CN110993917B

CN110993917B - Cathode material, preparation method thereof and lithium ion battery

Info

Publication number: CN110993917B
Application number: CN201911320314.9A
Authority: CN
Inventors: 王刚; 王凤英; 王冲; 韦浩民; 王万玺; 南辉; 李鑫
Original assignee: Qinghai Nationalities University
Current assignee: Qinghai Nationalities University
Priority date: 2019-12-19
Filing date: 2019-12-19
Publication date: 2021-10-15
Anticipated expiration: 2039-12-19
Also published as: CN110993917A

Abstract

The invention provides a positive electrode material, a preparation method thereof and a lithium ion battery. The positive electrode material includes: a base material; a spinel oxide; an olivine-type oxide, wherein the spinel oxide is located between the matrix material and the olivine-type oxide; reduced graphene oxide, wherein the olivine-type oxide is located between the spinel oxide and the reduced graphene oxide. The anode material with the three-layer coating material has excellent voltage stability, cycling stability and rate capability.

Description

Cathode material, preparation method thereof and lithium ion battery

Technical Field

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

Background

At present, fossil energy is the most dependent energy for human production and life. However, fossil energy is a non-renewable energy, and in the using process, harmful gases such as carbon monoxide, carbon dioxide, sulfur oxide, nitrogen oxide and particulate matters can be generated, so that the phenomena such as greenhouse effect and haze can be caused, and the living environment of human beings can be seriously damaged. Therefore, in order to reduce the dependence on fossil energy, the search for renewable energy sources to replace fossil energy is imperative. At present, secondary energy sources that can replace fossil energy sources include geothermal energy, wind energy, solar energy, tidal energy, and the like. However, most of these renewable energy sources are affected by the environment, geographical location, weather, region, and the like, and cannot continuously and stably generate energy. Therefore, the method is not suitable for being directly used in the production and life of people, and the storage, conversion and reutilization are needed. Among many storage devices, lithium ion batteries are receiving attention due to their characteristics of low cost, no memory benefit, and the like.

Although the current applications of lithium ion batteries are very wide and diversified, the power density and energy density of the lithium ion batteries need to be improved. The key to limiting the energy density of lithium ion batteries is the positive electrode material. Early positive electrode materials used LiCoO₂、LiMn₂O₄Subsequently, people developed LiNi with more excellent performance_xMn_yCo_1-x-yO₂Materials (nickel manganese cobalt ternary materials). Although the high-nickel ternary cathode material is higher than other cathode materials in energy density, the high-nickel ternary cathode material also has the insurmountable defect that: for example, the high-nickel ternary material is easy to deviate from the metering ratio in the synthesis process, easy to convert to a spinel structure in the circulation process, mixed cation arrangement or mixed lithium-nickel arrangement, poor in thermal stability, unstable in surface layer structure and the like, so that the circulation performance, the safety performance and the storage performance of the high-nickel ternary material are poor, and the large-scale application of the ternary material in the field of power batteries is hindered. In recent years, lithium-rich manganese-based ternary materials have higher energy storage density and low cobalt dosage (can be repeatedly used for many times and has specific discharge capacity of more than 250mA g)^-1) Are receiving attention and expectations from the industry.

Although the discharge capacity of the lithium-rich cathode material is close to twice that of the traditional cathode material, at present, the lithium-rich cathode material has a plurality of problems, mainly comprising the following four aspects: 1. the first charge-discharge efficiency is too low; 2. the rate capability is not ideal enough; 3. cycling is not stable enough at high voltage; 4. the cycling process produces a phase transition of the spinel, resulting in a sustained drop in the median voltage. In order to solve the problems of the lithium-rich manganese-based cathode material, methods such as doping of anions and cations and transition metal ions, inert oxide surface coating modification, electrolyte adjustment and the like can be adopted. The surface coating modification of the inert oxide surface coating can form a protective layer on the surface of the lithium ion anode material, so that the corrosion of HF gas generated by an anode material/electrolyte interface in a circulating process to the lithium ion anode material can be avoided, and the capacity attenuation in the circulating process is reduced. However, this method reduces the electron conductivity of the surface, and reduces the rate characteristics and large-current charge and discharge characteristics of the battery. How to compromise the surface protection of the cathode material, and maintain or improve the surface electron conduction of the material is a difficult task.

Disclosure of Invention

The invention adopts a unique three-layer structure to coat the lithium-rich manganese-based anode material. The inner two layers of the three-layer structure are respectively spinel oxide and olivine oxide from inside to outside, and the outermost layer is high-temperature reduced graphene oxide. The spinel oxide/olivine type oxide can well realize the protection of the lithium-rich manganese-based core material and simultaneously provide a fast channel of lithium ions; the reduced graphene oxide of the outer layer can provide sufficiently high electron conductivity for the modified cathode material.

The invention provides a positive electrode material, which comprises: a base material; a spinel oxide; an olivine-type oxide, wherein the spinel oxide is located between the matrix material and the olivine-type oxide; reduced graphene oxide, wherein the olivine-type oxide is located between the spinel oxide and the reduced graphene oxide.

In the above cathode material, the matrix material is a lithium-rich manganese-based cathode material.

In the above cathode material, wherein the spinel oxide is spinel-type LiMn₂O₄。

In the above positive electrode material, the olivine oxide is olivine LiMgPO₄。

The invention also provides a method for preparing the cathode material, which comprises the following steps: forming a base material; coating the base material with an olivine-type oxide; coating graphene oxide on the olivine-type oxide; annealing the base material after coating the olivine-type oxide and the graphene oxide, forming a spinel oxide between the base material and the olivine-type oxide, and reducing the graphene oxide to a reduced graphene oxide.

In the above method, wherein the thickness of the olivine-type oxide is 1nm to 20 nm.

In the above method, wherein the thickness of the graphene oxide is 1nm to 7 nm.

In the method, the annealing temperature is 450-500 ℃.

The invention also provides a lithium ion battery which comprises the cathode material.

The invention has the advantages that: the process method is simple, and large-scale preparation and production can be realized; the spinel layer and the olivine layer in the three-layer coating can effectively inhibit side reaction on the interface of the anode and the electrolyte, thereby improving the first-turn coulomb efficiency and the cycle stability of the device; the reduced graphene oxide layer in the three-layer coating can effectively reduce the series resistance of the anode, and improve the modification of the material on the lithium ion anode material, so that the obtained composite material has higher lithium ion transmission rate and lower impedance.

Drawings

Fig. 1 is an X-ray diffraction analysis (XRD) pattern and a Scanning Electron Microscope (SEM) pattern of the lithium-rich manganese-based positive electrode material precursor prepared in example 1.

Fig. 2 is an XRD spectrum and an SEM spectrum of the forsterite (LMP) -coated lithium ion positive electrode material prepared in example 2.

FIG. 3 is an XRD spectrum and a TEM spectrum of the LLO @ Spinel @ LMP @ rGO lithium ion cathode material prepared in example 3.

Fig. 4 is a first charge-discharge curve, a 1C cycle voltage curve, a 1C cycle curve, and a rate performance curve of lithium ion batteries of different lithium ion cathode materials prepared in examples 1, 2, and 3.

Detailed Description

The following examples are presented to enable those skilled in the art to more fully understand the present invention and are not intended to limit the invention in any way.

The invention provides a surface coating modification method for a lithium-rich manganese-based lithium ion positive electrode material, and the whole preparation process is efficient, environment-friendly, economic, simple in process and suitable for large-scale production.

According to the invention, the lithium-rich manganese-based lithium ion positive electrode material is coated by three layers from inside to outside, namely a spinel coating layer, an olivine coating layer and a reduced graphene oxide coating layer from inside to outside. Firstly, coating an olivine layer on the surface of a positive electrode material, and then coating a graphene oxide layer on the surface of the olivine. After carrying out high temperature treatment on the graphene oxide layer, carbon in the graphene oxide will penetrate through the olivine layer to induce the surface layer of the positive electrode material to generate a spinel layer, and the graphene oxide layer will be evolved into a reduction graphene oxide layer. Thus, a three-layer coating structure of spinel-olivine-reduced graphene oxide is formed. The specific implementation process is as follows:

(1) and (3) coating the metal M with olivine. Weighing quantitative lithium-rich manganese-based lithium ion positive electrode material, dispersing the material in a quantitative surfactant (the surfactant can be one of polyvinylpyrrolidone (k30) and cationic surfactant) solution, stirring for 1-2h, adding quantitative metal M salt (the metal M can be Fe or Mg, continuously stirring for 1h, adding quantitative phosphate, transferring the phosphate into a water bath, continuously reacting for 6h at 60 ℃, washing and drying the powder, mixing with quantitative lithium salt, and performing heat treatment in air (the typical temperature is 450 DEG, 500 DEG, and the heating rate is 1-5 ℃ for min)^-1) And 5h, obtaining the olivine modified lithium ion cathode material (the typical thickness of the olivine layer is 1nm-20 nm).

(2) Coating of the graphene oxide layer: weighing a certain amount of metal M olivine modified lithium ion cathode material, dispersing in a certain amount of surfactant solution (the surfactant can be one of polyvinylpyrrolidone (k30) and cationic surfactant), and stirring for 1-2 h. In addition, a certain concentration (concentration from 1mg ml) was added^-1-2mg ml^-1) And (3) carrying out ultrasonic treatment on the graphene oxide solution for 2 hours under ultrasonic waves, dropwise adding the solution into the mixed solution, transferring the mixed solution into a water bath, carrying out water bath at the temperature of 60 ℃ for 3 hours, separating solid particles from the solution, and drying. The graphene oxide layer formed is typically 1nm-7nm thick.

(3) The realization of three-layer coating: and (3) putting the solid particles obtained in the step (2) in a muffle furnace, introducing Ar gas as protective gas, and annealing at a lower temperature (typically 450-500 ℃) for 5 hours to obtain the three-layer coated modified lithium-rich manganese-based positive electrode material. Wherein the spinel layer typically has a thickness of 1-3nm and the reduced graphene oxide layer typically has a thickness of 1-5 nm.

The following description is given in conjunction with specific examples to better understand the present invention.

Example 1: preparation of lithium-rich manganese-based positive electrode material

Will contain the required stoichiometric amount of MnSO₄·H₂O(1.014g)、NiSO₄·6H₂O (0.5257g) and CoSO₄·7H₂O (0.5662g) in water was added to 50ml of water and stirred until the solution became clear, and 25ml of ethanol was added. After 1h, the precipitant NH₄HCO₃(0.8295g) the aqueous solution was added dropwise to the mixed solution. After 5h the precursor was separated using a centrifuge and washed three times and then dried at 60 ℃. Finally, the precursor is reacted with LiOH H₂O (5% excess lithium) is mixed thoroughly and pretreated in air at 400-600 ℃ for 2h, calcined at 800 ℃ for 5h to obtain black powder 0.4Li₂MnO₃·0.6LiMn_1/3Ni_1/3Co_1/3O₂(LLO). As shown in FIG. 1 (a), the XRD pattern and Li of the sample_1-xMn₂O₄The standard cards are consistent, and the diffraction peak shows obvious alpha-NaFeO₂Lamellar configuration characteristic peaks. A set of smaller diffraction peaks appears between the regions of about 20 and 2 theta, mainly because the material contains Li in the main body₂MnO₃The structure is that Li and Mn in the transition metal layer are orderly arranged to form LiMn₆、LiMn₅The Ni type superlattice has space group of C2/m, and belongs to monoclinic system. In addition, it can be seen in (b) of FIG. 1 that microspheres having a matrix material of 1.5um to 3.5um are obtained.

Example 2: olivine LMP (LiMgPO)₄) Is coated with

1.5g PVP (k30) was dissolved in 15ml deionised water (30 ml beaker vessel) and 0.0210g Mg (NO) was added₃)₂·6H₂After dissolving O with stirring, 0.5g of the positive electrode material prepared in example 1 was added. After 1h, a calculated amount of 0.0095g of NH was added₄H₂PO₄And the beaker was moved into a 60 ℃ water bath. After 6h, the powder was separated, washed and dried. Finally, the mixture was mixed with a fixed amount of 0.0062g CH₃COOLi was mixed and sintered at 450 ℃ in air for 5 hours. Thus obtaining the olivine-coated cathode material. As shown in fig. 2 (a), all samples have similar diffraction peaks, which means that there is no structural change after LMP coating. And XRD pattern and Li_1-xMn₂O₄The standard cards of (a) are consistent, indicating that the single-layer LMP coating causes no transformation to the structure and crystal form of LLO. FIG. 2 (b) (LLO), (c) (LLO @ LMP) are SEM images. As can be seen, the surface becomes smooth after the LLO is coated with a single layer of LMP.

Example 3: realization of three-layer cladding structure

0.5g of the forsterite modified lithium ion cathode material prepared in example 2 is weighed and dispersed in 0.01g of polyvinylpyrrolidone (k30) solution to be stirred for 1-2 h. In addition, after 1.85mg ml-1 of graphene oxide solution was sonicated under microwave for 2h, the graphene oxide solution was added dropwise to the above mixed solution, and it was transferred into a water bath, which was then water-bathed at 60 ℃ for 3 h. Finally, annealing for 5 hours at 400 ℃ in an atmosphere taking Ar gas as protective gas. A three-layer clad material was obtained. As shown in fig. 3 (a), it can be seen on the XRD curve of the sample obtained in this example that two weak peaks appear around 37 degrees at 2 θ of 30, which is the spinel-type LiMn closest to the positive electrode base material side₂O₄And the spinel layer is generated in the calcining process after the graphene oxide is coated. In addition, FIG. 3 (b) shows a TEM image of LLO @ Spinel @ LMP @ rGO.

The cathode materials of the above examples were prepared into lithium ion batteries by the same conventional method, and fig. 4 is a graph comparing electrochemical properties of the base materials prepared in examples 1, 2 and 3 and the modified materials. As shown in fig. 4 (a), the obtained three-layer coating material has a high first-turn coulombic efficiency (84%), and in fig. 4 (b), (c), and (d), it can be known that the three-layer coating material has excellent voltage stability, cycle stability, and rate capability.

Those skilled in the art will appreciate that the above embodiments are merely exemplary embodiments and that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the application.

Claims

1. A method of making a positive electrode material, comprising:

forming a matrix material, wherein the matrix material is a lithium-rich manganese-based positive electrode material;

coating olivine oxide on the substrate, wherein the olivine oxide is olivine LiMgPO₄；

Coating graphene oxide on the olivine-type oxide, including: dispersing the base material coated with the olivine oxide in a surfactant solution, and stirring for 1-2h to obtain a mixed solution; carrying out ultrasonic treatment on a graphene oxide solution for 2 hours under ultrasonic waves, dropwise adding the solution into the mixed solution, transferring the mixed solution into a water bath kettle, and carrying out water bath for 3 hours at the temperature of 60 ℃;

annealing the substrate material coated with the olivine oxide and the graphene oxide at a temperature of 450-500 ℃, forming a spinel oxide between the substrate material and the olivine oxide, and reducing the graphene oxide to reduced graphene oxide, wherein the spinel oxide is spinel-type LiMn₂O₄。

2. The method as claimed in claim 1, wherein the olivine-type oxide has a thickness of 1nm to 20 nm.

3. The method of claim 1, wherein the graphene oxide is 1nm-7nm thick.