Zehan Liu^1,a，Zhuyao Liu^2,b*

1College of Material and Metallurgy，Guizhou University, Guiyang 550025, Guizhou, China

2 Logistics and Support Department，Qingdao Central Hospital of Rehabilitation University, Qingdao 266042, Shandong, China

^aEmail: 993790900@qq.com

^bCorrespongding author Email: liuzhuyao@163.com

Abstract: In recent years, under the role of the strategic concept of energy conservation, emission reduction and green environmental protection, the Chinese government has ushered in the golden cycle of the development of the new energy automobile industry. At present, the power source of new energy vehicles is mainly some lithium-ion batteries and fuel cells, etc., although the use of these fuels has made initial achievements in the structural design of new energy vehicles, but compared with the traditional automobile manufacturing industry, there is still a gap in the cost of automobile construction, power and other aspects. This is one of the key factors that currently inhibit the development and promotion of the new energy vehicle industry, and the realization of high energy density lithium batteries has become the main way of modern new energy vehicle structure construction. Based on the above research, combined with the characteristics of lithium-ion battery, combined with the characteristics of lithium-rich manganese based anode materials, doping modification was studied to improve its energy density and service life. In addition, its conductivity and ion transport ability can be significantly improved by doping, which lays a foundation for improving its comprehensive performance. The implementation of this project will lay a certain theoretical and practical basis for the research and development and practical application of high-performance anode materials.

Key words: new energy vehicles; Lithium battery; Li-rich manganese based cathode material; Doping modification,

1 Introduction

As shown in Figure 1 (a), the classical crystal structure of LiMO2 (M = Ni, Co, Mn) layered oxide is composed of alternating layers filled with Li and M ions in octahedral positions, that is, the cubic close-packed O3 structure, whose oxygen layers are stacked in ABCABC sequence. In the Li2MnO3 component, some of the M in the transition metal layer is replaced by Li, and the Li ions in the transition metal layer tend to be arranged in a honeycomb pattern due to the large difference in charge and size between Li+ and Mn4+ (FIG. 1b). The current mainstream views on the structural models of Li-rich cathode materials can be divided into the composite two-phase model of layered Li2MnO3 and LiMO2 proposed by Thackeray et al., and the single-phase solid solution model proposed by Dahn et al., in which M and Li are uniformly mixed while maintaining the global honeycomb order. The key difference between the two structural models is the coherence length of the Li2MnO3 structure, which is much shorter in the solid solution model than in the composite two-phase model. In the solid solution model, Jarvis et al. believe that the transition metal M portion occupies the Li and Mn sites in the Li2MnO3 component, and the configuration is C2/m space group of monoclinic crystal system. Another view is that the solid solution configuration is R3 ̅m space group, that is, some sites in the transition metal layer are occupied by Li ions. The composite two-phase model has also been proved by some studies. For example, Yu et al. directly observed the co-existence of monoclinic phase Li2MnO3 and hexagonal phase LiMO2 through ABF-STEM, as well as the interface region between the two phases.

Figure 1 Crystal structures of (a) layered LiMO2 oxide, (b) Li-rich layered Li2MO3 oxide, and (c) Li-rich layered Li1+ XM1-XO2 oxide, where M represents the 3d transition metal

Policy orientation of energy consumption and environmental protection. With many reasons such as technological progress and market dividends, China’s new energy vehicle industry has entered a period of rapid development. Although new energy vehicles have made great progress in structural design and material research and development, they still lag behind conventional vehicles in terms of cost, power and mileage, so the development of high-capacity power batteries is a very important development direction. This is because it has a high energy storage density, a small self-discharge rate, good cycle performance and a high service life. At present, the driving range of electric vehicles is short and the safety is poor, which has become the main factor restricting its development and popularity. It is the most direct and effective method to add the positive and negative materials with larger energy than the positive and negative electrodes at the same time. High voltage, long life and no memory effect are important trends in the development of new energy vehicles. However, the specific capacity of the current commercial lithium ion battery (such as lithium diamond acid and lithium iron phosphate) is small, and it is difficult to meet the requirements of automotive power batteries. Lithium-rich manganese (xLi2MnO3• (1-x)LiMO2) has become a research hotspot due to its advantages of high specific energy and low price. However, its first conversion efficiency and magnification characteristics are poor, which become the bottleneck restricting its industrialization development.

Based on this, this paper proposes to study the lithium-rich manganese electrode, and use ion doping and other means to functionalize it, in order to lay a foundation for the development of lithium batteries with higher energy density. Promote its application in new energy vehicles.

2 Overview of lithium batteries

2.1 Features of lithium batteries

General commercial lithium-ion battery cathode materials are transition metal oxides or phosphates and other lithium-embedded compounds, such as LiCoO2, LiCo1/3Mn1/3Ni1/3O2, LiMnO4 and LiFePO4, the negative electrode is multi-purpose graphite carbon materials and metal oxides, Including natural graphite and synthetic carbon fiber, mesophase small carbon and tin monoxide, tin dioxide, tin composite oxide, etc. The current commercial battery anode material does not use lithium as a negative electrode, although lithium metal has a very high theoretical specific capacity (3860mAh/g). Lithium itself has high activity, and it is easy to form dendrites on the surface of lithium metal in the charge and discharge process. On the one hand, lithium dendrites irreversibly reduce the active lithium in the negative electrode material after being detached from the electrode surface, and reduce the specific capacity of the negative electrode material.

The electrolyte is mainly an organic solvent that dissolves lithium salt solutes (such as LiClO4,LiAsF6 and LiPF6, etc.). Organic solvents generally use alkyl carbonates such as diethyl carbonate (EDC), ethylene carbonate (EC) and propylene carbonate (PC). Generally, the best electrolyte is the mixed solvent of DMC and EC. The electrolyte also needs to meet certain conditions: ① good thermal stability, not easy to decompose; Good chemical stability, not easy to react with other substances in the electrolyte; (3) High conductivity of Li+; ④ with high decomposition voltage; ⑤ Ensure that the electrode reaction is reversible; ⑥ Safe, non-toxic; ⑦ Low cost. The diaphragm is generally a porous polymer (PP,PE), placed between the positive and negative electrodes, to ensure the passage of lithium ions, prevent short circuit of the battery, while still holding the electrolyte. The diaphragm needs to have acid resistance, good isolation, strong lithium ion conductivity and can prevent other ions from passing, rich material sources, low price, and certain mechanical strength to avoid short circuit inside the battery.

In recent years, lithium-ion batteries have been widely used in the development and construction of China’s new energy automobile industry because of their large battery capacity, long cycle life cycle and higher ratio function. Especially after the current lithium-rich manganese based cathode materials appear in the public view, lithium-ion batteries have been promoted and applied more widely in the technical level of the industry and the scale of market development, and are one of the important components of the new generation of energy lithium battery cathode materials with the most promising application prospects.

2.2 Working Principle

The operation of a lithium-ion battery is shown in Figure 2. In the charge and discharge of lithium-ion batteries, Li+ will continuously migrate to the negative electrode. In this case, by embedding Li+ in the cathode, it diffuses in the electrode, thereby improving its sodium storage capacity. On the contrary, in lithium salt batteries, Li+ will migrate to the negative electrode after it is separated from the crystalline state, thereby improving its specific emission capacity.

Figure 2. Working principle diagram of lithium-ion battery

2.3 Li-rich manganese cathode material

The preparation methods of Li-rich manganese based cathode materials mainly include solid phase method, hydrothermal method and sol-gel method. Solid phase method is the most common preparation method, by the lithium source, manganese source and other doped elements of the precursor mixed and calcined at high temperatures to form the target material. This method has the advantages of simple operation and low cost, but it may lead to poor uniformity and particle size distribution of the material. Hydrothermal method can effectively control the particle size and morphology of materials by using aqueous solution reaction under high temperature and high pressure. The method is usually synthesized using an aqueous solution of lithium and manganese salts at a specific temperature and pressure. By adjusting the reaction conditions, such as temperature, reaction time and pH value, Li-rich manganese based cathode materials with different structures and properties can be obtained. For example, lithium rich manganese based materials synthesized by hydrothermal method usually have higher specific surface area and better electrochemical properties. By sol-gel method, a uniform precursor solution is formed, which is then dried and calcined at low temperature to prepare Li-rich manganese based cathode materials. The advantage of this method is that it can prepare uniformly dispersed nanoparticles and improve the conductivity and charge-discharge performance of the materials. Specifically, the sol-gel method optimizes the microstructure of the material by adjusting the reactant concentration and drying conditions.

In practical applications, the selection of the appropriate preparation method needs to be comprehensively considered according to the end use and electrochemical performance requirements of the material. For example, the hydrothermal method is suitable for battery applications that require high specific capacity and stability, while the solid phase method is suitable for mass production. Through the comparative study of different preparation methods, a more suitable preparation scheme of Li-rich manganese based cathode materials can be explored to meet the demand of high-performance batteries for new energy vehicles.

2.3.1 Material structure

The LMNCO cathode material is a solid solution formed by two Li-rich phases Li2MnO3 and LiMO2, the general formula is xLi2MnO3·(1-x)LiMO2 or Li(Li1/3-x/3Mn1/3-2x/3Mx)O2(M=Ni, Co, Mn and Fe, etc.). LiMO2 phase is hexagonal layered structure, Li and M ions are in the lithium layer and TM layer, respectively. The Li2MnO3 phase is a monoclinic crystal system, in which 1/3 of the Li ions are in the Mn ion in the TM layer, which together form the LiMn6 superlattice structure with low symmetry, belonging to the C2/m monoclinic crystal system.

FIG. 3 Schematic diagram of LMNCO crystal structure

2.3.2 Chemical properties

The study of charge and discharge mechanism is the key to solve the problem of positive electrode materials. Figure 4 shows the electrochemical reaction route of xLi2MnO3·(1-x)LiMO2 positive electrode materials. In the first charging stage of the LMNCO cathode material, the charging curve can be clearly divided into two parts, one is the inclined area of V < 4.5, and the other is the long platform L-shaped area of V > 4.5. When V < 4.5, the Li+ in LiMO2 is detached from the lithium layer, and the Li+ in Li2MnO3 fills the previous Li vacancy by diffusing from the octahedral position in the TM layer to the tetrahedral position in the LiMO2 lithium layer, maintaining the structural stability of oxygen tightly packed. In this process, Li2MnO3 does not provide capacity, and the charge curve appears as a slanted line. When V > 4.5, Li+ is mainly removed from Li2MnO3, and O2- is released from the lattice surface in the form of O2 in order to maintain charge balance due to the valence of Mn to +4. The charge curve shows a long L-shaped platform, which is related to Li2MnO3, but this platform will disappear in the subsequent cycle.

Lu et al. conducted an in-depth study of LMNCO cathode materials with XRD, and proposed the view of irreversible O2 precipitation, suggesting that the generation of voltage platform at the first charging stage is related to the irreversible O2 precipitation in the material lattice. Armstrong et al. used SEMS to study the first charge and discharge process of LMNCO, and detected the release of O2 on the surface of the material. Therefore, the platform appearing at 4.5V can be seen as the LMNCO characteristic curve, as well as the source of its high first irreversible capacity and ultra-high capacity. At present, the most accepted explanation is that when V > 4.5, Li2MnO3 phase is activated, Li+ is removed from the lithium layer and TM layer of Li2MnO3, and O2- will be released to the particle surface along with Li+, and finally released in the form of Li2O to form O2 and MnO2. In the subsequent discharge process, due to the irreversible loss of oxygen, Li2O cannot return to the LMNCO lattice, which is one of the important reasons for the low efficiency of the first cycle of the material.

3. Experimental materials and methods

3.1 Material Preparation

According to the Li∶Ni∶Co∶Mn∶Mg ratio of Li[Li0.2Ni0.13Co0.13Mn0.54]1-xMgxO2(x=0,0.03,0.07,0.12) in the molecular formula, LiOH·H2O(56.5%, industrial grade), Ni(CH3COO)2·H2O(98%, Industrial), Co-(CH3COO)2·H2O(99.5%, Industrial grade), MgO(98%, Industrial grade), Mn-(CH3COO)2·4H2O(99%, industrial grade), Ni(CH3COO)2·H2O, Co(CH3COO)2·H2O, Mn(CH3COO)2·4H2O and CuO, in order to avoid volatilization of Li, Co and Ni at high temperatures, In the experiment, LiOH·H2O, C4H6NiO4·4H2O, MgO and C4H6CoO4·4H2O should be weighed in excess. First, C4H6NiO4·4H2O, C4H6CoO4·4H2O, MgO and C4H6MnO4·4H2O were ground, and then LiOH·H2O was added into the crucible to stir evenly, and finally the crucible was put into the high-temperature atmosphere box furnace (KFQ-1200) for calcination. The roasting temperature is divided into two periods. The first stage is carried out at room temperature, and the heating rate lasts from 5 to 500° C for 8 hours. The second stage, at 500 ° C, maintains the temperature at 850 ° C at a rate of 10 ° C/minute for 12 hours. Finally, the raw material is cooled to room temperature in the furnace and ground into a leaching solution of nickel 0.13 cobalt 0.13 Mn0.54.

3.2 Physical performance characterization

3.2.1 Height difference thermal analysis

Differential Thermal Analysis (DTA) is a very important means of thermal analysis. The results of DTA are mainly related to whether the sample to be tested can have a state change accompanied by thermal effect, so the test cannot determine whether the nature of the state change is a physical change or a chemical change. The mechanism of change also needs to be determined by other testing methods.

The analyzer used in this experiment is produced by Neishi Company, and the temperature range is: room temperature to 900 ℃; Heating speed: 5 ℃ min-1.

3.2.2 X-ray diffraction analysis

X-ray Diffraction (XRD) is a simple and effective means of analysis for determining the structure of a substance. The atoms or molecules in the crystal are in an ordered arrangement, and the X-ray is produced when the electrons in the inner layer of the atom transition under the bombardment of electrons moving at high speed. The crystal can be seen as a diffraction grating. When the X-ray incident on the sample to be measured at a certain Angle θ(the residual Angle of the incident Angle), a series of characteristic diffraction peaks of different intensification will be generated when the incident Angle is different. The crystal system, crystal structure and cell parameters of the sample are determined by analyzing the position, shape and intensity of the peaks.

In the experiment, the samples were analyzed by XRD with the D/ MAX-γBX-ray diffractometer of Nikko Electric. The test conditions are: Cu anode Kα ray (λ=0.154178nm), graphite monochromator, voltage 45kV, current 50mA, scanning 2θ Angle range 10° ~ 90°, scanning speed: 5°/min, step size: 0.02°, slit DS:1°,SS:1°,RS:0.15mm.

3.2.3 Scanning electron microscope

The microstructure and particle size distribution were studied by SEM and TEM. TEM can provide higher resolution images, reveal the internal microstructure and grain boundary information of materials, and further analyze the aggregation phenomenon of particles and its impact on electrochemical properties.

They were characterized by S4700 SEM (10 kV accelerated, 1000 nA, working interval 18700 µm).

3.2.4 X-ray photoelectron spectroscopy

The phase analysis and characterization of the synthesized Li-rich manganese based electrode were carried out by X-ray diffractometer. It is found that the lithium rich manganese base alloy has good crystal properties and good lamellar structure characteristics. In the XRD pattern, the intensity and position of characteristic peaks can reflect the crystallinity and phase purity of the material, and further analysis of its cell parameters can reveal the defects or deformation that may be introduced during doping or synthesis.

In this experiment, the ESCALAB 250Xi photoelectron spectrometer of American Physical Electronics Company was used to analyze the samples.

3.3 Electrochemical performance test of materials

3.3.1 Constant current charge and discharge test

The instrument model used in this experiment is CT3001A, and the test magnification range is 0.1C-5.0C, where 1.0C =250mAh·g-1. The fluctuation of the temperature of the battery during the test will have a negative effect on the electrochemical performance of the material, so the battery that needs to be electrochemical test should be placed in an incubator with a temperature of 25 ° C to ensure the stability of the external environment of the battery during the test.

3.3.2 Cyclic voltammetry test

In cyclic voltammetry, different scanning rates are used to analyze the electrochemical behavior of materials. The REDOX properties of the material can be obtained by analyzing the relationship between current and voltage. For example, in some studies, Li-rich mangan-based cathode materials have shown good current response over different potential ranges, indicating high electrochemical activity.

The instrument used in this experiment is the electrochemical Princeton workstation. The voltage span is set to 2.0-4.8V, and the scanning battery speed is 0.1mV ·s-1. The actual electrochemical reaction mechanism of Li-rich manganese based cathode materials was obtained by the measured volt-time curve.

3.3.3 Constant current intermittent titration test

The instrument model used in this experiment is CT3001A. The lithium-ion battery is stationary for 12h at a voltage window of 2.0-4.8V, and then automatically performs two charge and discharge cycles to make it fully activated, so that the battery needs to be stationary for 5h. Then the program automatically jumps to the set GITT program: (15min slow constant current charge at a low power rate of 0.1C) → (stand for 30min) → (cycle until the charging voltage is 4.8V) → Transition procedure → (15min slow constant current discharge at a low power rate of 0.1C) → (stand for 30min) → (cycle until the discharge voltage is 2.0V). The potential-time curves of several GITT monotitration processes formed by both working and reference electrodes were tested to determine the dynamic properties of lithium ions in Li-rich manganese based cathode materials.

3.3.4 Electrochemical impedance test

Resistance testing can help assess the internal resistance and interface resistance of the material, affecting ion migration and the overall performance of the battery. By comparing the resistance values of different materials under the same conditions, the influence of doping modification on the conductivity can be judged. Some studies have shown that after proper doping, the internal resistance of the material is significantly reduced, thus improving the energy efficiency of the battery. The instrument used in this experiment is the electrochemical Princeton workstation. The assembled battery is placed in a 25℃ incubator, and the corresponding electrochemical impedance test is carried out according to different conditions. The frequency span is 10-2Hz-105Hz, and the vibration amplitude is 5mV. The impedance/frequency characteristics and the electron transport characteristics between the cathode interface (SEI film) and the electrode were obtained by electrochemical reaction of different types of electrodes.

3.4 Electrode material preparation and battery assembly

Add the sample powder, acetylene black and PVDF to the weighing bottle according to the mass ratio of 8:1:1, and then add an appropriate amount of NMP solvent, put the weighing bottle on the magnetic mixer for about 12 hours to get a evenly mixed paste, prepare a clean surface of aluminum foil, fix it on the glass plate with tape, and evenly coat the stirred paste on the surface of aluminum foil. The thickness is similar to that of the tape. The glass plate is sent into a vacuum drying oven and dried at 120℃ for 12h. Then the dried aluminum foil is cut into circular pole pieces with a diameter of 14mm by the tablet press. The pole piece is pressed under 8MPa pressure for about 5min by an oil press, and the weight is accurately weighed after being pressed. Finally, the weighed pole sheet was sent to the oven for vacuum drying at 120℃ for 12h.

Assemble batteries in an Ar atmosphere glove box. The weighed positive electrode sheet, battery diaphragm, button battery case, lithium sheet, nickel foam, etc. are fed into the glove box. The electrolyte uses lmol/LLiPF6/EC+EMC+DEC(volume ratio 1:1:1). Lithium sheet as a negative electrode, nickel foam as a fluid collector, assembled into a button battery in the glove box, sealed the battery with a sealer, left for 12h for battery activation, and then tested.

4 Effect of doping modification on properties of Li-rich manganese based cathode materials

4.1 Selection and preparation of doping elements

Doping modification is an important way to improve its electrochemical performance. On this basis, the selection of appropriate doping elements and preparation process is the key to the optimization of material properties. This project intends to take aluminum, cobalt, magnesium and other elements as the research object, using their excellent electrical conductivity, structural stability and electrochemical properties to explore their applications in lithium batteries.

As shown in Figure 4, the characteristic diffraction peak of the Li-rich manganese based cathode material is composed of α-NaFeO2 (hexagonal R-3m) layered crystal structure and Li2MnO3 (monocline C2/m) structure caused by the regular arrangement of LiMn6 superlattice in the transition metal layer at a low Angle of 20°~25°. As can be seen from the figure, the two groups of characteristic diffraction peaks of the four materials I(006)/(102) and I(018)/(110) have obvious splitting, and the (110) crystal surface of the material has good growth, which can contribute to the rapid exit/embedding of Li+.

Figure 4 XRD pattern of Li-rich manganese based cathode material Li1.2Mn0.54+x Ni0.13-x Co0.13O2

In order to understand the cell structure characteristics of the materials carefully, Rietveld method was used to refine the obtained XRD data to obtain the cell constants and characteristic diffraction peak intensity ratios of the four materials, as shown in Figure 5 (a-d). The cell constants c values were 14.1183, 14.1203, 14.1153 and 14.1029, respectively. The value of cell constant c represents the interlayer spacing of hexagonal layered structures with Li+ exited/embedded. The higher the value of c, the easier the Li+ migration process and the better the electrochemical performance. At the same time, the c/a ratio is also a very important parameter to measure the hexagonal layered crystal structure. The c/a ratio of the four materials is 4.9779, 4.9814, 4.9791 and 4.9516, indicating that these four materials have good layered crystal structure (all greater than 4.899) [41,42]. The degree of Li+/Ni2+ mixing in Li-rich manganese based cathode materials is compared by the number ratio R of the characteristic diffraction peak I(003)/I(104). When R > 1.2, it indicates that the cationic mixing degree is weak and the arrangement is well ordered. When R < 1.2, it indicates that the cation arrangement is more chaotic, resulting in Li+ withdrawal/embedding difficulties, higher irreversible first-circle capacity loss and poor electrochemical performance. When R < 1.0, the Li-rich manganese cathode material is completely deactivated. The characteristic peak intensity ratio of I(003)/I(104) of Li-rich manganese based cathode material Mn0.56Ni0.11, which is 1.20, is higher than that of the other three materials Mn0.54Ni0.13 (1.13), Mn0.58Ni0.09 (1.18) and Mn0.60Ni0.07 (1.16). It can be seen that the lithium rich manganese based cathode material Mn0.56Ni0.11 has the highest R, c/a and c values, which proves that it has the most stable two-dimensional hexagonal layered structure, the smallest cation mixing and the lowest irreversible capacity loss in the first circle, so it presents the highest discharge specific capacity and the best long-term cycle stability.

Figure 5. Refined XRD spectra (a) Mn0.54Ni0.13; (b) Mn0.56Ni0.11; (c) Mn0.58Ni0.09; (d) Mn0.60Ni0.07

As can be seen from Figure 5 (a-d), the appearance of the four Li-rich manganese based cathode materials is basically the same, and the particle surface is smooth, which indicates that adjusting the proportion of nickel-manganese element does not change the material morphology. Agglomeration of the material can reduce the specific surface area of the active cathode material. When agglomeration is serious, the active material located in the central region cannot get adequate reaction, which inhibits the removal/embedding efficiency of Li+, resulting in poor specific discharge capacity. Compared with the other three materials, the lithium rich manganese based cathode material Mn0.56Ni0.11 has no obvious agglomeration phenomenon, complete crystal growth, reasonable particle size and suitable compact degree, so it shows higher specific discharge capacity and initial coulomb efficiency. As can be seen from Figure 5e, the crystal surface of the material successfully prepared by sol-gel method is relatively smooth, crystal edges are clear and crystallinity is good, and the three transition metals Mn, Ni and Co are evenly distributed in it. This is because the continuous agitation of the sol-gel method during the precursor synthesis process helps the elements to be evenly distributed and form a stable and orderly layered structure.

The addition of aluminum can effectively prevent the precipitation of manganese and improve the cyclic stability of the material. In the process of producing Li-rich manganese bases, the content of aluminum is regulated by adding metallic elements such as aluminum (aluminum nitrate) and manganese (Mn). For example, when the catalyst concentration is 0.5%-2%, the catalyst has better stability and larger specific capacitance. Adding Co is a method that can significantly increase its electrical conductivity and enhance its electrochemical performance. At present, the synthesis of Li-rich manganese based cobalt ion batteries is mainly prepared by Sol-Gel method. The main idea is to use a variety of cobalt (Co) sources, as well as Mn (Mn) sources, through reasonable high temperature heat treatment, to achieve uniform doping of metal elements. It was found that suitable Co (about 1%) doping can obviously improve the specific capacitance and cycle performance of the material. The addition of Mg can improve the microstructure and heat resistance of the alloy. This project intends to prepare Li-rich manganese base cathode by solid-phase synthesis method, and obtain Li-rich manganese base cathode material by sintering at high temperature using magnesium hydroxide and other magnesium sources as raw materials. The addition of magnesium can effectively prevent the oxidation-reduction reaction of manganese, so as to improve its safety. In terms of the selection of doping elements, the influence of doping elements on the material structure, the size and valence state of doping elements and their interaction with manganese were investigated. Due to the difference in the type and concentration of doped elements, Li-rich manganese anode materials have poor cycle stability in the cycle process. Therefore, it is urgent to carry out systematic experimental research in order to achieve the best results.

4.2 Material characterization after doping modification

The crystal structure, morphology, chemical composition and homogeneity of Li-rich manganese based electrode were studied. The crystal structure of the doped product was analyzed by X-ray diffraction (XRD). It was found that although some metal ions (such as Co, Mg, etc.) were modified, the layered structure did not change significantly. The lattice parameter values will change slightly. It is numerically simulated by Bragg theorem, which shows that it is stable during crystallization.

The surface morphology was studied by scanning electron microscopy. The particle size and dispersion of nano-powders can be improved by element doping. Among them, by adding different impurities, the nanoparticles can be made to grow more evenly, thereby reducing the generation of large particles, thereby increasing the surface area. In addition, through transmission electron microscopy (TEM) technology, the microstructure of dopants can be better observed, so as to better reflect the distribution of impurity atoms in it.

The components were tested by the energy spectrometer, and it was found that the added elements were all in line with the design requirements, indicating that the process was feasible. Through the analysis of the distribution of elements in the sample, it is found that the impurity atoms are uniformly distributed on the lithium-rich manganese electrode, which proves the uniformity of the electrode.

Raman analysis showed that the lattice vibration modes of the samples changed after the modification. The results show that in different samples, different ions form a new characteristic peak on the surface of the sample, indicating that there is a certain interaction between different metal ions under different conditions.

4.3 Comparison of electrochemical properties after doping modification

It was characterized by various electrochemical means and characterized. The samples were characterized by constant current charging and discharging experiments and AC impedance spectroscopy.

Through the cyclic voltammetry analysis of the sample, it was found that the oxidation-reduction peak of the sample surface was significantly enhanced at different speeds, indicating that the electrochemical performance of the electrode was improved. When aluminum alloy is added, its maximum current can be increased by 20%, mainly because of its conductivity to metal materials and ionic fluidity. The experimental results show that the sample in the high pressure region has a large peak value, indicating its stability and catalytic performance under high pressure conditions.

The results of constant current charging test show that the modified material has a higher capacity than the unmodified material. The results show that at 0.1℃, the ratio of high lithium manganese based cathode material containing Co can reach 180 mAh/g, which is 15% higher than that of raw material. In addition, the performance of the doped material is better than that of the unmodified material under high magnification conditions, especially at 1 and 5℃, the specific capacity attenuation is significantly reduced.

The impedance measurement of the specimen shows that the charge transfer resistance of the specimen decreases significantly during charging/discharging. After testing, the resistance loss of manganese was reduced to 100€, while the resistance loss of other elements was as high as 150€, showing good conductivity.

Compared with the above electrochemical properties, it is found that the electrochemical properties of the modified compounds are significantly improved. The research results will lay a foundation for further improving the design and preparation of such novel nanostructures.

Conclusion

It is the most promising new lithium-ion battery anode material at present. However, the cyclic stability and high non-reversible capacity of the Li-rich manganese electrode lead to a sharp decrease in the platform voltage during charging and discharging. On the basis of the above research, this project intends to introduce lithium ions into Li-rich manganese series negative electrode materials, and improve the stability of its crystal structure through chemical modification, so as to give play to its good magnification characteristics. In this regard, we will also improve its application in the field of new energy vehicles from many aspects and multiple dimensions.

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