Accelerating rate calorimetry (ARC) was used to investigate the impact of delithiated Li0FePO4 on the decomposition of LiPF6-based electrolyte. We used 1 M LiPF6 in a solvent mixture composed of dimethyl carbonate and ethylene carbonate ( 1:1 w/w). Commercially available LiFePO4-based 18650 lithium-ion cells were completely charged up to a cut-off voltage of 4.2 V and afterwards disassembled in an argon filled glove box. The whole sample preparation for an ARC experiment was carried out under argon atmosphere to prevent atmospheric influences. Beside the self heating rate, we also analysed the pressure rise during an experiment to evaluate the influence of delithiated Li0FePO4 on the electrolyte decomposition, which is primarily initiated by the conducting salt LiPF6. The results show both in the self heating rate and the pressure development an inhibiting effect of delithiated Li0FePO4 on the electrolyte decomposition. This effect is independent of the state of charge (SOC) and seems to be typical for (delithiated) Li0FePO4 in contrast to a commercial Li[Ni0.33Co0.33Mn0.33]O2/LiCoO2 blend. Besides that, we investigated the thermal behaviour of the bare Li0FePO4 and in presence of salt-free solvent. X-ray diffraction after measurements of delithiated Li0FePO4 in presence of electrolyte or salt-free solvent showed a new crystalline phase.