First-principles Study On Conductivity Properties Of A Series Of Compounds Based On La₂CoMnO₆

Energy shortage and environment pollution are the two problems faced by the whole world. Solid state fuel cells are considered to be important means to solve these problems because they can transfer chemical energy of fuels into electricity efficiently and environmentally friendly. The high operation temperature of traditional SOCFs results in premature aging of interconnect materials and large electrode polarization cause great performance loss of the fuel cells, which restrict the wide use of SOFCs. Now, researchers pay attention to materials that can operate efficiently at 400-700℃. And the mixed ionic electronic conductor (MIEC) cathode materials are the hotspot of research. Within this context, we attempt to find out some novel MIEC cathode materials based on the first-principles calculation. On one hand, we would figure out mechanisms for the experimental results, on the other hand, we would conclude some principles to conduct the synthesis of new cathode materials. This thesis can be mainly divided into four chapters.The first chapter can be divided into two parts. In the first part, we briefly introduced the history of fuel cells, the main constitutes and working principle of SOFCs, the sorts of materials used by the component parts of SOFCs and the current situation of cathode materials in experiment and in theory. In the second part, we introduce the structure characteristics of perovskite compounds, the theory of our calculating methods and the software we use.In the second chapter, we studied the effects of antisite defects (ADs) on electrical conductivity and the formation energy of oxygen vacation in La2CoMnO6 (LCMO). The results show that the antisite defects (like Co on Mn’s site (CoMn) and Mn on Co’s site (Mnco)) intrinsically exist in LCMO. When Co and Mn on their own sites, they are +2 and +4, respectively, but when on the antisites, they both tend to be +3. Ads results in more states in the Fermi level and there are strong hybridation of 3d orbitals of Co and Mn and 2p orbitals of O, which are benificial to the conduct of electrons. This indicates Ads would increase the conductivities of electrons. What’s more, the delocation of electrons also contributes to the formation of oxygen vacancies. The formation energies for oxygen vacancies are predicted to follow the trend Co2+ -O-Mn4+> Co2+ -O-Co3+> Mn3+ -O-Mn4+ and the underlying microscopic mechanism is attributed to the more electron delocalization between mixed-covalent transition metals (Co2+ -O-Co3+ and Mn3+ -O-Mn4+), which is beneficial to diminish the electronic repulsion and helps to stabilize the vacancy. Therefore, we could conclude that oxygen ionic conductivity should be enhanced in the compounds with higher degree of cation disorder.In the third chapter, we studied effects of Sr substitution on the conductivity properties of La1-xSrxCoMnO6(x=0.5,1,1.5) compounds. Sr is +2 and La is +3 in these compounds, then Sr replacing parts of La results in some holes. We find that before the ratio of Sr/La less than one, the holes introduced by Sr mainly affect Co and make the oxidation state of Co increase from +2 to +3, thus making the Co and Mn on the B sites of the compounds tend to be disordering, which results in smaller oxygen vacancy formation energy and improving the ionic conductivity. When the ratio of Sr/La exceeds one, the extra holes transfer to oxygen sublattice, leading the valence of oxygens to be less negative, which further decrease the formation energy of oxygen vacancy and increase the ionic conductivity. In La0.5Sr1.5CoMnO6-δ, the energy of formation of oxygen vacancy is only 0.62 eV, and the migration barrier is 0.58 eV, which indicates oxygens could diffuse easily in this compound. We conclude that introducing Sr in the A site of LCMO makes the electrons left by removing a neutral oxygen atom more delocalized, which diminish the electronic repulsion and helps to stabilize the vacancy.In the fourth chapter, we studied the effects of different cations on conducting properties of La2M0.5Mn1.5O6 (M=Co, Fe, Ni, Cu) compounds. Results show the band gap near the Fermi level of La2Cu0.5Mn1.5O6 is the lowest, implying it could be better in electronic conductivity. Because the valence of Cu is lower than Co、Fe、Ni in these compounds and it could easily transfer from +2 to+ 1, the electrons left by removing a neutral oxygen atom would prefer to inject to Cu, thus, promoting the formation of oxygen vacancies and diffusion of oxygen. And the calculation results show La2Cu0.5Mn1.5O6 has the lowest oxygen vacancy formation energy and migration barrier, indicating La2Cu0.5Mn1.5O6 would be a promising cathode material.