| Li-ion batteries(LIBs)have contributed greatly to the human society over the past 30 years,especially by dominating the portable electronics field.However,the energy density of commercial LIBs is still not sufficiently high to meet all these applications,which is mainly limited by the capacity of their cathode materials.To address this issue,over the past two decades,enormous efforts have been devoted to developing new cathode materials with higher eapacities.More recently,the discovery of anionic redox activity in Li-rich materials has been shown to offer a promising way to achieve this goal.Taking advantage of the cumulative cationic and anionic redox processes,materials such as Li-rich materials can exhibit a capacity of over 250 mAh/g.However,the asset provided by such staggering capacities is negated by the capacity fade and voltage decay upon cycling.The circumvention of this issue would provide huge opportunities to increase the energy density of commercial LIBs,but better understanding on the underlying mechanism during charge and discharge should be first obtained.In this dissertation,we performed a comprehensive study on the structural transformation and its influence on the electrochemical performance of Li-rich transition metal(TM)oxides by using the first-principles calculations.In the first part of the dissertation,by using the first-principles calculations combined with experimental measurements,the structural evolution of Li2RuO3 and its influence on the electrochemical performance were investigated.Our results indicate that a two-step structural transformation would occur for Li2RuO3 upon cycling,one is related to the rearrangement of oxygen array when half of the Li ions are extracted,and the other to the cation migration upon further delithiation.The calculated electronic structure reveals that the removal of electrons from the O 2p orbitals of LixRuO3(x<1)upon delithiation will lower the stability of its host structure,which triggers the cation migration from Li-TM to Li layers.As a consequence,the significant change of oxygen local environments can re-stabilize the delithiated structures and effectively suppress the O2 release.However,some drawbacks like the voltage decay and poor ionic kinetics are also found due to the TM migration by our theoretical simulations.In the second part of the dissertation,we performed a comprehensive study on the mechanism of redox reaction in Li1.2Ruo 6Ni0.2O2,Li2RuO3 and Li2Ru0.6Ni0.4O3-?.Calculated results show that the pDOS of the oxygen 2p states of Li2Ru0.6Ni0.4O3-? is most close to the Fermi level,which is followed by that of Li2RuO3 and Li1.2Ru0.6Ni0.2O2.This explains why Li2Ru0.6Ni0.4O3-? has a lowest oxygen oxidation potential among them.Furthermore,we find that the pDOS of the oxygen 2p states will be affected by the local oxygen environment.In these three systems,the difference in the content of substitution of Ni and structural transformation after substitution will lead to difference in the local oxygen environment,which will impact the pDOS of the oxygen 2p states.In the third part of the dissertation,the mechanisms of cation migration in Li2MO3(M=Mn,Ni,Mo,Ru and Ir)were studied.Our results show that the cation migration would occur in Mn and Ni-based layered oxides before the 50%state of charge.While,for Mo,Ru and Ir-based layered oxides,the cation migration only occurs upon high charge states.By analyzing the charge compensation and the formation energy of oxygen vacancy in these five Li-rich materials,we fourid that oxidation of oxygen would decrease the stability of host structure and release O2.As a result,the unstable structure will trigger the cation migration,which can re-stabilize the dehthiated structures by changing oxygen local environments.Therefore,cation migration can be viewed as a structural self-reguiation driven simultaneously by the anionic redox reaction in Li-rich layered oxides.This explain why the cation migration is related to the different elelnent of Li-rich layered oxides with different 0 redox reaction. |
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