The synthesis of the active material resulted in a phase-pure material (see Figure ) with no additional impurity phases. Iodometric titration further confirmed the formation of the desired LaSrMnO~4 phase, with the oxygen stoichiometry determined to be 4.06 (corresponding to Mn +3.12 ), which is close to the target of 4.0. Therefore, it will be referred to as LaSrMnO4 throughout the paper. The active material. crystallizes with a tetragonal crystal structure in the I4/mmm space group and the lattice parameters were determined to be a = 3.7928(6) Å and c = 13.1326(3) Å, which are comparable to the literature values of a =3.82759 (19) Å and c = 13.07090(79) Å. To be used in the all-solid-state FIB, the as-synthesized active cathode material was ball-milled with electrolyte and SWCNTs, as described in section 2.1.2. Figure a shows the typical charging curve of LaSrMnO4 II La0.9Ba0.1F2.9 II Pb-PbF2. During the charging process, when the potential reaches 3 V, the charging capacity is approximately 280 mAh/g, which surpasses the active material theoretical capacity of 155 mAh/g. This suggests the occurrence of secondary reactions, such as carbon fluorination . To obtain a deeper understanding, structural changes in the cathode material during charging of the LaSrMnO4 II La0.9Ba0.1F2.9 II Pb-PbF2 system were analysed using ex-situ X-ray diffraction A steep increase in the potential from 0 V to 0.12 V marks the beginning of the electrochemical reactions which involve processes like transport of F⁻ ions from the anode to the cathode and accumulation of charges at the electrolyte/cathode interface . In the range from 0.12 V to 0.2 V, a plateau typical of a two-phase domain was observed, leading to a capacity of approximately 50 mAh/g, which corresponds to a composition approximately equivalent to LaSrMnO4F~0.75. During this process there is a change in lattice parameters as shown in Figure followed by the shifting of the most intense reflection