New Insights Into Scientific Fundamentals Of Li- And Mn-rich Layered Oxide Cathode For Lithium-ion Batteries

Li- and Mn-rich layered oxide ?LMRO? with high discharge specific capacity of 280 mAh g^-1 and high energy density of 1000 Wh kg^-1 can promisingly become the next generation cathode material for lithium-ion batteries?LIBs?. However, Poor cycling stability and rate performance, low initial coulombic efficiency of the LMRO cathode severely inhibits its practical applications. In this thesis, Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ ?LNCMO? cathode materials were prepared by spray pyrolysis, and their structures and electrochemical performance were systematically studied by x-ray diffraction ?XRD? and Rietveld refinement, high resolution transmission electron microscopy ?HRTEM?, scanning electronic microscopy ?SEM?, Raman spectra, photoelectron spectroscopy ?XPS?, galvanostatic charge and discharge test, AC impedance spectroscopy?EIS? techniques. The mechanisms resulting in the fast capacity/voltage fading, poor rate performance, and low coulombic efficiency of the LNCMO cathode materials were investigated, and the mechanisms of synergic effect of various phases and metal elements in the LNCMO cathode materials were also systematically explored. Moreover, strategies of using new type binder of sodium carboxymethyl cellulose ?CMC?, Mg²⁺ ions substitution, and in-situ precipitation Er₂O₃ coating were used to significantly improve the electrochemical performance of the LNCMO cathode materials.Firstly, the CMC was used as a novel binder for the LNCMO cathode, which can effectively enhance its cycling stability because the higher stickness of the CMC binder can significantly stabilize the electrode structure and the Na+ ions in CMC can dope into cathode material to suppress its capacity/voltage fading and dissolution of metal elements.The mechanism of the phase transformation of the LNCMO cathode material during annealing process was investigated. The results of the XRD and Rietveld refinement found that the LNCMO cathode material underwent a gradual phase transformation process of hexagonal LiMO₂?R-3m? phase?monoclinic Li₂MO₃?C2/m? phase?orthogonal LiMn₂O₄?Fd-3m? phase ?M=Ni, Co, Mn? with the increase of the annealing temperature. The HRTEM indicated that the structure rearrangement of Li and transition metal?TM=Ni, Co, Mn? in the TM layer from a random arrangement to a LiTM2 super-lattice arrangement facilitated the formation of the Li₂MO₃?C2/m? with increasing of the annealing temperature. When the annealing temperature increased up to 1000 ?, the Li evaporated from the structure to decrease the Li amount in the LNCMO cathode material, as a result the ratio of the Li:M was lower than that of the LiMO₂ and Li₂MO₃, which led to the formation of the LiM₂O₄. The results clearly indicated that the LNCMO cathode material was an integrated nano-domain composite structure when annealed at 900 ?. The XPS showed that the values of Ni, Co, Mn are +2,+2 and+3,+4, respectively, for the LNCMO cathode material. Moreover, the LNCMO cathode material annealed at 900 ? demonstrated the best electrochemical performance.The results of the XRD and Rietveld refinement showed that the migration of the transition metal ions and structure rearrangement in the LNCMO cathode material resulted in the formation of the spinel like LixMn2O₄ phase during the first cycling. The monoclinic Li₂MO₃ phase with the super-lattice structure mostly transformed into the hexagonal LiMO₂ phase after the first cycle. Moreover, a part of hexagonal LiMO₂ phase transformed into the spinel like LixMn2O₄ phase, and the calculation of the spinel like LixMn₂O₄ phase in the LNCMO cathode material caused to its capacity/voltage fading during cycling. Studies on the different cut-off potential windows indicated that the LNCMO cathode delivered the best electrochemical performance when cycled in the potential window of 2.0?4.8 V. Moreover, the kinetic parameters ?Rct, Ea and DLi+? of the redox reaction of the Ni²⁺/Ni⁴⁺, Co³⁺/Co⁴⁺ in the LiMO₂ phase were much better than that of the action of the Li₂MnO₃ phase and redox reaction of the Mn⁴⁺/Mn³⁺ in the LiMnO₂ phase, which revealed that the action of the Li₂MnO₃ phase and redox reaction of the Mn⁴⁺/Mn³⁺ in the LiMnO₂ phase are the key controlled step of the kinetics of electrode process of the LNCMO cathode.The structure and electrochemical performance of the XLi₂MnO₃-?1-X?LiMO₂ ?M=Ni, Co, Mn,0?X?1? ?LNCMO? during cycling were also systematically investigated. The results revealed that there is a synergic effect between the LiMO₂ phase and the Li₂MO₃ phase in the LNCMO cathode material with different X. The Li₂MO₃ phase can effectively enhance the electrochemical capacity and cycling stability of the LNCMO cathode material, but decrease its coulombic efficiency and rate performance. However, the integrated nano-domain composite structure between the Li₂MO₃ phase and the LiMO₂ phase can activate the electrochemical activity of the Li₂MO₃ phase and simultaneously increase the coulombic efficiency and rate performance of the LNCMO cathode. Therefore, the synergic effect between the LiMO₂ phase and the Li₂MO₃ phase realized the higher electrochemical capacity, long cycling stability and higher rate performance of the LNCMO cathode, and the LNCMO with X=0.5 delivered the best overall performance.The effect of various metal elements M ?Li, Ni, Co, Mn? and LiM02 ?M=Li, Ni, Co, Mn? and their mechanisms in the LNCMO cathode material were investigated by XRD and charge and discharge tests. The results demonstrated that increasing the Li amount in the LNCMO cathode material can improve the percentage of the Li₂MO₃ phase and consequently improved the electrochemical capacity and cycling stability of the LNCMO cathode, but also decrease its initial coulombic efficiency and rate performance. On the contrary, decreasing the Li amount in the LNCMO cathode material can result in the formation of the LiM₂O₄ phase and consequently decrease the electrochemical capacity but can enhance its coulombic efficiency and rate performance, and the LNCMO cathode with over 5% Li amount showed the best overall electrochemical. Increasing the amount of the Ni and LiNiO₂ in the LNCMO cathode material can significantly suppress the capacity/voltage fading during cycling and enhance rate capability of the LNCMO cathode material. Increasing the amount of the Co and LiCoO₂ in the LNCMO cathode material can cause to the faster capacity/voltage fading of the LNCMO cathode material during cycling, but it can enhance the rate capability. Increasing the amount of the Mn and LiMn₂O₄ in the LNCMO'cathode material can facilitate the formation of the LiMn₂O₄ phase in the LNCMO cathode material and decrease its electrochemical capacity, but it can effectively improve the cycling stability and initial coulombic efficiency of the LNCMO cathode material.Based on the above studies, the ions substitution and in-situ precipitation oxide coating strategies were proposed to effectively enhance the overall electrochemical performance of the LNCMO cathode material. The results of the XRD and Rietveld refinement indicated that the Mg²⁺ substitution can suppress the migration of the transition metal ions of the LNCMO cathode material during cycling to inhibit the formation of the spinel like phase. The Mg²⁺ substitution can increase the percentage of the Li₂MO₃ phase in the LNCMO cathode material to improve its electrochemical capacity. Moreover, the EIS was carried out to study the kinetics of the LNCMO cathode with the Mg²⁺ substitution during cycling, and the results showed that the Mg²⁺ substitution can increase the conductivity, decrease the electrochemical reaction impedance and facilitate the Li+diffusion of the LNCMO cathode material. Therefore, the LNCMO cathode material with the Mg²⁺ substitution demonstrated the higher capacity, long cycling life, higher rate performance. The results of the XRD, HRTEM and XPS confirmed that the in-situ precipitation oxide coating strategy can obtain a LNCMO@Er₂O₃ composite cathode material with a uniform, complete, strong binding force nano-scale ErO₃ coating on the LNCMO surface. This Er₂O₃ coating can significantly prevent the dissolution of the metal elements into the electrolyte and consequently improve the initial coulombic efficiency, cycling stability and rate performance of the LNCMO cathode material.