Investigations On Electrochemical Properties Of Electrode Materials For Lithium Ion Batteries By Multi-scale Simulations

There is an intense,worldwide effort to develop durable lithium ion batteries with high energy and power densities for a wide range of applications,including electric and hybridelectric vehicles.To meet the increasing demands of energy storage,particularly for transportation applications such as plug-in hybrid electric vehicles,researchers will need to develop improved lithium-ion battery electrode materials that exhibit high energy density,high power,better safety,and longer cycle life.Density functional theory?DFT?has been the most successful,widely used method in condensed-matter physics,computational physics and quantum chemistry to describe properties of condensed matter systems,which include not only standard bulk materials but also complex materials such as molecules,proteins,interfaces and nanoparticles.These methods can prescreen previously unknown compounds and can explain complex phenomena observed with these compounds.Based density functional theory,the diffusion behavior and electronic structure of surface modified LiMnPO₄ using LiFePO₄ coating as cathode materials for lithium ion battery is studied.The calculated diffusion coefficient of Li along the?1?0?0?interface is in the range of 3.65?×?10^-115.28??×??10^-12 cm²/s,which is larger than that in the pure LiMnPO₄ with a value of 7.5??×??10^-14 cm²/s.Therefore,the charging/discharging rate performance of the LiMnPO₄ can be improved by surface coating with the LiFePO₄.We also investigate Li diffusion behavior in the bulk cathode LiNiPO₄ materials with defects.The energy barrier of Li is 0.28 eV in the pristine LiNiPO₄.The energy barriers of Li diffusion along the[010]direction in the LiNiPO₄ were not affected so much compared with that in the pristine system,however,the diffusion energy barrier of Ni diffusion was much larger than that of Li.This indicates that the existence of antisite defect will severely impede Li diffusion along the[010]direction and thus explains the poor cycling performance of LiNiPO₄ cathode materials for LIBs.Structural phase transitions of electrode materials are studied based on density funcitional theory.The origin of the Ni surface segregation for Li-rich Mn-based layered transition metal oxides is investigated by calculating the defect formation energies of antisite defects.The simulation results show that antisite defects are energy favorable in Li-rich Mn-rich layered oxide cathode.Al-doping is beneficial for improving the structure stability and preventing the surface segregation,thus Al-doping can improve cycle ability and rate capability of Li-rich and Mn-rich layered cathodes.Meanwhile,we aslo study the surface phase transition for LiNi_0.5Mn₁₅O₄.The Ni and Mn ions in the surface regions of the LiNi_0.5Mn₁₅O₄ easily occupy the tetrahedral Li-positions during delithiation process,which supports the experimental results and explains the surface structure changes of the LiNi_0.5Mn₁₅O₄ upon delithiation.Using a density functional theory,we studied the electrochemical propertieso of Sn anode.The diffusion barrier of single lithium atom in the?-Sn is 0.21 eV,which shows a little change as another lithium atom was next to it.However,the barrier is 0.39eV for lithium in the?-Sn and the lithium tend to bond together when more lithium atoms appear.The different behavior of diffusion barriers is attributed to a different binding energy of Li-Li atoms in Sn with the different crystal structures.The?-Sn used as an anode material for lithium ion batteries is preferable to?-Sn for the fast diffusion of lithium.Diffusion of Li in Li _x Sn alloys is also investigated in order to fully understand the lithiation process in these types of Li ion batteries.Variation of the calculated open-circuit voltages of the Li _x Sn alloys is found to agree well with experimental results.Diffusion coefficients of the Li in the Li _x Sn alloys are in the range between 6.6×10^-8 and 5.6×10^-7 cm²/s at room temperature according to our calcualtions,which is within the range between 8.0×10^-8 and 5.9×10^-7 cm²/s obtained from the experimental measurement.Li-Si intermetallics,Li₁₂Si₇ and Li-Si alloy as anodes of lithium ion batteries are studied using first-principles and first-principles molecular dynamics method.Vacancy formation and diffusion of lithium ions showed a strong dependence on crystallographic sites of the lithium ions.The thirteen crystallographic lithium ions in the Li₁₂Si₇ can be divided into three types based on Li mobility.These crystallographic lithium ions take part in the fast diffusion process and are distributed within one dimensional column.