The SEM micrograph of the as-synthesized compound is given in Figure . The as-synthesized particles show a coarse well-sintered structure which can be attributed to the high sintering temperatures used for the synthesis of the parent RP compound. Elemental mapping (see In order to be used as the cathode within the all-solid-state FIB, the as-synthesized La2Ni0.75Co0.25O4.08 was ball-milled together with the electronically conductive carbon matrix along with ionically conductive La0.9Ba0.1F2.9 solid electrolyte. This serves not only to mix all the components together to form an intimate interfacial contact, but also to reduce the particle size of the active material and to increase the electrochemically active surface area of the active material for the redox reactions. For the successful operation of the fabricated all-solid state FIB, elevated operation temperatures of around 170 °C are necessary in order to achieve sufficient fluoride ion conductivity of the La0.9Ba0.1F2.9 solid electrolyte, which is reported to be in the order of 10 -4 S cm . The temperature stability of the cathode composite mixture was also investigated by heating it at 170 °C for 100 hrs, i.e., conditions similar to the operating conditions of the FIB. From the comparison of the lattice parameters obtained from the Rietveld fits of the X-ray diffractograms of the as-synthesized parent oxide, ball-milled cathode, composite and heat-treated cathode composite, it is confirmed that ball milling and heating the composite does not influence the nature of the cathode active material, and that any structural changes induced are strictly from the electrochemical reaction (see Figure and Table S 1 of the supplementary material). Further, the weight fractions of La2Ni0.75Co0.25O4.08 and the solid electrolyte within the cathode composite determined from Rietveld analysis of the XRD data match well with the expected values.