Spin-polarized density functional theory calculations employing the plane-wave pseudopotential technique were used to investigate the atomic,electronic structures of ferromagnetic LaCoO3 and La1-xSrxCoO3 surfaces((001),(110)and(111)with different terminations).7-layer nonstoichiometric symmetrical slab models were adopted.The thermodynamic stability of these surfaces along with temperature and O2 partial pressure was analyzed with phase diagrams drawn with the total energies obtained from DFT calculations.The influence of Sr-doping on the thermodynamic stability of the surfaces was also examined.Results indicate that Sr ions prefer to substitute La ions in the outermost layers rather than those in bulk layers for both(001)surfaces.In undoped LaCoO3 system,CoO2-and LaO-terminated(001)surfaces are the most stable two of all considered surfaces under typical operational conditions of the SOFCs(T = 1100 K,pO2 = 0.2 atm),while Sr-doping in LaCoO3 crystal destabilizes the CoO2-terminated surface with respect to the La0.75Sr0.25O-terminated surface.These detailed results of La1-xSrxCoO3 surface properties at the atomistic level are of high importance for the further study of the oxygen reduction mechanism at the La1-xSrxCoO3 cathode.