The aging stability of several Li2FeSiO4 (LFS) polymorphs in LiPF6-based organic carbonate solvent electrolyte at elevated temperature was investigated. The different LFS polymorphs were obtained by hydrothermal reaction and subsequent annealing at different temperatures. In addition, a carbon-coated sample was prepared. The samples were aged separately from each other at 60°C in LiPF6-based organic electrolyte for 10 days and analyzed afterwards. The characterization of the residual powder by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) reveals a different aging behavior depending on the structure of the LFS polymorph. This influence of the polymorph on the stability of LFS powder against HF was confirmed by investigations of the aged electrolyte by nuclear magnetic resonance (NMR) spectroscopy and ion chromatography (IC).