The feasibility of using energy dispersive X-ray diffraction to characterize full size battery cells is demonstrated by unprecedented in situ measurements of the electrochemical processes taking place inside high temperature sodium metal halide (Na/MCl2 M Ni and/or Fe) cells during charge/discharge cycling Diffraction data provide phase information either via line scans across the 5 cm wide cells or via fixed location scans as a function of time The data confirm the propagation of a well-defined chemical reaction front as a function of charge/discharge time beginning at the ceramic separator and proceeding Inward Measurement of the temporal evolution of the phase abundances yields mechanistic understanding and reaction rates as a function of charge/discharge state In the case where M includes Fe the data also clearly show the appearance of an intermediate phase Na6FeCl8 during charging thereby underscoring the power of this technique to reveal subtle mechanistic information A number of additional detailed electrochemical kinetic effects are also discussed This study shows that in situ high energy X-ray diffraction characterization of advanced battery cells in space and time is eminently feasible on a routine basis and has great potential to advance the understanding of buried chemical processes (C) 2010 Elsevier B V All rights reserved