-4.9V. Then, the button battery was transferred to a high temperature environment of 55°C, and the cycle was continued for 50 cycles at a rate of 0.2C, while the charge-discharge voltage range of the control battery was still 3.5V to 4.9V.

Taking LiNi _0.5 Mn _1.5 O ₄ and LiNi _0.4 Mn _1.6 O ₄ as controls, the measured data are listed in Table 1.

Table 1. Electrochemical properties of positive active materials of various embodiments of the present invention

The results show that the batteries assembled from the original LiNi _0.5 Mn _1.5 O ₄ material, the original LiNi _0.4 Mn _1.6 O ₄ material and the cathode active materials prepared in Comparative Examples 1 and 2 were tested at a high temperature of 55 °C for 50 The capacity decays faster after 1 week, which is due to the decomposition of the electrolyte and the dissolution of the positive active material Mn/Ni, resulting in a faster capacity decay of the material; while the gradient P-doped materials prepared in Examples 1 to 4 are at a high temperature of 55 °C. Under the test environment, the capacity after 50 weeks is almost not attenuated. This is because the phosphorus gradient doping alleviates the harmful side reactions between the cathode active material and the electrolyte, and inhibits the decomposition of the electrolyte and the dissolution of Mn/Ni. , thereby improving the cycle stability of the battery.

It can be seen from the above table that phosphorus doping and surface modification with place-occupying elements (Al, Nb, B, Si, F, S) alleviate the harmful side reactions between the positive active material and the electrolyte, inhibit the decomposition of the electrolyte and The dissolution of the positive active material Mn/Ni improves the cycle stability of the battery. Simultaneous gradient phosphorus doping and placeholder elements (Al, Nb, B, Si, F, S) have a synergistic effect, compared to individual phosphorus element doping and placeholder elements (Al, Nb, B, Si, F, S) Surface modification, the cycling stability of the materials modified by the two is significantly improved.

Taking LiNi _0.5 Mn _1.5 O ₄ as a control, the measured electrochemical data are listed in Table 2.

Table 2. Electrochemical rate performance of cathode active materials of various embodiments of the present invention

It can be seen from the above table that the rate performance of the lithium nickel manganate material is increased by non-metal elements such as boron on the surface of the material.

The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity, all possible combinations of the technical features in the above-described embodiments are not described. However, as long as there is no contradiction between the combinations of these technical features, All should be regarded as the scope described in this specification.

The above-mentioned embodiments only represent several embodiments of the present invention, and the descriptions thereof are specific and detailed, but should not be construed as a limitation on the scope of the invention patent. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can also be made, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims (13)

1. A modified positive electrode active material, comprising:

the primary particles comprise a spinel phase and a rock-salt-like phase, the spinel phase is a core, and the rock-salt-like phase is distributed on the surface of the spinel phase to form a shell;

the spinel phase is formed from a lithium-containing compound having a spinel crystal structure;

the halite-like phase comprises at least one occupying element of Al, Nb, B, Si, F and S, and the occupying element occupies 16c or 8a vacancy of spinel octahedron or the position of oxygen ion in spinel octahedron;

the primary particles are doped with phosphorus elements, and the phosphorus elements are distributed in a gradient manner from outside to inside.

2. The modified positive electrode active material according to claim 1, wherein the lithium-containing compound is manganese phosphateA lithium compound having the chemical formula Li_1+xNi_0.5-yMn_1.5-zO_uWherein x is more than or equal to 0.2 and less than or equal to 0.2, y is more than or equal to 0.2 and less than or equal to 0.2, z is more than or equal to 0.2 and u is more than or equal to 3.8 and less than or equal to 4.2.

3. The modified cathode active material according to claim 1, wherein the spinel phase has a thickness of 0.1 to 30 μm.

4. The modified positive electrode active material according to claim 1, wherein the thickness of the rock-salt-like phase is 0.5nm to 50 nm.

5. The modified cathode active material according to claim 1, wherein the structure in which the phosphorus element in the primary particles is distributed in a gradient is a phosphorus gradient doped layer, and the thickness of the phosphorus gradient doped layer is 0.5nm to 40 nm.

6. The modified positive electrode active material according to claim 1, wherein the primary particles further contain one or more elements selected from the group consisting of magnesium, calcium, iron, copper, zinc, titanium, zirconium, cobalt, cadmium and vanadium.

7. A method for preparing a modified positive electrode active material according to any one of claims 1 to 6, comprising the steps of:

providing a lithium-containing compound;

mixing a phosphorus source, a rock-like salt phase inducer and the lithium-containing compound to obtain a mixture; and

heating the mixture to 600-1200 ℃ at the heating rate of 0.5-10 ℃/min, sintering for 0.5-10 hours, and cooling to room temperature at the cooling rate of 0.5-10 ℃/min.

8. The method for producing a modified positive electrode active material according to claim 7, wherein the phosphorus source includes one or more of nickel phosphate, cobalt phosphate, manganese phosphate, magnesium phosphate, calcium phosphate, iron phosphate, copper phosphate, zinc phosphate, titanium phosphate, zirconium phosphate, lithium phosphate, cobalt pyrophosphate, nickel pyrophosphate, manganese pyrophosphate, magnesium pyrophosphate, calcium pyrophosphate, iron pyrophosphate, copper pyrophosphate, zinc pyrophosphate, titanium pyrophosphate, zirconium pyrophosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, dilithium hydrogen phosphate, lithium pyrophosphate, pyrophosphoric acid, phosphoric acid, and phosphorus pentoxide.

9. The method for preparing a modified cathode active material according to claim 7, wherein the rock-salt-like phase inducer is one or more of an oxide, a simple substance, and a salt of the space-occupying element.

10. The method of claim 9, wherein the rock-like salt phase inducer further comprises one or more of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, formic acid, oxalic acid, and citric acid.

11. The method for preparing the modified cathode active material according to any one of claims 7 to 10, wherein the mass ratio of the phosphorus source, the rock salt-like phase inducer and the lithium-containing compound is (1 to 30): (1-30): 2000.

12. a positive electrode of a lithium ion secondary battery, comprising a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer comprises the modified positive electrode active material according to claims 1 to 6.

13. A lithium-ion secondary battery characterized by comprising:

a positive electrode according to claim 12;

a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector;

a separator and an electrolyte.

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