First-principles Investigation Of Surface Structures,Mechanical Stability,and Electrochemical Properties Of Pyrophosphates As Cathode Materials

In this work,first principles calculations were employed to investigate the electrochemical properties,elastic properties,and surface properties of Li2MnP2O7(?-and?-phases),and Li2 Fe P2O7 that are cathode materials of lithium ion batteries(LIBs).We did our best to reveal the microscopic properties of materials and predict the possibility,which are conducive to providing the theoretical basis and guidance for experiment by the theoretical calculation.This thesis includes two parts:With the aid of first principle calculations,structural characteristics,mechanical stability,and electronic and electrochemical properties of two polymorphs of manganese-based pyrophosphate ?-and ?-phases of Li2 Mn P2O7 and their relevant delithiated structures are explored for comparison.Our results indicate that,although these two polymorphs of Li2MnP2O7 belong to the monoclinic space group,considerable differences are discovered in Mn local environment of crystal structures.The cell voltage vs Li/Li+ are 4.68 and 4.16 V for?-and ?-phases of the Li2 Mn P2O7/Li Mn P2O7 platform,respectively,comparable to the experimental values(4.45 and 4.00 V)for first voltage plateaus.All of the lithium atoms are practically fully ionized in the ?-and ?-Li2MnP2O7 and their relative half delithiated states,charge transfer mainly concentrated upon Mn and O,which leads to the oxidization state of Mn from Mn2+ to Mn3+ and then from Mn3+ to Mn4+.The bond gaps of delithiated configurations decrease gradually with removing lithium ions,and the conductivity changed from insulator nature to conductor characteristic.By the elastic properties calculations,the Pugh ratios(B/G)are 3.28 and 2.86 for the ?-and ?-Li2 Mn P2O7,respectively,indicating their high mechanical stability.However,small B/G values are observed for the relevant delithiated phases.In addition,Young's modulus(E)and Poisson's ratio(?)for ?-and ?-phases of Li2 Mn P2O7 and their delithiated configurations are also presented to explore the hardness and bond characteristics.Using first principles calculations with the generalized gradient approximation(GGA)+U framework,we investigate the surface energy,surface structures,thermodynamic equilibrium shape,and electrochemical property of Li2FeP2O7.According to the experience of previous works and X-ray diffraction(XRD)results,we chose seven low-index surfaces and surface(210)as the research target in this work.The surface(111),(110),(011)and(210)are found to have the lower surface energies with smaller displacement of ions on these surfaces compared to other surfaces.As well,the ions presented on the surface change significantly during the relaxation for the coordination,bond length,and displacement along the z-axis.There are four surfaces,namely,surface(111),(110),(011),and(210)of Li2 Fe P2O7,presentedon the thermodynamic equilibrium morphology,indicating a better stability of these surfaces.To study the electrochemical properties of surfaces,we compare the redox potentials vs Li/Li+of surfaces with bulk structures.The results indicate that lower voltages of surfaces than the bulk's except for surface(100),showing a better energetic preference to extract Li from the surface structure with a fast ions transportation on the surface.Therefore,amplifying the areas of surface(111),(110),(011)and(210)of Li2FeP2O7 is in favour of the thermodynamic equilibrium and ions transfer.We also consider the effect of Li content for the surface energy,equilibrium morphology,and voltage,so we construct the slabs of Li Fe P2O7 and Fe P2O7,including seven low-index surfaces and surface(210),respectively.The surface(111),(110),(011),and(210)are still presented on the Wulff shape.Moreover,all the voltage of the surface of Li Fe P2O7 are greater than the corresponding values of relevant surfaces in Li2FeP2O7.