Phase-field modeling is not only a powerful simulation tool to
predict the microstructure evolution but also a useful
theoretical method to study the interface kinetics. In this paper,
the thermodynamic principles for and the recent progress in the
phase-field modeling of isothermal solidification of binary alloys
are reviewed. Different phase-field models with or without the
condition of equal concentrations or equal diffusion potentials
are reformulated from the entropy/free energy functional or the
thermodynamic extremal principle. Their physics behind,
problems and relation to the sharp interface models are
analyzed. The importance to solve directly the additional
constraints in the modeling system self-consistently in
thermodynamics is highlighted