Application of X-ray Diffractometer in Characterization of Ternary Cathode Materials for Lithium Ion Batteries
In recent years, many provinces and cities in China have continued to suffer from severe smog pollution, which has aroused the public's strong concern about ambient air quality. PM2.5 is the main component of smog. The contribution rate of fuel vehicles is between 20% and 30%, and the promotion of zero-emission new energy vehicles is one of the ways to effectively solve the smog problem. In May 2015, the State Council issued “Made in China 2025” and listed “Energy Saving and New Energy Vehicles” as one of the 10 key breakthrough areas. New energy vehicles have achieved unprecedented development opportunities. As the "heart" of new energy vehicles, the performance of lithium-ion batteries directly determines their endurance, service life, safety and reliability. As is well known, a lithium ion battery is composed of a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the positive electrode material is a key core material. The nickel-cobalt-manganate ternary material (hereinafter referred to as "ternary material") in the positive electrode material combines the high specific capacity of lithium nickelate (LiNiO2), the high magnification of lithium cobaltate (LiCoO2), and lithium manganate (LiMn2O4). Its high safety and low cost have become the material of choice for new energy vehicles with high cruising range.
The specific capacity, cycle performance and safety performance of lithium-ion batteries are closely related to the crystal structure of the materials. Studying the stability of ternary materials under different temperature conditions and structural changes during electrochemical cycling will help A good understanding of the ternary material charge and discharge mechanism and electrochemical process is of great significance for optimizing product solutions and developing high-performance ternary materials.
X-Ray Diffraction (XRD) is a device specially used to analyze the crystal structure of materials. It can be refined to obtain the unit battery parameters and ion mixing information of ternary materials. The field of ion batteries has been widely used. The ray generation principle of the XRD device is that the tungsten filament emits an electron beam to bombard the metal target, 99.9% of the energy is dissipated as heat, and only 0.1% is converted into X-ray, which generates a large amount of heat. The heat of the traditional XRD equipment is concentrated at the fixed position of the target, and the heat cannot be lost in time, which seriously affects the stability of the X-rays. Therefore, it can usually be used only at a small current, resulting in low signal resolution, which is not conducive to the microscopic crystal structure. Accurate characterization. In recent years, with the application of a new generation of "target-to-target" technology, electron beam bombardment of different parts of the rotating target is conducive to the rapid loss of heat, and the heat generation problem of the X-ray generator has been significantly improved, and the X-ray can be improved. The current and voltage are generated, which further enhances the signal resolution, improves the accuracy and reliability of quantitative analysis, and makes the application of XRD in the field of lithium ion batteries mature.
This paper introduces the application of XRD in ternary material preparation process and material doping modification, and also describes its in-situ high temperature thermal performance of ternary materials and battery charge and discharge mechanism. application.
I. Application of XRD in the preparation and doping of ternary materials
The diffraction spectrum of materials can be obtained by XRD, and the crystal form, unit battery parameters and content of each phase can be obtained by fitting the finishing. The crystal structure of the ternary material LiMO2 (M=Ni, Co, Mn) is similar to that of LiCoO2. It belongs to the hexagonal system and has a layered structure of α-NaFeO2. The space group is (R-3m) and Li+ occupies 3a. In the (000) position, the transition metal ions of Ni, Co, and Mn occupy the 3b (0 1/2) position irregularly, and O2- occupy the 6c (00 z) position. The O at the 6c position is cubically packed, and the transition metal ions (Ni, Co, Mn) at the 3b position and the lithium at the 3a position alternately occupy the octahedral voids, and are layered on the (111) crystal plane. Among the ternary compounds, the valences of the transition metal Ni, Co and Mn elements are +2, +3, and +4, respectively.
1.XRD application in ternary material preparation process
In recent years, layered high-nickel ternary material LiNi1-x-yCoxMnyO2 (0
Beijing Institute of Technology Wu Feng research group on LiNi0.6Co0. The sintering temperature of 2Mn0.6O2 material was studied . As shown in Figure 2, the samples of different sintering temperatures showed a hexagonal α-NaFeO2 layered structure with no impurity peaks, (006)/(102) and (108). The /(110) peaks showed obvious splitting, and the c/a values ??were all greater than 4.899, indicating that the materials obtained at different sintering temperatures were layered. As the sintering temperature increased, the (003) peak intensity increased, I003/I104 The value also increases gradually, indicating that the increase of temperature is beneficial to the increase of crystallinity and the decrease of cation mixing. It is also found that the LiNi0.6Co0.2Mn0.6O2 material obtained at a moderate sintering temperature of 850 °C exhibits the most superior electrical properties. The perfect crystal structure avoids the uneven structure of the material at higher sintering temperature and provides the best channel for lithium ion transport. On the other hand, the larger primary particles obtained by high temperature sintering reduce the side reaction of the positive electrode material and the electrolyte. Combine the above 2 points to make The battery made by the sample at 850 °C has the most superior capacity, rate and cycle performance.
The research of the sintering atmosphere on the properties of LiNi0.5Co0.2Mn0.3O2 by XRD was studied by XRD. Li: (Ni+Co+Mn)=1.08:1 was sintered in 10%, 20%, 30%, 40% (volume content) O2 atmosphere, and the samples were labeled as N-L523, A-L523, O-, respectively. L523, O2-L523. The results are shown in Fig. 3. With the increase of oxygen concentration, the (003) peak shifts to the left, the interplanar spacing increases, and the c value increases, the c/a value increases, and the I003/I104 value The increase (shown in Table 1) indicates that the layered structure of the material is more perfect, and the degree of ion mixing is gradually reduced, and the comprehensive performance of the material is better. The electrical property test also confirms that the material has the best capacity under the 40% concentration O2 atmosphere. Magnification and cycle performance.
Based on the existing literature reports, XRD characterization technology has been widely used in the optimization of ternary material sintering temperature , sintering atmosphere, ultrasonic  and other preparation processes, by characterization of materials Crystal structure, unit battery parameters, interplanar spacing, cation mixing degree, etc. Optimize the material preparation process and accelerate the development process of ternary cathode materials.
2.Application of XRD in doping modification of ternary materials
Element doping is one of the important ways to optimize the performance of ternary materials. The impurity effect can be characterized by XRD. The research group of Wang Zhixing of Central South University studied the properties of F-doped LiNi0.5Co0.2Mn0.3O2-zFz (z=0, 0.02, 0.04, 0.06) ternary materials . 0.2% F doping The 1C discharge capacity retention rate after the 100-week cycle of the sample can reach 81.1%, which is much better than 70.1% of the undoped sample. The 10C discharge capacity after doping at 0.2% F was 121 mAh/g, which was 11 mAh/g higher than that of the undoped sample at 110 mAh/g. The reason is that after F doping, F-occupies O2-position, is conserved by charge, some cations are reduced, and the reduced cation has a larger atomic radius, so F doping will cause unit battery parameters a, c, The V value increases as shown in Table 2. With the increase of F doping content, the (003) peak position shifts to a small angle. The corresponding interplanar spacing d is increased by the Bragg formula, and the corresponding activation energy of lithium ion migration is reduced. Therefore, F doping improves charge and discharge. The migration rate of lithium ions in the process, the EIS test reached a consistent conclusion. On the other hand, the Li-F bond can be 577 kJ/mol, while the Li-O bond energy is only 341 kJ/mol, and the F-doping enhances the structural stability of the material. Based on the above two factors, F doping improves the cycle and rate performance of materials under high voltage conditions.
Based on the existing literature reports, XRD characterization technology has been widely used in the optimization of ternary material sintering temperature , sintering atmosphere, ultrasonic  and other preparation processes, in order to characterize the crystal structure of the material, Parameters such as unit battery parameters, interplanar spacing, and cation mixing degree optimize the material preparation process and accelerate the development of ternary cathode materials.
Second, the application of in-situ XRD in ternary materials
1. Application of in-situ high temperature XRD in the characterization of ternary materials thermal stability
In situ high temperature XRD can monitor lithium ion during heating process in real time The structural evolution process of the positive electrode material is used to characterize the thermal properties of the material. For ternary materials, especially ternary materials for high-nickel and high-capacity power batteries, thermal performance is a bottleneck in practical applications, and an important problem that needs to be solved and solved in scientific research.
A variety of cathode materials are mixed in a certain proportion and then applied to lithium ion batteries to give advantage of each cathode material and obtain optimal overall performance. The research team of Hong Kong University of Science and Technology Shao Minhua used in-situ high temperature XRD and mass spectrometry to characterize the thermal properties of LiMn2O4(LMO)-LiNi1/3Co1/3Mn1/3O2(NCM) cathode materials with different mixing ratios under charge. Mixed scheme . As shown in Fig. 4, the mixed samples in different proportions are accompanied by the release of carbon dioxide (CO2) during the heating process, and the release of oxygen (O2) is not detected. It is presumed that the released O2 reacts with carbon, organic binder, etc., and finally Released in the form of CO2. When the temperature is less than 500 ° C, the phase transition process of the three materials is almost similar. However, when the temperature is higher than 500 °C, LMO shows two phases of Mn4O3 and MnO, LMO-NCM (3:1) increases the (Ni, Co, Mn)O phase, and LMONCM (1:1) increases NiO. phase. The formation of the NiO phase is accompanied by a decrease in the high oxidation state of Ni, suggesting the release of O2 during the phase change, which is why the high NCM mixture has more peaks in the CO2 release curve in the high temperature region. The XRD pattern also confirmed that NCM first undergoes a phase transition, and then LMO begins to phase change, and NCM stability is lower than LMO. It is worth noting that as the NCM content increases, the initial temperature of CO2 release also decreases, and NCM stability is poor, which is consistent with the conclusions obtained from the XRD pattern.
H i t o s h i Y a s h i r o The high temperature thermal stability of de-lithium Li0.35[Ni1/3Co1/3Mn1/3]O2 coated with AlF3 was studied . In-situ high temperature XRD (HT-XRD) data confirmed that the initial phase temperature of NCM material from hexagonal layered (R-3m) to cubic spinel (Fd3m) increased from 200 °C to 260 °C after AlF3 coating ( See Fig. 5), the combined temperature of (108) and (110) peaks is increased from 250 °C to 280 °C, the phase change completion temperature is raised from 450 °C to 500 °C, and the initial change temperature of the lattice parameters is also increased. (See Figure 6). The above data proves that the thermal stability of LiNi1/3Co1/3Mn1/3O2 material after AlF3 coating is better. Similarly, the in-situ high temperature XRD can be used to evaluate the effect of coating and doping processes on the thermal stability of ternary materials.
2. Application of in-situ battery XRD in ternary materials
In-situ battery XRD technology can monitor the structural evolution process of lithium ion cathode material in charge and discharge process in real time, which is of great significance for studying the mechanism of battery capacity decay. . In general, the charging of a ternary material lithium-ion battery to a high voltage is advantageous for the capacity, but the reaction between the electrode and the electrolyte is likely to occur at a high voltage, causing irreversible loss of capacity, so the corresponding cycle life is also poor, so the actual battery In applications, it is particularly important to select the appropriate operating voltage for the cathode material [10, 11].
It has been reported in the paper about the capacity decay mechanism of high nickel materials. The T.Nakamura team of Hyogo University of Japan used in-situ battery XRD technology for Li[Ni1/3Co1/3Mn1/3]O2, Li[Ni0.45Co0.1Mn0.45]O2, Li[Ni0.5Mn0.5]O2. The ternary materials with different nickel contents of Li[Ni0.5Co0.2Mn0.3]O2, Li[Ni0.6Co0.2Mn0.2]O2 and Li[Ni0.7Co0.2Mn0.1]O2 were studied . The XRD refinement results of the in-situ battery show that the unit battery volume changes greatly during the charge and discharge process as the nickel content increases (Fig. 7). The larger volume change is not conducive to the stability of the structure. The experiment proves that the nickel content is higher. The lower the material capacity is maintained, the higher the internal resistance after cycling and the more cracks. The attenuation mechanism of high nickel ternary materials can be explained by volume, internal resistance and microcracks. Large volume changes during charging and discharging of ternary materials are not conducive to structural stability, and internal resistance increases unnecessary capacity loss. The formation of microcracks does not affect the conductivity of the isolated particles, but forms a new interface between the positive electrode and the electrolyte, providing conditions for the occurrence of side reactions. As the cycle time is extended, the microcracks of the high nickel ternary material are more likely to grow, and the increase of the high valence nickel further accelerates the oxidative decomposition of the electrolyte. Therefore, between high capacity and long life, the best balance needs to be pursued.
Shu Jie et al. studied the structural changes of LiNi0.5Co0.2Mn0.3O2 during charging and discharging process by using in-situ battery XRD technology, and deeply analyzed the mechanism of material specific capacity attenuation under deep charge and discharge conditions, and explored the best material. Use voltage . As shown in Fig. 8, the repeatability of a, c, c/a, and V4 parameters during the charging and discharging process of 2 to 4.3 V is good, indicating that the material has good cycle performance in the voltage range of 2V to 4.3V. In the process of charging and discharging from 2 to 4.9 V, the recovery of the unit battery parameters is poor. The lattice constant of a decreases from 2.8933 to 2.8637 (\u003e0.681Li) and then to 2.8593 (\u003e0.979Li), and the value of Δa changes relatively. For the value of c, it first increases to 14.4152 (\u003e0.681Li) and then decreases to 14.3206 (\u003e0.979Li). The c/a ratio increased from 4.9413 to 5.0339 and then gradually decreased to 5.085 (shown in Figure 9). The peak positions of the c and c/a curves correspond to the initial phase transition voltage of the H2 phase to the H3 phase, ie 4.7 V (0.681 Li). The corresponding c/a value of H3 is small, indicating that the layered structure of the H3 phase is poor and the degree of crystallization is lowered, which is not conducive to the electrochemical performance of the material. In summary, in order to exhibit the best cycle performance of LiNi0.5Co0.2Mn0.3O2 material, the material can be used between 2.0 and 4.6V, and long life and high storage efficiency can be obtained by suppressing the formation of the H3 phase.
Many literatures have reported the use of in-situ battery XRD technology to study the variation of the structure of lithium ion battery during charge and discharge process, so as to characterize the coating and doping modification effect [14-15], and provide data support for the preparation of optimized materials.
The application of XRD technology in the cathode material of lithium ion battery has widely covered the determination of basic process parameters such as lithium ratio, sintering temperature and sintering atmosphere, and modification of coating and doping. the study. In-situ high temperature XRD can monitor the crystal structure of materials under different temperature conditions in real time, and is also applied to the study of crystal form change, material thermal performance evaluation and material composition optimization design. The in-situ battery XRD technology has also been used to explore the optimal potential range of ternary materials in actual use by real-time monitoring of the crystal structure of the positive electrode material during charging and discharging of the battery, thereby realizing the optimal design of life and capacity. In summary, XRD technology has been widely used in ternary cathode materials for lithium-ion batteries, and is expected to play a greater role in the future.
 Wang Lei, Wu Borong, Mu Daobin, et al. Single-crystal LiNi0.6Co0.2Mn0.2O2 as high performance cathode materials for Li-ion batteries[J].Journal of Alloys And Compounds,2016(674):360-367.
 Wang Fu,Yun Zhang,Jizhou Zou,et al.The structural mechanism of the improved electrochemical performances resulting from sintering atmosphere for LiNi0.5Co0.2Mn0.3O2cathode Material[J].Journal of Alloys and Compounds,2013(558):172-178.
 Yun FeiXia, MinNie,Zhen Bo Wang, et al.Structural morphological and electrochemical investigation of LiNi0.6Co0.2Mn0. 2O2 cathode material synthesized in different sintering conditions[J].Ceramics International,2015(41):11815-11823.
 Zheng Xiaobo,Li Xinhai,Guo Huajun,,et al.Enhanced electrochemical performance of LiNi0.6Co0 .2Mn0.2O2cathode materials by ultrasonic-assisted co-precipitation method[J].Journal of Alloys and Compounds,2015(644):607-614.\u003cb r\u003e  Wang Ding, Wang Zhixing, Li Xinhai, et al. Effect of surface fluorine substitution on high voltage electrochemical performances of layered LiNi0.5Co0.2Mn0.3O2cathode materials [J].Applied Surface Science, 2016(371): 172-179.
 Zeng Yu,Ke huiQiu,Yang Ziqi,et al.Influence of europium doping on the electrochemical performance of LiNi0.5Co0.2Mn0.3O2cathode materials for lithium ion batteries[J].Ceramics International, 2016(42):10433-10438.
 Huang Zhenjun, Wang Zhixing, Guo Huajun, et al. Influence of Mg2+doping on the structure and electrochemical performances of layered LiNi0.6Co0.2-xMn0.2MgxO2cathode materials [J].Journal of Alloys and Compounds, 2016(671):479-485.
 Hu Enyuan, Bak SeongMin, Shao Minhua, et al. Thermal stability in the blended lithium manganese oxide Lithium nickel cobalt manganese oxide Cathode materials: An in situ time resolved X-Ray diffraction and mass spectroscopy study[J]. Journal of Powe r Sources, 2015(277): 193-197.
 Seung TM, Lee KiSoo, Yoon Chong Seung, et al. Effect of AlF3 Coating on Thermal Behavior of Chemically Delithiated Li0.35[Ni1/3Co1/3Mn1 /3]O2[J].J.Phys.Chem.C.,2010(114):4710-4718.
 Yabuuchi.N,Ohzuku.T.Electrochemical behaviors of LiCo1/3Ni1/3Mn1/3O2 In lithium batteries at elevated temperatures [J]. J. Power Sources, 2005 (146): 636-639.
 KMShaju, G. Peter, Bruce. Macroporous Li (Ni1/3Co1/3Mn1/3) O2: A High-Power and High-Energy Cathode for Rechargeable Lithium Batteries [J]. Adv. Mater., 2006 (18): 2330-2334.
 K Ishidzu, Y Oka, T Nakamura. Lattice volume Change during charge/discharge reaction and cycle performance of Li[NixCoyMnz]O2[J].Solid State Ionics,2016,(288):176-179.
 Shu Jie,Ma ??Rui,Shao Lianyi,Ren Yuanlong , et al.In-situ X-ray diffraction study on the structural evolutions of LiNi0.5Co0.3Mn0.2O2 in different working potential windows[J ].Journal of Power Sources, 2014(245):7-18.
 Fell CR, Chi M, Meng YS, Jones JL, In situ X-ray diffraction study of the lithium excess layered oxide compound Li[ Li0.2Ni0.2Mn0.6]O2 during electrochemical cycling[J].Solid State Ionics,2012(207):44-49.
 Yu Haoxiang,QianShangshu,Shu Jie,et al.Morphological Electrochemistry and in -situ XRD study of LiNi0.6Co0.2Mn0.1Al0.1O2 as high potential cathode material for rechargeable lithium-ion batteries[J].Journal of Alloys and Compounds,2016(667):58-64.