First-principles Studies Of Doping Effects On Li₂MnO₃ Cathode Materials For Li-ion Batteries

Li2MnO3 is an important component of the Li-rich solid solution cathode material. The electrical conductivity of Li2MnO3 is low due to its intrinsic insulating electronic structure, which is unfavorable to the fast charge/discharge of the battery.Using the first principles calculations, this thesis studied the electronic structure of F,Cl and P doped Li2MnO3. The formation and migration of small polarons in F, Cl and P doped Li2MnO3 are studied, particularly. The results demonstrate that F doping can create polaron states in the Li2MnO3 lattice. The migration energy barriers of small polaron are low(~0.27 eV). But, F doping is not an efficient way to improve the electronic conductivity of Li2MnO3. There are two reasons:on one hand, the doped F atom has strong trapping effect(~0.6 eV) to the small polaron, which is bad for the migration of the small polaron. On the other hand, the concentration of the polaron states is dependent on the doping concentration. Considering the Cl and F are in the same main group, however the electronegativity are different. Further, we studied the influences of Cl doping on Li2MnO3. The results of Cl doping are similar to the results of F doping. Then, we studied the atomic structure and electronic structure of P doping at Li sites in Li2MnO3. The results show that: P doping can create small polaron, more efficiently. The trapping energy between doped P and small polaron is small, compared with that in F doped Li2MnO3. It is shown that the migration of single small polaron are easy, the energy barriers are within the scope of 0.14~0.33 eV, for different migration pathways. Furthermore, the migration energy barriers of two polarons were calculated, the migration energy barriers are about 0.46 eV. By comparing the Bader charge and integration charge of O atoms in delithiated Li2MnO3 with and without the P doping. We can make the conclusion that P doping is conductive to suppress the O loss electron upon Li removal at high potentials,improve the structural stability.All in all, our results indicate that F, Cl doping at O sites are not effective ways to improve the electrical conductivity, however, P doping at Li sites is an effective way to improve the electrical conductivity and structural stability of Li2MnO3.