First Principle Study Of The Electrochemical Properties Of LiVOPO₄, Orthosilicate And Carbonophosphate Cathode Materials

In this work, first principles calculations are performed to investigate the electrochemical properties of LiVOPO₄, orthosilicate materials and carbonophosphate materials, in order to propose new methods to improve the performance of these polyanion cathode materials.?1? First principles calculations are performed to investigate the surface energies, equilibrium morphology, surface redox potentials, and surface electrical conductivity for LiVOPO₄. Relatively low-energy surfaces are found in the ?100?, ?010?, ?001?, ?011?, ?111?, and ?201? orientations of the orthorhombic structure. Using the calculated surface energies, the thermodynamic equilibrium shape for LiVOPO₄ crystal is built through a Wulff construction. Surface ?001? and ?111? are the dominating surfaces in the Wulff shape. Similar calculations for VOPO₄ display a larger decrease in the surface energies for surface ?100? rather than those in the other surfaces. It suggests that the Wulff shape of LiVOPO₄ is in close relationship with the chemical environment around. Surfaces ?100?, ?010? and ?201? show lower Li surface redox potentials in comparison with the bulk material. Therefore, the Li migration rate on surfaces could be effectively improved by maximizing the exposure of these low redox potential surfaces. In addition, lower surface band gaps are found in all orientations compared to the bulk material. It indicates that the electrical conductivity for LiVOPO₄ can be significantly improved by enlarging surfaces with relatively low band gaps in the particle. Therefore, synthesizing ?201? and ?100? nanosheets will greatly improve the electrochemical properties of the LiVOPO₄ material.?2? First principles calculations are employed to compare the voltage plateaus, cycling stabilities, electrical conductivities and ionic conductivities between Li₂MSiO₄ and Na₂MSiO₄ ?M= Fe, Mn, Co and Ni?. The results give good explanaions for the poor capacity in Li₂MSiO₄ as well as the reason why Li₂FeSiO₄ and Li₂FeSiO₄ exchanges only one Li ion per formula unit. Li₂MSiO₄ shows higher voltages and cycling stabilities than Na₂MSiO₄. However, in comparation with Li₂MSiO₄, Na₂MSiO₄ presents higher the electrical and ionic conductivities. Na₂NiSiO₄ might be a good cathode material for Na ion batteries, which can exchange 1.5 Li ion per formula unit. To combine the advantages of Li₂MSi04 with Na₂MSiO₄, we further investgate the Na doped Li_1.5Na_0.5MSiO₄ system. The results suggest that the electrical and ionic conductivities increased when Li₂MSiO₄ are partially doped by Na ion. Compared with the values in Li₂MSiO₄, the voltages and cycling stabilites do not change much in Li_1.5Na_0.5MSiO₄. Therefore, Na ion doping should be an effective measure to improve the performance for Li₂MSiO₄ cathode materials.?3? First principles calculations are used to evaluate the voltage plateau, cycling stabilities, electrical conductivities and safeties for Na₃MPO₄CO₃ ?M= Fe, Mn, Co and Ni?. Our results suggest that Na₃FePO₄CO₃ and Na₃MnPO₄CO₃ can reversibly exchange two Na ions per formula unit, but Na₃CoPO₄CO₃ and Na₃NiPO₄CO₃ can exchange only one Na ion under safety. Additionally, carbonophosphate materials possess poor electrical conductivities. In order to improve the electrical conductivities, we take Na₃MnPO₄CO₃ as an example and explore its morphology and surface properties. The results indicate that maximizing the exposure of surface ?-101?, ?110? and ?12-1? or synthesizing ?-101?, ?110? and ?12-1? nanosheets can greatly improve the electrical conductivities of the Na₃MnPO₄CO₃ material.